The influence of biochar and black carbon on reduction and bioavailability of chromate in soils
ABSTRACT The widespread use of chromium (Cr) has a deleterious impact on the environment. A number of pathways, both biotic and abiotic in character, determine the fate and speciation of Cr in soils. Chromium exists in two predominant species in the environment, trivalent [(Cr(III)] and hexavalent [Cr(VI)]. Of these two forms, Cr(III) is non-toxic and is strongly bound to soil particles, while Cr(VI) is more toxic, soluble and readily leaches into groundwater. The toxicity of Cr(VI) can be mitigated by reducing it to Cr(III) species. The objective of this study was to examine the effect of organic carbon sources on the reduction, microbial respiration and phytoavailability of Cr(VI) in soils. Organic carbon sources such as black carbon and biochar were tested for their potential in reducing Cr(VI) in acidic and alkaline contaminated soils. An alkaline soil was selected to monitor the phytotoxicity of Cr(VI) in sunflower plant. Our results showed that using black carbon resulted in greater reduction of Cr(VI) in soils compared to biochar. This is attributed to the differences in dissolved organic carbon (DOC) and functional groups that provide electrons for the reduction of Cr(VI). When increasing levels of Cr were added to soils, both microbial respiration and plant growth decreased. The application of black carbon was more effective than biochar in increasing the microbial population and in mitigating the phytotoxicity of Cr(VI). The net benefit of black carbon emerged as an increase in plant biomass and a decrease in Cr concentration in plant tissue. Consequently, it was concluded that black carbon is a potential reducing amendment in mitigating Cr(VI) toxicity in soil and plants.
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ABSTRACT: Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars). Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and enviro-technical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700°C) show the highest capacities to accept and donate electrons. Combined electrochemical, elemental, and spectroscopic analyses of the thermosequence chars provide evidence that the pool of redox-active moieties is dominated by electron-donating, phenolic moieties in the low-HTT chars, by newly-formed electron accepting quinone moieties in intermediate-HTT chars, and by electron accepting quinones and possibly condensed aromatics in the high-HTT chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".Environmental Science and Technology 04/2014; · 5.48 Impact Factor
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ABSTRACT: This study was aimed to examine the efficiency of a novel bacterial consortium on the reduction of toxic hexavalent chromium [Cr(VI)] to non-toxic trivalent Cr [Cr(III)]. Six Cr(VI)-resistant bacteria (IS1-IS6) were isolated from a tannery waste disposal site at Mount Barker, South Australia, of which three viz., IS1, IS2 and IS3 were selected based on Cr(VI) reduction ability in minimal salt medium. The isolates were identified as Bacillus endophyticus (IS1), Microbacterium paraoxydans (IS2) and Bacillus simplex (IS3) by 16S rRNA gene sequencing. All three isolates were able to tolerate chromium (Cr(VI), 300–400 mg L−1), arsenic (As(V), 1,000 mg L−1), copper (Cu(II), 300–400 mg L−1) and lead (Pb(II), 1,000 mg L−1). The isolates were evaluated both as an individual and as a consortia for Cr(VI) reduction in minimal salt medium and storm water, both spiked with 100 mg Cr(VI) L−1. In both cases, the rate of Cr(VI) reduction was found to be significantly higher in the bacterial consortium inoculation (t ½ = 8.45 for minimal salt medium; 6.02 h for storm water), compared to inoculation with individual isolates (t ½ = 53.3–115.5 h for minimal salt medium; 8.77–9.76 h for storm water). The rate of Cr(VI) reduction in both minimal salt medium and storm water was found to be higher in bacterial consortium inoculation (IS1 + IS2 + IS3) than in individual isolate inoculation. This experiment demonstrated that bacterial consortium prepared by using B. endophyticus, M. paraoxydans and B. simplex was more effective in Cr(VI) detoxification than application of individual bacterium. This experiment also proved that a bacterial consortium was more effective in Cr(VI) detoxification than the application of individual bacterial strain.Water Air and Soil Pollution 11/2013; 224(12). · 1.69 Impact Factor
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ABSTRACT: Biochar, being produced in an oxygen-restricted environment, is chemically more reduced than the original feedstock. Consequently, it was hypothesized that reduced biochar components could participate in redox-mediated reactions in the soil. This hypothesis was tested by measuring the reducing capacities of aqueous extracts of biochars and the reduction and solubilization of soil Mn and Fe oxides by the extracts. The reduction capacity of extracts from biochars produced from three feedstocks (eucalyptus wood, EUC; olive pomace, OP; and greenhouse waste, GHW) at different highest pyrolysis treatment temperatures (HTT; 350, 450, 600 and 800°C) was less for the EUC feedstock than the others, and was greater for biochars produced at lower HTTs. The organic fraction of the extracts apparently was responsible for the major part of the reducing capacity. Extracts of smaller-HTT biochars, having greater dissolved organic carbon (DOC) contents, had greater reducing capacities than extracts of larger-HTT biochars from the same feedstock. Extracts of two GHW biochars (GHW-450 and GHW-600) solubilized Mn and Fe from soils at pH values below 8. The extract with the greater reducing capacity (GHW-450) solubilized both metals to a significantly greater extent. Smaller-HTT biochars produced from agricultural wastes, having a greater variety and concentration of soluble reducing agents, are expected to have more impact on soil redox reactions than larger-HTT biochars. By participating in chemical and biological redox-mediated reactions in the soil, biochar could influence microbial electron shuttling, nutrient cycling, pollutant degradation, contaminant mobilization and abiotic formation of humic structures.European Journal of Soil Science 08/2013; · 2.39 Impact Factor