Invasive Water Hyacinth Transformed Into Pellets That Scrub Dye From Wastewater

One of the world’s most notorious invasive aquatic plants may soon have a new job: cleaning up the very kind of industrial pollution that fouls the waterways it chokes. Researchers in Thailand have developed pelletized composite adsorbents that combine diatomite, metakaolin-derived geopolymer chemistry, and residues of water hyacinth, the fast-growing floating weed that blankets lakes and rivers across the tropics. The resulting pellets, described in the journal Results in Chemistry, were designed to capture methylene blue, a widely used cationic dye that resists biodegradation, darkens receiving waters, and disrupts aquatic ecosystems when released in textile and industrial effluents. Rather than simply reporting another adsorbent with an impressive capacity number, the team set out to map the full chain of relationships linking composition, structure, surface chemistry, mechanical integrity, and adsorption performance in a material that can actually be handled, recovered, and reused outside the laboratory.

The choice of ingredients is deliberate and each component plays a distinct structural role. Diatomite, a silica-rich sedimentary rock formed from the fossilized frustules of microscopic algae and sourced from Lampang in northern Thailand, supplies an intrinsically porous mineral framework that facilitates mass transfer of dye molecules into the pellet interior. Metakaolin, produced by calcining kaolin clay from Ranong, serves as the reactive aluminosilicate precursor that forms the chemically stable geopolymer network when activated with a mixture of sodium silicate solution and 10 M sodium hydroxide. Water hyacinth residue collected in Chiang Mai brings oxygen-rich functional groups and a fibrous morphology that modify the pore architecture and surface reactivity of the final composite. The researchers cast pastes containing 20 weight percent calcined diatomite, 15 weight percent metakaolin, and the alkaline activator, adding water hyacinth at 0, 1, 2, and 3 weight percent to produce a series labeled DG-WH0 through DG-WH3, which were then cured at room temperature for up to 28 days.

Structural analysis revealed how the biomass reshapes the material from the inside out. X-ray diffraction showed that quartz remained the dominant crystalline phase in all compositions, inherited largely from unreacted portions of the diatomite and metakaolin precursors, while a broad diffuse feature between roughly 20 and 35 degrees two-theta confirmed the presence of a poorly ordered aluminosilicate geopolymer matrix. Fourier-transform infrared spectroscopy identified the characteristic Si-O-T asymmetric stretching band near 1016 to 1021 per centimeter, along with hydroxyl and carbonate features, and small shifts in these bands after dye exposure hinted at interactions between methylene blue and the composite surface. Scanning electron microscopy told a more visually striking story: the biomass-free pellet was relatively dense, but increasing water hyacinth content progressively introduced interfacial gaps, larger voids, and interconnected pore channels, particularly in the 2 and 3 percent formulations.

Quantitative measurements reinforced what the micrographs suggested. Apparent porosity rose from 58.57 to 62.03 percent across the series, while water absorption climbed sharply from 58.30 to 79.55 percent, and bulk density fell from 1.22 to about 1.08 grams per cubic centimeter. Nitrogen physisorption on the two endpoint compositions showed that the BET specific surface area of the 3 percent pellet reached 14.843 square meters per gram, roughly 86 percent higher than the 7.986 square meters per gram of the biomass-free control, while total pore volume increased by about 75 percent. In other words, the invasive weed was effectively blowing open the internal architecture of the pellet, creating more pathways and more exposed surface for dye molecules to reach.

Surface charge added a second, electrostatic dimension to the story. Zeta potential measurements at pH 7 showed that all pellets carried negative surface charges, becoming progressively more negative as water hyacinth content increased, from minus 67.3 millivolts for the control to minus 74.9 millivolts for the highest-biomass pellet. The point of zero charge of the best-performing composition was approximately 2.22, meaning that at the pH used in the adsorption experiments the surface was decisively negative and therefore electrostatically attractive to positively charged methylene blue cations. Deprotonated surface hydroxyl groups on the aluminosilicate framework, generating negatively charged siloxide and aluminate sites, are thought to contribute to this favorable electrostatic environment.

