A metal-organic framework surrounded with conjugate acid-base pairs for the efficient capture of Cr(VI) via hydrogen bonding over a wide pH range.

Abstract

Given the large amount of toxic Cr(VI) wastewater from various industries, it is urgent to take effective treatment measures. Adsorption has been regarded as highly desirable for Cr(VI) removal, but the effectiveness of most adsorbents is significantly dependent on pH value, in which precipitous performance drop and even structural collapse generally occur in strong acidic/alkaline aqueous. Thus, maintaining high adsorption performance and structural integrity over a wide pH range is challenging. To efficiently remove Cr(VI), we designed and prepared of an acid-base resistant metal-organic framework (MOF) Zr-BDPO, by introducing weak acid-base groups (-NH-, -N= and -OH) onto the ligand. Zr-BDPO achieved a maximum adsorption capacity of 555.6mg·g-1 and retained skeletal structure at pH=1-11. Interestingly, all these groups can generate conjugate acid-base pairs by means of H+ and OH- in the external solution and then form buffer layer. The removal of Cr(VI) at a broad range of pH values primarily via hydrogen bonds between -NH- and -OH, and the oxoanion species of Cr(VI) is unusual. This strategy that insulating high concentrations of acids and bases and relying on hydrogen bonds to capture Cr(VI) oxoanions provides a new perspective for actual Cr(VI) wastewater treatment.