Adsorption and desorption of malachite green by using chitosan-deep eutectic solvents beads Abubakar Chadi Sadiq a,b , Nurul Yani Rahim a, ⁎, Faiz Bukhari Mohd Suah a,c, ⁎ a Green Analytical Chemistry Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Pulau Pinang, Malaysia b Department of Chemistry, Bauchi State University, P.M.B. 065, Gadau, Nigeria c Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom abstract article info Article history: Received 6 June 2020 Received in revised form 3 September 2020 Accepted 3 September 2020 Available online 07 September 2020 Keywords: Chitosan beads Deep eutectic solvent Malachite green Chitosan–deep eutectic solvent (DES) beads were prepared from chitosan and DESs. The DESs used were choline chloride–urea (DES A) and choline chloride–glycerol (DES B). Both chitosan–DES beads were used to remove malachite green (MG) dye from an aqueous solution. The optimum pH for chitosan–DES A was recorded at pH 8.0 while optimum pH for chitosan–DES B was pH 9.0. The maximum adsorption capacity obtained for chitosan–DES A and chitosan–DES B were 6.54 mg/g and 8.64 mg/g, respectively. The optimum conditions for both chitosan–DES beads to remove MG were 0.08 g of adsorbent and 20 min of agitation time. Five kinetic models were applied to analyse the data and the results showed that the pseudo-second-order and intraparticle diffusion model fitted best with R 2 > 0.999. For the adsorption capacity, results show that the Freundlich and Langmuir adsorption isotherms fitted well with chitosan–DES A and chitosan–DES B, respectively. The maximum adsorption capacities (q max ) obtained from chitosan–DES A and chitosan–DES B were 1.43 mg/g and 17.86 mg/g, respectively. Desorption indicated good performance in practical applications. © 2020 Published by Elsevier B.V. 1. Introduction Water pollution is a matter of great concern as it causes great dam- age to human, terrestrial and aquatic animals. Dyes are one of the most problematic pollutants in water. Malachite Green (MG), C 23 H 25 N 2 , is one dye that has many uses in commercial and industrial applications. In medicine, it is used as a treatment for fungal and protozoal infections and as a disinfectant [1]. MG is also used in the food industry as a colouring agent and additive. Additionally, it is used as a dye in the leather, cotton, paper and acrylic industries [2]. The high demand for bright colours has increased the discharge of dyes into our waterways. MG, though found useful in many industrial applications, can cause in- juries to humans and animals through inhalation and ingestion. Effects from MG exposure such as carcinogenesis, mutagenesis, teratogenesis, respiratory toxicity and reduced fertility in humans have been reported [3]. However, the removal of MG dye from wastewater is difficult due to its complex chemical structure, high resistance to oxidizing agents and light, and low efficiency of chemical and biological precipitation [4,5]. Previously, the removal of these dyes from water has been achieved through various methods, such as: photocatalysis [6], advance oxidation [7], coagulation [8], membrane filtration [9], and adsorption [10]. Com- pared to the other techniques, adsorption has been employed widely due to its simplicity in design, the ease of the procedure, and the high efficiency [11]. Many adsorbents such as activated carbon, clay, peat, chitin and silica have been tested for their ability to remove dyes from aqueous solutions [12,13]. Chitosan, (C 6 H 11 NO 4 ) n , is one of the unindustrialized adsorbents used for the removal of dyes and heavy metal ions by industries, at low concentrations of pollutants [14]. Chitosan is the deacetylated form of chitin, which is a linear polymer of acetylamino-D-glucose and contains amino and hydroxyl functional groups which serve as adsorp- tion sites for many adsorbates [15]. Other useful features of chitosan are biodegradability, biocompatibility, abundance, non-toxicity, hydrophi- licity and anti-bacterial properties [16]. Although it is an effective adsorbent, raw chitosan is not suitable for the industrial treatment of water because the flaked or powdered forms are nonporous and easily soluble in acidic media. The low internal sur- face area of the nonporous material limits access to the interior adsorp- tion sites and hence lowers the adsorption capacities and rates [17]. Furthermore, the flaked or powdered forms of chitosan are friable and prone to swelling. The reprocessing of flaked or powdered chitosan into a highly porous bead can overcome these limitations. However, chi- tosan only dissolves in acidic media of pH less than 6. Therefore, an al- kaline solution treatment process is necessary to remove the acid after dissolution. International Journal of Biological Macromolecules 164 (2020) 3965–3973 ⁎ Corresponding authors at: School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Pulau Pinang, Malaysia. E-mail addresses: nurulyanirahim@usm.my (N.Y. Rahim), fsuah@usm.my (F.B.M. Suah). https://doi.org/10.1016/j.ijbiomac.2020.09.029 0141-8130/© 2020 Published by Elsevier B.V. Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac