Magnetic Oakwood Biochar Sweeps Phosphate From Wastewater Before Membrane Distillation

Municipal wastewater is not supposed to be glamorous, but a new study from Prairie View A&M University may have found one of the more elegant tricks to emerge from the treatment plant in years. Researchers there have shown that ordinary oak chips, charred in an oven-like furnace and studded with magnetic iron oxide nanoparticles, can strip nearly all of the phosphate out of phosphate-rich wastewater while cutting the cloudiness of the water by more than three quarters. The pretreated water then flows into a thermal desalination unit known as air-gap membrane distillation, where it emerges dramatically cleaner. The combined process, described in Frontiers in Membrane Science and Technology, addresses two stubborn problems at once: the chemical leftovers that conventional coagulants leave behind, and the membrane fouling that steadily chokes the performance of advanced water purification systems.

The team, led by Prashan M. Rodrigo and Raghava R. Kommalapati at the Center for Energy and Environmental Sustainability, began with a simple observation about the state of the art. Water treatment plants routinely use iron salts such as ferric chloride and ferrous sulfate, or aluminum-based compounds like polyaluminum chloride, along with polyacrylamide polymers, to clump suspended particles together so they can settle out of the water. These coagulants work, but they come at a price. Excess chemicals can remain in the treated water, adding a residual chemical load that complicates downstream processing. The researchers wanted a coagulant that would do the same job, do it better, and then conveniently remove itself from the equation before the water reached the next treatment stage.

Their answer is a composite they call Fe3O4/BC. Oakwood chips were dried at 105 degrees Celsius and then pyrolyzed at 400 degrees Celsius for one hour in a low-oxygen environment, yielding a carbon-rich biochar with a yield of about 62 percent. The char is naturally porous, with a large surface area and an abundance of surface functional groups such as hydroxyl, carboxyl, and carbonyl groups that can grab onto pollutants. To supercharge it, the team mixed the biochar into a solution of iron(II) and iron(III) salts in a two-to-one molar ratio, then raised the pH with potassium hydroxide. This co-precipitation step nucleated magnetite, Fe3O4, directly onto the biochar’s porous scaffold. After a day of aging at room temperature, the finished composite carried a magnetite loading of 17.3 percent, confirmed by X-ray diffraction showing the characteristic face-centered cubic pattern of magnetite with crystallites averaging roughly 24 nanometers, and by infrared spectroscopy revealing new iron-oxygen vibrations at 560 and 620 wavenumbers.

What makes the composite special is the combination of mechanisms it deploys simultaneously. During coagulation, positively charged sites on the magnetite neutralize the negatively charged surfaces of colloidal particles in the wastewater, reducing the electrostatic repulsion that normally keeps those particles suspended. That allows larger flocs to form, while the biochar’s pores continue to adsorb dissolved and colloidal contaminants onto its extensive internal surface. Then comes the stagecraft: because magnetite is strongly magnetic, the spent coagulant and its captured cargo of pollutants can be pulled out of the water rapidly with an applied magnetic field, rather than being left to linger in the treated stream. The researchers tested doses ranging from 0.625 to 7.5 grams per liter and settled on 2.5 grams per liter as the sweet spot, a dose that cut phosphate from 18.4 milligrams per liter down to about 1.1 milligrams per liter, a 94 percent removal, and dropped turbidity by 77 percent.

Not everything yielded equally, and the data reveal the chemistry behind each removal. Total suspended solids fell by 70 percent, and chemical oxygen demand dropped by roughly 42 percent, with total organic carbon down about 29 percent, thanks to adsorption of organic matter onto the biochar surface and the chemisorption capacity of the iron oxide nanoparticles. But total dissolved solids declined only about 20 percent, because dissolved ions such as sodium, magnesium, potassium, calcium, sulfate, nitrate, and chloride simply do not adsorb onto the composite. That limitation matters, because it is exactly where the second stage of the process takes over. Electron microscopy of the spent composite showed a compact, contaminant-laden surface bearing sulfur, chlorine, potassium, sodium, magnesium, calcium, and phosphorus, a chemical receipt of everything the material had scavenged from the water.

