- Email: [email protected]
Chemical and physical speciation of arsenic in a small pond receiving gold mine waste effluent
Ecotoxicology and Environmental Safety 53 (2002) 370–375
Chemical and physical speciation of arsenic in a small pond receiving gold mine waste effluent Rachel Sproal, Nicholas J. Turoczy, and Frank Stagnitti* School of Ecology and Environment, Deakin University, P.O. Box 423, Warrnambool, Victoria 3280, Australia Received 12 June 2001; accepted 26 June 2002
Abstract The chemical and physical speciation of arsenic in a small pond that receives wastewater from a gold mine operation in western Victoria, Australia was studied using differential pulse polarography. By using different sample pretreatments, distinction between the physical states (dissolved or particulate As), between the oxidation states (As(III) or As(V)), and between the degrees of lability (labile or strongly bound) was achieved. The results are interpreted in terms of the physicochemical properties with reference to the use of the pond as a settlement dam for gold mining effluent. The speciation of arsenic was found to vary markedly with the physicochemical properties of the water. A model for the behavior of arsenic in the pond is proposed. r 2002 Elsevier Science (USA). All rights reserved. Keywords: Arsenic; Speciation; Polarography; Pollution; Fish; Gold; Mining
1. Introduction Much of the world’s total production of arsenic, around 30,000 tons per annum, is intentionally released into the environment as industrial or agricultural waste water (Phillips, 1990). Highly toxic forms of arsenic have also been found in significant concentrations in the natural environment resulting from excessive exploitation of groundwater. For example the latest statistics in Bangladesh indicate that as much as 80% of the country and an estimated 40 million people are at risk of arsenic poisoning and related diseases resulting from drinking arsenic-contaminated groundwater (Alam and Satter, 2000). Significant concentrations of arsenic are also found in many marine environments with appreciable quantities found in seafood (Phillips and Rainbow, 1993). Consequently considerable research into the impacts of arsenic in the marine environment and its effects on human health has been conducted (Phillips and Rainbow, 1993). However, comparatively little research into the impacts of arsenic in freshwater environments has been conducted. This paper addresses *Corresponding author. Fax: +61-35563-3462. E-mail address: [email protected] (F. Stagnitti).
the environmental fate of arsenic found in a pond used as a sedimentation dam in gold mining operations. The toxicity of arsenic to aquatic organisms critically depends on the specific chemical form of the arsenic. The naturally occurring organo-arsenic forms are generally found to be relatively nontoxic to aquatic life (Phillips, 1990). However, the inorganic forms are highly toxic. Arsenic in the form of As(III) (arsenious acid, H3AsO3, and its dissociation products) is considered to be the most toxic form, followed by As(V) (arsenic acid, H3AsO4, and its dissociation products). Whether the arsenic is in the dissolved or the particulate state may also be very important from an ecotoxicological viewpoint, depending on the modes of uptake utilized by a particular organism. Analytical methods capable of differentiating between the different forms of arsenic have been developed. The use of differential pulse polarography to determine arsenic speciation has been proposed by several researchers (Holak, 1976; Myers and Osteryoung, 1973; Sharma, 1995). Such determinations are attractive because polarography has good sensitivity and can distinguish between As(III) and As(V) with comparatively little sample preparation (Princeton Applied Research Corp., 1976). The basis of the technique is
0147-6513/02/$ - see front matter r 2002 Elsevier Science (USA). All rights reserved. PII: S 0 1 4 7 - 6 5 1 3 ( 0 2 ) 0 0 0 5 7 - X
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
2. Materials and methods 2.1. Study site The study site was a sedimentation pond at the Ballarat Goldfields mine in Ballarat, Victoria, Australia. The gold mining operations are in close proximity to residential areas in the city of Ballarat, Australia (see Fig. 1). The lining of the pond consists of clay, which has numerous sites for adsorption of arsenic ions. The water in this pond is pumped from the underground mine shaft to prevent flooding. The water contains high concentrations of arsenic and iron and so requires pretreatment before it can be released into the nearby Yarrowee River. After pumping from the mine shaft, the water is aerated, which converts the Fe2+ present to the less soluble Fe3+, which flocculates as Fe(OH)3 and acts to remove arsenic from the solution. The water then passes through a series of settlement ponds and wetlands. In this study we have investigated the speciation in the largest of the settlement ponds. 2.2. Reagents Deionized water (resistivity X18 mO/cm) was produced by passing singly distilled water through a Milli-Q Water Purification System. Univar-grade magnesium nitrate hexahydrate, Mg(NO3)2 6H2O, and concen-
NSW
N
Victoria SA
that dissolved As(III) is electrochemically active at a dropping mercury electrode, whereas As(V) is electrochemically inactive. By measuring an untreated sample, the concentration of dissolved As(III) can therefore be specifically determined. Using different methods of pretreatment, various other species or fractions of arsenic can be converted to dissolved As(III) and subsequently determined by polarography. Although several researchers have proposed and developed polarography or other electrochemically based methods for investigating the speciation of arsenic, it appears that few environmental studies have actually used these techniques. Most applied environmental studies have used atomic techniques such as inductively coupled plasma atomic emission spectrometry, graphite furnace atomic absorption spectrometry, or hydride generation atomic absorption spectrometry (e.g., see Ahmann et al., 1997; Allinson et al., 2000; Aurillo et al., 1994; Anderson et al., 1996; Kelsall et al., 1999; Li and Smart, 1996; Edmonds et al., 1997). In this paper we describe the polarographic investigation of arsenic speciation in a small pond that receives effluent wastewater from a gold mine operation. The utility of the polarography method in easily distinguishing between the various oxidation states and between the physical forms of arsenic is demonstrated.
