Characteristics of nutrients and heavy metals release from sewage sludge biochar produced by industrial-scale pyrolysis in the aquatic environment and its potential as a slow-release fertilizer and adsorbent.

To assess the application potential of sewage sludge biochar produced by industrial-scale pyrolysis (ISB), the release characteristics of nutrients (NH, PO, K, Ca, Mg and Fe) and heavy metals (Mn, Cu, Zn, Pb, Ni and Cr) were investigated. Their release amounts increased with decreasing initial pH and increasing solid-liquid ratios (R) and temperature. The release types of NH, K, Mg, and Mn were diffusion/dissolution, while those of Cu, Zn, Pb, Ni, and Cr were diffusion/resorption. The release types of PO and Ca varied with initial pH and R, respectively. The chemical actions played dominant roles in their release, while particle surface diffusion and liquid film diffusion determined the rates of diffusion and resorption phases, respectively. The release of NH, PO, K, Ca, Mg, Mn and Zn was a non-interfering, spontaneous (except PO), endothermic, and elevated randomness process. The release efficiency of NH, PO and K met the Chinese standard for slow-release fertilizers, while the total risk of ISB was low. The eutrophication and potential ecological risks of ISB were acceptable when the dose was less than 3 g L and the initial pH was no lower than 3. In conclusion, ISB had potential as a slow-release fertilizer and adsorbent.