Synthesis and characterization of the water-based polyacrylic resin
A water-based polyacrylic resin was synthesized via inverse emulsion polymerization using methacrylic ester, acrylic acid, and maleic anhydride as primary monomers. Sodium dodecyl sulfonate acted as the emulsifier, and ammonium persulfate initiated the polymerization. This resin proved effective for adsorbing Cr(VI) ions from wastewater by electrostatic adsorption and chemisorption via Cr-O bond formation. The synthetic route and adsorption mechanism are depicted in Fig. 2. To characterize the resin and the Cr(VI)-loaded product, we employed FTIR spectroscopy (Fig. 3a) and XRD (Fig. 3b) to study chemical structural changes, SEM imaging (Fig. 3c) to analyze surface morphology, and TG analysis (Fig. 3d) to evaluate thermal stability.
Figure 3a presents the FTIR spectra of the water-based polyacrylic resin before and after Cr(VI) adsorption. The spectrum of the resin shows a broad absorption band at 3444 cm, attributed to O-H stretching vibrations. Peaks at 1701 cm and 1161 cm correspond to the C=O and C-O stretching vibrations of carboxyl groups, respectively. The absence of the C=C monomer peak (1640 cm) confirms complete polymerization, consistent with findings in acrylic resin studies. After Cr(VI) adsorption, shifts are observed: the O-H stretching peak shifts to 3535 cm, while the C=O and C-O peaks shift to 1720 cm and 1146 cm, respectively, indicating coordination between chromium ions and the water-based polyacrylic resin. Additionally, peaks appearing in the 750-975 cm range are attributed to Cr-O bands formed after Cr(VI) adsorption.
The amorphous nature of the resin, confirmed by XRD (Fig. 3b) showing broad peaks near 9° and 20° for both the pristine and Cr(VI)-loaded resins, further supports this interaction model. The peak positions and shapes remain largely unchanged, indicating the resin's amorphous structure is preserved upon Cr(VI) adsorption. Potentially hindering crystallization, a minor reduction in peak intensities at 14° and 28° after adsorption might suggest the formation of a three-dimensional network complex with chromium ions.
SEM images (Fig. 3c) reveal the morphology of the resin before and after Cr(VI) adsorption. The resin exhibits an intricate, porous network of roughly circular pores with varying diameters, nested within each other to form a rugged three-dimensional structure. This architecture provides abundant active sites and diffusion channels, enhancing adsorption capacity and kinetics. After Cr(VI) adsorption, while the basic pore structure is retained, rounded protrusions indicating partial pore blockage are visible. This blockage is likely due to the formation of insoluble chromium deposits during adsorption. Nevertheless, the overall structural integrity of the resin is maintained, which is advantageous for potential reuse and recycling.
Thermogravimetric analysis (Fig. 3d) assessed the thermal stability of the resin and the Cr(VI)-loaded resin from 29 to 600 °C. Both resins exhibit two primary weight loss stages, suggesting that Cr(VI) adsorption does not significantly compromise thermal stability. For the resin, the initial loss (29-250 °C) corresponds to adsorbed water evaporation, followed by carboxyl group decomposition (250-430 °C). The Cr(VI)-loaded resin shows similar stages, occurring slightly at higher temperatures (29-300 °C) for water loss, 300-500 °C for decomposition. The water loss rate increases from 8.5 to 13.2% upon Cr(VI) adsorption, with the temperature range for this loss extending. The final residue content of 11.6% is attributed to chromium oxide, confirming effective Cr(VI) uptake by the resin.
