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

Abstract

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 Cajanus cajan 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 (R² = 0.998), indicating chemisorption. In contrast, the Freundlich and Langmuir isotherm models confirmed heterogeneous multilayer adsorption with a predominant monolayer interaction for NA (R² = 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.