The Fate of Heavy Metals and Risk Assessment of Heavy Metal in Pyrolysis Coupling with Acid Washing Treatment for Sewage Sludge

Abstract

Pyrolysis is an emerging and effective means for sludge disposal. Biochar derived from sludge has broad application prospects, however, is limited by heavy metals. In this study, the fate of heavy metals (HMs) in pyrolysis coupling with acid washing treatment for sewage sludge was comprehensively investigated for the first time. Most of the HMs redistributed in the pyrolyzed residues (biochar) after pyrolysis, and the enrichment order of the HMs was: Zn > Cu > Ni > Cr. Compared with various washing agents, phosphoric acid presented a superior washing effect on most heavy metals (Cu, Zn, and Cr) in biochars derived at low pyrolysis temperature and Ni in biochars derived at high pyrolysis temperature. The optimal washing conditions for heavy metals (including Cu, Zn, Cr, and Ni) removal by H3PO4 were obtained by batch washing experiments and the response surface methodology (RSM). The total maximum HM removal efficiency was 95.05% under the optimal washing specifications by H3PO4 (acid concentration of 2.47 mol/L, L/S of 9.85 mL/g, and a washing temperature of 71.18 °C). Kinetic results indicated that the washing process of heavy metals in sludge and biochars was controlled by a mixture of diffusion and surface chemical reactions. After phosphoric acid washing, the leaching concentrations of HMs in the solid residue were further reduced compared with that of biochar, which were below the USEPA limit value (5 mg/L). The solid residue after pyrolysis coupling with acid washing resulted in a low environmental risk for resource utilization (the values of the potential ecological risk index were lower than 20). This work provides an environmentally friendly alternative of pyrolysis coupling with acid washing treatment for sewage sludge from the viewpoint of the utilization of solid waste.

Advances in sewage sludge application and treatment: Process integration of plasma pyrolysis and anaerobic digestion with the resource recovery

“…HMs transportation via different applications of pyrolysis byproducts was feasible [ 106 ]. Low environmental risk for resource use was achieved by coupling pyrolysis with acid washing (possible ecological risk index values were less than 20) [ 143 ]. In their study, they reported the ideal washing conditions by H 3 PO 4 (acid content of 2.47 mol/L, L/S of 9.85 mL/g, and a washing temperature of 71.18 °C), the overall maximum HM removal efficiency was 95.05%.…”

“…Although, high temperature operational condition could induce inertization process of the HMs in the SS valorization, but the HMs transportation via different applications of pyrolysis byproducts are still feasible [ 106 ]. The HMs can be significantly reduced with combination of plasma pyrolysis with chemical process such as acid washing with H 2 PO 4 [ 143 ]. As reported by some studies, during plasma pyrolysis process some small amount of harmful gas can be emitted and also owing to the process parameters can increase or decrease hydrogen in syngas produced.…”

Modulatory Role of Biochar Properties and Environmental Risk of Heavy Metals by Co-Pyrolysis of Fenton Sludge and Biochemical Sludge

“…Studies have demonstrated the effectiveness of sludge pyrolysis in reducing sludge volume and using organic matter to produce sludge biochar at minimal cost [ 14 ]. Concurrently, pyrolysis of sludge has been shown to efficiently immobilize HMs [ 15 ], thereby lowering their bioavailability in sludge [ 16 ] and facilitating the reuse of sludge products [ 17 ]. Since the co-pyrolysis process of sludge produces catalytic and synergistic effects, the resulting co-pyrolysis biochar has found diverse applications in environmental treatments, including soil improvement [ 18 , 19 ], pollutant adsorption [ 20 ], and catalytic reactions [ 21 ].…”

Effects of woodland slope on heavy metal migration via surface runoff, interflow, and sediments in sewage sludge application

“…Urbanization and global industrialization have led to increases in the production of sewage sludge (SS). SS production has thus become a major environmental issue 1 , 2 . Approximately 120 million tons of SS were generated by the major economies worldwide in 2019 3 , and the amount of SS generated by these economies is projected to be between 150 and 200 million tons in 2025.…”