Porous materials – for the adsorption and complexation of fluoride ions

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This post was last edited by Maimai on 2025-2-7 at 13:50. Among the commonly used fluoride removal techniques, adsorption technology has been widely studied; it offers an efficient, simple, low-cost, and highly selective method for removing fluoride from liquids. It has become very important to develop effective and robust materials for removing excess fluoride from liquid phases; various metal oxide adsorbents have been used for fluoride removal in liquid phases, such as aluminum-based, calcium-based, and iron-based adsorbents. Forming molecular complexes is also a method for fixing fluorides in the system. Various porous materials with fluoride removal capabilities have been developed to date, but it is still necessary to scale up the removal process on an industrial scale. More research is needed on regeneration in order to recover the adsorbent under field conditions and improve the economic viability of this process. Adsorption and Complexation 1. Adsorption: Ion exchange. Adsorbents often contain polar functional groups with hydrogen atoms, such as hydroxyl groups (—OH), carboxyl groups (—COOH), amino groups (—NH2), and sulfonic acid groups (—SO3H). The hydrogen atoms in these functional groups interact with the lone pair of electrons of fluorine to form hydrogen bonds with an angle of approximately 180°, thereby enabling the adsorption of fluorides. Adsorbent: Aluminum oxide. During the adsorption of fluoride ions by aluminum oxide, this process is primarily achieved through the exchange of fluoride ions with OH-. Therefore, the acidity and alkalinity of the adsorbent surface, as well as anion exchange, have an impact on the fluoride removal process. Acidic alumina exhibits higher adsorption capacity and a faster fluoride removal rate than basic alumina. This is because the ion exchange processes involved in the adsorption of fluorine are completely different for acidic alumina and basic alumina. For acidic alumina, OH- and Cl- provide exchange sites for fluorides ； For basic alumina, the exchange sites are mainly provided by OH-. Adsorbent: Titanium oxide TiO2 is selective for fluoride ions, and the adsorption kinetics of fluorides are relatively fast. It adsorbs fluoride ions from electrolyte solutions, with 75% of the fluoride ions being adsorbed within 10 minutes. The lower fluoride absorption in alkaline solutions is due to the competition between hydroxyl groups on the active sites of the adsorbent and fluoride ions, whereas in acidic solutions, the higher number of positively charged sites increases fluoride absorption. 2. Complexation: Fluorine-organic amine complexes. The formation of molecular complexes on the surface of porous materials is another way to remove fluorides from organic systems; for example, the complexation of HF with organic bases (such as pyridine/HF complexes and triethylamine/HF complexes). However, the complexation of organic bases may cause a change in the system’s pH value, so the selection of functional groups is particularly important. HF and non-alkaline, non-nucleophilic, weakly coordinating hydrogen bond acceptors can form stable complexes through hydrogen bonding. There are already comprehensive databases on hydrogen bond basicity (measured by pKBHX); for most compounds, pKBHX ranges from 1 to 5, with higher values indicating greater hydrogen bond basicity. This database can serve as a guide for selecting ideal hydrogen bond acceptor functional groups that, when attached to porous carriers, can complex with HF to achieve defluorination. Data source: doi:10.1016/j.cej.2012.02.041.dx.doi.org/10.1021/ja508369zdoi.org/10.1016/j.jcis.2012.11.063