# Chromium(III) Removal from Wastewater by Chitosan Flakes
## 2 Materials and Methods
### Adsorption Tests  
Batch experiments (in triplicate) were carried out using 100 and 500 mg of adsorbent each time; chitosan was added to 50 mL of Cr solution in a conical flask. The stirring rate was set at 120 rpm for all adsorption/desorption tests using a temperature-controlled magnetic stirrer. Analysis of metal ions was carried out using an ICP-optical emission spectrometer (Inductively Coupled Plasma, Optima 2000 DV, Perkin Elmer, Waltham, MA, U.S.).  
For experiments on the pH effect, the solutions were initially adjusted with aqueous solutions of acid or base (0.01 M HNO\({}_{3}\) and/or 0.01 M NaOH) to reach pH values between 0.5 and 5, and thus avoid the precipitation of Cr(OH)\({}_{3}\). Isotherms were recorded during the execution of adsorption experiments with various initial metal concentrations (C\({}_{0}\) = 50-2000 mg L\({}^{-1}\)) at 20 \({}^{\circ}\)C. Kinetic tests were performed using 100 mg of chitosan flakes, 100 and 500 mg L\({}^{-1}\) as the initial metal concentration at 20 \({}^{\circ}\)C, and pH = 3.8 for fixed time intervals during adsorption (t = 0-24 h). The effect of the granulocyte of the flakes on the adsorption capacity was investigated using the two granulometric fractions of chitosan (\(<\)0.42 mm and \(>\)0.42 mm) under the same conditions. For all adsorption batch tests, a contact time of 120 min was set.  
The equilibrium amount of metal in the solid phase, expressed as Q\({}_{\rm e}\) (mg g\({}^{-1}\)), was determined with reference to the mass balance equation: Q\({}_{\rm e}\) = (C\({}_{0}\) - C\({}_{\rm e}\))\(\times\)(V/m), where C\({}_{0}\) and C\({}_{\rm e}\) (mg L\({}^{-1}\)) are the initial and equilibrium metal concentrations, respectively, V (L) is the volume of the aqueous solutions, and m (g) is the mass of the adsorbent.  
Dynamic tests were performed using a glass column with an internal diameter of 0.9 cm and a bed high of 40 cm, filled with 2 g of chitosan. Tests were performed using both a real wastewater solution and a 500 mg L\({}^{-1}\) chromium(III) solution at pH = 3.5, imposing a flow rate of 23.6 mL h\({}^{-1}\) (1 BV h\({}^{-1}\)) by using a peristaltic pump.  
The real wastewater, a tannry washing solution, had the following composition: pH = 3.2, Chemical Oxygen Demand, COD = 9.1 g L\({}^{-1}\), Total Suspended Solids, TSS \(<\) 1 g L\({}^{-1}\), Cr\({}^{3+}\) = 635 mg L\({}^{-1}\), Na\({}^{+}\) = 1050 mg L\({}^{-1}\), Mg\({}^{2+}\) = 760 mg L\({}^{-1}\), Ca\({}^{2+}\) = 300 mg L\({}^{-1}\), Zn\({}^{2+}\) = 115 mg L\({}^{-1}\), Cd\({}^{2+}\) = 87 mg L\({}^{-1}\), SO\({}_{4}\)\({}^{2-}\) = 1.820 mg L\({}^{-1}\), and Cl\({}^{-}\) = 818 mg L\({}^{-1}\).  
Desorption experiments were performed in batch mode (T = 20 \({}^{\circ}\)C, t = 24 h). In particular, after the end of the adsorption phase, the adsorbent material was separated from the supernatant using filtration membranes (0.22 \(\upmu\)m). Then, desorption tests were performed using 50 mL solutions of H\({}_{2}\)SO\({}_{4}\) and EDTA as desorption reagents at a concentration of 0.1 M and 0.05 M, respectively.  
Figure 1: Thermogravimetric analysis (TGA) of the chitosan sample.  
The quantitative evaluation of desorption was carried out using desorption percentages calculated from the difference between the amount of metal loaded on the adsorbent after adsorption and the amount of metal in solution after desorption. To investigate the reuse capacity of the adsorbents, the above procedure was repeated 5 times under the same conditions (first adsorption and then desorption).