arsenic is a metalloid element that is commonly found in new … · 2017. 4. 24. · a simple...
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- Arsenic is a "metalloid" element that is commonly
found in New England soils and groundwater.
- As occurs primarily in sulfide minerals in bedrock, and as a
secondary element on/in oxyhydroxides (e.g. FeOH3),
- As is distributed throughout the overburden by both
physical (e.g. glacial erosion and transport) and chemical
(e.g., dissolution, precipitation, adsorption) processes.
- Anthropogenic arsenic sources include waste incineration,
coal combustion, metal mining, pesticide and herbicide
applications, and use as a wood preservative. Potential
sources include apple orchards, and numerous historical
industrial operations (e.g. leather tanning), and landfills.
- The USEPA drinking water standard for As is 10 ug/L.
11. Arsenic, As (atomic no. 33)
Smedley, 2008
World-wide Distribution of As Problems
in Groundwater
- In some areas of Bangladesh and West Bengal
(India), concentrations of As in groundwater exceed
water standards (10 to 50 mg/L), and may reach
levels in the mg/L range (see below).
- These As levels are from reductive dissolution of
Fe oxyhydroxide coatings on sedimentary grains
and release of the As adsorbed on the Fe coatings.
- Recalculated to pure FeOOH, As concentrations
may be over 500 ppm in these coatings.
- Reduction of the Fe is driven by microbial
metabolism of sedimentary organic matter, which is
present in concentrations as high as 6% carbon.
Smedley, 2008
As in Bangladesh GW
Welch, 2000
Higher arsenic concentrations are found in higher pH waters
No. water samples
As in
GW
Samples
From
Across
U.S.
Ayotte, sir2011-5059
Geographic distribution of Arsenic concentrations in
groundwater collected from wells as part of the National
Water-Quality Assessment Program, 1992–2003
Ayotte, sir2011-5059
Arsenic by pH and Redox ranges
USGS
NAWQA
GW
Samples
Ayotte, sir2011-5059
Percent of As samples > 1 mg/L
by pH and redox
Ayotte, sir2011-5059
Percentage of
NAWQA
samples greater than
the reporting level
for As (1 mg/l)
and Fe (10 mg/L)
As in NE US NAWQA
Groundwater Samples,
(Ayotte, sir2011-5059)
Arsenic in Groundwater in Eastern New England:
Occurrence, Controls, and Human Health
Implications
J .D . A Y O T T E , D .L . M O N T G O M E R Y , S .M . F L A N A G A N ,
A N D K.W . R O B I N S O N
-In eastern New England, high concentrations (>10 mg/L) of
arsenic can occur in groundwater.
-Water from wells in meta-sedimentary bedrock units, primarily
in Maine and New Hampshire, have the highest arsenic
concentrations; nearly 30% of wells in these aquifers have
arsenic concentrations greater than 10 mg/L.
-Arsenic was also found at concentrations of 3-40 mg/kg in
whole rock samples in these formations, suggesting a possible
geologic source.
- Arsenic is most common in groundwater with high pH. High
pH is related to groundwater age and the presence of calcite in
bedrock.
Wells with
higher arsenic
concentrations
were
generally found
in bedrock wells
in a zone
that occurs
in eastern ME,
eastern NH,
and central MA
Wells in
Bedrock
Composed of
Calcareous
Metasediments
were found
to have
higher
arsenic
concentrations
- Arsenate [As(V)] desorption increases at progressively
higher pH on iron oxides and some common clays.
-As concentrations are affected by the presence of
competing anions, such as phosphate (as PO4) and
sulfate (as SO4), for adsorption sites.
- Groundwater
with high arsenic
concentrations
(>10 mg/L)
generally has
pH values of 8
or greater.
Mc – Meta-calcareous rocks
Mu – undifferentiated metamorphic rocks
If – Felsic Igneous rocks
Across N.E.
higher
background
concentrations
of arsenic are
found in
higher pH
ground waters
from aquifers
with specific
bedrock
mineralogies
Ayotte, sir 2012-5156
Locations and
concentrations of
Arsenic in
groundwater
from NH bedrock
wells; n = 1,715
Colman, SIR 2011-5013
Colman, SIR 2011-5013
Colman, SIR 2011-5013
- As is stable in four oxidation states (+5,+3, 0, -3),
however, As (0) rarely occurs, and the -3 valence
state is only found in very reducing conditions.
- Arsenate (V) is stable in oxidizing environments,
(ORP > +100 mV), and the predominate species in
solution are H2AsO4- for pH 2.2 - 6.9, and HAsO4
-2
for pH 6.9 - 11.5 (see below).
- Arsenite (III) is stable in moderately reducing
environments; H3AsO30 predominates up to pH 9.2,
and H2AsO3- from pH 9.2-12.
- As(III) is more toxic than As(V), and as it adsorbs
less strongly, is more mobile than As(V).
