Humic acid–Fe–kaolinite composite for robust phosphate removal from complex wastewater matrices

Abstract Consistently achieving high phosphate removal across diverse water chemistries remains a significant obstacle in adsorption-based nutrient management. This study developed a humic acid–Fe–kaolinite (HA–Fe–Ka) composite integrating Fe-bearing reactive phases, humic functional groups, and a kaolinite mineral framework. The composite was generated via ferrihydrite coating followed by humic acid immobilization and characterized using FTIR, XRD, SEM–EDX, and BET analyses. Structural characterization indicated retention of the kaolinite framework together with Fe-bearing phases and humic functional groups, yielding a mesoporous material with a surface area of 52.7 m 2 g −1 . Batch adsorption trials showed phosphate removal of 88.1–96.7% across pH 3–11, with the highest removal at pH 7. Adsorption kinetics were best described by the pseudo-second-order model (R 2 ≈ 1.00). Langmuir fitting gave R 2 = 0.9956, qmax = 89.206 mg g −1 , and K L = 0.0749 L mg −1 . Thermodynamic analysis based on the reported distribution coefficients gave ΔG values of − 1.68 to − 4.91 kJ mol −1 for HA–Fe–Ka, with ΔH = + 10.36 kJ mol −1 and ΔS = + 43.77 J mol −1 K −1 , consistent with an endothermic adsorption process. Application to phosphate-affected waters in Saudi Arabia gave removal efficiencies of 79–84% in freshwater matrices and 55–61% in marine matrices. Overall, the results identify HA–Fe–Ka as a promising sorbent for phosphate removal from chemically complex waters.