The second stage is air-gap membrane distillation, or AGMD, a thermal technique in which hot feed water touches a porous membrane while water vapor passes through it into an air gap and condenses on a chilled plate. Because only vapor crosses the membrane, salts and non-volatile contaminants are rejected with high efficiency, and the membrane’s selectivity does not depend on pressure the way reverse osmosis does. The catch is that membranes can foul, and the choice of membrane material shapes both the flow rate and the vulnerability. The researchers compared a hydrophilic polyethersulfone (PES) membrane against a hydrophobic polytetrafluoroethylene (PTFE) membrane, both with a 0.1-micrometer pore size, running the cell at a feed temperature of 70 degrees Celsius and a coolant temperature of 27.3 degrees Celsius with cross-flows of 2 liters per minute.

The results exposed a striking trade-off. With untreated wastewater, the PES membrane produced an initial flux of about 170 liters per square meter per hour, a full 21 times greater than the PTFE membrane’s 7.9 liters per square meter per hour. But that dazzling throughput came at a cost: the hydrophilic PES surface attracted foulants, and its flux fell by more than 32 percent within just 2.5 hours, with substantial salt deposition confirmed by a loss of characteristic infrared peaks on the used membrane. The PTFE membrane, chemically inert and water-repellent, resisted wetting and fouling far better, losing 26.6 percent of its much lower flux over five hours. After the biochar pretreatment, both membranes behaved better. Flux decline nearly vanished for the PES membrane, and infrared analysis showed membrane surfaces largely unchanged when the pretreated water was fed through.

The water quality at the end of the pipeline tells the story most clearly. Permeate from the PTFE membrane had no detectable phosphate at all, with or without pretreatment, and its conductivity dropped to about 64 to 70 microsiemens per centimeter, from 817 in the raw wastewater. The PES membrane, notably, drove residual phosphate down to 0.1 milligrams per liter when pretreatment was included. The best permeate quality overall came from the pretreatment-plus-PTFE configuration, followed by PTFE alone, then pretreatment-plus-PES, and finally PES alone. Energy consumption, however, ran the other way: the PES-based process used about 1.5 kilowatt-hours per liter, while PTFE-based configurations required roughly 4.2 to 4.6 kilowatt-hours per liter because of their slower flux. The pretreatment step itself consumed a negligible 0.00095 kilowatt-hours per liter, and by protecting membrane flux it actually trimmed energy demand in the distillation stage, saving 0.03 and 0.32 kilowatt-hours per liter for the PES and PTFE processes respectively.

Translated into money at Texas electricity rates of about 11.8 cents per kilowatt-hour, the integrated best-performing process, pretreatment plus PTFE membrane distillation, produced water at an energy cost of roughly 0.50 dollars per liter, down from 0.54 dollars without pretreatment, while delivering the cleanest permeate of the four configurations tested. That figure remains high compared with conventional treatment, a reminder that membrane distillation is still an energy-hungry technology whose economics improve with waste heat or solar input, neither of which was exploited here. The authors are candid about the limitations of their bench-scale study: the wastewater came from a single collection event at the university’s treatment plant, real municipal streams fluctuate in phosphate, solids, and foulant content, and key questions about the reusability of the magnetic coagulant, the long-term leaching of iron oxide nanoparticles from the biochar matrix, and performance under continuous operation remain open. Pilot-scale validation is the next frontier. Still, the concept carries real appeal: a coagulant grown from waste oak chips that removes its own chemical footprint with a magnet, then hands a cleaner, gentler stream to the membranes downstream, extending their lifespan and cutting the energy bill of every liter they purify.

Subject of Research: Magnetic biochar-assisted pretreatment and air-gap membrane distillation for phosphate-rich municipal wastewater treatment

Article Title: Phosphate-rich municipal wastewater treatment using magnetite nanoparticles dispersed oakwood biochar-assisted pretreatment followed by air-gap membrane distillation

Article References: Rodrigo, P. M., Pant, B., Madduri, S. B., & Kommalapati, R. R. (2026). Phosphate-rich municipal wastewater treatment using magnetite nanoparticles dispersed oakwood biochar-assisted pretreatment followed by air-gap membrane distillation. Frontiers in Membrane Science and Technology, 5, Article 1918400. https://doi.org/10.3389/frmst.2026.1918400

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.1918400

Keywords: wastewater treatment, phosphate removal, magnetite nanoparticles, biochar, membrane distillation, air-gap membrane distillation, coagulation, PES membrane, PTFE membrane, membrane fouling, water purification, energy consumption