Environ. Sci. Technol. 1986, 2 0 , 183-186
Insight into the Mechanism of Accumulation of Arsenate and Phosphate in Hydro Lake Sediments by Measuring the Rate of Dissolution with Ethylenediaminetetraacetic Acid John Aggett’ and Lynley Susan Roberts
Department of Chemistry, University of Auckland, Auckland, New Zealand The mechanism of accumulation of arsenic and phosphate on sediments in Lake Ohakuri in the North Island of New Zealand has been examined by comparing the rates of dissolution of the anions with the rates of dissolution of the matrix elements iron, aluminum, calcium, manganese, and silicon when samples were reacted with ethylenediaminetetraacetic acid (EDTA) a t the natural pH of the sediments. Very close correlation was observed between the rates of dissolution of the anions and that of iron. This, together with the observation that iron not dissolved by EDTA was subsequently dissolved by Tamm’s reagent, suggests that the most important mechanism of accumulation involves some interaction between hydrous iron oxides and the anions. From the constancy of the ironarsenic and iron:phosphate ratios during dissolution it has been concluded that arsenic and phosphate are incorporated into these sediments by coprecipitation at the time of formation of the hydrous oxides rather than by adsorption on existing surfaces. Introduction During the course of an investigation into the fate of arsenic released from geothermal sources into the Waikato River in the North Island of New Zealand, it became evident that substantial quantities of arsenic were being accumulated in the sediments of hydro lakes downstream from the geothermal areas (1). As a consequence further investigations have since been undertaken to determine the factors that control the initial accumulation process and the stability of the sediments formed. This communication is concerned with the use of interelement correlations to provide insights into the mechanism of accumulation for anions such as arsenate and phosphate. Although interelement correlations do not provide direct evidence for the mechanism of accumulation, they have been used extensively (e.g., see ref 2) as indirect evidence. One of the more common techniques for identifying accumulation processes utilizes the correlation between the concentrations of adsorbate and possible adsorbants obtained by analysis of a relatively large number of samples. While this technique may provide satisfactory answers, observations suggest that in many instances researchers are required to pass judgment on the significance of correlation coefficients that are not clearly discriminating. Furthermore, successful application of this technique requires the collection of a relatively large number of samples that cover a satisfactory range of adsorbate concentrations but that at the same time do not differ significantly in matrix composition or properties of the adsorbent which may affect its adsorption behavior. Faced with the prospect of applying such a technique to sediments in Lake Ohakuri on the Waikato River, it was decided to investigate the possibility of using a comparison of the rates of dissolution of adsorbate and matrix elements as a means of identifying the accumulation process, the principle being that if the presence of some reagent selected to dissolve the adsorbent caused its slow dissolution, then the rate of dissolution of the adsorbate would follow 0013-936X/86/0920-0163$01.50/0
closely that of the adsorbent. The potential advantages of such a method were seen to be the following: (1) The relevant information could be obtained from a very small number of samples, one possibly being sufficient. This would largely eliminate problems arising from matrix and adsorbent property variations and would also simplify the logistics associated with the endeavor. (2) The dissolution procedure could be applied at the natural pH of the sediments, thus reducing the disturbance to other accumulation processes. The sediments in Lake Ohakuri are best described as oozes generally containing between 85 and 95% water. Chemical analyses of the dried sediments indicated that they were composed largely of siliceous material (75-85%) with significant concentrations of iron (4-1270 ), aluminum (2-4%), organics (7-12%), calcium (0.1-0.2%), and manganese (0.05-0.2%). Of these, iron and aluminum species appeared most likely to be involved in the accumulation of arsenate and phosphate. Hence, the method reported here is based on the use of a reagent that dissolves iron oxyhydroxides at rates suitable for the envisaged application. Methods such as Tamm’s oxalate (3) and others which are conventionally used for selective dissolution of iron species react too rapidly to permit a convenient sequence of observations during the dissolution procedure. However, since Borggaard ( 4 ) had shown that ethylenediaminetetraacetic acid (EDTA) dissolved amorphous iron oxides in soils considerably more slowly than did oxalate, it was decided to investigate and compare the rates of dissolution of iron, aluminum, manganese, silicon, calcium, arsenic, and phosphorous in the sediments by EDTA at the natural pH of the