Synthesis and evaluation of tannin-thiosemicarbazide-formaldehyde resin for selective adsorption of silver ion from aqueous solutions

doi:10.21203/rs.3.rs-4210570/v1

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Synthesis and evaluation of tannin-thiosemicarbazide-formaldehyde resin for selective adsorption of silver ion from aqueous solutions

https://doi.org/10.21203/rs.3.rs-4210570/v1

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published 18 May, 2024

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In order to anchor chelating groups with nitrogen and sulfur atoms on tannin, a novel adsorbent (Tannin-thiosemicarbazide-formaldehyde resin) was prepared through Mannich reaction by using blank wattle tannin, thiosemicarbazide and formaldehyde as raw materials. And the adsorption behavior of Ag⁺ from aqueous solution on the resin was evaluated in batch adsorption experiments. Fourier transform infrared spectroscopy (FT–IR) and elemental analyzer were applied to verify the successful immobilization. The surface morphology, thermal stability and pore structure of the resin were also characterized. The results showed that the adsorption isotherm of Ag⁺ on the resin was described well by the Freundlich model. The Ag⁺ adsorption equilibrium achieved within 180 min and the kinetic data were fitted better by the Pseudo-second-order kinetic equation in comparison with the Pseudo-first-order and Intraparticle diffusion equations. The adsorption capacity increased first and then trended to be stable with the increase of pH ranging from 1.0 to 7.0 and the resin exhibited high selectivity toward Ag⁺ in relation to Pb²⁺, Cd²⁺, Ni²⁺ and Ca²⁺. After three cycles of regeneration and reuse, the adsorption capacity was 1.68mmol/g (84.0% of removal efficiency). Based on the experimental results and various characterization instruments, the adsorption mechanism of Ag⁺ on the resin was attributed to inner-sphere complexation and chelation between Ag⁺ and multi-electron-rich atoms ( N, O, and S), in which S atom played the most important role.

Tannin

adsorption

silver

thiosemicarbazide

mechanism

Silver is a precious metal as well as a heavy metal and has been widely used in electroplating, automotive, electronics, aerospace, shipbuilding, decoration, and other industries due to its excellent gloss, ductility, conductance and thermal conductivity [1, 2]. As a precious metal, silver is a non-renewable resource, and the amount of silver stored on Earth is very rare. According to the situation in the solar energy industry, it is expected that silver shortage will begin in 2030 with the depletion of Ag resources [3]. As a heavy metal, silver ions are very difficult to biodegrade, and can accumulate hundreds of times under the biological amplification of the food chain, posing potential risks to microorganisms, plants and human beings, thereby causing damage and impact on ecosystems [4–7]. From the perspectives of environmental protection, human health and sustainable resource development, it is urgent to effectively extract and recover Ag⁺ from industrial wastewater. Numerous techniques have been employed for separating Ag⁺ from wastewater, such as hydrometallurgy [8], electrochemical deposition/reduction [9–11], chemical precipitation [12], solvent extraction [13], Ion exchange [14], membrane filtration [15, 16], adsorption [17–19], etc. Among these techniques, adsorption is commonly applied and studied due to the advantages of simplicity, adaptability, effectiveness, and cost savings, and is regarded as the most attractive approach for removing contaminants from wastewater [20].

In recent years, chemical modified adsorbents from a variety of agricultural byproducts as starting materials have been received enormous potential for selective adsorption of precious metals owing to their widespread availability, cost-effectiveness, biodegradability, sustainable utilization and ecological adaptability [2, 19, 21–23]. Tannins are the most abundant source of natural aromatic compounds extracted from leaves, roots, bark, seeds and woods [24]. For the presence of a large number of adjacent phenolic hydroxyl groups in their molecules, tannins are considered one of excellent candidates for manufacturing biosorbents for removing heavy metals and recovering the precious metals [25, 26]. However, tannins are water-soluble compounds, which undergo a process of immobilization/insolubility before being used as adsorbents. Insolubility of tannins can be achieved through cross-linking reaction with formaldehyde in the presence of acid or base catalysts [27, 28], and immobilization of tannins can be achieved by modifying water-insoluble matrices with tannins. The water-insoluble matrices include collagen, mesoporous silica, activated carbon, nanocellulose, Fe₃O₄@SiO₂ microsphere, starch, graphene oxide, and so on [29–36].

