Enhancing Dissolved Phosphorous Capture by Gypsum-Incorporated Biochar: Synergic Performance and Mechanisms

Excess nutrients such as phosphorus (P) in watersheds jeopardize water quality and trigger harmful algal blooms. Capturing nutrients from wastewater and agricultural runoff is an essential step to develop a circular nutrient system, which can not only recover lost nutrients, but also prevent water pollution. In this study, a novel designer biochar was generated by pyrolyzing woody biomass pretreated with a flue gas desulfurization gypsum. The removal of dissolved P by the designer biochar was more efficient than the gypsum, suggesting the pretreatment of biomass with the gypsum results in a significantly synergic effect on enhancing dissolved P removal. The sorption kinetics study revealed that the designer biochar requires much less time to reach sorption equilibrium compared to the gypsum. The maximum adsorption capacity of dissolved P on the designer biochar was greater than 200 mg g-1 according to Langmuir or Langmuir-Freundlich model, which is significantly higher than that of the gypsum. These results clearly showed that designer biochar is a better phosphorus sorption material (PSM) to capture dissolved P from nutrient-containing water compared to the gypsum. Post-sorption characterization explored the mechanisms of enhancing dissolved P removal by the designer biochar. Although the sorption of dissolved P involves multiple mechanisms, the precipitation reactions of Ca ions and P anions to form CaHPO4and Ca3(PO4)2 precipitates on the highly alkaline surface of the designer biochar were identified as a main mechanism. By contrast, CaHPO42H2O is the only precipitated product for dissolved P sorption by the gypsum. In addition, the initial solution pH and the coexisting bicarbonate (HCO3-) had less effects on the dissolved P removal by the designer biochar in comparison with the gypsum, which further confirms the former is an excellent PSM to capture dissolved P from a variety of aquatic media. 2023, The Authors. All rights reserved.