The influence of pH on the adsorption efficiency of Cr(VI) onto the water-based polyacrylic resin is presented in Fig. 4a. The experiments were conducted under standardized conditions: a constant resin dosage of 0.8 g, an initial Cr(VI) concentration of 1 mg/L, a temperature of 318 K, and an adsorption duration of 12 h. A clear correlation between pH and removal efficiency was observed, with optimal performance achieved at pH 2, yielding a maximum removal efficiency of 98.73%. This indicates that acidic conditions strongly favor the adsorption process. Removal efficiency progressively decreased as the pH increased beyond the optimal point. This decline is primarily attributed to increased competition from hydroxide ions (OH) for limited active adsorption sites on the resin surface. Furthermore, under alkaline conditions, Cr(VI) may hydrolyze, form less adsorbable species, or precipitate as chromium hydroxides, further diminishing the resin's efficiency. Therefore, the optimal pH for maximizing Cr(VI) removal by this resin is approximately 2. This finding aligns with the general behavior of many Cr(VI) adsorbents, where low pH promotes positively charged surface sites and the presence of highly adsorbable Cr(VI) species (e.g., HCrO or CrO under acidic conditions), facilitating electrostatic attraction and potential complexation mechanisms.
Figure 4b illustrates the relationship between the removal efficiency of Cr(VI) and its initial concentration (Co) in the wastewater, maintained at pH 2, using a resin dosage of 0.8 g, an adsorption time of 12 h, and a temperature of 318 K. The data reveal an inverse relationship: as the initial Cr(VI) concentration increases, the percentage removal efficiency decreases. This trend occurs because the fixed amount of resin possesses a finite number of active adsorption sites and a limited maximum adsorption capacity. While the absolute amount of Cr(VI) adsorbed increases with higher initial concentrations (confirmed by adsorption isotherm studies below), the proportional removal decreases because the demand for adsorption sites outpaces the supply. This behavior is typical for adsorption processes where the adsorbent quantity is constant.
Figure 5 illustrates the correlation between the removal efficiency of Cr(VI) ions and the resin dosage. The experiments were conducted at a Cr(VI) concentration of 1 mg/L, pH 2, an adsorption period of 12 h, and a temperature of 318 K. The results show that the removal efficiency initially increases with increasing resin dosage, then plateaus (Fig. 5a). This is because a higher resin dose provides a larger surface area for adsorption, thereby enhancing the removal efficiency. The maximum removal efficiency is achieved at a resin dosage of 0.8 g, as depicted in Fig. 5b. Beyond this optimal dosage, further increases in resin amount do not lead to a significant improvement in removal efficiency. This is attributed to the resin becoming saturated as more Cr(VI) solution passes through, reaching a point where it can no longer adsorb additional Cr(VI). Consequently, the optimal resin dosage is identified as 0.8 g.
The influence of temperature on the adsorption efficiency of Cr(VI) by the resin is shown in Fig. 6. The study was conducted with an initial Cr(VI) concentration of 1 mg/L, pH 2, a resin dosage of 0.8 g, and an adsorption duration of 12 h, varying the temperature.
As depicted in Fig. 6, the removal efficiency increases with temperature, reaching a peak of 98.73% at 318 K. Interestingly, a decline in efficiency is observed at temperatures exceeding 318 K. This temperature-dependent behavior suggests that the adsorption process is endothermic over the studied range, meaning that higher temperatures provide the necessary energy to facilitate the adsorption process. However, the observed decrease at very high temperatures might indicate that excessive thermal energy could promote desorption of previously adsorbed Cr(VI) ions.
To further characterize the thermodynamics of the adsorption process, it is necessary to analyze the thermodynamic parameters such as the enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG) using Eqs. (3)-(5). Thermodynamic analysis provides critical insights into the feasibility, spontaneity, and nature (physical vs. chemical) of the adsorption process.
The distribution coefficient, K (L/g), quantifies the adsorption equilibrium at a given temperature (T) via Eq. (3), using the equilibrium adsorption capacity (Q, mg/g) and equilibrium concentration (C, mg/L). Enthalpy change (ΔH) and entropy change (ΔS) are determined by plotting lnKd against the inverse temperature (1/T, where T is in Kelvin, K). Utilizing the universal gas constant (R = 8.3145 J mol K), the slope and intercept of the linear regression yield ΔH and ΔS, respectively (Fig. 7). A comprehensive summary of the derived thermodynamic parameters is provided in Table 1.
