Valorization of Rice Husk into Efficient Adsorbents for Dye Removal: A Comparative RSM–CCD Optimization Approach

Rice husk, a globally abundant rice-milling by-product, is frequently burnt or landfilled and underexploited for valorization into low-cost adsorbents. Converting such agricultural residues into carbonaceous materials supports a circular economy and offers a green alternative to energy- and reagent-intensive activated carbon for removing cationic pollutants. In this work, rice husk was valorized into two chars using muffle-furnace (MFRH) and nitrogen-assisted (NARH) pyrolysis and benchmarked against raw husk (RH) for removal of crystal violet, a model cationic dye. A face-centered central composite design (FCCCD) coupled with response surface method (RSM) was used to model and optimize the process across a pH range (4–10), adsorbent dose (1–9 g·L−1), and CV concentration (10–200 mg·L−1) for each material. Quadratic modelling showed high goodness of fit for all adsorbents (R2 = 0.97–0.99), closely reproducing experimental removals and confirming the suitability of the prediction. Although removal increased with pH and dose for all three materials, the magnitude and curvature of these effects differed markedly between raw and pyrolyzed husk. Raw husk outperformed both chars across most of the design space despite a lower surface area, due to diminished active sites on carbonized material, and the chars exceeded it only under optimized alkaline, high-dose conditions. Numerical optimization identified distinct conditions for each adsorbent, where experimental removals of 96.32% (RH), 98.37% (MFRH), and 99.66% (NARH) were obtained, which are in close agreement with model predictions. These findings show that RSM–CCD can be used to both optimize operating conditions and quantitatively compare rice-husk-derived adsorbents prepared under different pyrolysis conditions.