Metal ion/metal hydroxide loaded resin for ammonia removal from solutions containing high salinity by ligand exchange: Stability of metal ions during decomplexation of ammonia ligand

3.1.1 Reusability and stability of Me(ion)-LR

In this experiment, three resins (Cu²⁺-D751, Cu²⁺/Ni²⁺-D751 and Cu²⁺/Zn²⁺-D751) were used to adsorb ammonia from solution containing high salinity. Subsequently, these spent resins were regenerated by 1 mol/L NaCl solution. Three runs of ammonia saturated Cu²⁺-D751, Cu²⁺/Ni²⁺-D751 and Cu²⁺/Zn²⁺-D751 resins were performed and the shedding percent of metal-ions during the adsorption–desorption process were investigated. The corresponding results are shown in Fig. 1 and Table 1.

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Fig. 1

The performance of ammonia removal for Cu²⁺-D751, Cu²⁺/Ni²⁺-D751 and Cu²⁺/Zn²⁺-D751 resins.

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From Fig. 1, after three rounds of adsorption–desorption process, the ammonia adsorption capacity of Cu²⁺-D751 resin decreased slightly and remained at roughly 81% of original value. However, the ammonia adsorption capacity of Cu²⁺/Ni²⁺-D751 and Cu²⁺/Zn²⁺-D751 resins at the second and third rounds were significantly higher than that of at the first round. Such an increment could be attributed to the conversion of Me(ion)-LR to Me(hydroxides)-LR after being desorbed by NaCl solution. For Me(ion)-LR, the ammonia adsorption mechanisms have been identified as the electrical interaction and coordination complexation. In an ammonia nitrogen solution under alkaline conditions, the metal adsorption sites on Me(ion)-LR absorbed portion of OH^– in water to make the resin surface showed electronegative, which was conducive to the adsorption of NH₄⁺ in the form of ionic bonds through electrostatic interaction. On the other hand, the metal adsorption sites on Me(ion)-LR and N atom in NH₃ form a coordination bond by sharing the lone pair of electrons provided by N, O atom, thus forming a metal ammonia complex loaded resin. The Me(ion)-LR $\left(\overline{R-M{e}^{2+}}\right)$ becomes an ammonia-saturated resin $\left(\overline{R-Me{\left(N{H}_3\right)}_n{\left(OH\right)}_m}\right)$ after reaching equilibrium in ammonia adsorption process. When NaCl solution is used as a neutral salt for the desorption of ammonia-saturated resin, it plays a role in reducing the stability of, which promotes the shift of $\overline{R-Me{\left(N{H}_3\right)}_n{\left(OH\right)}_m}$ into $\overline{R-Me{\left(OH\right)}_m}$ and then NH₃ be eluted from the resin. Therefore, partial metal ion adsorption site on Me(ion)-LR have been converted to Me(hydroxides)-LR after the first desorption process. Comparing with Me(ion)-LR, Me(hydroxides)-LR has better ammonia bonding ability probably because of the inhibition of aeolotropic metal hydroxides that is formed by the resin and the well-dispersed metal hydroxides (Dong et al, 2017).

Table 1 shows that the loading capacity of Cu²⁺ on Cu²⁺-D751 resin was 54.36 mg/g, while the loading capacity of Cu²⁺ and Ni²⁺ on Cu²⁺/Ni²⁺-D751 resin and Cu²⁺ and Zn²⁺ on Cu²⁺/Zn²⁺-D751 resin were 140.25, 52.75 mg/g and 124, 17.25 mg/g, respectively. In conjunction with Fig. 1, it can be seen that the ammonia adsorption capacities of Cu²⁺/Ni²⁺-D751 (5.80 mg/g) and Cu²⁺/Zn²⁺-D751 (5.65 mg/g) in the first adsorption process were slightly higher than that of Cu²⁺-D751 (5.39 mg/g), which is attributed to the bimetallic ion loaded resins have a higher metal loading, so it can provide more effective ammonia adsorption sites.

