Synthesis of hydroxyl-rich alginate adsorbents with selective adsorption performance of germanium from alkaline solution

Natural germanium (Ge)-containing minerals usually have low Ge concentrations, which poses many challenges for Ge enrichment and recovery. Therefore, it is imperative to explore efficient processes for Ge recovery from secondary resources. In this study, an alginate-based adsorbent (HCNTs-SA) with excellent adsorption performance for Ge was synthesized by optimal preparation conditions (2 % ( w / v ) sodium alginate (SA) concentration, a 1:15 mg HCNTs /mg SA hydroxylated carbon nanotubes (HCNTs) dosage, and 0.075 mol/L calcium chloride (CaCl 2 ) concentration). The adsorbent had a spherical shape with a layered void structure and improved swelling property (increased by 1200 %). The adsorption process of Ge by HCNTs-SA followed pseudo-second order kinetics and Langmuir adsorption isotherm, which indicated that the primary adsorption behavior was monolayer chemisorption with a maximum theoretical capacity of 151.44 mg/g at pH = 11 and 1.00 g/L adsorbent dosage, exceeding both pure alginate gels and previously reported materials. Meanwhile, HCNTs-SA exhibited high selectivity for Ge. In the Ge/Si binary system, the selectivity coefficients of Ge for HCNTs-SA were all greater than 1.00, indicating its exceptional affinity for GeO(OH) 3 − as compared to SiO 3 2− . According to FTIR, XPS analysis and DFT calculations, the primary adsorption mechanisms of Ge by HCNTs-SA were revealed as complexation and hydrogen bonding, with hydroxyl and carboxyl groups identified as key adsorption sites. This work demonstrates that HCNTs-SA is a promising adsorbent for selective Ge recovery from alkaline solutions.