Heterogeneous Interactions of Arsenic in Aquatic Systems - ACS

31

H e t e r o g e n e o u s I n t e r a c t i o n s of A r s e n i c in Aquatic S y s t e m s

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 1, 2017 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

THOMAS R. HOLM, MARC A. ANDERSON, DENNIS G. IVERSON, and ROBERT S. STANFORTH University of Wisconsin, Water Chemistry Program, Madison, WI 53706

The chemistry of arsenic in soil and aquatic ecosystems can best be described as a complex array of homogeneous and hetergeneous chemical, biochemical, and geochemical reactions that together control the dissolved concentrations of arsenic in these systems. Although many of these reactions have been studied independently, the total description of the combined major reaction mechanisms that control the cyclic behavior of arsenic in environmental systems has largely been undeveloped. In the paper presented here, results obtained in laboratory and f i e l d studies are used to describe some of the major control mechanisms that affect the distribution as well as transformations of arsenic in an aquatic environment. Arsenic can exist in several oxidation states, as both inorganic and organometallic species, and in dissolved and gaseous phases (Table I). Dissolved arsenic species can adsorb to suspended solids and be carried down to the sediments in an aquatic system. Since gaseous arsenic compounds can form, arsenic can be removed from the sediments as dissolved gas or in gas bubbles (e.g. CH ). Thus, arsenic can cycle within aquatic ecosystems and this cyclic behavior has been reviewed by Ferguson and Gavis (]_) and Wool son (2). In any given system, i t is necessary to understand the behavior of a variety of different arsenic compounds as well as a variety of environmental compartments in order to totally characterize the cyclic behavior of this element. Four arsenic species common in natural samples are arsenate, arsenite, methanearsonic acid (MMAA) and dimethylarsinic acid (DMAA). These species possess different chemical properties which affect the mobility of arsenic in natural systems. For example, methanobacterium form trimethyl arsine from DMAA faster than fromMMAAor arsenate ( 3 J and arsenate and MMAA are more strongly adsorbed than DMAA on alluvial soils ( 4 ) . Transformation between the different oxidation states and species of arsenic may occur as a result of chemical or biochemical reactions (J_, _5, 6 2, _7, 8, 9). Inorganic chemical 4

9

0-8412-0479-9/79/47-093-711$06.50/0 © 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

712

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

Table I

Arsenic Species Commonly Found in Environmental Samples Species As0 "

Arsenate

+5

As0 -

Arsenite

+3

Methanearsonic Acid Monomethyl Arsonic Acid

+3

4

3


Oxidation State

Name(s)

3

3

CH As0(0H) 3

2

(CH ) AsOOH

Hydroxydlmethyl Arsine Oxide Dimethyl Arsinlc Acid Cacodylic Acid

AsH

Arsine

3 2

3

0

3

+1

-3

(C^JgAsH

Dimethyl Arsineb

-3

(CH )3As

Trimethyl Arsineb

-3

3

IUPAC name

a

Gaseous

D

transformations may be thermodynamically predicted to be a function of Eh and pH Geochemically, arsenic may form insoluble precipitates with calcium, sulphur, iron, aluminum, and barium compounds in natural waters (10). These precipitates are, however, slow in nucleating and show slow growth rates. Arsenic species are more l i k e l y to be adsorbed on the surface of organic and inorganic substrates than as crystalline precipitates. Indeed, i t will be shown in this paper that not only do adsorption reactions affect the transport of arsenic but they additionally alter the apparent transformation rates of arsenicals in natural systems. Since most natural systems are heterogeneous, that i s , they have solid and solution phases, one can immediately recognize the importance of understanding the role played by adsorption reactions in controlling dissolved elemental concentrations and transformation rates. It is the purpose of this paper to describe some of the major mechanisms that control arsenic in aquatic systems. Particularly, this paper addresses the problem of arsenic speciation and compartmentalization in sediments. To this end, results obtained from speciation, compartmentalization, kinetic, and adsorption studies using both f i e l d and laboratory samples will be interfaced in a descriptive model for arsenic in heterogeneous systems. The model has particular significance

Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


31.

