Seaweed Waste Transformed Into High-Performance Material That Strips Toxic Dye From Water

Every year, Mediterranean beaches are buried under mountains of dead seagrass, and every year, textile factories discharge rivers of synthetic dye into waterways that communities depend on. A new study published in Environmental Science and Pollution Research brings these two problems together in a single elegant solution. Researchers Assia Hassen and Amel Ben Slimane, working at the University of Gafsa and the University of Kairouan in Tunisia, have shown that activated carbon made from the abundant marine seagrass Posidonia oceanica, alone and in combination with the conducting polymer polyaniline, can pull the dye Murexide out of contaminated water with remarkable efficiency. Their comparative evaluation of three adsorbents offers a detailed mechanistic picture of how sustainable, biomass-derived materials could reshape the way we clean industrial wastewater.

The target molecule in the study, Murexide, is an anionic dye with a complex aromatic structure, and it serves as a representative challenge for a much broader class of water pollutants. Synthetic dyes are notoriously difficult to remove with conventional biological treatment because their chemical stability and intense coloration persist through standard processing. Adsorption, the process by which dissolved molecules bind to the surface of a solid material, remains one of the most attractive alternatives because it is simple, scalable, and can be tuned by engineering the adsorbent itself. The question the Tunisian team set out to answer was which surface chemistry works best, and whether combining two very different materials produces something better than either alone.

The first adsorbent they tested was pure polyaniline, or PANI, a nitrogen-rich conducting polymer that has long fascinated materials scientists for its tunable electronic states and dense array of amine and imine functional groups. In its doped emeraldine state, the form confirmed in this study by physicochemical characterization, PANI carries protonated sites that can attract negatively charged dye molecules through electrostatic forces. The second adsorbent, labeled ACPO, was produced by converting raw Posidonia oceanica biomass into a hierarchically porous activated carbon. This seagrass, which forms vast underwater meadows across the Mediterranean and washes ashore in enormous quantities, has previously attracted attention as a low-cost precursor for carbon materials, including in studies targeting hexavalent chromium removal. The third material was a hybrid composite in which PANI was incorporated homogeneously throughout the porous carbon framework.

Characterization of the composite revealed something crucial about why the hybrid works. Strong interfacial interactions, mainly π–π stacking between the aromatic backbones of the polymer and the graphitic planes of the carbon, along with hydrogen bonding, ensured that the polymer phase dispersed evenly across the carbon surface rather than clumping into inaccessible aggregates. This good dispersion preserved the accessible adsorption sites of both components, meaning the composite retained the structural porosity of the activated carbon while gaining the surface functionality of the polymer. In practical terms, the hybrid material offers two complementary capture mechanisms in a single particle: the internal pore network of the carbon and the chemically active polymer coating on its surfaces.

The performance numbers tell a striking story. Pure PANI achieved a maximum adsorption capacity of 333.33 milligrams of Murexide per gram of material, while the PANI/ACPO composite reached 322.58 milligrams per gram, a nearly identical figure. The activated carbon alone, by contrast, managed only 149.25 milligrams per gram. The conclusion is unambiguous: the polymer phase dominates the uptake chemistry, and the carbon’s primary contribution is architectural rather than chemical. Yet this is precisely what makes the composite so interesting from a sustainability standpoint. By using the seagrass-derived carbon as a scaffold, the researchers can deliver PANI-level performance while consuming far less of the synthetic polymer, cutting both cost and the environmental footprint of adsorbent production.

Equilibrium modeling added another layer of insight. For both PANI and the composite, the experimental data fit the Langmuir isotherm model, which describes adsorption as a process approaching a finite monolayer of dye molecules on energetically uniform sites, with correlation coefficients above 0.96. The bare activated carbon behaved differently, following the Freundlich model with a correlation coefficient of approximately 0.966, a signature of surface heterogeneity and multilayer adsorption. This divergence makes physical sense. The raw carbon presents a patchwork of different pore sizes and surface chemistries, each binding dye with different strengths, whereas the polymer-coated materials present a more uniform population of binding sites dominated by the polymer’s functional groups.

