app a risk pdf

8/6/2019 App A Risk pdf

1/62

Appendix A

A Review of Risks Associated to Arsenic,Cadmium, Lead, Mercury and Zinc

Mikael JohannessonKalmar University, Sweden

The Market Implication of Integrated Management of Heavy

Metals Flows for Bioenergy use in the European Union

Thermie STR/1881/98-SE


2/62

2

Published 2002 by Kalmar University

Department of Biology and Environmental Science

Environmental Science Section

SE-391 82 Kalmar, SWEDEN

http://www.bom.hik.se/ess

ISBN 91-89584-07-4 (This report, appendix A)

ISBN 91-89584-06-6 (The main report)

This report is an appendix to the report The Market Implication of Integrated Management

for Heavy Metals Flows for Bioenergy use in the European Union.

The correct citation of this report is:

Johannesson, M.: A Review of Risks Associated to Arsenic, Cadmium, Lead, Mercury and

Zinc, p. 62. Appendix A in Johannesson, M. (ed.) et al.: 2002, The Market Implication of

Integrated Management for Heavy Metals Flows for Bioenergy use in the European Union.

Kalmar University, Department of Biology and Environmental Science, Kalmar, Sweden, p.115.
http://www.bom.hik.se/esshttp://www.bom.hik.se/ess


3/62


4/62

4

5. Mercury (Hg)..................................................................................................... 38 5.1 General information .................................................................................................... 38

5.2 Use ................................................................................................................................. 38

5.3 Production..................................................................................................................... 395.4 Emissions and deposition............................................................................................. 39

5.5 Concentrations (in nature and human)...................................................................... 41

5.6 Limit values .................................................................................................................. 43

5.7 Risk assessments, exposure and harmful effects ....................................................... 43

6. Zinc (Zn)................................................................................................................ 48 6.1 General information .................................................................................................... 48

6.2 Use ................................................................................................................................. 486.3 Production..................................................................................................................... 49

6.4 Emissions and deposition............................................................................................. 49

6.5 Concentrations (in nature and human)...................................................................... 50

6.6 Limit values .................................................................................................................. 52

6.7 Risk assessments, exposure and harmful effects ....................................................... 53

7. References............................................................................................................. 58


5/62

5

1. Summary

1.1 Introduction1

The environmental and health risk associated to heavy metals and how to reduce the risks hasbeen discussed on the scientific and political agenda for decades. Several efforts have beenmade by national and international bodies such as WHO, FAO, EU, OECD, and UN-ECE inorder to survey the risks and decide on measures to reduce the risks.

This first chapter includes a summary and conclusions made from the following review of therisks associated to the heavy metals cadmium, mercury, lead and zinc. Arsenic, althoughclassified as a metalloid, is also included in that study. Among the data that has been reviewedand put together for each element are: use and production, concentrations in human andnature, emissions and deposition, trends, limit values, harmful effects. There are two aims

putting all these data together: 1) Make an overall risk analysis of the elements in questionincluding all relevant environmental and human aspects, 2) analyse to what extent biofuelhave a relevance for the risks associated to these elements. If we shall reduce the use of fossilfuel in order to avoid the threat of an anthropogenic climate change, and partly replace thefossil fuel use by increased use of biofuel, we have to analyse the risks associated to asignificant increase of bioenergy. One important risk to analyse is then the risks of a changedflow and exposure of heavy metals in the society and environment.

Data mainly from review reports that summarizes the knowledge from different areas hasbeen put together for each element. Reports from international bodies such as WHO, OECD,EU, and FAO has preferably been used but also reports from national authorities andscientific books that deals with a specific area of interest. The use of review reports compared

to a deep review of the specialized literature saves a lot of time, especially in this kind ofstudy that include so many different aspects. When using review reports the data willinevitable be some years old. However, in this case we are most interested in the rightmagnitude of the figures and of qualitative data.

The picture given here describe in particular the general situation in the western world, whichis regarding to most aspects investigated quite similar in most of these countries. However, inorder to be cautious and not underestimate the risks, emphasize has when possible been put onthose data that indicate the biggest risks.

In this context it is important to note that the harmful effects of a substance is a combinationof the substance inherent properties and the exposure of the substance. Thus, also smallconcentrations/flows might constitute a significant risk to human and nature. Although small,these concentrations/flows must be taken into consideration. It is also important to note thatthese elements occur in many different forms, which have various properties and constitutedifferent levels of risks. Further, they change form and availability due to chemical and

physical conditions in the surrounding medium, and due to biological activity.

Due to lack of data and the fact that the elements/compounds undergoes transformation duringthe flows in the nature and the society it is not possible to make separate risk analysis for each

possible compound that could include these elements. However, we have to bear in mind thata comparatively harmless compound might undergo a transformation to a much more

potentially harmful compound during specific conditions. This means that it is wise to use a

1 It should be observed that this report was written in 1999 and some data concerning limit values, emissions etchave not updated. In the main report there may be more recent data available. However, most kinds of data arerelevant and do not get old in a couple of years. From the source it is also possible to identify the age of the data.


6/62

6

precautionary approach when identifying flows/concentrations of importance for the riskhumans and nature are exposed to.

1.2 Production, emission, concentration

The production of arsenic has gone up and down during the last 1015 years and it is hard tosay anything about the trend. The production of cadmium has been stable since 1985, whilethe zinc primary production has increased. The world primary production of lead has since the

beginning of 1980s remained on about the same level, while the secondary produced lead hasincreased. Only the production of mercury has decreased substantially during the last 1015years.

For all studied elements the emissions are decreasing and are expected to continue todecrease. However, large quantities are stored in products in the society and will graduallyreach the environment unless they are taken care of safely. Some uses, for instance the use oflead for ammunition, will inevitably end up in the environment. This means that the elementsflow from society to the environment will not stop even if the emissions from production etc.

are stopped. Further, large quantities of the lead, cadmium and mercury are stored in thenature. For instance, the very large amounts of mercury in soil constitute a chemical timebomb that can cause severe pollution of aquatic ecosystems if it should be mobilized.

However, the concentration in air and watercourses of the elements has in general decreasedsubstantially due to the decreased emissions. But the response is much slower in soils,sediments, big lakes etc. were the turn overtime is much longer. In spite of the decreasedemissions to air is the balance of the elements in soils still in many areas positive (increasing)since the input via fertilizer, sewage sludge and air deposition together is greater than thegenerally slow removal of the elements.

1.3 Accumulation2, biomagnification

3, mobility, availability

The elements tendency to accumulate or biomagnify in organisms is dependent both on whichform the element is present and on the organisms. Relative differences in the uptake of metalions between plant and cultivars are controlled genetically and by various factors, includingsurface area of the root, root CEC (cation exchange capacity), root exudates and the rate ofevatranspiration. The transfer coefficient (the metal conc. in plant tissue above ground divided

by the total metal concentration in the soil) differs between metals. It is in generalconsiderable higher for cadmium and zinc, 110, than for arsenic, mercury and lead 0.010.1.However, since a numerous soil and plant factors can affect the accumulation of metals in

plants the transfer figures are not precise values but can indicate accumulation differences.

Arsenic is subject to bioaccumulation but not to biomagnification. Arsenic concentration is

elevated in marine biota. Plants take up arsenic in proportion to the soil concentration.However, some crops and mushrooms may accumulate high levels of arsenic even atrelatively low soil arsenic levels. Arsenic in soils is highly mobile, resulting in possiblegroundwater contamination, but retention and accumulation by biota should remain low.

Of all toxic metals released in large quantities into the environment, cadmium is generallyregarded as the one most likely to accumulate in the human food chain. In certain situations,cadmium displays a propensity for marked bioaccumulation. Shellfish (e.g. crab and lobster),filter feeding molluscs and fungi can accumulate cadmium to considerable high levels. Plantspecies differ widely in their ability to accumulate cadmium. Leafy crops are capable of

2 Bioacumulation means that the concentration is considerably higher in the organisms than in their environmentor food.3 Biomagnification means that the concentration increases along food chains.


7/62

7

accumulating cadmium to relatively high levels. The most important factors influencing plantcadmium accumulation are soil pH and cadmium concentration.

The uptake of lead differs a lot between species and the accumulation of lead in differenttissues differs also a lot. Most plants seams to have a root barrier that prevents lead to reachthe sprout. In general there is a positive relationship between the concentration of lead in the

soil and the plant. However, only a small proportion of the lead in soil is in general availablefor uptake by plants. The lead concentration is in general higher in plants than in animals andhigher in herbivore animals than in predators. Thus, there is in general no biomagnification inthe lead case. When lead is released into the environment it has a long residence timecompared with most other pollutants. The proportion of bio-available lead is higher if the pH,content of organic matter and concentration of iron-hydroxide is low compared to the oppositecondition. This is essentially also true for the water environment.

Mercury is involved in a whole chain of environmental transformations where the physicaland chemical properties of mercury are changed. It is more mobile than most other heavymetals. Once released in the environment, mercury may be redistributed between and withinenvironmental compartments. Inorganic mercury may be methylated to more toxic organic

forms. Mercury is accumulated in soil invertebrates, aquatic invertebrates, and insects. Fishalso take up mercury and retain it in tissues, principally methylmercury. Due to bioaccumulation and biomagnification the mercury concentration could be comparativelyhigh in fresh and marine fish especially for fish of prey. Accumulation of mercury in plantsincreases with increasing soil mercury concentration. However, plant takes up only a small

portion of the total mercury in soils. Generally, the highest concentrations of mercury are likein the lead case found in the roots. Besides the species of mercury, the toxicity and uptake ofmercury to aquatic organisms is also dependent on for instance pH, organic matter, salinity,temperature, presence of selenium and sulphide ion, and water hardness.

