# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Sorption and Ion Exchange
#### 3.4.2 Sorption Using Polymer Resins
It is assumed that gallium forms a complex with the active groups of the resin via the reaction mechanism presented in  
Figure 10: Comparison of the structure of the CMA resin (**a**) with the EDTA complexone (**b**). Figure 11 [1]. Regeneration of the ion exchange bed was achieved with a 0.1 M solution of sulfuric acid, resulting in a Ga\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) concentration of 4.5 g/dm\({}^{3}\) in the final solution [29]. The effluent from the column was sent for further processing, during which, as a result of the addition of NaOH and iron(III) sulfate, a precipitate of Fe\({}_{4}\)O\({}_{3}\)(AsO\({}_{4}\))\({}_{2}\) was formed. The maximum amount of arsenic that could be removed in this way was 95% and required the process to be carried out at a pH of 6 and an iron/arsenic ratio of 3.6. Then, the precipitate was filtered and the solution was sent to a nitrate anion purification process, which is usually carried out using a membrane method. Then, the solution purified in this way could be returned to the beginning of the process, where it can be used to dilute the solution after leaching [29]. A schematic diagram showing the entire described technology is shown in Figure 12.  
Studies have shown that gallium adsorption in batch systems yields very good results, as long as the volume ratio of the resin to the solution is 1:20. The contact time between phases usually lasts an hour. The situation is similar in the dynamic systems, where the process parameters are set so that 20 units of solution volume pass through 1 unit of resin volume per hour. Many chelating resins can be used for gallium adsorption under these conditions. They are divided into three types [86]:  
Figure 11: Mechanism of gallium adsorption on the Diaion CR 11 resin. Figure 12: Schematic diagram of gallium recovery from the GaAs scrap waste using Diaion CR 11 chelating resin. Resins which have a -NOH group and a functional group capable of forming a chelate with it via gallium;
2. Resins which have active groups with a structure analogous to 8-hydroxyquinoline;
3. Chelating resins with a high affinity for gallium, which have groups containing several electron donor atoms (e.g., O, N, or P), for example -NHOH and -P(O)(OH)\({}_{2}\)H.  
Similarly to gallium, the use of iminodiacetate resins for indium recovery also gives very good results. Research has shown that this type of resin, commercially named Amberlite IRC 748, allows indium adsorption with an efficiency of 99.5% in the dynamic system. Then, the metal ions can be washed out with a solution of sulfuric acid, with a flow rate of 5 cm\({}^{3}\)/min. It is also possible to recover indium using cellulose functionalized with iminodiacetic acid. The adsorption capacity of chelating cellulose is 1.5 mmol/g and can be achieved at a pH of 3.5 in the presence of zinc(II) ions [87]. Other ion exchangers with high selectivity towards indium turned out to be basic anion exchangers with the trade names Amberlite IRA 400 AR, Amberlite IRA 420, Dowex 1, and Amberjet 4200 CI. They enable the adsorption of indium from solutions containing platinum and tin. To isolate indium from the bed, the resins are washed with 0.1 M EDTA solution. Then, indium is precipitated from this solution by dropwise addition of a Na\({}_{2}\)S solution with a concentration of 3 M at a temperature of 80 \({}^{\circ}\)C. The resulting yellow-brown In\({}_{2}\)S\({}_{3}\) precipitate contains indium with a purity of 99% (m/m) [37]. The chelating aminodiacetic resin with the trade name Ionac(r) SR-5 and the resin containing diphosphate acid groups with the trade name Ionac R12 show a similarly high degree of adsorption of indium. Ionac SR12 additionally shows a greater selectivity towards indium when iron ions are present in the solution: the loading percentage of resin for indium was 40%, while for iron it was 5%. Ionac SR5 can be used for the adsorption of indium from solutions containing sulfuric acid [37].