Interactions Of Fluoride With HyAl(Fe)-montmorillonite Complexes And Their Implications For Fluoride-contaminated Acidic Soils

Fluoride is one of necessary micronutrients for both humans and animals. However, excessive intake of fluoride causes dental and/or skeletal fluorosis. Nowadays, with the rapid development of industry and agriculture, fluoride pollution occurs widely in air, water and soil. Generally, soil is the main media into which fluoride-containing gases and particles are added. Moreover, gaseous fluoride absorbed by plant leaves also finally accumulates in the topsoil through defoliation. Accordingly, interactions of fluoride with soils play an important role in determining fluoride transport and bioavailability.Many studies indicate that sorption of fluoride on soils is closely related to the constituents of soil clays, solution pH, and Al-F complexation. In acidic soils and aqueous environments (pH= 4.0~5.5), polymeric hydroxyaluminum (HyAl) cations with various OH/Al ratios are ubiquitous. Some models of polymeric HyAl cations have been proposed, for instance single [Al6(OH)12]6+ or double [Al10(OH)22]8+ gibbsite-like rings and much complex Al13 polynuclear species [AlO4Al12(OH)24(H2O)12]7+ etc. Montmorillonite (Mt) is a kind of 2:1 type expansible silicate that widely exists in soils. Due to large CEC of Mt, HyAl cations could be adsorbed on the external or internal surfaces of Mt and form HyAl-montmorillonite complexes. HyAl-montmorillonite complexes are widely distributed in acidic to slightly acidic soils. The fixation of HyAl cations on the Mt could significantly modify the mineralogical and electrochemical properties of the host clays. This would substantially influence adsorption/desorption reactions and transport of some nutrients and pollutants in soils and much attention of environmental scientists has been drawn to the effects of the complexes on ecological environments. In spite of a wide geographic distribution of HyAl-Mt-like components in many soils and extensiveinvestigations on fluoride adsorption onto bulk soils, the interactions of fluoride with HyAl-Mt complexes has not yet been specifically investigated.In the present study, HyAl-Mt complexes with different OH/Al ratios (1.6, 2.1, 2.4) and HyAl/Fe-Mt complexes were synthesized and characterized. Interactions of fluoride with HyAl(Fe)-Mt were investigated at weak acid to strongly acidified conditions and their environmental implications for fluoride-contaminated soils were also addressed. Additionally, desorption experiments were also conducted to evaluate the stability of fluoride fixed on HyAl(Fe)-Mt complexes. The main results of the research are as folio wings:(1) The results of characterization of montmorillonite and its complexes show that HyAl and HyFe ions have been intercalated in interlayers of the Na-Mt. Both the addition of Fe and different OH/Al ratios have great effects on the layer structure and surface properties of HyAl-Mt complexes. The HyAl-Mt is thermally stable to some extent below 500 ℃.(2) Interactions of fluoride with HyAl-Mt 1.6 were investigated at weak acid to strongly acidified conditions. Results indicate that the effects of pH on fluoride sorption onto HyAl-Mt1.6 were slight in the pH range of 5.0~9.0, whereas sorption increases rapidly with decreasing pH when pH was below 4.5. At initial pH6.62, ligand exchange was the main mechanism for fluoride sorption. At initial pH3.02 and high initial fluoride concentration, several mechanisms such as coprecipitation, scavenging and surface adsorption were involved in fluoride removal, through which fluoride sorption was pronouncedly enhanced and a rapid and discontinuous increase in sorption capacity in the sorption isotherm was observed with increase in initial fluoride concentration. Compared with Na-Mt, sorption capacities of HyAl-Mt1.6 for fluoride were significantly enhanced by interlayering and coating with polymeric HyAl cations. Therefore, the presence of HyAl-Mt complexes in acidic soils may greatly retard the transport and bioavailability of fluoride in soil environments.(3) At weak acid to strongly acidified conditions, the adsorption behaviors of fluoride on HyAl-Mt with varying OH/Al ratios(1.6, 2.1, 2.4) and on HyAl/Fe-Mt complexes were investigated in detail. The results indicate that the adsorptioncapacity of HyAl-Mt increased with decreasing OH/Al ratio. At strongly acidified conditions, HyAl-Mt complexes have higher adsorption abilities than HyAl/Fe-Mt, while at weak acid conditions, defluoridation capacity of the former is somewhat lower than that of the latter.