Chitosan-Graphite Membrane Strips Toxic Chromate from Water in Minutes

Hexavalent chromium, the industrial pollutant made infamous by decades of contamination cases around the world, may have met an unusually elegant adversary. Researchers at Sree Narayana College Kannur, affiliated with Kannur University in Kerala, India, have fabricated a thin composite membrane built from chitosan, a biopolymer derived from crustacean shells, and expanded graphite, an inexpensive form of layered carbon. In tests reported in Environmental Science and Pollution Research, the membrane stripped roughly 91.68 percent of chromate ions from aqueous solution in just 40 minutes, and after a rapid regeneration cycle its performance actually improved, reaching 94.52 percent removal.

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