Valorization of fly ash and GGBS derived green concrete composite for simultaneous removal of nitrate and phosphate from aqueous systems

Abstract

Nutrient pollution caused by elevated nitrate (NO₃⁻) and phosphate (PO₄³⁻) concentrations is a major driver of eutrophication, while growing volumes of fly ash and ground granulated blast furnace slag (GGBS) from industrial activities presents pressing solid waste challenge. This study introduces dual-functional M40 grade of Green Concrete Composite (GCC) that functions both as structural material and as adsorbent for water treatment. GCC was fabricated by alkali-activating fly ash and GGBS, producing porous, amorphous aluminosilicate framework with tailored surface chemistry, hydroxyl and silicate groups, minor crystalline phases, thermal stability, and high point of zero charge (pH_pzc). Batch adsorption experiments examined effect of contact time (0–360 min), GCC dosage (0.5–10 g/L), initial concentration (1–100 mg/L), and solution pH (3–11). Maximum adsorption capacities of 14.08 mg/g for NO₃⁻ and 4.13 mg/g for PO₄³⁻ were achieved under optimal conditions (pH 6.7, 3 g/L and 60 min). Adsorption followed pseudo-second-order kinetic model and Langmuir isotherm, indicating chemisorption. Mechanistic analysis revealed NO₃⁻ removal occurred via electrostatic attraction and ion exchange, whereas PO₄³⁻ uptake involved electrostatic interactions, ligand exchange, and Ca²⁺ induced precipitation. In multi-pollutant systems, synergistic adsorption enhanced removal to 22.37 mg/g (NO₃⁻) and 12.90 mg/g (PO₄³⁻). GCC exhibited potential for regeneration and reusability over three cycles, with desorption efficiencies gradually decreasing for both pollutants across successive cycles. These findings highlight GCC as novel, low-cost, regenerable, and environmentally sustainable alternative to conventional adsorbents, presenting an integrated approach to water purification and industrial waste valorization.