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• Water pollution irritated by heavy metal ions is a severe environmental problem that arouses worldwide concerns [1]. Heavy metal ions can be accumulated in food chain and bring serious hazard to biological systems [2]. Hg(II) and Ag(I) are typical metal ions that usually released from the fields of electroplating, metallurgical, electronic, and battery industries [3]. The excess ingestion of Hg(II) would cause autoimmune diseases, insanity, paralysis and damage nervous system [4]. The uptake of Ag(I) would irritate the damages to human body including hypertension, breathing problems, and argyria [5]. Therefore, it is urgent and important to explore effective method or technique to eliminate Hg(II) and Ag(I) from aqueous solution.

  Chemical precipitation, membrane separation, solvent extraction, electrochemistry, and adsorption have been used for eliminating metal ions from water solution [6,7]. Adsorption is recognized as an effective method due to its low cost, high efficiency, convenient, and simple operation [8]. Various adsorbents including activated carbon [9], metal-organic frameworks [10], silica [11], polymer materials [6] and chelating resins [8] have been fabricated for removing Hg(II) and Ag(I). For example, Alsulami synthesized silica/thiourea-formaldehyde composite to decontaminate them from contaminated water samples [12]. Ryu synthesized thiol-modified porous organic polymer using melamine and 4-allyloxy benzaldehyde for the adsorption of Hg(II) [13]. Among the various adsorbents, the design of silica-based adsorbents has attracted great attentions due to its high surface area, distinctive mechanical stability, and adjustable porous structure [14,15]. However, the adsorption capacity of bare silica is poor which limits the application of silica as efficient adsorbent [16]. Surface chemical modification with special functional groups is a promising method to improve the adsorption performance [17]. Therefore, the design of appropriate functional groups for the targeted metal ions is of vital importance.

  PAMAM dendrimers delegate a group of attracting functional groups that contain abundant nitrogen and oxygen atoms [18]. The nitrogen and oxygen atoms are presented in different forms and located regularly in the dendrimer structure, which enable them exhibit good affinity for metal ions [19,20]. Therefore, they have been recruited as promising functional groups to construct silica-based adsorbents. For example, Augustus functionalized silica with the 3^rd and 5^th generation PAMAM dendrimers to remove Pb(II) from wastewater [21]. Qin synthesized phosphorous-containing PAMAM dendrimers modified silica to adsorb Zr(IV) and Hf(IV) [22]. Sun synthesized silica-gel supported PAMAM dendrimers for the removal of Hg(II) from water solution [23]. Qin fabricated PAMAM dendrimers modified attapulgite for the adsorption of aqueous Hg(II) [24]. Our group also synthesized a series of sulfur or Schiff base functionalized PAMAM dendrimers/silica composites to remove Hg(II) and Ag(I) from aqueous solution [25-28]. Previous results demonstrated the terminal groups of PAMAM dendrimer play important role during the adsorption. Polyhydroxyl groups contain abundant oxygen atoms and can accommodate more adsorption sites for metal ions [29]. Hence, it is assumed that the functionalization of PAMAM dendrimer with polyhydroxyl groups would further promote its adsorption performance.

  Therefore, polyhydroxyl‑capped PAMAM dendrimers/silica composites (G1-OH and G2-OH) were fabricated and used for the adsorption of aqueous Hg(II) and Ag(I). The structure of the composites was confirmed and their adsorption performance was investigated in detail. The adsorption mechanism was demonstrated and the reusability of the composites was also revealed.

  Silica (SiO₂, 200 mesh) was provided by Qingdao Makall Chemical Institute (China). 3-Aminopropyltrimethoxysilane (APTES) was purchased from Qufu Wanda Chemical Co., Ltd. (China). Analytical grade of nitric acid, tetrahydrofuran (THF), toluene, hydrochloric acid, anhydrous ethanol, methanol, dimethyl sulfoxide (DMSO), methyl acrylate (MA), tri(hydroxymethyl)aminomethane (THMMA), ethylenediamine (EDA) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). The characterization methods were given in Supporting information.

