Old Tyres Turned Phosphorus Magnets Could Feed Crops and Clean Wastewater

Every year, hundreds of millions of tyres reach the end of their lives, piling up in landfills, leaching chemicals into soil, and shedding microplastic particles into rivers and oceans. At the same time, the world’s farms face an uncomfortable paradox: phosphorus, the nutrient that underpins global food production, is finite, geopolitically concentrated, and rapidly lost from fields and sewage plants into waterways where it fuels toxic algal blooms. A new study published in Plant and Soil by researchers at The University of Western Australia offers an audacious way to attack both problems at once—converting discarded tyre rubber into a phosphorus sponge that can then be buried in poor soil as a slow-release fertiliser.

The research team, led by Hend A. Mohamed together with Zed Rengel, Kadambot H. M. Siddique and Zakaria M. Solaiman, started with a material most scientists have overlooked: biochar made from pyrolysed waste tyres. Australia alone generates more than 465,000 tonnes of used tyres annually, with Western Australia contributing over 60,000 tonnes, most of which are never recycled. At the Advanced Tyre Resource Recovery Facility in Western Australia, whole tyres are shredded, stripped of recyclable steel, and baked at 550 degrees Celsius for an hour, yielding carbon char, crude oil and fuel gas. The resulting tyre biochar, however, is not ready for the environment. Tyre-derived chars can carry polycyclic aromatic hydrocarbons, persistent compounds with toxic and carcinogenic potential, and are inherently enriched in zinc and other trace metals from the rubber formulation.

The researchers’ solution was deceptively simple: soak the tyre biochar in hydrogen peroxide. This mild, environmentally benign oxidant is prized precisely because it does not introduce metals or harsh residues. Approximately 12.5 grams of untreated tyre biochar, dubbed PTB, were treated with 100 millilitres of 10 percent hydrogen peroxide at a 1:10 weight ratio and left at room temperature for 48 hours—a deliberately extended contact time, five times longer than in the original method the team adapted, because the dense, highly carbonised, hydrophobic rubber matrix is slow to open up to oxidation. The treated material, MTB, was then rinsed, dried and characterised alongside the pristine version.

The chemical transformation was striking. The Brunauer–Emmett–Teller surface area of the modified biochar roughly tripled, meaning the oxidation carved out or exposed far more reactive surface per gram. Energy-dispersive X-ray spectroscopy revealed that the oxygen content of MTB nearly doubled relative to PTB, while zinc levels fell by half, suggesting the treatment simultaneously added oxygen-rich binding sites and leached exchangeable metals out of the carbon matrix. Interestingly, Fourier-transform infrared spectroscopy showed no dramatic proliferation of new functional groups—a subtle result the authors attribute to the extreme carbonisation of tyre char, where oxidation can enrich oxygen without producing large, spectrally detectable shifts. The point of zero charge also climbed from 5.7 to 7.2, tilting the surface chemistry toward forms that better retain negatively charged phosphate ions.

When the two biochars were pitted against each other in batch adsorption experiments, the difference was dramatic. At an application rate of 100 grams per litre, modified tyre biochar stripped 99 percent of phosphorus from a 75 milligram per litre solution, while untreated biochar managed only about 20 percent. Even at lower doses the modified material consistently outperformed, with removal efficiency rising from 33 to 99 percent across the tested range. Adsorption capacities tell a similar story: across initial phosphorus concentrations of 30 to 125 milligrams per litre, MTB captured 1.4 to 2.7 milligrams of phosphorus per gram, roughly four times the capacity of PTB. Notably, the adsorption was remarkably stable across pH 4 to 10, a practical advantage for real wastewater, whose pH varies widely.

The modelling work offered clues about the mechanism. Equilibrium data for the modified biochar fit the Langmuir isotherm exceptionally well, with a correlation coefficient of 0.97, indicating monolayer adsorption on a broadly homogeneous surface. Among four kinetic models tested, the pseudo-second-order and Elovich equations fit best, with R-squared values of 0.99 and 0.98 respectively. Both models describe chemisorption—binding that involves the sharing or exchange of electrons—pointing to strong, persistent phosphorus attachment through ligand exchange and inner-sphere surface complexation on the oxygenated surface, supplemented by electrostatic attraction. Time-course experiments reinforced this picture: whereas untreated biochar showed fluctuating, weakly bound phosphorus that desorbed over hours, the modified material climbed steadily to equilibrium within 10 hours, reaching 3.5 milligrams per gram and holding its plateau.

