# Investigation of Solution Microstructure in Ferric Sulfate Coagulation-Assisted Precipitation of Fluoride Ions

## TL;DR

This study explores how ferric sulfate helps remove fluoride ions by forming stable clusters during a chemical precipitation process.

## Contribution

The study reveals the formation of binuclear clusters between calcium fluoride and ferric hydroxide during coagulation.

## Key findings

- Ferric ions hydrolyze to form hydrated ferric hydroxide clusters at higher pH levels.
- The most stable hydrated cluster is [Fe(OH)3·(H2O)2] with the lowest binding energy.
- Calcium fluoride and ferric hydroxide clusters form binuclear clusters via electrostatic interactions.

## Abstract

The solution microstructure during the ferric sulfate-assisted precipitation of calcium fluoride was systematically investigated using molecular dynamics simulations and DFT methods. The microscopic behavior of various ions in a calcium fluoride box in the presence of ferric sulfate was simulated using MD. The corresponding hydrated cluster structures were extracted from the MD trajectory; then, the structure was optimized and the frequency was calculated at the B3LYP/6–311++G(d, p) level. The results show that no hydrated clusters had imaginary frequencies. Based on the topology, interaction region indicator, and surface electrostatic potential and binding energy analysis of the hydrated clusters, it was revealed that ferric ions are easily hydrolyzed to form hydrated clusters of ferric hydroxide at higher pH levels. The most stable of these structures is [Fe(OH)3·(H2O)2], which has the lowest binding energy. During the ferric sulfate coagulation process, calcium fluoride clusters and ferric hydroxide clusters could form binuclear clusters through electrostatic interaction. The two metal centers in the binuclear cluster, Ca and Fe, are connected by hydroxide ions.