Recovery of fluoride from wastewater in the form of cryolite granules by fluidized-bed homogeneous crystallization process

Abstract

Aquatic ecosystems are harmed by high fluoride levels, which can disturb the equilibrium of water, plants, and animals and reduce biodiversity. Fluoride contamination in water sources can pose health risks and it is crucial to remove fluoride to prevent adverse effects on the environment and public health. Green technology is employed to recover fluoride from simulated fluoride-rich wastewater. The fluidized bed homogeneous crystallization (FBHC) process is an advanced methodology that removes contaminants from wastewater through supersaturation to recover nontoxic granules that can be used for other purposes. This study compared the fluoride recovery between batch crystallization and the FBHC process in terms of the molar ratio (MR) of [F−]:[Al3+] between [1] and [3] and the pH influence on the recovery and crystallization. The concentration of [Al3+] was varied at 37, 74, and 111 mM, along with the influent flow rates of 4, 8, and 12 mL·min−1. FBHC systems offer improved control of pH levels, creating a stable environment for crystallization. Unlike batch systems, these continuous operating systems are more resilient to pH fluctuations due to efficient mixing, maintaining a consistent pH throughout the process. Using Box-Behnken Design (BBD) for the FBHC, optimal conditions were 74 mM initial [Al] concentration, MR of [F−]:[Al3+] of [2]:[1], and 8 mL·min−1 influent flow rate resulted in 93 % fluoride removal (FR) and 91 % crystallization ratio (CR). XRD and SEM-EDS data showed similarities between the peaks and compositions of cryolite (Na3AlF6). FBHC was able to produce granules based on the reaction mechanism in the synthesis of cryolite.