Green-engineered agricultural-based nano-adsorbent for efficient Cr( <scp>vi</scp> ) removal: batch mechanisms and continuous column performance

This study presents a green, cost-effective, and sustainable strategy for remediation of Cr(vi) from surface water. The approach employs a bio-based nano-adsorbent derived from watermelon leaves, integrated with green-synthesised TiO₂ nanoparticles using <i>Cajanus cajan</i> leaf extract. The acid-modified nano-adsorbent (A-WML/TiO₂) demonstrated enhanced adsorption efficiency compared to pristine A-WML due to increased surface area (98.25 m² g^-1), abundant active functional groups, and synergistic adsorption-photocatalytic reduction properties. Furthermore, the batch experiments confirmed the optimum removal conditions to be pH 4, 25 mg of adsorbent dosage, 10 ppm of Cr(vi), and a contact time of 3 h, achieving a 94.23% removal. Additionally, photocatalytic studies under sunlight further reduced Cr(vi) to Cr(iii) with an efficiency of 81%. Kinetic analysis followed pseudo-second-order behaviour (<i>R</i> ² = 0.998), indicating chemisorption. In contrast, the Freundlich and Langmuir isotherm models confirmed heterogeneous multilayer adsorption with a predominant monolayer interaction for NA (<i>R</i> ² = 0.9815). Reusability tests revealed excellent stability after five regeneration cycles using NaOH with minimal loss in performance, highlighting the sustainability of materials. The fixed-bed column studies using nano-adsorbent-modified sand showed a markedly prolonged breakthrough time, increasing from 400 to 800 min. This improvement was achieved with a 3 wt% TiO₂ loading at a 15 cm bed height under acidic pH, confirming the material's suitability for practical continuous water treatment systems. Thomas and Adams-Bohart model fitting confirmed the mass-transfer-controlled adsorption processes and predictive accuracy for column performance. A comparative evaluation with existing bio-adsorbents highlights a superior Cr(vi) removal capacity, dual functionality (adsorption and photocatalysis), and effective integration into sand filtration systems, thus enabling scalable deployment.