HSAB principle states that hard acids prefer to interact with hard bases, while soft acids prefer to combine with soft bases [37]. The phenolic hydroxyl groups in tannin molecules belong to hard bases, so the affinity of tannin-based bioadsorbents for soft acids like precious metals is expected to be low. In order to improve the affinity and selectivity of tannin-based bioadsorbent, further chemical modifications have been carried out. Gurung et al. modified persimmon tannin gel with tetraethylenepentamine, bisthiourea and N-aminoguanidine to adsorb Au³⁺, Pd²⁺ and Pt⁴⁺ [38–40]. Xiong et al. decorated tannin gel with diethylamine for adsorption of precious metals from a simulated wastewater [41]. In addition, there have been many reports on biomass adsorbents modified with N-atom and S-atom compounds to improve the adsorption performance of precise metals [42–44]. The functionalized adsorbents exhibit excellent selectivity and adsorption capacity towards precise metals owing to the inductive effect of the N and S atoms. Indeed, N and S atoms are softer bases than O atom, and they show stronger affinity and selectivity towards soft acids like precious metals. Although these tannin based adsorbents containing chelating groups with N and S atoms exhibit remarkable adsorption performance on precious metals, there are also some drawbacks in the preparation process, such as high cost, solvent toxicity, long time-consuming, low grafting efficiency and complicated synthesis processes. For example, anchoring tetraethylenepentamine onto persimmon tannins involved a three-step modification. Firstly, tannins gelled with formaldehyde in basic medium for 12 h. Then, the obtained tannin gel was subjected to an intermediate chloromethylation step in concentrate hydrochloric acid for 12 h. Finally, under the catalysis of sodium carbonate, the chloromethylation product reacted with tetraethylenepentamine in a heterogeneous system of dimethylformamide solvent for 24 h [38]. Therefore, it is necessary to design and develop new approaches for anchoring chelating groups with N and S atoms on tannins.

In this study, a tannin-thiosemicarbazide-formaldehyde resin (BTC) containing N and S atoms was synthesized through one-step Mannich reaction in aqueous solution and applied for Ag⁺ recovery. Some characterizations were performed to validate the reaction and elucidate the characteristics of the resin. To achieve the desirable adsorption capacity of Ag⁺, the influences of pH, contact time, initial concentration of Ag⁺ and ionic strength were investigated in details by batch experiments. The adsorption kinetics and isotherms were also studied through different models. The selectivity of BTC resin against Ag⁺ adsorption was also explored in solution containing multiple metals ions. Finally, adsorption mechanism of Ag⁺ on the resin was determined by various characterization instruments.

2.1 Materials and chemicals

Blank wattle tannin (BWT) was obtained from Guangxi Wuming tannin extract factory, China. Thiosemicarbazide (TC), 0.1 mol/L of AgNO₃ solution and 37 wt% of formaldehyde solution were purchased from Shanghai Titan Technology Co., Ltd., China. All other chemicals were of analytical grade without further purification.

2.2 Synthesis of tannin-thiosemicarbazide-formaldehyde resin

Tannin-thiosemicarbazide-formaldehyde resin (BTC) was prepared through Mannich reaction and the proposed reaction is shown in Fig. 1. Briefly, 3 g of BWT was dissolved in 50 mL of pure water, followed by adding 2.0 g of TC into the solution. After adjusting the pH to desired value, the temperature of the reaction was allowed to rise to 80°C and 2.5 mL of formaldehyde (37 wt%) was added into the flask within 30 min. The reaction was carried out for 5 h. Subsequently, the sample was separated from mixture by centrifugation and washed three times with deionized water and anhydrous ethanol, respectively. Afterward, the obtained product was dried by vacuum at 40°C for 24 h.

