Tiny Nanoparticles in Rivers May Outmuscle Organic Matter in Breaking Down Pollutants

For decades, environmental chemists have assumed that the sunlit chemistry of rivers and lakes is dominated by dissolved organic matter, the tea-colored brew of decayed plant and microbial material that drifts through nearly every natural water body on Earth. When sunlight strikes these waters, organic matter absorbs photons and spawns a cloud of short-lived but ferociously reactive molecules, including hydroxyl radicals, that can shred pesticides, pharmaceuticals, and industrial chemicals. A new study published in Nature Communications by Joshua Quinn, Aidan McClure, Pierre Herckes, and Paul Westerhoff of Arizona State University now argues that this long-standing picture is incomplete. Their experiments show that nanomaterials, present at vanishingly small concentrations of just a few micrograms per liter, can generate hydroxyl radicals at rates that rival or exceed those of organic matter, fundamentally changing how scientists model the fate of pollutants in surface waters.

The team focused on titanium dioxide, a nanomaterial that is both engineered for use in paints, sunscreens, and self-cleaning surfaces and produced incidentally by combustion and industrial processes. Titanium dioxide is a well-known photocatalyst: when illuminated with ultraviolet light, electrons in the material are promoted across its band gap, leaving behind holes that react with water and hydroxide ions to produce hydroxyl radicals. What has been missing, the researchers argue, is an assessment of this chemistry at concentrations that actually occur in the environment. Earlier laboratory studies often used milligram-per-liter nanoparticle loadings, concentrations far above the parts-per-billion levels measured in real rivers, which made it easy to dismiss nanomaterials as negligible players in natural water photochemistry.

Quinn and colleagues designed their experiments around environmentally realistic numbers. They tested titanium dioxide nanomaterials at 1 to 100 micrograms per liter, alongside dissolved organic matter at 1 to 5 milligrams of carbon per liter, the range typical of many natural surface waters. Rather than relying on a single ultraviolet wavelength, they illuminated their samples at 222, 265, and 365 nanometers, spanning the germicidal vacuum-ultraviolet region used in some advanced water treatment systems, the mid-ultraviolet band relevant to sunlight reaching the Earth’s surface, and the longer wavelengths that penetrate deepest into natural waters. This multiwavelength approach allowed them to probe how the balance of photochemical reactions shifts as the spectral character of the light changes.

The central result is striking. Although the nanomaterial concentrations were up to a thousand times lower than those of the organic matter by mass, the titanium dioxide produced hydroxyl radicals at rates ten to one hundred times higher per unit mass. In practical terms, this mass-normalized advantage meant that the tiny particle loadings generated steady-state hydroxyl radical concentrations comparable to those produced by the far more abundant organic matter. Hydroxyl radicals are among the most potent oxidants in chemistry, reacting near the diffusion-controlled limit with a vast range of organic molecules, so even modest changes in their steady-state concentration can translate into large changes in how quickly pollutants degrade.

Perhaps even more intriguing is what happened when the two components were combined. When titanium dioxide nanomaterials and dissolved organic matter were illuminated together, hydroxyl radical formation exceeded the sum of the contributions measured for each component alone, by 16 to 21 percent. This synergistic behavior suggests that the two photochemical systems interact in ways that simple additive models cannot capture. One plausible mechanism involves the organic matter acting as a scavenger of holes or an electron shuttle at the nanoparticle surface, or the nanoparticles altering the photophysics of the organic matter itself. Whatever the precise molecular pathway, the finding implies that real waters, which always contain both particles and dissolved organics, may be more photochemically active than current models predict.

To test whether this radical production matters for real contaminants, the researchers examined the degradation of 6PPD-quinone, a transformation product of a ubiquitous tire rubber antioxidant that made headlines several years ago when it was identified as the culprit behind acute coho salmon deaths in urban streams of the Pacific Northwest. The compound has become an emblematic micropollutant of concern because it is toxic to aquatic life at low concentrations and is continuously washed from roads into waterways. In the team’s experiments, dissolved organic matter had only a limited effect on the breakdown of 6PPD-quinone, but the presence of nanomaterials significantly enhanced its degradation, driven by the elevated steady-state hydroxyl radical concentrations the particles sustained under illumination.

This contrast is chemically telling. Dissolved organic matter plays a dual role in natural waters: it produces reactive species when illuminated, but it also quenches them, soaking up radicals before they can attack target pollutants. The net effect on any given contaminant therefore depends on the delicate balance between production and scavenging. Titanium dioxide, by contrast, appears to act as a cleaner radical source with less of a scavenging penalty at these low concentrations, allowing the extra hydroxyl radicals to be spent on pollutant molecules rather than on the organic matrix. For a compound like 6PPD-quinone, which is not efficiently attacked by the triplet excited states of organic matter, the hydroxyl radical pathway supplied by nanoparticles may be the dominant route to degradation.

The implications extend beyond natural waters into engineered treatment systems. Ultraviolet disinfection is a cornerstone of modern water treatment, and advanced oxidation processes deliberately generate hydroxyl radicals to destroy recalcitrant contaminants. The study’s findings suggest that trace nanomaterials already present in water, or the interplay between those particles and natural organic matter, could influence the efficiency of ultraviolet-based treatment at 222, 265, and 365 nanometers in ways that current design models do not account for. Conversely, the synergy between nanoparticles and organic matter hints at opportunities: understanding and harnessing these interactions could lead to treatment strategies that exploit naturally occurring or deliberately added photocatalysts to degrade micropollutants more effectively with less energy.

For environmental modeling, the message is that the photochemical budget of a sunlit stream cannot be computed from organic matter alone. Natural nanomaterials, including iron oxides and other mineral colloids, along with incidental and engineered nanoparticles released from consumer products, industrial effluents, and atmospheric deposition, all coexist at parts-per-billion levels in surface waters. If even a fraction of them behave as photocatalysts the way titanium dioxide does, then reactive intermediate production in the environment has been systematically underestimated. The Arizona State team’s work provides the first quantitative demonstration that these overlooked particles can contribute to, or even exceed, the hydroxyl radical production traditionally credited to dissolved organic matter.

The study also arrives at a moment of growing public and scientific attention to the invisible chemical traffic in the world’s waters, from tire-derived toxins to pharmaceutical residues. As urbanization continues to load rivers with both micropollutants and nanomaterials, the two streams of contamination may be more entangled than anyone realized, with the particles quietly shaping the chemical lifetime of the pollutants swimming among them. Quinn and colleagues have shown that at concentrations once dismissed as trivial, nanomaterials are not passive passengers in surface waters but active photochemical engines. Understanding that hidden catalysis, the authors suggest, will be essential for predicting how quickly contaminants vanish from sunlit waters, and for designing the ultraviolet treatment systems of the future.

Subject of Research: Photochemical production of reactive species by environmentally relevant nanomaterial concentrations and their role in micropollutant degradation in surface waters

Article Title: Environmentally Relevant Concentrations of Nanomaterials Drive Photochemical Production of Reactive Species and Pollutant Degradation in Surface Waters

Article References: Quinn, J., McClure, A., Herckes, P., & Westerhoff, P. (2026). Environmentally Relevant Concentrations of Nanomaterials Drive Photochemical Production of Reactive Species and Pollutant Degradation in Surface Waters. Nature Communications. https://doi.org/10.1038/s41467-026-78433-9

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78433-9

Keywords: nanomaterials, titanium dioxide, hydroxyl radicals, dissolved organic matter, photochemistry, surface waters, 6PPD-quinone, micropollutants, water treatment, ultraviolet light, environmental chemistry, photocatalysis