sediments. Calculations based on available equilibrium data (5) indicated that in theory EDTA was capable of dissolving simple insoluble species containing iron, aluminum, manganese, and calcium. Prior to its application to the problem of arsenic accumulation, experiments were carried out to determine the rate of dissolution of iron(II1)-hydroxy species in order to gain an insight into the likely time scale of the method and the effect of aging of the precipitate. Since several of the samples used in this study were selected from a larger number of samples obtained during studies on the mobility of arsenic in the sediments, it has also been possible to compare the data obtained from the dissolution rate experiments with those obtained by determination of the total iron and total arsenic concentrations in the sediments and thus to determine the relative merits of the procedure presented in this paper and the conventional method of establishing association between adsorbed species and the adsorbent. Experimental Section Preparation of Hydrous Ferric Oxide. A 2.15-g sample ammonium ferric sulfate was dissolved in 200 mL of deionized water. Sodium hydrogen carbonate (1 mol L-’) was added with stirring until the pH reached 7.0. Stirring was continued for another 5 min, and the product was then allowed to settle. The precipitate was aged in
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 2, 1986
183
the solution in which it had been prepared. Preparation of Hydrous Ferric Oxide Impregnated on Silica. A 21-g sample of ammonium ferric sulfate was dissolved in 100 mL of deionized water. To this was added 50 g of silica gel S for column chromatography. The mixture was left overnight. A total of 20 mL of water was then added to fluidize the mixture which was then transferred to a centrifuge tube and centrifuged to remove excess solution. The silica gel was then rinse-centrifuged twice to remove unadsorbed ferric ions; 50 mL of water was then added followed by ammonia (2 mol L-l) until the pH reached 7.0. The mixture was left for 1h, then poured into a 1-L measuring cylinder, shaken, settled, and decanted to remove unimpregnated iron species. This procedure was repeated until the supernatant was clear. The iron-impregnated silica gel was stored under deionized water. Sediment Samples. Samples were collected in a Jenkins core sampler. They were immediately placed on an extruder and separated into 1-cm sections each of which was transferred to a separate 50-mL stoppered centrifuge tube. Unoccupied space at the top of the tube was flushed with nitrogen. With 4 h the interstitial water was removed by centrifugation, and the solid material was dried at 50 “C for 16-24 h before being stored at room temperature. Sediment Dissolution Procedures. In initial experiments concerned only with the mechanism of adsorption of arsenic, sediment samples (0.5-1.0 g) were added to 250 mL of EDTA (10-1 mol L-I) at pH 7.0 in a Quickfit FV500 reaction vessel which was immersed in a water bath at 25 “C. The system was stirred at 300 rpm and a Radiometer Titrator TTTlC pH meter used as a pH stat to maintain the pH in the range 7.0 f 0.1 units, the natural pH of the sediments. At selected intervals 10-mL aliquots were removed. They were centrifuged briefly to separate the bulk of the sediment from the supernatant. The supernatant was then decanted from the centrifuge tube, filtered through a 0.20-pm membrane filter to remove finer particles, and analyzed. The small amount of sediment in the centrifuge tube was mixed with 10 mL of replacement EDTA reagent and returned to the reaction vessel. Dissolution was continued for 48 h. In later experiments involving analysis of phosphate it mol was necessary to lower the EDTA concentration to L-l in order to avoid problems in analysis. At the same time the temperature was increased to 35 “C to increase the dissolution rate. Dissolution of Hydrous Ferric Oxides. The precipitate and supernatant were agitated vigorously to “homogenize”the system, and 25 mL of the “homogenate” was then rapidly added to 225 mL of EDTA (10-1 mol L-l at pH 7.0). In other respects the experimental procedure was identical with that employed for sediment dissolution experiments. In order to determine the percentage of the total iron dissolved by EDTA and also to gain some idea of the type of iron that had been dissolved, sediment was subjected to the following additional sequence of extractions after extraction with EDTA. The EDTA solution was separated from the sediment by centrifugation and replaced by 250 mL of Tamm’s reagent (24.9 g L-’ ammonium oxalate and 12.6 g L-l oxalic acid, pH 3.25) (3). Dissolution with Tamm’s reagent was carried out at 25 “C for 2 h. A sample of the supernatant was then removed for analysis and the remaining reagent removed by centrifugation and replaced by 50 mL of concentrated hydrochloric acid in which the sediment was boiled gently for 1h. The supernatant from 184
Environ. Sci. Technoi., Voi. 20, No. 2, 1986
Y -60 a
1,
3
.r\r
0 v)
P
d
Figure 1. Percent hydrous iron oxide dissolved by EDTA at pH 7.0 as a function of time. Curve 1, iron oxide aged 1 week. Curve 2, iron oxide aged 4 weeks.
