Efficient Cr (VI) removal from tannery wastewater using Co-Zn ferrite-modified water hyacinth biochar: mechanisms and optimization

Chromium contamination from tannery effluents significantly threatens environmental sustainability and public health. This study aims to develop a Co-Zn ferrite-incorporated water hyacinth biochar (Co-ZnF@WHB) for efficient Cr (VI) removal from contaminated water. The biochar was synthesized through pyrolysis of water hyacinth steam at 450 ⁰C, followed by HNO₃ activation to enhance surface functionality. FTIR and XPS analysis confirmed the introduction of C = O and − OH groups, which created active sites and significantly improved Cr (VI) adsorption. XPS analysis indicated Cr (VI) reduction to Cr (III) via oxidation of − C = O to COO⁻ groups. The incorporation of Co-Zn ferrite introduced magnetic properties, facilitating easy separation. Adsorption experiments revealed a strong pH dependence, with maximum capacities of 52.15 mg/g at pH 2.0 (298 K) in synthetic solutions and 66.38 mg/g at pH 3.54 for tannery effluent. Kinetic modeling suggested chemisorption as the rate-limiting step, while isotherm analysis confirmed monolayer adsorption. Thermodynamic studies indicated an endothermic (ΔH⁰ ≈ 34 kJ/mol) and spontaneous (ΔG⁰ < 0) adsorption process. The response surface methodology (Box-Behnken Design) optimized preparation conditions, ranking factor influence as pH > Cr (VI) concentration > contact time. The primary Cr (VI) removal mechanisms involved surface complexation, electrostatic interactions, and reduction. This study demonstrates the potential of Co-ZnF@WHB as an effective, sustainable, and eco-friendly material for Cr (VI) remediation from polluted water sources.