Herbal plant licorice ( Glycyrrhiza glabra ) root-activated carbon-based material for methyl violet and crystal violet dyes removal: thermochemical synthesis and experimental design optimization

This study transformed solid waste from Glycyrrhiza glabra (licorice) roots into activated carbon (LIQ-AC) by a microwave-assisted chemical activation method utilizing phosphoric acid (H₃PO₄). The prepared LIQ-AC served as the adsorbent for cationic dyes (crystal violet; CV, and methyl violet; MV). The adsorption optimization was conducted using Response Surface Methodology with Box-Behnken Design (RSM-BBD) for the adsorption parameters (LIQ-AC dosage (0.02-0.1 g/100 mL, pH of 4-10, and contact time of 5-60 min, and 25 °C). The accuracy of the developed BBD model was confirmed by ANOVA analysis, with F-values of 138.27 for CV and 388.49 for MV, and corresponding p-values <0.0001, indicating that the models are highly significant. The optimum adsorption conditions predicted by the model were a LIQ-AC dosage of 0.1 g/100 mL, pH 7, and a contact time of 60 min, achieving maximum dye removal efficiencies of 93.3% for CV and 94.6% for MV. The kinetic and isotherm analyses demonstrated that the adsorption of CV and MV onto LIQ-AC followed the pseudo second order (PSO) kinetic model and conformed to the Freundlich isotherm. These results indicate that the adsorption process is governed by chemisorption on heterogeneous surface sites. The maximum adsorption capacity (qmax) of LIQ-AC was 125 mg/g for CV and 140 mg/g for MV. Overall, this study shows the successful application of circular economy approach via conversion of biomass solid waste to bioadsorbent, where the latter revealing outstanding adsorption performance toward cationic dyes.