Design and preparation of CaFe layered double hydroxides for efficient treatment of fluorinated water deduced by tailored sustained-release of Ca2+

Abstract

The pollution from fluorinated wastewater is increasingly severe, and its excessive intake can cause significant harm to human health. CaFe-LDH (Layered Double Hydroxides, LDH) is susceptible to hydrolysis, releasing Ca2+ that can cause secondary pollution to water sources, resulting in limited attention from researchers regarding its application in water treatment. This research explored the application of CaFe-LDH in fluorinated water treatment, harnessing its distinct properties for combined defluoridation through precipitation and adsorption. Various CaFe-LDHs with different Ca/Fe ratios were prepared using inexpensive raw materials and calcined to obtain various CaFe-CLDHs (Calcined Layered Double Hydroxides, CLDH). The calcination temperature and the Ca/Fe ratios influence the composition of CaFe-CLDH, thereby determining its hydrolysis capacity. The key intermediate product of CaFe-LDH/CLDH hydrolysis, CaFe2O4, was discovered for the first time, which slow hydrolysis provides crucial sustained-release capacity for Ca2+, increasing the utilization rate of free Ca2+ and significantly enhancing the defluoridation capacity. CaFe-LDH/CLDH exhibits excellent defluoridation capacity, which increases with the rise in Ca/Fe ratio and calcination temperature. The defluoridation capacity of Ca2Fe-LDH was 234.76 mg g−1, while Ca10Fe-CLDH-800 °C exhibited an astonishing defluoridation capacity of 463.16 mg g−1. The defluoridation of CaFe-LDH/CLDH involves a complex coupling reaction process, incorporating precipitation, coagulation, and adsorption, while the hydrolysis process encounters resistance, as confirmed by DFT simulation results that also verify the adsorption capacity of CaFe2O4 for F. This work provides guidance for the treatment of fluorinated water.