  G1-OH and G2-OH were synthesized as illustrated in Fig. 1.

  Figure 1

  

  Figure 1.  The synthetic procedures for G1-OH and G2-OH.

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  G0.5 was prepared as follows: 60 g SiO₂, 70 mL APTES and 150 mL toluene were suspended and mechanically stirred at 70 ℃ for 6 h under N₂ atmosphere, and amino groups functionalized SiO₂ (G0) was obtained after filtering and extracting with ethanol for 12 h. Subsequently, the mixture of 45 g G0, 20 mL MA and 150 mL methanol was mechanically stirred under N₂ atmosphere at 50 ℃ for 48 h. And G0.5 was obtained after filtering and extracting for 12 h by methanol.

  G1.5 was synthesized by the following two successive steps: under the protection of N₂, 25 g G0.5, 220 mL EDA and 150 mL methanol were mechanically stirred for 96 h at 25 ℃, and then amino-terminated PAMAM dendrimer modified SiO₂ (G1) was obtained after the same purification process as G0. Subsequently, 13 g G1, 40 mL MA and 100 mL methanol were suspended and stirred mechanically at 50 ℃ for 96 h under N₂ protection. And G1.5 was obtained after purification by the same procedures of G0.5.

  G1-OH and G2-OH were prepared by the amidation reaction of G0.5 and G1.5 with THMMA under N₂ protection. For the synthesis of G1-OH, the mixture of 5 g G0.5, 6.5 g THMMA, 5 g anhydrous K₂CO₃ and 200 mL DMSO was stirred for 72 h under 50 ℃, and G1-OH was achieved. Similarly, the mixture of 5 g G1.5, 13 g THMMA, 10 g anhydrous K₂CO₃ and 200 mL DMSO was stirred at 50 ℃ for 96 h, and G2-OH was obtained. The purification of G1-OH and G2-OH was the same to that of G0.5.

  The adsorption property and mechanism of the adsorbents for Hg(II) and Ag(I) was evaluated by similar methods described elsewhere [30] as described detail in Supporting information.

  The FTIR spectra of the composites are demonstrated in Fig. 2a. For SiO₂, the broad and intense absorption at about 3430 cm⁻¹ is belonged to the stretching vibration of Si-OH. The stretching vibration of Si-O-Si appears at 1098 cm⁻¹, while that of symmetrical vibration and bending vibration present at 801 and 456 cm⁻¹ [31]. After functionalization with APTES, the new peaks of CH₂ appear at 2923 and 2852 cm⁻¹ in the spectrum of G0, and the bending vibration of NH appears at 1640 cm⁻¹ [32,33]. The characteristic adsorption peak of carbonyl group appears in the spectrum of G0.5 at 1735 cm⁻¹, which suggests the synthesis of G0.5 was achieved [34]. After amidation reaction of G0.5 with THMMA, the carbonyl ester absorption peak disappears and the absorption of C-O at 1383 cm⁻¹ is strengthen, which demonstrates G1-OH was synthesized successfully [35,36]. G1.5 and G2-OH exhibit similar spectra to G0.5 and G1-OH, which proves G1-OH and G2-OH were also synthesized.

  Figure 2

  

  Figure 2.  (a) FTIR spectra of the composites (b); XPS spectra of G1-OH and G2-OH. High resolution XPS spectra of N 1s (c) and O 1s (d) of G2-OH.

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  The structure of the composites was also characterized by XPS and shown in Figs. 2b-d via chosen G2-OH as model. C, N, Si and O peaks can be found in Fig. 2b, further demonstrating the successful synthesis of G2-OH. The N 1s spectra can be divided into the peaks of CONH and C-N at 399.12 and 398.45 eV in Fig. 2c [37], while those of O 1s spectra of Si-O, OH and CONH can be observed at 532.10, 531.56 and 530.87 eV in Fig. 2d [38]. The XPS results further verify the targeted adsorbents have been synthesized.