Zinc is under special circumstance bioaccumulated in organisms but is not subject tobiomagnification in terrestrial or aquatic food chains. When the surrounding value is elevatedthe organisms on the lowest trophic level accumulate zinc (show higher concentrationcompared to the environment), while organisms on higher trophic levels (e.g. birds andmammals) accumulates zinc when the surrounding level of zinc is lower than normal. Theuptake/excretion of zinc is regulated by organisms on all trophic levels but to a higher degree

by organisms on the higher trophic levels.

1.4 Current load compared to toxicity and essentiality

The toxicity of the studied elements is strongly dependent on in which form they occur. Allinvestigated elements may occur in forms that are toxic to very toxic for living organisms.

Zinc and possibly also arsenic are the only ones that are observed to be essential for living

organisms in such amounts that there may be a risk for deficiency also in situations that couldnot be characterized as extreme. Zinc is essential for all living organisms and incomparatively large quantities. Arsenic is observed to be essential for animals, possiblyessential for humans but probably not for plants. Cadmium and lead is normally counted asnon-essential metals although there are studies that indicate that they may have a positiveeffect on growth for some organism at very low levels. Mercury has as far as we know nonecessary function in any living organism.

Most of the arsenic exposure is of natural origin. Humans may in special areas, due tohydrogeological conditions, be exposed to harmful levels of arsenic via drinking water. Theother important rout of exposure is via seafood. However, the bulk of the arsenic found inseafood exist in organic forms which are regarded as less toxic compared to the inorganicforms. Inorganic arsenic is classified as carcinogenic and there exist as far as we know no safelevel of exposure.


8/62

8

The major human exposure of cadmium for non-smokers is food and drinking water, whichamounts to about 80 % of the total intake. The critical effect for humans is probably tubulardamage. Even if the population average kidney concentration is relatively low for the general

population, a certain proportion may have values exceeding the value where renal tubulardamage can occur. It has been estimated that, at present average daily intake of cadmium inSweden, about 1 % of women with low body iron stores and smokers may experience adverse

renal effects related to cadmium. If the average daily intake of cadmium would increase to 30g/day, about 1 % of the entire population would have cadmium-induced tubular damage. Forwomen with low iron stores up to 5 % would have tubular damage. Some population groupsin Europe already exceed this intake and the margin is very narrow for large groups.

The major exposure of lead for the non-smoking exposed adult population is food and water.For infants older than about 5 month and young children dust/soil may also be a major sourceof exposure. Young children and foetus are more sensitive to lead exposure than olderchildren and adults. The toxic effects of lead have now been demonstrated at very low levels,and there is some suggestion that there may be no level of exposure below which lead isharmless. Lead is known to cause proximal renal tubular damage. For lead levels in blood lessthan 1.2 mol/l (25 g/dl) decrements in intelligence quotient (IQ) has been observed and

reduction in human peripheral nerve conduction velocity may occur with levels as low as 1.44mol/l (30 g/dl). Recent studies made by Bringmark and Bringmark at the Swedishuniversity of agricultural science (SLU) indicate that lead and mercury may distinctly affectmicrobiological soil processes at lower levels than previously found. Effects were observed atlevels that could be found in forest soils in the south of Sweden.

The general population is primarily exposed to mercury through diet (organic mercury fromfish and inorganic mercury from non-fish food) and dental amalgam (inorganic mercury). Theclinical and epidemiological evidence indicates that prenatal life is more sensitive to the toxiceffects of methylmercury than is adult life. The inhibition of protein synthesis is one of theearliest detectable biochemical effects in the adult brain. The kidney is the critical organfollowing the ingestion of inorganic divalent mercury salts. The central nervous system is thecritical organ for mercury vapour exposure. It is not possible to set a level for mercury (e.g. in

blood or urine) below which (in individual cases) mercury-related symptoms would not occur.Human hypersensitive reactions (dermatitis) to metallic mercury have also been identified.Certain groups with high fish consumption may attain a blood concentration of about 200 glitre/litre, (corresponding to 50 g/g of hair) associated with a 5 % risk of neurologicaldamage to adults.

Microorganisms responsible for decomposition of cellulose are usually very sensitive toheavy metals, especially mercury. There are indications that the regional enhanced concentra-tions of mercury in central Europe and southern Scandinavia may have an adverse effect onmicrobiological life processes, and indirectly on the recycling of important nutrients and onthe tree vitality.

Except for occupational exposure and point sources zinc do not probably occur in suchelevated concentrations that negative effects should be expected. On the contrary, if zincsupply is not given, there is a risk of zinc deficiency in arable soils and for humans withcertain needs. However, some aquatic organisms are very sensitive to zinc and elevatedconcentrations could be found downstream a zinc mine even long time after the mining isceased.

1.5 Concluding remarks

Table 1.5b summarizes important information regarding arsenic, cadmium, lead, mercury andzinc with respect to environmental and health risk analyses.

Further, the following 5 important concluding remarks could be made:


9/62

9

1) In the environment commonly found concentrations of arsenic, cadmium, lead and mercuryconstitute a risk for living organisms and may cause distinct negative effects on sensitiveorganisms and humans during specific conditions. At least for zinc and possible also forarsenic there may also be a risk for deficiency during specific conditions.

2) Although the emissions of cadmium, mercury and lead has decreased significantly during

the last decades and probably will continue to decrease, the concentrations of these elementsin for example forest soils and arable soils will continue to increase or decrease very slowlyduring the coming decades.

3) The plant uptake of zinc and cadmium ions is in general considerably larger than it is forarsenic, lead and mercury.

4) In order to eliminate or significantly reduce the risk and negative effects on organisms andhumans that are not occupationally exposed or exposed by point sources, the diffuseemissions of especially cadmium, lead and mercury have to be reduced significantly.However, the exposure from point sources may be of significant and crucial importance forindividuals and for the environment situated near them. This includes also zinc and arsenic.

Point sources contribute also significantly to the overall load.5) It is possible divide the five elements into four groups based on both the overall risk theyconstitute and on their relevance in the context of bio energy use. See table 1.5a below.

Table 1.5a Grouping of As, Cd, Hg, Pb and Zn with respect to risk and biofuelrelevance

High risk and high relevance

Cadmium

Moderate risk and high relevance

Zinc

High risk and moderate relevance

Lead, Mercury

Moderate risk and moderate relevance

Arsenic

1) High general risk and high relevanceCadmium: The current load constitutes a risk to human and the environment. The load willcontinue to be high for several decades. Cadmium is taken up in comparatively high amounts

by plants.

2) High general risk moderate relevanceLead and mercury: The current load constitutes a risk to human and the environment. Theload will continue to be high for several decades. The uptake by plants is moderate.

3) Moderate general risk and moderate/high relevanceArsenic and Zinc: The current load constitutes a moderate risk to human and the environmentexcept for occupationally exposed and exposure near point sources. Elevated arsenic exposurevia drinking water should be identified and eliminated. Zinc is taken up in comparatively highamounts by plants. But it constitutes in general no problem. The uptake of arsenic by plants ismoderate.


10/62

10

Table 1.5b Summary of some qualitative data of importance for the assessment of human an

associated to arsenic, cadmium, lead, mercury and zinc in relation to an increased use of bio

Arsenic Cadmium Lead Mer

Primary production. No trend last 15 years. Constant last 15 years. Constant last 15 years. Significant dlast decades

Emissions to air & water. Decreasing. Decreasing. Decreasing. Decreasing.

Change of concentration

in soil.

? Increasing/constant.

Decreasing in the upper

part of some European

forest soils.

Increasing/constant. Increasing/c

Soil uptake by plants. Low uptake by root. Some species, varieties

or clones accumulate

high concentrations.

Low uptake by root. Low uptake

Current load to humanand the environment

compared to toxicity.

In general low. Highhuman exposure via

drinking water in some

areas.

High. Human renaltubular damage and

osteoporosis (risk

groups). Probably

effects on sensitive

organisms.

High. No safe limit.Probably effects on risk

groups and sensitive

organisms.

High. No safProbably effe

groups and s

organisms.

The importance of

terrestrial plants for

element exposure and

circulation.

Moderate. High. Relatively high source of

exposure. Moderate

importance for

circulation.

Moderate.

Accumulation and bio-

magnification.

Bioaccumulation, yes.

Especially in the marine

environment. Bio-

magnification no.

Bioaccumulation, yes.

Both in the terrestrial

and in the marine

environment.

In general no

bioaccumulation or bio-

magnification.

Bioaccumula

bio-magnific

Especially o

mercury in a

organisms.

Essential for living

organisms?

Essential for animals.

Possibly also for

humans.

Non-essential. Possibly

essential for some

species.

Non-essential. Possibly

essential for some

species.

