Silver(I) recovery on thiomorpholine - modified functional polymer

: The functional polymer containing heterocyclic ligands was synthesized by microwave modification of a crosslinked poly(vinylbenzyl chloride – divinylbenzene) matrix with thiomorpholine. The modification yield was 89.3%. The sorbent was used to recover Ag(I) from the synthetic and real chloride solutions. The maximum sorption capacity of Ag(I) was approximately 180 mg Ag/g. The sorption kinetic data were well-fitted to the pseudo-first-order kinetic model. The degree of silver desorption was approximately 50.0% using a 1.0% potassium cyanide solution in a 0.50% hydrogen peroxide solution. The resin retained its capacity toward Ag(I) in five consecutive sorption/desorption cycles. The thiomorpholine modified resin was highly selective toward Ag(I) in relation to Cu(II), Pb(II), Co(II), Ni(II), and Zn(II) from the real chloride leaching solution.


Introduction
Silver is a precious metal found in lead-zinc, copper, tin, nickel, and gold deposits. Most silver comes as a by-product from ores that are mined and processed to make other metals. From a commercial point of view, native silver, which does not contain chemicals or is not associated with other minerals, is also important.
Silver has been known since ancient times. Originally, they were used for jewellery or for the production of everyday objects and for the production of coins. Today, its properties are used in many technical fields, such as electronics, medicine, and photovoltaics. Despite the great demand for this metal, resources are limited. It is estimated that by 2024, the demand for silver will exceed the production of natural raw materials and waste materials (Li and Adachi, 2018). Based on scenarios from the solar industry, the silver shortage will begin in 2030. Therefore, it is necessary to increase the number of potential sources for the recovery of this metal.
In recent years, a great deal of research has been carried out to develop hydrometallurgical methods for recovering silver from industrial waste, waste electrical and electronic equipment, or jewellery. Recycling is also beneficial for environmental reasons.
The most popular hydrometallurgical method of transferring silver into a solution was cyanide leaching, in which up to 90% of silver recovery processes were carried out in this way. This process was not an ideal one on account of its toxicity, low leaching rate, and its inefficiency in treatment refractory ores. Other leaching processes that use thiosulfate, acidified thiourea, hydrochloric acid, and ammonia solution have also been studied, and some of these processes have found applications in the industry. Chloride leaching has several advantages due to the higher dissolution rate of silver, low price of leaching reagents, and non-polluting character (Puvvada et al., 2000;Liu et al., 2010;Behnajady and Moghaddam, 2011).
Different separation processes such as cementation, membrane processes, solvent extraction, or sorption are used to recover metals from solutions after leaching. Different types of sorbents are chloride leach solution. The effects of hydrochloric acid and sodium chloride concentrations on Ag(I) sorption, sorption isotherms, the effect of contact time, and Ag(I) desorption were also studied.

Chemical reagents
The chemicals were Sigma-Aldrich products. A synthetic solution of Ag(I) was prepared by dissolving a known amount of silver nitrate in a chloride solution (4.0 M NaCl,0.10 M HCl). The initial concentration of Ag in the synthetic solution was 51.2, 55.6, or 60.0 mg/dm 3 .
The solution of Ag(I) and Cu(II) was prepared by dissolving a known amount of copper sulfate and silver nitrate in a chloride solution (4.0 M NaCl,0.10 M HCl). The concentration of Ag(I) and Cu(II) was 60.0 and 32.1 mg/dm 3 , respectively.
The concentration of metal ions in synthetic and real chloride solution was determined using Varian SpectrAA 20 Plus Atomic Absorption Spectrometer.

Preparation of polymeric resin
The resin presented in this work was prepared from the starting material, the vinylbenzyl chloride/divinylbenzene (VBC/DVB) copolymer. It contained 2.0 wt. % of crosslinker. The polymeric support was obtained by typical suspension polymerization (Trochimczuk, 1998). The prepared resin was characterized by an expanded gel structure.
The polymeric resin was prepared by microwave modification. The VBC/DVB (2.0 wt. % of DVB) copolymer was placed on a Petri dish and swollen with thiomorpholine (five times the molar excess with respect to the chlorine content in the polymer) for 1 h at room temperature. The reaction mixture was then placed in a microwave reactor for 10 min, at 90 W. The resin obtained was washed with acetone, acetone/water (1:1), water. The beads were placed in the column and washed with 1 M HCl, water, 1 M NaOH, and water.