HOLM

ET AL.

Heterogeneous Interactions of Arsenic

713

with respect to the biochemical transformation studies that have previously been performed by other researchers (11, 12, 13, J4, 3, ]j>). In order to investigate the complex array of mechanisms that control the transformations and distribution of arsenic in an aquatic system, i t was advantageous to study a system that was stressed by high concentrations of arsenicals. Then analytical measurements could be performed at levels well above detection limits and the rates of processes such as biotrans formations and sedimentation would be enhanced over corresponding rates in systems having background levels of arsenicals. The lower Menominee River (Figure 1) is an example of such a system and is the area chosen for this research. Due to the improper disposal of arsenical wastes, total arsenic concentrations are as high as 0.08 M in ground water, 1.6 χ Ι Ο " M in river water, 0.08 M in sediment pore f l u i d s , and 4000 mg kg" in sediment solTds (16). This paper is based on ongoing research in the lower Menominee River (16) and represents results to date. Each of the following sections has its own methods, results, and discussion subsections. 5

1

Speciation and Compartmentalization Methods of Speciation and Fractionation. It is apparent that in order to understand the mobility of arsenic and its availability for reactions, methods of speciation and fractionation must be applied to sediment samples in f i e l d and laboratory studies. In this paper speciation refers to the separation and quantitative determination of inorganic arsenic, methanearsonic acid, and cacodylic acid. Compartmentalization involves identifying the major compartments for arsenic in a heterogeneous system (e.g. aqueous, adsorbed, occluded,...) and determining the amounts of arsenic in each compartment. Fractionation involves the extraction of arsenic from operationally defined fractions of the solid phase of an aquatic system (e.g. sediment). Several analytical methods for speciating arsenic have been reported. They include chromatographic techniques such as electrophoresis and ion-exchange (17), paper chromatography (18) and HPLC (19); selective volatilization of arsenic compounds to analogous arsines followed by GC-MES (20); boiling point separation/spectral emission (21 hand atomic absorption (22). The above techniques have been applied to samples such as; commercial pesticides (20),coal and f l y ash (23),rocks, sediments, soils and mineraTT (24, 22),plant tissue (18), bovine liver (23),and water samples"T25). Studies in this laboratory require the use of analytical and speciation procedures which can be routinely performed on a variety of samples. Most of the above mentioned speciation techniques require that a considerable amount of equipment be



714

C H E M I C A L M O D E L I N G IN AQUEOUS SYSTEMS

dedicated to these particular analyses. In addition, some of them involve experimental parameters which are d i f f i c u l t to reproduce, hence, making routine analysis d i f f i c u l t . The speciation technique adopted for this study is a modification of Yamamoto's (25) chromatographic method which is simple to perform, requires a minimum amount of equipment and is reliable for separating inorganic-As ( i . e . arsenite and arsenate),monomethyl-arsonic acid,and dimethyl-arsincic acid in river water and i n t e r s t i t i a l water. The procedure involves: 1) conditioning a Dowex AG 50W-X8 resin column to the acid form and adjusting i t s pH to 1.5-1.8; 2) placing a sample (at pH ο3 ω υ

ο

CDCD "O

+->

03 J Q E S-

Ε

Ο

CD

•ι—

en CO s- -a C D C 0 3 C D «— OC 4->θ3 ->I — C CD+S STi •ι-

Ε

+J

ο

LO ο ^t-

CO I —

o

ο

o

OsJ

^·

ο ο ο

ο ο

Ο ο LO

r—

Ε ο •r— 4->

03

Ο

«Ι

Ο

CO -4-> r—

Ζ5 ce CD C0

Ο

•r—

03

•Γ-

S4-> Ε E

ΟD οθ 3cr C Ο ο

υ

Ι

Ε ο

CD S -C D Ο > 03 CD ο θ3 ΟίCD ^- TJ

o ο

o t—

Ο

cr» cr»

