Release behavior of fertilizers and heavy metals from iron-loaded sludge biochar in the aqueous environment

In this study, the release behavior of fertilizers (NH4+-N, PO43- and K) and heavy metals (Mn, Zn, Ni, Cu, Pb and Cr) from iron-loaded sludge biochar (ISBC) was investigated to evaluated the feasibility and risks of ISBC as a slow release fertilizer. Their release capacity was significantly enhanced with decreasing initial pH, increasing solid-liquid ratio (RS-L) and rising temperature (p < 0.05). When the initial pH, RS-L and temperature were separately 5 (fertilizers)/1 (heavy metals), 1:5 and 298 K, the final concentrations of NH4+-N, PO43-, K, Mn, Zn and Ni were 6.60, 14.13, 149.4, 53.69, 72.56, and 1.01 mg L-1, while the maximum concentrations of Cu, Pb and Cr were 0.94, 0.77, and 0.22 mg L-1, respectively. Due to the tiny difference between the R2 values, revised pseudo-first-order and pseudo-second-order kinetics models described their release behavior well, suggesting that physical and chemical interactions played an important role. Activation energies greater than 40 kJ mol-1 indicated that the rate-controlling steps of the release of NH4+-N, PO43- and Ni were chemical reactions, while chemical reactions and diffusion together determined the release rates of K, Mn, Zn, Cu, Pb and Cr because their activation energies were in the range of 20-40 kJ mol-1. The increasingly negative ΔG and positive ΔH and ΔS suggested that their release was a spontaneous (except Cr) and endothermic process with an increase of randomness between the solid-liquid interface. The release efficiency of NH4+-N, PO43- and K were in the ranges of 28.21%-53.97%, 2.09%-18.06% and 39.46%-66.14%, respectively. Meanwhile, the pollution index and evaluation index of heavy metals were in the ranges of 33.31-227.4 and 4.64-29.24, respectively. In summary, ISBC could be used as a slow-release fertilizer with low risk when the RS-L was less than 1:40.