Functionalizing Biochar with Layered Double Hydroxides for Phosphate Removal from Aqueous Solutions

Overview

Both biochar and layered double hydroxides (LDH) have received substantial attention for their abilities to adsorb chemical contaminants, yet little research has been reported on LDH/biochar composites for phosphate (P) removal from aqueous solutions. To test my hypothesis that biochar functionalized with LDH may provide a novel absorbent for treatment of phosphate-laden waters, LDH/biochar composites were synthesized through liquid phase co-precipitation of Mg-Al and Mg-Fe LDH on varying amount of biochar derived from bamboo biomass. A phosphate absorption screening test indicated that after being functionalized with LDH, the biochar exhibited much higher phosphate absorption than its pristine counterpart and the enhancement of adsorption with 40% LDH in the composites was more than with 25% LDH. Mg-Al LDH/biochar composites had higher affinity to phosphate than Mg-Fe LDH/biochar composites. Kinetics adsorption experiments of the 40% Mg-Al LDH/biochar composite with 10 and 50 mg P/L solutions indicated that over 95% of adsorption was completed within 1 h of contact time, following the second-order kinetics model. Solution pH increased with contact time, indicating releases of hydroxide accompanied phosphate adsorption. The adsorption isotherm suggested a shift from chemisorption/inter-layer ion exchange at low phosphate concentrations to precipitation at high phosphate concentrations. The adsorption characteristics confirmed that the LDH/biochar composites can serve as an effective adsorbent for wastewater treatment. A final lettuce seedling bioassay further suggested that the spent LDH/biochar composites can be recycled as a slow-release fertilizer to enhance vegetation growth.