Loading of metal cations on D-HD-H is a popular cation exchanger for removing metal ions from water because it has sulfonate functional groups for binding metal ions to it. ICP-OES measurements revealed that D-H was loaded with different concentrations of Cu(II), Ni(II), and Co(II). The loaded amount of these metals on 1 g of D-H was 296 mg/g, 90 mg/g, and 60 mg/g, respectively.CharacterizationFT-IRFigure 1a displays the FT-IR spectra of D-H, D-Cu2+, and D-Cu2+ after NH4+ adsorption (D-Cu(II)-ammine composite). The spectrum of pure D-H displayed an absorption peak at 3420 cm−1, which may be interpreted for the stretching OH vibrations of physically adsorbed water molecules. The broad peak of C–H stretching in aliphatic species (C–H and –CH2 groups) appeared at 2925 cm−1, while the band at 1640 cm−1 refers to the aromatic ring of C=C stretching vibration33. The stretching vibration of S=O in the sulphonic acid group is related to the band at 1420 cm−1. The bands at 1034 cm−1 and 1072 cm−1 are due to the symmetric and antisymmetric stretching vibrations of SO3 group, respectively34. The band at 836 cm−1 showed a bending vibration of C–H out of the plane of the aromatic ring, and the band at 664 cm−1 was related to C–S stretching vibration35. FT-IR spectrum of D-H after loading with copper ions indicates that D-Cu2+ was formed through hydrogen ion of the sulfonate group which is the exchangeable moiety with copper (II) ions36. This leads to the movement of the stretching vibration band of the hydroxyl group from 3420 to 3444 cm−1. The two bands at 1072 cm−1 and 999 cm−1 were moved to 1120 cm−1 and 1003 cm−1, respectively, indicating the development of the coordinating bond between Cu(II) and the SO3 groups of D-H. While, the peak at 664 cm−1 remained at the same position as in Fig. 1a of D-H, showing that some sulfonated groups of D-H resin remain free31. In addition, the spectrum of D-Cu2+ showed a new band at 445 cm−1 associated with Cu–O bending33. After the adsorption of NH4+ on D-Cu2+, the IR spectrum (Fig. 1a) indicates ammonia bound to the Cu(II) ions, which is confirmed by the blue shift of the stretching band at 1410 cm−1 of the SO2 group to 1425 cm−1. Additionally, a new band at 480 cm−1 was attributed to the stretching vibration of N–Cu, confirming that a complex had formed between the Cu(II) ion and the ammonia molecules31.Figure 1(a) FT-IR spectra of D-H, D-Cu2+, and D-Cu(II)-ammine composite, (b) TGA curves of D-H and D-Cu(II)-ammine composite.TGAThe pristine D-H and the prepared D-Cu(II)-ammine composite exhibit three distinct weight loss stages, according to the TGA thermogram in Fig. 1b. For the raw D-H resin, the first stage results in a weight decrease of about 15.86% at (50–118 °C) temperature range. This is mostly caused by the physical adsorption of water from the surface of D-H. At the temperature range of 118–329 °C, about 24.64% of the weight loss occurs during the second stage, which refers to the decomposition of sulfonated functional groups37. The third step in weight loss above 330 °C is the result of polymer backbone decomposition38. While the TGA of the D-Cu(II)-ammine composite showed a cumulative weight loss of approximately 55.72% during various steps. At temperatures between 50 and 109 °C, the first step revealed around 5.71% due to the dehydration of adsorbed water molecules. The second stage displayed the complete decomposition of amino ligands of the Cu(II)-ammine complex, with a weight loss of 10.99% at temperatures between 109 and 310 °C31. Above 310 °C, there was a 39.02% weight loss in the last stage of the degradation process. This is associated with the organic polymer degradation of D-H, leaving behind a thermally stable metal oxide. The D-Cu(II)-ammine composite has higher thermal stability than the D-H resin39.SEMSEM analysis was used to study the surface morphologies of D-H, D-Cu2+, and D-Cu(II)-ammine, as shown in Fig. 2a–c, respectively. SEM image of D-H showed randomly distributed spherical shapes with a smooth surface40. It was found that the surface of D-Cu2+ had covered spherical spots of irregular size (Fig. 2b), confirming that D-H had been impregnated with Cu(II) ions41. The surface of D-Cu(II)-ammine composite illustrated the appearance of some bright aggregate particles on the D-Cu2+ surface, which demonstrates the adsorption of NH4+ on