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 (NH4+, PO43−, 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 (RS-L) and temperature. The release types of NH4+, K, Mg, and Mn were diffusion/dissolution, while those of Cu, Zn, Pb, Ni, and Cr were diffusion/resorption. The release types of PO43− and Ca varied with initial pH and RS-L, 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 NH4+, PO43−, K, Ca, Mg, Mn and Zn was a non-interfering, spontaneous (except PO43−), endothermic, and elevated randomness process. The release efficiency of NH4+, PO43− 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−1 and the initial pH was no lower than 3. In conclusion, ISB had potential as a slow-release fertilizer and adsorbent. •Release behavior of nutrients and HMs significantly relied on initial pH, RS-L and T.•Pseudo-secondary kinetic model better described the release of nutrients and HMs.•Their release was a non-interfering, spontaneous, endothermic and entropic process.•Release rates and HSEI confirmed ISB's potential as a slow-release fertilizer.•ISB could be as an adsorbent for water treatment (initial pH ≥ 3, dose