Non-essenti


11/62

2. Arsenic (As)

2.1 General information

Arsenic is a grey element with a metallic lustre. Arsenic is classified as a metalloid andexhibits both metallic and non-metallic properties. Its chemical properties are similar to thoseof phosphorus and belong to the same group (Va) in the periodic table. Arsenic is present invalency -3, 0, +3, and +5. The valence 0 is unusual, and is not absorbed, while the -3 form(arsine) is a gas and would not account for any significant exposure under most environmentsettings. Elemental arsenic is insoluble in water but soluble in HNO3. The averageconcentration of arsenic in the earth crust has been estimated to 1.52 mg/kg. It is the 20thmost abundant element in the crust, fourteenth in the seawater, and twelfth in the human

body. Arsenic occurs naturally in more than 200 mineral species: from the native element oralloys to arsenides, sulphides and oxidation products (oxides, arsenites, and arsenates).Arsenic is present in many mineral deposits and in particular those containing sulphide

minerals. The most common mineral is arsenopyrites (FeSAs). Arsenic is in normal seawaterconditions present as HAsO42-. Arsenic has a rich and complex organic chemistry and this is

reflected to some extent in its environmental behaviour. Complex organic arsenic compoundssuch as tetramethylarsonium salts, arsenocholine, arsenobetaine, dimethyl (ribosyl) arsineoxides, and arsenic containing lipids have been identified in the marine environment. Avariety of complex organoarsenic compounds may be present at sites containing wastes fromcoal utilisation or oil production (WHO 1992:10; Park Ridge 1993:60; WHO 1986:17;Thornton & Farago 1997:2; North et al. 1997:407; Azcue & Nriagu 1994; Naqvi et al.1994:56).

Arsenic has been known for about 4 000 years and first gained notoriety as a poison. In 1885,it accounted for nearly one-third of the homicide poisonings in France (North et al. 1997:406).Arsenic differs from many of the common heavy metals in that the majority of organiccompounds are less toxic than inorganic arsenic compounds. Whilst having many chemicalsimilarities to phosphorus, the soil chemistry of arsenic is much more diverse because it canexist in more than one oxidation state under the normal range of soil conditions, and arseniccan form bounds with sulphur and carbon much ore readily than does phosphorus (ONeill1995:105, 110).

Arsenic compounds are often unstable, and in many cases not well defined materials. Forexample, the arsenites (+3) of the alkali metals are slowly converted in solution to arsenates(+5), by atmospheric oxygen. Arsenic trisulphide reacts vigorously with oxidizing agents, andhydrogen sulphide is generated on contact with strong acids. Arsenic trichloride is highlyreactive with water, strong oxidants, ammonia, and some alkalis; the reaction results in thegeneration of hydrogen chloride and chloride gas. Inorganic arsenic compounds may generate

highly toxic (and flammable) arsine gas when in contact with acids plus reducing metals (e.g.,zinc or iron), or with sodium hydroxide plus aluminium (WHO 1992:9-10).

As a result of the presence of arsenic in the parent rock, arsenic is present naturally in soils invarious quantities. Inorganic arsenic is found chiefly in the form of its compounds with metals(arsenides), which usually occur in isomorphous mixture with sulphides (WHO 1992:13).

Arsenic in soils is highly mobile, resulting in possible groundwater contamination, butretention and accumulation by biota should remain low. Any retention of arsenic by soilswould occur by adsorption especially if the soils contained iron or aluminium oxides(Bhumbla & Keefer 1994:73).

Arsenic is subject to bioaccumulation in marine organisms but in general not tobiomagnification. There is substantial evidence in animals that arsenic is an essential trace


12/62

12

element, but only a little data on its potential essentially in humans. Arsenic is not essentialfor plants (WHO 1986:51-52; North et al. 1997:407; Alloway 1995:3031).

2.2 Use

Metallic arsenic is used in alloys, in combination with lead and copper, in semiconductordevices, and in glasses. The most important use of arsenic(As3+)oxide is in the manufacture ofa variety of insecticides, herbicides, fungicides, algicides, sheep dips, and pharmaceutical

products. Arsenic sulphide is used for dehairing skins intanning, in the manufacture ofpyrotechnics and semiconductors, and in the manufacture of special optical glass. Calciumand lead arsenates have been used as insecticides but have largely been replaced. About onthird of the world arsenic production is used for wood preservation. They usually consist ofchromated copper arsenate. Other examples where arsenic compounds are used are solar-cells,light emitting diodes, paints, dyes, ceramics, algaecides and feed additives (for instance

prevention of swine dysentery) (WHO 1992:12; North et al. 1997:406; Azcue & Nriagu1994:12).

The addition of up to 3 % of arsenic hardens lead and minimizes the softening of lead-basebearing alloys used in internal combustion engines. Automotive body solders usually contain0.5 % of arsenic. The addition of arsenic (0.52 %) improves the sphericity of leadammunition (Azcue & Nriagu 1994:13).

From the 1860s until the introduction of DDT and other organic pesticides in the 1940s,inorganic compounds of arsenic remained the dominant insecticides available to farmers andfruit growers (Azcue & Nriagu 1994:12).

Arsenic compounds have been used in medicine since the time of Hippocrates ca. 400 B.C.E.Inorganic arsenicals have been used for centuries, and organoarsenicals have been used for atleast a century in the treatment of syphylis, yaws, amoebic dysentery and trypanosomiasis(Eisler 1994:188).

2.3 Production

The world production of white arsenic (arsenic trioxide) was in 1994 (estimate) about 43 000tonnes. In 1985 the world production was about 53 000 tonnes, and in 1989 it was 61 000tonnes. According to Chapman et al. (1999) the world production of arsenic trioxide wasabout 50 million tonnes in 1997 from the countries listed. However, in addition to thecountries listed eight more countries were believed to produce arsenic trioxide. It is fromthese date hard to say anything about the trend during the last ten years (Chapman et al.1999:21; Knight-Ridder financial/commodity research bureau 1996:7).

China and Chile was the worlds largest producer of arsenic and contributed to about half ofthe world production in 1997. In 1997 France and Belgium produced most arsenic among theEU-countries listed by Chapman et al. (Chapman et al. 1999:21).

Arsenic is recovered as a by-product of processing certain complex ores that are mainly forcopper, cobalt, lead, zinc, gold and silver, and its supply is dependent to a large extent on thedemand for these metals. Arsenic trioxide is the most important commercial compound. TheUnited States was the worlds largest consumer of arsenic in 1994, accounting for about one-half of the total world demand (Azcue & Nriagu 1994:58; Knight-Ridder financial/com-modity research bureau 1996:7).


13/62

13

2.4 Emission and deposition

The greatest amount of flux of arsenic has been estimated to occur from land (3 000 x 108g/yr.) and atmosphere (2 000 x 108 g/yr.) to oceans, followed by oceans to sediments (2 500 x108 g/yr.) and sediments to land (2 400 x 108 g/yr.). The amount of flux from oceans is fairlyhigh to sediments, biota (1.300 x 108 g/yr.) and dissolved phases (1 000 x 108 g/yr.). On the

other hand, the amounts of flux from land to biota (300 x 108 g/yr.), or terrestrial biota to land(300 x 108 g/yr.) is medium to low, and less than from mining and smelting (tot 500 x 108g/yr.). Very low flux has been estimated to take place from volcanoes to land (54 x 108 g/yr.),to sediments (40 x 108 g/yr.) and to atmosphere (3 x 108 g/yr) (Bhumbla & Keefer 1994:54).

Sources of arsenic into drinking/irrigation waters into soils, dust and the food chain may bederived from (a) naturally occurring arsenic-rich minerals and rocks, (b) from sulphidemineralization and waste materials resulting from mining and smeltering activities, and fromthe burning of arsenic-rich coal (Thornton & Farago 1997:7). One example of previousemission from a plant in Slovakia burning coal is mentioned by Bencko. The plant burnedlow-grade coal with an arsenic content ranging from 800 to 1 500 g arsenic/ton dry weightand emitted about half a ton arsenic every day around 1960 despite the use of electrostatic

eliminators (Bencko 1997:84).The roasting of arsenic-containing (sulphide) ores and burning of arsenic rich coal releasesarsenic trioxide, which may react in air with basic oxides, to form arsenates, which can then

be deposited to soils. For example, in mineralised areas of Cornwall, concentrations of arsenicin surface soils range up to 2 500 mg/kg, and in the vicinity of old roasting ovens and smelterstacks it may range from 0.1 to 1.0 % or more (Thornton & Farago 1997:3).

Estimates for 32 countries in Europe of past and likely future air emission of arsenic,cadmium, lead, and zinc have been made by European Environmental Agency (EEA). Theemissions of arsenic to air has decreased from about 2 000 tonnes in 1975 to about 1 000tonnes in 1991, and it is expected to continue to decrease (EEA 1998:112). In 1990 the airemissions of arsenic in EU-15 has been estimated to 575 tonnes of which 202 came fromItaly, 166 from UK, and 122 from Germany (Eurostat 1998:78).

Arsenic has been added to soils in many ways. In 1988, the major portion about 40 % wasfrom commercial products and about 25 % was from coal fly ash and bottom ash, 14 % fromatmospheric fallout, 10 % from mine tailings, 7 % from smelters, 3 % from agriculture, and2 % from manufacturing, urban and forestry wastes (Bhumbla & Keefer 1994:55, Nriagu &Pacyna 1988).

2.5 Concentrations (in nature and human)

More than 99 % of the total arsenic in the environment is present in rocks. Almost all the rest

are found in the oceans and in the soil, while biota and atmosphere contain very littlecompared to the other reservoirs (Bhumbla & Keefer 1994:5354).

The average concentration of arsenic in the earths crust has been estimated to be about 1.8mg/kg. The average concentrations of arsenic in igneous rocks, limestone, sandstone andshale are 1.5, 2.6, 4.1 and 14.5 mg/kg respectively. Arsenic is concentrated in some reducingmarine sediments which may contain up to 3 000 mg/kg. Soils overlaying sulphide-oredeposits may contain several hundred mg/kg (WHO 1992:13). Uncontaminated soils usuallycontain 140 mg arsenic /kg, with the lowest concentrations in sandy soils and those derivedfrom granites. However, there are also reports of high concentrations in soils from granitesand quartzite (up to 250 mg/kg for soils derived from granite). Higher concentrations arefound in alluvial soils and in organic soils. The estimated mean value of arsenic in

uncontaminated soils is 56 mg arsenic/kg, although divergent values (both higher and lowerconcentrations) could be found in specific countries or regions (WHO 1986:17; Thornton &Farago 1997:2-3; Huang YanChu 1994:18, 21; Bhumbla & Keefer 1994:5354, 62).


