Environ. Sci. Technol. 2004, 38, 4002-4011
In situ Metal Precipitation in a Zinc-Contaminated, Aerobic Sandy Aquifer by Means of Biological Sulfate Reduction G. M. C. M. JANSSEN AND E. J. M. TEMMINGHOFF* Department of Soil Quality, Wageningen University, P.O. Box 8005, 6700 EC Wageningen, The Netherlands
The applicability of in situ metal precipitation (ISMP) based on bacterial sulfate reduction (BSR) with molasses as carbon source was tested for the immobilization of a zinc plume in an aquifer with highly unsuitable initial conditions (high Eh, low pH, low organic matter content, and low sulfate concentrations), using deep wells for substrate injection. Batch experiments revealed an optimal molasses concentration range of 1-5 g/L and demonstrated the necessity of adding a specific growth medium to the groundwater. Without this growth medium, even sulfate, nitrogen, phosphorus, and potassium addition combined with pH optimization could not trigger biological sulfate reduction. In column experiments, precipitation of ZnS(s) was induced biologically as well as chemically (by adding Na2S). In both systems, zinc concentrations dropped from about 30 mg/L to below 0.02 mg/L. After termination of substrate addition the biological system showed continuation of BSR for at least 2 months, suggesting the insensitivity of the sulfate reducing system for short stagnations of nutrient supply, whereas in the chemical system an immediate increase of Zn concentrations was observed. A pilot experiment conducted in situ at the zinccontaminated site showed a reduction of zinc concentrations from around 40 mg/L to below 0.01 mg/L. Termination of substrate supply did not result in an immediate stagnation of the BSR process, but continuation of BSR was observed for at least 5 weeks.
Introduction Biological sulfate reduction (BSR) has proven to be an effective means in reducing heavy metal concentrations in contaminated water (1). BSR is the reduction of sulfate to sulfide catalyzed by the activity of sulfate-reducing bacteria (SRB) using sulfate as electron acceptor (2). The low solubility of metal sulfides will cause the sulfide formed to precipitate with present metal ions (3). Applications of BSR for the precipitation of heavy metals are mainly found in the fields of acid mine drainage (AMD) and wastewater treatment. AMD is characterized by low Eh, low to moderate pH, and high concentrations of sulfate, iron, and heavy metals, and the use of BSR is aimed at pH increase and sulfate and metal removal. BSR has been investigated for treatment of AMD on-site in reactive barriers (4-5) as well as off-site in anaerobic bioreactors (6-8) or (constructed) * Corresponding author phone: +31-317-482357; fax: +31-317483766; e-mail:
[email protected]. 4002
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 14, 2004
wetlands (9-10). Treatment of wastewaters with BSR is often focused on sulfate and metal removal in anaerobic bioreactors (11-13). By the studies mentioned, among others, BSR has been established as a very useful technique for heavy metal removal in both fields, and a wide range of electron donors has been proved to be useful in the process, varying from expensive and pure substrates such as ethanol (14), lactate (6), and hydrogen (12) to economically more favorable waste products, with or without enrichment with pure substrates or inoculating with monocultures or media (manure, sludge, soil) containing SRB (5-6, 11, 15-19). The two fields of BSR application (AMD treatment and wastewater treatment) have in common that the technique is usually tested in an environment that already has or can be designed to have favorable conditions for the growth of SRB. Furthermore, especially the off-site systems are characterized by a relatively high level of accessibility and controllability, which guarantees the continuation of the created conditions for as long as desired. If the heavy metal contamination, however, is situated in an aquifer far below the soil surface and this aquifer initially lacks BSR supporting conditions, new challenges have to be faced. Off-site treatment of the contaminated groundwater or installation of a reactive barrier is often not an option because of the long extraction times and/or the high costs involved. Then, tackling the contamination by creating a socalled “in situ reactive zone” can in some settings be a practical alternative (1). However, the affected area cannot easily be accessed, and environmental conditions can be changed only by injecting, through a limited amount of deep wells, substances that directly or indirectly alter these conditions (20-22). Furthermore, heterogeneity in chemical and physical soil parameters adds uncertainty to the fate of the applied substances and the environmental conditions that will apply. These two complications make the process much less controllable and predictable than is the case in the aforementioned applications. The aim of the present study is to investigate the applicability of in situ metal precipitation (ISMP) relying on BSR for the situation outlined above. Specifically, we strived for the immobilization of extremely severe Zn contamination (with Zn concentrations as high as 180 mg/L) located in an aerobic, acid sandy aquifer, low in organic matter, by means of injection of the necessary substances through deep wells. In contrast to what is usually the case in AMD and also often in wastewaters, sulfate concentrations were relatively low at our test site. The combination of these environmental conditions made the site, in principle, unsuitable for BSR and even constitutes sort of a “worst case scenario” for in situ remediation by means of BSR. In the present study, first the utility of molasses as carbon and energy source for the BSR was investigated in batch experiments. These experiments were also designed to yield information about the suitable molasses concentration range and the necessity of adding other nutrients to the groundwater to achieve BSR. Building on the knowledge gathered in the batch experiments, column experiments were performed to investigate the BSR process in a more realistic aquifer system. Special attention was given to the sensitivity of the sulfate reducing system to oxidizing periods. Finally, the ISMP principle is applied in situ at a zinc contaminated site in a field-scale pilot experiment, using deep wells for substrate injection. 10.1021/es030131a CCC: $27.50
2004 American Chemical Society Published on Web 06/12/2004
TABLE 1. Analysis of the Molasses Used in the Experiments component
content/value
water crude proteins crude fats crude cell substances crude ashes starch sugars volatile bases containing N (like NH3) calcium (Ca) phosphorus (P) potassium (K) sodium (Na) chlorides sulfur (S) arsenic (As) cadmium (Cd) mercury (Hg) lead (Pb) copper (Cu) zinc (Zn) nickel (Ni) chromium (Cr) pH density (F)
0.29 (M/M) 0.04 (M/M)
