Fluoride occurrence in environment, regulations, and remediation methods for soil: A comprehensive review.

Fluoride, a naturally occurring chemical element, is largely insoluble in soils. More than 90% of the fluoride in soil is bound to soil particles and is unable to be dissolved. As part of the soil, fluoride is predominantly located in the colloid or clay fraction of the soil, and the movement of fluoride is strongly affected by the sorption capacity of the soil, which is affected by pH, the type of soil sorbent present, and the salinity. The Canadian Council of Ministers of the Environment soil quality guideline for fluoride in soils under a residential/parkland land use scenario is 400 mg/kg. In this review, we focus on fluoride contamination in soil and subsurface environments, and the various sources of fluorides are discussed in detail. The average fluoride concentration in soil in different countries and their regulations for soil and water are comprehensively reviewed. In this article, the latest advances in defluoridation methods are highlighted and the importance of further research addressing efficient and cost-effective methods to remediate fluoride contamination in soil is critically discussed. Methods used to mitigate fluoride risks by removing fluoride from the soil are presented. We strongly recommend that regulators and soil chemists in all countries explore opportunities to improve defluoridation methods and consider adopting more stringent regulations for fluoride in soil depending on geologic conditions.

Defluoridation of water using dicarboxylic acids mediated chitosan-polyaniline/zirconium biopolymeric complex.

The present investigation describes the preparation of hydrogen bonded chitosan-polyaniline/zirconium biopolymeric matrix by grafting method under dicarboxylic acid medium for the removal of fluoride, first time. Herein, the dicarboxylic acids, oxalic acid, malonic acid, succinic acid were used as medium. The synthesized complex was characterized by usual analytical techniques like FTIR, XRD, SEM and EDAX analysis. From the batch equilibrium experiments, the maximum defluoridation capacity (DC) was found to be 8.713 mg/g at room temperature with the minimum contact time of 24 min at 100mg of the sorbent dosage. The temperature study results of adsorption kinetics showed the adsorption behavior could be better described by the pseudo-second-order equation than pseudo-first-order kinetic model. The adsorption isotherm was well fitted by the Freundlich equation rather than Langmuir and D-R isotherms. The mechanism of fluoride removal was ligand exchange at neutral pH and electrostatic attraction at acidic pH of the medium. Regeneration studies were carried out to identify the best regenerant which makes the process cost-effective. Conclusions of this work demonstrate the potential applicability of the dicarboxylic acid mediated chitosan-polyaniline/zirconium complex as an effective adsorbent for fluoride removal from water.