Chitosan and Humic Acid Nanocoatings Strip Herbicides from Water at Low Pressure

Herbicide residues in drinking water have become one of the most stubborn contamination problems of modern agriculture, and a new study suggests the solution may come from two of nature’s most humble materials. A researcher at Sree Narayana College in Kollam, India, working with the Advanced Centre of Environmental Studies and Sustainable Development at Mahatma Gandhi University, has built ultra-thin membranes from humic acid and chitosan, two naturally derived biopolymers, and shown that they can strip herbicides from water with remarkable efficiency while operating at pressures far lower than conventional high-end filtration systems demand.

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