Methods of analysis
The water regain was measured using a centrifugation method in which about 1 g of polymer swollen in water was placed in a column with filled-glass bottom and centrifuged at 2000 rpm for 5 min., weighted and dried at 100 o C for 24 h. After being cooled in a desiccator, the polymer was weighted again. The water regain was calculated as (mw -md)/md, where mw is the weight of the swollen polymer after centrifugation, and md is the dry weight of the polymer (Jermakowicz- Bartkowiak, 2005).
The nitrogen content was determined using the Kiejdahl method after mineralization of the resin sample (about 200 mg) in the concentrated sulfuric acid containing copper sulfate and potassium sulfate (Pilśniak-Rabiega and Trochimczuk, 2014).
The chlorine content was measured by burning about 20 mg of dry polymer sample in oxygen in a flask containing 25 cm 3 of 3.0% hydrogen peroxide solution. The content of chlorine was determined using the Volhard's method (Pilśniak-Rabiega and Trochimczuk, 2014).
The sulfur content was measured by burning about 20 mg of dry polymer sample in an oxygen-filled flask. The products of burning were adsorbed in 15 cm 3 of 3.0% hydrogen peroxide solution. After 1 h, the flask was heated in a water bath for 30 min to evaporate carbon dioxide. Then the content of the flask was titrated with 0.01 M NaOH in the presence of bromothymol blue.
The middle-infrared spectra (4000 -400 cm -1 ) of the resins were collected on a Fourier transform, Bruker VERTEX 70V vacuum spectrometer equipped with an air-cooled DTGS detector. The ATR accessory was used for the measurements. The spectral data were recorded at the resolution of 2 cm -1 with 64 scans collection.

Evaluation of the sorption properties
The sorption capacity of the resin towards Ag(I), Pb(II), Cu(II), Zn(II), Co(II), and Ni(II) was determined from synthetic and real chloride solutions by contacting the resin samples with 20.0 cm 3 of metal ions solution. Metal ions solution containing Ag (51.2, 55.6, or 60.0 mg/dm 3 ), Cu (450 mg/dm 3 ), Pb (4.2010 3 mg/dm 3 ), Zn (20.0 mg/dm 3 ), Co (16.2 mg/dm 3 ) and Ni (5.01 mg/dm 3 ) was used in the sorption experiments. The ratio of ligands in the resin to Ag in the solution was set to 10:1. After 24 h, the polymer was separated by filtration and the concentration of Ag was measured using the atomic absorption technique (AAS) on a Varian SpectrAA 20 Plus Atomic Absorption Spectrometer.
The sorption isotherm was determined by contacting various amounts of resin (containing 0.1-10×molar excess of the ligand) with 20.0 cm 3 of solution containing 51.2 mg/dm 3 of Ag, 4.0 M of NaCl, 0.10 M of HCl for 24 h, at 23±2 o C and plotting the sorption as a function of the equilibrium concentration.
For sorption kinetics, identical resin samples were shaken with 20.0 cm 3 of solution containing 55.6 mg/dm3 Ag in chloride solution (4.0 M NaCl, 0.10 M HCl). The samples were taken at different intervals of time, the resin and solution separated, and the concentration of Ag was determined by AAS.
The distribution coefficient (Kd) was calculated as the ratio of the amount of metal taken by 1 g of resin and the amount of metal remaining in 1 cm 3 of solution after sorption. The selectivity coefficient (αsel.) was calculated as the quotient of the distribution coefficients for each metal.
Desorption of Ag was determined by contacting an amount of resin, loaded with the known amount of Ag, with 20.0 cm 3 of eluent at room temperature (23 o C) and at 50 o C for 24 h. After that time, the concentration of Ag was determined by AAS and the percentage of Ag eluted was calculated.