The most consequential question, of course, was whether phosphorus-loaded tyre biochar could actually feed plants. To find out, the team ran a glasshouse pot experiment with maize grown for 28 days in nutrient-poor sandy soil from Shenton Park Field Station—soil with a pH of 4.8, just 3.5 milligrams per kilogram of Colwell-extractable phosphorus, and extremely low nitrogen. Four treatments were compared at rates of 0, 0.25, 0.5 and 1 percent by weight: pristine biochar, modified biochar, and both types pre-loaded with phosphorus from solution. The phosphorus-enriched modified biochar carried 4.7 to 19 milligrams per kilogram of phosphorus, far more than the enriched untreated version.

The results were unequivocal. Maize receiving phosphorus-loaded modified biochar at 0.5 and 1 percent application rates produced dramatically greater shoot dry weight, with statistical significance at p < 0.0001, and the 1 percent treatment also significantly boosted root dry weight. Overall, phosphorus-loaded modified biochar increased shoot and root biomass nearly threefold relative to phosphorus-loaded pristine biochar and the unamended control. Analysis of variance confirmed a significant interaction between biochar type and application rate for both shoot and root growth, meaning that the modification genuinely mattered—the benefit was not merely from adding bulk carbon to sandy soil, but from delivering phosphorus in a form plants could actually access.

The researchers are careful about what this does and does not prove. The 28-day pot trial demonstrates short-term seedling responses under controlled moisture, not multi-season field performance, and field outcomes for biochar amendments are known to be highly site-specific, sometimes persisting for years and sometimes fading. Crucially, the team measured no polycyclic aromatic hydrocarbon concentrations and no metal leaching in this study, so any claims about the environmental safety of tyre biochar as a soil amendment remain preliminary. Zinc, the dominant trace metal in tyre-derived materials, can accumulate in soils over time, and while surface oxidation has been reported in other work to reduce metal leaching and stabilise metal fractions, the authors explicitly call for quantification of PAH content, leaching behaviour and ecotoxicological effects before any field deployment.

There are also economic realities to confront. Adsorption capacities of 1.45 to 2.34 milligrams per gram, while vastly better than the pristine material, sit well below the 45 to 150 milligrams per gram reported for metal-modified biochars loaded with magnesium, aluminium, iron, calcium or lanthanum, because hydrogen peroxide upgrades the intrinsic carbon surface rather than adding high-affinity mineral phases. Techno-economic studies of tyre pyrolysis suggest profitability improves substantially at larger operating scales, and the authors recommend site-specific techno-economic assessment before commercialisation. Still, the conceptual achievement is hard to dismiss: a single low-cost treatment converts a hazardous waste stream into a water-cleaning adsorbent, then converts that loaded adsorbent into a fertiliser that triples maize biomass in depleted soil. It is a closed loop—waste cleaning water, water nutrient feeding crops—that points toward the kind of circular resource economy agriculture will increasingly need as phosphate rock dwindles and tyre mountains keep growing.

Subject of Research: Hydrogen peroxide-modified waste tyre biochar for phosphorus recovery from wastewater and reuse as a slow-release phosphorus fertiliser

Article Title: H₂O₂-modified tyre biochar for phosphorus adsorption and fertiliser application

Article References: Mohamed, H. A., Rengel, Z., Siddique, K. H. M., & Solaiman, Z. M. (2026). H₂O₂-modified tyre biochar for phosphorus adsorption and fertiliser application. Plant and Soil. https://doi.org/10.1007/s11104-026-09034-0

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09034-0

Keywords: tyre biochar, phosphorus recovery, hydrogen peroxide modification, adsorption, slow-release fertiliser, wastewater treatment, maize, circular economy, pyrolysis, soil amendment, eutrophication, waste tyres