# 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
New composite materials, created by combining carbon nanotubes (CNTs) with UiO-66 series metal-organic frameworks (MOFs), can improve the recovery efficiency for Gain solution [76]. CNT is a unique material composed of graphite multilayers, with a hollow tubular structure, excellent mechanical and structural properties, and a high specific surface area. MOF, on the other hand, is a type of porous crystal material characterized by a high specific surface area, high porosity, adjustable pore structure, and strong stability. UiO-66, which is one of the MOFs, has an ordered micropore structure that enhances the compatibility of the polymer matrix and creates more active sites. This can improve the transmission rate, and it has a high potential for use in the adsorption and recovery of metal ions. By combining CNT with UiO-66, better composite materials can be produced, which retain the original morphology and pore structure of MOF while being well dispersed in various solvents [76; 77]. Using 2-CNT/ UiO-66-NH\({}_{2}\) composite at 40 \({}^{\circ}\)C and a pH of 8, the maximum capacity of Ga adsorption is 925.44 mg/g, and the efficiency at which it is adsorbed can reach 99% after only 1 h [76]. Adsorption of indium via MOFs of UiO-66 is also possible (a maximum adsorption capacity of 11.75 mg/g was recorded). In a solution with a pH of 1, even common, coexisting ions, such as magnesium and aluminum, do not disturb the adsorption of In(III). Furthermore, UiO-66 can be easily regenerated to adsorb In(III) again in another cycle [77]. Indium can adsorb onto multiwalled CNTs too. An adsorption capacity of 40 mg/g can be achieved; however, the presence of other cations in the solution has a significantly negative impact on the process [78]. Indium recovery from aqueous solutions is also possible using an iron-magnesium composite (MgFe\({}_{2}\)O\({}_{4}\)). In optimum conditions, this material obtained a maximum adsorption capacity of 46.4 mg/g, and very efficient desorption with a 15% HCl solution was also possible, which allowed for 10 adsorption-desorption cycle tests [79]. Phosphoryl sawdust bead can be used as a sorbent in both batch and column systems, and the adsorption efficiency increases with its quantity. The highest adsorption capacity recorded was 0.95 mg/g; however, approximately 20% of the adsorbed indium cannot be washed out during the bed regeneration [80]. Hydroxapatite (HAP) is a naturally occurring mineral, that can be used in the recovery of indium from solutions. HAP was coupled with two surfactants: Pluronic P123 and Pluronic F127. Maximum adsorption capacity estimated using the Langmuir model was c.a. 11 mg/g for all sorbents: HAP, HAP P123, and HAP F127 at the optimum pH of 4 and with a time elapsed ranging from 15 to 45 min. Copper, cadmium, tin, and zinc did not interfere with the removal of indium. The studied adsorbents can be applied for multicycle wastewater treatment, demonstrating high adsorption and desorption capacity during three cycles [81]. Biomaterials consisting of living organisms or their remains can also be used to adsorb indium from solutions. It turned out that the marine gram-negative bacteria _Shewanella algae_ are capable of selectively adsorbing indium. They can function in the solutions derived directly from the leaching of indium-containing materials with an HCl solution. Using them is simple, but very time-consuming, so they are not expected to be used on a large scale [23].