Ultra-fast and robust capture of fluoride by an amino terephthalic acid-facilitated lanthanum-based organic framework: insight into performance and mechanisms

Abstract

A sorbent consisting of a lanthanum-loaded metal–organic framework, involving terephthalic acid (La@TPA) and amino terephthalic acid (La@ATPA) as organic linkers, was synthesized and evaluated for fluoride (F) removal from groundwater. F sorption on La@ATPA was unaffected by the initial pH and remained constant over pH 3–10. For a sorbent dose of 1.0 g L⁻¹ in deionized water, the fluoride concentration decreased from 10 mg L⁻¹ to 1.5 mg L⁻¹ within 20 min with an F sorption capacity of 238 mg g⁻¹. Besides, the fluoride removal rate remained unaffected by other anions as determined through binary sorption experiments. Leaching tests, including the toxicity characteristic leaching procedure (TCLP), revealed that an insignificant amount of fluoride (∼0.60 mg L⁻¹) was released from the fluoride-loaded sorbent. F uptake primarily occurred due to the formation of LaF_3(s), whose presence in fluoride-loaded La@ATPA was confirmed by transmission electron microscopy-selected area (electron) diffraction (TEM-SAED), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Besides, electrostatic and Lewis acid–base interactions were also suggested by Fourier-transform infrared spectroscopy (FTIR) and XPS analyses. Furthermore, the synthesized sorbent removed fluoride from 6.0 mg L⁻¹ to 1.2 mg L⁻¹ in groundwater within 90 min, with a relatively low dose of 2.5 g L⁻¹. These results were consistent with the thermodynamic favorability of the formation of LaF_3(s) over a wide range of conditions tested. The excessively high fluoride uptake, applicability to a broad pH range, recyclability, stability of the sorbed fluoride, and insignificant interference from common anions including those in fluoride-contaminated groundwater makes La@ATPA an attractive candidate for defluoridation.