# Removal of Chromium(VI) by Chitosan Beads Modified with Sodium Dodecyl Sulfate (SDS)
## 1 Introduction  
Toxic metal contamination in aquatic environments has attracted tremendous attention due to the rapidly increasing number of manufacturing industries. Heavy metals are of major environmental concern because they are non-biodegradable and cannot be decomposed or metabolized [1]. Several metals cause serious health and environment problems, and chromium (Cr) compounds are one of the most toxic contaminants in wastewater due to their high solubility and toxicity, and free transferability [2]. Cr has been widely applied in many industrial activities based on its excellent properties, for example, such as in electroplating, leather tanning, nuclear power plants, and textile industries [3, 4]. Moreover, it can be used for anodizing, corrosion control, and chemical manufacturing [5, 6, 7]. Chromium usually exists in two stable oxidation states: trivalent Cr(III) and hexavalent Cr(VI) in a natural environment, as its other oxidation states are not stable in aerated aqueous media [8]. In a natural aqueous environment, Cr(VI) may exist in the form of CrO\({}_{4}\)\({}^{-}\) or HCrO\({}_{4}\)\({}^{-}\), whereas Cr(III) is inclined to form [Cr(H\({}_{2}\)O)\({}_{6}\)]\({}^{3+}\), Cr(H\({}_{2}\)O)\({}_{5}\)(OH)\({}^{2+}\), Cr(H\({}_{2}\)O)\({}_{4}\)(OH)\({}_{2}\)\({}^{+}\), or Cr(III) organic complexes. It is well-known that Cr(III) is an essential material for organisms, whereas Cr(VI) is more toxic, carcinogenic, and mutagenic [8, 9]. Thus, the development of a recovery method for this metal (particularly Cr(VI)) is significant from the standpoint of environmental protection. Many physical andchemical technologies, such as ion exchange, precipitation, ultrafiltration, reverse osmosis, and electro dialysis, have been reported and used for removing heavy metals [10]. Nevertheless, these procedures have some disadvantages, such as a high consumption of reagents and energy, low selectivity, high operational costs and difficult further treatment due to the generation of toxic sludge [11]. Adsorption is considered to be an efficient method for removing metallic ions in aqueous solutions [12; 13]; in particular, biological adsorption (biosorption) is one of the most environmentally friendly, economically favorable, low cost, recyclable, and technically easy methods [14; 15]. Among the many biosorbents available, chitosan can be an outstanding biosorbent for metals for the reason that its amine (-NH\({}_{2}\)) and hydroxyl (-OH) groups may serve as coordination sites to form complexes with various heavy metal ions [16]. Chitosan has been proven to be very efficient as a biosorbent for the recovery of several toxic metals such as mercury (Hg), uranium (U), molybdenum (Mo), vanadium (V), and platinum (Pt) [17; 18; 19], and its full chemical name is called (1,4)-2-amino-2-deoxy-\(\beta\)-D-glucose. It can be employed as an environmentally friendly adsorbent due to it being economical and the fact that it does not result in secondary pollution. Chitosan can be produced by the alkaline deacetylation of chitin, which originates from the most abundant biopolymer--cellulose. Chitosan is a polymer that can be acquired from the shells of seafood, such as prawns, crabs, and lobsters [20]. The biopolymer is characterized by a high percentage of nitrogen, and exists in the form of amine groups, free amino groups, and hydroxyl groups, which are responsible for metal ion binding through chelation mechanisms [21]. The uses of chitosan in the removal of various pollutants have been adequately reviewed [22]. However, chitosan has some defects, such as notable swelling in aqueous media and nonporous structures, resulting in a very low surface area [23]. Therefore, many types of chemical modification can be undertaken to produce chitosan derivatives for improving the removal efficiency of the heavy metal [24]. For example, various chemical or physical modifications can be adopted for increasing the number of exposed active sites [25; 26].