Synthesis and characterization of the resin
The thiomorpholine resin (resin I) was prepared by the chemical modification of the VBC/DVB (2 wt.% of DVB) polymer. The VBC/DVB copolymer with expanded gel structure, containing 2 wt.% of DVB was obtained in the presence of toluene. Elemental analysis gave 5.30 mmol/g of chlorine, which corresponds to 83.0% yield of VBC incorporation into the polymeric network. The chloromethyl groups present in the VBC/DVB structure served as a reactive place for the immobilization of thiomorpholine ligands.
The polymeric resin was prepared by microwave modification. Elemental analysis gave 3.95 mmol of N/g, which corresponds to 89.3% yield (see Table 1). The sulfur content in this resin is 3.77 mmol/g. The yield calculated from the sulfur content is 85.0%. The degree of substitution of chlorine in chloromethyl groups by thiomorpholine calculated on the basis of nitrogen and sulfur content was similar. Modification efficiency and chlorine content (0.381 mmol/g) confirmed the effective immobilization of functional groups in the polymer matrix.
The microwave-modified immobilization efficiency of heterocyclic groups in the VBC/DVB copolymer was 50-90% (Pilśniak-Rabiega et al., 2019; Pilśniak-Rabiega and Wolska, 2020; Pilśniak-Rabiega and Wolska, 2021). Resin I had small values of water regain (1.06 g of water/g of polymer) due to the presence of sulfur atoms, which showed hydrophobic properties. The FTIR spectrum of the VBC/DVB copolymer (see Fig. 1) shows a characteristic band with a wave number of 1264 cm -1 derived from the C-Cl bonds of the chloromethyl group (Yaacoub and Le Perchec, 1988;Egawa et al., 1990). The disappearance of this band indicates the substitution of chloromethyl groups by thiomorpholine ligands. This range is not visible in the spectrum of the thiomorpholinemodified copolymer, suggesting that chlorine atoms in the chloromethyl groups have been substituted by sulfur-containing ligands. The broad band at 3427 cm -1 comes from the hydroxyl groups that are formed during the hydrolysis of vinylbenzyl chloride to vinylbenzyl alcohol. In both spectra between 1490 and 1513 cm -1 , two skeletal vibration bands emanating from the phenyl rings are visible. In the range from 2850-2920 cm -1 , bands are visible that are derived from the tensile vibrations of the aliphatic groups C-H. The FTIR spectrum of a modified polymer (see Fig. 1) shows a strip with a wave number of 1445 cm -1 indicating a deformed shear vibration of the N-CH2 bond. At a wave number of 1416 cm-1 , a stripe is visible, which may have originated from the -CH2 groups associated with sulfur atoms (Silverstein and Bassler, 1970;Stuart, 2004). Based on the spectrum, the chemical structure of the functional polymer looks as follows (see Fig. 2):

Fig. 2. Structure of the investigated resin
A similar structure (see Fig. 3) was achieved with the thiomorpholine-functionalized nanoporous mesopore (Ganjali et al., 2010). Construction of a selective sensor, modified with silica-based organicorganic hybrid materials containing thiol groups covalently attached to the solid framework via a propyl chain.

Effect of hydrochloric acid and sodium chloride concentration on the sorption of silver(I)
To determine the influence of acidity on Ag(I) sorption on the functional polymer, a series of experiments were carried out at 23±2ºC using chloride solutions of Ag(I) (60.0 mg of Ag/dm 3 ) with It was observed that the Ag(I) sorption decreased with increasing HCl concentration. An exception is the lower sorption capacity at 0.01 mol/dm 3 of HCl. This may be caused by the speciation of Ag(I) in chloride solution (Liu et al., 2010). The best Ag(I) sorption results can be obtained from the solution containing 0.10 mol/dm 3 of HCl. At relatively low acidity, the functional groups on the adsorbent exist in protonated form and exhibit high electrostatic interaction with the Ag(I) coordination anion according to the hard and soft acid and base (HSAB) theory (Xiong et al., 2016). At higher concentrations of HCl (0.30-2.0 mol/dm 3 ), the sorption of Ag(I) decreased due to competition with Clions due to their increased concentration in solution.
The effect of NaCl concentration on the sorption of Ag(I) at 23±2ºC on thiomorpholine -modified functional polymer was investigated in the concentration range of 1.0-5.0 mol NaCl/dm 3 , 0.10 mol HCl/dm 3 , and 60.0 mg Ag/dm 3 (see Fig. 5). The resin showed the highest affinity for Ag when the concentration of NaCl was 4.0 mol/dm 3 . The Ag(I) sorption decreased when NaCl concentration was greater or smaller than 4.0 mol/dm 3 . The decrease in sorption capacity is due to the speciation of Ag(I) (Liu et al., 2010).
The results presented in Figs. 4 and 5 show that the best Ag(I) sorption can be observed from the solution containing 4.0 mol/dm 3 of NaCl and 0.10 mol/dm 3 of HCl in the chloride solution of silver(I). Therefore, all subsequent experiments, including the study of sorption mechanism, sorption isotherm obtaining, and kinetic studies, were carried out using this solution.

