The urgency behind the work is easy to grasp. Textile manufacturing is among the largest industrial consumers of water and one of the biggest dischargers of colored effluent. Methylene blue, a cationic dye used in cotton, silk, paper, and ink production, and Congo red, an anionic diazo dye common in textiles and diagnostic tests, are both prized for their color stability and water solubility, which is precisely what makes them so persistent once they enter rivers and groundwater. Exposure to these compounds has been linked to respiratory irritation, damage to mucous membranes, dizziness, vomiting, diarrhea, genetic abnormalities, and harm to the nervous system. Because the dyes resist biodegradation, conventional biological treatment plants struggle to break them down, leaving adsorption as one of the most practical remediation routes.

Adsorption, in which pollutant molecules stick to the surface of a solid material, has long been favored for its simplicity, low cost, and versatility. The catch is finding an adsorbent with enough surface area, the right surface chemistry, and sufficient structural stability to perform well cycle after cycle. Previous studies have explored fly ash, activated carbons, crushed brick, cedar sawdust, and carbon nanotubes, and graphene-family materials have attracted particular attention because of their enormous surface-to-volume ratios and rich arrays of oxygen-containing functional groups. Zinc oxide, meanwhile, is cheap, widely available, and environmentally benign, but on its own it tends to form clusters that bury its active sites. The new research set out to combine the two materials so that each compensates for the other’s weaknesses.

The synthesis followed well-established chemistry. The team produced graphene oxide using the modified Hummer’s method, oxidizing graphite powder with sulfuric acid, sodium nitrate, and potassium permanganate under carefully controlled temperature steps, then purifying and drying the resulting sheets. Zinc oxide nanoparticles were prepared separately by chemical co-precipitation, dissolving zinc acetate dihydrate in water and raising the pH to 12 with sodium hydroxide, followed by washing, drying, and calcination at 400 degrees Celsius for four hours. Finally, the two components were merged by ultrasonication: graphene oxide was dispersed in deionized water, zinc oxide was gradually added at a three-to-one mass ratio, and the mixture was stirred at 60 degrees Celsius for three hours before being collected and dried.

Characterization confirmed that the composite had formed as intended. UV-visible spectroscopy showed the characteristic ZnO band-gap absorption near 376 nanometers alongside the graphene oxide peaks at roughly 244 and 363 nanometers. Fourier transform infrared spectroscopy revealed the oxygenated functional groups of graphene oxide, including C-O, C-OH, C=C, C=O, and O-H bands, together with a strong ZnO vibration near 650 wavenumbers, evidence that the nanoparticles were anchored to the sheets. X-ray diffraction displayed the full set of wurtzite ZnO crystal planes plus the graphene oxide (001) reflection, while electron microscopy showed crumpled, interconnected graphene sheets decorated with ZnO particles, forming a porous, wrinkled network. Energy-dispersive X-ray analysis put the composition at about 37.2 percent carbon, 38.1 percent oxygen, and 20.4 percent zinc by weight. A zeta potential of minus 21.5 millivolts indicated a negatively charged, moderately stable colloid.

The adsorption experiments revealed how sensitively performance depends on solution chemistry. For methylene blue, removal improved steadily as pH increased, because deprotonation of the composite’s surface groups accumulated negative charge that electrostatically attracted the cationic dye. Congo red behaved in the opposite fashion: removal rose as pH climbed from 2 to 6 and then fell at higher pH, where strong negative surface charge repels the anionic dye. The point of zero charge was measured at pH 7.1, meaning the surface is positive under acidic conditions and negative under alkaline ones. Adsorbent dose mattered too, with removal climbing as dosage increased from 10 to 40 milligrams and then plateauing once all available dye molecules had been captured. Raising the initial dye concentration from 30 to 150 milligrams per liter reduced the percentage removed, since a fixed number of active sites becomes saturated, and equilibrium was reached after about 80 minutes of contact.

Modeling of the equilibrium data showed that the Langmuir isotherm, which assumes monolayer adsorption on a homogeneous surface, described the process better than the Freundlich, Temkin, or Dubinin-Radushkevich models. According to the Langmuir fit, the composite achieved a maximum adsorption capacity of 320.12 milligrams of methylene blue per gram, more than double the 175.24 milligrams per gram recorded for Congo red. Kinetic analysis told a complementary story: the pseudo-second-order model fit best, with the Elovich model also performing well, indicating that chemisorption, the formation of actual chemical bonds between dye and surface, dominated the uptake. An intra-particle diffusion constant that did not pass through the origin showed that diffusion inside the pores was only one of several rate-controlling steps.

Thermodynamic measurements added further depth. Gibbs free energy values ranged from minus 2.98 to minus 10.95 kilojoules per mole across temperatures from 5 to 30 degrees Celsius, confirming that adsorption was spontaneous at every temperature tested. Positive enthalpy changes of 87.34 kilojoules per mole for methylene blue and 89.94 kilojoules per mole for Congo red marked the process as endothermic and pointed to strong adsorbent-dye interactions, possibly involving dehydration of both surfaces before binding. Positive entropy changes exceeding 10 joules per mole per kelvin suggested a dissociative mechanism at the solid-liquid interface. Optimal removal occurred at 20 degrees Celsius, after which the surface saturated.

The proposed mechanism is genuinely multi-pronged. Electrostatic attraction between the negatively charged composite and cationic methylene blue drives much of that dye’s uptake, while hydrogen bonding, pi-pi stacking between dye aromatic rings and graphene domains, surface complexation at ZnO sites, and penetration of dye molecules into the pores all contribute. Infrared spectra taken after adsorption showed new bands corresponding to methylene blue’s C-N and trimethylammonium groups and Congo red’s sulfonate groups, along with shifts in the O-H and C=O features, direct spectroscopic fingerprints of the binding interactions. The synergy between the two components appears central: graphene oxide’s vast surface area and functional groups disperse the zinc oxide nanoparticles, preventing the aggregation that cripples standalone ZnO, while the nanoparticles add extra active sites and structural stability.

Perhaps most importantly for real-world deployment, the material can be regenerated. Washing with ethanol followed by 0.1 molar hydrochloric acid released the bound dyes, and the recovered composite retained useful performance over five consecutive adsorption-desorption cycles, with methylene blue efficiency declining from 97.25 to 68.06 percent and Congo red from 93.89 to 63.35 percent. Compared against previously reported adsorbents, including pine tree bark, sunflower-husk-derived silica materials, and various graphene and zinc oxide systems, the ZnO-GO composite stands out for combining high capacity for both a cationic and an anionic dye with straightforward synthesis and reusability. The authors argue that this balance of performance, cost, and sustainability makes the nanocomposite a credible candidate for scaling up to industrial wastewater treatment, where a single material that can capture dyes of opposite charge could simplify remediation trains considerably.

Subject of Research: Adsorptive removal of cationic and anionic textile dyes from water using a zinc oxide-graphene oxide nanocomposite

Article Title: Enhanced adsorptive removal of methylene blue and Congo red from aqueous solution using zinc oxide-graphene nanocomposite

Article References: Kadian, J., Yadav, S., Dhawan, M., Kumar, A., & Chahar, M. (2026). Enhanced adsorptive removal of methylene blue and Congo red from aqueous solution using zinc oxide-graphene nanocomposite. Discover Chemistry, 3(1), Article 527. https://doi.org/10.1007/s44371-026-00984-z

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00984-z

Keywords: zinc oxide, graphene oxide, nanocomposite, adsorption, methylene blue, Congo red, wastewater treatment, dye removal, chemisorption, Langmuir isotherm, water pollution, nanomaterials