The work, conducted by researchers at Sule Lamido University in Nigeria together with collaborators at Universiti Putra Malaysia, addresses a stubborn class of water pollutant. Methylene blue is an aromatic heterocyclic basic dye, chemically designated [3,7-bis(dimethylamino) phenothiazine chloride], with a molecular weight of 319.85 g/mol and a characteristic absorption maximum at 664 nanometers. It is classified as toxic and carcinogenic, and crucially it is non-biodegradable — conventional treatment plants cannot reliably break it down. Textile, pharmaceutical, petrochemical, and chemical industries discharge vast quantities of dye-laden effluent annually, and tightening discharge regulations worldwide have intensified the search for remediation technologies that are cheap, robust, and effective under ambient conditions.

The Nigerian-Malaysian team turned to advanced oxidation processes, a family of techniques capable of mineralizing complex organic pollutants into carbon dioxide, water, and inorganic ions. Their photocatalyst of choice, molybdenum trioxide, is an n-type semiconductor transition-metal oxide with a band gap of roughly 3.0 electronvolts — narrow enough to be excited by visible wavelengths, which constitute the bulk of the solar spectrum. MoO3 is already prized in electrocatalysis, batteries, gas sensing, and supercapacitors, and it serves as a precursor for molybdenum disulfide, molybdenum dioxide, and molybdenum metal. What the new study adds is a careful, head-to-head comparison of how annealing temperature reshapes both the crystal phase and the pollutant-destroying performance of the same parent material.

Synthesis proceeded by an ultrasonic-assisted hydrothermal route. Four grams of ammonium heptamolybdate tetrahydrate were dissolved in deionized water, acidified dropwise with nitric acid under vigorous stirring and ultrasonication, and sealed in a Teflon-lined autoclave at 150 degrees Celsius for 24 hours. The recovered precipitate was washed, dried, and then split into two thermal futures: one sample calcined in air at 450 degrees Celsius, the other at 650 degrees Celsius, each for two hours at a ramp rate of 5 degrees per minute. That single difference in annealing temperature proved decisive, steering the material into two distinct crystallographic identities with distinctly different morphologies.

X-ray diffraction confirmed the phase split with textbook clarity. The 450-degree sample indexed entirely to monoclinic β-MoO3, with cell constants a = 3.9540 Å, b = 3.6870 Å, and c = 7.0950 Å, matching reference pattern JCPDS 00-047-1320. The 650-degree sample converted completely to orthorhombic α-MoO3, with parameters a = 3.962 Å, b = 13.858 Å, and c = 3.697 Å, matching JCPDS 05-0508. Field-emission scanning electron microscopy revealed the morphological consequences: smooth, homogeneous plate-like β-MoO3 structures with thicknesses of 60 to 120 nanometers, versus rod-like α-MoO3 aggregates assembled from stacked nanoplates of similar thickness. Energy-dispersive X-ray spectroscopy confirmed a clean 1:3 molybdenum-to-oxygen ratio in both, with no detectable impurities.

The decisive physical differences emerged in the surface and optical measurements. BET analysis gave the α-MoO3 nanorods a specific surface area of 5.4 square meters per gram against just 3.3 square meters per gram for the β-MoO3 nanoplates, along with a richer population of mesopores concentrated in the 2-to-6-nanometer range. Diffuse reflectance spectroscopy placed the band gaps at 2.8 eV for the rods and 2.86 eV for the plates — both firmly in visible-light territory. Most tellingly, photoluminescence spectroscopy showed a lower emission intensity for the α-MoO3 rods, signaling that photogenerated electrons and holes recombine less frequently there. In photocatalysis, every recombined electron-hole pair is a wasted photon, so lower recombination translates directly into more oxidative power at the catalyst surface.

Performance testing bore this out. Under visible illumination from the 23-watt lamp, with a 0.6 gram-per-liter catalyst dose, a solution pH of 8, and 10 milligrams per liter of dye, the α-MoO3 nanorods degraded 97 percent of the methylene blue in 150 minutes, with a pseudo-first-order rate constant of 0.0211 per minute — roughly twice the 0.0151 per minute achieved by the β-MoO3 nanoplates. Both systems fit the Langmuir-Hinshelwood kinetic model with exemplary linearity, yielding correlation coefficients above 0.999. Photolysis alone removed a negligible 2.01 percent of the dye, and dark adsorption accounted for only 4 to 5 percent, confirming a genuine synergistic partnership between light and catalyst. Chemical oxygen demand measurements fell in parallel, verifying that the dye was being genuinely mineralized rather than merely bleached.

