Fluoride enters water through natural geological processes and industrial activities, including semiconductor manufacturing, electroplating, and glass production. Excessive exposure can damage teeth and bones, making effective removal an environmental priority. Adsorption offers a relatively simple treatment approach, and nanoscale magnesium oxide has attracted interest because of its availability and fluoride-binding properties. However, nanoparticles can clump together, limiting accessible reaction sites and complicating recovery. Carbon supports can improve their dispersion, but developing materials that combine high fluoride uptake with tolerance to changing water chemistry remains challenging. Invasive plant biomass offers an accessible feedstock for exploring this combination.
A study (DOI: 10.48130/bchax-0026-0021) published in Biochar X on 30 September 2026 by Jiunian Guan's team, Northeast Normal University, develops a biomass-supported composite with high fluoride uptake through multiple complementary capture mechanisms.
The researchers used Rhus typhina residues to prepare hydrochar, then applied magnesium chloride-assisted pyrolysis to produce a nanoscale magnesium oxide-modified pyro-hydrochar composite, abbreviated nMgO/Py-HyC. They conducted batch adsorption experiments to examine how fluoride uptake changed with concentration, contact time, temperature, pH, competing anions, and ionic strength.
Microscopy, elemental analysis, X-ray diffraction, infrared spectroscopy, and X-ray photoelectron spectroscopy characterized the material before and after treatment. Separate tests with the carbon support and magnesium oxide assessed whether combining them improved performance. Microscopy revealed a porous framework covered with stacked nanoscale sheets resembling coral-reef folds, while diffraction confirmed the presence of crystalline magnesium oxide.
Both Langmuir and Freundlich models described the adsorption data well, with the Langmuir model yielding the maximum capacity of 469.64 milligrams per gram at 298 kelvin. Uptake increased sharply during the first 24 hours and approached equilibrium after 48 hours. Thermodynamic analysis indicated that adsorption was spontaneous and involved substantial chemical interactions. Performance remained statistically stable between pH 5 and 11, but declined under strongly acidic or alkaline conditions. Nitrate and sulfate concentrations up to 10 millimoles per liter caused no significant interference, whereas bicarbonate above 1 millimole per liter inhibited fluoride capture.
Increasing ionic strength also reduced uptake, demonstrating that water composition can affect treatment performance. In a comparison using an initial fluoride concentration of 100 milligrams per liter, the composite adsorbed 86.44 milligrams per gram, exceeding the reported sum of uptake by the separately tested components. Analyses after adsorption detected fluoride and supported the formation of magnesium-fluoride bonds and fluoride-containing solids. Together, the findings supported a mechanism involving electrostatic attraction, surface complexation, anion or ligand exchange, precipitation, and hydrogen bonding. The carbon framework helped expose reactive sites and supported interactions between fluoride and magnesium-containing phases.
The study presents a materials strategy that connects fluoride removal with the productive use of invasive plant residues. Its contribution extends beyond adsorption capacity by showing how a porous carbon support and magnesium oxide can work together under varied solution conditions. The findings establish a laboratory basis for further development, while the authors identify engineering evaluation under different actual wastewater conditions as a next step. Such work will help determine how the composite's adsorption behavior translates into practical treatment systems.