Preparation of beta-cyclodextrin decorated magnetic lignin by citric acid cross-linker for water treatment

• A practical and eco-friendly multifunctional green adsorbent was prepared by grafting beta-cyclodextrin on magnetic kraft lignin (beta-CD@ML) for the removal of methylene blue. • beta-CD@ML shows higher selectivity to cationic dyes than to anionic dyes. • The synthesized adsorbent is reusable during several adsorption-desorption cycles. • Kinetics, thermodynamics, isotherms and disposal of the removal process are thoroughly investigated. In this research, a straightforward method was used to prepare a multifunctional adsorbent for selective removal of cationic dyes. To synthesize this adsorbent, beta-cyclodextrin (beta-CD), due to the outstanding and unique structure of their caves, which have the appropriate size for trapping and adsorption of cationic dyes, was grafted onto magnetic lignin (ML) via citric acid cross-linker for the first time. Various techniques were applied to characterize magnetic adsorbent (beta-CD@ML), including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and nitrogen adsorption-desorption analysis. To check the performance of the beta-CD@ML, the adsorption process was investigated to remove methylene blue from aqueous solutions. The best result for methylene blue removal was approximately 97 % under conditions of pH 7, beta-CD@ML dosage of 0.6 g L −1 , pollutant concentration of 20 mg L −1 , and time of 45 min, while the removal of methylene blue for magnetic lignin under the same conditions was 55 %. The appropriate model used in adsorption was the Langmuir isotherm, which showed a maximum adsorption capacity of 80 mg g −1 at 25 °C. The stability and reusability of beta-CD@ML adsorbent were also investigated, and the results showed a slight decrease in adsorption efficiency.