Environ. Sci. Technol. 2000, 34, 3094-3100
Carbonate Ions and Arsenic Dissolution by Groundwater MYOUNG-JIN KIM,† J E R O M E N R I A G U , * ,† A N D SHERIDAN HAACK‡ Department of Environmental Health Science, School of Public Health, University of Michigan, Ann Arbor, Michigan 48109, and Water Resources Division, U.S. Geological Survey, Lansing, Michigan 48911
Samples of Marshall Sandstone, a major source of groundwater with elevated arsenic levels in southeast Michigan, were exposed to bicarbonate ion under controlled chemical conditions. In particular, effects of pH and redox conditions on arsenic release were evaluated. The release of arsenic from the aquifer rock was strongly related to the bicarbonate concentration in the leaching solution. The results obtained suggest that the carbonation of arsenic sulfide minerals, including orpiment (As2S3) and realgar (As2S2), is an important process in leaching arsenic into groundwater under anaerobic conditions. The arsenocarbonate complexes formed, believed to be As(CO3)2-, As(CO3)(OH)2-, and AsCO3+, are stable in groundwater. The reaction of ferrous ion with the thioarsenite from carbonation process can result in the formation of arsenopyrite which is a common mineral in arsenic-rich aquifers.
Introduction The chemistry of arsenic in groundwater remains murky despite the large number of publications on the topic (1-8). In many parts of the world, elevated levels of arsenic are found in groundwaters that are mostly reducing, as evidenced by low redox potentials and dissolved oxygen concentrations (typically < 0.5 mg L-1), high concentrations of dissolved iron and manganese and a preponderance of As(III) in the soluble phase. The groundwater environments are often characterized by high alkalinity. The most common arsenicbearing minerals in the host rocks, believed to be the source of the arsenic in groundwater, include arsenic-rich (arsenian) pyrite and various arsenic sulfides and sulfosalts (2, 6, 7, 9-11). Although arsenic can be released from these thiomineral phases by an oxidation process (12-15), there is no known abiotic mechanism for leaching the arsenic from these minerals in the host rocks under reducing conditions. The purpose of this investigation is to ascertain the likely factors and mechanisms involved in leaching arsenic from the fresh core samples of Marshall Sandstone (MSS), the principal bedrock aquifer in southeastern Michigan. Groundwater in MSS is reducing, has elevated levels of arsenic, and shares most of the characteristics noted above (16). Similar geochemical conditions prevail wherever elevated levels of naturally occurring arsenic are found in groundwater of midwestern United States (1, 17, 18). The origin of the * Corresponding author phone: (734)936-0706; fax: (734)764-9424; e-mail:
[email protected]. † University of Michigan. ‡ Water Resources Division, U.S. Geological Survey. 3094
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
anomalous arsenic in groundwater in this region of the country has been a matter for much conjecture and speculation. In this report, the role of bicarbonate in leaching arsenic into groundwater was investigated by conducting batch experiments using core samples of MSS and NaHCO3 solutions. The effects of pH and redox conditions on arsenic dissolution were evaluated. Nickson et al. (7, 8) have recently proposed that arsenic in groundwater derives from reductive dissolution of As-rich iron oxyhydroxides that exist as dispersed phase in the aquifer rock. The sorbed arsenic is released and reduced during the dissolution of the oxyhydroxides. The relationship between dissolved As and HCO3was assumed to be indirect and attributed to the catenated effect of microbial reduction of organic matter (CH2O):
4FeOOH + CH2O + 7H2CO3 f
4Fe2+ + 8HCO3- + 6H2O (1)
The present study suggests that the leaching of As into groundwater is driven by direct interaction between HCO3and As minerals in the aquifer rocks.
