The determination of metal speciation in natural waters by electrochemical techniques

Øyvind Mikkelsen

Mikkelsen 2003

Mikkelsen 2003

Overview

• Theoretical aspects- Natural water- Speciation, and importance of speciation studies- Available techniques for speciation studies- Electrochemical techniques

• Some practical examples- Cu, Cd, Pb and Zn speciation in natural water- Fe(II) and Fe(III) speciation in seawater- Al(III) speciation in natural water

• Conclusions

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Theoretical considerations

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Natural water

• Natural water includes e.g. rivers, lakes, ground water, wells, seawater,….

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Mikkelsen 2003

What is speciation?

In water trace metals are present in a wide range of chemical forms, in both the particulate and dissolved phases. The dissolved phase comprises the hydrated ions, inorganic and organic complexes, together with species associated with heterogeneous colloidal dispersion and organometallic compounds.In some instances these metals are present in more than one valency state.

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Possible forms of trace elemementsSimple ionic species: Zn(H2O)6

2+

Valency states: As(III), As(V), Cr(III), Cr(IV)Weak complexes: Cu-fulvic acid

Adsorbed on colloidal particles: Cu-Fe(OH)3-humic acidLipid-soluble complexes : CH3HgClOrganometallic species: CH3AsO(OH2), Bu3SnClParticulate : Metals adsorbed onto or contained within clay particles

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G.E Batley, Trace element speciation; analytical methods and problems, CRC Press, Inc., 1989

Interactions affecting trace metal speciation

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An example, lead.

Free metal Pb2+

Ion pair PbHCO3

Complexes with organic pollutants Pb2+/EDTA

SOLUTION

Complexes with natural acids Pb2+/fulvic acid SUSPENSION

Ion adsorbed onto colloids Pb2+/Fe(OH)3

COLLODIAL

Metal within decomposing Pb in organic soilsorganic materialIonic solids Pb2+ held with the

clay structure, PbCO3

SOLID

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Why speciation studies?

Generally basic reasons for speciation measurements:Study transport and biogeochemical cycling processesPredict biological impact (identify those metal species which are likely to have adverse effects on biota and includes measurements both of bioavailability and toxicity)

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Toxicity

In general, the toxicity of metals stems from the fact that they are biological non-degradable and have a tendency to accumulate in vital organs, e.g. brain, liver, etc. and their accumulation become progressively more toxic

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Ionic copper are fare more toxic towards aquatic organisms than organically-bounded copper, and that more stable the copper complex, the lower is its toxicity.

Toxicity, some examples.

As(III) is fare more toxic than As(V)

Alkyl compounds of mercury and lead are especially toxic because they are lipid-soluble

Ni, Cr, Cu and Se are known to display carcinogenic effects due to their interactions with nucleic acids – e.g. whereas Cr(VI) is anionic and highly toxic Cr(III) is nontoxic, this because negative charge on CrO4

- makes it able to pass the cell membrane

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Detection of trace metal speciation?

Lipid soluble forms

Particle bond forms

Ionic forms and labile complexes

Information of speciation can be obtained even near the total limit of detection because separation methods can be used prior to the measurements of the actual species

These species are in principle more difficult to measure because any separation methods or attempts of pre- concentration will shift the distribution of the species. Molecular spectroscopy ? Fails due to detection limit Potentiometry ? Fails due to detection limit ?Voltammetry ? ICP-MS ?

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Detection of trace metal speciation?

Technique ResponseAtomic spectrometryFlame AAS, Flameless AAS

All the metal species in the sample, i.e. the total metal determined

Visible absorption spectrometry

Free metal ions plus ions released from complexes by the color forming reagent

ICP-MS Total and isotopes

Voltammetry Free metal ions plus any ions released from complexes during analyses. Total electrochem. cont.

Chromatography Non-labile species can sometime be determined separately

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Linear range

Maintaince

Online

Cost

Capacity

Efficiency

Speciation

Interference

Sensitivity

Voltammetri

ICP/MSEl. term. AAS

Flame AAS

Detection of trace metal speciation?

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Metals of common enviromental concern

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Electrochemical methods

Principle; information about the analyte is achieved from measurements of e.g.

potential, current, resistance or conductance. There at several methods available:

- Coulometry (measurements of current and time)

- Conductometry (measurements of conductance)

- Potentiometry (measurements of potential at zero current)

- Polarography / Voltammetry (measurements of current as function of an applied potential)

In particular voltammetry is suitable for analyses of trace metal and speciation studies. Detection limit for the most common heavy metals are in the range of 10-6 to 10-12 M.