The positive ΔH value, calculated to be 138.47 kJ/mol, falls within the range typically associated with chemical adsorption (20-400 kJ/mol), strongly suggesting that the interaction between Cr(VI) and the polyacrylic resin involves chemical bonding, such as complexation or ion exchange, rather than merely physical van der Waals or electrostatic forces. The positive ΔS value indicates an increase in disorder or randomness at the solid-liquid interface during adsorption. This increase can be attributed to two main factors: while Cr(VI) ions, particularly in their hydrated forms, become more ordered upon adsorption onto the resin surface, the desorption of previously bound water molecules from the adsorbent sites significantly contributes to the overall rise in entropy. Such thermodynamic profiles are commonly observed in chemisorption processes involving ion exchange or complexation, where the liberation of structured water molecules plays a key role. The negative ΔG values observed at and above 318 K confirm the spontaneity and feasibility of the adsorption process within this temperature range, in accordance with the second law of thermodynamics. The increasingly negative ΔG with rising temperature further underscores the enhanced spontaneity of the adsorption at higher temperatures.
The adsorption kinetics of Cr(VI) onto the water-based polyacrylic resin were investigated under conditions of 1 mg/L initial Cr(VI) concentration, pH 2, 0.8 g resin dosage, and 318 K, as illustrated in Fig. 8. The removal efficiency was found to increase progressively with adsorption time, reaching a maximum of 98.46% after 12 h. Beyond this duration, extending the contact time did not result in a significant further increase in removal efficiency, indicating that the adsorption process had reached equilibrium. The initial rapid increase in removal efficiency suggests that accessible surface sites are quickly occupied by Cr(VI) ions. The subsequent slower rate until equilibrium reflects the diffusion of Cr(VI) ions into the internal pores of the resin particles or onto less accessible sites. The plateau observed after 12 h confirms that the resin's adsorption sites have become saturated under these conditions. Therefore, an adsorption time of 12 h is determined to be sufficient for achieving near-complete removal.
To elucidate the rate-controlling mechanism of the adsorption process, kinetic data were fitted to pseudo-first-order and pseudo-second-order models (Eqs. （6） and （7）, Fig. （9a,b). Kinetic modeling is essential for understanding how quickly the adsorption process occurs and identifying the dominant mechanism, whether it be surface adsorption, intra-particle diffusion, or film diffusion. The pseudo-first-order model assumes that the rate is proportional to the difference between the equilibrium adsorption capacity (Q) and the adsorption capacity at time t (Q). The pseudo-second-order model, proposed by Ho and McKay, assumes that the rate is proportional to the square of the amount of adsorbate remaining in the solution, suggesting chemisorption as the rate-limiting step.
Here, K is the pseudo-first-order rate constant (1/min), while K is the pseudo-second-order rate constant (L/mg min), and KQ represents the initial adsorption rate (mg/g min).
The kinetic parameters calculated from the data in Fig. 9 using Eqs. (6) and (7) are presented in the Table 2.
As shown in Table 2, the pseudo-second-order model provided a significantly better fit to the experimental data, evidenced by a much higher correlation coefficient (R > 0.99) compared to the pseudo-first-order model. Furthermore, the equilibrium adsorption capacity (Q) calculated using the pseudo-second-order model aligns more closely with experimental observations than that derived from the pseudo-first-order model. These findings suggest that the pseudo-second-order model provides a more accurate description of Cr(VI) adsorption onto the resin, indicating that chemisorption -- potentially involving electron sharing or transfer between Cr(VI) species and the resin's functional groups -- is the dominant rate-limiting step in the process. The relatively high values for the pseudo-second-order rate constant (K) and the initial adsorption rate (KQ) further corroborate the rapid kinetics of Cr(VI) uptake by this polyacrylic resin.
The competitive adsorption of heavy metal ions from a mixed solution was investigated using a flame-graphite atomic absorption spectrophotometer. In this study, 0.1 g of resin was employed, and the initial concentrations of the five metal ions were uniformly set at 50 mg/L. The resulting adsorption capacities for each ion are presented in Fig. 10.
