Interaction of fluoride with hydroxyaluminum-montmorillonite complexes and implications for fluoride-contaminated acidic soils

In acidic soils and aquatic environments, polymeric hydroxyaluminum (HyAl) cations with various OH/Al ratios are ubiquitous. Hydroxyaluminum-montmorillonite (HyAl-Mt) complexes are widely distributed in acidic to slightly acidic soils and aquatic environments. The fixation of HyAl cations on Mt can significantly modify the mineralogical and electrochemical properties of the host clay, thereby substantially influencing adsorption/desorption and transport of many nutrients and pollutants. In the present study, HyAl-Mt complexes were synthesized through adsorption of polymeric HyAl cations with OH/Al ratio of 1.6 on Na-saturated Mt (Na-Mt). Interaction of F− with HyAl-Mt was investigated in acidic conditions and the environmental implications for F−-contaminated soils were also addressed. Results indicated that the effects of pH on F− sorption by HyAl-Mt were slight in the pH range of 5.0–9.0, whereas sorption increased rapidly with decreasing pH when pH was below 4.5. At initial pH >4.0, ligand exchange was the main mechanism for F− sorption. At initial pH 3.02 and high initial F− concentrations, several synergic mechanisms, such as coprecipitation, scavenging and surface adsorption were involved in F− removal from solution, which resulted in an abrupt and discontinuous increase in sorption capacity of HyAl-Mt for F− with increasing initial F− concentration, and a slight dependency of F− sorption on HyAl-Mt dosage. Compared with Na-Mt, sorption capacities of HyAl-Mt for F− were significantly enhanced by interlayering and coating with polymeric HyAl cations. HyAl-Mt would be important natural scavengers for F−. The presence of HyAl-Mt in acidic soils could greatly retard F− transport and bioavailability in soil environments. Interaction of F− with HyAl-Mt may mitigate acidification of F−-contaminated acidic soils. Application of synthetic HyAl-Mt may be one alternative for remediation of acidic F−-contaminated soils.