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Potentiometry

Voltammetry Anodic stripping

voltammetry Adsorptive cathodic stripping voltammetrySquare wave stripping voltammetry

Ion selective electrodes

Electrochemical detection of trace metals

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Electroanalysis is a powerful technique for the study of trace element speciation, and has been applied to over 30 elements Four to six metals of prime environmental concern; Cu, Pb, Cd, Ni, Zn an Co can be detected simultaneously and with a sensitivity in the range of ng/LStudy of the kinetics of metal complex dissociation at en electrode is supported by well-established theory

Speciation study can be performed in the field within minutes, with low-cost equipment

Electrochemical techniques requires minor sample pretreatment, resulting in fewer potential sources for contaminations

Some advantages for el.chem. techniques

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Range of applicability, el.chem. speciation methodsDirect applications, determination of

redox state

fraction bound in inert organic complexes or to organic colloids, by measurements before and after UV irradiation after UV irradiation and

half wave or peak potential shifts

labile and inert metal fraction

Indirect applications, determination of

size distribution after ultra filtration

lipid soluble complexes, after extraction of water samples with e.g. n-octanol or 20% n-butanol in hexane

Pre concentration prior to e.g. Carbon Furnaces AAS

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Range of applicability; labile/inert metal fraction

Discrimination between labile and inert metal fraction in the sample- Labile metal compromise free metal ion and metal that can dissociate in the double layer (near electrode surface) from complexes or colloidal particles- For natural waters the most used techniques are ASV, AdCSV and SWV- Applied to e.g. Cu, Pb, Cd, Zn, Mn, Cr, Tl, Sb and Bi- Often the labile metals have been found to correlate well with the toxic fraction of the metal

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Range of applicability; redox state

Determination of the redox state of an element in solution is very important because it can drastically affect the toxicity, adsorptive behavior, and metal transport Applied to distinguish between e.g. Fe(III)/Fe(II), Cr(VI)/Cr(III), Tl(III)/Tl(I), Sn(IV)/Sn(II), Mn(IV)/Mn(II), Sb(V)/Sb(III), As(V)/As(III), Se(VI)/Se(IV), V(V)/(IV), Eu(III)/Eu(II), U(VI)/U(IV)

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Species Toxicity Electrochemical lability

Arsenic (III) HIGH HIGH

Arsenic (V) LOW LOW

Chromium (III) LOW LOW

Chromium (IV) HIGH HIGH

Thallium (I) HIGH HIGH

Thallium (III) LOW LOW

Cu2+ HIGH HIGH

CuCl2 HIGH HIGH

CuCO3 HIGH HIGH

Cu2+ -fulvic acid LOW LOW

Cu2+ /humic-Fe2O3

MEDIUM MEDIUM

Cu2+ -DMP HIGH LOW

Redox state, toxicity vs. el.chem. lability

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Range of applicability; half wave potential shifts

Shift in the polarographic half wave potential or ASV peak potential of metal ions in presence of complexing agents can provide information about the thermodynamic stability of complexes in solution.

Quantitative deductions may be difficult due to the high number of possible present ligands and metals in natural or polluted water Sometime however it is possible to do such quantitative deductions (e.g. the ASV peak for copper(I)-chloro complex in seawater)

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Limitations of el. chem. speciation techniques

Unable to measure the concentration of individual ionic species

Also, electrochemical techniques like polarography and ASV are dynamic systems which draw current through the solution and disturb ionic equilibrium. However with microelectrodes the current flowing is reduced to nA or pA

E.g. one peak only will appear for a mixture of Cd2+, CdSO4, CdCl+, and CdCO3 (which all may coexist in a river water sample)

Ion-selective electrode potentiometry is the only method that can measure the activity of a individual ion – but the sensitivity has up to nowbeen poor

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Limitations, however….

