Metal nanoparticles are entering the underground world in growing quantities, and scientists are warning that the soil beneath our feet may be far more dynamic than previously assumed. A new review in Environmental Chemistry Letters examines how these extremely small particles move through soils, sediments, aquifers and other subsurface environments. The particles can originate from industrial manufacturing, consumer products, agriculture, wastewater, environmental remediation and natural geological processes. Once released, they do not simply remain in one place. They can attach to mineral grains, dissolve into ions, aggregate into larger clusters, react with organic matter, or hitchhike on naturally occurring colloids. These transformations determine whether nanoparticles are trapped near the surface or transported into groundwater and deeper geological formations.
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
