Influence of biochar application methods on the phytostabilization of a hydrophobic soil contaminated with lead and acid tar.

A hardwood biochar was examined for its potential use as an amendment to aid in the phytostabilization of a severely-contaminated soil at a former sulfuric acid recycling factory site. The soil, which has remained unvegetated for nearly a century, contained high pseudo-total concentrations of lead, arsenic and antimony and was both highly acidic and hydrophobic due to the presence of petroleum-based acid tar. Three application approaches were tested with 10 and 20% (vol/vol) biochar: Incorporation into soil, top-dressing on the surface, and layering within the soil. The results suggest that the homogeneous mixing of the hardwood biochar into soil would not promote the long-term restoration at this site due to its inherently low alkalinity relative to the very high net acidity of the existing soil. In contrast, surface application of biochar resulted in the most successful growth of Canada wild-rye grass by exploiting the properties inherent to biochar alone.

Bacteria in gel probes: comparison of the activity of immobilized sulfate-reducing bacteria with in situ sulfate reduction in a wetland sediment.

A novel method was used to examine the microbial ecology of iron-rich wetland sediments receiving neutral-pH coal mine drainage. Gel probes inserted into the sediments allowed analysis of the distribution and activity of bacterial sulfate reduction (BSR). A mixed population of sulfate-reducing bacteria enriched from anoxic wetland sediments was immobilized in low temperature-gelling agarose held in grooved rods or probes. The probes were inserted vertically into sediments and were allowed to incubate in situ for 48 h. After their retrieval, the gels were sectioned and analyzed for residual BSR activity and were compared to in situ BSR rates and chemical porewater profiles. The depth distribution of residual BSR activity in the immobilized cell gel probes differed significantly from the BSR measured in situ. Approximately 51% of the total integrated residual sulfate reduction activity measured in the gel probes occurred between 0 and 7 cm of the upper 20 cm of sediment. In contrast, ca. 99% of the integrated in situ BSR occurred between 7- and 20-cm depth, and only 1% of the total integrated rate occurred between 0- and 7-cm depth. Lactate-enriched bacteria immobilized in the gel may have been atypical of the majority of sulfate-reducing bacteria in the sediment. Agarose-immobilized sulfate-reducing bacteria might also be able to proliferate in the otherwise inhospitable zone of iron reduction, where sulfate and labile carbon compounds for which they are usually outcompeted can diffuse freely into the gel matrix. Gel probes containing particulate iron monosulfide (FeS) indicated that FeS remained stable in sediments at depths greater than 2 to 3 cm below the sediment-water interface, consistent with the shallow penetration of oxygen into surface sediments.

Use of poly(lactic acid) amendments to promote the bacterial fixation of metals in zinc smelter tailings.

The ability of poly(lactic acid) (PLA) to serve as a long-term source of lactic acid for bacterial sulfate reduction activity in zinc smelter tailings was investigated. Solid PLA polymers mixed in water hydrolyzed abiotically to release lactic acid into solution over an extended period of time. The addition of both PLA and gypsum was required for indigenous bacteria to lower redox potential, raise pH, and stimulate sulfate reduction activity in highly oxidized smelter tailings after one year of treatment. Bioavailable cadmium, copper, lead and zinc were all lowered significantly in PLA/gypsum treated soil, but PLA amendments alone increased the bioavailability of lead, nickel and zinc. Similar PLA amendments may be useful in constructed wetlands and reactive barrier walls for the passive treatment of mine drainage, where enhanced rates of bacterial sulfate reduction are desirable.

Treatment of metal-contaminated water using bacterial sulfate reduction: results from pilot-scale reactors.

Simple anaerobic reactors were installed to treat metal-contaminated water in an underground coal mine and at a smelting residues dump in Pennsylvania. The reactors consisted of barrels and tanks filled with spent mushroom compost, within which bacterial sulfate reduction became established. Concentrations of Al, Cd, Fe, Mn, Ni, and Zn were typically lowered by over 95% as contaminated water flowed through the reactors. Cadmium, Fe, Ni, and some Zn were retained as insoluble metal sulfides following their reaction with bacterially generated H(2)S. Aluminum, Mn, and some Zn hydrolyzed and were retained as insoluble hydroxides or carbonates. Reactor effluents were typically circumneutral in pH and contained net alkalinity. The principal sources of alkalinity in the reactors were bacterial sulfate reduction and limestone dissolution. This article examines the chemistry of the reactor systems and the opportunities for enhancing their metal-retaining and alkalinity-generating potential.