Γ -

•Γ—

>>

-M Ε

Ο

ο

^

Ο

4->

CO

Ο

ο

ρ—-

ο

LO

cr»

•Γ-

Ο ο < : -Μ


719

C H E M I C A L M O D E L I N G IN AQUEOUS SYSTEMS


720

portion of the sediments, was mixed with fresh sediment in approximately a 1:2 sediment:pore water ratio. A 2 ml sample of the sediment slurry was placed in a small glass tube and stoppered with a rubber septum. The sediment transfers were done under an N2 atmosphere to avoid oxidation. A volume of standard arsenic or phosphate solution was injected with a syringe through the septum. The tubes were shaken for approximately 24 hours. The solids were separated by centrlfugation and f i l t r a t i o n through a 0.45 ym f i l t e r , and the pore water analyzed for arsenic or phosphorus. Blanks were run using arsenic and phosphorus in d i s t i l l e d water and pore water to investigate sorption to the glass tubes and precipitation in the pore waters. Both factors were found to be unimportant in removing arsenic or phosphorus from the experimental solution. Results. The experimental results (Figure 4) show significant differences in the loss from solution of the various arsenic species and phosphate. Clearly, some species are removed from solution to a much greater extent than others, with the relative removals being as follows: P0 > AS0 > MMAA > As0 > CA 4

4

most strongly removed

3

least strongly removed

where MMAA = monomethyl arsonic acid and CA = cacodylic acid. Phosphate and arsenate were both removed to a great extent, while cacodylic acid was removed only slightly and in some cases not at a l l . Since the loss from solution occurred only when the sediments were present, either sorption or surface precipitation must be controlling the concentrations. Since adsorption is probably the major process controlling the observed loss from solution, 1t is instructive to f i t the data obtained to a Langmuir isotherm. Adsorption constants for the various species calculated from the data obtained are given in Table IV. Constants for arsenite were not calculated because the data indicated that arsenite loss from solution does not follow an adsorption isotherm. Inspection of the arsenite loss from solution isotherm shown in Figure 4 indicates that the loss Is linearly dependent on concentration over the concentration range used. The constants given In Table IV show the strong adsorption of phosphate and arsenate relative to cacodylic acid, with monomethyl arsonic acid intermediate. Constants for the adsorption of arsenate on amorphous iron and aluminum hydroxide are also included in Table IV to compare the sediments with well-characterized adsorbents. It was assumed that loss from



Figure 4. Sorption of phosphate and arsenic species by anaerobic Menominee River sediments


to


722

CHEMICAL

MODELING

IN AQUEOUS

SYSTEMS

Table IV Sorption of Phosphate and Arsenic Species by Anaerobic Sediments Species PO

4

As0

4

MMAA

Γ

*-ι

ymol g

K a

-i

Ν

vimol 1

b

d

9.1

15

7.7

32

3

3.6

28

11

3

d

Correlation Coefficient? 0.86 0.98 0.38

As0 ·

h

1250

137

~

--

As0

h

1700

51

—

—

4

CA

f

4

g

'

3.6

320

4

e

0.85

?Langmuir c o e f f i c i e n t s . Number of p o i n t s . C o r r e l a t i o n of l i n e a r i z e d form of data to a l e a s t squares ^approximation of the best s t r a i g h t l i n e through the p o i n t s . Ignoring one data p o i n t shown i n Figure 4 due to p o s s i b l e background i n t e r f e r e n c e . Ignoring three points i n which l i t t l e or no adsorption was JC observed. Adsorption on amorphous A l ( 0 H ) g . rAdsorption on amorphous Fe(0H)o. Data from Ferguson e t ajk ( 3 1 j .


31.

HOLM


ET AL.

