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

Abstract

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.