Abstract
Fluoride contamination is a major problem in wastewater treatment. Metaettringite (which has previously shown enhanced anion adsorption) was investigated as a possible adsorbent to remove fluoride from low-concentration solution (25 mg-F/L). The fluoride removal properties of ettringite and metaettringite were first compared at pH 10, and metaettringite was found to be more effective. The dominant reaction mechanism for fluoride adsorption in metaettringite was found to be recrystallization of metaettringite by rehydration; this was accompanied by precipitation of calcium fluoride. The adsorption kinetics followed the pseudo-second order model. Metaettringite was also able to remove fluoride effectively in low pH environment (i.e., at pH 3.5). The influence of coexistence of sulfate ions in solution on the fluoride removal performance was investigated, and a small decrease in performance was noted. The residual fluoride concentrations obtained with higher doses of metaettringite were lower than those specified by the Japanese effluent standard (non-coastal areas: 8 mg-F/L; coastal areas: 15 mg-F/L). The fluoride removal capacity of metaettringite was compared with those of other solid materials. The observed maximum capacity was 174.7 mg-F/g-metaettringite. In the case of high fluoride concentration solution, the main removal mechanism will be changed to calcium fluoride precipitation. In general, metaettringite is regarded as promising material for fluoride removal in wastewater treatment.
Highlights
The ettringite structure was confirmed by X-ray diffraction (XRD; RINT2000, Rigaku, Tokyo, Japan)
To determine the adsorption kinetics using metaettringite as the adsorbent, the results of metaettringite were fitted to the pseudofirst order model, pseudo-second order model, and intra particle diffusion model
It has been suggested that the ion exchange of sulfate ions in ettringite might only occur in the surface layer [30]
Summary
In the past, compared with the attention given to heavy metals and organic pollutants, the importance of fluoride contamination in effluent has usually been underestimated. People have recently begun to notice that the impact of water and soil pollution from fluoride is a serious problem. Many industries produce wastewater that exceeds allowable fluoride levels for human consumption or environmental discharge [3]. Fluoride removal is a critical part of water treatment. High fluoride content is commonly found in wastewaters from industries including the mining [4], semiconductor [5, 6], fertilizer [7, 8] and photovoltaic [9] sectors.
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