Recovery of phosphate from aqueous solution by dewatered dry sludge biochar and its feasibility in fertilizer use

Discharging of dewatered dry sludge into the environment can trigger irreversible environmental damage. The treatment of dewatered dry sludge into a valuable biochar product is essential for the sustainability of resources and environmental safety. In this study, we examined the removal of phosphate from water bodies by adsorption of biochar that had been prepared from dewatered dry sludge with different pyrolysis temperatures. We showed that being a composite material rich in carbon, CaO and MgO were produced in the biochar preparation when the pyrolysis temperature was increased to 700 °C. The phosphate adsorption of biochar has strengthened with the increase of pH. The phosphate adsorption of composite was fitted with the pseudo-second-order kinetic model, while the Langmuir adsorption isotherm model yielded a maximum phosphate adsorption of 51.79 mg/g. The phosphate adsorption by sludge biochar was mainly affected by the deposition of phosphate crystals and electrostatic attraction. Overall, biochar prepared from dewatered dry sludge demonstrated acceptable phosphate removal performance and phosphate-loaded biochar had a slow release of phosphorus, therefore, can be used as a phosphate fertilizer to promote plant growth.