Synthesis of Mn/Al double oxygen biochar from dewatered sludge for enhancing phosphate removal

Abstract This study presents a simple one-step hydrothermal method to produce a new Mn/Al double oxygen sludge-derived biochar (MA-SBC) from dewatered sewage sludge, which is used for enhancing phosphate removal. Characterization results showed that a cubic spinel-type MnAl2O4 was successfully embedded in the matrix of MA-SBC. A series of systematically designed batch experiments were performed to evaluate the effect of initial pH, contact time, and initial phosphate concentration on the adsorption behavior of MA-SBC. The results revealed that the adsorption process of phosphate onto MA-SBC conformed to a pseudo-second-order model, and the rate of adsorption was mainly dominated by a mechanism of film diffusion. A Langmuir isotherm model accurately described the adsorption isotherm, with a maximum adsorption amount of 28.20 mg-P/g at 298 K (50 mg/L for the initial phosphate solution), which was more efficient than those of previously reported composites. Moreover, X-ray diffraction, Fourier transform infra-red and X-ray photoelectron spectroscopy analyses proved that the mechanism of phosphate removal from aqueous media were the inner-sphere surface complexation reaction of phosphate with ≡M-OH groups and a subsequent precipitation reaction of aluminum phosphate. Cycle stability and metallic ion leaching tests demonstrated the strong stability of MA-SBC. In general, the results of this research suggest that MA-SBC has potential applicability for removing phosphate from aqueous media as an alternative, eco-friendly, and economic adsorbent for environmental restoration.