Removal of Heavy Metals From Aqueous Solutions Using Biochar Derived From Agricultural Waste: Kinetics, Mechanisms, and Cost Analysis.

Abstract

This study evaluates the adsorption performance and mechanisms of Pb(II), Cu(II), Cd(II), and Zn(II) removal from water using coconut-fiber-derived biochar (CFBC). The physicochemical properties of CFBC were characterized by proximate and ultimate analyses, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET) analysis, and determination of the point of zero charge (pHpzc). Batch experiments were conducted to investigate the effects of contact time, solution pH, adsorbent dosage, and competing metal ions on removal efficiency. Under optimal conditions, removal efficiencies reached 88.21%-93.48% after 480 min at a CFBC dose of 5 g/L and pH 6.0-7.0. Competitive adsorption decreased the equilibrium adsorption capacities of all four metals, with the adsorption selectivity following the order Pb(II) > Cu(II) > Cd(II) > Zn(II). Adsorption kinetics were analyzed using four models: pseudo-first-order (PFO), pseudo-second-order (PSO), Weber-Morris intraparticle diffusion, and the Boyd model. In addition, adsorption isotherms were evaluated using both linear and nonlinear forms of the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models. The results indicate that the PSO model best describes the adsorption kinetics. Within the investigated range, adsorption proceeds through a complex process involving simultaneous intraparticle diffusion and film diffusion. Equilibrium data fit the Langmuir model best, with maximum monolayer adsorption capacities (qmax, mg/g) of Pb(II) (35.95 ± 2.94), Cu(II) (26.73 ± 1.39), Cd(II) (19.22 ± 1.36), and Zn(II) (18.90 ± 2.45). Cost analysis suggests that CFBC is a low-cost and environmentally friendly adsorbent, indicating strong potential for application in the treatment of heavy-metal-contaminated wastewater.