The study was led by Ziheng Wang of The University of Manchester, working with Majid Sedighi, and examined what happens when extremely fine particles shed from carbon-based adsorbents remain suspended in treated water. The team’s central discovery is what researchers call co-transport: dissolved contaminants bind to microscopic carbon fragments, and those fragments then travel through sand filters and even membranes with pores far smaller than the particles themselves appear to be. In their experiments, the overwhelming majority of lead remaining after fine-sand filtration was not dissolved in the water at all but riding on residual carbonaceous particles.

The numbers are striking. After fine-sand filtration, particle-bound lead accounted for 84.5 percent of the total lead detected in filtrates from activated carbon, and reached as much as 90.2 percent for biochar produced from corn straw. In other words, when the researchers measured what was actually left in the water after treatment, nearly nine parts in ten of the model contaminant were attached to particles rather than floating freely as dissolved ions. Any assessment that looked only at the dissolved fraction would have dramatically underestimated how much lead remained in the system.

Activated carbon is a staple of drinking water and wastewater treatment worldwide, prized for its enormous internal surface area and its affinity for a broad range of pollutants. Biochar, a close cousin produced by heating biomass such as wood and crop residues in low-oxygen conditions, is attracting growing interest as a potentially lower-cost alternative, with the added appeal of turning agricultural waste into a useful remediation material. Both work by adsorption: pollutant molecules and ions adhere to surfaces and pores, effectively pulled out of the water column and immobilized on the solid.

But solids are not immortal. Physical breakdown, erosion, and the simple presence of very fine material in the original product mean that some carbonaceous fragments inevitably escape the adsorbent bed and move downstream. The Manchester team set out to quantify this under controlled conditions, testing activated carbon alongside three biochars made from hardwood, wheat straw, and corn straw, with lead serving as a representative heavy-metal contaminant. Their experimental toolkit combined adsorption experiments with fixed-bed filtration, stirred suspensions, sand filtration, and membrane filtration at pore sizes of 0.45 and 0.02 micrometers.

What they found in the filtrates was a population of residual particles with size features clustered around 100 to 200 nanometers, around 0.5 to 1 micrometer, and an apparent fraction near 5 micrometers. The researchers are careful about that largest figure. It is unlikely, they caution, that intact 5-micrometer particles passed directly through the pores of a 0.45-micrometer membrane. A more plausible explanation is that smaller particles aggregated during filtration, sample handling, concentration, or measurement, creating larger apparent structures. Further experiments, they note, are needed to verify exactly how this aggregation occurs.

The most consequential result emerged when the team separated dissolved lead from particle-carried lead. Even after 0.45-micrometer membrane filtration, a standard step in many analytical and treatment workflows, lead remained associated with particles in the 0.02 to 0.45 micrometer size range. That means conventional filtration and dissolved-phase measurements can overlook part of the contaminant load entirely. A water sample that appears clean by dissolved-metal standards may still carry a substantial hidden burden of pollutant, provided it is attached to particles small enough to pass through the filter.

This distinction has implications that reach well beyond the laboratory bench. Common analytical procedures often filter water samples before measuring dissolved metals, precisely to remove particles and obtain what is considered the truly dissolved concentration. But if a significant share of the contaminant is particle-bound, that pre-filtration step removes the contaminants along with the particles, potentially giving an incomplete, even misleading, picture of contaminant transport. In practical terms, a treatment plant could report low dissolved lead while substantial lead continues moving through the system on carbon fragments too fine to see. The authors argue that water-treatment performance should therefore be evaluated using both dissolved contaminants and contaminants carried by residual particles, treating the two fractions as distinct components of the total load.

The researchers are careful to frame their findings within the limits of the study. All experiments were conducted under controlled laboratory conditions, and real treatment systems are messier places. Water chemistry, including pH, ionic strength, natural organic matter, and dissolved ions, could change how stable the residual particles are and how readily they and their attached contaminants move through actual treatment trains. Natural organic matter, for instance, is known in colloid science to alter particle surfaces and aggregation behavior, so the fractions measured in the laboratory may shift in different water matrices. The authors call for future work to test representative drinking water and wastewater matrices rather than idealized solutions.

They also point toward practical countermeasures that could be evaluated in follow-up research. These include pre-washing adsorbents before deployment to remove loose fines, mechanical stabilization of the carbon materials, granulation to reduce the generation of mobile fragments, improved coagulation and flocculation downstream to capture escaping particles, and combined sand and membrane filtration schemes designed to intercept the finest fractions. None of these measures is proposed as a proven fix; each is a candidate for testing against the co-transport pathway the study has now documented. The broader message is a recalibration of expectations for two of the most widely studied materials in water treatment. Activated carbon and biochar remain excellent adsorbents, and the study does not challenge their capacity to bind pollutants. What it challenges is the assumption that binding equals removal. As Wang put it, if very small carbon particles remain mobile, the contaminants attached to them may also continue moving through the treatment system. For engineers, regulators, and researchers, that means the particles leaving an adsorbent bed deserve the same scrutiny as the water flowing past them, and the dissolved fraction alone can no longer stand in for the whole story of what a filter has, and has not, removed.

