No abstract

Physical weathering can modify the stability of biochar after field exposure. The aim of our study was to determine the potential carbon sequestration of the two chars at different timescales. We investigated the modification in composition and stability resulting from physical weathering of two different chars produced (i) at low temperature (250°C) by hydrothermal carbonization (HTC); and (ii) at high temperature (1200°C) by gasification (GS) using contrasting feedstocks. Physical weathering of HTC and GS placed on a water permeable canvas was performed through successive wetting/drying and freezing/thawing cycles. Carbon loss was assessed by mass balance. Chemical stability of the remaining material was evaluated as resistance to acid dichromate oxidation, and biological stability was assessed during laboratory incubation. Moreover, we assessed modification in potential priming effects due to physical weathering. Physical weathering induced a carbon loss ranging between 10 and 40% of the total C mass depending on the feedstock. This C loss is most probably related to leaching of small particulate and dissolved compounds. GS produced from maize silage showed the highest C loss. The chemical stability of HTC and GS was unaffected by physical weathering. In contrast, physical weathering strongly increased the biological stability of HTC and GS char produced from maize silage. After physical weathering, the half-life (t 1/2 ) of GS was doubled but only slight increase was noted for those of HTC. During the first weeks of incubation, HTC addition to soil stimulated native soil organic matter (SOM) mineralization (positive priming effect), while the GS addition led to protection of the native SOM against biologic degradation (negative priming effect). Physical weathering led to reduction in these priming effects. Model extrapolations based on our data showed that decadal C sequestration potential of GS and HTC is globally equivalent when all losses including those due to priming and physical weathering were taken into account. However, at century scale only GS may have the potential to increase soil C storage.

The microbial decolourization of Reactive Red 2 (RR2) dye has been studied under anaerobic conditions. Three semicontinuous bioreactors were operated with dye concentrations-R1 (control: 0 mg RR2 l −1 ), R2 (100 mg RR2 l −1 ) and R3 (200 mg RR2 l −1 ). The parameters monitored were, oxidation-reduction potential (ORP), methane production, colour and chemical oxygen demand (COD) removal during the feeding cycles. The oxidation-reduction potential values for the first few days were above −150 mV, which later on decreased to less than −275 mV in all the reactors. Colour removal during the first few days of operation was due to adsorption of dye on to anaerobic biomass. However, under steady state conditions, colour removal was above 76% for both the dye containing reactors and it was due to biologically mediated degradation. Methane production and chemical oxygen demand removal in the control and dye containing reactors were almost the same. Integrated analysis of the monitored parameters indicated that, the primary mechanism of colour removal was adsorption of RR2 on to anaerobic biomass and subsequent degradation. Decolourization rates were found to be first order with respect to dye concentration, although an increase in the influent dye concentration resulted in a decrease in the rate from 0.0074 (g volatile suspended solid, VSS) −1 h −1 (100 mg RR2 l −1 ) to 0.0039 (g VSS) −1 h −1 (200 mg RR2 l −1 ). Based on total methane production no inhibition effect of dyes was observed but total methanogenic activity (TMA) results exhibited inhibition of methanogenesis.