Understanding aliphatic and phenolic hydroxyl functionalities in UiO-66-p-OHs for selective boron extraction from saline water

Boron is both an essential trace element and an important industrial raw material, rendering its efficient extraction from brines highly valuable. This study synthesized UiO-66-p-PhOH (phenolic hydroxyl) and UiO-66-p-AlOH (aliphatic hydroxyl) by grafting ortho-hydroxyl functional groups onto UiO-66-p, achieving adsorption capacities of 2.28 and 3.65 mmol/g in 4000 mg/L B solution, respectively. Both adsorbents reached adsorption equilibrium within 20 mins, enabling efficient boron extraction. UiO-66-p-OHs maintained structural stability in complex brines and exhibited higher selectivity coefficients ( K B were between 1.5 and 18.1) compared to that in binary solutions ( K B were between 0.09 and 5.2). Moreover, boric acid bound by each phenolic hydroxyl was 2.5–4.3 times of aliphatic hydroxyls. DFT calculations revealed that phenolic hydroxyls exhibited stronger affinity, with an adsorption energy of −29.01 compared to −27.24 kJ/mol for aliphatic hydroxyls. UiO-66-p-PhOH and UiO-66-p-AlOH could be regenerated through a simple water-washing and mild-acid soaking process, achieving regeneration rates of 77.5% and 95.4%, respectively. This work demonstrated the boron extraction efficiency of phenolic hydroxyl and aliphatic hydroxyl groups in saline water, providing important reference for designing boron adsorbents under high salinity conditions.