# Chromium(III) Removal from Wastewater by Chitosan Flakes
## 1 Introduction  
The removal of heavy metal ions from aqueous solutions, both for pollution control and for the recovery of raw materials, has assumed increasing importance in recent years. Among the many metals potentially harmful to the environment and human health, chromium pollution is of considerable concern, as the metal is widely used in many industrial activities such as electroplating, leather tanning, nuclear power plants, and textile industries [1, 2]. To address this problem, numerous separation techniques are currently available (e.g., adsorption, ion exchange, selective precipitation, nanofiltration, etc. ), the selection of which, however, is far from trivial and deserves extensive attention to avoid a suboptimal choice or the failure of the reclamation activity [3]. In general, adsorption-based technologies have proven to be among the most viable alternatives proposed for the treatment of industrial wastewater contaminated by a wide variety of pollutants, both organic [4] and inorganic [5, 6], due to the low processing andinstrumentation costs, the simplicity of operation, and the availability of different types of low-cost and environmentally friendly adsorbents. A wide range of materials, including activated carbon [7], metal oxides, carbon nanotubes, polymers, agricultural residues [8], and natural and modified clays [9], have been used successfully to adsorb heavy metals from aqueous solutions. This is even more interesting when it is possible to exploit low-cost adsorbent materials from industrial waste [10], with a double advantage for the environment, in line with operational guidelines such as the Circular Economy and the \"near-zero discharge\" of hazardous waste [11] established by the most recent European laws. In this context, a very promising and inexpensive material is chitosan (poly-\(\beta\)-(1\(\rightarrow\)4)-2-amino-2-deoxy-D-glucose), a nitrogenous polysaccharide prepared from chitin by partially deacetylase its acetoamine groups using strong alkaline solutions at about 70 \({}^{\circ}\)C. Chitosan has a high potential for the adsorption of metal ions, since it has both amino and hydroxyl groups that can act as chelation sites for metal ions. One of the most interesting advantages of chitosan is its versatility, since the material can be easily physically modified to obtain different forms of polymers such as beads [12], membranes [13], or sponges [14] for different applications. Chitosan can also be easily chemically modified to increase its applications [15]. Recently, several critical reviews have been published on the many applications of chitosan as an environmentally friendly biomaterial [16], ranging from the medical field [17] to food technology [18] and environmental protection [19]. Chromium can be found in the environment in the forms Cr(III) and Cr(VI), as its other oxidation states are not stable in aerated aqueous media [20]. The trivalent state is the most stable form in reducing conditions and is present as a cationic species (Cr(OH)\({}^{2+}\), Cr(OH)\({}_{2}\)\({}^{+}\)), with the first or second hydrolysis products dominating at pH values from 4 to 8. The low solubility of Cr(OH)\({}_{3}\) (log k = \(-\)16.19) considerably limits the concentration of Cr(III) for pH values above about 5. Given its high danger to biological systems, many studies have focused on the removal of Cr(VI), while very few articles deal with the adsorption of Cr(III) by chitosan. [PERSON] et al. [21] reported the uptake of Cr(III) ions by chitosan flakes and the effect of PO\({}_{4}\)[3]- on the adsorption mechanism. [PERSON] and collaborators [22] studied the removal of Cr(III) using a packed column filled with crab chitosan. More recently, [PERSON] & [PERSON] [23] reported comparative studies on the sorption of Cr(III) on chitin and chitosan in terms of a comparison between Langmuir and Freundlich isotherms. Overall, the papers often concern the application of chitosan membranes or beads, and not of chitosan flakes.