Cadmium removal from water by green synthesized nanobioadsorbent [SiO2@DOPP]: Mechanism,isotherms, kinetics and regeneration studies

In this study, dried orange peel powder DOPP] is chemically modified with nanosilica (SiO₂) employing sonication technique to produce nanobioadsorbent SiO₂@DOPP]. SiO₂@DOPP] This nanoadsorbent was evaluated for Cd²⁺ removal from aqueous systems. Successful functionalization of DOPP] into nanosilica was confirmed by various analytical techniques like XRD, FTIR, SEM, EDX, TEM, DLS, pH_zpc and TGA. XRD, FTIR and EDX confirmed the emergence of new peaks after modification of DOPP] by nanosilica and adsorption of Cd²⁺ onto SiO₂@DOPP]. Further, TGA spectrum suggested that SiO₂@DOPP] nanoadsorbent is thermally more stable than DOPP]. pH plays a major role to Cd²⁺ adsorption onto SiO₂@DOPP]. The optimum conditions for Cd²⁺ removal include pH = 6.5 and 0.03g adsorbent dose with 100 min contact time. Different adsorption isotherms models best fitted-(Langmuir adsorption model)], adsorption kinetics best fitted–(Pseudo second order and Intraparticle diffusion)] were examined for the removal of Cd²⁺. The maximum monolayer adsorption capacity q_max] was 142 mg/g. Thermodynamic evaluation indicates the endothermic and spontaneous nature of Cd²⁺ adsorption onto SiO₂@DOPP]. Furthermore complexation mechanism of Cd²⁺ onto SiO₂@DOPP] is discussed in detail. The results indicate involvement of functional group interactions, π–metal interactions, proton exchange, chelate complexes and electrostatic interactions during adsorption of Cd²⁺ onto SiO₂@DOPP]. Based on the results it has been inferred that SiO₂@DOPP] is a promising nanobioadsorbent to manage environment burden of Cd²⁺ from aqueous systems.