Bioavailability and eco-toxicity of heavy metals in chars produced from municipal sewage sludge decreased during pyrolysis and hydrothermal carbonization

Abstract

The rapid increase in municipal sewage sludge production due to quick urbanization and economic development raised significant socio-economic and environmental issues in recent years. Although the increase in heavy metals' (HMs) immobilization through pyrolysis and hydrothermal carbonization (HTC) of municipal sewage sludge are reported in previous investigations, but there is still limited information available regarding the potential environmental risks associated with these processes. This study investigated the chemical speciation, bioavailability and eco-toxicity of HMs (Zn, Cu, Ni and Pb) in biochars (pyrolysis temperatures of 450 °C and 600 °C) and hydrochars (HTC temperature of 180 °C) produced from two sludge feedstocks (sludge cake and stabilized sewage sludge). The HTC generally resulted in higher char yield compared to pyrolysis process. The raw sewage samples showed a flat morphological surface texture, while HTC process increased the porosity and surface area of produced hydrochars. The pyrolysis process also increased the porous structures and surface morphology of biochars, with higher porosity in biochars produced at 600 °C. The pH value of hydrochars was significantly (P < 0.05) lower than the sludge feedstock, while pyrolysis process significantly (P < 0.05) increased the pH value of produced biochars. The H/C and O/C ratios of char products were gradually decreased by increasing production temperature, illustrating the formation of higher aromaticity and stronger carbonization. The concentration order of HMs in sludge feedstock was Zn > Cu > Pb > Ni, and with thermochemical processes the concentration of HMs increased in biochar and hydrochar products. The enrichment of HMs in all biochars was significantly (P < 0.05) higher than hydrochars. The concentration of HMs significantly (P < 0.05) reduced in bioavailable fraction and increased in stable fraction during pyrolysis and HTC processes. Consequently, the potential environmental risk and eco-toxicity of HMs significantly (P < 0.05) decreased in hydrochars and biochars with low or no environmental risk of direct land application.