The research, led by Nor Jumanah Said Mohamed of Universiti Kebangsaan Malaysia together with colleagues including Ahmad Razi Othman, Siti Rozaimah Sheikh Abdullah and Nur ‘Izzati Ismail, with Clara Manno of the British Antarctic Survey and Mohd Yunus Abd Shukor of Universiti Putra Malaysia, focuses on chitosan, a biopolymer derived from chitin. Chitin is the structural material that gives crustacean shells their strength, and it is abundantly available as waste from the seafood industry. When chitin is deacetylated, meaning that acetyl groups are chemically removed from its long molecular chains, it becomes chitosan, a positively charged polymer that can bind negatively charged particles suspended in water and draw them into settleable clumps. That charge behaviour is precisely what makes chitosan attractive as a coagulant for microplastics, which tend to carry surface charges that keep them dispersed.

The catch lies in how chitosan is produced. Conventional extraction relies on strong acids and alkalis, processes that consume chemicals and energy and can themselves create environmental burdens that offset the benefits of using a bio-based coagulant. The Malaysian team therefore turned to deep eutectic solvents, or DESs, a family of designer solvents formed by mixing a hydrogen bond donor and a hydrogen bond acceptor, in this case all based on choline chloride. The resulting liquids have unusual solvation properties, low volatility and a reputation for being far gentler on the environment than traditional industrial solvents. Because their composition can be tuned by choosing different donor molecules, DESs offer chemists a kind of molecular dial for controlling extraction outcomes.

The researchers tested four choline chloride-based deep eutectic solvents, pairing the choline chloride with glycerol, malonic acid, glucose or urea. Each combination interacts differently with the mineral and protein components of shrimp shells, and the team wanted to know which would best liberate chitin and convert it toward chitosan. The winner was choline chloride combined with malonic acid, abbreviated CCMA. Shells of the whiteleg shrimp Litopenaeus vannamei, one of the most farmed shrimp species in the world and therefore a plentiful waste stream, treated with this solvent yielded 19.09 percent chitosan with a solubility of 90.68 percent, the best figures among the four solvents examined.

Characterisation confirmed that the extracted material was genuine chitosan with useful structural properties. Fourier transform infrared spectroscopy, which probes the chemical bonds in a sample, verified the functional groups expected of deacetylated chitin. X-ray diffraction revealed a semicrystalline structure, a balance of ordered and disordered regions that is typical of chitosan and relevant to how the polymer behaves in solution. Scanning electron microscopy added a striking visual detail: the DES-extracted chitosan displayed a porous, honeycomb-like morphology. Such an open architecture increases the surface area available for contact with suspended particles, a feature that plausibly supports the material’s performance as a coagulant.

Getting the most out of the extraction required careful optimisation. The team first used a one-variable-at-a-time approach, adjusting temperature, reaction time and the ratio of shrimp shell material to solvent individually, and then refined the process with response surface methodology, a statistical technique that maps how several variables interact simultaneously and locates the true optimum rather than a series of local improvements. The optimal conditions emerged as a temperature of 93.91 degrees Celsius, a reaction time of 8.15 hours and a shell-to-solvent ratio of 0.042 grams per millilitre, corresponding to 21 grams of shell per 500 millilitres of solvent. Under these conditions the chitosan reached a degree of deacetylation of 89.05 percent, and the statistical model describing the process showed a strong fit with an R-squared value of 0.9521, indicating that the chosen variables explain most of the variation in the outcome.

The degree of deacetylation matters enormously for coagulation. The more acetyl groups removed, the more free amino groups the polymer carries, and at acidic to neutral pH these amino groups become protonated, giving the chain its positive charge. A chitosan with nearly 90 percent deacetylation is therefore a highly charged molecule, well suited to neutralising the negative surface charges that keep microplastic particles suspended. Charge neutralisation and the subsequent bridging of particles by long polymer chains are the two principal mechanisms by which coagulants turn a cloudy suspension into settleable flocs.

With the optimised chitosan in hand, the team put it to the test against polyethylene, one of the most common plastics in the waste stream and a frequent component of microplastic pollution. In a controlled suspension of polyethylene microparticles in deionised water, the CCMA-derived chitosan achieved a maximum reduction of 58.10 percent in turbidity and 63.55 percent in total suspended solids at a dose of 50 milligrams per litre. Importantly, these figures were corrected against a no-coagulant control, ensuring that the measured removal reflected genuine coagulation rather than simple settling. The performance was comparable to that of commercial chitosan under the same conditions, a significant result for a material produced through a greener extraction route.

