# 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
Of the available complexes, two, in particular, have been shown to have the ability to form exceptionally stable complexes with these metals: iminodiacetic acid (IDA) and ethylenediaminetetraacetic acid (EDTA). Although they cannot be directly attached to a polymer surface, other reagents have been used to obtain resins whose functional groups mimic IDA and EDTA structures and behaviours. A polystyrene resin with active groups that have a structure analogous to IDA is called IDA resin, while another polystyrene resin with active groups that are similar to EDTA is called CMA resin. Figures 9 and 10 show the structures of the complexes and the corresponding resins [1]. Figure 9: Comparison of the structure of the IDA resin (**a**) with the IDA complexone (**b**). Both resins allow for the quantitative adsorption of gallium and indium from the solutions. The maximum capacity of the ion exchanger for the given metal depends on the pH at which the process is carried out. The maximum capacity of the IDA resin in the case of gallium is 0.9 mmol/g of resin at a pH of 2.8, and in the case of indium, it is 1.2 mmol/g of resin at a pH of 2. The CMA resin is capable of adsorbing a maximum of 1.5 mmol of gallium/g of resin at a pH of 2 and 1.4 mmol of indium/g of resin at a pH of 1.8. Both metals can be removed from the resins by regenerating the beds with a 2 M hydrochloric acid solution. It is worth noting that the complexes formed with the functional groups of the CMA resin are much more stable than those formed with the IDA resin. Moreover, a breakthrough point of the CMA bed is reached later than that of the IDA bed [85]. Tests have shown that both resins can effectively separate gallium or indium from solutions additionally containing Al(III) ions. Moreover, the CMA resin is also able to selectively adsorb them from solutions containing Zn(II) and Ca(II) ions as well. However, it shows greater selectivity towards Fe(III) ions than towards In(III) and Ga(III), and its selectivity towards Cu(II) is very similar to its selectivity towards Ga(III). Therefore, if the presence of Fe(III) or Cu(II) is suspected in the raw solution, it is necessary to remove them beforehand. Most often, hydrogen sulfide gas is passed through the solution, which causes the precipitation of copper sulfide and the reduction of Fe(III) to Fe(II)--towards which the CMA resin is much less selective [85]. The chelating resin with the trade name Diaion CR 11 has a similar structure to the IDA resin, but in the active groups, the mobile ions are Na\({}^{+}\), not H\({}^{+}\). A method of arsenic and gallium recovery was developed using Diaion CR-11 from waste containing gallium arsenide, which allows for the obtainment of a concentrated gallium solution with a purity of approximately 99.3%. Gallium and arsenic were leached with a 2 M nitric acid solution for about 30 min at 30 \({}^{\circ}\)C and an S/L ratio of 2 g/100 cm\({}^{3}\). About 98% of gallium and arsenic present in the waste were transferred to the resulting solution, which was very acidic and its pH was usually lower than 0. Because at the higher pH, the breakthrough point of the bed is reached more slowly and the capacity of the ion exchange for gallium increases, the solution was diluted before passing it through the column. Dilutions ranging from 1/5 to 1/40 were tested, and the capacity of the ion exchanger increased with the dilution and amounted to 0.14 mmol/cm\({}^{3}\) of resin and 0.38 mmol/cm\({}^{3}\) of resin within this range, respectively. The twenty-fold dilution was chosen because, in addition to the satisfactory ion exchange capacity of 0.37 mmol/cm\({}^{3}\) of resin, it also allowed for the obtainment of a good Ga/As partition coefficient of 115. Ga\({}^{3+}\) ions were present in the solution, while arsenic appeared in the form of H\({}_{3}\)AsO\({}_{4}\). It was passed through the column with a hydraulic loading of 1 BV/h, and the process ended after 35 BV had flowed.