2.3 Characterization

Fourier transform infrared spectroscopy (FT–IR) experiments of samples were examined by a FT–IR spectrometer (FT-IR, Nicolet 6700). Thermogravimetric analysis (TGA) was measured by a TGA instrument (Netzsch, STA 449C) in a range of temperature 30–800°C by 10°C/min under the protection of nitrogen. The content of C, H, N and S of BWT and BTC was determined by an elemental analyzer (EA, Vario Macro Cube, Elementar). Specific surface area and pore size were measured on a specific surface area tester (ASAP2020) through N₂ adsorption–desorption isotherms. The surface morphology and elemental distribution of the samples were observed by scanning electron microscopy and energy dispersive spectroscopy (SEM-DES, FEI QUANTA 200). X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi) was carried out to measure the surface state and electronic structure of BTC resin before and after Ag⁺ adsorption, and the spectra were fitted using the XPSPEAK4.1 software. X-ray diffraction (XRD) patterns of BTC before and after Ag⁺ adsorption were recorded by X-ray diffractometer (D8-Advance). Zeta potential values were obtained from a nanoparticle size and Zeta potential analyzer (Litesizer 500).

2.4 Adsorption experiments

Batch adsorption experiments were performed to evaluate the adsorption behavior of Ag⁺ on BTC. In typical experiments, 50 mg of BTC was mixed with 50 mL silver ion solution and continuously shaken at 150 rpm and 25°C. The effects of pH, initial concentration of Ag⁺, contact time, coexisting metal ions and ionic strength were investigated by varying one variable while keeping all other parameters fixed. The study of pH influence was conducted by varying the solution pH regulated by either HNO₃ or NaOH between 1.0 and 7.0. The mixture with 2 mmol/L of Ag⁺ concentration was shaken in a thermostatic shaker bath for 24 h at 25°C. Adsorption isotherms experiments were conducted with the Ag⁺ concentrations ranging from 1 to 6 mmol/L at pH 5.0 for 24 h to reach adsorption equilibrium. For the kinetics study, samples were withdrawn at periodic time intervals from the reaction mixture with 4 mmol/L of Ag⁺ concentration. The effect of coexisting metal ions was investigated by mixing 2 mmol/L of Pb²⁺, Ni²⁺, Cd²⁺ and Ca²⁺ with 2 mmol/L Ag⁺ at pH 2. The effect of ionic strength was evaluated by adding different dosages of NaNO₃ ranging from 0 to 0.5 mol/L with 4 mmol/L Ag⁺. The residual Ag⁺ concentrations after adsorption were determined by using an atomic absorption spectrophotometer (WFX 200). The adsorption capacity (q_t) of Ag⁺ on the BTC was calculated according to the following Eq. (1),

$${q}_{\text{t}}=({C}_{0}-{C}_{\text{t}})V/m$$

where C₀ and C_t (mmol/L) are the initial concentration and concentration of the adsorbate after time t, respectively, q_t (mmol/g) is adsorption capacity after time t, m (g) is the weight of the BTC used and V (L) is the volume of the solution.

3.1 Structure characterization

Table 1 displays the results of the elemental analysis of C, N, O as well as S of BWT and BTC. The contents of N and S in BTC were 10.65% and 7.282% respectively, which far exceeded the contents of N and S in BWT. In addition, the C value of BTC decreased after chemical modification, due to the lower C content of TC. These findings confirmed that TC had been successfully attached onto BWT.

Table 1

Elemental analysis of BWT and BTC
Sample	Element content (%)
Sample	C	H	N	S
BWT	49.19	4.938	0.36	0.718
BTC	44.88	4.934	10.65	7.282

Figure 2 presents the FT-IR spectra of BWT, TC and BTC. In the spectrum of BWT, the adsorption band at 3412 cm^− 1 corresponded to O–H stretching vibration, the peaks at 1617, 1508 and 1452 cm^− 1were characteristic adsorption band of benzene ring, the peaks at 1343 cm^− 1 were related to O–H bending vibration, and 1201 and 1029 cm^− 1 were attributed to the C = C-O stretching vibration and C-O-C stretching vibration [45, 46]. For TC, the adsorption peaks at 3369, 3264 and 3179 cm^− 1 were corresponding to N-H stretching vibration, and 1644 and 1620 cm^− 1 corresponding to NH₂ bending vibration. The stretching vibration bands of N-C = S(Ⅰ), N-C = S(Ⅱ) and N-C = S(Ⅲ) were observed at 1532 and 1483 cm^− 1, 1316, 1285 and 1163 cm^− 1, and 1001 cm^− 1, respectively [47, 48]. The peaks at 800 and 600 cm^− 1 were attributed to out-plane vibration and wagging vibration of N-H. After chemical modification, the spectral structures of BTC and BWT were similar but had a slight shift in wavenumbers, which demonstrated that the core structure of BWT remained unchanged. In addition, the spectrum of BTC presented new peaks approximately at 1291 and 1092 cm^− 1, which were assigned to –(C = S)–N–of TC moiety [49, 50]. It was also observed that the peaks of N-H out-plane vibration and wagging vibration appeared at 801 and 606 cm^− 1, respectively [51]. These observations revealed that BTC adsorbent was successfully prepared.

