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 the sewage sludge during the microwaveassisted thermal hydrolysis process were investigated under different temperatures (80 degrees C, 100 degrees C, 120 degrees C, 140 degrees C, and 160 degrees 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 solid, 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 solubilization 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 degrees C.

Automated summary

The study found that the migration of heavy metals between solid-liquid phase mainly depended on temperature, and the percentage of mobile forms of heavy metals decreased after treatment. NH4+-N showed a strong negative correlation with the bio-availability of most heavy metals, while VS had a positive correlation. The total potential ecological risk index decreased significantly after microwave treatment at 160 degrees C.