Zinc-Doped Ferrite Wrapped in Polypyrrole Pulls Excess Fluoride Out of Drinking Water

Excess fluoride in drinking water is one of the world’s most widespread yet underreported water crises. While small amounts of fluoride protect teeth, concentrations above the World Health Organization guideline of 1.5 milligrams per litre can cause dental and skeletal fluorosis, conditions that permanently damage teeth and bones. More than 200 million people worldwide are estimated to drink water with elevated fluoride, and affected regions span China, India, Poland, Iran, Pakistan, Nigeria, Tanzania and Senegal. In Senegal’s groundnut basin, in the central and western parts of the country, groundwater fluoride concentrations routinely exceed 2 milligrams per litre, well above safe limits. A new study published in the Journal of Materials Science: Polymers by a team led by researchers at Université Cheikh Anta Diop in Dakar, working with collaborators in France, reports a promising engineered solution: a hybrid nanocomposite that combines a zinc-doped magnesium ferrite spinel with the conducting polymer polypyrrole, capable of stripping fluoride from contaminated water in about half an hour.

The material, abbreviated Mg-Zn-Fe@PPy, was built in two stages. First, the researchers used a sol-gel method to create the ferrite core. Stoichiometric amounts of zinc sulfate heptahydrate, iron(II) sulfate heptahydrate, magnesium sulfate heptahydrate and citric acid monohydrate were dissolved in absolute ethanol and stirred at 70 to 85 degrees Celsius for roughly four hours until a viscous gel formed. The gel was dried at 100 degrees Celsius for 16 hours, ground into a fine powder and calcined at 500 degrees Celsius for four hours in static air, with a carefully controlled heating rate of 5 degrees Celsius per minute to ensure gradual thermal decomposition. The result was a crystalline spinel ferrite in which zinc ions substitute into the magnesium-iron oxide lattice, a doping strategy chosen because iron, zinc and magnesium all show strong affinity for fluoride during adsorption.

The second stage wrapped these nanoparticles in polypyrrole through oxidative polymerization. Four grams of the Mg-Zn-Fe nanoparticles were dispersed in deionized water, ferric chloride oxidant was added and allowed to equilibrate, and freshly distilled pyrrole monomer was introduced dropwise. After six hours of vigorous stirring, a black colloidal suspension formed, which was vacuum filtered, washed with ethanol-water mixtures and dried at 80 degrees Celsius. The choice of polypyrrole is scientifically deliberate: the polymer is electrically conductive, environmentally stable, non-toxic, inexpensive and easy to prepare. Crucially, during chemical polymerization, exchangeable counter-anions from the reaction solution become incorporated into the polymer matrix to maintain charge balance, giving the material a built-in ion-exchange capacity that fluoride ions can exploit.

Characterization confirmed the hybrid structure at every step. Raman spectroscopy of the bare ferrite showed the five characteristic vibrational modes of a spinel structure, with high-frequency metal-oxygen modes shifted by 15 to 25 wavenumbers relative to unsubstituted ferrites, evidence of zinc substitution in the lattice. The composite spectrum additionally displayed the disorder-induced D band and the graphitic G band of polypyrrole, with an intensity ratio of 0.851 indicating partial graphitization and sp2-hybridized domains. Most tellingly, the ferrite’s own vibrational modes were completely attenuated in the composite, showing that the polymer coating exceeded the Raman probe depth of roughly 50 nanometres and that no metal oxide surfaces remained exposed. Scanning and transmission electron microscopy revealed spherical and elliptical particles with surface cavities from volatile release during calcination, and TEM images confirmed a distinct core-shell morphology: a dark spinel ferrite core surrounded by a lighter polypyrrole shell.

Adsorption performance was assessed in batch experiments using sodium fluoride solutions quantified with SPADNS reagent by ultraviolet-visible spectroscopy. The kinetics were strikingly fast. Within the first ten minutes, adsorption jumped from zero to 1.375 milligrams per gram, driven by the abundance of readily accessible surface sites. Between ten and thirty minutes, capacity crept up from 1.375 to 1.9 milligrams per gram as fluoride migrated into less accessible internal sites, and beyond thirty minutes the curve plateaued, marking equilibrium. The overall adsorption quantity reached 1.9 milligrams per gram at an initial fluoride concentration of 5.4 milligrams per litre and a natural pH of 6.5. Kinetic modelling showed the pseudo-second-order model fit best, with a correlation coefficient of 0.9956 against 0.9823 for the pseudo-first-order model, a signature of chemisorption in which chemical bonds form between fluoride and the adsorbent.

