Effects of fly ash modification on phosphorus adsorption of sludge-based biochar from aqueous solution

Fly ash (FA) was used to modify sludge-based biochar (SBC) to improve its phosphorus (P) adsorption capacity, which was conducted by adding FA in the dried sewage sludge before 600 °C pyrolysis. FA modification increased the surface area of SBC, decreased its average pore diameter, increased its Al and Fe contents, improved its crystallinity, and enhanced its maximal P adsorption capacity from 12.37 mg adsorbed P/g SBC (mg/g) to 14.05 mg/g. The P adsorption process was well fitted by the pseudo-second-order kinetic and Langmuir isotherm models, indicating that P adsorption onto SBC was a single molecular layer adsorption dominated by chemical adsorption. The P adsorption onto SBCs was spontaneous and endothermic. FA modification improved the reusability of SBC by increasing the retention rate from 74.9 ± 5.8% to 80.8 ± 5.4% after four adsorption and desorption cycles. FA modification changed the P adsorption mechanism of SBC from ligand exchange, electrostatic attraction, and ion exchange to surface inner-sphere complex formation, electrostatic attraction, and ion exchange.