371
Ballarat
Melbourne
Warrnambool
100 km
Fig. 1. Site location.
trated nitric acid were obtained from Ajax Chemicals. Sodium sulfite, Na2SO3, and concentrated hydrochloric acid (both AnalaR grade) were obtained from BDH Chemicals as was IRA-401 ion exchange resin. Arsenic oxide (As2O3) was used to prepare a standard 1.00 g/L As(III) stock solution. Working standards of 1.00 mg/L As(III) were prepared from the stock solution on the day of use. 2.3. Field measurements and sample collection Samples were collected from a boat over the deepest part of the pond (10 m). At this point, vertical profiles of temperature, dissolved oxygen, oxidation–reduction potential, pH and turbidity were measured using a Yeo-Kal Model 611 Water Quality Analyzer that was calibrated according to the manufacturer’s instructions. Samples were collected at 1-m intervals using a horizontal Nisken bottle. Water was transferred to high-density polyethylene bottles which had previously been soaked in 1 M HNO3 for at least 24 h and rinsed several times with deionized water. Samples were stored at approximately 41C in a refrigerator until analysis. 2.4. Suspended solids The suspended solids content of the samples was determined by filtering suitable aliquots through predried (1051C) and weighed Millipore GF/C filters. The filters were then redried and reweighed, and the mass of suspended material was calculated. It should be noted that as the effective pore size of the GF/C filters is not the same as the 0.45-mm filters that were used to distinguish between dissolved and particulate arsenic (described later) the results of the suspended solids analysis are not directly comparable with the results of the arsenic analysis. 2.5. Polarography Arsenic concentrations were determined using a PAR Model 364 Polarographic Analyzer in combination with
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
372
a PAR Model 303A static mercury dropping electrode (SMDE) electrode stand. The working electrode was used in the SMDE mode with a medium drop size selected. A Ag/AgCl 3 M KCl electrode was used as the reference electrode and a platinum wire was used as the auxiliary electrode. Samples were purged with highpurity N2 (BOC Gases) for 5 min before measurement. Samples were analyzed using a differential pulse scan (scan rate 5 mV/s, pulse height 50 mV) over the potential range from –0.35 to –0.75 V vs Ag/AgCl. Polarograms were recorded on an HP 7045A XY recorder (Princeton Applied Research Corp., 1976). 2.6. Speciation scheme The polarographic procedure described above specifically detects labile As(III). Other forms of arsenic can be determined by converting them to labile As(III) and remeasuring. Fig. 2 shows the forms of arsenic that were distinguished in this study. The various fractions were determined as follows. 2.6.1. Free arsenic(III) Free arsenic(III) was determined using the methods described by Myers and Osteryoung (1973) and Holak (1976). Essentially, 1.0 mL of concentrated HCl was added to 10.0 mL of filtered sample in a polarographic cell, approximately 0.25 g of IRA-401 resin was added, and the As(III) reduction peak was measured as described earlier. The peak heights were quantified using a calibration curve produced by adding increments of 1 g/L As(III) standard to 10.0 mL of deionized water to which 1.0 mL of concentrated HCl had been added. 2.6.2. Total arsenic Since polarographic methods are extremely dependent on the chemical form of the analyte, thorough digestion methods are required to determine total arsenic. The method described by Higham and Tomkins (1993) was therefore used to digest the samples. Essentially, 10 mL of concentrated nitric acid and 4 g of Mg(NO3)2 6H2O were added to 1.00 mL of sample in a beaker. The
sample was evaporated to dryness over several hours and then heated to 5001C in a muffle furnace for 30 min. The beaker was then placed on a steam bath, and the residue was redissolved using 5.0 mL of concentrated HCl. At this point in the procedure, the arsenic should be present as H3AsO4 (i.e., free As(V)). Then 0.1 g of Na2SO3 was added to convert the As(V) to As(III). After dilution to 50.0 mL, arsenic was then determined polarographically as described above. 2.6.3. Dissolved arsenic Samples were filtered through 0.2-mm cellulose acetate disposable syringe filters (Micro Filtration Systems) and the method described above for total arsenic was applied to the filtrate. 2.6.4. Particulate arsenic Particulate arsenic was determined by mass balance calculation: particulate As ¼ total arsenic dissolved arsenic ¼ ðbÞ ðcÞ: 2.6.5. Free arsenic Free arsenic was determined after converting free As(V) in a filtered sample to As(III) using the procedure described earlier. 2.6.6. Free arsenic(V) Free arsenic(V) was determined by mass balance calculation: free AsðVÞ ¼ free As free AsðIIIÞ ¼ ðeÞ ðaÞ: 2.6.7. Bound dissolved arsenic Bound dissolved arsenic was determined by calculation: bound As ¼ dissolved As ðfree AsðIIIÞ þfreeAsðVÞÞ ¼ ðcÞ ððaÞ þ ðfÞÞ:
3. Results Total Arsenic (b)
Dissolved Arsenic (c)
Bound Arsenic (g)
Particulate Arsenic (d)
Free Arsenic (e)
Free Arsenic (III) (a)
Free Arsenic (V) (g)
Fig. 2. Speciation forms recognized in this study.