The authors are careful about the limits of their findings. The coagulation experiments were conducted in deionised water, a clean and simplified matrix that lacks the organic matter, salts and competing particles found in real wastewater. Natural waters can dramatically alter coagulation behaviour, since dissolved organic carbon competes for the polymer’s binding sites and ionic strength changes the electrostatic interactions at play. The study itself notes that further validation under representative wastewater conditions is required before the approach can be considered ready for practical deployment. That caveat is standard for laboratory-scale coagulation research, but it is an essential one for readers hoping to gauge how quickly this technology might reach treatment plants.

Even with those caveats, the study contributes a meaningful piece to the puzzle of sustainable microplastic remediation. It links two waste streams in a single solution: shrimp shells that would otherwise be discarded become a high-performance coagulant, and the solvent used to extract it is designed to be environmentally benign and potentially recoverable. It also demonstrates a rigorous workflow, from solvent screening through statistical optimisation to materials characterisation and application testing, that other groups can adapt to different feedstocks and different pollutant targets. As regulators and utilities worldwide grapple with how to remove microplastics from effluents and drinking water, approaches that avoid swapping one pollution problem for another will be increasingly valuable. This work suggests that the answer to a plastic problem might, in part, come from the seafood counter, provided the chemistry behind it stays as green as the ambition.

Subject of Research: Green extraction of chitosan from shrimp shell waste with deep eutectic solvents for microplastic coagulation

Article Title: Deep eutectic solvent-assisted extraction of chitosan for polyethylene microplastic coagulation

Article References: Mohamed, N. J. S., Othman, A. R., Manno, C., Shukor, M. Y. A., Abdullah, S. R. S., & Ismail, N. I. (2026). Deep eutectic solvent-assisted extraction of chitosan for polyethylene microplastic coagulation. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38253-2

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38253-2

Keywords: chitosan, deep eutectic solvents, microplastics, polyethylene, coagulation-flocculation, shrimp shell waste, chitin, degree of deacetylation, response surface methodology, water treatment, green chemistry, Litopenaeus vannamei

The wastewater at the heart of the study came straight from the dyeing process outlet of a textile factory in Isfahan, in central Iran, and it was as dirty as the researchers expected. Chemical oxygen demand, a measure of organic pollution, stood at 8100 milligrams per liter, roughly 135 times the permissible limit for discharge to surface water in Iran. Color intensity registered 23,480 platinum-cobalt units, turbidity 620 nephelometric turbidity units, and total suspended solids 3890 milligrams per liter. Elemental analysis added another layer of concern. The effluent carried 5500 micrograms per liter of aluminum, 5300 of iron, an extraordinary 4200 of the toxic metal thallium, along with antimony, manganese, zinc, copper and lead at levels far above natural background. In Iran only about 20 percent of industrial effluents receive proper treatment before discharge, so the stakes for finding an affordable, effective recipe are high.

Polyaluminum chloride, or PACl, has become a favored coagulant in water treatment because it works across a wide pH range, forms dense and fast-settling flocs and produces less sludge than traditional alum. It is sold in both powdered and liquid form, and the two differ in ways that matter to plant operators. The powdered grade used here contained 28.05 percent aluminum oxide, more than double the 10.48 percent of the liquid grade, but liquid PACl arrives ready to dose, skipping on-site dissolution. The researchers optimized each coagulant independently through standard jar tests on 1-liter samples, defining the optimum as the minimum dose that pushed residual turbidity to 20 NTU or below with the smallest settled sludge volume. Powdered PACl settled on 0.7 grams per liter while liquid PACl required 3.0 milliliters per liter. Every scenario also received sodium hypochlorite at 75 milliliters per liter, chosen because color removal plateaued beyond that dose, and calcium oxide at 1.0 gram per liter, enough to push pH above 10.5 so that dissolved metals could precipitate as hydroxides.

Five treatment configurations were tested in triplicate. The first used powdered PACl, the second liquid PACl, the third a blend of the two, and the fourth and fifth replaced commercial coagulant entirely, or nearly so, with clarifier sludge collected from a local drinking water plant that itself uses PACl. The idea behind the sludge experiments was elegantly circular: the sedimentation basin sludge contains residual polyaluminum chloride and amorphous aluminum hydroxides, so it might act as a free, waste-derived coagulant that simultaneously reduces chemical purchases and diverts waste from landfills. The sludge was mixed into wastewater at a ratio of one part sludge to five parts effluent, the ratio that preliminary trials showed produced maximum floc formation. For the fifth scenario, a modest half-milliliter dose of liquid PACl was added on top of the sludge to test whether virgin and recycled coagulants could work synergistically.

The results split the metals into two camps. Zinc and chromium were the consistent success stories, removed at better than 96 percent in every configuration, with residual zinc concentrations falling below 0.2 micrograms per liter in most scenarios. Thallium, iron, manganese and silicon also dropped by more than 98 percent in the liquid and mixed PACl scenarios. Barium and strontium, by contrast, proved stubbornly recalcitrant, with strontium removal never exceeding 33 percent anywhere in the study, marking these two elements as priority targets for future work. Most striking was the fate of aluminum itself. Clarifier sludge alone removed 99.8 percent of aluminum, the best figure of any scenario, and near-complete removal of lithium and barium besides. But in the fifth scenario, where a small PACl dose was layered onto the sludge, aluminum removal collapsed to zero, and the treated water actually carried more aluminum than the mixed inlet, 4800 micrograms per liter against 4567 going in.

