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: 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; · 2.65 Impact Factor
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ABSTRACT: Biochar has great potential as a soil amendment to immobilize heavy metals, thereby reducing their bioavail-ability. In this study, biochars derived from chicken manure and green waste were compared with commercial activated carbon (AC) and laboratory produced black carbon (BC) for the sorption of Pb and Cd. Sorption kinetics and equilibrium sorption isotherms for Pb and Cd were obtained for the char materials and the data were fitted to kinetic and sorption isotherm models.. Chicken manure-derived biochar (CM) showed the highest sorption capacity for both Pb and Cd, and the Pb sorption by biochars was higher than the Cd sorption because of the precipitation of Pb with various ions released from the biochars such as carbonate, phosphate, and sulfate. The sorption data for both Pb and Cd were better represented by the pseudo-second order kinetic model than the pseudo-first order kinetic model, which indicates chemical sorption between biochar and metals. For the isotherm studies, char materials was mixed with various amount of Pb or Cd solutions and the remaining metal concentration was measured. The equilibrium sorp-tion data followed a Langmuir isotherm with a maximum sorption capacity of 6.8–11 and 1.7–8.0 mg/g by biochars for Pb and Cd, respectively. Furthermore, CM immobilized Pb and Cd up to 93.5 and 88.4 %, respec-tively, while BC was not effective in the immobilization of Pb in soil. Overall, the sorption experiments in solution and the immobilization experiment in soil showed that biochars are more effective than AC in the sorption of Pb and Cd, and that they have the potential to be used as a soil amendment to remediate metal-contaminated soil.Water Air and Soil Pollution 11/2013; · 1.75 Impact Factor
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ABSTRACT: Soil contamination with heavy metals and organic pollutants has increasingly become a serious global environmental issue in recent years. Considerable efforts have been made to remediate contaminated soils. Biochar has a large surface area, and high capacity to adsorb heavy metals and organic pollutants. Biochar can potentially be used to reduce the bioavailability and leachability of heavy metals and organic pollutants in soils through adsorption and other physicochemical reactions. Biochar is typically an alkaline material which can increase soil pH and contribute to stabilization of heavy metals. Application of biochar for remediation of contaminated soils may provide a new solution to the soil pollution problem. This paper provides an overview on the impact of biochar on the environmental fate and mobility of heavy metals and organic pollutants in contaminated soils and its implication for remediation of contaminated soils. Further research directions are identified to ensure a safe and sustainable use of biochar as a soil amendment for remediation of contaminated soils.Environmental Science and Pollution Research 04/2013; · 2.76 Impact Factor