Chemical and microbial control of pyrite weathering and its implications to arsenic mobility and sulfur and iron geochemistry

Controls on pyrite deposition and weathering were investigated in Newark Basin (Lockatong formation) black shale. Inorganic sulfur species including pyritic S and acid volatile S were quantified in early, middle, and late Lockatong formation black shale samples obtained from the Newark Basin rock core collection. Pyritic S accounted for more than 50% of total S in Lockatong black shale and was strongly correlated to total S, yet no correlation was found between acid volatile S and total S. An inverse relationship was identified between pyritic S and acid volatile S at various depths in the formation which may indicate their redox potential-controlled interconversion. Further analysis of the redox potential index, Th/U ratio, showed that redox potential was one of the controlling factors for the deposition of the reduced sulfur species in the Newark Basin sedimentary environments.;Trace metal enrichment factors showed that As and Mo were two highly enriched elements in Lockatong formation black shale. Correlation analyses show that As was closely linked to pyritic S and Mo was associated with organic matter in Lockatong black shale. The hypothesized mobilization of arsenic from pyritic black shale by a sulfide-arsenide exchange and oxidation reaction was tested with FeAsS, homogenized Lockatong formation black shale and Lockatong formation black shale pyrite incubated under oxic, hypoxic and anoxic conditions. Incubation results showed that sulfide increased arsenic mobilization to the dissolved phase from all three solids under oxic and hypoxic, but not anoxic conditions. Moreover, XANES results show that arsenic in Newark Basin black shale pyrite has the same oxidation state as that in FeAsS (-1) and thus extend the sulfide-arsenide exchange mechanism of arsenic mobilization to sedimentary rock, black shale pyrite. This process will be most important to arsenic mobilization in aquifers where pyrite is exposed to a supply of sulfide from zones of anoxic groundwater which supports sulfate reducing bacteria and oxic groundwater or other oxidants.;The direct role of microorganisms in pyrite weathering in Newark Basin black shale was examined through microbial colonization experiments. Thick biofilms were observed on the surface of pyrite bearing, polished black shale thick sections during their incubation in a subsurface groundwater well and the microbial community structure of the colonized microbes was analyzed. Colonizing microbes preferentially attached to pyrite rather than the shale matrix and the co-occurrence of bacteria-shaped pits and secondary iron minerals on pyrite were observed. 16S rDNA sequence analysis of pyrite and arsenopyrite biofilm communities indicated that most of the mineral colonizing microorganisms were members of the Fe(III)-reducing Geobacteraceae (delta-proteobacteria). Other sequences showed high similarity (>99%) with species in the beta and &egr;-proteobacteria that are able to oxidize iron or sulfur. These results indicate that microbes may play a critical role in the transformation of iron sulfides and their secondary minerals such as iron oxides, as well as the mobilization of trace elements from such minerals in slightly acidic black shale aquifers.