Harnessing Prunella vulgaris L. residues for enhanced biosorption of dyes in wastewater

Wastewater containing hazardous dyes, such as methylene blue (MB) and malachite green (MG), poses a significant threat to both the aquatic environment and human health. Therefore, timely treatment of dye wastewater is crucial prior to its discharge into the environment. In this research, Prunella vulgaris L. residues obtained from decoction were selected as a novel raw adsorbent for the efficient removal of methylene blue (MB) and malachite green (MG) from aqueous solutions. The four operational factors, namely adsorbent dosage, initial concentration of target dyes, solution pH, and contact time were optimized using Box-Behnken design of response surface methodology. Under the optimized conditions, the adsorbent demonstrated significantly improved removal efficiency for both MB and MG, achieving 99.62% and 93.29%, respectively. Furthermore, the Freundlich model (Adjusted R-2 of MB = 0.9757 and Adjusted R-2 of MG = 0.9881) and pseudo-second-order kinetic equation (Adjusted R-2 of MB and Adjusted R-2 of MG both exceeded 0.9700) were found to accurately describe the adsorption processes of MB and MG, indicating that their adsorption behaviors exhibited favorable multilayer nonuniformity primarily governed by physicochemical interactions. The maximum adsorption capacities of MB and MG in aqueous solutions on the adsorbent were found to be 238.84 and 300.42 mg.g(-1), respectively. The adsorption mechanisms of MB and MG on the adsorbent were confirmed through various techniques including SEM, EDS, N-2 adsorption-desorption test, Zeta potential class, XPS and FT-IR analysis. These mechanisms include electrostatic adsorption, pore filling, ion exchange, hydrogen bonding, and pi-pi interaction between the adsorbent and MB/MG. In summary, the residuals of Prunella vulgaris L. after decoction can be considered as a potentially efficient adsorbent material for the removal of MB and MG from aqueous solutions, offering a novel approach to regeneration of Chinese herbal residue.