723


solution followed a Langmuir isotherm. Since a small number of points were used and there was some scatter in the data, there is some uncertainty in the constants. However the constants are useful for calculations, as will be seen later in this paper. Discussion. Wauchope (4) reported that adsorption of phosphate and various arsenic species on alluvial soils varied as follows: As0 « MMAA > DMAA > P0 4

most strongly adsorbed

4

least strongly adsorbed

The arsenic species adsorbed in the same pattern in both studies, while phosphate changed from being the most strongly adsorbed in this study to the least strongly adsorbed in Wauchope's study. The soils used in Wauchope's study were presumably aerobic, with iron and aluminum hydroxides present. The sorption of phosphate and the arsenic species on the twelve soils used in Wauchope's experiment correlated well with both the clay content and the iron content of the s o i l s . The Menominee River sediments, however, were anaerobic, so iron should have been present as Fe(OH)? rather than the Fe(0H)3 which was, presumably, present in Wauchope's alluvial s o i l s . The solubility of Fe(0H)2 is greater than that of Fe(0H) (30) and its surface properties may d i f f e r from those of Fe(0H)3. Tfius, differences in amount and speciation of iron may have accounted for differences in phosphate sorption. Ferguson and Anderson (31) investigated the adsorption of arsenate and arsenite on Tron and aluminum hydroxides and found that for both adsorbents the adsorption of arsenate followed a Langmuir isotherm while the sorption of arsenite varied linearly with concentration. In the concentration range used in the experiment described above, they found that arsenate was much more strongly adsorbed than arsenite. They did not explain the rather surprising difference in behavior between arsenate and arsenite; however, their results give support to the results found in this experiment. Combining the results of the three studies, i t appears that arsenate is more strongly adsorbed than monomethyl arsonic acid, which is more strongly adsorbed than cacodylic acid, and that a l l three probably follow Langmuir isotherms. Arsenite sorption, on the other hand, varies linearly with concentration over concentration ranges of environmental significance. Phosphate adsorption also follows a Langmuir isotherm, but the strength of adsorption relative to the arsenic species varies depending on the adsorbent. 3


CHEMICAL MODELING IN AQUEOUS SYSTEMS


724

Adsorption processes are very dependent on the solid surface characteristics and pH, as well as other factors (adsorbate and adsorbent identity and concentration, solution ionic strength, identity of other ions present, e t c . ) . Since neither surface characteristics nor pH were measured or controlled, the numerical values obtained in the experiment need to be interpreted with care, as was discussed previously. The relative trends, however, will probably be valid for sediments similar to those used in the experiment, i . e . , anaerobic river sediments where sediments control the concentration of the species in solution. Wauchope's findings Indicate that the relative trends may not be valid for aerobic sediments or s o i l s . Adsorption in sediments or soils occurs primarily on various clays and on amorphous iron and aluminum hydroxide particles and coatings on other particles. The concentration of the adsorbing species obviously varies greatly in different sediment samples. Thus the constants determined for one sediment sample will apply only to the particular concentrations and composition of adsorbing species found in that sample, i . e . , the constants will be sample specific. As long as the adsorbing species remain the same in different sediment samples, although at different concentrations, the relative adsorbing patterns found for different species will remain the same, and the results from one set of experiments can be used as an aid to understanding the processes occurring in a similar sediment sample. If, however, the adsorbing species change between samples, then the relative adsorptive strengths of the sediments for different anions may change, and the results from experiments with one sediment cannot be applied to the different sediments. Further experiments are planned to better elucidate the relative adsorbing strengths of phosphate and arsenic species on aerobic sediments. Rates of Species Transformations in Anaerobic Sediments Kinetics and Adsorption. If microorganisms only take up dissolved arsenic species, then adsorption can affect rates of species transformations by lowering concentrations of reactants. In this section calculations of changes in concentrations of reactants and products of arsenic species transformations in sediments are presented. Rates of transformation are assumed to be proportional to species concentrations and the rate of another reaction, V, coupled to the transformation. That 1s, the transformation reaction is not assumed to be a source of energy or structural material for the microorganisms. Thus, -

d

(y*0

= kV (Species) .



31.

HOLM ET AL.


