Modified plant leaves/chitosan composite: adsorption modeling of crystal violet dye using Box–Behnken design

The utilization of sustainable materials derived from biopolymers and plant waste is a compelling approach to the removal of organic dyes from wastewater. In this study, a sustainable adsorbent (hereinafter, CHI/FL-OXA) generated from chitosan and activated Fig (<i>Ficus carica</i> L.) leaves using oxalic acid was created for effective organic dye (crystal violet, CV) adsorption. Box-Behnken Design (BBD) helped to accomplish the modeling and optimization of the adsorption variables comprising A: CHI/FL-OXA dose (0.02-0.08 g/L), B: pH (4–10), and C: time (10–40 min). The best variables for maximal CV uptake (93.1%) were as follows: CHI/FL-OXA dose = 0.064 g/L; pH ∼ 9.2; contact duration = 27.6 min. Agreements with pseudo-first-order and Freundlich models were shown by the experimental results of CV adsorption by CHI/FL-OXA. The intraparticle diffusion plots reveal three linear stages, indicating a multi-step adsorption process where initial dye transport is followed by intraparticle diffusion and surface adsorption, with the non-zero intercept (<i>C</i> ≠ 0) confirming that intraparticle diffusion is not the sole rate-controlling mechanism. The CHI/FL-OXA biomaterial, with an adsorption capacity of 375.72 mg/g, exhibited strong potential for adsorbing cationic dyes like CV dye. Adsorption of CV cationic dye on the CHI/FL-OXA enfolds numerous interactions, including electrostatic forces, Yoshida H-bonding, n-π, and H-bonding. The current work supports the Sustainable Development Goals (SDGs) like Clean Water and Sanitation (SDG 6), Responsible Consumption and Production (SDG 12), Climate Action (SDG 13), and Life Below Water (SDG 14). This study introduces a novel sustainable adsorbent (CHI/FL-OXA) created from chitosan and treated plant (<i>Ficus carica</i> L.) leaves using oxalic acid for efficient removal of crystal violet dye. Box-Behnken Design (BBD) was employed to optimize adsorption variables. The research highlights the potential of plant leaves as a valuable resource for environmental remediation. This innovative approach supports green chemistry and sustainable development, while promoting the circular economy concept.