Aqueous Geochemistry of Arsenic
Smedley, 2008
Arsenate (V)
Arsenite (III)
As Adsorption - The oxidation state of As has a significant effect on
its rate of transport in groundwater, with As(V)
absorbing to a greater extent, especially to iron
hydroxides (e.g. Fe(OH)3) , Mn, and Al, than As(III) at
lower pH values and As(III) absorbing to a greater
extent than As(V) at higher pH values.
- Adsorption of arsenic on to iron oxides is affected by
pH, amount of iron oxide present, and concentrations
of competing ions (e.g. phosphate).
-Studies of arsenic adsorption reveal a rapid uptake,
followed by a longer process that may be diffusion-
limited into iron-oxide colloidal material.
Smedley, 2008
Absorbed
As in/on
Hydrous
ferric
oxide
(Hfo)
As Adsorption (Stollenwerk, 2004)
- Adsorption and redox conditions are the predominate
mechanisms controlling transport of As in groundwater.
- Hydrous oxides of Fe, Al, Mn, and clay minerals have been
shown to be significant absorbents of As. The extent of As
absorption is affected by aqueous chemistry including pH,
As speciation, and the concentration of competing ions.
Phosphate, sulfate, carbonate, silica, and other anions have
been shown to decrease adsorption of As.
- Under moderately reducing conditions, arsenite (III) is
stable and adsorption increases with increasing pH. Under
oxidizing conditions, arsenate (V) is stable and adsorption
decreases with increasing pH.
As(III)
As(IV)
- Oxidation of As-bearing sulfides is a source
of As from geologic materials. Oxidation of
sulfides in mined areas throughout the world has
led to high concentrations of As in soils, surface
water, and groundwater; including occurrences
in western Canada, the western USA, and the
Bolivian Altiplano.
- Sulfide oxidation has also resulted in high As
concentrations (up to 215 μg/L) in groundwater
in the eastern USA, in parts of the Piedmont
rocks in Pennsylvania and New Jersey . (Barringer, 2013)
- Reductive dissolution of Fe hydroxides and
release of sorbed As explains much of the observed
mobilization of As from sediments to groundwater.
- Organic matter is likely an important component of
the reduction process. Studies have shown that metal-
reducing microbes can enhance mobilization of As,
and that oxidation of the organic matter drives the
redox reactions whereby Fe hydroxides are
reductively dissolved and sorbed As is released.
- Arsenic can also be released from Mn oxides as they
reductively dissolve, but may not remain in the
groundwater, instead resorbing to Fe hydroxide.
(Barringer, 2013)
Reactions affecting As in sediments
and ground water (Barringer, 2013)
A Simple Qualitative Model of Arsenic
Geochemistry (Hounslow, GW, 1980)
-Three key hydro-geochemical environments
control As fate and transport, these are based on the
oxidation-reduction state of ground water as
defined by the presence or absence of dissolved
oxygen (DO) and hydrogen sulfide (H2S).
-The mobility of arsenic is greatly influenced by
these dissolved gases and the behavior of iron in
these same environments.
Very Generalized Redox diagram!
Oxic
Suboxic/Anaerobic
Anaerobic/H2S present
Aqueous Environments
The key environments (see figure 1) are:
Aerobic Waters
These are defined as containing measurable
concentration of dissolved oxygen. Hydrogen
sulfide is absent. This environment is typical of
oxygenated surface streams, water in the
unsaturated zone, and some shallow ground water.
Anaerobic Waters -Water that lacks measurable DO, as oxygen has typically
been removed by reaction with organic matter.
- Anaerobic environments can be further subdivided:
A) Hydrogen sulfide absent – Water of this type is mildly
reducing and characteristic of shallow ground water.
B) Hydrogen sulfide present – this type of water is strongly
reducing and usually owes its hydrogen sulfide to sulfate
reduction.
- Under sulfate-reducing conditions, bacteria reduce sulfate
to H2S and HS- and, at the same time, carbon is oxidized to
the bicarbonate ion. This reaction is primarily dependent
upon the availability of oxidizable organic matter. The
environment is characteristic of either contaminated or
deeper ground water.
Importance of Iron - In mildly anaerobic water (i.e. lacking hydrogen sulfide),
soluble ferrous iron exists and moves readily in the subsurface.
It’s in this zone that the greatest degree of arsenic mobility
would be expected and be in the more toxic arsenite state.
- In strongly anaerobic water (i.e. containing hydrogen
sulfide), iron will precipitates as iron sulfides (marcasite or
pyrite), depending on the pH. When this occurs, many other
metal sulfides may co-precipitate with it. Under acidic
conditions As would precipitate as arsenic sulfide. Under
alkaline conditions the As forms sulfur-arsenite ions that may
combine with heavy metals to form insoluble compounds.
- Under extremely reducing and acidic conditions, and in the
presence of sulfur, As2S3 (orpiment) or AsS (realgar) may
form.
