Molecular identification guided process design for deep removal of fluoride from electroplating effluent

Abstract

Fluoride was ubiquitous in effluents from various industries; however its specification in wastewater was poorly understood, hindering the rational design of a water defluoridation process. In this study, we developed a molecular identification method for fluoride in industrial wastewater by combining NMR analysis and chemical tests. Given the complicated composition and the huge quantity, electroplating effluents were chosen as the representative of fluoride-containing wastewater. ¹⁹F NMR analysis suggested F⁻ and BF⁻₄ as the two main fluoride forms in raw wastewater and effluents from wastewater treatment sections, and chemical tests suggested F⁻ concentration in the effluents ranging from 5.1 to 7.3 mg L⁻¹. We also explored the adsorptive removal of BF⁻₄ by using the strongly basic ion exchange resin D201, i.e., quaternary ammonated poly (styrene-co-divinylbenzene) beads. The adsorption of D201 toward BF⁻₄ followed pseudo-first-order kinetics (k₁ = 2.45 h⁻¹) with an equilibrium time of less than 3 h. Adsorption isotherms were fitted well with the Langmuir model, with the maximum adsorption capacity of 257.25 mg g⁻¹ at 298 K. D201 exhibited a greatly enhanced adsorption selectivity toward BF⁻₄ compared to common anions including Cl⁻ and SO²⁻₄, mainly owing to the low hydration energy (ΔG_hyd° = −220 kJ mol⁻¹) of BF⁻₄. In the fixed-bed mode, the D201 column could generate 850 and 460 bed volume (BV) clean water ([BF⁻₄] < 1.5 mg-F/L) from synthetic and realistic electroplating effluents, respectively. The exhausted D201 column was fully refreshed with NaCl solution for five defluoridation cycles with a constant effective treatment amount of ∼1400 BV.