Microwave-assisted synthesis of catechol-based functionalized adsorbents for selective adsorption of critical element germanium

The critical element germanium lacks efficient and economical recovery process to recover it from dilute sources such as coal fly ash. In the current study, a novel microwave-based synthesis method was used to functionalize polystyrenic bead with catechol (A-Cat), nitro-catechol (A-Cat-N), and pyrogallol (A-Py). The adsorbents were used to selectively adsorb Ge from dilute acidic solutions and were investigated for pH dependency, adsorption isotherm, adsorption kinetics, Ge selectivity, and regeneration of adsorbent. The functionalization was confirmed with FTIR, and the grafting yields of 50 %, 41.7 %, and 62 % were achieved for catechol, nitro-catechol, and pyrogallol functionalization, respectively. The Ge adsorption followed Langmuir isotherm with Langmuir adsorption capacities of 29.76 mg/g, 39.14 mg/g, and 37.13 mg/g, for A-Cat, A-Cat-N, and A-Py, respectively, at pH 3. The adsorbents were highly selective for Ge against other elements at pH 1–3, showing no adsorption for many competitive ions at pH 1–2. The A-Cat was more selective than A-Cat-N and A-Py. The adsorbents followed pseudo-second-order kinetics. The adsorbents were reusable for multiple cycles; however, A-Cat-N and A-Py lost capacity due to incomplete desorption in the first cycle. The adsorption mechanism was surface complexation of germnaium with catechol ligand resulting in high selectivity. Linear free energy relationship was used to correlate the apparent adsorption equilibrium with metal hydrolysis constants, and these LFERs can be used to predict the relative selectivity of different cations on A-Cat, A-Cat-N, and A-Py.