Other speciation methods, including ion exchange chromatography, Solvent extraction, dialysis, and ultrafiltration also disturb the natural ionic equilibrium in water samples during the speciation process

In addition often the question is only the discrimination between to species, where one is electrochemically active (labile) and the other species inert

Some practical examples

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Measurements of Cu, Pb, Cd and Zn in waters

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-free and weakly complexed metals are transported across the cell membrane and provide bioconcentration factors between 102 and 105

-when the capacity of a cell to detoxify accumulated metal is exceeded, damage to cyroplasmic constituents will occur, e.g. ultrastructural deformities, as well as reduction of cell division rate, respiration, photosynthesis, motility, electron transport activity, and ATP production

-may replace Mg at sulfhydryl binding sites-possible intracellular reaction between Cu and reduced glutathione which defend the cell against peroxide damage-loss of lysosomal membrane stability, which may lead to a leakage of hydrolytic enzymes into the cytosol and catabolic breakdown of the cell

Measurements of Cu, Pb, Cd and Zn in watersHeavy metals have a influence on the biological life, and may cause serious damage due to toxicity effects

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The surface area of an organism is critical to the passive metal diffusion process into cell, therefore bacteria and algal communities frequently have the highest metal concentrations in the food web. There are a magnifying through the food web E.g. Periphyton has been found to contain up to 1g/kg Cd, while the normal Concentration are 22 g/kg indigenous bryophyte populations in rivers draining old metal mines have been shown to contain up to16 mg/g Pb and 7 mg/g Zn

Cu, Pb, Cd and Zn

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Cu, Pb, Cd and Zn

In fresh water- inorganic fraction computed to be present

mainly as CuCO3 (over 90%), colloidal particles and hydrated iron oxides In seawater

- dominant inorganic species computed to be carbonato and hydroxy complexes (CuCO3 up to 80%), in addition CuOH+ and Cu(OH)2

0 (approx. 6,5%), Cu(OH)(CO3)- (approx. 6,5%), CuHCO3

+ (approx. 1%) and Cu Coastal surface seawater usually has 40 to 60% of total copper present as inert organic complexes. In unpolluted seawater ASV-labile copper is usually less than 50% of dissolved copper, even at pH as low as 4,7. Most freshwater streams also has little ASV-labile copper (organically bound)

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In fresh water- Computed to exist as PbCO3 and Pb2(OH)2CO3

(often > 90% of the inorganic lead species)

- in general lead has a stronger affinity for some inorganic adsorbents, especially iron oxide (pH 7), than for organic ligands,

- at pH 6.0 or lower most lead is found as electro inactive Pb2(OH)2CO3

In seawater- Pb is found as carbonato complexes (83%)

and chloro species (11%)- 40 to 80% of dissolved lead is found in the

inorganic colloid fraction

Cu, Pb, Cd and Zn

Alkyllead in natural waters may be determined by ASV after selective organic phase extraction

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In fresh water- Dominant form is computed to be Cd2+ and

CdCO3 depending on pH- Cd adsorbs to colloidal particles only at

relatively high pH values, so very little Cd is present as pseudocolloidsIn seawater

- Cd is computed to exist as CdCl+ and CdCl20 complexes (92%)

Cu, Pb, Cd and Zn

A high portion (over 70%) of Cd is found to be ASV labile both in seawater and freshwater. In anoxic water Cd may exist as no-labile CdHS+

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Cu, Pb, Cd and Zn

In fresh water- dominant inorganic forms are computed to

be Zn2+ (50) and ZnCO3 (38%)In seawater

- main species are computed to be Zn2+ (27%), chloro complexes (47%), and ZnCO3 (17%)

- open ocean waters contains as little as 10 ng/L Zn at the surface

The carbonato complexes of Zn, especial the basic carbonates, may have low ASV lability. About 59% of the total zinc in seawater and river water is ASV labile.

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Cu, Pb, Cd and Zn

Most suitable techniques are ASV and AdCSV

ASV AdCSV

1. Step The electrode are set to a potential about 300 mV more negative than the first expected metal peak Mn+ + ne- M (deposited on the electrode)

2. Step Cd is than stripped of by reverse the potential over the electrode towards more positive value M Mn+ + ne-

1. Step Cations are complexed with surface active

complexing agents (L) Mn+ + xL MLx

n+

2. Step Metal-complex adsorbs to the electrode

surface MLx

n+ + Met MLxn+

,ads(Met)

3. Step The cation is released from complex by

reduction MLx

n+,ads(Met) + me-

M(n-m)+ + xL + Met

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Speciation scheme for Cu, Pb, Cd and Zn in waters

Aliquot No.