Fig. 3 shows depth profiles for temperature, dissolved oxygen, and redox potential at the time of sample collection. The pond was stratified in terms of each of these parameters, with a layer of warmer, well-oxygenated water overlying a layer of cooler, anoxic (and reducing) water. The water was considerably less turbid in the anoxic layer. Suspended solids (Fig. 4) increased with depth as anticipated for a sedimentation pond. Figs. 5–7 show the depth profiles for the concentrations of the major arsenic species.
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
0
Dissolved Oxygen (mg/L)
0
0
4
Temperature (oC) 20 8 12 16
24 2
2
4
Depth (m)
Depth (m)
373
4
6
6
8
8
10 0.0
0.5 1.0 1.5 Suspended Solids (mg/L)
2.0
Fig. 4. Suspended solids with depth.
10 -200 -100 0 100 200 Redox Potential (mV) Fig. 3. Profiles for temperature, dissolved oxygen, and redox potential with depth.
0
4. Discussion
Depth (m)
Not surprisingly, the highest concentrations of total arsenic, around 3.4 mg Total As/kg, were found in the clay lining of the pond. Fish and plants are able to excrete arsenic (Phillips and Rainbow, 1993). The concentrations of arsenic found in these are consequently lower than those found in the clay; around 1.5 mg Total As/kg in fish and 0.5 mg Total As/kg in aquatic plants. The concentrations of arsenic in the discharge waters from the mining operation is below EPA limits; however, the arsenic found in the fish tissues is higher than recommended limits for human consumption (Harte et al., 1991). Comparison of Figs. 5–7 with Figs. 3 and 4 shows that the speciation of arsenic varied markedly with the physicochemical properties of the water. About twothirds of the total arsenic (at any depth) is present in the dissolved phase and about one-third in the particulate phase. However, the total concentration of arsenic is much higher in the oxic layer than in the anoxic layer, suggesting that although suspended particles sink through the redox cline, much of the arsenic does not penetrate this barrier. In the oxic layer virtually none of the dissolved arsenic is labile, whereas in the anoxic layer about one-third is labile. Therefore, although a significant fraction of the arsenic present may be described as being in the dissolved state, it appears that much of this arsenic is probably adsorbed onto colloidal
2
4
6
8
Particulate Dissolved Total
10
0
2
4
6 8 10 [As] (mg/L)
12 14
Fig. 5. Particulate, dissolved, and total arsenic with depth.
iron-oxyhydroxide particles small enough to pass through a 0.45-mm filter. This observation is in contrast with previously reported research that suggests that arsenic in natural waters are generally present in solution (e.g., Phillips and Rainbow, 1993, p. 69). Of the dissolved arsenic that is labile, almost all is As(V) in the oxic layer, and almost all is As(III) in the anoxic layer.
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
374
redox/pH conditions present in the pond, the transformation is likely to be
0
0 þ HAsO2 4 þ 4H þ 2e -H3 AsO3 þ H2 O:
2
The uncharged arsenic species rapidly desorb from the iron-oxyhydroxide particles before fully penetrating the redox cline, and the arsenic therefore largely remains in the oxic layer. This model explains the distribution of arsenic species that exists within the pond and explains why the pond does not remove as much arsenic as expected. To increase the sedimentation efficiency it would be necessary to prevent the development of the redox cline in the water body either by mixing or by oxygenating the lower depths of the water body.
Depth (m)
4
6
8
Labile Non-labile (ie bound) Total dissolved
10
0
2
4
6 8 10 [As] (mg/L)
12
14
Fig. 6. Labile, nonliable, and total arsenic with depth.