  The thermal stability of SiO₂, G1-OH and G2-OH were determined by TGA as illustrated in Fig. 3a. SiO₂ exhibits two stages weight loss that attributed to the dehydration of physically adsorbed water and the condensation of silanol groups [39,40]. The first stage weight loss is about 4.0% and occurs bellow 110 ℃, while the second stage accounts for 2.5% weight loss from 110 ℃ to 800 ℃. Unlike SiO₂, three stages of weight loss can be observed for G1-OH and G2-OH. The first stage is also attributed to the evaporation of physically adsorbed water under 110 ℃. Then, the weight loss between 110 ℃ and 220 ℃ is mainly caused by the condensation of silanol group [25]. The weight losses are 4.5% and 5.7% for G1-OH and G2-OH in this stage. The last stage is happened in the range of 220-800 ℃ due to the decomposition of polyhydroxyl-capped PAMAM dendrimers that grafted onto the surface of SiO₂ [41].

  Figure 3

  

  Figure 3.  TGA (a) and N₂ adsorption-desorption plots of the composites (b).

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  N₂ adsorption-desorption plots and the physical parameters of SiO₂, G1-OH and G2-OH are illustrated in Fig. 3b and Table S1 (Supporting information). The plots are similar and belonged to type IV isotherm, which indicates G1-OH and G2-OH have similar porous structure to that of SiO₂ [42]. The BJH desorption pore size distribution is shown in Fig. S1, the pores between 25 nm and 125 nm are dominant for all samples. The physical parameters in Table S1 show that the pore sizes of SiO₂, G1-OH and G2-OH are distributed between 35 and 45 nm. Compared with SiO₂, the pore volume of G1-OH and G2-OH are reduced from 0.76 cm³/g to 0.73 and 0.54 cm³/g, while the surface areas of them are decreased by 13.09% and 26.42%. The occupation of the pores by the dendrimers is responsible for the reduction of pore volume and surface area [43].

  The SEM images of SiO₂ and the composites are shown in Fig. 4. The morphology of G1-OH and G2-OH is not changed significantly as compared with SiO₂ in Figs. 4a-c, which indicates the mechanical stability of SiO₂ is excellent and it was not destroyed during the functionalization. Compared with SiO₂ (Fig. 4d), the surface of G1-OH and G2-OH in Figs. 4e and f become rougher due to the introduction of polyhydroxyl-capped PAMAM dendrimer. The rough surface of the adsorbents is beneficial for the adsorption.

  Figure 4

  

  Figure 4.  SEM images of SiO₂ (a, d), G1-OH (b, e), G2-OH (c, f); and elements distribution of N (g-i) and O (j-l) that belonged to SG (g, j), G1-OH (h, k), G2-OH (i, l).

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  The nitrogen element distribution of SiO₂ and the composites are presented in Figs. 4g-i. It can be seen that the nitrogen element cannot be detected on the surface of SiO₂ in Fig. 4g. After decoration with polyhydroxyl-capped PAMAM dendrimer, the nitrogen element can be found on the surface of G1-OH and G2-OH as shown in Figs. 4h and i. The nitrogen distribution density of G2-OH is denser than that of G1-OH, which indicates the nitrogen content of G2-OH is higher. The distribution of oxygen element also displays similar trend as shown in Figs. 4j-l. It can be seen that the density of oxygen of SiO₂ is relatively low. However, it becomes denser on the surface of G1-OH and G2-OH. The variation of element distribution also proves the composites was successfully synthesized.

  The effect of solution pH on the adsorption property of G1-OH and G2-OH for Hg(II) and Ag(I) is illustrated in Fig. 5. The adsorption capacity of the composites for both ions increases with solution pH increases from 1 to 6 with the optimum adsorption capacity appears at pH 6. For example, the adsorption capacity of G2-OH for Hg(II) and Ag(I) are 0.11 and 0.04 mmol/g at pH 1. And it increases to 0.25 and 0.41 mmol/g at pH 6, which is promoted by 0.14 and 0.37 mmol/g. The weak adsorption at low pH is due to the protonation of OH and NH groups of polyhydroxyl-capped PAMAM dendrimer, which prevents the metal ions from contacting with the adsorbents [28]. With the increase of solution pH, more functional groups would be deprotonated and available for binding the metal ions [44]. Hence, the adsorption capacity increases at higher solution pH. The adsorption capacity of G2-OH is higher than G1-OH for both Hg(II) and Ag(I) as it contains high content of functional groups. Therefore, the following experiments were conducted under pH 6.