I
Y
P
100
200
300
400
5oo
I
24
l
48
l
HWRS
MINUTES
Figure 2. Percent hydrous iron oxide impregnated on silica dlssolved by EDTA at pH 7.0 as a function of time. Curve 1, iron oxide aged 1 week. Curve 2, iron oxide aged 4 weeks.
Table I. Sequential Dissolution of Hydrous Iron Oxide length of aging
percentage dissolved EDTA” Tamm HC1 Pure Hydrous Oxide
10 min 1 week 4 weeks
1oob
56.0 11.9
0
44.0 37.8
0 0
50.3
Hydrous Oxide Impregnated on Silica Gel 1 week 68.5 31.5 0 4 weeks 43.1 54.5 2.4
” 48-h dissolution.
In 20 min.
this digest was separated from the remaining sediment and diluted to 250 mL for analysis. On some occasions sediment was also digested with hydrochloric acid-nitrc acid (4:l). The objective of this sequence of digestions was to follow the EDTA dissolution with dissolution of the remaining amorphous iron oxyhydroxide (6)and then mineralized iron. Digestion with hydrochloric acid-nitric acid was intended to identify organoarsenic species. Analysis. Iron, aluminum, silicon, manganese, and calcium were determined by atomic absorption spectroscopy with flame atomization. Arsenic was determined by atomic absorption spectroscopy with hydride generation from 1 mol L-l hydrochloric acid (7). Phosphorus was determined by the molybdenum blue method (APHA with minor modifcation).
Results and Discussion The dissolution rates of hydrous ferric oxides were found to be markedly dependent both on aging and the envi-
Table 111. 1ron:Arsenic Ratios' during Dissolution with EDTA
100
E: 3 0 VI
E
iron:arsenic ratio IA1, July 1981 IA1, March 1982
dissolution time, min
eo
30 60 90 120 150 180 240 300 360 480 24b 48b
60
z co v
% 0. 20
loo
200
300
400
MINUTES
500
40
24
HOURS
Figure 3. Rates of dissolution of calcium, manganese, aluminum, Iron, and arsenic from sample IB1, Jan 1982, by EDTA at pH 7.0. ((3) Calcium; (a)manganese; (0)aluminum; (0)iron; (0)arsenlc.
18.5 18.0 18.0 17.2 17.2 18.4 17.0 21.8 21.9 23.4 20.7 20.8
22.6 21.7 20.0 24.8 25.0 26.4 22.0 23.5 24.6 22.4 25.5 20.6
"Ratio of mass of iron dissolved to mass of arsenic dissolved. bTime in hours.