On the other hand, as shown in Table 1, the Cu²⁺ shedding percents in the three adsorption–desorption process of ammonia removal by Cu²⁺-D751, Cu²⁺/Ni²⁺-D751 and Cu²⁺/Zn²⁺-D751 resin were detected not to exceed 0.09%. For Cu²⁺/Ni²⁺-D751 resin, the Ni²⁺ shedding percent was detected only in the first adsorption process and the Ni²⁺ shedding percent was 0.17%. Three-round desorption process in ammonia removal by Cu²⁺/Ni²⁺-D751 resin, the maximum Ni²⁺ shedding percent was 1.65%. For Cu²⁺/Zn²⁺-D751 resin, the Zn²⁺ shedding percent was detected only in the first adsorption process and the Zn²⁺ shedding percent was 0.64%. However, the maximum Zn²⁺ shedding percent reached 4.08%.

From the above results, it can be found that when using Me(ion)-LR to remove ammonia from water containing high salinity during the adsorption–desorption process, there is indeed a problem of metal ions dissolving from the Me(ion)-LR into the liquid phase. However, the shedding percent of Cu²⁺ is lower than that of Ni²⁺ and Zn²⁺, because the stability order of the chelate formed by metal ion and iminodiacetate moieties on D751 resin is: Cu²⁺>Ni²⁺>Zn²⁺ (Zhang et al, 1990).

3.1.2 Characterizations of Me(ion)-LR

The SEM images (a-j) of the Na⁺-D751 resin (a), Me(ion)-LR (b ∼ d), and Me(ion)-LR after first adsorption and desorption (e ∼ j) are shown in Fig. 2. As indicated in Fig. 2 (a), the Na⁺-D751 resin clearly exhibits a skeleton structure connected by small particles. After metal(ion) loading, it contained film-like species coating on the resin skeleton structure, and the coating degree of the film-like species on the Cu²⁺/Zn²⁺-D751 resin (Fig. 2 (d)) was highest, followed by Cu²⁺-D751 (Fig. 2 (b)) and Cu²⁺/Ni²⁺-D751 resin (Fig. 2 (c)). After ammonia adsorption, the film-like species was more evenly coated on the resin surface, as shown in Fig. 2 (e)∼(g). Meanwhile, compared to Me(ion)-LR (Fig. 2 (b)∼(d)), the surface morphology of the Me(ion)-LR after first desorption (Fig. 2 (h ∼ j)) changed insignificantly.

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Fig. 2

SEM images of (a) Na⁺-D751, (b) Cu²⁺-D751, (c) Cu²⁺/Ni²⁺-D751, (d) Cu²⁺/Zn²⁺-D751, (e) Cu²⁺-D751 after the first adsorption, (f) Cu²⁺/Ni²⁺-D751 after the first adsorption, (g) Cu²⁺/Zn²⁺-D751 after the first adsorption, (h) Cu²⁺-D751 after the first desorption, (i) Cu²⁺/Ni²⁺-D751 after the first desorption, (j) Cu²⁺/Zn²⁺-D751 after the first desorption.

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Fig. S1 shows that the N₂ adsorption and desorption isotherms of the seven resins exhibited typical type IV with H1 hysteresis loop according to IUPAC classification (Saha and Deng, 2010). The initial part of the type IV is attributed to monolayer–multilayer adsorption according to Nawrocki et al (1993). This H1 hysteresis loop probably arises from the agglomerates loading to narrow pore size distribution. The specific surface areas, pore volume and mean pore diameter of the Na⁺-D751 resin, Me(ion)-LR after first adsorption and Me(ion)-LR after first desorption were summarized in Table 2. It can be seen that after metal ion loading, the specific surface area and pore volume of the resins were increased, while the mean pore diameter decreased. This changes in BET surface area and pore textural properties can be explained by the formation of bridge complex between metal ion and the iminodiacetate moieties. It was worth noting that the specific surface area of three Me(ion)-LR after the first adsorption decreased obviously, especially the latter two resin, from 18.2 to 14.8 m²/g and 18.3 to 15.4 m²/g, respectively, which could be interpreted by the formation of complex between ammonia and transition metals. Moreover, the BET surface areas, pore volume and mean pore diameter of Me(ion)-LR after first desorption were smaller than the Na⁺-D751 resin, Me(ion)-LR after first adsorption, which further indicates that the properties of Me(ion)-LR after regeneration have changed.