The operational parameter study added practical nuance. Raising the catalyst dose from 0.2 to 0.6 grams per liter lifted degradation from 55 to 97 percent, but further increases backfired as particle compaction and light scattering shaded the active sites. Solution pH mattered enormously: at pH 2, only 43 percent of the dye disappeared, while pH 8 delivered near-complete removal. The explanation lies in electrostatics. Molybdenum trioxide carries a point of zero charge at pH 8, so in alkaline media its surface is negatively charged and attracts the positively charged methylene blue cations, while in acidic media repulsion drives them apart. Increasing dye concentration worked against degradation, with 20 milligrams per liter solutions reaching only 53 percent removal in the same irradiation window, as concentrated dye blocked photon penetration and saturated adsorption sites.

Scavenger experiments identified the chemical executioners. When benzoquinone was added to trap superoxide radicals, degradation collapsed to 38.2 percent; EDTA, a hole scavenger, suppressed it to 24.6 percent; and tert-butanol, which quenches hydroxyl radicals, cut it to 29.3 percent. The authors conclude that photogenerated holes and hydroxyl radicals are the dominant reactive species attacking the dye molecules, with superoxide radicals playing a supporting role. Band-edge calculations placed the conduction band at +0.451 eV and the valence band at +3.29 eV, consistent with prior literature and with a mechanism in which visible photons excite electrons across the 2.8-eV gap, leaving holes that oxidize water into hydroxyl radicals while electrons reduce dissolved oxygen into superoxide species.

Perhaps the most industrially consequential finding is durability. The α-MoO3 nanorods were recovered by simple centrifugation and water washing — no chemical regeneration — and redeployed across five consecutive cycles. Degradation efficiency declined gently from 97 to 93, 86, 86, and finally 83 percent, demonstrating that the catalyst neither dissolves nor deactivates under repeated visible-light duty. Combined with synthesis from inexpensive ammonium heptamolybdate, operation under a low-wattage lamp, and ambient conditions, the results position α-MoO3 nanorods as a credible candidate for scaled-up photocatalytic wastewater treatment, particularly in regions where sunlight itself could stand in for the fluorescent lamp. The study is a reminder that in materials chemistry, sometimes the most powerful lever is simply how hot you bake the crystal.

Subject of Research: Visible light-driven photocatalytic degradation of methylene blue dye using α-MoO3 nanorods and β-MoO3 nanoplates synthesized by hydrothermal methods

Article Title: Visible light-assisted photocatalytic degradation kinetics of methylene blue (MB) dye by β-MoO3 nanoplates and α-MoO3 nanorods

Article References: Ibrahim, Y., Saidu, U., Abdullah, A. H., Abdul Rashid, S., Muhamad, E. N., & Muhammad, Z. (2026). Visible light-assisted photocatalytic degradation kinetics of methylene blue (MB) dye by β-MoO3 nanoplates and α-MoO3 nanorods. Discover Chemistry, 3(1), Article 510. https://doi.org/10.1007/s44371-026-00967-0

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00967-0

Keywords: photocatalysis, molybdenum trioxide, methylene blue, wastewater treatment, nanomaterials, visible light, advanced oxidation processes, α-MoO3 nanorods, β-MoO3 nanoplates, degradation kinetics, hydrothermal synthesis, water purification

The review focuses on metal and metal oxide nanoparticles, including silver, titanium dioxide, zinc oxide, cerium dioxide, iron-based materials and nanoscale zero-valent iron. Their environmental behavior is governed by a combination of particle properties and subsurface conditions. Size, shape, surface coating, crystal structure, density and electrical charge all influence mobility. So do pH, ionic strength, dissolved oxygen, natural organic matter, clay minerals, competing particles and groundwater velocity. Because nanoparticles have enormous surface areas relative to their mass, even small changes in their surroundings can alter their behavior. A particle that remains stable in clean laboratory water may rapidly aggregate, dissolve or bind to soil minerals in a natural aquifer.