  Figure 5

  

  Figure 5.  The effect of pH on adsorption of Hg(II) (a) and Ag(I) (b) (C = 0.002 mol/L, T = 25 ℃).

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  The adsorption kinetics of G1-OH and G2-OH for the two ions are shown in Fig. 6. The adsorption of Hg(II) is rapid in the first 70 min and approaches saturation at 150 min. The adsorption capacity of G1-OH and G2-OH are 0.17 and 0.18 mmol/g at 70 min, which accounts for 94.44% and 85.41% of the total adsorption capacity for Hg(II). Similarly, the uptake of Ag(I) by G1-OH and G2-OH is very rapid in 0–70 min and the adsorption capacity can achieve 0.39 and 0.40 mmol/g. Then, the equilibrium is reached at about 120 min with the adsorption capacity of 0.40 and 0.43 mmol/g for G1-OH and G2-OH, respectively. The variation of the adsorption rate with time is closely related to the number of metal ions and active functional groups. At first, both the ions and functional groups are abundant, which could realize the rapid adsorption for metal ions [45]. Then, the number of available functional groups and metal ions are all decreased, which decrease the contact chances between the adsorbents and metal ions [18]. Hence, the adsorption rate decreases gradually. Moreover, the chelates that formed by the adsorbed metal ions and polyhydroxyl-capped PAMAM dendrimer would hinder the metal ions to diffuse into the internal of the adsorbents, which also reduces the adsorption rate [46].

  Figure 6

  

  Figure 6.  Adsorption kinetics for Hg(II) (a) and Ag(I) (b) (C = 0.002 mol/L, pH 6, T = 25 ℃).

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  The kinetic data were fitted by pseudo-first-order (PFO), pseudo-second-order (PSO) and Boyed film diffusion (BFD) models as described in Supporting information [8]. The results are demonstrated in Table 1 and Table S2 (Supporting information). The parameters in Table 1 indicate PSO model is suitable to fit the adsorption kinetics of the two ions as the R₂² values are higher than the R₁² values obtained from PFO model. Moreover, the adsorption capacity calculated (q_{e, cal}) by PSO model is in accordance with experimental result (q_{e, exp}), which also proves PSO model is more suitable. The result indicates the adsorption of the two ions by G1-OH and G2-OH is a chemisorption process [9]. The adsorption-diffusion process is analyzed to further understand the adsorption process. The adsorption process for Hg(II) was mainly divided into three steps: (1) The mass transfer of Hg(II) from liquid phase to the adsorbent's surface by boundary layer (film diffusion); (2) Hg(II) diffuses into the pores of the adsorbent (intraparticle diffusion); (3) The capture of Hg(II) by G1-OH and G2-OH. The last step is usually very rapid and is not the rate controlling step [30]. Therefore, BFD model was used to identify whether film diffusion or intraparticle diffusion is the rate controlling step. The fitting results of BFD model in Table S2 demonstrates film diffusion is the rate-controlling step as the fitting plots exhibit excellent linearity without going through the original points.

  Table 1

  

  Table 1.  Fitting result of PSO and PFO models.

