The significance of the work lies less in any single number than in the combination of properties the membrane brings together. Conventional approaches to chromate contamination typically rely on activated carbon, ion-exchange resins, or chemical precipitation, each of which treats adsorption and filtration as separate steps requiring separate equipment. The new material merges the two operations into a single system: contaminated water passes through the membrane, and the chromate is captured on the membrane surface as it goes. That integration could simplify treatment trains for industries such as metal finishing, leather tanning, and electroplating, where chromium-laden effluent is a persistent regulatory headache.

Chromate ions are not a minor nuisance. The hexavalent form of chromium, in which the metal exists as the oxyanion CrO4 2- under neutral to alkaline conditions, is classified as carcinogenic, mutagenic, and teratogenic. Chronic exposure has been linked to lung cancer and DNA damage, with studies documenting double-strand breaks in human lung cells exposed to chromate compounds. Because the ion is soluble and mobile in water, it travels readily through groundwater and municipal systems, which is why regulators worldwide impose strict limits on its discharge. Removing it at the source, quickly and cheaply, remains one of the more urgent engineering challenges in water treatment.

The Kerala team’s membrane is built on chitosan, a polysaccharide obtained by deacetylating chitin from shellfish waste. Chitosan carries abundant amine groups along its polymer backbone, and these nitrogen sites are protonated under acidic conditions, giving the polymer a positive charge that electrostatically attracts the negatively charged chromate anion. Chitosan alone, however, tends to swell in water and can lack mechanical robustness, which is where the expanded graphite comes in. Expanded graphite is produced by thermal treatment that forces graphite layers apart into a worm-like, highly porous structure with enormous internal surface area. Embedded within the biopolymer matrix, it reinforces the film, adds additional adsorption sites, and creates conductive, thermally stable pathways through the material.

To confirm that the composite actually possessed the architecture the researchers intended, they subjected it to a battery of characterization techniques. Infrared spectroscopy identified the functional groups responsible for binding, X-ray diffraction probed the crystalline structure of the embedded graphite, and scanning electron microscopy revealed the membrane’s surface morphology and pore structure. Raman spectroscopy, a technique particularly sensitive to carbon-based materials, verified the graphitic component, while thermogravimetric analysis measured how the material behaved under heating, an important indicator of its thermal stability in service. Together, these methods established that the expanded graphite was successfully integrated into the biopolymer rather than merely mixed in.

The adsorption experiments themselves were tracked by ultraviolet spectroscopy, a standard analytical method for quantifying chromate concentrations in solution. The researchers systematically varied the experimental parameters, including contact time, solution chemistry, and temperature, to find the conditions under which the membrane performed best. The kinetics were strikingly fast: equilibrium was reached within 40 minutes, a time scale that matters enormously in practical treatment, where slower adsorbents can require hours of residence time and correspondingly larger treatment infrastructure.

Thermodynamic analysis added a layer of mechanistic insight. The calculated Gibbs free energy change fell between 0 and -20 kilojoules per mole, a range that signals predominantly physical adsorption rather than the formation of strong chemical bonds. The negative enthalpy change indicated that the process is exothermic, releasing heat as chromate binds to the membrane, and the overall thermodynamic parameters confirmed that adsorption is spontaneous and feasible under the tested conditions. In plain terms, the chromate ions are being captured by comparatively weak but numerous physical interactions, electrostatic attraction and van der Waals forces chief among them, which is good news for regeneration, since weakly bound ions can be released again without harsh chemistry.

Among the isotherm models fitted to the equilibrium data, the Dubinin-Radushkevich model described the membrane’s behavior best, with a coefficient of determination of 0.998. The D-R model, rooted in the potential theory of adsorption on energetically heterogeneous surfaces, is particularly useful because it distinguishes between physical and chemical adsorption mechanisms, and its superior fit here is consistent with the thermodynamic picture of a physically driven process. The near-perfect fit suggests the membrane surface presents a fairly uniform distribution of adsorption energies, likely a consequence of the well-dispersed graphite within the chitosan matrix.

Perhaps the most commercially relevant result concerns reuse. The membrane could be regenerated in only 15 minutes, and remarkably, the regeneration did not degrade performance but enhanced it, lifting removal efficiency to 94.52 percent. Even after seven consecutive adsorption-desorption cycles, the membrane still removed 89.62 percent of the chromate, retaining nearly all of its original capacity. For real-world deployment, where adsorbent replacement and disposal costs often dominate operating budgets, that kind of cycling stability is a decisive advantage over single-use adsorbents that must be discarded once saturated.