0
Depth (m)
2
4 As (III) As (V)
6
8
10 0.0
0.1
0.2 0.3 [As] (mg/L)
0.4
0.5
5. Conclusions The chemical and physical speciation of arsenic in a pond that receives gold mine wastewater was studied using differential pulse polarography. The pond was stratified at the time of sample collection, with a layer of warmer, well-oxygenated water overlying a layer of cooler, anoxic (and reducing) water. The speciation of arsenic was found to vary markedly with the physicochemical properties of the water. Approximately twothirds of the total arsenic (at any depth) was present in the dissolved phase and about one-third in the particulate phase. The total concentration of arsenic in the oxic layer was much higher than that in the anoxic layer with virtually none of the dissolved arsenic in the oxic layer labile and about one-third in the anoxic layer labile. Although a significant fraction of the arsenic was present in the dissolved state, much of this arsenic was adsorbed onto colloidal iron-oxyhydroxide particles. Of the dissolved arsenic that was labile, almost all was in the form of As(V) in the oxic layer and in the anoxic layer almost all was As(III). The efficiency of the sedimentation pond as a mechanism to settle arsenic can be improved either by mixing or by oxygenating of the lower depths of the water body.
Fig. 7. Arsenic(III) and (V) with depth.
References Considering these observations, the following model for the behavior of arsenic in the pond is proposed: arsenic enters the pond associated with the particulate matter, i.e., adsorbed onto iron-oxyhydroxide particles. The particles sink and pass through the redox cline. However, much of the arsenic does not penetrate into the anoxic zone due to the changes in speciation associated with the change in physicochemical conditions resulting from pond stratification. Under the
Ahmann, D., Lee, R.K, Hemond, H.F., Lovely, D.R., Morel, F.M.M., 1997. Microbial mobilization of arsenic from sediments of the Aberjona watershed. Environ. Sci. Technol. 31, 2923–2930. Alam, M.B., Satter, M.A., 2000. Assessment of arsenic contamination in soils and waters in some areas of Bangladesh. Water Sci. Technol. 42, 185–193. Allinson, G., Turoczy, N., Kelsall, Y., Allinson, M., Stagnitti, F., Nishikawa, M., 2000. Mobility of the constituents of chromated copper arsenate in a shallow, sandy soil. N. Z. J. Agric. Res. 43, 149–156.
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375 Anderson, S., Rasmussen, G., Snilsberg, P., Amundsend, C.E., Westby, T., 1996. Assessing the toxicity and mobilization of impregnation salts at a contaminated site. Fresenius J. Anal. Chem. 354, 676–680. Aurillo, A.C., Mason, R.P., Hemond, H.F., 1994. Speciation and fate of arsenic in three lakes of the Abjerona watershed. Environ. Sci. Technol. 28, 577–585. Edmonds, J.S., Shibata, Y., Francesconi, K.A., Rippingale, R.J., Morita, M., 1997. Arsenic transformations in short marine food chains studied by HPLC-ICP MS. Appl. Organometal. Chem. 11, 281–287. Harte, J., Holden, C., Schneider, R., Shirley, C., 1991. Toxics A to Za Guide to Everyday Pollution Hazards.. University of California Press, Berkeley. Higham, A.M., Tomkins, R.P.T., 1993. Determination of trace quantities of selenium and arsenic in canned tuna fish by using electroanalytical techniques. Food Chem. 48, 85–93. Holak, W., 1976. Determination of arsenic and selenium in foods by electroanalytical techniques. J. Assoc. Off. Anal. Chem. 59, 650–654.
375
Kelsall, Y., Allinson, M., Allinson, G., Turoczy, N., Stagnitti, F., Nishikawa, M., Morita, M., 1999. Leaching of copper, chromium and arsenic in a soil of South West Victoria, Australia. Toxicol. Environ. Chem. 70, 375–384. Li, H., Smart, R.B., 1996. Determination of sub-nanomolar concentration of arsenic(III) in natural waters by square wave cathodic stripping voltammetry. Anal. Chem. 325, 25–32. Myers, D.J., Osteryoung, J., 1973. Determination of arsenic(III) at the parts-per-billion level by differential pulse polarography. Anal. Chem. 45, 267–271. Phillips, D.J.H., 1990. Arsenic in aquatic organisms: a review, emphasizing chemical speciation. Aquat. Toxicol. 16, 151–186. Phillips, D.J.H., Rainbow, P.S., 1993. Biomonitoring of Trace Aquatic Contaminants. Chapman & Hall, London. Princeton Applied Research Corp., 1976. Application Brief A-6. Determination of arsenic by differential pulse polarography, AB A-6-10M-12/76, Princeton, NJ, 4pp. Sharma, P., 1995. Sequential trace determination of arsenic(III) and arsenic(V) by differential pulse polarography. Anal. Sci. 11, 261–266.
