Efficient removal and recovery of phosphate by biochar loaded with ultrafine MgO nanoparticles

Biochar loaded with MgO is a promising adsorbent for the removal and recovery of phosphate from aqueous solutions. However, its phosphate adsorption capacity is unsatisfactory, especially at low phosphate concentrations. Loading nanoscale MgO onto biochar is an effective strategy. Here, ultrafine MgO nanoparticles and MgO nanosheets were loaded onto biochar from steam-exploded straw (UMB and SMB) via an impregnation-precipitation-pyrolysis method. The crystal sizes of ultrafine MgO nanoparticles and MgO nanosheets were about 6-8 nm and 10-16 nm, respectively. The phosphate adsorption capacity of UMB at C₀ = 100 mg P L^-1 was 219.4 mg P g^-1, which was higher than that of SMB (164.9 mg P g^-1). The results suggest that surface precipitation was the dominant adsorption mechanism and the hydration process and the smaller particle size of MgO may play a key role in the superior phosphate removal by UMB. Removal tests in real low-concentration phosphate water samples showed that 0.05 g L^-1 UMB could reduce the phosphate concentration from 0.17 mg P L^-1 to 0.01 mg P L^-1. In addition, phosphate could be desorbed from UMB in varying environments, and therefore has the potential to be used in fertilizer production or directly as a slow-release fertilizer.