Discharge printing is a subtractive art. Rather than adding pigment to undyed cloth, the printer applies a paste to fabric that has already been dyed a uniform shade, and the paste chemically destroys the dye in selected areas. The result is a crisp white or differently colored pattern emerging from a solid ground. The chemistry hinges on breaking the chromophore, the light-absorbing part of the dye molecule, which in the reactive azo dyes commonly used on cotton is a conjugated system built around an azo linkage of two nitrogen atoms joined by a double bond. Reducing agents donate electrons to this linkage, cleaving it and collapsing the conjugated system, so the molecule no longer absorbs visible light and the fabric appears white where the paste was applied.

The industry’s preferred reducers, however, come with a heavy environmental and toxicological price. Thiourea dioxide, sodium sulfoxylate formaldehyde, and sodium dithionite are the workhorses of reductive discharge, but conventional discharge pastes typically contain 20 to 30 percent hazardous chemicals per printing cycle. Sulfoxylate formaldehyde compounds release formaldehyde, a human carcinogen associated with nasal sinus and nasopharyngeal cancers. Sodium dithionite generates non-regenerable oxidation products such as sulfite and sulfate that complicate the disposal of residual paste and wash water. Stannous chloride formulations can introduce heavy metal ions and decompose to produce corrosive hydrogen chloride, while other by-products including carbon monoxide, sulfur dioxide, and zinc oxide add to the pollution burden. Oxidative alternatives such as potassium permanganate risk damaging the cotton substrate itself, and ozone treatment, though gentler, produces blurry printed outlines.

The Bangladesh team’s insight was to exploit the phytochemical wealth of fruit peels that are normally discarded as waste. Orange peels account for 30 to 50 percent of the fruit’s mass and pineapple peels for 40 to 50 percent, and both are rich in polyphenols, flavonoids, and ascorbic acid. These compounds are natural electron donors, which is precisely the defining property of a reducing agent. In the discharge paste, the polyphenols and flavonoids transfer electrons to the azo bonds of the reactive dye under alkaline conditions, breaking the conjugated system responsible for the color in much the same way synthetic reducers do, but with far lower ecotoxicity. Previous studies had already shown that antioxidant-rich extracts from banana, orange, pineapple, watermelon, and papaya peels could reduce and solubilize vat dyes, but their use in reactive discharge printing on cotton remained largely unexplored.

To prepare the extracts, the researchers separated the peels, dried them at 60 degrees Celsius for 24 hours, ground them into a moderately fine powder, and extracted them with distilled water at a 1:3 weight-to-volume ratio at 95 degrees Celsius for 30 minutes in a laboratory dyeing machine. After filtration, the orange peel extract and pineapple peel extract were formulated into white discharge print pastes alongside a synthetic thickener, urea as a hygroscopic agent, and soda ash to maintain the alkaline conditions needed both to activate the reducers and to promote hydrolysis of the covalent bond between the reactive dye and the cellulose fiber. The pastes were screen-printed onto cotton knit fabric dyed with two commercial vinyl sulfone reactive dyes, Remazol Ultra Carmine and Remazol Navy, then steamed at 105 degrees Celsius for 10 minutes and washed through a multi-step cycle.

Infrared spectroscopy provided the first line of chemical evidence. Both extracts showed broad absorption bands near 3355 inverse centimeters, consistent with hydroxyl stretching from phenolic and alcoholic groups, and a band around 1651 inverse centimeters attributable to carbonyl stretching, a functional profile consistent with phytochemicals capable of reducing activity. When the researchers compared the spectra of dyed fabrics before and after discharge printing, they found noticeable increases in transmittance at dye-related absorption bands, particularly in the fingerprint region between roughly 1500 and 600 inverse centimeters, where aromatic, sulfonate, and carbon-sulfur vibrations of the dye structure reside. The weakened or eliminated dye-associated signals indicated that both extracts had genuinely altered the chromophores in the printed areas rather than merely masking them.

The quantitative results were striking. On fabric dyed with the carmine shade, pineapple peel extract achieved a color strength reduction of 95.60 percent, slightly outperforming thiourea dioxide at 95.36 percent and sodium dithionite at 95.21 percent, while orange peel extract reached 93.01 percent. On the navy shade, the conventional agents led, with sodium sulfoxylate formaldehyde achieving 94.78 percent reduction, followed by thiourea dioxide and sodium dithionite, while orange peel extract managed 92.87 percent and pineapple peel extract 89.63 percent. Crucially, blank print pastes without any reducing agent produced reductions of only 9.46 and 10.45 percent on the two shades, confirming that the fruit extracts themselves were responsible for the discharge effect. The carmine dye discharged more readily than the navy dye across all treatments, likely because its chromophoric system was less resistant to the reducing environment.