As the data in Fig. 10 reveals, the resin demonstrates the highest adsorption selectivity for Cr(VI), followed by Pb(II) and Cu(II), then Zn(II), with Mn(II) showing the lowest selectivity. This selectivity can be explained as follows: Under pH 2 conditions, the adsorption of Cr(VI) by the resin occurs through both electrostatic adsorption (facilitated by protonation) and chemisorption involving Cr-O bond formation. The combined effect of these two mechanisms enhances the resin's adsorption for Cr(VI). For Pb(II) and Cu(II), their high charge density and strong complexation ability with the -COO groups on the resin result in good adsorption selectivity. In contrast, Zn(II) has moderate affinity and its adsorption is hindered by competition from H ions, affecting the resin's selectivity for it. Finally, Mn(II) is adsorbed less effectively due to its larger ionic radius and higher hydration energy.
Figure 11 illustrates the isothermal adsorption of Cr(VI) by the resin under the conditions: pH 2, resin dosage 0.01 g, and temperature 318 K. Data shows that within the initial concentration range of 0.5-30 mg/L, the adsorption capacity increases significantly with rising Cr(VI) concentration. This is attributed to the greater availability of adsorption sites on the resin at lower Cr(VI) concentrations. As the initial Cr(VI) concentration increases further, the adsorption capacity plateaus, indicating that all available adsorption sites on the resin become saturated. Consequently, increasing the Cr(VI) concentration beyond this point does not lead to additional adsorption.
To further understand the adsorption mechanism and predict the adsorption capacity under different equilibrium conditions, the experimental data were fitted to the Langmuir and Freundlich isotherm models, using Eqs. (8) and (9), respectively.
Q (mg/g) is the theoretical maximum adsorption capacity, b (L/mg), K, and n are isothermal model constants. The model fitting curves for the Langmuir (shown in Fig. 12a) and Freundlich (shown in Fig. 12b) are presented below.
The model parameters calculated from the data in Fig. 12 using Eqs. (8) and (9) are presented in the Table 3.
The analysis presented in Table 3 reveals that the Langmuir model exhibits a better fit, with a correlation coefficient (R) of 0.9911, compared to R = 0.8904 for the Freundlich model. The superior fit of the Langmuir model strongly suggests that the adsorption of Cr(VI) onto the water-based polyacrylic resin follows a monolayer adsorption mechanism, where Cr(VI) ions are adsorbed onto a finite number of specific active sites on the resin surface until all sites are occupied. The Langmuir model also allows for the estimation of the maximum adsorption capacity (Q), calculated to be 142.86 mg/g. This high value underscores the considerable potential of the resin for Cr(VI) removal. Additionally, the Freundlich model, which describes multilayer adsorption on heterogeneous surfaces, was also applied. In the context of the Freundlich model, an 'n' value of 1.55, being greater than 1, generally indicates a favorable adsorption process, meaning the adsorbate has a preference for the adsorbent surface under the given conditions. This further corroborates the favorable nature of Cr(VI) adsorption onto the resin.
The practical applicability and economic viability of an adsorbent are significantly influenced by its recyclability. Therefore, the regeneration and reusability of the water-based polyacrylic resin for Cr(VI) adsorption were evaluated. Following adsorption, desorption was achieved using 0.2 mol/L hydrochloric acid. The resin was then collected, thoroughly rinsed with distilled water to remove residual acid and desorbed Cr(VI), soaked in a NaCl solution, and rinsed again prior to the next adsorption cycle. The results of these regeneration and reusability tests are illustrated in Fig. 13. Impressively, the resin maintained a removal efficiency of (93.58 ± 1.25) % even after five consecutive adsorption-desorption cycles. Statistical analysis (independent t-test, n = 3) confirms that the decrease in Cr(VI) removal efficiency from (97.87 ± 0.93) % in the first cycle to (93.58 ± 1.25) % in the fifth cycle is statistically significant (p = 0.012), demonstrating statistically significant but limited performance degradation after multiple reuses. These results highlight the regenerative capabilities of the polyacrylic resin and its potential for sustainable reuse in practical applications.