FTIR analysis was used to provide evidence of comparable difference between the (a) Na⁺-D751, (b) Cu²⁺-D751, (c) Cu²⁺/Ni²⁺-D751(c) and (d) Cu²⁺/Zn²⁺-D751 resins (Fig. 3(A)). The adsorption peak at 3457 cm⁻¹ assigned to the –OH stretching vibration peak with respect to –COOH groups (Prabhu et al, 2014). After loading metal ion on D751, the intensity of peak was obviously weakened and shifted to 3453, 3468, 3475 cm⁻¹. It might be that the forming of coordination bond between the O atom on the –OH and metal ion, as shown in Fig. S2 (Chen et al, 2017). The peaks appeared 3025, 2924 and 2849 cm⁻¹ belonged to the C–H stretching vibrations of saturated alkyl groups (Yuan et al, 2006). After loading metal ion, the peaks were changed insignificantly, probably because saturated alkyl groups were not participated in the adsorption reaction. The absorption peak at 1630 cm⁻¹ and 1402 cm⁻¹ attributed the vibration absorption peak about the carboxylate on iminodiacetate moieties (Prabhu et al, 2014), and the peaks after loading metal ion were also shifted to 1625, 1615 cm⁻¹ and 1395, 1388 cm⁻¹, respectively. It could be classified to the formation of a coordination bond between metal ion and O from carboxylate on iminodiacetate moieties, as shown in Fig. S2 (Chen et al, 2017; Zhang et al, 2020). In addition, the C-N peak at 1191 cm⁻¹ was shifted to 1215, 1218 and 1224 cm⁻¹ may be due to the binding of metal ion to N-donor atom for iminodiacetate-type exchangers. (Igberase et al, 2017; Sengupta et al, 1991).

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Fig. 3

FTIR spectra of (A) Me(ion)-LR ((a) Na⁺-D751, (b) Cu²⁺-D751, (c) Cu²⁺/Ni²⁺-D751 and (d) Cu²⁺/Zn²⁺-D751), (B) Me(hydroxides)-LR ((a) Cu(OH)_x-D751, (b) Cu(OH)_x/Ni(OH)_x-D751, (c) Cu(OH)_x/Zn(OH)_x-D751).

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3.2.1 Effect of NaOH concentration

