Natural material-modified absorbent for phosphorus removal from waters

Excessive phosphorus accumulation in reservoirs is a major driver of eutrophication, necessitating sustainable remediation strategies that mitigate the limitations associated with metal-modified adsorbents. We propose a biochar-laterite-bentonite (BLB) composite as a synergistically engineered and low-cost natural adsorption system for phosphorus removal from waters. The scientific contribution of this work lies in demonstrating that effective phosphorus adsorption can be achieved through modification using natural materials, without reliance on rare-earth or heavy metal-modified adsorbents. By combining the high porosity of biochar, the structural stability and cation-exchange properties of bentonite, and the Fe/Al oxide-rich surfaces of laterite, the composite achieves enhanced adsorption through controlled exposure of active sites. Batch experiments identified BLB (4:4:1) as the optimal formulation, achieving an experimental adsorption capacity of 19.24 mg/g and 76.96% removal efficiency under batch conditions (10 mg/L of P, pH 6.5). Kinetic analysis was well described by both pseudo-first-order and pseudo-second-order models ( $R^2>0.99$ ), whereas the Langmuir model best described the adsorption isotherm. Langmuir isotherm modeling predicted a maximum monolayer capacity ( $q_{\max }$ ) of 48.39 mgP/g at 45 °C ( $R^2=0.9567-0.9948$ ). Spectroscopic characterization indicated that phosphate uptake could be governed primarily by inner-sphere Fe/Al-O-P complexation, with precipitation contributing under selected conditions. Competitive inhibition experiments showed the interference order ${\text{CO}}_3^{2-}>{\text{SO}}_4^{2-}>{\text{NO}}_3^{-}>{\text{Cl}}^{-}$ at constant conditions (temperature, P concentration, and pH), indicating that multivalent anions compete strongly for Fe/Al adsorption sites. Collectively, this study provides mechanistic insight into how modified-composite design with natural materials can enhance phosphorus adsorption, offering a sustainable and scalable alternative to conventional metal-modified adsorbents for eutrophication control.