The thermal stabilities of BWT, TC and BTC were conducted and compared, and the results are shown in Fig. 3. The BTC showed three major steps transient of weight loss at temperature (1) 85, (2) 210 and (3) 307°C, corresponding to (1) moisture evaporation, (2) decomposition of TC moiety on BTC and (3) BWT degradation, respectively [52]. Comparatively, the BWT exhibited higher the residue weight than BTC. These results indicated that TC was successfully anchored on BWT.

The surface morphology of the prepared BTC adsorbent was characterized by SEM and the result is shown in Fig. 4(a). It was seen that BTC had a spherical porous structure with rough surface and uneven size. The pore structure and morphology are crucial parameters for adsorbent to adsorb metal ions [53, 54], so BET test was carried out and the nitrogen adsorption–desorption curve is presented in Fig. 4(b). The curve indicated BTG exhibit typical type-IV curve as defined by IUPAC [55]. The BET surface area, total pore volume and average pore size were 67.34 m²/g, 8.069 nm and 0.1358 cm³/g, respectively, suggesting that BTG was not a totally porous adsorbent.

3.2 Influence of pH on Ag⁺ adsorption on BTC resin

It is widely known that pH value is a dominant parameter for the metal ions adsorption, because it not only affects the metal species, but also influences the surface charge of adsorbent [56]. The influences of pH on adsorption capacity and zeta potential of BTC were investigated at 2 mmol/L of AgNO₃ for 24 h. The pH values were regulated over the range of 1.0 to 7.0 to avoid the possibility of silver precipitation [57], and the results are presented in Fig. 5(a). The adsorption capacity of Ag⁺ increased first and then trended to be stable with the increase of pH value, and the maximum removal rate is more than 95% at pH 4–7. The pH_ZPC of BTC was measured to be 2.5, indicating that the surface of BTC was positively charged at pH below 2.5 due to protonation of functional groups. The positively charged BTC was unfavorable for the adsorption of cationic silver because of electrostatic repulsion. When the pH value increased from 3.0 to 4.0, the functional groups on the surface of BTC dissociated and the surface was mainly negative charged, which was conductive to the Ag⁺ adsorption, thus increasing capacity. However, with further increasing pH, the Ag⁺ adsorption capacity changed insignificantly even if the zeta potential continued to decrease.

3.3 Influence of contact time and adsorption kinetics

The influence of contact time on Ag⁺ adsorption capacity is presented in Fig. 5(b). The adsorption capacity increased rapidly in the first 60 min, then slowed down and reached equilibrium at 180 min. Pseudo-first order, pseudo-second order and intraparticle diffusion models [35] were employed to fit the experimental data, and the fitting results and calculated kinetics parameters are shown in Fig. 5(b) and Table 2, respectively. In contrary to the pseudo-first-order model, which overestimated the adsorption capacity of Ag⁺ in the initial stage and underestimated at the equilibrium stage, the pseudo-second order model was more suitable to describe Ag⁺ adsorption kinetic behavior onto the BTC, because of higher correlation coefficient and closer equilibrium adsorption capacity between fitting value and experimental value, revealing that chemisorption was the rate control step. The adsorption kinetic data was also described by intraparticle diffusion model and the adsorption process was separated into three steps as presented in Fig. 5(c). The first initial fast step corresponds to the external surface adsorption within the initial 60 min, the subsequent slow step represented the intraparticle diffusion during the adsorption time ranging from 60 to 180 min, and the final equilibrium step after 240 min reflected the equilibrium adsorption. Obviously, the fitting result of the first linear was off the origin, indicating that Ag⁺ adsorption on BTC was not completely governed by the intraparticle diffusion [35, 58].