Whiteness told a more nuanced story. Conventional reducers generally produced brighter, whiter discharged areas, with whiteness index values on the navy-dyed fabric exceeding 100 for all three synthetic agents, compared with 57.16 for orange peel extract and 45.38 for pineapple peel extract. The researchers note that the whiteness index depends on the final reflectance balance and chromaticity of the surface rather than simply on the percentage of dye removed, so a fabric can discharge well yet retain a slight tint that lowers its measured whiteness. The superior and more reproducible whiteness of the synthetic agents may reflect their controlled synthesis and purification, whereas natural extracts vary in composition with fruit source, maturity, and extraction efficiency. Mechanical testing showed moderate but acceptable losses of bursting strength, ranging from 14.73 to 21.65 percent relative to the original undyed fabric’s 217.60 kilopascals, with pineapple peel extract actually preserving strength better than several conventional agents on the navy-dyed fabric.

The study’s authors are careful to frame the comparison honestly: the amounts of the conventional reducers were optimized across multiple trials to achieve visually similar discharge effects, so the results reflect practical performance rather than an exact equal-dose chemical comparison. Even so, the message is clear. Water extracts of two abundant fruit wastes, prepared with nothing more elaborate than drying, grinding, and boiling, delivered discharge performance comparable to optimized doses of thiourea dioxide, sodium sulfoxylate formaldehyde, and sodium dithionite, while avoiding formaldehyde release, heavy metals, and problematic sulfur-based effluent. As the textile industry faces mounting pressure to adopt circular and sustainable processing, the prospect of turning kitchen-scale waste into a functional printing chemical is exactly the kind of elegant, low-tech solution that could travel quickly from the laboratory bench to the print floor, transforming both the industry’s waste stream and its chemical one at the same time.

Subject of Research: Use of orange and pineapple peel extracts as natural reducing agents for sustainable discharge printing on reactive-dyed cotton fabric

Article Title: Discharge printing on cotton fabric using fruit peel extracts as natural reducing agents

Article References: Raafi, S. M., Islam, Z., & Kumar, D. (2026). Discharge printing on cotton fabric using fruit peel extracts as natural reducing agents. Discover Chemistry, 3(1), Article 568. https://doi.org/10.1007/s44371-026-01019-3

Image Credits: AI Generated

DOI: 10.1007/s44371-026-01019-3

Keywords: discharge printing, cotton fabric, natural reducing agents, orange peel extract, pineapple peel extract, reactive dyes, sustainable textiles, fruit waste valorization, phytochemicals, azo dye reduction, textile chemistry, green chemistry

The study, published in Discover Green Chemistry by M. H. N. Revon and N. Priyantha, tackles a persistent gap in corrosion science. Traditional inhibitors for stainless steel, including nitrite- and phosphate-based compounds, have come under scrutiny for environmental and health concerns, while metal oxide nanoparticles synthesized by conventional chemical routes typically rely on toxic reducing agents and stabilizers. Plant extracts have emerged as greener alternatives, but very few reports have explored fruit waste, and orange peel in particular, as a vehicle for making MgO nanoparticles aimed at protecting stainless steel surfaces in aggressive media. The mechanism by which such biosynthesized particles defend steel had also remained underexplored.

The synthesis itself is disarmingly simple. Fresh peels of Citrus reticulata were crushed and refluxed in deionized water at 90 degrees Celsius for an hour to produce an extract rich in flavonoids, polyphenols and organic acids. That extract was then added to a magnesium nitrate solution, and the pH was raised to 10 by dropwise addition of sodium carbonate. After four hours of stirring, centrifugation and calcination at 500 degrees Celsius, a pale powder of magnesium oxide nanoparticles emerged. Powder X-ray diffraction confirmed the crystalline periclase structure, with characteristic peaks at 36.9, 42.9 and 62.3 degrees matching the (111), (200) and (220) planes. Broad diffraction features between 18 and 23 degrees betrayed residual phytoconstituents clinging to the particle surfaces, a detail that would prove central to the inhibitor’s performance.

Fourier transform infrared spectroscopy painted a complementary picture. A band at 522 wavenumbers confirmed the Mg-O stretching vibration, while features at 1744 and 1219 wavenumbers corresponded to carbonyl and carbon-oxygen stretching from the adsorbed flavonoids and polyphenols. Particle size analysis revealed primary nanoparticles roughly 37.6 nanometers in diameter, although scanning electron microscopy showed that the particles tend to cluster into larger, irregular flaky aggregates upon drying. The sharp-edged, high-surface-area morphology is precisely what corrosion scientists look for in an inhibitor, since more surface area means more contact points for adhesion to the metal.