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  The adsorption isotherms of the adsorbents for the two ions are illustrated in Fig. 7. The increase of ion concentration and adsorption temperature can promote the adsorption for the two ions by G1-OH and G2-OH. For example, the adsorption capacity of G1-OH under 35 ℃ is elevated from 0.19 mmol/g to 0.49 mmol/g when Hg(II) concentration increases from 0.001 mol/L to 0.008 mol/L. The adsorption capacity of G2-OH is promoted from 0.35 mmol/g to 0.82 mmol/g under this condition. For the influence of adsorption temperature, the adsorption capacity of G1-OH and G2-OH for Hg(II) is increased from 0.14 and 0.35 to 0.36 and 0.64 mmol/g when the temperature raises from 15 ℃ to 35 ℃ under 0.003 mol/L. High concentration would promote the diffusion of metal ions to the functional groups and increases the chelating chance between them [36]. Therefore, the adsorption capacity is increased by raising metal ion concentration. The favorable adsorption under high temperature indicates the adsorption is an endothermic process [31].

  Figure 7

  

  Figure 7.  The adsorption isotherms of G1-OH for Hg(II) (a) and Ag(I) (c) as well as G2-OH for Hg(II) (b) and Ag(I) (d) (pH 6, C₀ = 0.001–0.008 mol/L).

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  The isotherm data were analyzed by Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models as described in Supporting information [4]. The results are summarized in Table 2 and Table S3 (Supporting information), R_L² values of Langmuir model are higher than R_F² values of Freundlich model in Table 2, indicating the adsorption for the two ions at different temperatures could be better simulated by Langmuir model [18]. These results indicate the adsorption of the two ions by G1-OH and G2-OH is carried out by monolayer adsorption behavior [29]. The mean free energy values (E) calculated by D-R model are all between 8 and 16 kJ/mol in Table S3, suggesting the adsorption of the two ions is chemical adsorption. ΔH, ΔS and ΔG of the adsorption were calculated by Eqs. S11 and S12 (Supporting information) as described in Supporting information and are summarized in Table 3 [46]. The negative values of ΔG indicate the adsorption is spontaneous. The positive values of ΔH and ΔS demonstrate the adsorption was endothermic and entropy increased process. The reason for the positive ΔS is mainly as follow: Hg(II) and Ag(I) are existed in the solvation form with H₂O molecules surrounded in the solution before adsorption, and then the hydration H₂O molecules would be released by the functional groups of G1-OH and G2-OH during adsorption, which would increase the randomness of the system, and hence the ΔS value is positive [25,30].

  Table 2

  

  Table 2.  The fitting results of Langmuir and Freundlich models.

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  Table 3

  

  Table 3.  Thermodynamic parameters of the adsorption process.

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  The comparison of q_m with alternative reported adsorbents are shown in Table 4, it is obviously that G1-OH and G2-OH exhibit competitive adsorption performance [47-62]. For example, the q_m of G2-OH is 0.76 mmol/g for Hg(II), which is 5.85, 3.30 and 2.71 times to that of aluminum-pillared magnetic bentonite [48], sulfurized magnetic biochar [49] and activated carbon [50]. The q_m of G2-OH for Ag(I) is 0.81 mmol/g, which is 3.38, 1.72 and 1.31 times to that of PAMAM dendrimer based gels [55], sulfur-encapsulated silica nanocapsules [56] and thiourea modified PVA [59].

  Table 4

  

  Table 4.  The comparison of q_m with other adsorbents.

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  The adsorption selectivity of G1-OH and G2-OH for the two ions is presented in Tables S4 and S5 (Supporting information). The results show that they display good selective adsorption performance for Hg(II) and Ag(I). They can adsorb Hg(II) selectively with the presence of Fe(III), Co(II), Cu(II), and G2-OH can solely capture Ag(I) with the coexistence of Ni(II), Cd(II), Co(II). According to Evert Nieboe theory, metal ions could be separated into class A (oxygen-seeking), class B (nitrogen/sulfur-seeking) and intermediate class metal ions. Hg(II) and Ag(I) are belonged to class B metal ions and other metal ions are attributed to intermediate ions. Therefore, Hg(II) and Ag(I) are easy to be interacted with nitrogen to form stable chelates [63].

