# 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
#### 3.4.1 Sorption Using Silica Gels, Carbons, Minerals, and Biomaterials  
Silica gels demonstrate high adsorption efficiency for gallium. Impregnating them with appropriate solvents further improves this ability. The choice of solvent also depends on the type of acid which was used to leach the metal and what other metal ions are present in the solution. The pH of the solution is also very important, because in some cases, for example when silica gel is impregnated with D2 EHPA, above a certain pH value (in this case 2) partial leakage of D2 EHPA from the gel may occur [71]. The use of unmodified silica gel for adsorption of gallium(III) from solutions very often requires pH correction with a buffer. Studies have shown that adsorption is quantitative at the pH of 3. Gel impregnated with n-octylaniline provides quantitative recovery of gallium from solutions containing hydrochloric acid. Its advantage is that there is no need to add a buffer, and metal elution is easy. The use of 2-hydroxy-5-(2-hydroxybenzylideneamino)benzoic acid also allows for the obtainment of high process yields [31]. In the case of indium, unmodified silica gels have much lower effectiveness of adsorption and do not show sufficient efficiency even at a very elevated pH [31]. However, silica adsorbent modified with D2 EHPA, named D2 EHPA/SiO\({}_{2}\)-P, exhibits excellent adsorption selectivity and kinetics for In(III), even from a solution which contains a large number of impurities, especially Fe(III). In(III) from D2 EHPA/SiO\({}_{2}\)-P can be desorbed with a 3 M HCl solution, and D2 EHPA/SiO\({}_{2}\)-P still has high adsorption and desorption efficiency for In(III) after five cycles of adsorption-desorption experiments. During the process of leaching of LCD waste with 0.1 M H\({}_{2}\)SO\({}_{4}\), no toxic gases are released. Iron(III) ions can be reduced using oxalic acid which allows for the separation of Fe(III) from In(III) [72]. Gel modified with a nonionic surfactant called TritonX-100, containing D2 EHPA micelles (known as DTS), also shows a high In(III) adsorption capacity. It is believed that the metal is bonded with the solvent by forming a chelate, which is then adsorbed onto the silica gel [31]. Probably, the O-H bond in D2 EHPA participates in the substitution reaction and bonds with In(III), forming the In-O bond at the adsorption site. Furthermore, the bond order value of In-O was the lowest in D2 EHPA; therefore, during the desorption process, the In-O bond was easy to break [72]. Impregnating the surface of silica gel with n-octylaniline or 2-hydroxy-5-(2-hydroxyphenylideneamino)benzoic acid also allows for the obtainment of high sorption efficiencies [31]. Research was also carried out on the adsorption of gallium from solutions using activated bentonite. Bentonite is a clayy montmorillonite rock that swells in water. It is also known as \"bleaching earth\". In addition to montmorillonite, it also contains quartz, feldspars, and mica [73]. In the studies, bentonite was activated with sulfuric acid, and as a result of sorption, 99.1% of gallium was recovered from a gallium(III) nitrate solution. The research shows that the adsorption efficiency is positively influenced by both an increase in the pH (the research was carried out in a pH range from 1 to 3 so that the Ga(H\({}_{2}\)O)\({}_{6}\)NO\({}_{3}\)\({}^{2+}\) form dominated in the solution) and an increase in the mass of bentonite involved in the process. However, increased temperature significantly reduces the efficiency of the process and should be maintained at around 20 \({}^{\circ}\)C. The optimal sorption conditions were as follows: a temperature of 20 \({}^{\circ}\)C, a solution pH of 2.50, and a bentonite mass of 3.50 g per 1 L of solution [74]. Modified bentonite has also been used as an indium sorbent. The so-called CCB, i.e., chitosan-coated bentonite, turned out to be a promising solution, and its adsorption capacity for indium is approximately 5.79 mg/g [75].