the surface of D-Cu2+.Figure 2SEM micrographs of (a) D-H, (b) D-Cu2+, (c) D-Cu(II)-ammine composite, EDX images of D-Cu2+: (d) before and (e) after the NH4+ adsorption.EDXFigure 2d,e shows EDX spectra of the D-Cu2+ before and after the NH4+ adsorption. The elemental composition of the D-Cu2+ is C, O, S, and Cu, which demonstrates the loading of copper ions on the surface of D-H resin (Fig. 2d). A new peak for N was observed after the adsorption process, Fig. 2e, and the ratio of N is about 37.53%. These results confirm the adsorption of NH4+ onto the D-Cu2+ surface.Kinetics of removing ammonia using metal supported on Dowex-50WX8 (D-Mn+)The effectiveness of the removal of NH4+ from an aqueous solution using D-Cu2+, D-Ni2+, and D-Co2+ was compared utilizing an identical initial concentration of NH4+. It was noticed that the equilibrium adsorption capacity of D-Cu2+ (qe = 95.58 mg/g) towards NH4+ was greater than that of D-Ni2+ (qe = 57.29 mg/g) and D-Co2+ (qe = 43.43 mg/g), as can be seen in (Fig. 3a). The highest performance of D-Cu2+ in removing NH4+ is due to the highest loaded amount of copper on D-H surface compared to nickel and cobalt42,43. The adsorption process of NH4+ on the loaded D-H was controlled by two techniques. One included the ion exchange between hydrogen ions of a sulfonated group attached to the polymeric resin and NH4+. The other represented the ligand exchange of NH4+ with hydrated water and formed a complex with transition metal cations loaded on the D-H surface. Since D-Cu2+ was represented as the best applicable adsorbent for removing NH4+ from aqueous solutions, detailed experiments aimed at studying its efficiency in getting rid of NH4+ were carried out under various experimental conditions.Figure 3(a) The impact of contact time on the adsorption of NH4+, (adsorbent dose = 0.03 g, [NH4+]o = 136.8 mg/L, 120 rpm and 30 °C). (b) Pseudo 2nd order kinetic plot of the NH4+ adsorption using (0.03 g) of D-Cu+2 at 30 °C.The adsorption of NH4+ on D-Cu2+ is influenced by contact time, (Fig. 3a). Within the first 15 min, NH4+ was rapidly adsorbed onto D-Cu2+, reaching a plateau after 20 min. It is possible to attribute the rapid initial adsorption of NH4+ onto D-Cu2+ to a vast number of replaceable active sites, which promote accelerated ion exchange and complex formation rates. Then the adsorption process was continued at a slower rate until equilibrium was attained. This is due to the active sites on the surface became gradually occupied, and there was a noticeable decline in the rate of adsorption, which led to the equilibrium state8.The maximum ammonia adsorption capacity (qmax) of D-Cu2+ is comparable to that of other common NH4+ capture adsorbents, such as cation exchange resin, clay minerals, their modifications, nanocomposite, and Cu(II)-loaded adsorbents described in the literature, as seen in (Table 1). D-Cu2+ showed the highest NH4+ adsorption capacity within 20 min. This means that D-Cu2+ is a viable adsorbent for removing NH4+ from an aqueous solution.Table 1 The maximum ammonia adsorption capacity of D-Cu2+ and several adsorbents are described in the literature.To numerically describe the adsorption kinetics and fit the experimental data of the adsorption, linear pseudo 1st model, linear pseudo 2nd order, and intraparticle diffusion models were evaluated according to (Supplementary Eqs. S1, S2, and S3), respectively. More information about these models was given in the online resource. The low value of the correlation coefficient of the pseudo-first-order model (R2 ≈ 0.95), as appeared in (Table 2), proved that this model is inappropriate and unsuitable for the prediction of the adsorption of NH4+ onto D-Cu2+. However, the pseudo 2nd order model has higher R2 values (> 0.99), which was nearly equal to one. Additionally, the calculated adsorption capacity (qe,cal) of the pseudo 2nd order model was extremely close to the experimental one (qe,exp). This indicates that the pseudo 2nd order model predicts the adsorption of NH4+ onto D-Cu2+ more accurately than the pseudo 1st order model, as presented in Fig. 3b. The pseudo 2nd order model’s superior fit demonstrates how the adsorption mechanism is dependent on the ratio of adsorbent to adsorbate. According to the above results, the NH4+ adsorption by D-Cu2+ is a chemisorption process44,45,46,47,48.Table 2 Parameters of