Kinetic analysis showed that uptake over time followed the pseudo-second-order model for all three materials, indicating that the rate-limiting step involves chemisorption-like interactions between the dye and the surface rather than simple mass transfer alone. However, the researchers also found that intraparticle diffusion and boundary layer effects contributed to the overall adsorption rate, particularly for the activated carbon. This observation highlights the role of pore diffusion in mass transfer: dye molecules must physically travel through the tortuous internal network of the carbon before reaching binding sites deep within its structure. For the composite, the polymer’s presence on external and near-surface sites shortens this journey, which helps explain why the hybrid matches pure PANI despite containing less of the high-capacity polymer phase.

Thermodynamic measurements completed the mechanistic portrait. The Gibbs free energy change was negative, confirming that adsorption is spontaneous; the enthalpy change was positive, meaning the process is endothermic and actually improves at elevated temperatures; and the entropy change was positive, indicating increasing randomness at the solid–liquid interface as dye molecules are released from solution and immobilized on the surface. Taken together, these parameters point to adsorption dominated by physical interactions rather than strong chemical bond formation. The mechanistic picture that emerges is one of coexisting uptake pathways: electrostatic attraction between protonated polymer sites and the anionic dye, π–π donor–acceptor associations between aromatic moieties on the adsorbent and the dye’s ring system, hydrogen bonding, and diffusion through the porous network.

What makes this work resonate beyond the laboratory is the circular economy logic at its core. Posidonia oceanica is not a cultivated crop; it is a naturally shed biomass that accumulates on beaches in quantities large enough to create disposal problems for coastal municipalities. Converting this waste stream into a hierarchically porous activated carbon transforms a nuisance into a functional material, and pairing it with a modest loading of polyaniline yields performance comparable to the pure polymer at a fraction of the polymer cost. The authors report that the research received no specific grant funding, and they declare no competing interests, framing the work as a straightforward contribution to the growing field of biomass-derived adsorbents for wastewater remediation.

The study also arrives amid intensifying global concern about water pollution and its health consequences, with recent literature documenting human health risks from contaminated water and the challenges that conventional biological and physicochemical treatments face against persistent organic pollutants. Adsorption with sustainable, low-cost materials is increasingly viewed as a practical complement to advanced treatment trains, particularly in regions where expensive imported adsorbents are out of reach. By systematically comparing a conducting polymer, a marine-biomass carbon, and their hybrid across isotherms, kinetics, and thermodynamics, Hassen and Ben Slimane have provided a template for how such materials should be evaluated before deployment. The PANI/ACPO composite, combining structural porosity with rich surface functionality, stands out as a sustainable candidate for efficient Murexide removal, and the mechanistic framework established here should transfer readily to other anionic dyes and ionic pollutants. As Mediterranean coastlines continue to receive their annual deliveries of dead seagrass, the idea that this humble biomass could anchor next-generation water treatment technology is no longer speculative; it is now supported by quantitative, mechanistically grounded evidence.

Subject of Research: Development and comparative evaluation of polyaniline, Posidonia oceanica-derived activated carbon, and their composite as sustainable adsorbents for Murexide dye removal from water

Article Title: From marine biomass to sustainable adsorbents: comparative evaluation of polyaniline, Posidonia oceanica-derived activated carbon, and their composite for Murexide removal

Article References: Hassen, A., & Ben Slimane, A. (2026). From marine biomass to sustainable adsorbents: comparative evaluation of polyaniline, Posidonia oceanica-derived activated carbon, and their composite for Murexide removal. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38260-3

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38260-3

Keywords: adsorption, polyaniline, activated carbon, Posidonia oceanica, Murexide, dye removal, wastewater treatment, marine biomass, composite materials, isotherm modeling, adsorption kinetics, water pollution