14/62

14

High levels of arsenic may be present in some sulphide-rich coals (up to 1 500 mg/kg).Arsenic is present in much larger amounts in fly ash than in bottom ash. Average arsenicconcentrations in coal ashes have been reported to vary from 56 mg/kg and 128 mg/kg to 156mg/kg, but some samples may exceed 1 000 mg/kg. Arsenic can be present in sewage sludge.Some researchers have reported concentrations from 3 to 46 mg of As/kg in sludge from theUnited States and the Netherlands (Bhumbla & Keefer 1994:56; 62).

The airborne concentration of arsenic in unpolluted areas is generally less than a fewnanograms per m3, while as much as 1 g/m3, or more, has been recorded near point sources(WHO 1986:47). The concentration of arsenic in fresh water varies greatly. Theconcentrations in unpolluted fresh waters typically ranges from 110 g/litre, rising to 1005000 g/litre in areas of sulphide mineralization and mining. Natural high concentrations ofarsenic in drinking water constitute a serious health problem in several regions around theworld (see chapter 2.7) (Thornton & Farago 1997:2).

Arsenic in aquatic systems is preferentially distributed to the sediment. Arsenic associatedwith soils particles can be a major source of arsenic contamination when soil particles aredetached and carried as sediments during erosion. Sediments can contain substantial (100300

mg/kg) amounts of total arsenic. In addition to the particle-bound arsenic originating fromland, new suspended solids with large amounts of arsenic are generated within bodies ofwater due to precipitation of salts (WHO 1992:13; Bhumbla & Keefer 1994:70).

Typically reported Arsenic concentrations in the open sea lies in the range of 1.3 g to 2.4g/litre of which arsenate is the dominant species, although arsenit, due to biological activity,occurs at concentrations greater than those expected from purely thermodynamicconsiderations. Only a very minor fraction of total arsenic in the oceans remains in solution inseawater, as the majority is sorbed on to suspended particulate material. The concentration inuncontaminated sea sediments varies considerable from less than 1 mg/kg d.w. to over 100mg/kg, with an average for world oceans of about 40 mg/kg and in the range of 3 to 15 mg/kgfor coastal regions and estuaries. The concentration in uncontaminated sediments in thecoastal areas is in general lower than in the deep-sea sediments Sediment (WHO 1986:24,Thornton & Farago 1997:2; Francesconi & Edmonds 1994:223224).

There is a significant difference in the level and chemical forms of arsenic in terrestrial andmarine organisms. In terrestrial flora and fauna, and fresh water biota the background levelsof arsenic are usually less than 1 mg As/kg (dry weight) whereas marine organisms containfrom 1 mg As/kg to more than 100 mg (dry weight). Arsenic concentrations in the tissues inmarine biota show a wide range of values, being highest in lipids, liver, and muscle tissues,and varying with age of the organism, geographic locale. Arsenic appears to be elevated inmarine biota because of their ability to accumulate arsenic from seawater and food sources,not because of localized pollution (Maeda 1994:156; Eisler 1994:201, 213214).

Marine algae can accumulate high concentrations of arsenic of which a significant part may

be inorganic arsenic. Most of the arsenic in algae is present as arsenic-containing ribosides(both water-soluble and lipid-soluble). In general, brown algae contain considerable amountsof arsenic, typically in the range 10 to 100 mg/kg d. w. while green and red algae contains 1to 20 mg/kg d. w. Generally, the arsenic content in freshwater algae is lower than in marinealgae. However, some fresh water algae accumulate arsenic to a large degree (WHO 1986:25;Maeda 1994:166168; Francesconi & Edmonds 1994:253; Phillips 1994:265266).

Arsenic levels in human food products, with exception of seafood, are generally low (lessthan 0.25 mg As/kg). The concentration in edible plants is generally low even when the cropsare grown on contaminated land. Seafood commonly contains 120 mg arsenic/kg mainly asorganic arsenic on a wet weight basis, although higher values have been reported (up to 50 100 mg/kg). Highest values are found in bottom-feeding fish and crustacean. Freshwater fish

and pelagic marine fish have in general total arsenic concentration around or below 1 mg/kg.Elevated arsenic levels have been found in wine (WHO 1992:14; WHO 1986:25-30, 47-48;Phillips 1994, ONeill 1995:115).


15/62

15

The level of arsenic in terrestrial plants is generally well below the concentrations in soil. Ingeneral roots contain higher levels than stems, leaves or fruit (ONeill 1995:115).

Human hair and nails show the highest arsenic concentrations, while fairly highconcentrations are found in skin and lungs. Examples of reported median concentrations:

blood (whole) 38 g/kg, bone 57 g/kg, brain 13 g/kg, hair 460 g/kg, kidney 33 g/kg,

liver 28 g/kg, lung 82 g/kg, and nail 300 g/kg The high concentrations in nails and hairhas been explained by the high content in these tissues of keratin, the SH-groups of whichmay bind trivalent inorganic arsenic. The methylated metabolites of inorganic arsenic and theseafood arsenic are not accumulated in hair (WHO 1986:40, 42).

The average concentration of total arsenic in urine is general in the range of 15-30 g/litre.Intake of seafood meal may give rise to arsenic concentrations of more than 1 000 g/litrewithin 24 hours. After 23 days, the concentrations have decreased to almost normal values.The concentration of inorganic metabolites is generally less than 10 g/litre urine in theEuropean countries (WHO 1986:43-45).

Eisler (1994) has put together an extensive list of arsenic concentrations found in both non-

biological and biological materials.

2.6 Limit values

In 1993 the WHO made a provisional guideline recommendation of 10 g/litre for arsenic indrinking water (North et al. 1997:416). The EU has (1998) adopted 10 g/litre as limit valuefor inorganic arsenic in drinking water. The limit value should be met within five years(European Union 1998). Before 1993, the WHO drinkingwater guideline value for arsenic(total) was 50 g/litre which corresponds to a daily intake of 100-200 g (WHO 1992:14;WHO 1986:50).

In the WHO air quality guidelines for Europe it was concluded that, because inorganic arsenic

is carcinogenic and there is no safe threshold, no safe level for arsenic can be recommended(WHO 1992:25). The WHO has estimated the human lifetime risk of getting lung cancer fromarsenic in air as 0.0015 per g/m3.

WHO has recommended that the daily oral intake of inorganic arsenic should not exceed 2 garsenic/kg body weight. The Joint FAO/WHO Expert Committee on Food Additives assigneda provisional tolerable weekly intake (PTWI) of 15 g inorganic arsenic/kg body weight, butstressed that there is a narrow margin between the PTWI and intakes reported inepidemiological studies to have toxic effects (WHO 1992:19, 25). There are indications thatarsenic may be essential, at least for some animals. If it is essential also for humans, thehuman requirement for arsenic is probably close to 20 g/day (WHO 1996:219). The PTWIvalue (15 g) corresponds to maximum daily intake of 150 g for a human weighing 70 kg.

Yamane reported in 1979 that the maximum allowable limit of arsenic residue at 15 mg/kg, asdetermined by the 1 N HCl extraction method. It has been estimated that rice yield decreases

by 10 % at 25 mg arsenic/kg (Huang YanChu 1994:40).

Eisler has put together a list of criteria for arsenic that has been proposed by authorities andexperts from different countries in order to protect natural resources and human health (Eisler1994:245251).

2.7 Risk assessments, exposure and harmful effects

As a result of its natural occurrence, humans are universally exposed to arsenic in various

forms. The various naturally occurring inorganic and organic arsenic compounds areinterlinked through complex biotic and abiotic transformations in the environment. Inorganic


16/62

16

arsenic can be converted to methylated species in soil and water by a number ofmicroorganisms (e.g. fungi and bacteria in soils, algae in water) under aerobic, as well asanaerobic, conditions. In anoxic parts of the soil layer, arsenic can be immobilized as thesulphide. In water at moderate or high redox potentials arsenic can be stabilized as a series of

pentavalent (arsenate) oxyanions. However, under most reducing conditions, the trivalentarsenite species dominates. The reduced form of inorganic arsenic, arsenite (As3+), is

considered to be much more toxic, more soluble and mobile than the oxidised form, arsenate(As5+). Soluble arsenic concentrations are usually controlled by redox conditions, pH,biologically activity and adsorption reactions but not by solubility equilibria. In general thetoxicity effects of arsenic on plants increases as soils becomes more acid (WHO 1992:13;Thornton & Farago 1997:2; Bhumbla & Keefer 1994:6364; ONeill 1995:116).

The main sources of arsenic in soils are the parent materials from which they are derived.However, arsenic may also originate from industrial waste discharges or agricultural use(fertilizers, lime and pesticides). In many regions of long agricultural use, soils haveaccumulated residues of arsenic. Under the ranges of Eh and pH in soils, arsenic may be

present as either As5+ arsenate or As3+ arsenit, with microbial activity causing methylation,demethylation and/or change in oxidation state. Arsenates of iron and aluminium are the

dominant phases in acid soils and are less soluble than calcium arsenate, which is the mainchemical form in many calcareous soils. Arsenic sulphide minerals may form if sulphurspecies are present and if redox potential is low enough (Thornton & Farago 1997:3; HuangYanChu 1994:1819; Alloway 1995:24-25).