That counterintuitive collapse points to a phenomenon known as overdosing or charge reversal. Coagulants work by neutralizing the negative charges that keep colloidal particles suspended, and beyond the optimum dose the excess positive charge can flip particle surfaces back to a stable, restabilized state, re-suspending material that had already clumped. The sludge, already laden with residual aluminum hydroxides, plus the added PACl apparently tipped the system past that threshold. The authors caution that they did not measure zeta potential, so the mechanism remains a hypothesis, but the practical lessons are unambiguous: more coagulant is not better, combining waste-derived and virgin coagulants without re-optimization can backfire, and fixed-dose recipes are inadequate for wastewater whose composition shifts from batch to batch.

On the conventional pollutants, the combined PACl scenario proved the most robust all-rounder, cutting chemical oxygen demand by 95 percent to a residual 380 milligrams per liter and turbidity by 95 percent, while removing nearly 97 percent of suspended solids. Clarifier sludge alone, despite its dazzling color removal of 99.6 percent, managed only 19.8 percent turbidity removal and 79.9 percent for chemical oxygen demand, confirming that recycled sludge cannot substitute for commercial coagulant on high-strength textile effluent. Electrical conductivity rose in every scenario because calcium oxide and PACl both add dissolved ions, and final pH ranged from 10.74 to 12.51, values that would require neutralization before any discharge or reuse.

The study’s most distinctive move was its scoring system. Rather than ranking treatments by individual removal percentages, the team calculated the grey water footprint, an indicator that translates each pollutant load into the volume of freshwater needed to dilute it to regulatory limits. The critical pollutant for the raw effluent was chemical oxygen demand, driving a footprint of 121,500 cubic meters per month at the facility’s assumed discharge of 750 cubic meters per month, meaning the factory’s pollution load would demand dilution water more than 160 times its own flow. Liquid PACl delivered the best outcome: a 92.96 percent reduction in the footprint, statistically the top performance by analysis of variance, with the combined PACl scenario statistically comparable at 90.85 percent and powdered PACl at 88.27 percent. The sludge-only scenario ranked last at 83.33 percent, despite its standout aluminum and color numbers, because its weak performance on organic matter and turbidity dominated the overall environmental burden.

The grey water footprint also exposed a hidden bottleneck that conventional single-parameter assessment would have missed. In the combined PACl scenario, chemical oxygen demand was cut so effectively that antimony, with only 49 percent removal, quietly became the limiting pollutant, its footprint climbing above the organic load’s. Optimizing one pollutant, in other words, can mask the factor that actually constrains environmental performance. The researchers argue that the metric offers an objective common currency for comparing heterogeneous treatment trains, and note that their chemically enhanced primary treatment approached the footprint reductions typically associated with secondary biological processes such as activated sludge.

For plant operators, the study lands on a concrete protocol: liquid PACl at 3.0 milliliters per liter, sodium hypochlorite at 3750 milligrams per liter of active chlorine, and calcium oxide at 1.0 gram per liter, a combination that cut the grey water footprint by 93 percent while stripping more than 99 percent of thallium and zinc. Clarifier sludge, meanwhile, earns a narrower but genuine role as a free, selective coagulant aid for specific targets such as aluminum and color, and as a landfill-diversion strategy, so long as its dose is optimized independently and its reactive aluminum content is properly characterized. The authors flag open questions, including chlorinated byproducts from hypochlorite oxidation, the economics of full-scale deployment, and the mechanistic differences between PACl formulations, but the headline conclusion stands: when environmental impact is tallied honestly across every pollutant, the liquid form of a familiar coagulant is the strongest tool yet tested for taming textile wastewater.

Subject of Research: Comparative evaluation of PACl coagulant variants and recycled clarifier sludge for treating real textile wastewater using the grey water footprint metric

Article Title: Comparative assessment of PACl variants and clarifier sludge for textile wastewater treatment: A grey water footprint approach

Article References: Tabandeh, L., Arastou, K., & Ebrahimi, A. (2026). Comparative assessment of PACl variants and clarifier sludge for textile wastewater treatment: A grey water footprint approach. Cleaner Engineering and Technology, 34, Article 101319. https://doi.org/10.1016/j.clet.2026.101319

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101319

Keywords: textile wastewater, polyaluminum chloride, grey water footprint, coagulation-flocculation, clarifier sludge, heavy metal removal, chemical oxygen demand, sodium hypochlorite, calcium oxide, circular economy, water treatment, industrial effluent