Microbial studies of a selenium-contaminated mine site and potential for on-site remediation

Abstract

Surface water Selenium (Se) concentrations are above regulatory standards at several active and inactive phosphate mine sites in the US Western Phosphate Resource Area. The focus of the present study was to examine the impacts of the microbial communities on the oxidation state of Se in overburden waste from the Smoky Canyon phosphate mine in Idaho, USA. Microbial populations were found that reduce soluble selenate (SeO (4) (2-) ) to insoluble elemental Se. Microcosm experiments were conducted for molecular genetic analysis of this microbial community by rRNA gene profiling. An acetone pretreatment step was developed to remove interfering pre-petroleum hydrocarbons from the samples prior to extraction. PCR was used to amplify 16S and 18S rRNA genes present in the microbial community DNA. The amplified products were subjected to denaturing gradient gel electrophoresis (DGGE). Isolates and excised DGGE bands were amplified and sequenced for identification to determine the relative importance of culturable isolates to the total microbial population. Analysis of samples from different sites at the mine showed how Se contamination and previous remediation treatments changed the microbial populations across the site. Members of the family Enterobacteriaceae were dominant among the selenate reducing isolates from the site containing high Se levels. In particular, Serratia fonticola was isolated repeatedly from contaminated Smoky Canyon Mine site samples. Packed column studies were performed with seleniferous waste rock fractions from Smoky Canyon Mine. Column amendments consisted of combinations of iron, compost, and whey. Eh, pH, and extractable Se measurements were taken. Tests with infiltrated water showed columns containing an organic amendment combined with iron metal were the most resistant to Se leaching. Iron-based compounds from the corroding metal are thought to strongly bind the Se reduced by microbial activity, thereby stabilizing the Se in an insoluble form. We conclude that long-term stabilization of selenium at contaminated mine sites may require reductive microbial processes combined with abiotic immobilization by iron, either natural or engineered, to stabilize the Se and retard re-oxidation and release. Iron-selenide or iron-selenite compounds are more stable and resistant to leaching, especially when removed from active weathering.

Persistent Bacterial and Fungal Community Shifts Exhibited in Selenium-Contaminated Reclaimed Mine Soils

“…Many of the Alphaproteobacteria and Actinobacteria sequences matched known taxa related to N cycling, including Rhizobiales and Bradyrhizobiaceae ( Alphaproteobacteria ) and Frankiales and Micrococcales ( Actinobacteria ). Many known N-cycling bacteria are also able to reduce Se oxyanions, either aerobically or anaerobically ( 61 , 64 ), and at least one Rhizobiales ( Agrobacterium tumefaciens ) has been isolated from nearby reclaimed mines in the Phosphate Patch and shown to reduce selenate ( 15 ). Some organisms can reduce selenate via nitrate reductases ( 66 ), and several nitrate-reducing OTUs were seen to increase significantly in a saturated soil experiment following selenate amendment ( 35 ).…”

“…Considering the importance of microorganisms in the Se biogeochemical cycle and the implications of altering Se speciation on human and environmental health, there is a clear knowledge gap surrounding microbial-Se interactions. While culture-based approaches have succeeded at identifying individual Se-tolerant and/or Se-transforming organisms ( 15 – 19 ), information regarding whole-microbial-community responses to Se contamination is lacking. Most studies show that microbial abundance, activity, diversity, and distribution are severely depressed in metal(loid)-contaminated soils compared with uncontaminated soils ( 20 , 21 ).…”

“…Elevated concentrations of Se have also been measured in soils, surface waters, and vegetation ( 32 ), though how the soil microbial community is interacting with, or is influenced by, this contamination is unclear. Microbial community dynamics may mediate metalloid speciation in soils, but to date, the only studies investigating microbial communities in Se-impacted natural environments have focused on communities in anaerobic environments ( 15 , 33 – 35 ). The resident communities in Se-contaminated oxic soil environments remain completely unknown.…”

Microbial community analysis of two field‐scale sulfate‐reducing bioreactors treating mine drainage

“…As sulfate-reducing bioreactors are microbiologically driven, an improved understanding of the microbial community involved is fundamental for improved design and performance (Kaksonen et al, 2004;Labrenz and Banfield, 2004;Hallberg and Johnson, 2005a). In the 1990s, researchers began to investigate the microbiology of the systems using culture-based techniques (Lyew et al, 1994;Hard et al, 1997;Benner et al, 2000;Labrenz and Banfield, 2004;Hallberg and Johnson, 2005a) and, more recently, biomolecular techniques (Clarke et al, 2004;Kaksonen et al, 2004;Labrenz and Banfield, 2004;Hallberg and Johnson, 2005a;Morales et al, 2005;Bruneel et al, 2006;Chandler et al, 2006;Geets et al, 2006;Knotek-Smith et al, 2006;Nicomrat et al, 2006a,b;Pruden et al, 2007). A relationship between performance and the type of microbial inoculum has recently been demonstrated (Pruden et al, 2007), providing evidence that the role of the microbial community is important for design and operation.…”

Bayer Electrofilter Fines as Potential Se(VI) Adsorbents

“…The adsorption method is considered the most efficient and economical because of its fast removal rate and minimum pretreatment of samples. [9][10][11][12][13][14][15][16][17][18][19][20] Aluminum, iron, and silica oxides are usually used as adsorbent in the adsorption process to remove heavy metals and metalloids from water. Iron oxides and aluminum oxide have been shown to adsorb Se.…”