Adsorption and photocatalytic desorption toward Cr(vi) over defect-induced hierarchically porous UiO-66-(OH)2: a sustainable approach

Abstract

A facile modulation strategy was adopted to fabricate hierarchically porous HP-UOH-X (HP, UOH and X represented the hierarchical pores, UiO-66-(OH)₂ and the dosage of benzoic acid, respectively) via introducing benzoic acid with different dosages into the precursor solution of UiO-66-(OH)₂. The formation of hierarchical pores boosted the exposure of –OH groups and the Cr(VI) mass transfer in HP-UOH-X, which vastly enhanced its sorption capacities and sorption rates. The optimal adsorbent (HP-UOH-80) displayed better sorption capacity (266.74 mg g⁻¹) toward Cr(VI) (T = 308 K, pH = 2.0) and faster diffusion rate (k₁ = 14.21 mg g⁻¹ min^0.5, k₂ = 6.25 mg g⁻¹ min^0.5) than those of the pristine UiO-66-(OH)₂ and other HP-UOH-X adsorbents. Interestingly, HP-UOH-80 exhibited good selective uptake ability toward Cr(VI) in different simulated water samples containing various competing anions. The corresponding mechanism was proposed that the –OH groups and the defect sites played the dominant contribution to Cr(VI) adsorption, which could be affirmed by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, the strategy of photocatalytic Cr(VI) reduction for desorption was introduced to replace traditional chemical desorption. In all, this work presented an effective adsorbent and a sustainable approach for Cr(VI) elimination, which can regenerate the adsorbent via a photocatalytic process rather than chemical washing.