Fig. 4 (a) shows the effect of Me(hydroxides)-LR prepared under different NaOH concentrations (from 0 to 0.02 mol/L) on ammonia removal. It can be seen from Fig. 4 (a) that the adsorption capacity of Cu(OH)_x-D751 resin for ammonia increased firstly and then decreased with the increasing of NaOH concentration, and reached maximum at 0.005 mol/L. The results can be interpreted as follow: initially, the partial of metal adsorption sites on Me(ion)-LR absorbed OH^– to form Me(hydroxides)-LR $\overline{(R-Cu{\left(OH\right)}_x)}$ , which makes the resin surface showed electronegative, thus facilitating the adsorption of NH₄⁺ in the form of ionic bonds through electrostatic interaction. Meanwhile, the other partial of metal adsorption sites on Me(ion)-LR absorbed NH₃ in the form of coordination bond through coordination complexation to form copper ammonia complex. When the NaOH concentration was 0.005 mol/L, the synergistic effect of electrostatic interaction and coordination complexation on ammonia adsorption reaches the maximum. Further increasing the concentration of NaOH, the metal adsorption sites on Me(ion)-LR were more occupied by OH^–. The adsorption capacity of Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resin for ammonia increased with the increasing in NaOH concentration, and attained maximum at 0.01 mol/L. Compared with Cu(OH)_x-D751 resin, bimetallic hydroxide loaded resin has a higher metal loading, so it can provide more effective ammonia adsorption sites. Fig. 4 (b) exhibits the changes of pH value of reaction solution after ammonia adsorption by Me(hydroxides)-LR prepared under different NaOH concentrations (the corresponding pH value of modifier at each concentration from 7 to 12.3) on ammonia removal. The results suggested that the pH values of solution after ammonia adsorption by either Me(ion)-LR or Me(hydroxides)-LR were lower than that of the initial solution (pH = 11). On the one hand, when the pH of the initial solution was 11, most of ammonia was presented as molecular NH₃(aq) (Eq. (6)) (Yu et al., 2021; Chen et al., 2018). The coordination complexation between NH₃(aq) and metal adsorption sites on the resin were carried out continuously, which reduced the content of NH₃(aq) in the solution, promoted the proportion of NH₃(aq)/NH₄⁺(aq) to shift towards NH₃(aq), thus leading to the decrease of the pH value of the solution.

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Fig. 4

Effect of (a) NaOH concentration and (b) changes of pH value of solution after ammonia adsorption by Me(hydroxides)-LR, (c) modification time on ammonia removal by Me(hydroxides)-LR.

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On the other hand, it can also be found from Fig. 4(b) that the adsorption of ammonia by Me(hydroxides)-LR would also provide OH^– and promoted the conversion of NH₄⁺(aq) to NH₃(aq), resulting in a further increase in ammonia removal. Consequently, the change in solution pH values should be a trade-off between above two effects.

3.2.2 Effect of modification time

Fig. 4(c) shows that the influence of Me(hydroxides)-LR obtained by varying modification time on ammonia removal. The adsorption capacity of Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resin for ammonia in solution increased firstly and then decreased with the increasing of modification time, and reached maximum when the modification time was 15 min. One reasonable explanation is that a combination of OH^– and metal ions on Me(ion)-LR formed metal hydroxide in a relatively short time. The existing form of the loaded metal ion on resin changed, which was conducive to the adsorption of ammonia. With the extension of NaOH treatment time, the metal ions on Me(ion)-LR fell off, resulting in the decrease of the ammonia adsorption performance.

3.2.3 Reusability and stability of Me(hydroxides)-LR

In this experiment, three resins (Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resin) were used to adsorb ammonia. Subsequently, these spent resins were desorbed using 1 mol/L NaCl solution and used to treat ammonia again. Three successive desorption and regeneration processes were carried out. The performance of ammonia removal for Me(hydroxides)-LR and the shedding percent of metal ions during the adsorption and desorption process were investigated. The results are shown in Fig. 5 and Table 3, respectively.

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Fig. 5

The adsorption capacity of Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751, Cu(OH)_x/Zn(OH)_x-D751 resin for ammonia after three times of adsorption–desorption process.

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Fig. 5 indicates that the adsorption capacity of three resins for ammonia decreased gradually after three times of desorption and regeneration. The third adsorption capacity of Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resin for ammonia were 4.04 mg/g, 6.05 mg/g and 6.14 mg/g, respectively, which remained at 70%, 73% and 77% of the first adsorption capacity, respectively.

