Measurement of in situ rates of selenate removal by dissimilatory bacterial reduction in sediments

Abstract

vertical distribution in sediments relative to other forms • A radioisotope method for measurement of bacterial of anaerobic bacterial respiration. An understanding of respiratory reduction of selenate to elemental selenium in how the process of dissimilatory selenate reduction oper­ aquatic sediments was devised. Sediments were labeled ates in these respects should ultimately aid the design of with [75Se]selenate, incubated, and washed, and 75SeO(S) treatment schemes and the rehabilitation of selenium­ was determined as counts remaining in the sediment. Core contaminated regions. In order to answer these questions profiles of selenate reduction, sulfate reduction, and den­ and to achieve these long-term goals, it was f1I'St necessary itrification were made simultaneously in the sediments of to devise a method for measuring in situ selenate reduction. an agricultural wastewater evaporation pond. Most of the In situ measures of bacterial activities have been a focus in situ selenate reduction (850/0) and all the denitrification of microbial ecology for the past 25 years. A variety of activities were confined to the upper 4-8 em of the profile, whereas sulfate reduction was greatest below 8 em (890/0 techniques employing radioisotopes, gas chromatography, of total). The integrated areal rate of selenate reduction and/or enzyme inhibitors or analogues have been exploited was 301 J,tmol m-2 day, which results in a turnover of to assess environmental rates of methanogenesis (8), den­ water column selenate in 82.4 days. itrification (9, 10), nitrogen fixation (11), and sulfate re­ duction (12). This latter process is analogous to the methods we report herein to measure selenate reduction. Introduction However, in the case of selenate reduction, our task was Oxyanions of selenium have been identified as toxic simplified because 75Se is a 'Y-emitting radioisotope (t l / 2 constituents in drainage waters from irrigated, seleniferous =120 days), which circumvents the quenching/efficiency agricultural soils (1). This environmental problem is ap­ problem associated with liquid scintillation counting of parently quite common in many regions of the western weak ~-emitters like 35S. Thus, there was no need to United States (2). Therefore, methods that address the volatilize and trap the product 75SeO(S) as there is for question of removal of selenium oxyanions from soils [35S]sulfide, but merely to wash away the added [75Se]­ and/or drainage waters by various means have received selenate. Our results with core profiles from a selenium­ considerable attention because they offer the prospect of rich agricultural wastewater evaporation pond represent restoring water or soil quality while allowing for the con­ quasi in situ assays because they were incubated in the tinuation of irrigated farming. With regard to possible laboratory under conditions approximating the field. microbiological methods, volatilization of selenium by However, our results indicate that selenate reduction oc­ formation of alkylated gases (e.g., dimethyl selenide) has curs in the surficial layers of these sediments, in proximity been suggested for treatment of soils (3) or impacted marsh to the region of active denitrification, yet spatially sepa­ waters, such as those of the Kesterson Wildlife Refuge (4, rated from sulfate reduction occurring deeper in the core. 5). Areal rates of selenate removal obtained from this core J Recently, we reported on a novel process by which indicate its rapid removal by reduction to SeO(s), and a anaerobic bacteria respire selenate, which in turn bio­ quick turnover of water column selenate. When combined chemically reduces this oxyanion to selenite and ultimately with previous observations with regard to inhibitors and to elemental selenium (6). This dissimilatory reduction stimulators of selenium reduction, the conceptual design was demonstrated to occur in sediments, was independent of an efficient anaerobic selenate digester can be dis­ of sulfate, and was inhibited by nitrate and chromate. cerned. Bacterial cultures capable of selenate respiratory growth have been isolated (6, 7). Although we were able to show Experimental Section that this process holds the potential to rapidly remove and Sites and Sampling. Surficial (upper ro.J20 em) sedi­ sequester considerable quantities (millimolar) of added ments were collected from Hunter Drain, an agricultural selenate from sediment slurries, the questions remained drain located in Stillwater, NV, where selenium contam­ as to how rapidly it proceeds in nature, and what was its ination is documented (2), as well as from the littoral zone of Big Soda Lake, an alkaline/saline environment (13)