THE ROLE OF CARBONATE ION IN DISSOLUTION OF ARSENIC FROM AQUIFER MATERIAL

Myoung-Jin Kim*, Jerome Nriagu*^a and Sheridan Haack**

* Department of Environmental Health Science, School of Public Health, University of Michigan, Ann Arbor, MI 48109

** Water Resources Division, U.S. Geological Survey, Lansing, MI 48911

a, jnriagu@umich.edu

ABSTRACT

The purpose of this study is to investigate the factors controlling arsenic leaching from the aquifer material into groundwater. Rock samples from a new well in southeast Michigan were reacted with NaHCO₃ under various conditions. It was shown that the arsenic leaching increased with NaHCO₃ concentration. Arsenic leaching rate was dependent on oxic/anoxic conditions. The results suggested that carbonation of arsenic minerals is an important arsenic dissolution process in groundwater under anaerobic conditions. Formation of stable arseno-carbonate complexes such as As(CO₃)₂^-, As(CO₃)(OH)₂^-, and As(CO₃)⁺ is proposed.

INTRODUCTION

Elevated levels of arsenic in groundwater have been usually associated with arsenic-containing minerals such as arsenopyrite, realgar, orpiment, arsenic-rich iron oxyhydroxide, or arsenic within the pyrite crystal lattice (Nickson et al., 2000; Smedley et al., 1996; Thornton, 1996; Williams et al., 1996; Welch et al., 1988). Little information is available on the factors controlling arsenic dissolution, the mechanism of arsenic leaching, and the relation between arsenic species in aquifer material and those in groundwater.

It has been reported that bicarbonate ion plays an important role in dissolution of minerals such as pyrite. The potential role of HCO₃^- in abiotic pyrite oxidation has been reported (Evangelou et al., 1998), and there is molecular evidence for pyrite surface–Fe(II)HCO₃ complexes during the reaction between pyrite and bicarbonate. It is suggested that the complexes on the surface of pyrite increased its oxidation rate by accelerating the abiotic oxidation of Fe²⁺ (Evangelou et al., 1998). Pantsar-Kallio and Manninen (1997) have investigated how bicarbonate ion affects arsenic dissolution in contaminated soils, and found that the amounts of arsenic species extracted increased with NaHCO₃ concentration.

In this study, the role of carbonate ion in arsenic dissolution from aquifer material into groundwater was investigated. The study of arsenic leaching in groundwater was conducted through batch experiments using rock samples from a new drilled well.

METHODS

A new well was drilled in Bad Axe, Huron County, Michigan in October 1997 by the U.S. Geological Survey (USGS) and core samples were taken at different depths between 50 ft and 350 ft. The range of total arsenic concentrations was from 0.8 mg/kg to 70.7 mg/kg at different depth (Kim, 1999).

In order to determine which major ions most effectively cause arsenic leaching, tests were conducted using water (deionized water and groundwater) and 0.1M solutions of KCl, Na₂SO₄, MgSO₄, CaSO₄, NaHCO₃, KHCO₃, and FeCl₃. One gram of core sample and 20 mL of each solution were placed in three polyethylene bottles, and the bottles were shaken using a platform shaker. One bottle of each solution was removed from the shaker after set intervals (4 hours, 1 day, and 3 days). The sample mixtures were filtered through a 0.45 mm membrane. The arsenic concentrations in the filtrates were determined by a graphite furnace atomic absorption spectrophotometer (GFAAS).

Preliminary tests above showed that NaHCO₃ leached arsenic from core samples most efficiently. Subsequently, varying concentrations of NaHCO₃, ranging from 0.02 to 0.6M were used to test the rate of arsenic leaching. The tests were performed for 3 days using the same experimental method described above.

To investigate how the rate of arsenic leaching changes under oxic or anoxic condition, three kinds of solution were prepared as follows. (1) 0.04M NaHCO₃ made in deionized water under air, (2) 0.04M NaHCO₃ made in deionized water under nitrogen, and (3) 0.04M NaHCO₃ made in groundwater under nitrogen. Oxic conditions were maintained by preparing the solution in air-saturated deionized water and under standard atmospheric conditions. 2.4g of rock sample and 20 mL of air-saturated solution (1) were placed in seven polyethylene bottles. To provide anoxic conditions for experiments with solutions (2) and (3), the solution preparation and experimental setup were conducted using a vacuum pump and a glove bag under nitrogen. In order to set up an experiment with solution (2), 2.4g of rock sample from each Schlenk tube and 20 mL of deaerated 0.04M NaHCO₃ solution were placed in seven polyethylene bottles, each bottle was tightly sealed. For experiments with solution (3), the same procedure was used except that untreated groundwater instead of deionized water was used. A total of 21 bottles was shaken using a platform shaker, and one bottle of each solution was removed from the shaker after each of the time intervals (from 4 hours to 4 days). The mixtures were filtered through a 0.45 mm membrane filter and the arsenic concentrations in the filtrates were determined by GFAAS.

RESULTS AND DISCUSSION

We found that NaHCO₃, KHCO₃, and FeCl₃ solutions extracted arsenic from the core samples most efficiently; 5.9% of total arsenic content was leached with 0.1M NaHCO₃, 4.6% with 0.1M KHCO₃, and 1.9% with 0.1M FeCl₃. These results indicate that bicarbonate ion and ferric ion play important roles in arsenic leaching in groundwater.

The arsenic leaching rate was noticeably dependent on NaHCO₃ concentration and increased with reaction time for each concentration. After three-day reaction time, 1.5-14.8 % of total arsenic was leached: 1.5% in a 0.02M solution and 14.8 % in a 0.6M solution. Nickson et al. (2000) reported that arsenic concentration was significantly correlated with bicarbonate ion concentration in anoxic groundwater. In order to confirm this finding, data of arsenic and bicarbonate ion in several previously published papers were plotted (Figure 1). Significant relations between arsenic and bicarbonate ion concentration in groundwater were found in some studies (Nimick, 1998; Smedley et al., 1996), but not in others (Williams et al., 1996). The reason for the inconsistency may be due to the difference in arsenic content of the source material. This cannot be checked because pertinent data were not reported.

Figure 1. Relation between bicarbonate ion and arsenic in groundwater from previously

published papers (a)Nimick, 1998; (b)Smedley et al., 1996; (c) Williams et al., 1996

Figure 2 shows arsenic concentrations over time under the three different conditions. The highest arsenic leaching rate was obtained from the air-saturated solution (1), next was from anoxic solution (2), and the lowest rate was from anoxic groundwater (3). The result indicates that the presence of bicarbonate ion can increase the leaching of arsenic from aquifer material under both aerobic and anaerobic conditions.

Figure 2. Arsenic leaching rate over time under oxic/anoxic conditions, air: 0.04M NaHCO₃

made in deionized water under air, N₂: 0.04M NaHCO₃ made in deionized water under nitrogen, and GW: 0.04M NaHCO₃ made in groundwater under nitrogen

Based on the entire experimental results, it is suggested that bicarbonate ion enhances the rate of arsenic dissolution from aquifer material into groundwater by producing arseno-carbonate complexes such as As(CO₃)₂^-, As(CO₃)(OH)₂^-, and As(CO₃)⁺. Further studies using an ion chromatography, C-NMR, and Raman spectroscopy to identify the exact forms of arseno-carbonate complexes present are underway.

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