A novel core–shell bimetallic ZrAl-MOF simultaneously boosting electrostatic attraction and ion exchange to eliminate excessive fluoride†

Abstract

Excess fluoride in drinking water can cause poisoning. To solve this problem, a porous metal–organic framework (MOF) was fabricated via the solvothermal approach and employed for the removal of fluoride ions from water. ZrAl-MOF was fabricated by self-assembly of polyvalent Zr⁴⁺, Al³⁺metal ions and 4,4-biphenyldicarboxylic acid (BPDC). The developed metal-based ZrAl-MOF was used to remove fluoride ions from water and could remove fluoride ions to a maximum of 109.2 mg g⁻¹ (308 K). ZrAl-MOF has a special core–shell structure with a layer of small balls stacked outside and cobweb-like structure inside. The properties of the bimetallic MOF can be adjusted by Zr⁴⁺, Al³⁺ metal ions and BPDC to meet the demand for maximum adsorption performance. Bimetallic MOFs commonly have a substantial specific surface area, capable of providing numerous active sites and being favorable for the adsorption reaction of substances. The electronic properties of different metals may lead to stronger electrostatic attraction and enhanced adsorption of fluoride ions. The factors affecting the adsorption effect, such as solution pH, ZrAl-MOF dosage, reaction time, initial fluoride concentration, temperature, and coexisting anions, were optimized. The fluorine adsorption capacity of ZrAl-MOF was less affected by the adsorbent under acidic conditions and by the presence of sulphate and nitrate ions in the water. In addition, the experimental data were fitted with various adsorption kinetic and isotherm models. It is shown that fluorine adsorption is feasible and spontaneous. The fluorine adsorption mechanism of ZrAl-MOF is mainly electrostatic attraction and ion exchange.