Simultaneous recovery of phosphate, ammonium and humic acid from wastewater using a biochar supported Mg(OH)2/bentonite composite

Abstract

With the motivation of achieving resource reclamation and its sustainable utilization as fertilizer, a biochar supported Mg(OH)₂/bentonite composite (PMRB) was prepared using an integrated precipitation-pyrolysis method for the simultaneous capture of phosphate, ammonium and humic acid from wastewater. The results showed that PMRB was composed of Mg(OH)₂ nanoflakes and a porous bentonite-biochar frame produced by the pyrolysis of rice husks as a pore-generation agent. The adsorption process by PMRB did not need additional pH adjustment, and PMRB presented maximum nutrient adsorption capacities of 125.36 mg g⁻¹ for phosphate, 58.20 mg g⁻¹ for ammonium and 34.57 mg g⁻¹ for humic acid at pH 8.0. A pseudo-second order equation and the Weber–Morris model were able to accurately describe the kinetics of phosphate, ammonium and humic acid adsorption by PMRB. A mechanistic study revealed that the adsorption mechanism involved struvite crystallization, co-precipitation, π–π interactions, ion exchange, electrostatic interactions and inner-sphere complexation. The recovered precipitate (HMCC) contained approximately 13% P₂O₅ and the production cost of HMCC was estimated as 0.16 $ per kg. The results and concept from this study suggest a new sustainable and economical strategy to synthesize and apply PMRB to simultaneously recycle nutrients and fertilize soils using the recovered precipitate.