Chemical and physical speciation of arsenic in a small pond receiving gold mine waste effluent Rachel Sproal, Nicholas J. Turoczy, and Frank Stagnitti* School of Ecology and Environment, Deakin University, P.O. Box 423, Warrnambool, Victoria 3280, Australia Received 12 June 2001; accepted 26 June 2002
Abstract The chemical and physical speciation of arsenic in a small pond that receives wastewater from a gold mine operation in western Victoria, Australia was studied using differential pulse polarography. By using different sample pretreatments, distinction between the physical states (dissolved or particulate As), between the oxidation states (As(III) or As(V)), and between the degrees of lability (labile or strongly bound) was achieved. The results are interpreted in terms of the physicochemical properties with reference to the use of the pond as a settlement dam for gold mining effluent. The speciation of arsenic was found to vary markedly with the physicochemical properties of the water. A model for the behavior of arsenic in the pond is proposed. r 2002 Elsevier Science (USA). All rights reserved. Keywords: Arsenic; Speciation; Polarography; Pollution; Fish; Gold; Mining
1. Introduction Much of the world’s total production of arsenic, around 30,000 tons per annum, is intentionally released into the environment as industrial or agricultural waste water (Phillips, 1990). Highly toxic forms of arsenic have also been found in significant concentrations in the natural environment resulting from excessive exploitation of groundwater. For example the latest statistics in Bangladesh indicate that as much as 80% of the country and an estimated 40 million people are at risk of arsenic poisoning and related diseases resulting from drinking arsenic-contaminated groundwater (Alam and Satter, 2000). Significant concentrations of arsenic are also found in many marine environments with appreciable quantities found in seafood (Phillips and Rainbow, 1993). Consequently considerable research into the impacts of arsenic in the marine environment and its effects on human health has been conducted (Phillips and Rainbow, 1993). However, comparatively little research into the impacts of arsenic in freshwater environments has been conducted. This paper addresses *Corresponding author. Fax: +61-35563-3462. E-mail address: [email protected] (F. Stagnitti).
the environmental fate of arsenic found in a pond used as a sedimentation dam in gold mining operations. The toxicity of arsenic to aquatic organisms critically depends on the specific chemical form of the arsenic. The naturally occurring organo-arsenic forms are generally found to be relatively nontoxic to aquatic life (Phillips, 1990). However, the inorganic forms are highly toxic. Arsenic in the form of As(III) (arsenious acid, H3AsO3, and its dissociation products) is considered to be the most toxic form, followed by As(V) (arsenic acid, H3AsO4, and its dissociation products). Whether the arsenic is in the dissolved or the particulate state may also be very important from an ecotoxicological viewpoint, depending on the modes of uptake utilized by a particular organism. Analytical methods capable of differentiating between the different forms of arsenic have been developed. The use of differential pulse polarography to determine arsenic speciation has been proposed by several researchers (Holak, 1976; Myers and Osteryoung, 1973; Sharma, 1995). Such determinations are attractive because polarography has good sensitivity and can distinguish between As(III) and As(V) with comparatively little sample preparation (Princeton Applied Research Corp., 1976). The basis of the technique is
0147-6513/02/$ - see front matter r 2002 Elsevier Science (USA). All rights reserved. PII: S 0 1 4 7 - 6 5 1 3 ( 0 2 ) 0 0 0 5 7 - X
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
2. Materials and methods 2.1. Study site The study site was a sedimentation pond at the Ballarat Goldfields mine in Ballarat, Victoria, Australia. The gold mining operations are in close proximity to residential areas in the city of Ballarat, Australia (see Fig. 1). The lining of the pond consists of clay, which has numerous sites for adsorption of arsenic ions. The water in this pond is pumped from the underground mine shaft to prevent flooding. The water contains high concentrations of arsenic and iron and so requires pretreatment before it can be released into the nearby Yarrowee River. After pumping from the mine shaft, the water is aerated, which converts the Fe2+ present to the less soluble Fe3+, which flocculates as Fe(OH)3 and acts to remove arsenic from the solution. The water then passes through a series of settlement ponds and wetlands. In this study we have investigated the speciation in the largest of the settlement ponds. 2.2. Reagents Deionized water (resistivity X18 mO/cm) was produced by passing singly distilled water through a Milli-Q Water Purification System. Univar-grade magnesium nitrate hexahydrate, Mg(NO3)2 6H2O, and concen-
NSW
N
Victoria SA
that dissolved As(III) is electrochemically active at a dropping mercury electrode, whereas As(V) is electrochemically inactive. By measuring an untreated sample, the concentration of dissolved As(III) can therefore be specifically determined. Using different methods of pretreatment, various other species or fractions of arsenic can be converted to dissolved As(III) and subsequently determined by polarography. Although several researchers have proposed and developed polarography or other electrochemically based methods for investigating the speciation of arsenic, it appears that few environmental studies have actually used these techniques. Most applied environmental studies have used atomic techniques such as inductively coupled plasma atomic emission spectrometry, graphite furnace atomic absorption spectrometry, or hydride generation atomic absorption spectrometry (e.g., see Ahmann et al., 1997; Allinson et al., 2000; Aurillo et al., 1994; Anderson et al., 1996; Kelsall et al., 1999; Li and Smart, 1996; Edmonds et al., 1997). In this paper we describe the polarographic investigation of arsenic speciation in a small pond that receives effluent wastewater from a gold mine operation. The utility of the polarography method in easily distinguishing between the various oxidation states and between the physical forms of arsenic is demonstrated.