  The adsorption performance of the adsorbents for metal ions in simulated wastewater was investigated by choosing Hg(II) as model ion. The industrial wastewater which contains Hg(II), Cd(II), Pb(II), Na(I), Mg(II), Ca(II) and other anionic ions such as NO₃⁻, SO₄²⁻ and Cl⁻ was prepared according to previous reported as described in Supporting information [30]. The removal rates of G1-OH and G2-OH for Hg(II) are 83.43% and 98.74%, which are higher than previous reported adsorbents such as bifunctional polysilsesquioxane microspheres [30], surface-tailored nanocellulose aerogels [64], and polyethyleneimine functionalized magnetic graphene oxide [65]. The result indicates G1-OH and G2-OH can be used for decontaminating Hg(II) from real water system with great potential.

  The FTIR spectra of the adsorbents before and after adsorption are illustrated in Fig. 8a. The stretching vibration of OH and NH around 3430 cm⁻¹ becomes very weak after adsorption, and the bending vibration of CONH and C-O at 1650 and 1383 cm⁻¹ disappears. The results demonstrate OH, C-N and CONH groups took part in the adsorption. The XPS spectra of G2-OH are selected as models and illustrated in Figs. 8b-f. Hg 4f, Ag 3d and Cl 2p peaks appear in the spectra of G2-OH after adsorption in Fig. 8b, which demonstrates Hg(II) and Ag(I) have been captured by G2-OH [66]. The XPS spectra of Hg 4f can be divided into Hg 4f_7/2 and Hg 4f_5/2 at 100.06 and 104.04 eV [35,67], and that of Ag 3d is divided into Ag 3d_5/2 and Ag 3d_3/2 at 367.64 and 373.65 eV [68]. N 1s spectra of C-N and CONH in Fig. 8c shift from 398.45 and 399.12 eV to 398.94 and 399.99 eV after adsorption Hg(II), while O 1s spectra of CONH and OH move from 530.87 and 531.56 eV to 531.07 and 531.48 eV in Fig. 8d after adsorption. For the adsorption of Ag(I), N 1s spectra of C-N and CONH in Fig. 8e shift from 398.45 and 399.12 eV to 396.75 and 399.20 eV, while O 1s spectra of CONH and OH increase from 530.87 to 531.56 eV to 531.20 and 531.81 eV in Fig. 8f. XPS analysis further proves OH, C-N and CONH groups participate in the adsorption.

  Figure 8

  

  Figure 8.  (a) FTIR spectra and (b) XPS spectra of G2-OH before and after adsorption Hg(II) and Ag(I). High resolution XPS spectra of N 1s (c) and O 1s (d) after adsorption Hg(II) as well as N 1s (e) and O 1s (f) after adsorption Ag(I).

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  2 mol/L HNO₃–5% thiourea solution was used for the regeneration of the adsorbents after adsorption and the result is illustrated in Fig. 9. For G1-OH, the regeneration rates are 100% for the first three adsorption-desorption cycles for Hg(II), and it decreases to 96.94% and 95.69% for the fourth and fifth cycles. Similar to that of Hg(II), the regeneration rates of G1-OH after adsorption of Ag(I) are 100% for the first to fourth cycles, and it decrease to 91.00% for the fifth cycle. For G2-OH, the regeneration rates are also 100% for the first and second cycles for both Hg(II) and Ag(I). Then, it decreases to 97.66% and 85.72% for the third and fourth cycles after adsorption Hg(II), and it still maintains 80.40% for the fifth cycles. Similar regularity could also be observed for Ag(I). The lower regeneration rate of G2-OH for the last cycles is due to the good affinity for the metal ions. The good regeneration property suggests G1-OH and G2-OH could be reused with practical value.

  Figure 9

  

  Figure 9.  The regeneration rate of G1-OH (a) and G2-OH (b).