Compared with Me(ion)-LR (Table 1), the loading capacity of metal ions on the Me(hydroxides)-LR (Table 3) were not changed obviously. In addition, it can be observed from Table 3 that for Cu(OH)_x-D751 resin, the leakage of Cu²⁺ was detected only in the first adsorption process and the Cu²⁺ shedding percent was 0.05%. The Cu²⁺ shedding percent in three desorption processes were lower than that of in the Cu²⁺-D751 resin. In addition, compared with Cu²⁺/Zn²⁺-D751 resin, the Zn²⁺ shedding percent of Cu(OH)_x/Zn(OH)_x-D751 resin in first desorption process was significantly reduced from 4.08% to 0.71%. However, the Ni²⁺ shedding percent of Cu(OH)_x/Ni(OH)_x-D751 resin in three adsorption and desorption processes were higher than that of Cu²⁺/Ni²⁺-D751 resin.

3.2.4 Characterizations of Me(hydroxides)-LR

Fig. 6 presents SEM micrographs of Me(hydroxides)-LR (a ∼ c) and desorption process (d ∼ f). As seen in Fig. 6 (a ∼ c), the film-like species coating on the surface of Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751, Cu(OH)_x/Zn(OH)_x-D751 resin and it still has obvious porous structured. Compared Fig. 6 (a ∼ c) with Fig. 2 (b ∼ d) reveals that a large number of small blocks were dispersedly distributed on the surface of Me(hydroxides)-LR. After the first ammonia desorption, the porous structured on the resin surface was reduced, and the distribution of film-like species was more evenly compared Fig. 6 (d ∼ f) with (a ∼ c).

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Fig. 6

SEM images of (a) Cu(OH)_x-D751, (b) Cu(OH)_x/Ni(OH)_x-D751, (c) Cu(OH)_x/Zn(OH)_x-D751, (d) Cu(OH)_x-D751 after desorption, (e) Cu(OH)_x/Ni(OH)_x-D751 after desorption, (f) Cu(OH)_x/Zn(OH)_x-D751 after desorption.

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As depicted in Fig. S1(b), the N₂ adsorption and desorption isotherms of six resins also shown typical type IV with H1 hysteresis loop. This result was indistinguishable from that in Fig. S1(a). The specific surface area, pore volume and mean pore diameter of the Me(hydroxides)-LR were decreased as compared to Me(ion)-LR due to the reaction between OH^– with metal ion loaded on D751 resin, as shown in Table 4.

