Germanium is a critical element used in high-tech applications such as semiconductors, fiber optics, and solar panels. Due to its limited primary sources, recent research has focused on recovering germanium from secondary resources, including zinc production residues, spent optical fibers, and other industrial wastes. Hydrometallurgical processes play a key role in recycling germanium efficiently while minimizing environmental impact. Various adsorbents have been developed to enhance germanium recovery, each with unique properties influencing adsorption efficiency, kinetics, and desorption performance.
One of the promising materials for germanium adsorption is poly-dopamine (PDA) coated magnetic nano-Fe₃O₄ (Fe₃O₄@PDA), further modified with polyethylenimine (PEI). This composite performs best at pH 6, following the pseudo-second-order kinetic model. The adsorption mechanism involves both single-layer and multilayer adsorption, with chemisorption playing a dominant role. The presence of phenolic hydroxyl and amino groups on the material surface facilitates complexation coordination, which is essential for germanium removal.
TA-CS has been investigated for germanium recovery from zinc residue leachates. The introduction of tartaric acid introduces protonable hydroxyl and carboxyl groups, enhancing buffering capacity under acidic conditions. The material exhibits an adsorption capacity of 57.28 mg/g for Ge(IV), showing a preference for germanium over zinc with a separation factor of 3.22. The adsorption mechanism is influenced by electrostatic adsorption, complexation, hydrogen bonding, and ion exchange. While the presence of anions such as silicate, phosphate, and carbonate reduces adsorption efficiency, chloride has minimal impact. Applied to real leachates, TA-CS achieves an 80% germanium recovery rate with a dosage of 15 g/L. Nitric acid proves to be the most effective desorption agent, although repeated adsorption-desorption cycles reduce efficiency.
Chitosan-based adsorbents grafted with hydroxybenzoic acids, including p-hydroxybenzoic acid (HBA-CS), 3,4-dihydroxybenzoic acid (DBA-CS), and 3,4,5-trihydroxybenzoic acid (TBA-CS), have been explored for germanium adsorption. The adsorption efficiency follows the order TBA-CS > DBA-CS > HBA-CS, correlating with the number of o-phenolic hydroxyl groups. The adsorption process aligns with the pseudo-second-order kinetic and Langmuir isotherm models, indicating a chemisorption-based single-layer adsorption. The presence of competing anions negatively affects adsorption, with silicate exerting the greatest impact. When iron(III) is present at a 1:1 ratio with germanium, it is preferentially adsorbed; however, as the ratio increases, germanium adsorption improves. Nitric acid effectively desorbs germanium, but adsorption efficiency declines over repeated cycles.
Commercial N-methylglucamine-based sorbents (S108, CRB03, CRB05) have demonstrated high efficiency in removing trace elements, including germanium, from solar saltworks brines. Germanium and boron exhibit rapid adsorption kinetics, reaching equilibrium within an hour. Desorption is achieved using hydrochloric acid, though separating individual metals from the eluate remains a challenge. The anion-exchange resin DIAION CRBO2, containing hydroxyl and amine groups, has also been studied for germanium and copper removal from chloride solutions. Germanium is adsorbed via complexation with hydroxyl and amino groups, primarily through germanate acid and metagermanate anions. While copper is desorbed using hydrochloric acid, germanium requires sodium hydroxide for desorption.
Titanium dioxide nanoparticles modified with organic acids have been explored for germanium recovery, with adsorption efficiency dependent on the number of hydroxyl groups present. Tartaric acid-modified TiO₂ (TA-TiO₂-OH) exhibits the highest adsorption capacity of 122 mg/g at pH 3. While desorption using hydrochloric acid is effective, adsorption efficiency declines after multiple cycles due to incomplete germanium removal.
During the neutralization leaching of zinc oxide dust, germanium is lost due to its adsorption onto colloidal iron hydroxide. The application of ultrasonication has been shown to reduce the redox potential of the system, inhibiting colloidal iron hydroxide formation. Under optimized conditions—30 g/L sulfuric acid, 80°C temperature, and 500 W ultrasonic power—germanium and iron losses are significantly lower compared to conventional processing methods.
Despite the development of various adsorbents and recovery techniques, disparities in adsorption capacities and long-term stability remain key challenges. Many studies are conducted using synthetic solutions, which do not accurately replicate real industrial leachates containing high concentrations of sulfuric acid. The decline in adsorption efficiency after multiple adsorption-desorption cycles further complicates large-scale applications. Future research should focus on optimizing these technologies for real waste streams to bridge the gap between laboratory experiments and industrial requirements.
Bio-hydrometallurgical approaches for germanium recovery remain underexplored but hold promise for sustainable extraction methods. Continued advancements in these recovery processes will be crucial for ensuring a stable and reliable supply of germanium, particularly as demand increases in the semiconductor and fiber optic industries. The development of efficient and environmentally friendly recycling technologies will contribute to a circular economy, reducing dependence on primary germanium sources while minimizing environmental impact.