Physico-chemical characterization of biochars from solid municipal waste for use in soil amendment

• Biochars were successfully produced from solid organic wastes of municipal origin Properties of biochars (BC) varied with the precursor used and pyrolysis condition.
• Low temperature BC had high yield and charge but low pH, surface, carbon, and ash.
• High temperature BC had low yield and charge but high pH, surface, carbon, and ash.
• BC properties can be tailored to address specific soil amendment needs.

The fast economic and demographic growth lead to generation of large amounts of solid wastes placing Qatar on top of most nations with a per capita solid waste generation of nearly 2.5 million tons/year of which 60% is organic. A substantial amount of this solid waste ends up in landfills generating greenhouse gases that contribute to global warming. At the same time, the soil in most of the country is depleted and/or naturally poor. These issues can be addressed through conversion of this waste into biochar to improve soil quality and act as carbon sink. The objectives of this study were to (1) produce biochar from 4 different groups (paper, soft materials, hard wood, and mixed materials) at 3 different pyrolysis temperatures (300, 500, and 750 °C) and residence times (2, 4, and 6 h), and (2) evaluate biochars’ properties relevant to soil applications, namely physico-chemical properties [yield, pH, bulk density, ash, total surface area (TSA), surface charge (SC), and electrical conductivity (EC)] and elemental composition. Feedstocks were ground and pelleted then pyrolyzed under N2 using a Lindberg furnace equipped with a retort using the above conditions. Results showed that biochars’ pH, TSA, and ash content increased with temperature while the yield recovery and SC were higher at low temperature, with 94% biomass recovery observed for hard wood at 300 °C versus 23% at 750 °C. The pH of the four types of biochar increased from 5.7 at 300 °C for hard wood to 12 at 750 °C for mixed materials which make them suitable for a range of pH remediation in both acidic and alkaline soils. The TSA was limited in all biochars produced at 300 °C but reached 241 m2 g−1 and 163 m2 g−1 for hard wood and mixed materials produced at 750 °C, respectively. This suggests that biochars produced at high temperature can provide an internal surface area for soil microbiota while contributing to retention of water and nutrients. The C content increased as the temperature increased to reach 97% and 62% at 750 °C for HW and mixed materials, respectively, suggesting that biochars obtained at high temperature could increase the soil CEC and sequester carbon in the soil for long term. SEM analysis clearly showed the development of well-defined pores as the temperature increases. This study suggests that solid waste-based biochars have the potential to enhance soil properties, if produced under careful selection of precursor and pyrolysis conditions.