Subject of Research: Co-transport of particle-bound contaminants by residual superfine activated carbon and biochar particles in water filtration

Article Title: Tiny carbon particles may carry contaminants through water treatment filters

Article References: Tiny carbon particles may carry contaminants through water treatment filters. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: activated carbon, biochar, water treatment, co-transport, lead contamination, sand filtration, membrane filtration, adsorption, particle-bound pollutants, dissolved metals, colloids, water quality

The technique at the heart of the work is called layer-by-layer assembly, a method that builds films one molecular layer at a time by alternately dipping a charged substrate into solutions of positively and negatively charged polymers. In this case, the substrate was a commercially available nylon microfiltration membrane with a positively charged surface. Each dipping cycle deposits a nanometer-thin pairing, or bilayer, of negatively charged humic acid followed by positively charged chitosan, and repeating the cycle builds up a controlled nano-composite coating whose thickness and composition can be tuned with molecular precision.

Humic acid, a major component of the natural organic matter found in soils and waterways, is rich in oxygen-containing functional groups such as carboxyl, carbonyl, and hydroxyl moieties that can complex with dissolved solutes. Chitosan, a linear polysaccharide derived from chitin, carries protonated amino groups under acidic conditions that form strong electrostatic salt bridges with the carboxylate groups of humic acid. Together, the two biopolymers create a dense, interactive separation skin on top of a membrane whose pores would otherwise be far too large to catch small organic molecules like herbicides.

Characterization of the coatings confirmed the assembly proceeded as designed. Ultraviolet-visible spectroscopy showed the absorbance at 256 nanometers rising linearly with each deposited bilayer, indicating uniform growth. Infrared spectroscopy revealed peak shifts consistent with electrostatic bonding between the carboxylate groups of humic acid and the ammonium groups of chitosan. Spectroscopic ellipsometry measured bilayer stacks growing from about 7 nanometers at three bilayers to roughly 19 nanometers at nine bilayers. Atomic force microscopy showed surface roughness increasing from 163 to 321 nanometers after modification, while the effective pore diameter shrank from 0.448 micrometers to 0.2 micrometers, and thermogravimetric analysis confirmed the modified membranes remained thermally stable up to around 450 degrees Celsius.

The filtration tests focused on four herbicides representing distinct chemical classes: the chlorophenoxy compounds 2,4-D and 2,4,5-T, the phenyl urea herbicide buturon, and the neutral amide herbicide diphenamid. All were tested at concentrations of 10 to the minus 4 moles per liter in a dead-end ultrafiltration cell operating at just 20 pounds per square inch and 500 revolutions per minute. The bare nylon membrane barely rejected any of the compounds, with removal rates between roughly 7 and 16 percent. Once coated, performance improved steadily with each added bilayer, and the nine-bilayer membrane delivered the best results of all.

The standout result came from 2,4-D, one of the most widely used weed killers in the world and a suspected human carcinogen, which was rejected at approximately 99 percent. Buturon followed at around 97 percent, 2,4,5-T at about 85 percent, and diphenamid at roughly 33 percent. The differences among these compounds reveal the physics of the separation. The two chlorophenoxy acids are anionic at neutral pH and hydrophobic, so they are retained through a combination of electrostatic repulsion from like charges in the polyelectrolyte matrix, hydrophobic adsorption, and steric blocking. The more polar 2,4-D experienced stronger repulsion than its less polar cousin, explaining its superior rejection.

Buturon, though non-ionic, carries a high dipole moment of 5.44 debyes and a log octanol-water partition coefficient near 3, indicating substantial hydrophobicity. Its rejection appears to arise mainly from hydrophobic adsorption onto humic acid sites combined with steric hindrance from the highly charged bilayer stack. Diphenamid fared worst because this neutral molecule has low polarizability, a modest dipole moment of 3.60 debyes, and limited hydrophobicity, leaving steric effects as its only barrier. Infrared spectra taken after filtration showed a new carbonyl peak at 1717 wavenumbers on the used membranes, direct evidence that herbicide molecules had been adsorbed within the bilayer architecture rather than simply screened by pore size.

