arsenic is a metalloid element that is commonly found in new … · 2017. 4. 24. · a simple...

- 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


Arsenic by pH and Redox ranges

USGS

NAWQA

GW

Samples


Percent of As samples > 1 mg/L

by pH and redox


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

- 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


• Br nitrate Fe 1 m downgradient

0 20 40 60 80 100 120


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


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



• Br nitrate Fe As 1 m downgradient

0 20 40 60 80 100 120


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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Lead, Pb

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