371
Ballarat
Melbourne
Warrnambool
100 km
Fig. 1. Site location.
trated nitric acid were obtained from Ajax Chemicals. Sodium sulfite, Na2SO3, and concentrated hydrochloric acid (both AnalaR grade) were obtained from BDH Chemicals as was IRA-401 ion exchange resin. Arsenic oxide (As2O3) was used to prepare a standard 1.00 g/L As(III) stock solution. Working standards of 1.00 mg/L As(III) were prepared from the stock solution on the day of use. 2.3. Field measurements and sample collection Samples were collected from a boat over the deepest part of the pond (10 m). At this point, vertical profiles of temperature, dissolved oxygen, oxidation–reduction potential, pH and turbidity were measured using a Yeo-Kal Model 611 Water Quality Analyzer that was calibrated according to the manufacturer’s instructions. Samples were collected at 1-m intervals using a horizontal Nisken bottle. Water was transferred to high-density polyethylene bottles which had previously been soaked in 1 M HNO3 for at least 24 h and rinsed several times with deionized water. Samples were stored at approximately 41C in a refrigerator until analysis. 2.4. Suspended solids The suspended solids content of the samples was determined by filtering suitable aliquots through predried (1051C) and weighed Millipore GF/C filters. The filters were then redried and reweighed, and the mass of suspended material was calculated. It should be noted that as the effective pore size of the GF/C filters is not the same as the 0.45-mm filters that were used to distinguish between dissolved and particulate arsenic (described later) the results of the suspended solids analysis are not directly comparable with the results of the arsenic analysis. 2.5. Polarography Arsenic concentrations were determined using a PAR Model 364 Polarographic Analyzer in combination with
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
372
a PAR Model 303A static mercury dropping electrode (SMDE) electrode stand. The working electrode was used in the SMDE mode with a medium drop size selected. A Ag/AgCl 3 M KCl electrode was used as the reference electrode and a platinum wire was used as the auxiliary electrode. Samples were purged with highpurity N2 (BOC Gases) for 5 min before measurement. Samples were analyzed using a differential pulse scan (scan rate 5 mV/s, pulse height 50 mV) over the potential range from –0.35 to –0.75 V vs Ag/AgCl. Polarograms were recorded on an HP 7045A XY recorder (Princeton Applied Research Corp., 1976). 2.6. Speciation scheme The polarographic procedure described above specifically detects labile As(III). Other forms of arsenic can be determined by converting them to labile As(III) and remeasuring. Fig. 2 shows the forms of arsenic that were distinguished in this study. The various fractions were determined as follows. 2.6.1. Free arsenic(III) Free arsenic(III) was determined using the methods described by Myers and Osteryoung (1973) and Holak (1976). Essentially, 1.0 mL of concentrated HCl was added to 10.0 mL of filtered sample in a polarographic cell, approximately 0.25 g of IRA-401 resin was added, and the As(III) reduction peak was measured as described earlier. The peak heights were quantified using a calibration curve produced by adding increments of 1 g/L As(III) standard to 10.0 mL of deionized water to which 1.0 mL of concentrated HCl had been added. 2.6.2. Total arsenic Since polarographic methods are extremely dependent on the chemical form of the analyte, thorough digestion methods are required to determine total arsenic. The method described by Higham and Tomkins (1993) was therefore used to digest the samples. Essentially, 10 mL of concentrated nitric acid and 4 g of Mg(NO3)2 6H2O were added to 1.00 mL of sample in a beaker. The
sample was evaporated to dryness over several hours and then heated to 5001C in a muffle furnace for 30 min. The beaker was then placed on a steam bath, and the residue was redissolved using 5.0 mL of concentrated HCl. At this point in the procedure, the arsenic should be present as H3AsO4 (i.e., free As(V)). Then 0.1 g of Na2SO3 was added to convert the As(V) to As(III). After dilution to 50.0 mL, arsenic was then determined polarographically as described above. 2.6.3. Dissolved arsenic Samples were filtered through 0.2-mm cellulose acetate disposable syringe filters (Micro Filtration Systems) and the method described above for total arsenic was applied to the filtrate. 2.6.4. Particulate arsenic Particulate arsenic was determined by mass balance calculation: particulate As ¼ total arsenic dissolved arsenic ¼ ðbÞ ðcÞ: 2.6.5. Free arsenic Free arsenic was determined after converting free As(V) in a filtered sample to As(III) using the procedure described earlier. 2.6.6. Free arsenic(V) Free arsenic(V) was determined by mass balance calculation: free AsðVÞ ¼ free As free AsðIIIÞ ¼ ðeÞ ðaÞ: 2.6.7. Bound dissolved arsenic Bound dissolved arsenic was determined by calculation: bound As ¼ dissolved As ðfree AsðIIIÞ þfreeAsðVÞÞ ¼ ðcÞ ððaÞ þ ðfÞÞ:
3. Results Total Arsenic (b)
Dissolved Arsenic (c)
Bound Arsenic (g)
Particulate Arsenic (d)
Free Arsenic (e)
Free Arsenic (III) (a)
Free Arsenic (V) (g)
Fig. 2. Speciation forms recognized in this study.