kinetic models along with their correlation coefficient (R2) for the NH4+ adsorption onto D-Cu2+ (0.03 g) at 30 °C.According to the intra-particle diffusion plot, which is depicted in (Supplementary Fig. S1), the adsorption mechanism is divided into two stages. The quick adsorption phase of the first phase exhibited a sharp slope. It was consistent with the liquid film’s surface diffusion. The initial phase involved a straight line that failed to intersect with the point of origin. This shows that the adsorption of NH4+ occurred over the exterior surface. Furthermore, increasing the initial concentrations of NH4+ from 63.33 to 297.5 mg/L caused the adsorption rate to accelerate and increased values of kp1. Table 2 lists the values of kp1 and kp2, with kp1 being larger than kp245,49.The effect of solution pHOne of the most important factors in the removal of NH4+ by D-Cu2+ is the medium’s pH. Because it plays an important role in the ratio of two forms of ammonia and the adsorbent surface. To evaluate the NH4+ removal percentage as a function of pH, the solution pH was changed from 2 to 12 using a universal buffer (Fig. 4a). However, the initial concentration of NH4+, the dose of D-Cu2+, and the temperature were kept constant. The pHPZC of D-Cu2+ was determined to be around 5.5 in water, as shown in Supplementary Fig. S2. When the pH of the medium lower than 5.5 demonstrated low removal efficiency. This can be attributed to the surface was protonated and high concentration of H+ ions in the solution competed with the uptake of NH4+, lowering the adsorbed amount of NH4+ by D-Cu2+ through the ion exchange process50. The removal efficiency (R%) increased noticeably from 30.13 to 76.48% when the pH rose from 2 to 6. The highest removal efficiency was achieved at pH (6–8)51,52. Conversely, the R% of NH4+ dramatically dropped above pH 8.00, from 81.23% to 40.36% at pH 12. This is due to the Cu2+ on D-H surface becoming hydrolysis and producing Cu(OH)2, which decrease the uptake of NH4+ through ligand exchange and complex formation. The obtained results are completely consistent with our previous work31.Figure 4(a) The impact of initial pH on NH4+ removal ([NH4+]o = 136.8 mg/L using 0.03 g of D-Cu2+, pH = 8.4 at 30 °C), (b) Effect of D-Cu2+ dose on the NH4+ removal efficiency ([NH4+]o = 136.8 mg/L, pH = 8.4 at 30 °C), (c) The impact of the initial concentration of NH4+ on the removal efficiency with the contact time, (D-Cu2+ dose = 0.03 g, pH = 8.4 at 30 °C).Influence of D-Cu2+ dosageThe effect of D-Cu2+ dose on the removal efficiency of NH4+ is illustrated in (Fig. 4b). Variable doses of D-Cu2+ (5–70 mg) were used to investigate the effect of the adsorbent dosage. The removal efficiency of NH4+ significantly increased from 27.77 to 83.84% as the dose of D-Cu2+ was raised from 5 to 30 mg. The reason for this pattern is that, when the adsorbent dosage increased, a large number of exchangeable active adsorption sites became available. Furthermore, the rate of coordination complexation increased as the concentration of loaded copper on the D-H surface increased25. The occupation of extra-active sites caused the removal efficiency to reach the plateau at 30 mg of D-Cu2+ and did not significantly increase up to 70 mg, these results agree with that obtained by Mousavi et al.51. As a result, a 30 mg dose of D-Cu2+ was chosen to study the influence of the other experimental factors on the uptake of NH4+ by D-Cu2+.Effect of initial concentration of ammoniaThe effect of the initial concentration of NH4+ was investigated by varying its concentration from 63.33 to 297.5 mg/L, (Fig. 4c). When the NH4+ concentration raised from 63.33 to 136.8 mg/L, the NH4+ removal rate utilizing D-Cu2+ (0.03 g) during the stated contact time (20 min) got up to 83.84%. This is caused by an increase in the NH4+ concentration gradient in the solution, which creates a strong driving force for mass transfer53. Moreover, the saturation of the active adsorption sites (Cu(II) ions) with the adsorbed NH4+ to form Cu(II)-ammine complexes, and then the system reached an equilibrium state51. When [NH4+]o increased beyond 136.8 mg/L, the removal of NH4+ decreased. This occurred due to the lack of free adsorption sites on the D-Cu2+, and all Cu2+ ions were bound to NH4+ molecules.Adsorption isothermsFour isothermal models were applied to analyze