Arsenic adsorption in soils is related to the pH, chemical and physical properties, and cationexchange capacity. In one study it was found that the maximum adsorption of arsenite took

place at pH interval 68. The adsorption of arsenate is dependent on the ratio of (CaO +MgO) to (Al2O3 + Fe2O3. Arsenic and phosphate in soil exhibit similar behaviour; both are forinstance strongly adsorbed by amorphous iron oxide. Phosphate substantially suppressesarsenic adsorption by soils and the extent of suppression varies from soil to soil. Arsenic ismore strongly bound to soils that have a high clay or high organic matter content. In thesecircumstances arsenic is less available to plants. Plants take up arsenic in proportion to thesoil concentration (passive uptake), except at very high soil concentrations. In order for plantlevels to reach 1 mg As/kg on fresh weight bases, soil levels must in general exceed 200 to300 mg As/kg. However, some crops may accumulate high levels of arsenic even at muchlower soil arsenic levels. For instance, arsenic concentrations in crops grown on soilscontaining 25 to 50 mg As/kg were 6 to 12 mg As/kg on a fresh weight bases for alfalfa and

pasture grass. Also some mushrooms can accumulate high concentrations of arsenic (WHO1992:13; Thornton & Farago 1997:4; Huang YanChu 1994:30, 3235; Bhumbla & Keefer1994:73; Maeda 1994:162).

In a study where vegetables (beetroot, lettuce, onion, pea, carrot and bean) concentrations ofarsenic were compared with the arsenic in soils (range 144-892 mg/kg) it was found that theconcentration in the plant increased with the soil concentration except for carrots and beans.

However, only lettuce exceeded 1 mg/kg dry weight. A positive correlation betweenextractable arsenic in soil and the content of arsenic in rise and wheat has also been found.The distribution of arsenic in plants, in general, is in descending order from root to stem andleaf to edible parts. It has been suggested that low levels of phosphates displace arsenic fromsoil particles which increases the uptake and phytotoxicity, whereas, larger amount of

phosphates compete with arsenic at root surfaces to decrease uptake and phytotoxicity.Arsenic concentrations in leaves may also be elevated if arsenic pesticides have been used(Thornton & Farago 1997:4; Huang YanChu 1994:3839).

Arsenic is subject to bioaccumulation but not to biomagnification. Rather the arsenic leveldecreases successively in freshwater organisms and marine organisms with elevation in thetropic level. Algae actively take up naturally occurring arsenate, and transform inorganic

arsenic into a variety of organic arsenic compounds. The concentration of arsenic inunicellular algae has been reported to reach up to 3 000 times the concentration in thesurrounding water. This is a major source of arsenic for higher organisms. Fish and


17/62

17

crustaceans accumulate arsenic compounds via the food-chain. The accumulation direct viaseawater is low. A number of studies have demonstrated positive relation between size ofmarine animals and their arsenic concentration. Arsenic is in general not biomagnified inmarine food-chains. Arsenobetaine (CH3)3As+CH2COO

-, has been found to be the major formof arsenic in marine animals contributing to the human diet. In general organoarsenic speciesare dimethylarsenic compounds in aqueous plants such as algae, and trimethylarsenic

compounds in molluscs, crustaceans and fish, among marine organism (WHO 1992:14; WHO1986:21- 23, 51-52; Maeda 1994:180181).

Arsenobetaine has also been found in higher organisms in brackish water but not in freshwater environments, except in shrimp. In fresh water organisms several other organiccompounds than arsenobetaine has been found. While relatively high proportion of arsenicmay be present in algaes in the more toxic inorganic form the organic form is predominant(more than 80 %) in for instance crustaceans and fish (WHO 1986:25, 28; Maeda 1994:181).

Exposure of the general population occurs mainly through arsenic present in food anddrinking water. Seafood is in general the dominant source of arsenic to humans. People thatoften consumes seafood may reach several thousands g of total arsenic per day. However,

8595 % of the arsenic in marine products is in general present as the much less toxic, organicarsenic compounds. In some areas, the natural high arsenic content of the drinkingwater hascaused endemic, chronic arsenic poisoning. South-west Taiwan, West Bengal India,Langunera northern Mexico, Northern Chile, South-east Argentine, Inner Mongolia areexamples of areas where naturally elevated concentration of arsenic in drinking water andirrigation water has caused chronic arsenic poisoning resulting in for instance, hyperkeratosis,hyperpigmentation and skin cancer. Millions of people in these areas are exposed to arseniclevels that lies above the present WHO drinkingwater guideline value for arsenic of 50g/litre. The drinking water used in these areas often contain 100 g As/l or more, andsometimes up to twenty times more (Thornton & Farago 1997:7-10; Luo et al. 1997:55-68;Chowdhury et al. 1997:93-111). Excluding such areas the daily intake of arsenic throughdrinking normally contributes to less than 10 g As/day (WHO 1992:14; WHO 1986:47, 49,52).

Soil and dust ingestion may in arsenic contaminated areas significant contribute to the overallexposure of arsenic for young children. However, to assess the risk from that source one haveto examine the bioavailibilty which may be much less for the arsenic species found in soilthan those in water (Farago et al. 1997:221-223).

It has been estimated that the normally daily intake of inorganic arsenic is less than 50 g inEurope and USA and more than 100 g in Japan. Although most of the arsenic in seafood is

present in organic form, the contribution of inorganic arsenic from seafood may besignificant. A daily consumption of 150 g seafood would give rise to an intake of about 15-75g inorganic arsenic per day, the higher figure relating to a daily intake of flatfish, crustaceanand molluscs. The daily intake of seaweed in Japan is about 5 g. With an assumed average

arsenic concentration of about 100 mg As/kg dry weight, or about 20 mg As/kg wet weight,this means that the daily intake of arsenic from seaweed might exceed 100 g, out of which60-80 % probably is in the form of inorganic arsenic. Daily consumption of seafood wouldgive rise to an intake of about 135 1 500 g organic arsenic. However, it can be estimatedthat the world average daily intake of organic arsenic from seafood (most of which isarsenobetaine) is 15-20 g (WHO 1986:49, 52).

In the working environment high inhalation exposures may be associated with the smelting ofnon-ferrous sulphide ores, glass manufacturing, wood preservation plants, and the agriculturalapplication of arsenic-containing pesticides (WHO 1992:14).

Studies on experimental animals, as well as on humans, have shown that over 90 % on an

ingested dose of dissolved inorganic trivalent or pentavalent arsenic is absorbed from thegastrointestinal tract. In 15 human subjects, each ingesting about 10 mg of arsenic (mostlyarsenobetaine) with witch flounder, less than 1 % was recovered in the faces within 8 days


18/62

18

indicating a high absorption. In the lungs, water-soluble arsenic compounds are rapidlyabsorbed (WHO 1992:14; WHO 1986:37-38).

Although high levels of arsenic are maintained for long periods of time in the bone, hair, andnails of exposed individuals, most inorganic arsenic is eliminated at much higher rate with theurine, manly as dimethyl-arsenic acid and methane-arsonic acid. Depending on the

administrated dose, the half-life in man, after short-term exposure, is in the range of 13 days.There is no long-term accumulation of arsenic in soft tissues. With increasing arsenic intake,the proportion of arsenic detoxified (methylated) is reduced. Increases in the inorganic arsenic

body burden may be expected at daily intakes exceeding about 200 g/person. In persons witha low (stable) dietary arsenic intake, the urinary levels may be used to monitor exposure toinorganic arsenic. Since the elimination of arsenic takes place mainly via the kidneys, theconcentration of arsenic in the urine is a good indication of exposure to inorganic arsenic(WHO 1992:14).

Arsenic is a common, for human, toxic substance with exceedingly diverse manifestations of poisoning. Different species of arsenic have different degrees of toxicity, with arsine andtrivalent (arsenite) causing the most damage. The bodys toxic response depends on the route

and dose of exposure plus individual and local tissue susceptibilities. Eisler has put togetheran extensive list of at what level lethal and sublethal effects of arsenic have been found indifferent organisms (Morton & Dunette 1994:29; Eisler 1994).

According to Eisler (1994) most experts agree on 10 points: (1) inorganic arsenicals are moretoxic than organic arsenicals, and trivalent forms are more toxic than pentavalent forms; (2)episodes of arsenic poisoning are either acute or subacute; cases of chronic arsenosis arerarely encountered, except in humans; (3) sensitivity to arsenic is greatest during the earlydevelopmental stages; (4) arsenic can traverse placental barriers; as little as 1.7 mg As+5/kg

body weight at critical stages of hamster embryogenesis, for example, can produce foetaldeath and malformation; (5) biomethylation is the preferred detoxification mechanism forinorganic arsenicals; (6) arsenic is bioconcentrated by organisms, but not biomagnified in thefood chain; (7) in soils, depressed crop yields were recorded at 3 to 28 mg water-solubleAs/L, or about 25 to 85 mg total As/kg soil; adverse effects on vegetation were recorded atconcentrations in Air > 3.9 g As/m3; (8) some aquatic species were adversely affected atwater concentrations of 19 to 48 g As/L, or 120 mg As/kg in the diet, or tissue residues of1.3 to 5 mg As/kg fresh weight; (9) sensitive species of birds died following single oral dosesof 17.4 to 47.6 mg As/kg body weight; and (10) adverse effects were noted in mammals atsingle oral doses of 2.5 to 33 mg As/kg body weight, and at feeding levels of 50 mg, andsometimes only 5 mg, As/kg in the diet (Eisler 1994:214215).

