Pristine and modified radix Angelicae dahuricae (Baizhi) residue for the adsorption of methylene blue from aqueous solution: A comparative study

In the present study, samples derived from radix Angelicae dahuricae (Baizhi, BZ) residue, a traditional Chinese herb, were investigated for their adsorption behavior and mechanism of methylene blue (MB). Optimization of factors like SDS impregnation ratio, contact time and pH on the adsorption were conducted firstly and it showed that the highest adsorption capacity of BZ samples when the SDS impregnation ratio equals to 1:1 at pH 10.5 at equilibrium. According to the study on pH_pzc and pH, the SDS loading on BZ contributed to reducing the repulsion force between BZ sample and MB molecule and thus promoting the absorption. The adsorption kinetics was found to follow the pseudo-second-order kinetic model, suggesting chemisorption plays the dominant role in the adsorption process. The Langmuir model is adequate to represent the MB adsorption data, the maximum monolayer adsorption capacity (Q_L) for BZ 11 (SDS impregnation 1:1) was 1411 mg/g. By studying adsorption kinetics and thermodynamics, it is concluded that SDS impregnation increased the adsorption capacity by strengthening the adsorbent-adsorbate bonding force rather than augment the numbers of available active sites. Elemental analysis (EA), fourier transform infrared spectrometry (FT-IR) and scanning electron microscopy (SEM) analysis of BZ samples confirmed the SDS impregnation and the adsorption of MB molecules. Based on experimental results, Brunauer-Emmett-Teller (BET) and FT-IR, it is reckoned that after ring opening under alkaline conditions, the coumarin could produce cis-o-coumaric acid as the strong electron-donating structure which reacted with the strong electron acceptor MB molecule to produce the charge transfer complex and enhanced adsorption. The remarkable adsorption capacity of BZ samples compared to various natural and artificial materials and the high resorption rate as 97.8% indicated the radix Angelicae dahuricae residue as a potential adsorbent material.