Fig. 3 shows depth profiles for temperature, dissolved oxygen, and redox potential at the time of sample collection. The pond was stratified in terms of each of these parameters, with a layer of warmer, well-oxygenated water overlying a layer of cooler, anoxic (and reducing) water. The water was considerably less turbid in the anoxic layer. Suspended solids (Fig. 4) increased with depth as anticipated for a sedimentation pond. Figs. 5–7 show the depth profiles for the concentrations of the major arsenic species.
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
0
Dissolved Oxygen (mg/L)
0
0
4
Temperature (oC) 20 8 12 16
24 2
2
4
Depth (m)
Depth (m)
373
4
6
6
8
8
10 0.0
0.5 1.0 1.5 Suspended Solids (mg/L)
2.0
Fig. 4. Suspended solids with depth.
10 -200 -100 0 100 200 Redox Potential (mV) Fig. 3. Profiles for temperature, dissolved oxygen, and redox potential with depth.
0
4. Discussion
Depth (m)
Not surprisingly, the highest concentrations of total arsenic, around 3.4 mg Total As/kg, were found in the clay lining of the pond. Fish and plants are able to excrete arsenic (Phillips and Rainbow, 1993). The concentrations of arsenic found in these are consequently lower than those found in the clay; around 1.5 mg Total As/kg in fish and 0.5 mg Total As/kg in aquatic plants. The concentrations of arsenic in the discharge waters from the mining operation is below EPA limits; however, the arsenic found in the fish tissues is higher than recommended limits for human consumption (Harte et al., 1991). Comparison of Figs. 5–7 with Figs. 3 and 4 shows that the speciation of arsenic varied markedly with the physicochemical properties of the water. About twothirds of the total arsenic (at any depth) is present in the dissolved phase and about one-third in the particulate phase. However, the total concentration of arsenic is much higher in the oxic layer than in the anoxic layer, suggesting that although suspended particles sink through the redox cline, much of the arsenic does not penetrate this barrier. In the oxic layer virtually none of the dissolved arsenic is labile, whereas in the anoxic layer about one-third is labile. Therefore, although a significant fraction of the arsenic present may be described as being in the dissolved state, it appears that much of this arsenic is probably adsorbed onto colloidal
2
4
6
8
Particulate Dissolved Total
10
0
2
4
6 8 10 [As] (mg/L)
12 14
Fig. 5. Particulate, dissolved, and total arsenic with depth.
iron-oxyhydroxide particles small enough to pass through a 0.45-mm filter. This observation is in contrast with previously reported research that suggests that arsenic in natural waters are generally present in solution (e.g., Phillips and Rainbow, 1993, p. 69). Of the dissolved arsenic that is labile, almost all is As(V) in the oxic layer, and almost all is As(III) in the anoxic layer.
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375
374
redox/pH conditions present in the pond, the transformation is likely to be
0
0 þ HAsO2 4 þ 4H þ 2e -H3 AsO3 þ H2 O:
2
The uncharged arsenic species rapidly desorb from the iron-oxyhydroxide particles before fully penetrating the redox cline, and the arsenic therefore largely remains in the oxic layer. This model explains the distribution of arsenic species that exists within the pond and explains why the pond does not remove as much arsenic as expected. To increase the sedimentation efficiency it would be necessary to prevent the development of the redox cline in the water body either by mixing or by oxygenating the lower depths of the water body.
Depth (m)
4
6
8
Labile Non-labile (ie bound) Total dissolved
10
0
2
4
6 8 10 [As] (mg/L)
12
14
Fig. 6. Labile, nonliable, and total arsenic with depth.