the equilibrium data for NH4+ adsorbed on D-Cu2+, involving Langmuir, Freundlich, Temkin, and D-R. The linear and non-linear plots of the Freundlich and Langmuir isotherms were presented in Supplementary Figs. S3, S4, S5, and S6, respectively. All parameters derived from the isotherm models were reported in Table 3, together with the values of (R2). A glance at Table 3, revealed that the non-linear Langmuir model was the best model to describe the adsorption of NH4+ on D-Cu2+, particularly at higher temperatures. A dimensionless constant known as the separation factor (RL), which describes the Langmuir isotherm, is written as RL = 1/(1 + KLCe)54. According to the information in (Table 3), the values of (RL) in the range of 0.12–0.35 showed favorable Langmuir adsorption, at all temperatures. The values of (qmax) decreased from 333.6 to 280.9 mg/g as the temperature rose from 293 to 303 K, which was consistent with an exothermic reaction55.Table 3 Isotherm parameters and the correlation coefficient, R2 for the adsorption of NH4+ onto (0.03 g) D-Cu2+.Both Temkin and D-R models predicted that the chemisorption behavior predominated for NH4+ adsorption. It was found that the value of mean sorption energy (E), which was computed from the data of the D-R model, was more than 40 kJ/mol, suggesting that chemical action is the predominant adsorption mechanism21. This proves that removing NH4+ from an aqueous solution using D-Cu2+ included coordination complexation that formed between aqueous ammonia and the loaded Cu(II) ions on the surface of D-H.Adsorption thermodynamicsAt three distinct temperatures (293, 303, and 313 K), the adsorption of NH4+ on D-Cu2+ was investigated. The enthalpy change (\(\Delta {\text{H}}_{\text{ads}}\)) and entropy change (\(\Delta {\text{S}}_{\text{ads}}\)) of adsorption were calculated using the Van’t Hoff equation (Supplementary Eq. S4). However, the change in Gibbs-free energy of adsorption (ΔGads) was calculated from equations (Supplementary Eqs. S5, S6). Supplementary Figure S7 and Table 4, respectively, provided the Van’t Hoff plot and the values of thermodynamic parameters. As seen in Table 4, the value of Kd decreased as the temperature increased. This demonstrates the exothermic characteristics of the adsorption process27. At all reaction temperatures, ΔGads showed negative values for the removal of NH4+ using D-Cu2+, confirming the spontaneous and favorable nature of the adsorption process8. The value of ΔHads was also negative, indicating exothermic adsorption process. Additionally, the negative value of ΔSads, reveals the randomness at the solid/liquid interface and entropy decreased throughout the NH4+ adsorption process56.Table 4 Thermodynamic parameters of NH4+ adsorbed onto (0.03 g) D-Cu2+.Effect of coexisting ionsIndustrial and agriculture wastewater typically contains inorganic salts; these ions may compete with ammonia for adsorption sites on the D-Cu+2 surface. To assess the impact of these species (cations and anions) on the adsorption process, several ions, including Na+, K+, Ca2+, Cl−, NO3−, and SO42− were spiked. Experimental batches were carried out using 0.03 g of D-Cu2+ and 136.8 mg/L of NH4+. After 60 min. of adsorption time, the residual NH4+ concentration was then measured. In the presence of Na+, K+, and Ca2+, the R % of NH4+ decreased from 83.84 to 67.84%, 61.30%, and 39.43%, respectively. This was mostly due to the competitive adsorption between these cations with NH4+ on D-Cu2+ during the ion exchange process in simulated wastewater. Ca2+ ion has a high valence form, causing its influence to be notably stronger than that of other cations57,58. As can be seen in Fig. 5a–c, increasing the concentration of cations increases the competition between these cations and NH4+, which leads to a gradual decrease in the amount of NH4+ uptake59. In the presence of different anions, such as Cl−, NO3−, and SO42−, the R % of NH4+ decreased to 71.30%, 72.91%, and 69.84%, respectively. Therefore, these anions had a little minor impact on the removal efficiency of NH4+. According to these results, which are shown in Fig. 5a–c, both cations and anions have a limited influence except the Ca2+ ion on the removal efficiency of NH4+ by D-Cu2+. Since the majority of NH4+ removal occurs as a result of coordination complexation with copper ions at the D-H surface.Figure 