Before testing the inhibitor, the team documented just how hostile acidic chloride environments are to Grade 202 steel. Ring-shaped specimens, cleaned with deionized water and acetone and left unpolished to mimic industrial conditions, were immersed in hydrochloric acid solutions of varying strength and weighed at 24-hour intervals over at least a week. In 1.00 molar hydrochloric acid, the steel dissolved so completely within two days that its mass could no longer be tracked. Adding sodium chloride made matters dramatically worse: after seven days in 0.10 molar sodium chloride, mass loss reached about 37 percent, compared with roughly 20 percent without the salt, an increase of about 85 percent. In the harshest combination tested, 1.00 molar sodium chloride with 0.50 molar acid, the steel lost essentially all of its mass. Electron microscopy of exposed surfaces showed deep pits and rough, fractured corrosion products where the polished alloy had once been smooth.

Against this bleak backdrop, the magnesium oxide nanoparticles produced remarkable results. In 0.25 molar hydrochloric acid, a 0.05 molar dose of the particles cut the corrosion rate from 4.02 millimeters per year to 0.136, an inhibition efficiency of 96.6 percent. Raising the dose to 0.10 molar drove the corrosion rate down to a startling 0.00395 millimeters per year. Even in the more aggressive 0.50 molar acid, where the uninhibited steel corroded at 7.80 millimeters per year, the nanoparticles delivered efficiencies of 91.9 percent at 0.05 molar and higher at 0.10 molar. The team notes that both the particles themselves and the electron-donating phytochemicals from the extract contribute to this protection, since the flavonoids and polyphenols adsorb alongside the oxide and reinforce the barrier layer.

Electrochemical impedance spectroscopy provided mechanistic depth. In the three-electrode cell, with the steel as working electrode, a platinum counter electrode and a silver/silver chloride reference, the inhibited samples produced much larger semicircles in Nyquist plots, indicating substantially higher polarization resistance. Bode phase-angle plots showed the intermediate-frequency peak growing and shifting to lower frequencies in the presence of the nanoparticles, the signature of a more capacitive, resistive surface film that retards charge transfer, a film that remained stable throughout a week of testing. The solution resistance itself changed when nanoparticles were present, suggesting that the particles adsorb ions and alter ionic conductivity at the electrode surface. Double-layer capacitance dropped markedly, further evidence of dense adsorption of particles and phytochemicals onto the steel.

Tafel polarization analysis and open circuit potential measurements rounded out the electrochemical case. Adding the inhibitor shifted the corrosion potential toward more positive, more noble values while sharply lowering the corrosion current density, and because both the anodic metal dissolution branch and the cathodic hydrogen evolution branch were suppressed, the nanoparticles act as a mixed-type inhibitor. The positive shift in open circuit potential, which grew with increasing inhibitor concentration and stabilized over thousands of minutes, pointed to the formation and consolidation of a protective surface film rather than any transient effect.

The proposed mechanism hinges on synergy between the oxide core and its organic shell. The hydroxyl and carboxyl groups of the peel phytochemicals, which cap the particles during synthesis and remain adsorbed on their surfaces, promote adhesion of the nanoparticles to the steel through hydrogen bonding and electrostatic interactions. The resulting layer blocks the diffusion of hydronium and chloride ions toward the metal while the extract simultaneously improves dispersion stability, preventing the agglomeration that would otherwise reduce surface coverage. Optimal dosing reflects this balance: 0.10 molar suffices in 0.25 molar acid, whereas 0.50 molar acid demands 0.20 molar for full protection, since lower doses cover too little surface and excessive loadings risk particle clumping. Compared with previously reported inhibitors, the magnesium oxide system performs on par with gold nanoparticles at a fraction of the cost, and far outpaces zinc oxide, which dissolves in acid.

The implications extend well beyond the laboratory. Stainless steel Grade 202 is normally confined to mild service because of its vulnerability to localized corrosion, but the authors suggest that phytochemically protected surfaces could survive moderate and even aggressive industrial conditions, from pickling baths to marine settings. The approach also exemplifies circular economy chemistry, converting a ubiquitous food-processing waste stream into a functional nanomaterial without toxic reagents. And the same phytochemical-capped magnesium oxide particles hold promise in drug delivery, agricultural fertilizers, active food packaging, ultraviolet protection in cosmetics and water purification. If the green synthesis scales as readily as it performed in acid, the humble orange peel may prove to be one of the most valuable byproducts in the fight against corrosion.

Subject of Research: Green synthesis of MgO nanoparticles using orange peel extract for corrosion inhibition of stainless steel Grade 202 in acidic media

Article Title: Phytochemically-aided synthesis of MgO nanoparticles facilitated by orange peel extract for corrosion inhibition of stainless-steel Grade 202 in acidic media

Article References: Phytochemically-aided synthesis of MgO nanoparticles facilitated by orange peel extract for corrosion inhibition of stainless-steel Grade 202 in acidic media. (n.d.). https://doi.org/10.1007/s44509-026-00009-z

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00009-z

Keywords: corrosion inhibition, magnesium oxide nanoparticles, orange peel extract, stainless steel Grade 202, green synthesis, hydrochloric acid, electrochemical impedance spectroscopy, Tafel polarization, pitting corrosion, phytochemicals, open circuit potential, sustainable materials