Concurrent removal of phosphate and ammonium from wastewater for utilization using Mg-doped biochar/bentonite composite beads

Abstract

This study reports a strategy for concurrent removal of phosphate and ammonium by a new adsorbent of Mg-doped biochar/bentonite composite bead (SA-Mg@BC/BT), with a high specific surface area and abundant functional groups to facilitate the adsorption process. The maximum phosphate and ammonium adsorption capacities of SA-Mg@BC/BT reached 132.2 mg/g and 39.5 mg/g respectively, apparently surpassing those of SA-BC/BT in this study and other relevant adsorbents in literatures. The removal ability for phosphate kept stable with coexisting monovalent anions (NaCl, MgCl2, KNO3 and KF), while decreased slightly in the presence of divalent anions (MgSO4, CaSO4 and K2SO4). Interestingly, the adsorption capacity for the ammonium removal was even found enhanced by divalent salts (12.0%, 23.9% and 15.0 for MgCl2, MgSO4 and CaSO4 respectively) and K2SO4 (17.9%). For the actual wastewater, the excellent concurrent removal ability of 98.3% for phosphate and 35.6% for ammonium was validated, and adsorption capacity in actual wastewater was almost the same as that in simulated wastewater. Such an excellent concurrent removal ability for phosphate and ammonium by SA-Mg@BC/BT can be ascribed to the struvite crystallization, ion exchange and Mg-P complexation formation. Finally, the leaching and pot experiments indicated the slow-release ability of the adsorption saturated SA-Mg@BC/BT as a fertilizer, providing a new possibility of nutrient recovery.