Table 2

Kinetics parameters for the Ag⁺ adsorption on BTC.
Kinetic models	q_e (mmol/g) calculated	Constant		R²
${q}_{\text{t}}={q}_{\text{e}}(1-{\text{e}}^{-{k}_{1}t})$ (Pseudo-first-order)	2.8375	k₁ = 0.05145 (min^− 1)		0.981
${q}_{\text{t}}=\frac{{q}_{\text{e}}^{2}{k}_{2}t}{1+{q}_{\text{e}}{k}_{2}t}$ ( Pseudo-second-order)	3.0177	k₂ = 0.03146 (mmol/g min)		0.999
${q}_{\text{t}}={k}_{3}\sqrt{t}+C$ ( Intraparticle diffusion)	—	k₃₍₁₎ = 0.9815 (mmol/g min^0.5)	C₁ = 0.2138	0.982
		k₃₍₂₎ = 2.3822 (mmol/g min^0.5)	C₂ = 0.0360	0.986
		k₃₍₃₎ = 2.8323 (mmol/g min^0.5)	C₃ = 0.0049	0.848

3.4 Influence of Ag⁺ initial concentration and adsorption isotherm

To evaluate the maximum adsorption capacity, the change of adsorption capacity with different initial concentration of Ag⁺ had investigated. As seen in Fig. 5(d), the adsorption amount of Ag⁺ rose with the increase of initial concentration of Ag⁺. Langmuir and Freundlich models [35] were applied for describing the experimental data of Ag⁺ on the BTC, and the fitting curves and calculated parameters obtained from two models are presented in Fig. 5(d) and Table 3. The Langmuir model was not appropriate for describing the adsorption isotherm according to the correlation coefficient (R²). In contrast, the Freundlich model offered better description to the adsorption isotherm data. In addition, the value of n lied between 1 and 10, indicating that the adsorption was favorable throughout the entire concentration range under research.

Table 3

Parameters calculated from Freundlich and Langmuir models for Ag⁺ adsorption on the BTC.
Langmuir model: (${q}_{\text{e}}=\frac{{Q}_{\text{e}}{k}_{\text{L}}{C}_{\text{e}}}{1+{k}_{\text{L}}{C}_{\text{e}}}$)			Freundlich model: (${q}_{\text{e}}={k}_{\text{F}}{C}_{\text{e}}^{1/\text{n}}$)
Q_e (mmol/g)	k_L (L/mmol)	R²	k_F (mmol/(g (L/mmol)^1/n))	n	R²
3.1825	15.1236	0.839	2.8553	6.2890	0.992

3.5 Influence of ionic strength on adsorption capacity

The effect of ionic strength on Ag⁺ adsorption was investigated with the concentrations of NaNO₃ ranging from 0.005 to 0.5 mol/L. As presented in Fig. 6(a), the adsorption capacity of Ag⁺ gradually increased with the increase of NaNO₃ concentration. Generally, if an outer-sphere surface complex is formed between adsorbent and adsorbate by ion exchange or electrostatic force, the adsorption capacity decreases with increasing ionic strength, while for inner-sphere surface complex, the adsorption capacity increases with the increase of ionic strength or is independent of ionic strength [59, 60]. Therefore, the positive influence of ionic strength suggested that Ag⁺ adsorption onto BTC was an inner-sphere adsorption mechanism through coordinate-covalent bond between ligand and Ag⁺, which was consistent with earlier research reported by Dong et al [56].

3.5 Adsorption selectivity

Considering the complexity of the real aquatic environment, the selectivity of BTC resin for Ag⁺ adsorption was evaluated in a mixture solution containing Ag⁺, Pb²⁺, Ca²⁺, Ni²⁺, and Cd²⁺ with the same concentration. Figure 6(b) showed the Ag⁺ removal rate of BTC resin was more than 95%, while the removal rate of other metal ions was less than 10%, revealing that BTC exhibited high selectivity to the Ag⁺ adsorption from the mixed solution.

3.6 Regeneration and Reuse study for the BTC

To regenerate and reuse the BTC, the desorption experiment was carried out by immerging the BTC-Ag in 0.5 mol/L thiourea–0.1 mol/L HNO₃ mixed solution for 1 h and washed with ultrapure water for five times [48, 51]. The regeneration BTC was reused for the next cycle. After three cycles of regeneration and reuse, the adsorption capacities were 1.91, 1.82 and 1.68 mmol/g, corresponding to 95.5, 91.1 and 84.1% of removal efficiency. Therefore, BTC adsorbent possessed good stability and reusability for the Ag⁺ recovery from wastewater.