Materials and Methods Each apparatus and bottle utilized in the experiment was washed using nitric acid. All reagents were of analytical grade, and all solutions were prepared with deionized water. Arsenic and iron in solution were determined by a graphite furnace atomic absorption spectrophotometer (GFAAS, Perkin-Elmer 4100 ZL), and an electrodeless discharge lamp was used for arsenic analysis. During the GFAAS analysis, a matrix modifier of 5 µg of Pd and 3 µg of Mg(NO3)2 was used for each 20 µL of sample. The detection limit of arsenic analysis was 3 µg/L. A flame atomic absorption spectrophotometer (Varian Techtron Model 1200) was used to determine total iron concentration in acid-digested solutions of rock samples. Ion chromatography (IC) used was made by Alltech and connected to a conductivity detector. The redox potential (Eh) was determined using a platinum electrode (Ag-AgCl) and reference hydrogen ion electrode (Orion Co.). The pH of each solution was measured with an Orion Model 250A pH meter. A dissolved oxygen (DO) meter (YSI 5000) was used to measure DO in solution. For the speciation of As(III) and As(V), ion exchange methods of Ficklin (19) and Grabinski (20) were adapted and modified (21). Water samples were acidified using HCl to pH 4-7 and allowed to pass through a column packed with strong anion-exchange resin (AG1-X8, 100 to 200 mesh chloride form), followed by 0.1 M HCl as eluent. It has been shown that adjustment of pH to 4-7 does not affect the As(III)/ As(V) ratio in samples (16). The As(III) species from the sample was in the pass-through solution, and the As(V) species was adsorbed into the resin and later eluted with 0.1 M HCl. A new well was drilled in Bad Axe, Huron County, MI in October 1997 by the U.S. Geological Survey (USGS), and core samples were taken at different depths between 50 and 350 ft. This interval is in the Marshall Sandstone, the primary aquifer for potable water in southeast Michigan (22). The location was chosen because it was close to existing wells with high dissolved arsenic concentration (total As ) 167278 µg/L, As(III) ) 73-92%). Immediately after the core samples (4′′ diameter) were collected, they were placed in sealed plastic bags filled with nitrogen. Subsequently, the MSS samples were crushed and placed in a freeze-drier (Virtis Freezemobile model 12 SL) to dry for 3-4 days. The freeze10.1021/es990949p CCC: $19.00
2000 American Chemical Society Published on Web 06/21/2000
TABLE 1. Average Concentrations of Major and Minor Ions Found in Groundwater of Southeast Michigan ion -
HCO3 Ca2+ Na+ Mg2+ ClSO42K+ FFe (total)
concn (M) 10-3
5.0 × 1.8 × 10-3 1.7 × 10-3 1.0 × 10-3 8.7 × 10-4 3.9 × 10-4 1.6 × 10-4 3.0 × 10-5 1.8 × 10-5
ion -
NO3 Fe2+ NH4+ BrHSPO43Mn H+
concn (M) 1.4 × 10-5 1.2 × 10-5 1.1 × 10-5 1.2 × 10-6 8.8 × 10-7 8.8 × 10-7 6.0 × 10-7 4.0 × 10-8
TABLE 2. pH of Deionized Water, Groundwater, and 0.1 M Solutions Used in Leaching Experiments solution
pH
solution
pH
deionized water groundwater KCl Na2SO4 MgSO4
6.08 7.03 6.04 6.51 6.21
CaSO4 NaHCO3 KHCO3 FeCl3
8.76 8.50 8.26 2.16
dried samples were sieved through a 0.35 mm mesh and homogenized by thorough mixing. The samples were stored in a freezer until further analysis. To determine total arsenic concentration, the pulverized rock samples were digested using concentrated nitric acid (16). One gram of dry core sample and 10 mL of concentrated nitric acid were placed in a 100 mL glass beaker. Then the beaker was covered with a watch glass and put on a heater which was set at 100 °C for 1 h. After the beaker was cooled, approximately 30 mL of deionized water was added to the beaker. The suspended mixture was filtered through a 0.45 µm membrane. Finally the filtrate was transferred to a 100 mL volumetric flask and brought up to the volume using deionized water. The same procedure was performed with a blank (nitric acid without core sample) and a standard reference material (2709, San Joaquin Soil) in each batch of digestion. The digestion procedure was effective in extracting As from rock samples, showing 89-98% recoveries of standard material. The range of total arsenic concentrations in rock samples was from 0.8 mg/kg to 72.1 mg/kg. Because the mass of each sample at different depth was not enough to conduct the entire experiment with each sample separately, 10 core samples with high arsenic concentrations (>9 mg/ kg) were homogenized by mixing them together. The arsenic concentration of the mixture was 26.4 mg/kg. Unless otherwise specified, all further leaching experiments were performed with this mixed MSS sample. Determination of Effects of Major Ions on Leaching of Arsenic. To determine which major ions most efficiently cause arsenic leaching, deionized water, groundwater, and synthetic solutions containing the major ions found in groundwater of study area, namely Ca2+, K+, Mg2+, Na+, Cl-, HCO3-, and SO42-, were prepared. Table 1 shows average concentrations of major and minor ions in groundwater of southeast Michigan. Groundwater used in this leaching experiment was collected from a local well in Ann Arbor known to contain negligible amounts (
