Mineralogical and geomicrobiological investigations on groundwater arsenic enrichment in Bangladesh |
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| Institution: | 1. Department of Natural Sciences, Castleton State College, 233 South Street, Castleton, VT 05735, United States;2. Department of Geology, Middlebury College, 276 Bicentennial Way, Middlebury, VT 05753, United States;1. Department of Marine Science and Environmental Studies, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, United States;2. School of Earth and Environmental Sciences, Queens College, City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, United States;3. Maine Geological Survey, 93 State House Station, Augusta, ME 04333, United States;4. Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, United States;1. Department of Environmental Science and Engineering, State Key Laboratory of Multiphase Flow in Power Engineering, Xi''an Jiaotong University, Xi''an 710049, China;2. Department of Civil and Environmental Engineering, Penn State University, University Park, PA 16802, USA;1. Geology Department, Middlebury College, Middlebury, VT 05753, USA;2. Department of Earth Sciences, University of Ottawa, Ottawa K1N 6N5, Canada;3. Biology Department, Middlebury College, Middlebury, VT 05753, USA;4. Vermont Geological Survey, Montpelier, VT 05620-3902, USA;1. Delft University of Technology, Faculty of Civil Engineering and Geosciences, Stevinweg 1, Delft, The Netherlands;2. University of Dhaka, Department of Geology, Dhaka 1000, Bangladesh;3. North South University, Department of Environmental Science and Management, Bashundhara, Dhaka 1229, Bangladesh;4. VU University Amsterdam, Department of Molecular Cell Physiology, De Boelelaan 1085, NL-1081 HV Amsterdam, The Netherlands;5. VU University Amsterdam, Department of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands |
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| Abstract: | Sources of As in the Ganges sediments and microbial mechanisms of its release in groundwater were examined in the present study, where the authors have systematically examined the pertinent mineral species present in the sediments using XRD, TEM-EDS and EPMA techniques. The results show several As-bearing minerals in the Ganges sediments, in western Bangladesh. Iron-sulfide minerals consist of near-amorphous and/or crystalline precursors of framboidal pyrite and pyrite both of which contain As. Several types of Fe oxyhydroxides (oxides), which contain variable amounts of As were also found in muddy sediments. The content of As increases from Fe-oxides to the precursors of framboidal pyrite and pyrite. Four different chemical forms of As from the core sediments were determined. The sequentially extracted chemical forms are as follows: (1) acid soluble form (As mainly fixed in carbonates), (2) reducible form (As fixed in Fe- and/or Mn-oxides), (3) organic form, (4) insoluble form (As fixed mainly in sulfide and rarely in silicate minerals). Arsenic is dominantly sorbed on to Fe- and/or Mn-oxides, organic forms and sulfide minerals in most samples, although their relative abundances differ in different samples. Geomicrobial culture experiments were carried out to test the hypothesis that microbial processes play a key role in the release of As in groundwater. Batch culture and circulating water system experiments were designed using the sediments from Bangladesh. In the batch experiments, As was released at low Eh values a few days after adding nutrients containing glucose, polypepton and yeast extract, urea and fertilizer under a dominantly N2 atmosphere. This contrasts with the control experiments without nutrients. Circulating water experiments with sand layer in a N2 atmosphere showed similar results. These results support the hypothesis that microbial processes mediate the release of As into groundwater under reducing conditions. Glucose and polypepton used in the experiments may correspond to C and N sources, respectively. Younger sediments contain abundant organic matter, which is easily used by bacteria. So, the enhanced bacterial activity may correspond to simulation of accelerated natural diagenetic process using organic matter, or some fertilizer/wastewater effects. |
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