3.7 Adsorption mechanism

The variation of surface morphology and elements distribution on the BTC after the Ag⁺ uptake were characterized by SEM-EDS. As shown in Fig. 7(a), the spherical morphology of BTC remained unchanged after Ag⁺ adsorption. The elements on BTC after Ag⁺ uptake were examined by EDS (Fig. 7(b)), and the EDS spectrum exhibited that the main constituents were C, O, N, S and Ag, demonstrating the successful Ag⁺ adsorption on the BTC surface. The corresponding atomic percentages were 56.47%, 18.11, 9.06%, 7.54% and 8.82%, respectively. The atomic percent ratio of S and Ag is less than 1, indicating other functional groups involved in the Ag⁺ adsorption. The distributions of the C, O, N, S and Ag on the BTG-Ag surface were also explored and are displayed in Fig. 7 (c-g). Elemental Ag exhibited consistency with S, providing evidence that S atom on the BTC played important role in Ag⁺ adsorption. Based on the soft and hard acid-basic (SHAB) theory, Ag⁺ (soft metal ion) exhibited higher affinity to hard bases with donor atoms as O < N < S [37, 49, 61, 62].

To further examine the adsorption mechanism, the TF-IR spectrum of BTC after Ag⁺ adsorption (BTC-Ag) was explored. As shown in Fig. 2, compared with the spectrum of the BTC, a new peak at 1384 cm^− 1 related to Ag-S stretching vibration was observed in the spectrum of BTC-Ag [63]. In addition, the peak attributed to phenolic hydroxyl groups became narrow and shifted from 3375 to 3404 cm^− 1. The peaks caused by N-H out-plane vibration and wagging vibration moved from 801 and 606 cm^− 1 to 824 and 620 cm^− 1, respectively. These observations indicated that the atoms of O, S and N on the BTC involved in the Ag⁺ adsorption.

XRD analysis of the BTC before and after Ag⁺ adsorption was performed and the result is presented in Fig. 8(a). BTC exhibited an amorphous structure with a broad hollow peak at 2θ = 22°, which represented the presence of polar functional groups [64]. After Ag⁺ adsorption, not only no new adsorption peak was observed, but also the intensity of the peak at 2θ = 22° substantially decreased. This could be ascribed to formation of complexes between the phenolic groups on the BTC and silver ions, which reduces the polarity of the functional group [64].

For better assessing the mechanism by which the Ag⁺ could chelate with the functional groups onto the BTC, XPS measurement of the BTC before and after Ag⁺ adsorption was studied. Figure 8(b) shows the XPS survey spectra of the raw BTC and BTC loaded Ag⁺ (BTC-Ag). Before Ag⁺ adsorption, four peaks at 284.8, 533.07, 400.58 and 162.93 eV corresponded to C 1s, O 1s, N 1s and S 2p were observed in fresh BTC, respectively, further illustrating the success introduction of TC on the BWT. After Ag⁺ adsorption, a new peak at binding energy of 368.7 eV related to the representative peak of Ag 3d appeared. The strong signals in deconvolution of Ag 3d orbital (Fig. 8(c)) provided the evidence of Ag⁺ adsorption on the BTC, which was well consistent with the SEM-EDS results. In addition, the shift of electron binding energy in high-resolution XPS spectra deduces the change of bonding around an atom. Figure 8(d), (e) and (f) exhibited S 2p, O 1s and N 1s XPS spectra of BTC and BTC-Ag. The S 2p XPS spectra were deconvoluted into two different component peaks, which were attributed to S = O and C = S. The binding energies of C = S found at 162.82 and 164.01 eV in the pristine BTC shifted to 162.65 and 163.83 eV after Ag⁺ adsorption, respectively, suggesting a possible chemical change of the S once Ag⁺ was adsorbed on the BTC [53]. Similar phenomena were further confirmed by the shift of binding energies of O 1s and N 1s before and after Ag⁺ adsorption, revealing that N, O and S participated in the Ag⁺ adsorption, which was well consistent with the FT-IR results.

