A hydrogel made from sodium carboxymethyl cellulose, gelatin, and citric acid removes methylene blue from water with a reported capacity near 943 milligrams per gram—and still removes more than 85 percent after reuse. Methylene blue is a cationic dye with a stable structure, and it has been extensively used in the textile dyeing and printing industry.

In dye wastewater, it is a major source of pollution. Other treatment methods include electrolytic coagulation, chemical oxidation, electrochemical methods, photocatalytic methods, air flotation, membrane separation, and biodegradation. Despite being more effective, these methods have strict process conditions, unsatisfactory stability, and high treatment cost.

Adsorption offers easy operation, high treatment efficiency and speed, and is described as more economical and efficient, provided that new pollutants are not introduced. But some common adsorbents are costly to produce and may trigger secondary pollution.

The hydrogel can be employed as a new adsorbent to quickly adsorb and separate organic dyes, helping address dye wastewater treatment. The special functional groups of the hydrogel network can interact with dye molecules. In that case, the hydrogel can quickly adsorb and separate organic dyes.

Hydrogels prepared from natural polymers are abundant and renewable in nature, non-toxic and biocompatible, and can be biodegraded. As adsorbents with low environmental impact, they can lessen environmental pollution while improving resource utilization. Sodium carboxymethyl cellulose is a natural hydrogel raw material with degradability, hygroscopicity, and satisfactory biocompatibility.

Its surface contains large amounts of carboxyl and hydroxyl groups that provide active sites for pollutant adsorption. Gelatin is a water-soluble polymeric peptide polymer with desirable biocompatibility and degradability. Its carboxyl and amino groups can dissociate or hydrolyze in water into charged groups that can react with dyes.

Citric acid is a non-toxic cross-linking agent for cellulose derivative hydrogels. At high temperature, it forms an anhydride and reacts with hydroxyl groups to form ester cross-links between polymer chains. A previous citric-acid-cross-linked carboxymethyl-cellulose membrane had undesirable structural strength and a maximum methylene-blue adsorption capacity of 25 milligrams per gram.

Another sodium-carboxymethyl-cellulose and hydroxyethyl-cellulose membrane reached 769.23 milligrams per gram. The study prepared previously unreported environmentally friendly sodium carboxymethyl cellulose and gelatin semi-interpenetrating hydrogels by solution casting.

The hydrogels were characterized using X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy, while swelling was investigated at different salt-ion concentrations and pH values. The best hydrogel was selected for methylene-blue adsorption studies across different time, adsorbent dosage, pH, temperature, and initial methylene-blue concentration conditions.

The mixture was stirred at three hundred revolutions per minute for one point five hours, then left overnight without stirring while air bubbles were removed. The homogeneous solution was poured into a petri dish with a diameter of nine centimeters and dried at fifty degrees Celsius for forty-eight hours.

The dried xerogel was cured at one hundred twenty degrees Celsius for eight hours, promoting cross-linking between carboxymethyl-cellulose molecular chains. The products were soaked in deionized water for twenty-four hours to remove remaining cross-linker.

Finally, the hydrogel was dried at forty degrees Celsius for forty-eight hours and named the modified hydrogel sample C/G1. The steps were repeated to prepare the other hydrogels according to Table 1. Table one lists the preparation conditions for three CMC and gel hydrogel formulations.

Each formulation uses three grams of CMC and zero point one three five grams of citric acid, while the gel amount varies from one gram in C slash G one, to two grams in C slash G two, and three grams in C slash G three. These defined compositions provide the basis for comparing the hydrogels after drying and curing, followed by structural and swelling characterization.

Figure 1 schematically shows how citric acid forms a hydrogel with CMC and Gel during heating. Citric acid first dehydrates into cyclic anhydrides, which esterify hydroxyl groups on CMC; further reactions produce a semi-interpenetrating network involving the Gel component.

This visual matters because it connects the chemical cross-linking steps with the resulting network structure that supports the prepared hydrogel. Citric-acid cross-linking occurs in several stages. During heating, citric acid undergoes intramolecular dehydration to form a more active five-membered-ring anhydride.

The anhydride reacts with a hydroxyl group on the polysaccharide chain to form an ester bond. Remaining carboxyl groups form another cyclic anhydride for further esterification. The process eventually forms a diester bridge for cross-linking between polymer chains, and this hypothetical mechanism is illustrated in Figure 1.