The study also mapped how preparation and operating conditions shape performance. The pH of the chitosan deposition bath proved critical: at pH 1.7, chitosan is fully protonated and forms well-fabricated bilayers densely populated with solute-accessible interactive sites, delivering maximum rejection, while higher deposition pH values produced weaker coatings and lower efficiency. Adding salt to the deposition medium screened the charges on the polyelectrolytes, causing them to coil and thicken the multilayer while weakening electrostatic rejection of anionic herbicides. Similarly, anions such as phosphate, sulfate, nitrate, chloride, and acetate in the feed water reduced the rejection of the negatively charged herbicides but left the non-ionic compounds largely unaffected. Flipping the membrane so humic acid formed the exposed outer layer instead of chitosan slightly altered performance for several compounds, underscoring that solute interactions with the outermost layer matter.

Practical durability is where the results become genuinely compelling. Nine-bilayer membranes stored for six months retained nearly all of their original rejection efficiency, and repeated filtration cycles over the same membrane showed only a slight, gradual decline attributed to a reversible fouling layer and the progressive occupation of active sites rather than any mechanical failure of the coating. Because the system operates at low pressure, it consumes far less energy than reverse osmosis or nanofiltration, produces less waste brine, and avoids the aggressive chemical cleaning cycles that shorten the life of high-pressure membranes. The entire separating layer is made from natural, biodegradable materials, giving the approach an environmental profile that synthetic polyelectrolyte coatings struggle to match.

The implications reach well beyond the four herbicides tested. Layer-by-layer coatings of humic acid and chitosan have previously been adapted to capture pesticides as diverse as atrazine, picloram, and metolachlor, and the present work extends that toolbox to chlorophenoxy, phenyl urea, and amide chemistries under a single platform. Because the assembly process works on substrates of varying geometry and can be scaled with straightforward dipping procedures, the author suggests the system could inform the design of pilot plants for membrane-based removal of chemical contaminants from drinking water, bringing affordable, low-energy herbicide filtration closer to real-world deployment for communities whose water supplies carry agricultural residues.

Subject of Research: Nano-composite biopolymer membranes for herbicide removal from water via layer-by-layer assembly under ultrafiltration

Article Title: Low pressure nano-composite biopolymer membranes for the removal of herbicides from water under ultrafiltration conditions

Article References: P., N. C. (2026). Low pressure nano-composite biopolymer membranes for the removal of herbicides from water under ultrafiltration conditions. Journal of Materials Science: Polymers, 1(1), Article 7. https://doi.org/10.1007/s44493-026-00007-4

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00007-4

Keywords: ultrafiltration, layer-by-layer assembly, chitosan, humic acid, herbicides, water purification, nanocomposite membranes, biopolymers, 2,4-D, membrane filtration, micropollutants, low-pressure membranes

The metals in question are among the most frequently detected pollutants in industrial wastewater. Lead and copper ions originate from mining, electroplating, battery manufacturing, and metallurgical processing, and even at trace concentrations they can trigger neurological, renal, and cardiovascular disorders in humans while accumulating in aquatic food chains. Conventional remediation techniques such as chemical precipitation, ion exchange, coagulation–flocculation, and activated carbon adsorption all carry drawbacks, including high operational costs, the generation of contaminated sludge, and poor performance at low metal concentrations. Membrane-based separation has emerged as an appealing alternative because of its operational simplicity, compact footprint, and potential for continuous treatment, but the performance of any membrane depends heavily on how it is made.

That is where the Brazilian team’s choice of fabrication method becomes significant. Most nanofiber membranes described in the literature are produced by electrospinning, a technique that draws fibers from a polymer solution using high voltage. Electrospinning produces excellent fibers but suffers from low production rates and high energy consumption, which have limited its industrial adoption. The researchers instead used solution blow spinning, or SBS, a technique that replaces the electric field with a stream of compressed air. In SBS, the polymer solution is fed through a concentric nozzle while pressurized air simultaneously stretches the emerging jet into fine fibers, which are collected on a rotating drum. The method requires no high-voltage equipment, offers higher throughput, and still yields highly porous micro- and nanofibrous mats, making it an attractive route to membranes that could one day be manufactured at scale.

To give the inherently hydrophobic PVDF an affinity for metal ions, the team dispersed montmorillonite clay into the polymer solution at loadings ranging from 3 to 30 percent by weight relative to the polymer. Montmorillonite is a layered silicate with a high specific surface area and a substantial cation exchange capacity. Its negatively charged interlayer spaces and surface hydroxyl groups attract divalent metal cations such as Pb²⁺ and Cu²⁺ through electrostatic interactions and ion exchange, effectively turning the clay particles scattered through the fibrous network into a dense population of active adsorption sites. The researchers fabricated two families of membranes: pristine PVDF nanofiber mats and PVDF–clay composites, keeping the solution volume constant at five milliliters for every production run so that thickness differences could be attributed to the clay content itself.