The FTIR spectrum of Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751, Cu(OH)_x/Zn(OH)_x-D751 resin are shown in Fig. 3 (B). Compared with Me(ion)-LR (Fig. 4), the –OH stretching vibrations of –COOH groups was shifted from 3453 cm⁻¹ to 3449, 3468 to 3448 cm⁻¹ and 3475 cm⁻¹ to 3451 cm⁻¹, respectively, suggesting that the –OH were participated in the metal hydroxide loading process. The X-ray photoelectron spectroscopy (XPS) measurements were also conducted to further probe the possible ammonia adsorption mechanism onto Me(hydroxides)-LR. The results of XPS spectra of Me(hydroxides)-LR and ammonia saturated Me(hydroxides)-LR are exhibited in Fig. 7 (A ∼ F), respectively. As shown in Fig. 7 (A), two main peaks at 934.2 and 953.8 eV can be assigned to Cu 2p_3/2 and Cu 2p_1/2, which were corresponding to Cu(OH)_x and CuO, respectively (Ivanova et al., 2020, Li et al., 2020). And there are two satellite peaks located at 941.0 and 944.0 eV. For the N 1 s region, the peak at 400.0 eV proves the –NH₂ groups coming from D751 resin and the peak located at 402.0 eV is attributed to the metal-nitrogen bonds (-N-Cu) coming from the complexation of the –NH₂ groups to the Cu ions (Min et al, 2012; Mahata et al, 2021). Furthermore, The O 1 s spectrum of Cu(OH)_x-D751 resin is composed of three peaks, with a main peak at 531.5 eV and two short peaks located at 530.5 and 532.6 eV. The peaks at 531.5 and 530.5 eV, which are activated by O atoms in C = O bands (Ou et al, 2006) coming from D751 resin and the -Cu–O bonds (Liang et al, 2018) coming from Cu(OH)_x, respectively. Besides, the peak at 532.6 eV, which could be attributed to the absorption of O₂ (Xu et al, 2017). In the case of Ni 2p (Fig. 7(B)) on Cu(OH)_x/Ni(OH)_x-D751 resin, two main peaks located at 855.9 and 873.8 eV are observed assigned to Ni 2p_3/2 and Ni 2p_1/2 (Álvaro et al, 2019), confirming the existence of Ni(OH)_x and NiO, respectively (Pan et al, 2019). And there are two satellite peaks located at 861.2 and 879.6 eV. In the case of Zn 2p (Fig. 7(C)) on Cu(OH)_x/Zn(OH)_x-D751 resin, the binding energies existed at 1022.3 eV is corresponding to Zn(OH)_x (Xie et al, 2020). In addition, after ammonia adsorption, the Cu2p spectrum at Cu 2p_3/2 and Cu 2p_1/2 in Cu(OH)_x-D751-NH₃ (Fig. 7(D)), Cu(OH)_x/Ni(OH)_x-D751-NH₃ (Fig. 7(E)) and Cu(OH)_x/Zn(OH)_x-D751-NH₃ (Fig. 7(F)), respectively have a chemical shift toward higher binding energy. And the O1s peak at 530.5 eV was disappeared. Thus, it can be inferred that a coordination exchange has occurred between NH₃ and OH^– on Cu adsorption sites $\overline{(R-Cu{\left(OH\right)}_x)}\to \overline{(R-Cu{\left(N{H}_3\right)}_x)}$ . Moreover, it can be seen that the Ni2p peak in from Fig. 7(E) shifts 0.3 eV (from 855.9 eV to 856.2 eV) and the Zn2p peak in from Fig. 7(F) shifts 0.4 eV (from 1022.3 eV to 1022.7 eV) to higher binding energy, indicating that a coordination exchange of NH₃ and OH^– on Ni, Zn adsorption sites were also achieved ( $\overline{R-Ni{\left(OH\right)}_x}\to \overline{R-Ni{\left(N{H}_3\right)}_x}$ , $\overline{R-Zn{\left(OH\right)}_x}\to \overline{R-Zn{\left(N{H}_3\right)}_x}$ .

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Fig. 7

XPS spectra of Me(hydroxides)-LR before and after ammonia adsorption (A) Cu(OH)_x-D751 (B) Cu(OH)_x/Ni(OH)_x-D751, (C) Cu(OH)_x/Zn(OH)_x-D751, (D) Cu(OH)_x-D751-NH₃, (E) Cu(OH)_x/Ni(OH)_x-D751-NH₃, (F) Cu(OH)_x/Zn(OH)_x-D751-NH₃.

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3.3.1 Effect of different desorbents

A static desorption experiment was performed on ammonia-saturated resins using HCl, NaCl, and NaOH as desorbents to investigate the desorption effects of different desorbents, determine the shedding percent of metal ions, and analyze the stability of metal ions in the process of ammonia ligand decomplexation, as shown in Fig. 8 (a) and Table 5. The adsorption condition: room temperature (298 ± 2 K), concentration of ammonia solution 100 mg/L, initial pH of solution 11, dosage of NaCl 4 g/L, dosage of resin 6 g/L, stirring speed 120 r/min and reaction time 150 min.

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Fig. 8

The ammonia desorption effects of (a) different desorbents, (b) NaCl concentration, (c) desorption time.