In man, the smallest recorded fatal dose of inorganic arsenic is in the range of 70180 mg, butrecovery has been reported after much larger doses. Common acute symptoms include:nausea, vomiting, abdominal pain, rice-water diarrhoea, progressive general weakness, andsevere dehydration leading to collapse and heart failure. If the patient survives, hepatic and

renal impairment and central nervous and peripheral nervous system damage may becomeevident (WHO 1992:16).

With long-term exposure of inorganic arsenic, significant toxic effects in humans can beexpected to occur above a daily oral intake of 100-200 g. Tolerance to arsenic can developafter repeated exposure. The chronic signs of toxicity are insidious and may be difficult todiagnose. They are chiefly related to the skin, gastrointestinal tract, respiratory tract andnervous system, but also to the mucous membranes, blood, heart and liver. Inorganic arsenicis a proven human carcinogen (skin cancer, bladder cancer and other internal cancers) afterlong-term oral intake, as well as after inhalation. Arsenic is a unique carcinogen. It is the onlyknown human carcinogen for which there is adequate evidence of carcinogenic risk by bothinhalation and ingestion. Oral exposure of arsenic to human beings is usually not the result of

anthropogenic activity as it is with many carcinogens, but the result of natural contaminationof well-water supplies by arsenic-rich geologic strata. Furthermore, the evidence forcarcinogenity of arsenic is very strong in humans, but weak in animals, a different scenario


19/62

19

found with most carcinogens. A daily lifetime intake of 0.4 mg inorganic arsenic via drinkingwater has been found to correspond to a 5 % prevalence of skin cancer. Correlations, betweenelevated atmospheric arsenic levels and mortalities from cancer, bronchitis, and pneumoniawere established in an epidemiological study in England and Wales, where death fromrespiratory cancer increased at air concentrations > 3g As/m3 (WHO 1992:1618, 25; WHO1986:44, 46, 50, 53; Abernathy et al. 1997; North et al. 1997:410, 41417; Eisler 1994:186;

Guha Mazumber et al. 1997:112123).

In general, the toxic action of arsenic in experimental animals resembles that seen in man.However, animals are in general not as sensitive to arsenic as humans, which can be partiallyattributed to differences in gastrointestinal absorption. The oral LD50 of arsenic ranges from15 to 293 mg/kg body weight in rats, and from 11 to 150 mg/kg body weight in other animals.Inorganic arsenic is in general considerably more toxic than organic arsenic compoundswhich are the dominant species found in seafood. And trivalent arsenics are, in general, moretoxic than pentavalent arsenic. However, the relative difference in toxicity between the twoinorganic forms is reasonably small (2- to 3-fold). Different forms of arsenic may beinterconverted, both in the environment and in the vertebrate body (WHO 1992:1516; Naqviet al. 1994:5758).

There are increasing evidence that arsenic may be essential, at least for some species. Lowdoses (< 2 g/day) have been found to stimulate the growth and metamorphosis in tadpolesand increased viability and cocoon yield in silkworm caterpillars. Arsenic deficiency has beenobserved in rats, goats and pigs, the latter fed on a diet containing less than 0.05 mg As/kg. Inthese animals, reproductive performance was impaired, neonatal mortality was increased,

birth weight was lower and weight gain in second-generation animals was decreased. Severalbiochemical changes accompanying the signs of arsenic deficiency have been described, butthe fundamental mode and site of action of the element are not yet known. Extrapolation fromanimal experiments suggest that human adult intakes in the range 1215 g/day are probableadequate to meet any possible requirement. A substantial number of organic arsenicals, mostof them derivatives of phenylarsonic acid, are used as feed additives in poultry and swine

production. Inorganic selenium and inorganic arsenic are antagonistic in several animalspecies. In rats, dogs, swine, cattle and poultry, the arsenic protects against selenium

poisoning if arsenic is administrated in the drinking water and the selenium in the diet.However, the toxicity of naturally methylated selenium compounds is markedly enhanced byinorganic arsenic (WHO 1996:218; Eisler 1994:194).

For most aquatic animal species, the acute toxicity of inorganic arsenic compounds ismoderate to low (LC50 10100 mg/litre). However, long-term exposure of immature fish

populations to sublethal doses may result in toxic effects at about 4 mg/litre, and exposure ofDaphnia may lead to slightly impaired reproduction at 0.5 mg/litre. In aquatic ecosystems,algal communities seem to suffer most from exposure to arsenic. The growth of some speciesof unicellular algae is inhibited at arsenate concentrations as low as 75 g/litre. Communitiesof some species of marine algae (seaweed) may be eliminated at exposures of about 10

g/litre. The growth of some marine phytoplankton is inhibited by concentrations of arsenateat, or little above, ambient levels when phosphate concentration is low (WHO 1992:14; WHO1986:31).

Arsenic is also toxic to terrestrial plants. Although arsenic is commonly not regarded as anessential element for plants, small yield increases have sometimes been observed at low soil-arsenic levels, especially for tolerant crops such as potatoes, corn, rye and wheat. Arsenic ischemically similar to phosphorus, an essential plant nutrient; it behaves very much like

phosphate in the plantsoil system. Arsenate can enter into reactions in place of phosphorus,thereby becoming a toxicant. Their is strong evidence that arsenate is normally absorbed in amanner similar to the phosphate uptake mechanism (Bhumbla & Keefer 1994:73; Eisler1994:195).

The relationship between soil arsenic and growth of plants depends on the form andavailability of the arsenic. The toxicity of arsenic to plants varies (as i does for animals an


20/62

20

humans) with its form and valence, its toxic order being AsH3 > As3+ > As5+ > organic As.

Phytotoxicity studies have shown that 1 ppm soluble arsenic causes injury to cowpeas, 2 ppmcauses injury to barley, 7 ppm causes injury to rice and 9 ppm to peas, and beans. The yield ofoat was found to be reduced when 20 ppm arsenic in soil was applied (Huang YanChu1994:36-38, 40).

Plants growing on mine or smelter waste have developed resistance to arsenic toxicity. Forinstance, Cynodon dactylon was found to grew on a mine waste with 1 980 mg As/kg present.Plants growing on contaminated soils sometimes have concentrations of arsenic (6 000 mg/kghas been found) that may be toxic to animals eating the plants (WHO 1992:13; Bhumbla &Keefer 1994:73).

Environmental contamination with arsenic emitted from coal burning power stations has beenreported from several countries, including Slovakia, China and India. The implications ofthese arsenic emissions to human health have still to be fully assessed. One example ofecological effects from burning arsenic-rich coal is the mass extinction of honeybees that took

place within 30 km of a power plant in Slovakia when local coal containing 900 to 1 500mg/kg of arsenic was burned. In a province in China endemic arsenosis caused by burning

arsenic-rich coal has been reported (Thornton & Farago 1997:12; Bencko 1997:85).


21/62

21

3.Cadmium (Cd)

3.1 General information

Cadmium is a metal that belongs, together with zinc and mercury, to group IIb in the PeriodicTable. It is a relatively rare element (being 67th in order of element abundance) and is notfound in the pure state in the nature. Cadmium minerals (principally greenockite, CdS) do notoccur in concentrations and quantities sufficient to justify mining them in their own right.Cadmium is mainly associated with sulphide ores of zinc, lead and copper. Residual levels ofcadmium in lead refining are much lower than in zinc, and still lower in copper refining. It isnot possible to produce refined zinc metal without generating cadmium as a by product(WHO 1992a:36; OECD 1994:35; Alloway 1995b:122). Cadmium can form a number ofsalts but there are no evidence that organocadmium compounds occur in nature (WHO1992b:16).

Cadmium has a relatively high vapour pressure and volatilises at a comparatively lowtemperature. Its vapour is oxidized rapidly in air to produce cadmium oxide. When reactivegases or vapour, such as carbon dioxide, water vapour, sulphur dioxide, sulphur trioxide orhydrogen chloride are present, cadmium vapour reacts to form cadmium carbonate,hydroxide, sulphite, sulphate or chloride, respectively. These compounds may be formed instacks and emitted to the environment (WHO 1992a:24).

Cadmium is highly toxic to plants and animals and is normally counted as a non-essentialmetal for biological functions, even though it can probably be essential for certainmushrooms, according to recent findings. There are also studies on rats and goats that indicatethat to low intake of cadmium have negative effects on the growth. However, furtherevidence, accompanied by adequate statistical verification of the data, is needed beforecadmium according to WHO can be regarded as physiologically essential (Landner &Lindestrm 1998:98; WHO 1992a:36; Alloway 1995b:122; WHO 1996:210211).

The mobility of cadmium in the environment and the effects on the ecosystem depends to alarge extent on the nature of its compounds. Further, the speciation of cadmium in soil, plantsanimal tissues, and foodstuffs may be of importance for the evaluation of the health hazardsassociated with areas of cadmium contamination or high cadmium intake. However, very fewdata on the occurrence and speciation of cadmium compounds in nature are available (WHO1992a:24-25).

Relative differences in the uptake of metal ions between plant and cultivars are controlledgenetically and by various factors, including surface area of the root, root CEC (cationexchange capacity), root exudates and the rate of evatranspiration. The transfer coefficient

(the metal conc. in plant tissue above ground divided by the total metal concentration in thesoil) differs between metals. It is in general considerable higher for cadmium and zinc, 110,than for arsenic, mercury and lead 0.010.1. However, since a numerous soil and plant factorscan affect the accumulation of metals in plants the transfer figures are not precise values butcan indicate accumulation differences (Alloway 1995:27).