Table 11. Sequential Dissolution of Iron and Arsenic in Sample IB1, Jan 1982 reagent
percentage dissolved iron arsenic
EDTA Tamm HCl
57.9 29.3 12.8
60.6 33.7
5.7
ronment of the precipitate (Figures 1 and 2) (Table I). Whereas pronounced aging effects were observed with the pure precipitate, these were considerably decreased when the precipitate was impregnated on silica. Nevertheless, these experiments indicated that EDTA was capable of dissolving a significant percentage of precipitate in 48 h, and since much of the iron not dissolved by EDTA was subsequently dissolved by Tamm's reagent, it can be inferred that the material dissolved by EDTA is amorphous (6). When the technique was applied to oxic sediment samplee, it was observed that calcium and manganese dissolved quite rapidly, the maximum extent of dissolution being observed within 30 min of the commencement of the procedure; iron dissolved at rates comparable with those observed for the hydrous iron oxides impregnated on silica; aluminum dissolved at a much slower rate and silica scarcely at all. Figure 3 contains the results obtained with a typical sediment sample. Data for silica have been omitted because less than 1%of the total silica in the sample was dissolved by the application of the EDTA procedure. The close associaton between iron and arsenic in this sample is very clearly evident from the data in Figure 3, the data points for the two curves being so close that only alternative points for each curve have been included in the diagram. For further comparison the percentages of the total iron and arsenic in this sample which were dissolved during dissolution with the different reagents are listed in Table 11. Examination of these data leads to the conclusion that the arsenic is primarily associated with hydrous iron oxides and is distributed reasonably evenly through them. Although the nitric acid-hydrochloric acid dissolution procedure was not applied to sample IB1, Jan 1982, results from application of this procedure to other samples lead to the conclusion that there is little organically bound arsenic in these sediments. The constancy of iron:arsenic concentration ratios during the application of the EDTA dissolution procedure to surface sediment samples obtained at other times of the year is demonstrated by data in Table 111. These data indicate that there are no significant seasonal differences
Table IV. 1ron:Phosphorus Ratios" during Dissolution of Sample IB1, Jan 1982 dissolution iron:phosphorus dissolution iron:phosphorus time, min ratio ratio time, min 30 60 90 120 150 180
7.7 8.0 7.4 7.8 7.7 8.3
240 300 360 480 24b 48b
8.2 7.8 7.9 8.3 8.7 11.8
"Ratio of mass of iron dissolved to mass of phosphorus dissolved. Time in hours.
associated with the distribution of arsenic in these sediments. Returning to the discussion of data in Figure 3, although comparison of the dissolution rate curve for arsenic with those for calcium and manganese leads directly to the conclusion that arsenic is not associated with either of these two elements, any conclusion regarding the involvement of aluminum or any other species that dissolves more slowly than arsenic requires additional consideration since it is possible that the arsenic is associated with one particular form of such an adsorbent and that this is the only form which dissolves during the dissolution procedure. For those possible adsorbents that dissolve more slowly than arsenic the necessary additional information can be obtained by considering the ratio of the number of moles of potential adsorbent dissolved relative to that of the adsorbed species. In doing this it needs to be recognized that in the sediments under consideration phosphate is also accumulated and that its behavior is likely to be similar to that of arsenic. Evidence for this latter point is contained in Table IV. For sample IB1, Jan 1982, the combined number of moles of phosphate and arsenic entering solution during the EDTA dissolution procedure was 2.8 times the number of moles of aluminum dissolved, and hence, it appears from stoichiometric considerations that aluminum oxyhydrates are not the major adsorbents for phosphate and arsenic in these sediments. During the course of an investigation into the mobility of arsenic in these sediments (8) a large number of sediment cores were analyzed for total iron and total arsenic as a function of depth in the core. These data have also been used to examine the extent of the correlation between the iron and arsenic concentrations in the sediments and thus to compare the two methods for establishing association. The data for the conventional method (Table V) Environ. Scl. Technol., Vol. 20,
No. 2, 1986 185
Table V. Linear Least-Squares Analysis of the Relationship between the Iron and Arsenic Concentrations of Sediments depth of sample
regression coefficient
correlation coefficient
1 cm (20)“ 2 cm (20) 3 cm (20) 6-10 cm (100)
18.8 20.8 41.7 34.5
0.75 0.85 0.85 0.89
Number of samples.,
10
20
30
,40
,50
PERCENT TOTAL I R O N DISSOLVED
Figure 4. Variation of iron:arsenic ratio during dissolution of sample 1-10, May 1978, by EDTA at pH 7.0.