The researchers also report high water flux across the membrane with chromate retention of about 89 percent, indicating that the material does not sacrifice throughput for selectivity, a common trade-off in membrane design. Because the membrane is composed of environmentally friendly polymers and cheap expanded graphite, and because it regenerates rapidly without exotic chemicals, the authors position it as a sustainable alternative to conventional adsorbents. The work, led by Sruthi John and supervised by Jitha Kunhikrishnan Maniath, was conducted with analytical support from DST-SAIF Cochin and the instrumentation centre at Kannur University’s Payyannur campus. If the membrane’s performance holds up in continuous-flow testing with real industrial effluent, a chitosan film laced with humble graphite could become a quietly powerful tool against one of the world’s most dangerous water pollutants.

Subject of Research: Chromate ion removal from water using an expanded graphite-embedded chitosan composite membrane

Article Title: Fabrication of expanded graphite–embedded biopolymer-based composite membrane for chromate ion removal from aqueous solutions

Article References: John, S., & Maniath, J. K. (2026). Fabrication of expanded graphite–embedded biopolymer-based composite membrane for chromate ion removal from aqueous solutions. Environmental Science and Pollution Research, 33(30), 15743-15757. https://doi.org/10.1007/s11356-026-38226-5

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38226-5

Keywords: chitosan, expanded graphite, chromate, hexavalent chromium, water treatment, adsorption, composite membrane, biopolymer, regeneration, isotherm modelling, pollution remediation, heavy metals

The team, led by Fengxia Zhai, Liqi Zhuang, Xinyu Wei, Fahai Cao, and Shicheng Zhao, worked with sulfonated polyimides, a family of aromatic polymers prized for their thermal stability, mechanical robustness, and film-forming ability. By attaching sulfonic acid groups to the polymer backbone, chemists create the acidic sites that attract water and enable protons to hop through the material. The catch is dosage. To reach conductivities useful in a fuel cell, sulfonated polyimides typically need a high degree of sulfonation, and that very density of acidic groups makes the polymer drink up water voraciously. The membrane swells, its dimensions drift, and over time water attacks the imide linkages themselves, degrading the film hydrolytically until it fails. This long-standing trade-off between conductivity and durability has kept sulfonated polyimides largely out of commercial fuel cells despite their attractive chemistry.

The Shanghai group’s strategy was to sidestep the trade-off rather than fight it head-on. Instead of pushing sulfonation ever higher, they kept the polymer matrices at moderate sulfonation levels and introduced an inorganic proton conductor, phosphotungstic acid, as a dispersed filler. Phosphotungstic acid belongs to the polyoxometalate family, inorganic clusters with a distinctive Keggin structure in which a central phosphorus atom is caged by twelve tungsten-oxygen octahedra. These clusters are dense with acidic protons and can conduct them efficiently, particularly when hydrated. Crucially, the acid is strong enough to contribute conductivity without requiring the polymer host to be saturated with sulfonic acid groups, which means the membrane can stay dimensionally stable and hydrolytically tough while still moving protons quickly.

Building the composite required care, because polyoxometalates tend to aggregate and can leach out of polymer films. The researchers synthesized a series of sulfonated polyimides with varying degrees of sulfonation and two different chain architectures, random copolymers in which sulfonated units are scattered along the backbone, and block copolymers in which sulfonated and unsulfonated segments are grouped into distinct blocks. Block architectures are known to encourage microphase separation, creating connected hydrophilic channels that can act as proton highways. The phosphotungstic acid was then blended into each matrix using ultrasonic-assisted mixing followed by solution casting, a processing route in which ultrasound helps disperse the inorganic clusters evenly through the polymer solution before it is cast into a film and dried.

Characterization confirmed the approach worked at the molecular level. Fourier-transform infrared spectroscopy detected the vibrational fingerprints of the Keggin clusters inside the composite films, showing that the acid’s characteristic structure survived the blending and casting process intact. Scanning electron microscopy revealed a uniform dispersion of the filler throughout the polymer matrix, with no large aggregates that would have compromised mechanical integrity or created defects. That uniformity matters because proton conduction in a composite depends on connectivity: isolated pockets of conductor do little, but a well-dispersed phase can form continuous pathways that supplement the polymer’s own sulfonic acid network.