The cadmium content of crops is depending on farming practices and can be reduced byliming and maintaining a high organic matter content in the soil, and by reducing the input ofcadmium into soils via phosphorus fertilizer, sewage sludge and atmospheric deposition(Tahvonen & Kumpulainen 1993:253). However, there are reasons to be cautious with liming.One Swedish study has demonstrated that liming more than what is generally recommendedmay increase the cadmium uptake (Andersson & Simn 1991).

Several studies have reported that the concentration of cadmium in soils and crops haveincreased during the last century as an effect of atmospheric cadmium deposition and input


22/62

22

via phosphate fertilizers. With the estimated half-life for cadmium in soils varying between 15and 1 100 years this is obviously a longterm problem and pollution needs to be prevented orminimised wherever possible (WHO 1992a:49; KemI 1995:22; Alloway 1995b:122).

The pattern of cadmium consumption has changed in recent years with significant decreasesin electroplating and increases in batteries and specialized electronic uses. Between 1981 and

1989 the production and consumption of refined cadmium metal in the western world (notCentral and Eastern European countries) showed a general increase. However, during the firstyears of 1990s it decreased slightly (WHO 1992a:17, OECD 1994:23).

3.2 Use

Unlike lead, mercury and copper, which have been utilised for centuries, cadmium has onlybeen widely used this century. Cadmium has a limited number of applications but within thisrange the metal is used in a large variety of consumer and industrial materials. The principalapplications of cadmium fall into five categories: protective plating on steel; stabilizers for

poly vinyl chloride (PVC); pigments in plastics and glasses; electrode material in nickel

cadmium batteries; and as a component of various alloys. The production of rechargeablenickel-cadmium batteries accounted for about 60 % of the consumption of cadmium in thebeginning of 1990s. The patterns of use vary considerably from country to country, amongother things due to restrictions in use (WHO 1992a:38; OECD 1994:15; Alloway 1995b:122).

In 1990 the estimated use of cadmium for different applications in the Western world was: Ni-Cd batteries 55 %, pigments 20 %, stabilizers 10 %, coatings 8 %, alloys 3 %, andmiscellaneous uses 4 % (OECD 1994:25-32). Although the amount of cadmium used in

batteries is much greater than that used in pigments, stabilizers and plating, the latter usescause the major part of the cadmium flow to waste deposits at present due to differences in

product life span, degree of recycling etc. (OECD 1994:35).

3.3 Production

The world production of cadmium was in 1997 19 800 tonnes. The world production has beenabout the same during the last ten years and was in 1985 about 19 000 tonnes. Estimatedworld resources of cadmium are about six million tonnes (Knight-Ridder financial/commodityresearch bureau 1996:16; Chapman et al. 1999:43). Commercial cadmium production startedat the beginning of this century (WHO 1992a:17).

The most abundant sources of cadmium are the ZnS minerals sphalerite and wurtzite andsecondary minerals, such as ZnCO3 (smithsonite) which typically contain 0.20.4 % cadmiumalthough concentrations of up to 5 % cadmium can be found Alloway 1995b:123). Cadmiumis a by-product of zinc production and the amount produced is more dependent on zinc

refining than on market demand. As a result, the level of cadmium output has closelyfollowed the pattern of zinc production. The percentage of cadmium in zinc concentratesvaries from mine to mine, ranging from 0.07 to 0.83 per cent with an average of 0.23 per cent.Since the average zinc content of these concentrates is 55 per cent, approximately 3 kg ofcadmium will be produced for every tonnes of refined zinc (WHO 1992a:38; OECD 1994:15,23).

The production in the western world (Central and Eastern Europe excluded) in 1993 wasabout 15 000 tonnes. Over 10 % of the production is from secondary production i.e. recoveryand recycling. Cadmium recycling has been practical only from nickel-cadmium batteries,some alloys, and dust from electric arc furnaces operated by the steel industry. Cadmium canin these cases be separated from other materials in a comparatively uncomplicated fashion,with fairly low expenditure. However, cadmium in the form of compounds that are present at

low concentrations constrains the recycling of cadmium. The production in countries witheconomies in transition was estimated to be around 4 500 tonnes in 1990, but fell to 3 000


23/62

23

tonnes more recently with more than half produced in countries that were formerly part of theSoviet Union (OECD 1994:2327, WHO 1992a:17; Knight-Ridder financial/commodityresearch bureau 1996:16).

3.4 Emission and deposition

Volcanic activity is a major natural source of cadmium release to the atmosphere. The globalyearly flux from this source has been estimated to 820 tonnes. Deep-sea volcanismscontribute to the flux of cadmium to the sea but it is difficult to estimate to what extent. In thefirst half of the 1980s about 1015 % of total airborne cadmium emissions arise from natural

processes, the major source being volcanic action. Weathering results in a global riverinetransport of cadmium to the sea in an estimated order of 15 000 tonnes/year (WHO 1992a:36-40; OECD 1994:33). The atmospheric deposition of cadmium to fresh and marine watersrepresents a major input of cadmium at the global level (WHO 1992a:43).

Cadmium is released to the air, land and water by human activities. In general, the two majorsources of contamination are the production and consumption of cadmium and other non-

ferrous metals and the disposal of wastes containing cadmium (WHO 1992a:17). The globalanthropogenic emissions of cadmium to air was for the year 1983 estimated to lie in the rangeof 3 100 to 12 000 tonnes (Nriagu & Pacyna 1988).

The replacement of thermal melting by electrolyte refining as method of zinc production hasconsiderable reduced the cadmium emission to air. Emissions of cadmium to air also arisefrom, iron production, fossil fuel combustion, cement manufacture, and incineration of waste.(WHO 1992a:4041). The production of iron and steel may contribute to significant cadmiumemissions. Although the concentration of cadmium is low in both secondary and primarysteel-making the volumes of materials handled are very high. The cadmium content of coalvaries from 1 g/g (lignite) to 2 g/g. However, also in this case it is a question of hugequantities. Most of the cadmium from combustion of fossil fuels will be found in the fly ashand will be captured by emission control devices (OECD 1994:36).

Non-ferrous metal mines represent a major source of cadmium release to the aquaticenvironment. Cadmium release are not restricted to active mine sites, and mines disused formany years can still be responsible for continuing water contamination. At the global level,the smelting of non-ferrous metal ores has been estimated to be the largest human source ofcadmium release to the aquatic environment (WHO 1992a:42).

Although the contribution of emissions from point sources in Europe are of importance,cadmium emissions comes mainly from diffuse sources. Point sources are generally lessimportant than for other heavy metals. A European map of the cadmium concentrations inmosses during the early 1990s shows however, that the concentration varies considerably overEurope and is elevated around point sources (EEA 1998:112114; EEA 1995:45, 159).

Worldwide, between 15 000 and 17 000 tonnes of cadmium per year is transformed intoproducts belongings. The products have a potential of releasing cadmium to the environment.The significance of releases during manufacture is dependent upon the processes utilized andthe emission control techniques. Releases from these processes may mainly occur to both airand water, but solid manufacturing wastes may also be generated (OECD 1994:34).

Solid wastes from variety of human activities (e.g. ashes from fossil fuel combustion, wastefrom cement manufacture, municipal refuse, sewage sludge, solid wastes from non-ferrous

production and from the manufacture of cadmium-containing articles and, ashes from refuseincineration) resulting in large cadmium inputs at the national and regional levels. The threelatest mentioned sources probably constitute the greatest risk (WHO 1992a:43).

Phosphate fertilizers and atmospheric deposition are significant sources of cadmium input toarable soils in some parts of the world; sewage sludge can also be an important source at the


24/62

24

local level. Plants take up cadmium and cadmium is removed by harvest. Cadmium is anatural constituent of rock phosphates and deposits from some regions of the world containmarkedly elevated levels of the metal. Continuous application of phosphate fertilizers has

been shown to cause increased soil cadmium concentrations. The total input of cadmium inEuropean arable land has during several decades in general been significantly larger than theoutput. Thus, the concentration of cadmium in the arable soils has increased. Also the

concentration of cadmium in crops has increased during the same period (WHO 1992a:18, 42,44; Statistiska centralbyrn 1995:31).

The cadmium input to agriculture soil from P fertilizers has in UK been estimated to 4.3g/ha/y and in the former West Germany to 3.5 g/ha/y. In the EU, fertilizer inputs areestimated to be around 300 tonnes cadmium per year (Alloway 1995b:127).

Animal manure has a very heterogeneous composition. The concentration of cadmium insewage sludge varies considerably. However there are examples where the use of sewagesludge has lead to inputs as high as 80 grams of cadmium/hectare. At normal rates ofapplication of animal manure, up to 3 grams of cadmium per hectare may be added per year.Some cadmium will be subject to recirculation on the farm, although additional cadmium can

be introduced through imported feeding stuffs (OECD 1994:36-38; Alloway 1995b:128).According to the European Environment Agency, the air emissions of cadmium was inEurope about 12 000 tonnes in 1965 and less than 3 000 tonnes in 1991 (EEA 1998:112).However, according to Eurostat the total air emissions of cadmium in 38 European countrieswas in 1990 only 613 tonnes. In EU-15 the air emission was about 200 tonnes 1990 of whichmore than half came from Italy, Spain, and Germany (Eurostat 1998:78; UN/ECE 2000). Thecadmium emissions from 12 EU-countries (no figures available for Greece, Ireland andSpain) has decreased by 40-50 % from 1990 to the mid 1990-ties (UN/ECE/ 2000). The airemissions of cadmium are expected to continue to decrease (EEA 1998:112).