Operation Interpretation

1. (20 mL)

Acidify to 0,05 M HNO3, add 0.1% H2O2 and UV irradiation for 8 h, than ASV a

Total metal

2. (20 mL)

ASV at natural pH for seawater add 0.025 M acetate buffer, pH 4,7 for freshwaters

ASV-labile metal

3. (20 mL)

UV irradiate with 0,1% H2O2 at natural pH, than ASV b

(3)-(2)=organically bound labile metal

4. (20 mL)

Pass through small column of Chelex

100 resin, ASV on effluent c

Very strongly bound metal

5. (20 mL)

Extract with 5 mL of hexane-20% n-butanol, ASV on acidified, UV- irrad. aqueous phase d

(1)-(5)=lipid soluble metal

Sample (unacidified), filter through a 0.45-m membrane filter, reject particulates and store filtrate unacidified at 4C a) Bring to pH 4.7 with acetate buffer, b) Not valid if Fe > 100 g/L, c) Optional step, d) Solvent dissolved in aqueous phase must be removed first

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Measurements of Fe in seawater

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Fe in seawater

Iron is one of the most important bioactive trace metal in the oceans. The first-row transition metal plays a key role in the biochemistry and physiology of oceanic phytoplankton.

Low iron concentrations are suggested to limit phytoplankton growth and biomass in certain oceanic regions

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Fe in seawater

The oceanic chemistry is highly complicated, and still not fully understood.Dissolved iron can exist in two different oxidation states, Fe(III) and Fe(II).Thermodynamically Fe(III) is the stable form in oxygenated water, however several processes reduce Fe(III) to Fe(II). Fe(II) may exist forseveral minutes in surface water(pH 8) before it is oxidized back toFe(III).

Presence of Fe2+ may cause an increase in the dissolved iron fraction making more iron available for use by biota.

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Inorganic speciation of dissolved Fe(III) and Fe(II) differ considerably.Inorganic Fe(III) species are dominated by hydrolysis products, Fe(OH)2

+, Fe(OH)3

0, and Fe(OH)4-. Free hydrated Fe3+

ion is extremely rare.Inorganic Fe(II) however exists in primarily as Fe2+ ion.

Fe in seawater

Evidence is also found for complexing of Fe(III) and Fe(II) with organic ligands.

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Since total dissolved iron in oceanic surface waters can be very low (down to a few pM), there is a need for highly sensitive techniques.

Fe in seawater

Iron(II) at nanomolar levels has been determine by e.g. and colorimetry preceded by preconcentration of iron(II) using octadecyl silica as stationary phase

However the most suitable technique is AdCSV

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Fe in seawater

Fe(III) complexed with 1-nitroso-2-napthol is preconcentrated onto a hanging mercury drop electrode (adsorption). (Addition of H2O2 secures that all iron is oxidized to Fe(III)

Concentration of Fe(II) is calculated from the difference between analyses with and without added 2,2-dipyridyl, which masks Iron(II).

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Fe in seawater

Recent results from our laboratory has shown a new ASV technique that can be used for detection of Fe(II) down to 50 ng/L on solid dental amalgamelectrode.

Analyses can be performed directly in the sample with only the additions of citrate or oxalate

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12

17

22

27

32

37

42

47

52

57

-1200 -1000 -800 -600 -400 -200 0

E (mV)

I (

A)

R2 = 0.9998

0

10

20

30

40

0 20 40 60

Conc (g/L)

Pea

k h

eig

ht

(

A)

Detection of iron (II) with DPASV in tri-sodium citrate-5,5-hydrate (0.02M) solution. Addition of iron (II) standard to solutions of 1,67 ppb, 3,34 ppb, 5 ppb, 15 ppb, 25 ppb, 50 ppb, pre-deposition time 180 s.

Fe in seawater

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Measurements of Hg in water

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Mercury has no known essential functions, though it has been used to treat syphilis, actually with some success.

Hg in water

Mercury probably affects the inherent protein structure which may interfere with functions relating to protein production. Mercury has a strong affinity for sulfhydryl, amine, phosphoryl, and carboxyl groups, and inactivates a wide range of enzyme systems, as well as causing injury to cell membranes.

Main problems seem to result from its attack on the nervous system. Mercury may also interfere with some functions of selenium, and can be an immunosuppressant

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Hg in water

Mercury dissolved in water is present in many forms, including organomercurials, such as methylmercuric chloride, phenylmercuric chloride and other alkyl- and arylmercury compounds.

