Using sewage sludge with high ash content for biochar production and Cu(II) sorption

The ash content of municipal sewage sludge is generally high. However, the manner in which the composition of ash affects biochar properties and sorption remains unclear. Sewage sludge from two cities, Chongqing and Kunming, were pyrolyzed at different temperatures to produce biochar in this work. The physicochemical properties of biochar were investigated by bulk chemical characteristics (such as FTIR, XPS, Raman analysis, and elemental analysis) and benzene polycarboxylic acid (BPCA) molecular biomarkers, after which they were correlated with sorption characteristics. In comparison with biochar from Chongqing sewage sludge (CSS), biochar from Kunming sewage sludge (KSS) showed stronger polarity, a larger specific surface area (SSA) and more functional groups, but a lower degree of graphitization and aromatization. These differences may result from the higher aluminum (Al) content of KSS. The single-point sorption coefficient Kd values of biochar derived from CSS and KSS were analyzed together. Kd was positively correlated with the SSA and pore volume of sewage sludge and biochar produced at 200–300 °C. For biochar produced at 300–700 °C, the Kd value was positively correlated with the O content, O/C and (O + N)/C. The pyrolysis temperature of 300 °C was a threshold temperature for Cu(II) sorption onto biochar, at which there was a balance between decreased oxygen-containing functional groups and increased SSA. The findings of this study show that higher Al content in sewage sludge was beneficial to pore volume enlargement and functional group retention during the pyrolysis process. [Display omitted] •Higher Al content enhanced pore volume and functional group retention.•B6CA/BPCA was positively correlated with the IG/ID in the Raman spectra.•300 °C was a threshold for biochar properties and sorption characteristics.•Sorption of Cu(II) on biochar was mainly limited by SSA below 300 °C.•Sorption of Cu(II) on biochar was dominated by functional groups above 300 °C.