show that for oxic (surface) samples the correlation is poor but that there is an improvement with increasing depth. This ineffectiveness of the conventional approach to reflect the association between iron and arsenic in the oxic sediment is in all probability due to significant variations in the composition of the sediment samples studied. Despite the fact that the sediment were obtained from the same lake the iron:arsenic ratios in the sediments varied from 17.5 to 122. This problem associated with the large variations frequently encountered in natural systems is largely eliminated in the dissolution rate method. Preliminary studies on the dissolution of anoxic sediment samples from deeper in the sediment cores by EDTA have indicated a further difference between the two methods. From the results to date it appears that the better correlation between iron and arsenic concentrations observed for these samples by the conventional method (Table V) may be superficial. During the EDTA dissolution procedure the iron:arsenic ratios in several of these samples were not constant as they were in the oxic samples, but they fell from values that were initially between 150 and 200 to values between 50 and 100 (Figure 4). Although insufficient data are as yet available on these deeper samples to enable quantitative interpretation, they do appear to indicate the presence of at least two different forms of iron associated with arsenic, and this is probably associated with the reduction of iron(1II) to iron(I1) in the anoxic zone of the sediments. Such information cannot be gained from gross analytical data. From the observation that for most of the surface sediment samples examined the ironarsenic ratio in solution was reasonably constant during the dissolution of the bulk of the arsenic, it can be inferred that arsenic is distributed
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Envlron. Scl. Technol., Vol. 20, No. 2, 1986
reasonably evenly through the iron oxyhydroxide. This suggests that arsenic is incorporated into the sediments by coprecipitation onto the iron oxyhydroxide at the time of its formation rather than by adsorption onto previously formed surfaces.
Summary An association between iron and anionic arsenate and phosphate in Lake Ohakuri sediments has been demonstrated by comparison of the dissolution rates of these species by EDTA at the natural pH of the sediment. On the basis of dissolution rate data alone it has been concluded that solid calcium and manganese species are not involved in the accumulation process. The same conclusion has been reached for aluminum oxides after additional consideration of the relative molar amounts of aluminum, arsenic, and phosphate dissolved. The use of the EDTA dissolution procedure has provided two further insights into the adsorption process, viz., that the major adsorbent appears to be hydrous iron oxyhydroxide and that arsenic is probably coprecipitated at the time of formation of the precipitate. The EDTA dissolution rate method offers a relatively simple method for investigating the mechanism of adsorption of arsenate and phosphate to hydro lake sediments. Although it appears to fulfill the expectations outlined in the introduction, experience to date suggests that the application of the method presented is limited to samples from which it is capable of dissolving possible adsorbent species within a convenient time. Thus, this particular method does not look promising for application to soils where the dissolution of hydrous ferric oxides by EDTA has been shown to be very slow ( 4 ) . Other reagents capable of dissolving adsorbents at faster rates are present under study, and it is hoped to develop a series of reagents in order to apply the rate of dissolution technique to samples of widely varying activity. Acknowledgments We are grateful to Glennys O’Brien for provision of analytical data for application of the conventionalmethod for establishing the extent of association between iron and arsenic and to Stephen de Mora for useful discussions during the course of the project. Registry No. EDTA, 60-00-4; HCl, 7647-01-0;iron, 7439-89-6; calcium, 7440-70-2; manganese, 7439-96-5; aluminum, 7429-90-5; Tamm’s reagent, 37368-16-4; iron oxide, 11115-92-7.
Literature Cited (1) Aggett, J.; Aspell, A. C . N. 2. J. Sei. 1980, 23, 77. (2) Williams, J. D.; Syers, J. K.; Shulka, S. S.; Harris, R. F.; Armstrong, D. E. Environ. Sci. Technol. 1971, 5, 1113. (3) Tamm, 0. Medd. Statens Skogsforskningsinst. (Swed.) 1922,19,3a7. (4) Borggaard, 0. K. J . Soil Sei. 1979, 30, 727. (5) Soec. Pub1.-Chem. SOC.1964, No. 17. 25. (6) P’arfitt, R. L. Adu. Agron. 1978, 30, 1 . ’ (7) Aggett, J.; Aspell, A. C. Analyst (London) 1976,101, 341. (8) Aggett, J.; OBrien, G. A. Environ. Sei. Technol. 1985, 19, 231.
Received for review November, 4, 1983. Revised manuscript received February 8, 1985. Accepted June 3, 1985.