Figure two compares the hydrogels C slash G one, C slash G two, and C slash G three with their starting components, CMC and gelatin, using FT-IR in panel a and X-ray diffraction in panel b. The FT-IR plot retains characteristic component features, including CMC peaks at 3238, 2927, and 1588 inverse centimeters, and gelatin features near 3284 and 2870 inverse centimeters, while the hydrogel peak shapes change.

In panel b, CMC and gelatin show diffraction near twenty point zero and twenty point three degrees, respectively, providing evidence of their semi-crystalline or locally ordered structures. Figure three presents scanning electron microscopy images of the C/G1, C/G2, and C/G3 hydrogels, revealing their internal porous morphologies.

C/G1 appears rough and irregular, with an incomplete pore structure, which the authors associate with closely spaced CMC chains and electrostatic repulsion between carboxyl groups. As gelatin content increases, the images show more developed structural features, consistent with mesh formation through citric-acid cross-linking and hydrogen bonding between CMC and gelatin.

The C/G1 hydrogel had a rough, irregular surface with incomplete pore structure. With increased gelatin content, C/G2 exhibited a relatively regular surface and a distinct porous network structure, showing uniform dispersion and strong interactions between the polymers.

Adding an appropriate amount of gelatin could improve hydrogel structural stability by constructing a semi-interpenetrating network. But with further gelatin content, C/G3 became rougher, with fewer and larger pores that were not favorable for dye adsorption. C/G2 was therefore considered more suitable for dye adsorption studies because its relatively regular, distinct porous network had a high surface area and large mass-transfer channels.

Figure five compares hydrogel swelling in distilled water and zero point nine percent sodium chloride, and then tracks swelling across pH. In panel a, the reported distilled-water values for C/G1, C/G2, and C/G3 are three point nine one, five point nine three, and six point nine seven grams per gram, while the saline values are three point three seven, four point two eight, and four point nine six.

Panel b shows that swelling varies with pH, with particularly large changes for C/G3, highlighting how solution conditions influence hydrogel water uptake. In deionized water, the equilibrium swelling ratios of C slash G one, C slash G two, and C slash G three were three point nine one, five point nine three, and six point nine seven grams per gram.

These values were slightly higher than those in zero point nine percent sodium chloride solution. In the salt solution, the corresponding swelling ratios were three point three seven, four point two eight, and four point nine six grams per gram.

Increasing gelatin content could improve the swelling ratio. The proposed reason is that gelatin hindered cross-linking between citric acid and adjacent carboxymethyl-cellulose chains, lowering cross-linking and making the network looser. Figure six first calibrates methylene blue concentration using a highly linear absorbance standard curve: y equals zero point one five eight six x minus zero point zero zero two four three, with an R-squared of zero point nine nine nine eight.

It then compares equilibrium adsorption and swelling across C/G1, C/G2, and C/G3, before showing that C/G2 adsorption rises rapidly and approaches equilibrium after about three hundred minutes. Panels d and e plot pseudo-first-order and pseudo-second-order kinetic representations, providing models for interpreting that adsorption behavior.

Figure 6b displays the equilibrium adsorption capacity and deionized-water swelling ratio of C/G1, C/G2, and C/G3 hydrogels. The adsorption capacity became smaller as gelatin content increased. This might result from reduced carboxymethyl-cellulose content per unit mass, and therefore fewer carboxylate active sites.

For C/G2, adsorption increased sharply at the beginning, then gradually decreased, and reached adsorption equilibrium after about three hundred minutes. The initial behavior might result from numerous free adsorption sites and high methylene-blue concentration.

At equilibrium, adsorption no longer increased because adsorption and desorption reached a dynamic equilibrium. Table 3 reports fitted parameters for pseudo-first-order and pseudo-second-order models of methylene blue adsorption by the C/G2 hydrogel.

The pseudo-first-order fit gives a correlation coefficient of zero point nine nine two six and an estimated equilibrium capacity of three hundred twenty-nine point eighty-four milligrams per gram, while the pseudo-second-order fit reports zero point nine nine nine five and five hundred eighteen point thirteen milligrams per gram.

These parameters help identify which kinetic description better represents the adsorption data. The correlation coefficient from the pseudo-first-order model was lower than that from the pseudo-second-order model, revealing that the pseudo-second-order model matched the kinetics data better.

