Valorization of agriculture residues into novel mesoporous surface engineered (ZnO@RHBC) nanocomposites for sustainable sequestration of hexavalent chromium from electroplating industrial wastewater

• A Zinc oxide impregnated biochar nanocomposite was prepared using rice husk as a raw material for effective sequestration of Cr (VI). • Mesoporous nanocomposites enabled a high surface area (105.74m 2 /g) and excellent adsorption capacity (207.03 mg/g). • XPS spectra of nanocomposite revealed reduction of Cr (VI) into Cr (III) due to oxygen-containing functional groups. • Adsorbent followed the Langmuir isotherm (R² =0.9873) and Pseudo second order kinetics (R 2 = 0.996) indicating monolayer homogenous chemisorption. • The study is associated with SDGs 6, 9, and 12. Hexavalent chromium [Cr (VI)] is highly toxic in nature and poses serious threats to ecology and human health. In this study, cost-effective and sustainable ZnO@RHBC Nanocomposites were synthesized via a facile impregnation approach for the effective adsorption of Cr (VI) from wastewater. Characterization techniques, including UV-visible spectra, FTIR, XRD, FE-SEM with EDX, TEM, BET, TGA, pHzc, XPS, Raman spectra, Zeta potential, and particle size, were used to characterize the nanocomposites. Results confirmed the crystalline nature, mesosporous structure, abundant functional groups, high surface area (105.74 m 2 /g), and good thermal stability of the synthesized nanocomposites. The batch adsorption finding revealed that optimum adsorption was achieved at pH 3, adsorbent dose 0.1 g, initial Cr (VI) ion concentration 40 mg/l, contact time 90 mins, and temperature 35 °C with excellent adsorption capacity 207.03 mg/g and adsorption efficiency 92.42% Adsorption followed the Langmuir isotherm (R² = 0.9873) and Pseudo-second order kinetics (R² = 0.996), favouring monolayer chemisorption. Thermodynamic analysis showed a spontaneous, random, and exothermic nature of adsorption process. Electrostatic interaction, hydrogen bonding, coordinate binding reduction, and π–π interaction were the main mechanisms for Cr (VI) adsorption. The regeneration study of nanocomposites revealed that adsorption decreased by 91.22% to 62.33% over five successive cycles, primarily due to pore blockage. Furthermore, nanocomposites could adsorb 87.47% of Cr (VI) from electroplating wastewater. The cost-benefit analysis at the lab scale was also examined, which is crucial for the large-scale application of adsorbents. This research supports SDGs goals 6, 9, and 12 by improving water quality, promoting agricultural waste recycling, and advancing sustainable nano-adsorbents in environmental applications.