One of the central scientific frameworks used to explain this behavior is the Derjaguin–Landau–Verwey–Overbeek, or DLVO, theory. DLVO theory describes the balance between attractive van der Waals forces and repulsive electrostatic forces acting between nanoparticles and mineral surfaces. When repulsion dominates, particles may remain suspended and travel with flowing water. When attractive forces become stronger, nanoparticles can attach to sand, clay, iron oxides or organic coatings. However, the review emphasizes that classical DLVO theory is not sufficient on its own. Hydration forces, steric interactions, surface roughness, chemical heterogeneity and specific ion binding can all create non-DLVO effects. These forces may either promote retention or unexpectedly enhance transport, making simple predictions unreliable.

Movement through porous media is also controlled by physical processes. Advection carries nanoparticles with the average flow of groundwater, while hydrodynamic dispersion spreads them through variations in water velocity and flow paths. Brownian diffusion causes very small particles to move randomly, allowing them to reach surfaces that might otherwise be missed. Interception occurs when flowing particles pass close enough to grains to make contact, while gravitational sedimentation becomes more important as particles aggregate and grow heavier. Straining can trap particles when their effective size approaches the dimensions of pore throats. A particle may therefore travel rapidly through a coarse sandy layer but become immobilized in finer soil, compacted sediment or a region containing narrow pore spaces.

The ratio between nanoparticle size and grain size is particularly important. When particles are much smaller than the grains forming a porous medium, they may pass through larger pores, although surface chemistry still determines whether they attach. As the size ratio increases, straining and physical trapping become more likely. Rough mineral surfaces can either capture particles in microscopic valleys or, under some conditions, reduce contact with reactive sites and promote passage. Iron oxides and clays are especially influential because they possess chemically active surfaces and may carry charges opposite to those of nanoparticles. Such mineral coatings can act as powerful filters, but they can also become saturated, change charge with pH, or be covered by organic matter, altering their filtering capacity over time.

The review highlights a phenomenon that could transform how scientists think about nanoparticle pollution: colloid-facilitated transport. Natural colloids, including clay particles, iron oxides, humic substances, biochar fragments and other nanoscale materials, can act as mobile carriers. Instead of attaching directly to stationary sand grains, metal nanoparticles may bind to these suspended particles and travel with them. This process can extend the distance nanoparticles move through soil and groundwater. In mixed suspensions, particles can also undergo hetero-aggregation, joining together with materials of a different composition. For example, iron-based nanoparticles may interact with other engineered particles or natural mineral colloids, changing their size, density, charge and reactivity.

Competition between particles creates another counterintuitive effect. When natural colloids or one type of nanoparticle occupy available attachment sites on grain surfaces, they may block those sites and prevent other particles from being retained. This process, known as competitive blocking, can increase the mobility of nanoparticles that would otherwise be filtered out. Co-transport can therefore produce results that are not predictable from single-particle experiments. One material may improve the movement of another by modifying grain surfaces, changing aggregation behavior or creating mobile carriers. In some cases, the presence of colloids increases the release of previously retained nanoparticles, especially when groundwater chemistry shifts or flow conditions intensify.

Chemical transformation is equally important because a nanoparticle’s identity does not remain fixed after release. Oxidation can convert metallic particles into oxide layers or dissolved ions. Reduction may transform metal oxides into lower-valence compounds. Sulfur-rich environments can convert silver nanoparticles into silver sulfide, often a less soluble form with different mobility and toxicity. Organic matter may coat particle surfaces, stabilize suspensions or promote aggregation, depending on its composition and concentration. Changes in pH and redox conditions can alter dissolution rates, surface charge and mineral structure. In floodplains, wetlands, paddy soils and aquifers affected by fluctuating oxygen levels, nanoparticles may repeatedly change between more mobile and more strongly retained forms.

These transformations directly affect environmental risk. A particle that is immobilized in soil may later be remobilized when the pH changes, ionic strength decreases, organic coatings are degraded or water flow becomes stronger. Conversely, dissolution can reduce the number of particles while releasing metal ions that may be more biologically available. Aggregation can make particles easier to settle but may also create larger reactive surfaces or facilitate transport through preferential flow channels. The review notes that nanoparticle behavior cannot be judged solely by measuring total metal concentration. Researchers must distinguish between individual nanoparticles, aggregates, dissolved ions and nanoparticle–colloid complexes. Their chemical form, size distribution and surface state may be more important than their overall abundance.