The experimentally determined methylene-blue sorption values differed greatly from the pseudo-first-order estimate but fitted the values derived from the pseudo-second-order model. The pseudo-second-order model dominated methylene-blue adsorption by C/G2.

This suggests that adsorption may be governed by chemisorption involving sharing or exchange of electrons between hydrogel functional groups and dye cations. Figure seven shows that C/G2 hydrogel’s equilibrium adsorption capacity rises as the initial methylene blue concentration increases, while the removal percentage declines notably beyond about five hundred milligrams per liter.

Panels c through e compare Langmuir, Freundlich, and adsorption-isotherm representations, supporting the authors’ analysis of the adsorption mechanism. The before-and-after photographs in panel f provide a visible demonstration of color removal across solutions labeled from fifty to five hundred milligrams per liter.

The Freundlich correlation coefficient was noticeably lower than the Langmuir correlation coefficient, revealing that the Langmuir model was the better model for describing methylene-blue adsorption. The experimental adsorption capacity of C/G2 for methylene blue was nine hundred forty-one point two five milligrams per gram, nearly reaching the Langmuir theoretical maximum of nine hundred forty-three point four zero milligrams per gram.

The experimental results indicate Langmuir-dominated adsorption and saturated monolayer adsorption of methylene blue by C/G2 hydrogel. Table four reports the fitted parameters for Langmuir and Freundlich isotherm models describing methylene blue adsorption by the C/G2 hydrogel.

The Langmuir model gives a maximum adsorption capacity of nine hundred forty-three point four zero milligrams per gram, a K L value of zero point one one one eight liters per milligram, and an R-squared value of zero point nine nine four two. The Freundlich fit reports a K F of one hundred forty-one point nine five milligrams per gram, n of two point five six six zero, and an R-squared value of zero point eight five zero eight.

Table five compares the maximum methylene blue adsorption capacity, in milligrams per gram, for several reported adsorbents and the C/G2 hydrogel developed in this work. The listed values range from nineteen point twelve for CMC-g-poly(AA-co-IA)/MMT to nine hundred forty-three point forty for C/G2, with CMC-HEC at seven hundred sixty-nine point twenty-three.

This comparison matters because it places the present material’s measured capacity alongside prior literature values using the same adsorption-capacity metric. Figure eight maps how operating conditions affect methylene blue adsorption by the C/G2 hydrogel.

Increasing dosage raises removal from 75.24 percent at zero point four grams per liter to 98.85 percent at one point two grams per liter, while adsorption capacity declines; pH, temperature, and thermodynamic fitting are shown in panels b through d. Panel e tracks adsorption–desorption across three cycles, supporting evaluation of the hydrogel’s reuse.

Table 6 summarizes the thermodynamic parameters for methylene blue adsorption by the C/G2 hydrogel. The reported Gibbs free energy values are negative at all three temperatures: minus nine point three one at 303 kelvin, minus eight point four zero at 323 kelvin, and minus seven point seven three at 333 kelvin, indicating favorable, spontaneous adsorption.

The table also gives an enthalpy of minus 24.80 kilojoules per mole and an entropy of minus 51.05 joules per mole per kelvin, supporting the authors’ conclusion that the process is exothermic. The methylene-blue-loaded hydrogel was treated with two hundred milliliters of one molar hydrochloric acid for five hours to desorb the dye.

The hydrogels were then washed with distilled water and reused for adsorption. Adsorption–desorption cycles were conducted three times, with fresh solution in each trial. After three adsorption–desorption processes, the equilibrium adsorption capacity decreased from four hundred sixty-eight point two five to four hundred thirty-four point zero zero milligrams per liter, while the removal rate decreased from ninety-three point six five to eighty-five point nine two percent.

The decrease was attributed to chemisorption: when some adsorption sites were occupied by methylene-blue molecules, the adsorbent and adsorbate mass combination became more stable, leaving some methylene blue difficult to elute. Even after three adsorption–desorption processes, the removal rate remained above eighty-five percent, indicating that the hydrogel could still effectively remove methylene blue from aqueous solution after repeated use.

The C/G2 semi-interpenetrating hydrogel combined a regular porous structure with high methylene-blue uptake, Langmuir-type adsorption, and more than 85 percent removal after three cycles, supporting its potential for wastewater treatment.