Activation Chemistry-Driven Enhancement of Calotropis procera -Derived Fe3O4/Activated Carbon Nanocomposites for Efficient Treatment of Paint Industry Wastewater

Abstract The wastewater generated from the paint industry consists of very complicated organic contaminants that result in higher biochemical oxygen demand (BOD) and chemical oxygen demand (COD). In order to solve such wastewater problems, there is a need to develop efficient techniques. This paper focuses on the preparation of Fe3O4/activated carbon (AC) nanocomposites via activation by H3PO4 and KOH and comparative analysis of efficiency in treating paint industry wastewater. For characterization purposes, the obtained nanocomposites were analyzed using UV–vis spectroscopy, dynamic light scattering, X-ray diffraction, Brunauer–Emmett–Teller (BET) surface analysis, vibrating sample magnetometry, thermogravimetric analysis, scanning electron microscopy, and Fourier-transform infrared spectroscopy. It was found from BET analysis that H3PO4-AC/Fe3O4 had a higher surface area of 393.58 m2 g–1 compared to KOH-AC/Fe3O4 with a surface area of 368.92 m2 g–1. Both exhibited mesoporous structures with pore diameters of 2.98 and 2.87 nm, respectively. The highest efficiency (95.05% BOD and 97.34% COD) was recorded under optimal conditions (30 min; 2 mg of adsorbent; pH 4 for H3PO4-AC/Fe3O4 and pH 6 for KOH-AC/Fe3O4). Reusability studies demonstrated sustained adsorption performance over eight cycles, with H3PO4-AC/Fe3O4 exhibiting superior regeneration stability and pollutant removal efficiency compared to KOH-AC/Fe3O4. According to the Langmuir isotherm analysis, the highest adsorption capacities (165.93 mg g–1 of BOD and 213.94 mg g–1 of COD) were achieved using H3PO4-AC/Fe3O4, whereas the pseudo-first-order kinetics modeling revealed the best fit (R2 = 0.99936–0.99992). The process of intraparticle diffusion contributed to the adsorption but did not control the process. Thermodynamic parameters proved the spontaneous nature of the adsorption process as being endothermic and driven by entropy. The proposed mechanism of adsorption included the following processes: film diffusion, pore filling, intraparticle diffusion, hydrogen bonding, π–π interaction, van der Waals interactions, and interactions with oxygen-containing surface groups. These findings demonstrate that phosphoric acid activation significantly enhances the physicochemical properties and adsorption performance of biomass-derived Fe3O4/AC nanocomposites for efficient treatment of complex industrial wastewater.