A stepwise microwave synergistic pyrolysis approach to produce sludge-based biochars: Optimizing and mechanism of heavy metals immobilization

Abstract

A technical approach of stepwise microwave synergistic pyrolysis (SMSP) had been proposed to produce value-added sludge-based biochar. The approach includes a conventional pre-pyrolytic stage and a sequential microwave pyrolysis stage. It is vital to adjust conventional pre-pyrolysis parameters (i.e. temperature and time) to improve immobilization effect of heavy metals and decrease toxicity of sludge-based biochar. In the paper, sludge-based biochars were generated from different pre-pyrolysis temperature (400 ∼ 700 °C) and time (5 ∼ 20 min) by the SMSP approach. For heavy metals (Cu, Zn, Ni, Pb) in biochars, residual ratio, leaching ratio, chemical fractions and ecotoxicity were measured by using microwave digestion method, toxicity characteristic leaching procedure (TCLP) method, modified european community bureau of reference (BCR) method and risk assessment code (RAC) method respectively. The results demonstrated that 600 ∼ 700 °C and 10 ∼ 20 min are more suitable pre-pyrolysis temperature and time for immobilization of heavy metals via vitrification. Heavy metals in optimum sludge-based biochar (SBC) would have high residual ratio, low leaching ratio and low ecotoxicity risk simultaneously. Residual ratio of heavy metals in SBC are about 15% higher than that in optimum conventional sludge-based biochar (CSC). And leaching ratio of heavy metals in SBC are about 60% lower than that in CSC. These are mainly attributed to the shortened pyrolysis time and the high-energy sites effect induced by microwave heating.