The research addresses a waste stream that the study places at approximately 6–8 million tonnes of shrimp and crab shells worldwide each year, with only a small proportion used for animal feed or other applications. The authors describe discarded shells as a resource and environmental concern: degradation on land can consume soil nitrogen and release malodorous gases, while shell deposits offshore can contribute to sediment siltation and disturb benthic habitats. Shrimp shells are composite biomasses containing, typically, 20–30% chitin, 30–50% calcium carbonate and 15–25% structural proteins. Hydroxyl, acetamido and amino groups occur along chitin chains, while the shell’s layered structure can permit solvent penetration but also includes mineral material that may impede infiltration and reaction.

For the experiments, the team used a 4:1 weight ratio of polyethylene glycol 400 to glycerol. PEG 400 has low volatility and suitable viscosity, and the authors say it forms strong hydrogen-bonding interactions with chitin macromolecules. Blending it with glycerol was intended to improve swelling and infiltration of the chitin-rich feedstock. Shells from a nearby processing plant were dried at 105 °C, crushed to 60–80 mesh, dried again and ball-milled at 500 rpm for 30-minute cycles, with each cycle followed by a 60-minute standing period and the sequence repeated eight times. The resulting powder was combined with the liquefying agent in a stirred, refluxed three-necked flask. Sulfuric acid was added after the mixture reached its target temperature, and an ice bath terminated the reaction at the assigned time. Solid residues were vacuum-filtered, repeatedly washed with ethanol and dried at 105 °C to constant weight.

The optimization experiment examined four core process variables: reaction temperature, liquefaction time, liquid-to-solid ratio and catalyst dosage. Following single-factor experiments, the researchers adopted a four-factor, three-level Box–Behnken response-surface design and performed all experiments in triplicate. The design ranged from 160 to 180 °C, from 80 to 140 minutes, from a 12:1 to 16:1 liquid-to-solid ratio and from 6% to 8% catalyst. The response variable was the measured liquefaction rate. The resulting quadratic regression model was highly significant, with an F value of 19.76 and a p-value below 0.0001. It explained 95.18% of the observed variation in liquefaction rate, while the adjusted and predicted coefficients of determination were 0.9037 and 0.7744, respectively.

The model’s lack-of-fit test gave a p-value of 0.4811, which the authors interpreted as no significant lack of fit, and the coefficient of variation was 1.07%. The adequate-precision value was 16.31. Temperature and catalyst dosage were the strongest linear factors in the investigated domain, with p-values below 0.0001 for each. The liquid-to-solid ratio had a smaller but significant linear effect, with a p-value of 0.0076, while reaction time did not have a significant linear effect, with a p-value of 0.3015. All four factors had significant quadratic terms. Response surfaces showed that liquefaction could vary nonlinearly as temperature, time and other settings changed, and the paper states that excessively high or low values of the independent variables could hinder the process.

From the fitted response surface, the model predicted a theoretical optimum of 43.09% at 173.05 °C, a 14.67:1 liquid-to-solid ratio, 7.40% catalyst dosage and 107.90 minutes. The researchers rounded these values for practical control in the laboratory, using 173 °C, a 14:1 ratio, 7% catalyst and 108 minutes. Three parallel validation experiments produced a liquefaction rate of 42.35% ± 0.84%, corresponding to a relative prediction error of approximately 1.72% relative to the model optimum. The close result supports the response model within the tested design space. It does not establish that these settings are optimal for shell material with different composition, particle size, processing history or mineral content, because the experiments used shells collected from one nearby processing plant.

The kinetic experiments used the 14:1 liquid-to-solid ratio and 7% catalyst dosage while examining reaction temperatures from 150 to 180 °C. At each temperature, the liquefaction rate rose rapidly during the early stage, when shell material, catalyst and solvent had extensive contact. The increase then slowed as reactive components were consumed, and the measurements eventually approached a plateau. At the same reaction time, higher temperatures consistently corresponded to higher liquefaction rates. To fit the data, the researchers treated liquefaction as an irreversible bimolecular reaction under four assumptions: the liquefying-agent and catalyst concentrations remained effectively constant, the rate constant followed the Arrhenius relationship with temperature, and the reaction was irreversible. Because solvent was present in large excess, they reduced the rate equation to the changing solid fraction.

Linear fits of the kinetic data gave apparent reaction orders of 4.90 at 150 °C, 3.55 at 160 °C, 3.56 at 170 °C and 3.82 at 180 °C. The apparent rate constant increased monotonically across the four temperatures, from 0.0091 at 150 °C to 0.0173 at 160 °C, 0.0261 at 170 °C and 0.0305 at 180 °C. The authors interpret the varying apparent orders as evidence that shrimp-shell liquefaction is not a single elementary reaction but a complex, multistage process involving multiple structural transformations and intermediate stages. An Arrhenius fit of the rate constants against reciprocal absolute temperature yielded a correlation coefficient of 0.9708 and an activation energy of 64.52 kJ/mol.

The researchers then applied transition-state theory and the Eyring relationship to estimate apparent activation parameters. Gibbs free energy of activation was positive throughout the tested range, increasing from 121.39 kJ/mol at 150 °C to 122.04 kJ/mol at 160 °C, 123.43 kJ/mol at 170 °C and 125.71 kJ/mol at 180 °C; its average was 123.14 kJ/mol. Apparent activation enthalpy changed comparatively little, declining from 30.98 to 30.73 kJ/mol across the same range, with an average of 30.85 kJ/mol. Activation entropy was negative, ranging from −213.65 to −209.60 J/(mol·K) and averaging −211.66 J/(mol·K). The authors associate the negative value with increased order during activated-complex formation and possible solvation of intermediates by the excess polyols. They also interpret the relatively low activation enthalpy as consistent with simultaneous bond cleavage and new bond formation, rather than a process limited to breaking one biological macromolecular bond.

Taken together, the study characterizes shrimp-shell liquefaction in this mixed-solvent, sulfuric-acid system as a temperature-dependent multistage process. It supplies both an experimentally validated operating point and an apparent kinetic description: 173 °C, a 14:1 liquid-to-solid ratio, 7% catalyst and 108 minutes yielded 42.35% ± 0.84%, while the fitted activation energy was 64.52 kJ/mol. The scope remains laboratory-based and the measured rate primarily represents organic-biomass conversion because calcium carbonate dissolution was limited. The experiment did not test continuous or pilot-scale operation, solvent recovery, life-cycle impacts, varied commercial feedstocks or the performance of industrial products made from the liquefied material. Its principal result is therefore a quantified optimization and kinetic model for converting shrimp-shell powder under a controlled set of conditions.

Subject of Research: Optimization and chemical kinetics of shrimp-shell liquefaction using mixed polyols and sulfuric acid

Article Title: Kinetic analysis and parameter optimization of shrimp shell liquefaction

Article References: Zhang, J., Liu, X.-C., Chen, A., Zhang, H.-Y., & Zhang, Y.-C. (2026). Kinetic analysis and parameter optimization of shrimp shell liquefaction. Frontiers in Bioengineering and Biotechnology, 14, Article 1936936. https://doi.org/10.3389/fbioe.2026.1936936

Image Credits: AI Generated

DOI: 10.3389/fbioe.2026.1936936

Keywords: shrimp-shell waste, biomass liquefaction, polyethylene glycol 400, glycerol, sulfuric acid, response surface methodology, chemical kinetics, activation energy, transition state theory, marine biomass, Kinetic, analysis

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