Chemical Modeling in Aqueous Systems - ACS Publications

4 I n f l u e n c e of R e d o x E n v i r o n m e n t s Arsenic

in

Ground

o n the

Mobility

of

Water

Downloaded by UNIV OF PITTSBURGH on April 26, 2016 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch004

J. GULENS and D. R. CHAMP Atomic Energy of Canada Ltd., Chalk River Nuclear Laboratories, Chalk River, Ontario, CanadaK0J1J0 R. E. JACKSON Inland Waters Directorate, Fisheries and Environment Canada, Ottawa, Ontario, Canada K1A 0E7 The concept of a sequence of redox r e a c t i o n s i n ground water flow systems has been developed ( 1 ) . This concept i s based on a modified v e r s i o n of the theory presented by Stumm and Morgan ( 2 p. 326-339), and uses the flow system as the h y d r o g e o l o g i c a l framework onwhich a thermodynamically based sequence of redox r e a c t i o n s is superimposed. I n a confined a q u i f e r c o n t a i n i n g excess d i s s o l v e d organic carbon (DOC) and some solid-phase F e ( I I I ) and Mn(IV) m i n e r a l s , redox r e a c t i o n s can occur between the DOC and the o x i d i z e d species present i n the ground water. As the water flows from recharge t o discharge w i t h i n t h i s confined a q u i f e r , the o x i d i z e d species w i l l be reduced i n the f o l l o w i n g sequence: d i s s o l v e d oxygen, n i t r a t e , ( s o l i d ) manganese oxides, ( s o l i d ) f e r r i c hydroxides, s u l f a t e , d i s s o l v e d carbon d i o x i d e and f i n a l l y dissolved nitrogen (1). As a consequence of t h i s concept of redox sequences, i t can be concluded (1) that three s e q u e n t i a l zones or environments may e x i s t i n confined a q u i f e r systems: an o x i d i z i n g zone ( i n the recharge area), a " n e u t r a l " zone ( i n the t r a n s i t i o n area), and a reducing zone ( i n the discharge a r e a ) , Figure 1. The m o b i l i t y and concentration of m u l t i v a l e n t t r a n s i t i o n metals and nonmetals v a r i e s i n each of these zones, a p r i n c i p l e that can be u s e f u l l y a p p l i e d to the abatement and p r e v e n t i o n of ground water p o l l u t i o n . As an example, Fe i s immobilized as the F e ( I I I ) hydrous oxide i n the o x i d i z i n g zone; f u r t h e r downstream i n a l e s s o x i d i z i n g environment, the " n e u t r a l " zone, F e ( I I ) i s s t a b l e and as i t i s a l s o more s o l u b l e than the F e ( I I I ) hydrous oxide, the l a t t e r becomes reduced and m o b i l i z e d as F e ( I I ) . In the reducing zone, s u l f a t e reducing b a c t e r i a may e x i s t and Fe may again be immobilized as the i n s o l u b l e s u l f i d e . Hydrous oxide surfaces of sand immobilize As by a d s o r p t i o n processes ( 3 ) . The r e s u l t s of our s t u d i e s show that the extent of adsorption v a r i e s w i t h the o x i d a t i o n s t a t e of the As, the redox environment and/or the pH of the e l u t i n g water. The i n f l u e n c e of these parameters on the m o b i l i t y of As was s t u d i e d by e l u t i n g As through sand columns: waters of d i f f e r e n t redox 9

0-8412-0479-9/79/47-093-081$05.00/0 © 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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


82

Figure

1.

Sequential

redox zones within a confined aquifer: "neutral" (Fe \ Mn ); reducing (S ~) 2

2+

oxidizing

(O , t

2

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

NO ~); s

GULENS E T A L .

4.