To investigate these processes, scientists use a combination of laboratory experiments and field-scale observations. Packed-column experiments allow researchers to control grain size, flow rate, pH, electrolyte concentration and nanoparticle coatings while tracking breakthrough curves and retention profiles. Lysimeters provide a more realistic bridge between laboratory columns and natural soils by allowing rainfall, vegetation, microbial activity and long-term aging to influence transport. Quartz crystal microbalance with dissipation monitors the attachment of nanoparticles to model mineral surfaces and can reveal whether deposited layers are rigid, soft or viscoelastic. Parallel-plate systems simplify flow and surface interactions, while atomic force microscopy maps nanoscale roughness and measures forces between particles and collectors.

Advanced chemical analysis is becoming essential because conventional measurements often cannot reveal what is actually moving. Single-particle inductively coupled plasma mass spectrometry can count individual metal-containing nanoparticles, estimate their sizes and distinguish them from dissolved metal. Asymmetrical flow field-flow fractionation coupled with ICP-MS separates particles according to hydrodynamic size while simultaneously identifying their elemental composition. Diffusive gradients in thin films can measure labile and potentially bioavailable metal species within soils and sediments. Microscopy combined with spectrometry can show where nanoparticles accumulate and what chemical phases form around them. Together, these methods provide a more detailed picture of nanoparticle fate than bulk concentration measurements alone.

Researchers also use mathematical models to predict transport, retention, dissolution, aggregation and co-transport. Advection–dispersion equations describe the movement of particles through flowing water, while colloid filtration theory estimates the probability that particles will collide with and attach to porous-media grains. More advanced models include attachment and detachment, blocking, straining, ripening, aggregation and chemical transformation. Reactive transport models attempt to connect groundwater flow with redox reactions, dissolution and changes in particle surface chemistry. Yet the review warns that model predictions remain limited because most models simplify natural soils and aquifers. Real subsurface environments contain irregular pores, layered structures, preferential flow paths, fluctuating water saturation, biological coatings and chemically diverse mineral surfaces.

The consequences extend beyond nanoparticle transport itself. Mobile metal nanoparticles may carry adsorbed contaminants, alter the movement of heavy metals and organic pollutants, or influence the distribution of nutrients and microorganisms. Conversely, nanoparticles may acquire new coatings and chemical properties as they move, changing their ecological effects. Particles reaching plant roots can interact with soil minerals, microbial communities and root exudates before entering plants or remaining in the rhizosphere. In groundwater, their persistence and mobility may determine whether drinking-water resources are exposed. The same properties that make nanoparticles useful for pollution cleanup—high reactivity, small size and engineered surface chemistry—can also allow them to travel beyond their intended treatment zones.

The review’s central message is that there is no universal rule stating whether a metal nanoparticle will move or remain trapped. Retention is generally favored by fine-grained porous media, rough collector surfaces and opposite surface charges, while stable colloidal conditions, natural carriers and competitive blocking can promote transport. Particle aging can make these relationships change with time. The authors call for experiments that combine realistic soils, longer observation periods, fluctuating chemical conditions and multiple nanoparticles rather than relying exclusively on simplified laboratory systems. They also emphasize the need to link nanoscale measurements with field-scale hydrology. As metal nanoparticles continue to enter the environment, understanding their underground journey will be essential for assessing risks, designing safer technologies and preventing invisible contamination from moving through the world’s most important groundwater pathways.

Subject of Research: Transport, transformation, retention, co-transport and modeling of metal nanoparticles in subsurface porous media, soils, sediments and groundwater.

Article Title: Metal nanoparticles transport in the subsurface: a review

Article References: Wan, Q., Zhang, M., Zhao, M. et al. “Metal nanoparticles transport in the subsurface: a review.” Environmental Chemistry Letters 24, 201–227 (2026).

Image Credits: AI Generated

DOI: https://doi.org/10.1007/s10311-025-01879-8

Keywords: Stability; colloid-facilitated transport; aggregation; competitive blocking; transformation; modeling