Mechanisms for interactions of fluoride with hydroxyaluminum-montmorillonite complexes and the environmental implications for fluoride-contaminated soils
ZHU Mao-xu1, XIE Mei2, YU Hong1, JI Hong-wei11. College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266003, China; 2. College of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China
Hydroxyaluminum-montmorillonite (HyAl-Mt) complexes with OH/Al ratio of 1.6 were synthesized by irreversible adsorption of polymeric hydroxyaluminum cations on Na-saturated montmorillonite (Na-Mt). The interactions of fluoride with the HyAl-Mt are investigated under weak acid and strongly acidified conditions, and the environmental implications for fluoride-contaminated soils are also addressed. The results indicate that the effects of pH on fluoride sorption on the HyAl-Mt are slight in the pH range of 5.0～9.0, while sorption increases rapidly with decreasing pH when pH is lower than 4.5. At initial pH 6.62, ligand exchange is the main mechanism of fluoride sorption on the HyAl-Mt; at initial pH 3.02, several mechanisms such as surface adsorption, mineral dissolution and coprecipitation-sweeping are involved in fluoride sorption, through which fluoride removal is greatly enhanced. Compared with the Na-Mt, the sorption capacities of the HyAl-Mt for fluoride are significantly enhanced at similar conditions. The presence of HyAl-Mt complexes in acidic soils may greatly decreases the mobility and bioavailability of fluoride in soil environments. The interactions of fluoride with HyAl-Mt complexes may mitigate acidificationof fluoride-contaminated acidic soils to some extent. Application of synthetic HyAl-Mt complexes is one of effective ways for remediation of acidic fluoride-contaminated soils.