Redox

Environments

and

Arsenic

83

c h a r a c t e r i s t i c s wereused f o r e l u t i o n , and the e l u t i o n behaviour of As(V) was compared to that of A s ( I I I ) . Studies i n buffered s o l u t i o n s showed that an Fe-As complex was formed, the s o l u b i l i t y of which was a l s o dependent on the o x i d a t i o n s t a t e of the As and the s o l u t i o n pH. Experimental E l u t i o n Studies. The e l u t i o n p r o f i l e s of As(V) and A s ( I I I ) through sand columns were obtained using r a d i o a c t i v e a r s e n i c . A s was prepared by i r r a d i a t i n g reagent grade s o l i d A S 2 O 3 and A s 0 i n the Chalk River NRU r e a c t o r . The i r r a d i a t e d s o l i d s were d i s s o l v e d and loaded immediately. *As was obtained from Amersham/Searle Corp. as a s o l u t i o n of a r s e n i c a c i d i n 0.04 M HC1; a p o r t i o n of t h i s so_lution was converted to As ( I I I ) by heating i n the presence of HSO3 (4) (estimated conversion was 70%). The e l u t i o n s t u d i e s were conducted i n p l e x i g l a s columns (2.5 cm I.D. and 28 cm long) packed w i t h sand (60-230 mesh). The Fe and Mn content of the sand was 0.6% and 0.01% (w/w) r e s p e c t i v e l y , determined by atomic absorption spectrometry f o l l o w i n g an HC1 e x t r a c t i o n . The columns were hand packed w i t h the sand, using techniques to minimize d e n s i t y and s i z e segregation of the sands (.5) and thereby m i n i m i z i n g flow inhomogeneities ( 6 ) , and then p r e - e q u i l i b r a t e d by f l u s h i n g w i t h the e l u a t e f o r 3 days p r i o r to l o a d i n g . An a l i q u o t of r a d i o a c t i v e a r s e n i c ( A s or A s ) was loaded onto the base of the column and eluted w i t h upward flow. Eluant f r a c t i o n s were c o l l e c t e d i n an automated f r a c t i o n c o l l e c t o r and analyzed f o r As by γ-ray spectrometry. The As(V) and A s ( I I I ) e l u t i o n p r o f i l e s were obtained from separate columns, e l u t e d i n p a r a l l e l . The r e s u l t s have been corrected f o r the decay r a t e ( A s , t i . = 26.4 h, A s , t j . = 17.7 days). The columns were eluted a t 0.5 mL m i n both i n l a b o r a t o r y and f i e l d s t u d i e s . E l u t i o n s t u d i e s i n the l a b o r a t o r y were conducted at room temperature 298 K, using a i r - s a t u r a t e d d i s t i l l e d water, pH 5.7. E l u t ions i n the f i e l d were c a r r i e d out at ambient temperatures ranging from 278 Κ to 298 K, using ground water a t a temperature of 2 79 K. Ground water was pumped continuously from piezometers by a M a s t e r f l e x p e r i s t a l t i c pump at 50 mL m i n i n t o a 500 mL p l e x i g l a s sampling c e l l where E and pH values were measured; water was then pumped from t h i s c e l l i n t o the columns. The measured p o t e n t i a l of a platinum e l e c t r o d e , Eg, was determined r e l a t i v e to a saturated calomel r e f e r e n c e electrode, but values are quoted r e l a t i v e to a normal hydrogen e l e c t r o d e .


7 6

2

5

yt

7 6

7 6

7 4

7 4

- 1

- 1

H

Ground Water Geochemistry. Ground water from the lower a q u i f e r i n the lower Perch Lake Basin a t the Chalk R i v e r Nuclear L a b o r a t o r i e s , Figure 2, was used f o r f i e l d e l u t i o n s t u d i e s to provide water of d i f f e r e n t redox c h a r a c t e r i s t i c s . Water from piezometer "0" i n the t r a n s i t i o n area, and from KNEW i n the discharge area was used as being r e p r e s e n t a t i v e of " n e u t r a l " and



Figure 2. The lower Perch Lake basin, Chalk River Nuclear Laboratories, showing the location of piezometers HA, "O," and KNEW. The screens of these piezometers are 60 cm long and are located at the bottom of the piezometer, adjacent to the numbers on the figure.

Journal of Earth Sciences



4.

Redox Environments and Arsenic

GULENS E T A L .