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Fig. 8 (a) shows that the desorption percent of HCl and NaOH as desorbents was higher than that of NaCl. When the concentration of HCl increased from 0.01 mol/L to 1 mol/L, the desorption percent increased. Combing with Table 5, a large amount of metal ions loaded on the resin dissolution. Using HCl as the desorbent of ammonia-saturated resin, its desorption mechanism is that the NH₃ adsorbed on the resin first reacts with H⁺ to convert into NH₄⁺ and then dissociates from the coordination with metal ions. To further increase the HCl concentration, the metal ions loaded on the resin were replaced by H⁺, which were subsequently released into the solution, causing heavy metal pollution. Moreover, the desorption percent and metal ion shedding percent of NaOH as desorbent were slightly lower than those of HCl. Using NaOH as the desorbent of ammonia-saturated resin ( $\overline{R-Me{\left(N{H}_3\right)}_n{\left(OH\right)}_m}$ ), it reduces the stability of $\overline{R-Me{\left(N{H}_3\right)}_n{\left(OH\right)}_m}$ , which promotes the shift of $\overline{R-Me{\left(N{H}_3\right)}_n{\left(OH\right)}_m}$ into $R-Me{\left(OH\right)}_x$ and then NH₃ be eluted from the resin. In addition, when the concentration of NaOH increased to 1 mol/L, the resin appears to blacken according to naked eye observation (Fig.S3), which probably caused by combination of the Me(hydroxides)-LR and excessive OH^– to form black CuO_x ( $\overline{R-Cu{\left(OH\right)}_x}\to \overline{R-CuOx}$ ). Meanwhile, as can be seen from the data in Table 5, the phenomenon of metal ion shedding is still obvious. When 1 mol/L NaCl solution was used as a neutral salt for the desorption of ammonia-saturated Me(ion)-LR, the ammonia desorption percent of Cu(OH)_x-D751, Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resins were 67%, 64%, 70%, respectively. The ammonia desorption percent of NaCl as desorbent is lower than that of the other two desorbents, but those resins maintained good stability and low metal shedding percent.

3.3.2 Effect of NaCl concentration

The desorption experiment was conducted on ammonia-saturated adsorbents (Cu(OH)_x-D751-NH₃, Cu(OH)_x/Ni(OH)_x-D751-NH₃ and Cu(OH)_x/Zn(OH)_x-D751-NH₃ resins) using NaCl as desorbents to investigate the influence of NaCl concentration on ammonia desorption. The results can be seen from Fig. 8(b) that the ammonia desorption percent increased with the increasing of NaCl concentration. When NaCl concentration was 1.5 mol/L (87.66 g/L), the maximum desorption percent of Cu(OH)_x-D751 resin for ammonia was 67.29%. For Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resin, the maximum desorption percents were 69.53%, 76.54% when the NaCl concentration was 2 mol/L (116.88 g/L). The concentration of NaCl solution as desorbents was much higher than the initial concentration of NaCl in the reaction solution (4 g/L), indicating that the Me(hydroxides)-LR has a good stability and is suitable for the treatment of ammonia wastewater containing high salinity.

3.3.3 Effect of desorption time

In this experiment, the ammonia-saturated Cu(OH)_x-D751 resin (0.6 g) was soaked in 100 mL 1.5 mol/L NaCl solution and Cu(OH)_x/Ni(OH)_x-D751 and Cu(OH)_x/Zn(OH)_x-D751 resins (0.6 g) were soaked in 100 mL 2.0 mol/L NaCl solution, respectively, under shaking at 120 r/min for desired time (30 min, 60 min, 90 min). The results presented in Fig. 8(c) shows that the desorption percent of Cu(OH)x-D751, Cu(OH)x/Ni(OH)x-D751 and Cu(OH)x/Zn(OH)x-D751 resin for ammonia in solution increased firstly and then decreased with the increasing of desorption time, and the maximum desorption percent of three resins reached the maximum at 60 min, which were 67.29%, 69.53% and 76.54%, respectively. A probably reason is that when NaCl solution is used as desorbent for the desorption of ammonia-saturated resin, the maximum transition from $\overline{R-Me{\left(N{H}_3\right)}_n{\left(OH\right)}_m}$ into $\overline{R-Me{\left(OH\right)}_x}$ is achieved, and then NH₃ be eluted from the resin. And further prolonging the desorption time, the desorbed ammonia was re-adsorbed on the resin.