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

## TL;DR

A sustainable nano-adsorbent made from watermelon leaves and TiO2 effectively removes Cr(vi) from water in both batch and column systems.

## Contribution

A novel green-synthesized nano-adsorbent with dual adsorption and photocatalytic reduction capabilities for Cr(vi) removal is developed and tested.

## Key findings

- The nano-adsorbent achieved 94.23% Cr(vi) removal under optimal conditions.
- Photocatalytic reduction under sunlight converted Cr(vi) to Cr(iii) with 81% efficiency.
- The material showed excellent reusability and prolonged breakthrough time in fixed-bed column tests.

## 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.

This study presents a green, cost-effective, and sustainable strategy for remediation of Cr(vi) from surface water.