Biochar-Reinforced Rubber Foam as an Adsorbent for Simultaneous Removal of Multiple Nutrients from Wastewater: Ion-Dependent Mechanisms and Engineering Performance.

Simultaneous removal of ammonium (NH4 +), nitrate (NO3 -), nitrite (NO2 -), and phosphate (PO4 3-) from secondary effluent remains challenging in advanced wastewater treatment. Biochar-reinforced natural rubber latex foam (NRLF) composites were fabricated via the Dunlop process as structured, particle-retaining adsorbents for multicomponent nutrient removal. Among formulations tested (0-8 phr biochar), the NR-B2 composite (2 phr biochar) achieved the most balanced simultaneous removal under optimal conditions (pH 7, 10 g·L-1, 28 °C), with PO4 3- removal reaching 92.58%. The pH-responsive, charge-switchable surface (pHpzc = 7.3) enabled concurrent cationic and anionic uptake within a single monolithic framework. Model fitting revealed maximum adsorption capacities (qm) of 5.50, 20.62, 1.27, and 0.03 mg·g-1 for NH4 +, PO4 3-, NO3 -, and NO2 -, respectively. Equilibrium was reached between 4 and 12 h depending on the specific ion. NH4 + followed pseudo-first-order kinetics and Langmuir isotherm behavior, while anionic species conformed to pseudo-second-order kinetics and Temkin isotherm behavior. Furthermore, thermodynamic evaluations indicated that the adsorption processes were spontaneous (ΔG° < 0) and endothermic (ΔH° > 0), consistent with predominantly weak-to-moderate adsorption interactions under the investigated conditions. Postadsorption BET, ATR-FTIR, and XPS analyses confirmed phosphorus-containing surface species and preserved structural integrity following nutrient uptake. The NR-B2 composite requires no chemical modification, eliminates biochar particle loss, and is directly applicable as a structured adsorptive medium for tertiary nutrient polishing.