The adsorption experiments bore out the design logic. At pH 2, performance was poor, with removal efficiencies between roughly 49 and 54 percent, because the flood of hydrogen ions competed with dye cations for adsorption sites. Performance peaked at pH 4, where the 3 percent pellet removed 77.25 percent of methylene blue with an equilibrium capacity of 19.31 milligrams per gram, and remained high but essentially flat from pH 6 through 10, indicating that electrostatics alone do not govern the process. Kinetic measurements showed rapid initial uptake followed by a gradual plateau between about 800 and 1080 minutes, with 24-hour removal reaching 77.03 percent and a capacity of 19.26 milligrams per gram for the best pellet. Increasing the initial dye concentration from 50 to 250 milligrams per liter lowered the removal percentage from 95.26 to 77.40 percent while raising the per-gram capacity from about 4.76 to 19.35 milligrams per gram, the classic signature of a fixed number of sites being progressively saturated.

Equilibrium data fit the Langmuir isotherm model better than the Freundlich model, with a linearized correlation coefficient of 0.9998 and a maximum monolayer capacity of 22.32 milligrams per gram, while both linearized and nonlinear kinetic analyses consistently favored the pseudo-second-order model. Against the backdrop of published geopolymer adsorbents, the 22.32 milligrams per gram figure is competitive, outperforming lightweight geopolymer-expanded glass composites, metakaolin/slag geopolymers, red mud and rice husk ash systems, and coal gangue-based materials, and landing close to reduced graphene oxide/geopolymer composites, though some fly ash and metakaolin systems report higher capacities. The authors caution that direct comparisons across studies are complicated by differences in material form and experimental conditions, and their pellets carry an inherent mass-transfer penalty because dye molecules must diffuse through the pellet interior rather than contacting a loose powder directly.

That penalty is the price of practicality, and the study is unusually candid about the trade-offs involved. Compressive strength after 28 days of curing fell from 21.5 megapascals for the biomass-free pellet to 9.1, 6.5, and 5.5 megapascals at 1, 2, and 3 percent water hyacinth, because the fibrous additive and its interfacial discontinuities disrupt the continuity of the load-bearing geopolymer matrix. The modest gain in equilibrium capacity at the highest biomass loading does not fully compensate for the substantial loss of mechanical robustness, so the researchers suggest that an intermediate water hyacinth content may offer the best balance for applications where pellets must survive handling. Reusability testing over five adsorption-desorption cycles showed another honest limitation: removal efficiency declined to about 33 percent of its initial value by the fifth cycle, even though the pellets retained roughly 99 percent of their mass, pointing to a need for improved regeneration protocols.

What makes this work resonate beyond its numbers is the circular economy story it tells. Water hyacinth is a global nuisance, clogging waterways, blocking navigation, and starving aquatic ecosystems of light and oxygen, yet here its residue becomes a functional ingredient in a low-cost ceramic adsorbent built from abundant local minerals. The study also models a kind of scientific honesty that is increasingly valued in materials research: rather than claiming a single dominant mechanism, the authors attribute dye uptake to coupled effects, including electrostatic attraction to the negatively charged surface, possible hydrogen bonding through hydroxyl-containing groups, and improved pore accessibility, while explicitly declining to claim mechanisms, such as pi-pi interactions, that their characterization could not verify. They note that methylene blue served only as a model cationic dye, and that real textile wastewater, with its mixtures of dyes, salts, and organic matter, will pose stiffer competitive challenges. Future work on mixed-pollutant systems, thermodynamic analysis, and regeneration chemistry will determine whether these weed-powered pellets can graduate from elegant laboratory concept to working technology for the dye-polluted rivers that need them most.

Subject of Research: Pelletized diatomite-based geopolymer composites incorporating water hyacinth biomass for the adsorptive removal of methylene blue dye from wastewater

Article Title: Pelletized diatomite-based geopolymer-water hyacinth composites for methylene blue adsorption: insights into structure-property-performance relationships

Article References: Thammarong, S., Zhang, W., Pimpha, N., Saenkam, K., Boontakam, W., Tandorn, S., Randorn, C., Srichumpong, T., Pengpat, K., & Rujijanagul, G. (2026). Pelletized diatomite-based geopolymer-water hyacinth composites for methylene blue adsorption: insights into structure-property-performance relationships. Results in Chemistry, 31, Article 103923. https://doi.org/10.1016/j.rechem.2026.103923

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103923

Keywords: geopolymer, water hyacinth, diatomite, methylene blue, adsorption, wastewater treatment, dye removal, metakaolin, porous materials, biomass composites, isotherm modeling, mechanical strength