85

reducing environments r e s p e c t i v e l y . Piezometer HA i n the recharge area represents an o x i d i z i n g environment but i n a c c e s s i b i l i t y to e l e c t r i c i t y precluded i t s use i n the present s i t u a t i o n . Thus a i r saturated d i s t i l l e d water was used i n the l a b o r a t o r y e l u t i o n s t u d i e s to simulate an o x i d i z i n g environment ( i t i s r e a d i l y admitted that the absence of DOC and major and minor elements from the d i s t i l l e d water may a l t e r the e l u t i o n p r o f i l e ) . The geochemical parameters c h a r a c t e r i z i n g the ground water at these l o c a t i o n s are presented i n Table I . I t should be noted that at piezometer "0", w h i l e the d i s s o l v e d Fe c o n c e n t r a t i o n i s low and i s e n t i r e l y due to F e ( I I ) (polarographic d e t e r m i n a t i o n ) , the ground water here a l s o c o n t a i n s l a r g e amounts of suspended hydrated f e r r i c oxide. S o l u t i o n Studies. Reactions between Fe and As i n s o l u t i o n were a l s o s t u d i e d . An amount of FeCl3 s o l u t i o n was added to a buffered s o l u t i o n c o n t a i n i n g As so that the t o t a l a r s e n i c and i r o n c o n c e n t r a t i o n s were equal at 5 χ 10"" M. The s o l u t i o n s were s t i r r e d f o r 16-24 hours, allowed to s e t t l e and a l i q u o t s of the supernatant analyzed p o l a r o g r a p h i c a l l y f o r Fe and As, using a P r i n c e t o n A p p l i e d Research Model 174 Polarograph i n the sampled DC mode (2 mV s " scan r a t e , ΔΕ = 50 mV, 1 second drop t i m e ) . A 0.2 M sodium o x a l a t e (pH 4) s o l u t i o n was used to determine F e ( I I ) and F e ( I I I ) Ç 7 ) , w h i l e As(V) was determined i n a 2 M p e r c h l o r i c a c i d - 0.5 M p y r o g a l l o l s o l u t i o n (8^) and As ( I I I ) i n a 2 M sodium hydroxide - 0.2 M sodium t a r t r a t e s o l u t i o n Ç 7 ) . For heterogeneous s t u d i e s , A s ( I I I ) was adsorbed onto s i l i c a g e l impregnated w i t h f e r r i c hydroxide, according t o the batch method of Yoshida et a l . ( 9 ) . The i n i t i a l supernatant s o l u t i o n , and the supernatant s o l u t i o n r e s u l t i n g from an a c i d r i n s e (1 M HC1) of the s i l i c a g e l were analyzed p o l a r o g r a p h i c a l l y f o r As and Fe. 1

R e s u l t s and D i s c u s s i o n Column E l u t i o n s . The e l u t i o n behaviour of A s ( I I I ) i s s i g n i f i c a n t l y d i f f e r e n t from that of A s ( V ) , i n terms of both i t s time of i n i t i a l appearance and the q u a n t i t y of As e l u t e d . These parameters v a r y f o r each species w i t h the redox c h a r a c t e r i s t i c s of the water used, F i g u r e 3. In an o x i d i z i n g environment, A s ( I I I ) i s detected i n the column e l u a t e 5-6 times sooner than A s ( V ) , and the amount of A s ( I I I ) eluted (^60% of l o a d i n g ) i s ^8 times l a r g e r than that of As(V). In the " n e u t r a l " environment, the r e l a t i v e amounts of both species eluted are unchanged; however, the As(V) moves through the column much more r a p i d l y than before but s t i l l i s retarded w i t h r e s p e c t to the A s ( I I I ) . In the reducing zone, the m o b i l i t y of As (V) i s a c c e l e r a t e d : both species now appear i n the column e f f l u e n t a f t e r l e s s than one column volume i s d i s p l a c e d . Both species are a l s o e l u t e d almost q u a n t i t a t i v e l y (VL00% f o r A s ( I I I ) , ^80% f o r A s ( V ) ) .


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

TABLE I Ground Water Geochemical D a t a


PH

HA

"0"

5.4

6.9

580 DO

140

a

KNEW 8.3 75

2.4

Chemical Modeling in Aqueous Systems - ACS Publications

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