Sustainable use of Ca(OH)2 modified biochar for phosphorus recovery and tetracycline removal from water

To prevent possible secondary pollution from phosphorus-loaded biochar (BC) in agricultural systems, this study investigated the sustainable use of Ca(OH)2 modified wood biochars (Ca-BCs) for P recovery while significantly lowering the tetracycline (TC) adsorption onto Ca-BCs. Ca-BCs were prepared via calcination of mixtures of Ca(OH)2 and wood BC under 100-500 °C for removing P and TC from water. Compared to the pristine BC (without Ca(OH)2 modification), Ca-BC100 (Ca-BC prepared at 100 °C) showed a significant increase of P adsorption capacity from 4.00 to 138.70 mg/g due to reactive interaction between P and Ca(OH)2 on Ca-BC while decrease of TC adsorption capacity from 62.17 to 20.86 mg/g owing to decrease of surface area from 260.50 to 120.26 m2/g. Batch adsorption tests implied that the P adsorption on Ca-BC100 would occur mainly via electrostatic attraction (pH > 2.1) and formation of hydroxylapatite (Ca5(PO4)3(OH)) between phosphate and Ca(OH)2. In addition, Ca-BC100 reacted with TC via electrostatic attraction (pH > 7.6), complexation, hydrogen bond, and π-π interactions. P and TC adsorption onto Ca-BC100 was a chemical, endothermic, and spontaneous process. The dynamic adsorption experiments using a fixed bed column filled with Ca-BC100 indicated that Ca-BC100 could continuously and effectively remove P and TC from water. Ca-BC100 also effectively lowered P and COD in the dairy wastewater. Under the environmentally relevant conditions, continuous treatment of water containing P and TC using the pristine BC followed by Ca-BC100 showed the pristine BC removed 96% of TC and only 6% of P from water while Ca-BC100 made high recovery of P (94% of P) with negligible TC. Therefore, Ca-BC100 could be used for effective recovery of P with negligible TC from wastewater, and then applied to agricultural systems as a sustainable and safe P-rich biofertilizer.