5(a–c) Effect of coexisting ions with different concentrations on NH4+ adsorption, [NH4+]o = 136.8 mg/L using D-Cu2+ (0.03 g) at 30 °C.Removal of ammonia from synthetic wastewaterA synthetic wastewater sample that was prepared according to the procedures in the experimental part was used to evaluate the performance of D-Cu2+ for the removal of NH4+. Due to interference from other ions present in the industrial wastewater, (0.03 g) of D-Cu2+ only succeeded in removing 44.35% of NH4+ from the effluent after 60 min, this is lower than its removal efficiency in the aqueous medium (83.83%).Application of the catalytic performance of D-Cu(II)-ammine composite for degradation of dyesDyes have been globally produced in huge quantities as a result of substantial growth in modern industries for use in dying fabrics, leather, paper, plastics, cosmetics, printing, food, pharmaceutical industry, and others60. However, they impact the survival of organisms. Large amounts of effluent from the dyeing process color the wastewater. This effluent impairs photosynthesis and negatively affects human and marine organisms’ health61. Optimization of the D-Cu(II) ammine composite obtained from the NH4+ adsorption process using D-Cu2+ was performed. This product was used as a catalyst to break down two types of dyes (AB and MV2B) present in polluted water. The decomposition of the two dyes occurred in the presence of an environmentally friendly oxidizing agent, H2O2. The UV-vis spectra and time-dependent absorbance decreases of the two dyes are shown in Fig. 6a,b. The oxidative degradation of AB and MV2B in the presence of H2O2 using D-Cu2+ and D-Cu(II)-ammine individually as catalysts was illustrated in Fig. 6c. Within 15 and 90 min, it was found that approximately 95.83% and 93.27% of AB and MV2B were degraded, respectively in the presence of D-Cu(II)-ammine composite. However, only 26.01% and 90.39% of AB and MV2B were degraded, respectively in the presence of D-Cu(II) as a catalyst. These results showed that the D-Cu(II)-ammine composite had better catalytic activity than D-Cu2+ for the degradation of both AB and MV2B. Moreover, CO2 evolved from the reaction of AB and MV2B with H2O2 in the presence of D-Cu(II)-ammine composite was captured by an aqueous solution of barium hydroxide. As the reaction was carried out, a white precipitate of BaCO3 emerged, signifying the generation of CO2 as a catalytic decomposition product of AB and MV2B.Figure 6UV-vis spectra, as a function of time during catalytic decomposition of dyes in aqueous solution using 0.05 g of D-Cu(II)-ammine composite, [H2O2]o = 0.01 mol/L at 30 °C. (a) [AB]o = 7.5 × 10–5 mol/L, (b) [MV]o = 1.86 × 10–4 mol/L, (c) The degradation efficiency of the AB and MV2B by D-Cu2+ (0.05 g) and D-Cu(II)-ammine composite (0.05 g), [H2O2]o = 0.01 mol/L at 30 °C, and (d) D-Cu(II)-ammine composite recycling during the degradation of AB and MV2B with H2O2. (e) XRD of D-Cu(II)-ammine composite before and after the consecutive four catalytic cycles.Recycling of D-Cu(II)-ammine compositeFrom a practical standpoint, lowering the processing cost is critical for ensuring sustainable economic growth. As a result, the recovery and reusability of the D-Cu(II)-ammine composite as a catalyst were assessed utilizing the oxidative degradation of AB and MV2B with H2O2 as an experimental reaction. After the reaction ended, the catalyst was thoroughly rinsed with distilled water and H2O2 solution, dried, and then reused in the subsequent reaction cycle. To demonstrate the excellent catalytic activity and stability of the catalyst, four successive cycles of catalyst reusability were run. Figure 6d indicated a very slight loss in catalytic activity within the four cycles. From the first to the fourth cycle, the degradation efficiency of the AB and MV2B varied from 94.64 to 91.57% and 92.62 to 86.92%, respectively. This finding demonstrates that the D-Cu(II)-ammine composite is stable and may be recycled effectively several times with only a slight decrease in its catalytic activity up to four cycles. Moreover, XRD pattern (Fig. 6e) of D-Cu(II)-ammine composite before and after four reusability cycles indicates no change in its structure. This confirms the stability of the D-Cu(II)-ammine composite under the reaction conditions and can be a good candidate for other catalytic applications.
