Allophane compared with other sorbent minerals for the removal of fluoride from water with particular focus on a mineable Ecuadorian allophane

Worldwide F− contaminated groundwaters pose a serious health problem. For purification of these waters different adsorber materials are used such as activated aluminium oxide, iron oxyhydroxide (e.g. GEH®), or synthetic OH-Apatite (e.g. Fluorolith®). In some regions, however, inexpensive solutions are required motivating scientists to study the F− sorption of natural materials such as clays. The clay mineral allophane possesses a high affinity towards F−. In the present study different allophane samples are compared with other clay minerals as well as with Fluorolith®, a technically produced OH-apatite that is most commonly used in practice.Results obtained confirmed that allophane has the highest F− sorption capacity compared with kaolinite and goethite. The F− sorption capacity of some allophane samples is similar to the well-established F− adsorber material Fluorolith® and ranges from 3 to 5 mg F−/g. Sorption depends on the experimental conditions, particularly the pH value (present study: 8–9). The different sorption capacity of the different allophanes could be explained by the amount of aluminol groups, which is independent of the specific surface area. The comparison of the fluoride uptake plots of allophanes with those of Fluorolith®, however, proved that Fluorolith® is particularly advantageous at low F− concentrations suggesting a higher affinity of the Fluorolith® surface towards F−. Sorption kinetics, however, were similar. Fluorolith® adsorbs F− by exchange of the structural anion OH−. The comparison of the uptake plots, however, suggests that different reactions have to be considered in the case of allophane. The sorption mechanism of fluoride on allophane could not be characterized further.

Research Highlights
► In principle allophane is a candidate for an alternative F− adsorber material.
► Although Fluorolith® is particularly advantageous at low F− concentrations.