Unraveling the steric hindrance roles of the phenolic hydroxyl position on the selective Ge(IV) recovery from zinc residue leachate

Understanding the structure–activity relationship between the adsorption performance and accessible adsorption sites plays a vital role in developing efficient adsorbents for Ge(IV) recovery. Herein, three chitosan-based adsorbents were designed by grafting chitosan with p -hydroxybenzoic acid (HBA-CS), 3,4-dihydroxybenzoic acid (DBA-CS), and 3,4,5-trihydroxybenzoic acid (TBA-CS), and the phenolic hydroxyl amounts induced steric effect on Ge(IV) recovery was investigated. The results show that phenolic acids were successfully grafted onto the chitosan surface by an amidation reaction. The adsorption behavior of DBA-CS and TBA-CS conforms to the pseudo-second-order kinetic and Langmuir isotherm models, indicating that it is a single layer adsorption and chemisorption is the main adsorption process . The Ge(IV) recovery performance is correlated with the arrangement of the active adsorption sites and follows the order of TBA-CS > DBA-CS > HBA-CS. The steric effect of TBA-CS is more ideal than that of DBA-CS due to more phenolic hydroxyl groups, which endows it better affinity toward Ge(IV). Additionally, TBA-CS exhibits superior selectivity in the presence of competitive cations and interfering anions, and the adsorption efficiency was not significantly decreased after four regeneration cycles by the acid elution. The chelation adsorption sites for Ge(IV) provided by the o -phenolic hydroxyl groups are the dominant contribution, and the hydrogen bond and ideal steric effect further enhance the adsorption performance of TBA-CS. The structure regulation designed by steric hindrance optimization strategy to achieve high affinity is an attractive approach, which could be extended in the recovery of precious metal ions.