Co-pyrolysis of sewage sludge/cotton stalks with K_2CO_3 for biochar production: Improved biochar porosity and reduced heavy metal leaching

摘要

The co-pyrolysis of sewage sludge and biomass is considered a promising technique for reducing the volume of sewage sludge, adding value, and decreasing the risk associated with this waste. In this study, sewage sludge and cotton stalks were pyrolyzed together with different amounts of K_2CO_3 to evaluate the potential of chemical activation using K_2CO_3 for improving the porosity of the biochar formed and immobilizing the heavy metals present in it. It was found that K_2CO_3 activation effectively improved the pore structure and increased the aromaticity of the biochar. Moreover, K_2CO_3 activation transformed the heavy metals (Cu, Zn, Pb, Ni, Cr, and Cd) into more stable forms (oxidizable and residual fractions). The activation effect became more pronounced with increasing amount of added K_2CO_3, eventually resulting in a significant reduction in the mobility and bioavailability of the heavy metals in the biochar. Further analysis revealed that, during the co-pyrolysis process, K_2CO_3 activation resulted in a reductive atmosphere, increased the alkalinity of the biochar, and led to the formation CaO, CaCO_3, and aluminosilicates, which aided the immobilization of the heavy metals. K_2CO_3 activation also effectively reduced the leachability, and thus, the environmental risks of the heavy metals. Thus, K_2CO_3 activation can improve the porosity of the biochar derived from sewage sludge/cotton stalks and aid the immobilization of the heavy metals in it.