Impact of Aluminum and Iron (III) Ions on Removal of Fluoride Ion (F − ) by Calcite

Abstract

Abstract

Matrix acidizing adds high concentrations of hydrofluoric acid into the oil production well for the purpose of well stimulation and thus, the return fluids could contain high fluoride ion (F⁻) concentrations. Depending on the fate of the return fluids, high F⁻ concentrations, if untreated, could negatively affect public health and/or the environment. This article shows that calcite (CaCO₃) can remove F⁻ ions from solutions containing 200 mg/L of F⁻ and 200 mg/L of aluminum ion (Al³⁺) or 200 mg/L of iron (III) ion (Fe³⁺) by precipitation mechanism in batch mode. Al³⁺ and Fe³⁺ are among metallic ions that are present in the subsurface and, therefore, may appear in a typical matrix acidizing return fluid. Al³⁺ and Fe³⁺ can potentially form complexes with F⁻ and negatively affect F⁻ precipitation. F⁻ concentration was measured at fixed intervals during a 3-h period. The results showed that F⁻ removal efficiencies of up to 95% could be achieved when dissolved calcite is used on distilled water containing 200 mg/L of F⁻ at [Ca²⁺]/[F⁻] ratio of 2:1. In addition, the presence of Al³⁺ or Fe³⁺ and their complexes did not hinder, and in some instances improved the F⁻ removal efficiency. The PHREEQC geochemical software was utilized as a predictive tool, which verified the study findings regarding the complexes and their role in chemical precipitation of F⁻ compounds.