The performance data are the heart of the study. Among the many formulations tested, one stood out: a composite built from a block copolymer designated BB (3:1)-s, loaded with ten percent phosphotungstic acid. This membrane achieved a proton conductivity of 0.161 siemens per centimeter, a figure comparable to Nafion 117, the widely used commercial reference material. For a hydrocarbon-based polymer membrane to reach that benchmark is significant, since Nafion’s perfluorinated structure and phase-separated morphology have long given it an edge in hydrated proton transport. Equally important, the conductivity gain came without sacrificing strength: the same membrane recorded a tensile strength of 15 megapascals, indicating that the filler reinforced rather than plasticized the film.

Durability results may prove even more consequential than the conductivity numbers. The optimized composite membrane withstood more than 1,600 hours of hydrolytic stability testing, an endurance figure that addresses the Achilles heel of sulfonated polyimides. Because the membrane does not rely on extreme sulfonation, it absorbs water in a controlled way, limiting the swelling stresses and backbone hydrolysis that normally destroy highly sulfonated films. The authors also report favorable methanol barrier properties, an attribute that matters for direct methanol fuel cells, where fuel crossing the membrane from anode to cathode wastes fuel and poisons the cathode reaction. A membrane that conducts protons well but resists methanol crossover addresses one of the central design tensions in liquid-fuel cell development.

The work fits into a broader international effort to replace or augment perfluorinated ionomers with cheaper, more tunable hydrocarbon membranes. Recent years have seen sulfonated polyimides, polybenzimidazoles, and polyarylene ethers modified with cross-linking, branching, acid-base blending, and a parade of inorganic fillers including graphene oxide, metal-organic frameworks, mesoporous carbon nitride, and various heteropolyacids. Phosphotungstic acid in particular has been anchored into chitosan films, trapped inside metal-organic frameworks, and covalently linked to mesoporous supports in earlier studies. What distinguishes the new report is the systematic pairing of chain architecture with filler loading: by comparing random and block copolymer matrices across a sulfonation series, the researchers could identify a formulation where polymer morphology and inorganic conduction reinforce each other rather than compete.

There remain familiar hurdles between laboratory membranes and fuel cell stacks. Polyoxometalates are water-soluble, and long-term leaching under real operating cycles, with their humidity swings and thermal excursions, will need to be demonstrated beyond the hydrolytic soaking tests reported here. Membrane-electrode assembly integration, chemical stability in the presence of peroxide radicals generated at the electrodes, and manufacturability at scale are all questions the Polymer Bulletin paper does not settle. The authors acknowledge support from the National Natural Science Foundation of China, and the study was conducted at the Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, with co-author Zhai also affiliated with Sushui Energy Technology, a Shanghai company, hinting at an interest in commercial translation.

Still, the result offers a template for how composite design can dissolve a classic trade-off. By letting an inorganic superacid carry part of the proton-transport burden, the polymer matrix can stay modestly sulfonated, and therefore tough and dimensionally stable, while the composite as a whole conducts protons at Nafion-level rates. A membrane that simultaneously delivers 0.161 siemens per centimeter, 15 megapascals of tensile strength, more than 1,600 hours of hydrolytic endurance, and methanol resistance is a rare combination in the hydrocarbon membrane literature. If follow-up work confirms that the phosphotungstic acid stays put over thousands of fuel cell cycles, sulfonated polyimide composites of this kind could move from polymer chemistry journals toward the membranes that actually sit inside hydrogen and methanol fuel cells, where the balance of properties, not any single figure of merit, decides what succeeds.

Subject of Research: Composite proton exchange membranes combining sulfonated polyimide with phosphotungstic acid for fuel cells

Article Title: Sulfonated polyimide/phosphotungstic acid composite proton exchange membranes with balanced properties for fuel cells

Article References: Zhai, F., Zhuang, L., Wei, X., Cao, F., & Zhao, S. (2026). Sulfonated polyimide/phosphotungstic acid composite proton exchange membranes with balanced properties for fuel cells. Polymer Bulletin, 83(12), Article 653. https://doi.org/10.1007/s00289-026-06725-7

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06725-7

Keywords: proton exchange membrane, sulfonated polyimide, phosphotungstic acid, fuel cells, composite membrane, proton conductivity, hydrolytic stability, block copolymer, methanol crossover, Keggin structure, hydrogen fuel, polymer electrolyte