Migration and transformation of heavy metals during the microwave-assisted thermal hydrolysis of sewage sludge

The migration, transformation and ecological risk of heavy metals (Cr, As, Ni, Cu, Zn, Cd and Pb) in sewage sludge during the microwave-assisted thermal hydrolysis process were investigated under different temperatures (80 °C, 100 °C, 120 °C, 140 °C, and 160 °C). The potential relationship between the bio-availability of heavy metals and the variables of microwave treatment, including pH, ammonium-nitrogen, soluble chemical oxygen demand, pH, soluble protein, soluble polysaccharide and volatile solids, was also explored. The results showed that the migration of heavy metals between solid–liquid phase mainly depended on the temperature. The percentage of all heavy metals (except Cu) in mobile (acid-soluble/exchangeable and reducible) forms decreased after microwave-assisted thermal hydrolysis treatment. The solubilisation of compounds with C = O and O-H accompanied with the generation of organic and inorganic metal halides were also observed in the treated sludge through Fourier transform infrared spectroscopy analysis. NH4+-N showed the highest negative correlation to the bio-availability of most heavy metals (except Cu and Cr) with coefficients (absolute value) over 0.87 (P < 0.05). VS showed a positive correlation to the bio-availability of most heavy metals (except Cu). The total potential ecological risk index (RI) decreased by 46.65% after microwave treatment at 160 °C. HIGHLIGHTS Effect of microwave treatment on heavy metals speciation in sludge was reported.; The metal halides related to organic and inorganic compounds were observed.; NH4+-N showed the greatest correlation to the bio-availability of most heavy metals.; Environmental risk of treated sludge after microwave treatment was alleviated.