The pH of the solution proved decisive. The point of zero charge of the nanocomposite was measured at 6.3, meaning the surface is positively charged below that pH and negatively charged above it. Optimal removal occurred between pH 6 and 7, peaking at 6.5. In more acidic conditions, protons react with fluoride ions to form hydrofluoric acid, a weak molecular acid that the composite cannot effectively capture, dropping capacity to 0.775 milligrams per gram. In alkaline conditions, two mechanisms conspire against removal: the negatively charged surface electrostatically repels fluoride anions, and abundant hydroxide ions compete for the same adsorption sites. Adsorbent dosage also mattered, with removal efficiency rising from 25.9 to 71.7 percent as the dose increased, though capacity per gram fell as sites saturated beyond 100 milligrams of adsorbent.

Temperature told its own story. As conditions warmed from 25 to 45 degrees Celsius, uptake rose from 1.9 to 2.299 milligrams per gram, and thermodynamic analysis revealed a positive enthalpy change exceeding 20 kilojoules per mole, further evidence of chemisorption, alongside positive entropy and negative Gibbs free energy values confirming the process is spontaneous and becomes more so at higher temperatures. Among the isotherm models tested, the Freundlich model gave the best fit with a correlation coefficient of 0.9837, indicating a heterogeneous adsorbent surface with multilayer adsorption, while the heterogeneity factor below one confirmed a favourable process. Intraparticle diffusion modelling identified three sequential stages, with film diffusion dominating early and pore diffusion slowing markedly as active sites saturated.

Real water rarely contains fluoride alone, so the team tested interference from chloride, nitrate, bicarbonate, carbonate, sulfate and phosphate anions. The pattern was clear: the higher the negative charge of the competing ion, the greater the reduction in fluoride uptake, because multi-charged species such as sulfate and carbonate carry greater electron density and bind more strongly to the positively charged surface. Monovalent ions like chloride, nitrate and bicarbonate had little effect. Infrared spectroscopy before and after adsorption revealed the mechanism directly: the disappearance of the N-H band near 3400 wavenumbers points to ion exchange between chloride dopants in the polymer and fluoride ions, while changes in the 400 to 800 wavenumber region of the metal oxide bands indicate fluoride also swaps with hydroxide groups on the ferrite and binds electrostatically to protonated surface sites.

Practicality hinged on regeneration and real-world testing. The spent composite was regenerated with sodium hydroxide followed by hydrochloric acid treatment and reused across five consecutive cycles, with removal efficiency declining from 70.3 percent to 49.65 percent, a decrease attributed to incompletely desorbed chemisorbed fluoride and modest mass loss, but the stabilized capacity demonstrated genuine reusability. The decisive test came with actual tap water collected from localities across Senegal’s Arachidier basin and the department of Touba, all of which exceeded both WHO and Senegalese fluoride standards. After treatment, the Touba, Fatick and Koutal samples fell below the WHO limit, while Kaolack and Sibassore remained slightly above it, likely due to salinity and interfering ions, though every sample met the Senegalese standard after adsorption.

The study positions Mg-Zn-Fe@PPy as a low-dose, fast-acting adsorbent that outperforms many literature alternatives on contact time and dosage, even where some materials boast higher raw capacities at far larger masses. Because it is non-toxic, inexpensive to synthesize from commodity salts and effective on natural groundwater rather than just synthetic solutions, the nanocomposite offers a realistic pathway for communities in Senegal and across the fluoride belt of the developing world. As climate change, urbanization and industrialization continue to strain groundwater quality, materials that marry simple chemistry with demonstrated field performance may prove essential to turning a silent epidemic of fluorosis into a solvable engineering problem.

Subject of Research: Fluoride removal from drinking water using a zinc-doped magnesium ferrite oxide and polypyrrole nanocomposite adsorbent

Article Title: Engineering zinc-doped magnesium ferrite oxide/polypyrrole to remove excess fluoride from drinking water

Article References: Engineering zinc-doped magnesium ferrite oxide/polypyrrole to remove excess fluoride from drinking water. (n.d.). https://doi.org/10.1007/s44493-025-00005-y

Image Credits: AI Generated

DOI: 10.1007/s44493-025-00005-y

Keywords: fluoride removal, drinking water, polypyrrole, magnesium ferrite, nanocomposite, adsorption, chemisorption, Senegal, groundwater, water treatment, fluorosis, sol-gel synthesis