Aqueous Zone descriptions for figure 1
Conclusion:
Arsenic Mobility Greatest under
Mildly Reducing Conditions
“The Goldilocks zone: Arsenic is mobile when
not too oxidizing, and yet not too anaerobic”
Biogeochemical processes influencing
arsenic mobility using tracer tests
• As(V) adsorption vs. Competition from P
• As(V) reduction
D. Kent, 2004
Geochemical considerations
• Arsenic occurs naturally in pyrite and
other sulfide minerals
• Arsenic released during weathering of
sulfide minerals in oxic groundwater
• As(V) and As(III) adsorb extensively
onto Fe oxides
• As(V) adsorbs on aluminum oxides
and silicates
• Phosphate competes for sites
Arsenic in sediments:
competition with P
• Cape Cod Test site - total As in sediments:
2-5 nmole/g (0.1-0.4 ppm)
• Adsorbed As on sediments: 1 nmole/g
• 20-50% of arsenic in sediments is adsorbed
• Tracer test - groundwater (445 Liters) with
phosphate (PO4) injected at 620 mM
• Injection into oxic zone Kent and Fox (2004)
Background chemistry, AsTT03
0.00 0.02 0.04 0.06 0.08 0.10
Arsenic (mM)
8
9
10
11
12
13
14
15
Altitu
de,
mete
rs
AsT
DO
0 100 200 300 400 500
Dissolved Oxygen (mM),
As(V)
Injection port
As(III)
Breakthrough port
AWater Table
UZ
SZ
5.0 6.0 7.0
pH
8
9
10
11
12
13
14
150 50 100
Phosphate (mM)
pH
P
Injection portBreakthrough port
BWater Table
Phosphate injection results
0
100
200
300
400
500
SpC
(m
S/c
m)
5.2
6.2
pH
SpC
pH
Expected arrival time
End of injection
A
0 2 4 6 8 10
Hours after start of injection
0
100
200
300
400
500
600
Phosphoru
s (m
M)
0.00
0.02
0.04
0.06
0.08
0.10
Ars
enic
(m
M)
Phosphorus
As(V)
As(III)
B
Kent and Fox (2004)
As(V) reduction tracer test Hohn, et al., 2006
• Injected into anoxic (Fe) zone
• Injected 25 liters/hour, 4 weeks
• 6.7 mM As(V)
• 1.6 mM bromide
• Nitrate ~ 100 mM;
• dissolved oxygen 5-20 mM
• phosphate ~ 50 mM
As(V) tracer test
• Picture injection
stuff
Arsenic and phosphate
• F343 profile with P and As
5 6 7
pH
pH
0 100 200O2, Nitrate (mM)
-5
0
5
10
15
Altitude,
mete
rs t
o s
ea level
Fe
Nitrate
0 100 200 300 400Dissolved Iron (mM)
O2
0.00 0.10 0.20Arsenic (mM)
As(V)
F343
6/02
As(III)
0 50 100
Phosphate (mM)
AsT Phosphate
Injection
Zone
As(V) tt: 1 meter downgradient
• Br 1 m downgradient
0 20 40 60 80 100 120
Days after beginning injection
0.0
0.5
1.0
1.5
Br
(C/C
0)
Br
As(V) tt: 1 meter downgradient
• Br nitrate Fe 1 m downgradient
0 20 40 60 80 100 120
Days after beginning injection
0.0
1.0
2.0
3.0
4.0
Br
(C/C
0)
Br
0
100
200
300
NO
3- ,
Fe (m
M)
NO3-
Fe
• Br nitrate Fe phos 1 m
downgradient
0 20 40 60 80 100 120
Days after beginning injection
0.0
1.0
2.0
3.0
4.0
Br
(C/C
0)
Br
0
100
200
300
NO
3- ,
Fe, P
O4 (m
M)
NO3-
Fe
PO4
As(V) tt: 1 meter downgradient
As(V) tt: 1 meter downgradient
• Br nitrate Fe As 1 m downgradient
0 20 40 60 80 100 120
Days after beginning injection
0.0
1.0
2.0
3.0
4.0
Br
(C/C
0);
As
(m
M)
Br
0
100
200
300
NO
3- ,
Fe (m
M)
NO3-
FeAs(V)
As(III)
Longitude profiles showing concentrations of a) As(V) and b) As(III) 30, 45, 63 and 104
days after starting injection.
As(V)
As (III)
USEPA,
2007b
Attenuation
Processes
for Arsenic
- Arsenic is a metal that is commonly found in New England
soils and groundwater in sulfide minerals in bedrock, and as
a secondary element on/in oxyhydroxides (e.g. FeOH3),
- As sources also include waste incineration, coal burning,
metal mining, pesticide and herbicide applications, as a wood
preservative, and in landfills.
- As is commonly found in As(III) and As(V) redox states.
- As(III) is more toxic than As(V), sorbs less strongly, and so
is more mobile than As(V).
- As can be mobilized under low redox conditions as As(V) is
reduced to As(III), desorbs from mineral (iron oxide)
surfaces, and then moves as a dissolved form.
- As can be immobilized under strongly reducing conditions.
Summary
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