As described in this section, the adsorption mechanism of Ag⁺ on the as-prepared BTC resin as shown in Fig. 1 was attributed to the chelation between Ag⁺ and multi-electron- rich atoms ( N, O, and S),in which S atom played the most important role.

A tannin-based adsorbent (BTC) with nitrogen and sulfur atoms was successfully fabricated through Mannich reaction. The BTC exhibited high adsorption capacity and selectivity for Ag⁺ from solution in the presence of other metals, such as Pb²⁺, Ni²⁺, Ca²⁺ and Cd²⁺. The adsorption isotherm of Ag⁺ on the BTC was described well by the Freundlich model and the adsorption kinetic data of Ag⁺ on the resin were well fitted by the Pseudo-second-order model. Furthermore, interaction mechanism between BTC resin and Ag⁺ was inner-sphere complex and chelation between Ag⁺ and multi-electron- rich atoms (N, O, and S) from batch experiments and various characterization instruments. Among the multi-electron- rich atoms, S atom played the most important role. To conclude, this work provided a one-step method in aqueous solution for developing a tannin based absorbent containing nitrogen and sulfur atoms for the recovery of silver.

Acknowledgements

This work was supported by the Science and Technology Support Program of Sichuan Province (No. 2019YJ0399).

Funding

This work was supported by the Science and Technology Support Program of Sichuan Province (No. 2019YJ0399).

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethical approval

Not applicable.

Authors' contributions

X.B. Sun developed the concept, designed the experiments and drafted the manuscript. S.Y. Yin performed the experiments and acquired the data. Y.H. You reviewed the manuscript.

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No competing interests reported.

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Synthesis and evaluation of tannin-thiosemicarbazide-formaldehyde resin for selective adsorption of silver ion from aqueous solutions

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Experimental and Methods

2.1 Materials and chemicals

2.2 Synthesis of tannin-thiosemicarbazide-formaldehyde resin

2.3 Characterization

2.4 Adsorption experiments

3 Results and discussion

3.1 Structure characterization

3.2 Influence of pH on Ag⁺ adsorption on BTC resin

3.3 Influence of contact time and adsorption kinetics

3.4 Influence of Ag⁺ initial concentration and adsorption isotherm

3.5 Influence of ionic strength on adsorption capacity

3.5 Adsorption selectivity

3.6 Regeneration and Reuse study for the BTC

3.7 Adsorption mechanism

4 Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

Kinetic models	q_e (mmol/g) calculated	Constant		R²
\({q}_{\text{t}}={q}_{\text{e}}(1-{\text{e}}^{-{k}_{1}t})\) (Pseudo-first-order)	2.8375	k₁ = 0.05145 (min^− 1)		0.981
\({q}_{\text{t}}=\frac{{q}_{\text{e}}^{2}{k}_{2}t}{1+{q}_{\text{e}}{k}_{2}t}\) ( Pseudo-second-order)	3.0177	k₂ = 0.03146 (mmol/g min)		0.999
\({q}_{\text{t}}={k}_{3}\sqrt{t}+C\) ( Intraparticle diffusion)	—	k₃₍₁₎ = 0.9815 (mmol/g min^0.5)	C₁ = 0.2138	0.982
		k₃₍₂₎ = 2.3822 (mmol/g min^0.5)	C₂ = 0.0360	0.986
		k₃₍₃₎ = 2.8323 (mmol/g min^0.5)	C₃ = 0.0049	0.848

Synthesis and evaluation of tannin-thiosemicarbazide-formaldehyde resin for selective adsorption of silver ion from aqueous solutions

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Experimental and Methods

2.1 Materials and chemicals

2.2 Synthesis of tannin-thiosemicarbazide-formaldehyde resin

2.3 Characterization

2.4 Adsorption experiments

3 Results and discussion

3.1 Structure characterization

3.2 Influence of pH on Ag+ adsorption on BTC resin

3.3 Influence of contact time and adsorption kinetics

3.4 Influence of Ag+ initial concentration and adsorption isotherm

3.5 Influence of ionic strength on adsorption capacity

3.5 Adsorption selectivity

3.6 Regeneration and Reuse study for the BTC

3.7 Adsorption mechanism

4 Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

3.2 Influence of pH on Ag⁺ adsorption on BTC resin

3.4 Influence of Ag⁺ initial concentration and adsorption isotherm