The influence of biochar and black carbon on reduction and bioavailability of chromate in soils.

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 nontoxic and is strongly bound to soil particles, whereas Cr(VI) is more toxic and 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 (BC) 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 BC resulted in greater reduction of Cr(VI) in soils compared with biochar. This is attributed to the differences in dissolved organic carbon 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 BC was more effective than biochar in increasing the microbial population and in mitigating the phytotoxicity of Cr(VI). The net benefit of BC emerged as an increase in plant biomass and a decrease in Cr concentration in plant tissue. Consequently, it was concluded that BC is a potential reducing amendment in mitigating Cr(VI) toxicity in soil and plants.

Potential value of phosphate compounds in enhancing immobilization and reducing bioavailability of mixed heavy metal contaminants in shooting range soil.

Shooting range soils contain mixed heavy metal contaminants including lead (Pb), cadmium (Cd), and zinc (Zn). Phosphate (P) compounds have been used to immobilize these metals, particularly Pb, thereby reducing their bioavailability. However, research on immobilization of Pb's co-contaminants showed the relative importance of soluble and insoluble P compounds, which is critical in evaluating the overall success of in situ stabilization practice in the sustainable remediation of mixed heavy metal contaminated soils. Soluble synthetic P fertilizer (diammonium phosphate; DAP) and reactive (Sechura; SPR) and unreactive (Christmas Island; CPR) natural phosphate rocks (PR) were tested for Cd, Pb and Zn immobilization and later their mobility and bioavailability in a shooting range soil. The addition of P compounds resulted in the immobilization of Cd, Pb and Zn by 1.56-76.2%, 3.21-83.56%, and 2.31-74.6%, respectively. The reactive SPR significantly reduced Cd, Pb and Zn leaching while soluble DAP increased their leachate concentrations. The SPR reduced the bioaccumulation of Cd, Pb and Zn in earthworms by 7.13-23.4% and 14.3-54.6% in comparison with earthworms in the DAP and control treatment, respectively. Bioaccessible Cd, Pb and Zn concentrations as determined using a simplified bioaccessibility extraction test showed higher long-term stability of P-immobilized Pb and Zn than Cd. The differential effect of P-induced immobilization between P compounds and metals is due to the variation in the solubility characteristics of P compounds and nature of metal phosphate compounds formed. Therefore, Pb and Zn immobilization by P compounds is an effective long-term remediation strategy for mixed heavy metal contaminated soils.

Stabilization of carbon in composts and biochars in relation to carbon sequestration and soil fertility.

There have been increasing interests in the conversion of organic residues into biochars in order to reduce the rate of decomposition, thereby enhancing carbon (C) sequestration in soils. However energy is required to initiate the pyrolysis process during biochar production which can also lead to the release of greenhouse gasses. Alternative methods can be used to stabilize C in composts and other organic residues without impacting their quality. The objectives of this study include: (i) to compare the rate of decomposition among various organic amendments and (ii) to examine the effect of clay materials on the stabilization of C in organic amendments. The decomposition of a number of organic amendments (composts and biochars) was examined by monitoring the release of carbon-dioxide using respiration experiments. The results indicated that the rate of decomposition as measured by half life (t(1/2)) varied between the organic amendments and was higher in sandy soil than in clay soil. The half life value ranged from 139 days in the sandy soil and 187 days in the clay soil for poultry manure compost to 9989 days for green waste biochar. Addition of clay materials to compost decreased the rate of decomposition, thereby increasing the stabilization of C. The half life value for poultry manure compost increased from 139 days to 620, 806 and 474 days with the addition of goethite, gibbsite and allophane, respectively. The increase in the stabilization of C with the addition of clay materials may be attributed to the immobilization of C, thereby preventing it from microbial decomposition. Stabilization of C in compost using clay materials did not impact negatively the value of composts in improving soil quality as measured by potentially mineralizable nitrogen and microbial biomass carbon in soil.