725

If V is constant, for example i f V is described by the Michelis-Menten equation and the substrate concentration is high enough that V approaches Vmaxs then kV is a lumped constant and the transformation rate is described by a f i r s t order equation. Calculating the effects of adsorption on kinetics necessarily involves mixing kinetic and equilibrium data. However, the time scales for adsorption and desorption reactions are much shorter than those for mlcrobially mediated arsenic species transformations. Adsorption and desorption reactions reach equilibrium over a period of 24 hours or less (32, 33). On the other hand, Woolson (2) estimated conversion rates of 0.067 to 0.404 % day" for oxidative metabolism of cacodylic acid to arsenate in model aquatic systems. Assuming f i r s t order kinetics, these conversion rates translate to half lives of 5.8 to 34.7 months. Thus, Woolson's model ecosystems were probably at equilibrium at a l l times with respect to adsorption and desorption of arsenic species. 1

Examples of the Influence of Adsorption on Kinetics. Three cases of arsenic species transformations influenced by adsorption are considered in this section. The three cases considered are: 1) reactant not adsorbed, product adsorbed (e.g. demethylation of cacodylic acid to arsenate), 2) reactant adsorbed, product not adsorbed (e.g. methylation of arsenate to cacodylic acid), 3) both reactant and product adsorbed and competing for adsorption sites (e.g. demethylation of methanearsonic acid to arsenate). The equations for each case are shown in Tables V through VII, respectively. Most of the symbols in Tables V, VI, and VII have their common meanings. The symbol [B] stands for the concentration of adsorbed species Β and is the product of Γ, the number of moles of Β adsorbed per gram of adsorbent, and U, the number of grams of adsorbent per l i t e r . Thus, [B] has the units moles per l i t e r . An example of each case was calculated using the adsorption constants for anaerobic Menominee River sediments listed in Table IV. The f i r s t order rate constant used was 0.04 day" , corresponding to a half l i f e of 17 days. Initial reactant concentrations of 0.001 M were chosen to approximate conditions in moderately contaminated sediments (16). A sediment solids concentration of 60 g l " (corresponding to 95% water) was used. In the f i r s t two cases, setting up and solving the differential equation was straightforward. In case 2 a closed form solution could not be obtained, so the equation was solved numerically. Setting up the differential equation for case 3 involved solving a cubic equation, so a different approach was taken. For case 3 the system of mass balance and Langmuir equations was solved for [A] and [B]. Then the system was allowed to s

s

1

1


CHEMICAL

726

MODELING IN AQUEOUS

SYSTEMS


Table V Case 1. F i r s t Order Reaction. Product Adsorbed According to Langmuir Isotherm. Reactant not Adsorbed A

-> Β

=JP

- k [A]

C = [A] + [B] U Γ„ IB].

+ [B]

s

[B]

Κ + [B]

S

at t = 0

^[A] = C,

[Β] = 0

-kt [A] = Ce B

=

(independent of adsorption)

-N + (N

2

+ 4KC(l-e" )) k t

where Ν = Ur Example:

1 / 2

+ Κ - C(l-e~ ) kt

m

demethylation of cacodylic acid (weakly adsorbed) to arsenate (strongly adsorbed).

Key to Symbols:

A

reactant, Β product

[A]

aqueous concentration of [A]

U

concentration of adsorbent in g

Κ

constant in Langmuir isotherm adsorption capacity of adsorbent in mol g"l

[B]

c

S

concentration of adsorbed product t^s

=

U r

B


31.

H O L M E T AL.


727

Table VI Case 2. F i r s t Order Reaction. Reactant Adsorbed According to Langmuir Isotherm. Product Not Adsorbed. dt

dt

K

l

M

J

C = [A] + [A] + [B]

at t = 0 C = [k]s,o + [A]


s

UF [A]

M-

[A]

œ

Q

0

Κ + [A]

s

Ν,

α N

α

+

l

2

(^2-1

n

= "

where N = 2 κ + y-2

k

c°

t+

n s t a n t

α = [A]/C

1

H = 2 + κμ

K

2

=

K

/

C

y = r u/c

ά

œ

Other symbols described in Table V. Solve numerically using Newton iteration. Constant calculated by substituting a

Q

Case 3. A

t Q t

= [A]

B

s

t o t

=

Q

+

roA

[A]

at t = 0

1

s

[A]

2

K + [A] + [B] K /K A

A

- [Β]

+

[B]

B

3

s

[B] - i k i L _ _ S

C

Table VII First Order Reaction. Product and Reactant Adsorbed According to Competitive Langmuir Isotherm.