Among the co-existing forms of mercury in natural water the most toxic to man and biota are organomercurials (up to 46% of the total mercury content has been found in this form in river water samples, and up to 63% in unfiltered samples)

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Hg in water

Organomercurials as methyl-mercury has high lipid solubility, something that makes bioaccumulation a serious problem. Bioaccumulation up to 103 to 104 have been reported for mercury in fish .

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Hg in waterLD50 of different organomercuric compounds

Compound Anions

LD50 rat (mg/kg)

Vapour pressure 20C (g/L)

HgCl 210

HgCl2 37

Methylmercury Br- 94000

Cl- 10 94000

I- 90000

Acetate 75000

Hydroxide

10000

Ethylmercury I- 9000

Cl- 40 8000

Methoxyethylemercury

Cl- 2600

Acetate 2

Phenylmercury Acetate 17

Cl- 60 5

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Organic and inorganic mercury can be detected with a glassy carbon electrode modified with thiolic resin. Detection limits in low g/L

Hg in water

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Hg in water, detection of Hg2+, MeHg+, EtHg+, PhHg+

Sample

not treated treated

Hg2+

MeHg+ EtHg+

PhHg+

(EtHg2+)(PhHg2+)

(MeHg2+)

Hg2+

(Hg2

+)

Hg2+ total

(TMS)R. Agraz et al.

E1= -0,5V

Hg2+

MeHg+

E2= -1,0V E3= -1,35V

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- Fe or Mn particulate in suspension can interact (0,06 mg/L Fe and 0,12 mg/L Mn)

Some advantages,

- may be performed in presence of high conc. of a varity of anions and cations

- good sensitivity

- good selectivity

- pH changes in the sample is unnecessary

- possible use also for salt or brackish water

Some disadvantage

Hg in water

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Measurements of AL(III) speciation in water

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Al(III) speciation in natural water

Many natural waters are affected of serious acidification problems due to acid precipitation and other ecological problems, resulting in Al mobilizationThe impact of Al highly depend on its existing chemical form, therefore speciation measurements of Al is very important

Graphite Furnace atomic adsorption spectrometry involving Driscoll’s Method is maybe the most used technique.

- use of hazardous organic solvent methyl isobutyl ketone - expensive - greater errors for the indirectly detection of inorganic monomeric Al at low conc. of total Al

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Al(III) speciation in natural water

At pH 5,2 citrate, oxalate tartrate, salicylate, humic an fulvic acids display very strong complexation ability with Al(III)

ASV at HMDE, solochrome violet RS (SVRS)

Therefore at pH 5,2, SVRS is only able to sequester sulfato, silicato and fluoro complexes in addition to a small portion of unstable organic complexes

At pH 8,5 SVRS shows much stronger complexation ability than citrate, oxalate…..

Therefore at pH 8,5, the total monomeric Al will be able for electrochemical determination

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Al(III) speciation in natural water

pH 5,2 and SVRS

Al3+, AlOH2+, Al(OH)2+, AlSO4

+, AlF2+, AlF2+, AlHPO4

+,

… Al-SVRS

SVRS

pH 8,5 and SVRS

Al3+, AlOH2+, Al(OH)2+, AlSO4

+, AlF2+, AlF2+, AlHPO4

+, Al-NOM, … Al(OH)4

SVRS

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Original water samples(untreated)

Acid reactive Al (Alr)Acidification to pH 1.0, than determination at pH 8.5

Total monomeric Al (Ala)Determinated at pH 8.5Labile monomeric Al (Ali)Determination at pH 5.2

Acid soluble aluminium (Als)Als = Alr - Ala

Non-labile monomericAl (Alo)Alo = Ala - Ali

Al(III) speciation in natural water

Untreated

Filtered0.45 m

ASV at HMDE, solochrome violet RS

X. Wang et. al

Conclusions

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Conclutions

Electrochemical techniques are a important tool for measuring speciation of metals in e.g. natural watersDetection of free metal ions plus any ions released from complexes during

analyses (total electrochem. cont.)

Can also be used to detect other speciation forms, e.g. total metal, organically bound labile metal, strongly bound metal, and lipid soluble metal after different types of sample pretreatments.

Low cost instruments, good detection limits (ng/L), may be used online in field

Thank you for your attention!