0
Depth (m)
2
4 As (III) As (V)
6
8
10 0.0
0.1
0.2 0.3 [As] (mg/L)
0.4
0.5
5. Conclusions The chemical and physical speciation of arsenic in a pond that receives gold mine wastewater was studied using differential pulse polarography. The pond was stratified at the time of sample collection, with a layer of warmer, well-oxygenated water overlying a layer of cooler, anoxic (and reducing) water. The speciation of arsenic was found to vary markedly with the physicochemical properties of the water. Approximately twothirds of the total arsenic (at any depth) was present in the dissolved phase and about one-third in the particulate phase. The total concentration of arsenic in the oxic layer was much higher than that in the anoxic layer with virtually none of the dissolved arsenic in the oxic layer labile and about one-third in the anoxic layer labile. Although a significant fraction of the arsenic was present in the dissolved state, much of this arsenic was adsorbed onto colloidal iron-oxyhydroxide particles. Of the dissolved arsenic that was labile, almost all was in the form of As(V) in the oxic layer and in the anoxic layer almost all was As(III). The efficiency of the sedimentation pond as a mechanism to settle arsenic can be improved either by mixing or by oxygenating of the lower depths of the water body.
Fig. 7. Arsenic(III) and (V) with depth.
References Considering these observations, the following model for the behavior of arsenic in the pond is proposed: arsenic enters the pond associated with the particulate matter, i.e., adsorbed onto iron-oxyhydroxide particles. The particles sink and pass through the redox cline. However, much of the arsenic does not penetrate into the anoxic zone due to the changes in speciation associated with the change in physicochemical conditions resulting from pond stratification. Under the
Ahmann, D., Lee, R.K, Hemond, H.F., Lovely, D.R., Morel, F.M.M., 1997. Microbial mobilization of arsenic from sediments of the Aberjona watershed. Environ. Sci. Technol. 31, 2923–2930. Alam, M.B., Satter, M.A., 2000. Assessment of arsenic contamination in soils and waters in some areas of Bangladesh. Water Sci. Technol. 42, 185–193. Allinson, G., Turoczy, N., Kelsall, Y., Allinson, M., Stagnitti, F., Nishikawa, M., 2000. Mobility of the constituents of chromated copper arsenate in a shallow, sandy soil. N. Z. J. Agric. Res. 43, 149–156.
R. Sproal et al. / Ecotoxicology and Environmental Safety 53 (2002) 370–375 Anderson, S., Rasmussen, G., Snilsberg, P., Amundsend, C.E., Westby, T., 1996. Assessing the toxicity and mobilization of impregnation salts at a contaminated site. Fresenius J. Anal. Chem. 354, 676–680. Aurillo, A.C., Mason, R.P., Hemond, H.F., 1994. Speciation and fate of arsenic in three lakes of the Abjerona watershed. Environ. Sci. Technol. 28, 577–585. Edmonds, J.S., Shibata, Y., Francesconi, K.A., Rippingale, R.J., Morita, M., 1997. Arsenic transformations in short marine food chains studied by HPLC-ICP MS. Appl. Organometal. Chem. 11, 281–287. Harte, J., Holden, C., Schneider, R., Shirley, C., 1991. Toxics A to Za Guide to Everyday Pollution Hazards.. University of California Press, Berkeley. Higham, A.M., Tomkins, R.P.T., 1993. Determination of trace quantities of selenium and arsenic in canned tuna fish by using electroanalytical techniques. Food Chem. 48, 85–93. Holak, W., 1976. Determination of arsenic and selenium in foods by electroanalytical techniques. J. Assoc. Off. Anal. Chem. 59, 650–654.
375
Kelsall, Y., Allinson, M., Allinson, G., Turoczy, N., Stagnitti, F., Nishikawa, M., Morita, M., 1999. Leaching of copper, chromium and arsenic in a soil of South West Victoria, Australia. Toxicol. Environ. Chem. 70, 375–384. Li, H., Smart, R.B., 1996. Determination of sub-nanomolar concentration of arsenic(III) in natural waters by square wave cathodic stripping voltammetry. Anal. Chem. 325, 25–32. Myers, D.J., Osteryoung, J., 1973. Determination of arsenic(III) at the parts-per-billion level by differential pulse polarography. Anal. Chem. 45, 267–271. Phillips, D.J.H., 1990. Arsenic in aquatic organisms: a review, emphasizing chemical speciation. Aquat. Toxicol. 16, 151–186. Phillips, D.J.H., Rainbow, P.S., 1993. Biomonitoring of Trace Aquatic Contaminants. Chapman & Hall, London. Princeton Applied Research Corp., 1976. Application Brief A-6. Determination of arsenic by differential pulse polarography, AB A-6-10M-12/76, Princeton, NJ, 4pp. Sharma, P., 1995. Sequential trace determination of arsenic(III) and arsenic(V) by differential pulse polarography. Anal. Sci. 11, 261–266.