U r [ A ]

= [A] /

Kg + [B] + [A] K /K B

4 A

Symbols described in Table V. Procedure:

1. 2. 3.

Solve equations 1-4 for [A] and [B]. Compute ΔΑ = [A] ( l - e " ) for a short interval t. (Choose t such that ΔΑ [A]). Subtract ΔΑ from A and add ΔΑ to B . kt

t Q t

t Q t

Chemical Aqueous Systems 4. Jenne; Go to step Modeling 1 and inrepeat the procedure.

ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL


728

MODELING IN AQUEOUS

SYSTEMS

react as i f there were no solid present and [A] was the total concentration of A. In other words, an amount of Α, ΔΑ, was changed to Β according to the equation ΔΑ = [AlO-e-kt), where t was chosen to be less than 0.1 ti/2. Then A was subtracted from C A and added to C B and [A] and [B] were recalculated. The entire process was repeated until the desired time interval was covered. The method used for the solution of case 3 was tested on systems for which the differential equation could be set up. The approximate solution was within 0.1% of the exact solution in each case. Demethylation of cacodylic acid to arsenate was chosen as an example of case 1, product adsorbing with reactant not adsorbing, because cacodylic a d d is only weakly adsorbed by anaerobic sediments and arsenate is strongly adsorbed. For the same reason, methylation of arsenate to produce cacodylic acid was chosen as an example of case 2, reactant adsorbed and product not adsorbed. Demethylation of methanearsonic acid to arsenate was chosen as an example of case 3, reactant and product both adsorbing. The predicted arsenate vs time curves for demethylation of cacodylic acid are shown Tn Figure 5. Both curves are for f i r s t order decay with a rate constant of 0.04 day"' but with different amounts of adsorbent present. Adsorption of arsenate results in dissolved arsenate concentrations that are lower than total arsenate. An s-shaped curve results because arsenate produced during the f i r s t few days is almost entirely adsorbed. Predicted arsenate and cacodylic acid vs time curves for methylation of arsenate to produce caco*c|yMc acid are plotted in Figures 6 and 7, respectively. Adsorption of arsenate lowers the dissolved arsenate concentration and, therefore, results in a slower rate of methylation. The apparent rate constant for this reaction in sediment containing 60 g of solids per l i t e r is 0.03 d a y compared with 0.04 d a y ' in the absence of sediment. Demethylation of monomethylarsonic acid to produce arsenate with the two species competing for adsorption sites is depicted in Figure 8. Adsorption of MMAA results in slower conversion rates and the apparent rate constant in this case is 0.03 day-1. Since arsenate is adsorbed more strongly than MMAA the final arsenate concentration is less than the i n i t i a l MMAA concentration. The apparent rate of decomposition of adsorbed reactant is quite sensitive to the product of the parameters U and Γ^. For MMAA demethylation using the parameters of case 3, for which Ur^ is 216 μΜ, the apparent rate constant is 0.04 day" . If ΙίΓοο is increased to 500 the apparent rate constant drops to 0.03 day" . Dilution of sediment decreases U and, thus, 1

1

1



31.

HOLM


ET AL.

Ο

ΙΟ

20

30 TIME

40

50

729

60

(days)

Figure 5. Dissolved arsenate from demethylation of cacodylic acid, k = day , = 7.7 ^mol g~\ Κ = 32 mol L . (a) U = 0; (b) U = 60g Is . 1

1

μ

0.04

1

Heterogeneous Interactions of Arsenic in Aquatic Systems - ACS

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