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  In conclusion, polyhydroxyl-capped PAMAM dendrimer/silica composites (G1-OH and G2-OH) were synthesized and confirmed by FTIR, XPS, BET, TGA and SEM. The adsorption property of the as-prepared adsorbents for Hg(II) and Ag(I) was tested via batch method. The optimal adsorption solution pH is 6 for both metal ions. The adsorption is rapid in the first 90 and 75 min for Hg(II) and Ag(I), and finally reach equilibrium at 150 and 120 min, respectively. The adsorption for both metal ions favors high temperature and concentration. Adsorption kinetic and isotherm process can be fitted by PSO and Langmuir model, respectively. Thermal dynamic parameters indicate the adsorption process of the adsorbents for Hg(II) and Ag(I) is spontaneous, endothermic and entropy-increased chemical process. G1-OH and G2-OH can 100% selectively adsorb Hg(II) with the presence of Fe(III), Co(II), Cu(II). And they can 100% selectively capture Ag(I) with the coexistence of Ni(II), Cd(II) and Co(II). The adsorbents also exhibit good removal efficiency for Hg(II) from simulated industrial wastewater with the coexistence of Cd(II), Pb(II), Na(I), Mg(II), Ca(II) and other anionic ions. Adsorption mechanism demonstrates C-N, CONH and OH groups play the dominated role during the adsorption for Hg(II) and Ag(I). The excellent adsorption and regeneration property of G1-OH and G2-OH suggest they could be potentially employed for the selective adsorption and preconcentration of Hg(II) and Ag(I) from aqueous solution with practical values.

  CRediT authorship contribution statement

  Jiaxuan Wang: Methodology, Writing – original draft, Investigation. Tonghe Liu: Methodology, Software. Bingxiang Wang: Formal analysis. Ziwei Li: Investigation. Yuzhong Niu: Project administration, Supervision, Writing – review & editing. Hou Chen: Writing – review & editing. Ying Zhang: Writing – review & editing.

  Acknowledgments

  The work is supported by National Natural Science Foundation of China (No. 22278201), Youth Innovation Team Development Plan of Universities in Shandong Province, Fundamental Research Projects of Science & Technology Innovation and Development Plan in Yantai City (No. 2022JCYJ030), the Science Fund of Shandong Laboratory of Advanced Materials and Green Manufacturing at Yantai (No. AMGM2023F09), and the Innovation Project for graduate students of Ludong University (IPGS2024-056).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109900.

  
  
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• 

  Figure 1  The synthetic procedures for G1-OH and G2-OH.

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  Figure 2  (a) FTIR spectra of the composites (b); XPS spectra of G1-OH and G2-OH. High resolution XPS spectra of N 1s (c) and O 1s (d) of G2-OH.

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  Figure 3  TGA (a) and N₂ adsorption-desorption plots of the composites (b).

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  Figure 4  SEM images of SiO₂ (a, d), G1-OH (b, e), G2-OH (c, f); and elements distribution of N (g-i) and O (j-l) that belonged to SG (g, j), G1-OH (h, k), G2-OH (i, l).

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  Figure 5  The effect of pH on adsorption of Hg(II) (a) and Ag(I) (b) (C = 0.002 mol/L, T = 25 ℃).

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  Figure 6  Adsorption kinetics for Hg(II) (a) and Ag(I) (b) (C = 0.002 mol/L, pH 6, T = 25 ℃).

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  Figure 7  The adsorption isotherms of G1-OH for Hg(II) (a) and Ag(I) (c) as well as G2-OH for Hg(II) (b) and Ag(I) (d) (pH 6, C₀ = 0.001–0.008 mol/L).

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  Figure 8  (a) FTIR spectra and (b) XPS spectra of G2-OH before and after adsorption Hg(II) and Ag(I). High resolution XPS spectra of N 1s (c) and O 1s (d) after adsorption Hg(II) as well as N 1s (e) and O 1s (f) after adsorption Ag(I).

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  Figure 9  The regeneration rate of G1-OH (a) and G2-OH (b).

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  Table 1.  Fitting result of PSO and PFO models.

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  Table 2.  The fitting results of Langmuir and Freundlich models.

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  Table 3.  Thermodynamic parameters of the adsorption process.

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  Table 4.  The comparison of q_m with other adsorbents.

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•