The evaluation of immobilization behavior and potential ecological risk of heavy metals in bio-char with different alkaline activation.

Abstract

The bio-char was prepared by co-pyrolysis of municipal sewage sludge and biomass with chemical activation. The alkaline activating agents of KOH and K2CO3 were used to develop multilevel pore structure without heavy metal. The proximate analysis, ultimate analysis, SEM, and surface area and porosity analyzer were applied to present the physico-chemical properties and multilevel pore structure of bio-char. After impregnation pretreatment, the KOH provided more functional ingredients and reacted with C to expand pore structure for bio-chars. It was confirmed the specific surface area reached 2122.43 m2/g, and micropore area was 1674.85 m2/g after co-pyrolysis at 800 °C. Through the pretreatment of alkaline activation, the novel evaluation of heavy metal immobilization behavior in bio-char matrix were investigated by BCR sequential extraction and leaching tests. The KOH activation showed prominent immobilization behavior relatively, and the K2CO3 activation had more noticeable effects on leaching behavior. For Cu, Ni, Cr, Cd, Pb, and Zn, after co-pyrolysis at 900 °C, the proportion of unstable fraction decreased significantly, and the residual fractions of heavy metals were above 89.44% according to BCR sequential extraction procedure. Under optimal pyrolysis temperature, the Er value of bio-char reduced to 41.93, and the potential ecological risks decreased from considerable risk to low risk to ensure the further eco-friendly application.