Scanning electron microscopy confirmed that the process worked as intended. The membranes displayed uniform, smooth, cylindrical fibers with only occasional bead-like imperfections, a sign that the spinning parameters—30 percent polymer concentration, a solution flow rate of 76 microliters per hour, air pressure of 140 kilopascals, a working distance of 21 centimeters, and a collector speed of 80 revolutions per minute—were well chosen. X-ray diffraction revealed the characteristic crystalline phases of PVDF alongside clay-derived peaks corresponding to quartz and aluminum oxide, confirming that the filler was genuinely incorporated into the fibers. At the highest loading of 30 percent, however, clay agglomerates began to appear, producing structural defects, thinner and more fragile films, and, above that threshold, outright clogging and failure of fiber formation.

Before testing adsorption, the team determined the point of zero charge of the membranes, the pH at which the surface carries equal numbers of positive and negative charges. This value came out at 6.9 in pure water and 6.4 in a water–ethanol mixture, and the adsorption experiments were conducted near these values to minimize interference from the liquid medium. Batch tests, in which membrane pieces of roughly 50 milligrams were stirred with 20 milliliters of metal solution at 5 milligrams per liter, showed a familiar pattern: rapid uptake in the first hours as abundant adsorption sites were available, followed by a gradual slowdown as those sites filled. Adsorption was consistently higher in the water–ethanol mixture, which the authors attribute to ethanol acting like a surfactant, lowering the surface tension of water and helping the solution wet the hydrophobic polymer and penetrate the spaces between fibers to reach buried clay sites.

The kinetic and equilibrium analysis told a coherent mechanistic story. A pseudo-first-order model fit the data poorly, with correlation coefficients as low as 0.674, whereas the pseudo-second-order model tracked the experiments closely in both media, indicating that the rate-limiting step involves chemical interaction—chemisorption—between the metal ions and functional sites on the clay surface. Equilibrium data were best described by the Langmuir isotherm, which assumes monolayer adsorption on a finite number of identical sites, and the calculated maximum adsorption capacities matched the experimental values well. The dimensionless separation factor derived from the Langmuir constant was below one in every test, confirming that adsorption of both metals is thermodynamically favorable. Still, the Langmuir constants were low, between 0.044 and 0.298, and more than half of the dissolved metal remained in solution at equilibrium in the static tests—a limitation the researchers trace directly to the hydrophobic PVDF matrix, which impedes ion access to clay sites in stagnant water.

The picture changed dramatically when the membranes were operated as filters. In dead-end filtration experiments, in which the metal solution was forced through a 12.5 square centimeter membrane area under roughly one bar of pressure from a vacuum pump, removal efficiencies soared above 90 percent, with the membrane containing 10 percent clay achieving 92 percent copper removal in aqueous medium. The lead removal performance matched that of comparable electrospun membranes reported in the literature, but with the advantage of a faster, cheaper production process. Even pure PVDF removed a meaningful fraction of the metals, demonstrating that the porous fibrous architecture itself contributes to capture. The combined mechanism—physical retention by the dense fiber network working in tandem with adsorption at clay sites under pressure-driven flow—proved far more effective than batch adsorption alone, because the applied pressure overcomes the wetting resistance that limits ion penetration in static conditions.

Intriguingly, more clay was not always better. At 30 percent loading, filtration performance deteriorated: aggregates identified by energy-dispersive X-ray analysis acted as bypass routes that let water, and the metals it carried, slip through without contacting adsorbent sites, while the weakened mechanical properties of the highly loaded membranes allowed pores to widen under pressure. The study also revealed a classic trade-off between selectivity and permeability, with thicker, less permeable membranes delivering the highest selectivity. The authors are candid about the work’s limits: no regeneration or reuse studies were performed, long-term stability remains untested, and performance in multicomponent, real-world wastewater has yet to be assessed. Future work, they note, should address membrane reuse, metal leaching, permeability–selectivity trade-offs, and continuous-flow operation. Even so, the message is clear: solution blow spinning can rapidly produce PVDF–clay nanofiber membranes that excel as filtration materials, and with moderate clay loadings of around 10 percent, they offer a promising, scalable route to stripping toxic lead and copper from contaminated water.

Subject of Research: PVDF–montmorillonite clay nanofibrous membranes produced by solution blow spinning for the removal of lead and copper ions from contaminated water

Article Title: Removal of Pb and Cu metals by PVDF/clay membranes obtained through solution blow spinning technique

Article References: da Cruz Dias, G., Zadorosny, L., Sanches, A. O., dos Santos, M. C., & Malmonge, L. F. (2026). Removal of Pb and Cu metals by PVDF/clay membranes obtained through solution blow spinning technique. Journal of Materials Science: Polymers, 1(1), Article 18. https://doi.org/10.1007/s44493-026-00020-7

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00020-7

Keywords: solution blow spinning, PVDF, montmorillonite clay, nanofibrous membranes, heavy metal removal, lead, copper, water treatment, adsorption kinetics, Langmuir isotherm, membrane filtration, wastewater remediation