Cadmium is removed from the atmosphere by dry and wet deposition. Annual deposition ratesin rural areas in Scandinavia ranged from 0.4 to 0.9 g/ha during the 70s. It was estimated that3 g/ha/year was a representative value of the deposition of cadmium to agricultural soils inrural areas of the EEC around 1980. This could be compared with a corresponding estimatedinput of 5 g/ha per year for these areas from the application of phosphate fertilizers (WHO1992a:46).

At present, it is not possible to determine whether human activities have caused a historicincrease in cadmium levels in the polar ice caps. However, samples from the Arctic containon average more than ten times higher value (5 pg/g respectively 0.3 pg/g) possible indicatingan suspected greater anthropogenic influence in the Arctic compared to Antarctic (WHO1992a:17, 37).

3.5 Concentrations (in nature and human)Cadmium is widely distributed in the earths crust at average concentration of about 0.1 0.2mg/kg. However, higher levels may accumulate in sedimentary rocks, and marine phosphatesoften contain about 15 mg cadmium /kg. Cadmium is strongly associated to zinc minerals.High soil concentrations are more commonly found in areas containing deposits of zinc, leadand copper ores. Some black shale also contains elevated cadmium levels leading to high soilcadmium concentrations. Naturally elevated concentrations of over 22 mg/kg have been foundin soils from shales in California and in the Pennine Hills, UK (WHO 1992a:36; OECD1994:33).

Typically concentrations of cadmium in soils range between 0.1 and 0.4 mg/kg, while freshwater contains < 0.010.06 ng/litre. The concentration of lead, cadmium and mercury in thehumus layer of forest soils in 50 % of Sweden has increased by a factor between three and tensince the pre-industrial era; concentrations fall from south to north. However, for some


25/62

25

Swedish forest podzol soils the cadmium balance is now negative in the upper organic richlayer (the cadmium concentration is decreasing). The average cadmium content of seawater isabout 5-20 ng/litre in open seas, while concentrations in European unpolluted rivers roughlyvaried from 20 to 100 ng/litre. Average cadmium concentrations for the period 1991 to 1993were about 50 times higher in polluted European rivers than in clean rivers. However, ingeneral the concentrations in European rivers have decreased since 1985. Cadmium levels of

up to 5 mg/kg have been reported in river and lake sediments, and from 0.03 to 1 mg/kg inmarine sediments. Atmospheric concentrations in remote areas are typically in the range of0.010.04 ng/m3, while the concentrations in European rural areas vary from 0.1 to 0.5 ng/m3according to measurements reported in 1992 (WHO 1992a:36-37; OECD 1994:3334; EEA1998:112, 114115; Bergbck & Johansson 1996:46-48).

Phosphate fertilizers are widely regarded as being the most ubiquitous source of cadmiumcontamination of agricultural soils. It has been estimated that for individual western countriesthe relative contribution from the major anthropogenic sources was for phosphate fertilizers5458 %, atmospheric deposition 3941 %, and sewage sludge 25 % (Alloway 1995b:131).

A survey of agricultural soils in the USA comprising 3 045 samples representing 307 different

soil series collected from sites remote from obvious sources of metal contamination gave amean topsoil cadmium concentration of 0.265 mg/kg, a median of 0.2 mg/kg and a range of


26/62

26

higher in older organisms. Invertebrates it is especially accumulated in the liver and kidneys.Due to the lack of concentration data on nutrient such as calcium or, zinc it is not possible tocalculate any purification factors for the studies examined in the 1992 WHO report (WHO1992b:30, 52-58).

Concentrations of cadmium have been measured in mussels, fish and sediments from sites

located in both clean and contaminated areas in Europe. Cadmium concentrations in musselsranged from 10 to 1 700 g/kg wet weight. Concentrations up to about 300 g/kg can occureven far from known discharge points. Concentrations in fish ranged from very low, up to 15g/kg wet weight in the Gulf of Finland, the Gulf of Bothnia and the open waters of CentralMediterranean, to 560 g in samples from the Greek coast. Concentrations in sedimentsvaried between 10 and 9 000 g/kg dry weight. Excluding some samples collected very closeto point sources, the highest concentrations were measured near the mouth of the Rhine.Generally, concentrations below 200 g/kg can be considered as background levels (EEA1998:215219).

Meat, fish and fruit generally contain similar cadmium levels and values of 510 g/kg freshweight are representative for these food classes. Most plantbased foodstuffs contain higher

cadmium concentrations and a value of 25 g/kg fresh weight is considered representative forthe staple items, cereals and root vegetables. Although, cadmium residues in plants arenormally less than 1 mg/kg, plants growing in soils amended with cadmium (e.g. from sewagesludge) may contain significantly higher levels (WHO 1992a:56; WHO 1992b:31).

A great deal (35-65 % of heavy metals is in the outer layer of the seed, e.g. in the germ andbran, and can be removed from the caryopsis by certain methods of cereal technology. Wholegrain flours contain higher cadmium levels than lower extraction flours (Tahvonen &Kumpulainen 1993:251). High content of Cadmium have been found in wheat and productsmade of wheat for instance pasta. Finland: pasta 117 g/kg (mean), 182 g/g (max) in 1990,104 g/kg (mean) 145 g/kg (max) in 1991. High values in durum wheat have also beenfound in Italy (Tahvonen & Kumpulainen 1993:251).

In early 1990s the production of dry sludge from wastewater was about 6.3 million tonnes inthe whole European community (Alloway 1995a:45). High maximum levels of cadmium have

been measured in sewage sludge. Levels of cadmium in dry sludge reported in the literaturerange from less than 1 mg/kg to about 3 400 mg/kg. In UK the median cadmium value ofsludge used in 1990/91 was 3.2 mg/kg (Alloway 1995a:46; Alloway 1995b:129).

3.6 Limit values

The present PTWI (provisional tolerable weekly intake) for cadmium recommended byJEFCA (Joint Expert Committee on Food Additives) is 7 g/kg body weight. For a 65kgman this corresponds to a dietary intake of 65 g/day. The PTWI does not include a safety

factor and there is only a relatively small safety margin between exposure in the normal dietand exposure that produces deleterious effects (WHO 1996:206; Tahvonen 1995:15;FAO/WHO 1993).

At an average daily intake of 70 g/day, corresponding to the present PTWI, 7 % of the adultgeneral population would be expected to develop cadmium-induced kidney lesions. For high-risk groups the percentage would be even higher (up to 17 %). This has made a Swedishgroup of cadmium-health experts to claim that the current PTWI value is unacceptable andneeds to be lowered (Jrup et al. 1998:8).

The WHO air quality guideline for cadmium is (1999) 5 ng/m3 (year average). The WHOguideline for drinking water is 3 g/l (WHO 1996:206). The EU has (1998) adopted 5 g/litreas limit value for cadmium in drinking water. The limit values should be met within five years(European Union 1998). The limit value for cereals is on EU-level 0.1 mg/kg except for


27/62

27

wheat, bran, germs and rice for which it is 0.2 mg/kg. The limit values for vegetables andfruits on the EU-level vary between 0.05 and 0.2 mg Cd/kg (European Communities 2001).

The maximal allowable cadmium concentrations in sewage sludge in some countries listed byAlloway was in 1995: Denmark 0.8 mg/kg, Finland 1.5 mg/kg, and Sweden 2.0 mg/kg, whileUSA (1993) allowed 8.5 mg cadmium/kg. The great differences of the accepted values

depend on different risk management approaches. While the Nordic countries are unwilling toaccept an increase of the cadmium concentration in the arable soil in the long run, the USEPA has made an analysis of the extra risk the input of sewage sludge cadmium constitutetaking into account how much additional cadmium in the soils that will be bioavailable to

plants (Alloway 1995b:130).

3.7 Risk assessments, exposure and harmful effects

Of all toxic metals released in large quantities into the environment, cadmium is generallyregarded as the one most likely to accumulate in the human food chain (Tahvonen 1995:13).It has been shown repeatedly that an increase in soils cadmium result in an increased plant

uptake of the metal. It is this basic relationship that makes the soil-crop pathway of humanexposure susceptible to increased levels of soil cadmium (WHO 1992a:47-48). A studycomprising 30 countries in different parts of the world showed a good correlation between theextractable content of cadmium in soil and the cadmium content in plants. The level ofcadmium in cultivated soils and the concentration of cadmium in plants varied a lot betweencountries and areas in a country. The sample with the highest cadmium content in plantsexceeded the lowest about 1 500-times (6 037 g/kg respectively 4 g/kg, median value 61g/kg) while the extractable soil cadmium maximum value was 125 times higher than thelowest value (FAO 1992:15).

The most important factors influencing plant cadmium accumulation are soil pH andcadmium concentration. Increases in soil cadmium content result in an increase in the uptakeof cadmium by plants: the pathway of human exposure from agricultural crops is thus

susceptible to increase in soil cadmium. Cadmium is rather immobile in soil. However, adecreased soil pH increases the availability of cadmium in the soil, since it affects alladsorption mechanisms and the speciation of metals in the soil solution. Acidification of soilsand lakes may result in enhanced mobilization of cadmium from soils and sediments and leadto increased levels in surface and ground water. An other important factor that influences theuptake is the redox conditions. The uptake decreases with increased content of organic matter,clay, and the hydrous oxides of manganese and iron. Zinc has been found to have anantagonistic effect on cadmium uptake in soils with low cadmium concentrat

app a risk pdf

Documents