Sorption mechanism
The thiomorpholine resin was applied for the recovery of Ag(I) from the synthetic chloride solution (4.0 M NaCl, 0.10 M HCl). The sorption properties of resin I toward Ag(I) were studied under the conditions presented in the Experimental part. The results are shown in Table 2.  The sorption of silver chloro complexes by resin I was 13.94 mg Ag/g resin. These complexes were mainly sorbed in the form of AgCl4 3-. The formation of these complexes was suggested by Liu (Liu et al., 2010). According to the thermodynamic calculation, the silver complex species in the tested solution (4.0 M NaCl, 0.10 M HCl) mainly exist in the form of AgCl4 3-, whose proportion is 83%, when [Cl -] is 4.10 mol/dm 3 . The proportion of AgCl2 -and AgCl3 2-is 3.0 and 14%, respectively. Therefore, the Ag(I) complexes were mainly sorbed by this resin in the form of AgCl4 3-.
Based on the hard and soft acids and bases (HSAB) theory, noble metals are considered "soft acids" and tend to form stable complexes with sorptive materials containing "soft" donor atoms ("soft bases") such as sulfur. Sulfur atoms have an easily polarizable lone pair of electrons and a large atomic/ionic radius. Nitrogen atoms are borderline bases also tend to form complexes with precious metals.
Resin I displayed affinity towards Ag(I) due to the presence of donor N-atoms and especially S-atoms in the functional groups. Free pairs of electrons on nitrogen and sulfur atoms should be able to coordinate with Ag(I), which suggested that the main mechanism of metal uptake was the coordination.
It seemed probable that during the sorption of Ag(I) on resin I, the thiomorpholine ligands formed specific metal complexes by the ligand exchange mechanism. Ag(I) can be then coordinated to two sulfur donor-ligands and has a linear coordination geometry (see Fig. 6). The formation of these complexes might be the reason of a lower Ag(I) sorption, because two thiomorpholine groups take part in the formation of one complex. 6. Suggested mechanism for Ag(I) sorption on thiomorpholine resin

Isotherm of sorption
During the study of the sorption characteristics of the synthesized resin, the sorption isotherm at room temperature was also determined (see Fig. 7) from the silver(I) solution with a concentration of 51.2 mg/dm 3 and with a different molar ratio of the ligand to the silver ions. As can be observed, the initial concentration of the ions is an essential driving force to overcome any resistance to mass transfer between the solution and resin I. At a lower concentration of Ag(I), the solution can interact with the binding sites; therefore, the adsorption capacity increases rapidly with increasing initial concentration. At higher concentrations, the adsorption capacity is almost constant as a result of the saturation of the adsorption site. The maximum sorption capacity of Ag(I) for resin I was approximately 180 mg Ag/g. The Freundlich, Langmuir, and Dubinin-Radushkevich isotherm models were used to describe the equilibrium data in this study. The adsorption isotherm is given in Fig. 6 and the calculated parameters of the adsorption isotherms along with the coefficient of determination (R 2 ), are summarized in Table  3.
where qe (mg/g) is the uptake at equilibrium concentration, qm (mg/g) is the maximal uptake, Ce (mg/dm 3 ) is the equilibrium concentration, and bL (dm 3 /mg) is the constant related to the binding energy of the sorption system. Parameters qm and bL were calculated from the slope and intercept of the linear plot of 1/qe vs. 1/Ce , Esteves et al. 2022). Additionally, a separation factor (dimensionless) called the equilibrium or separation parameter (RL) is determined during the analysis, which allows one to determine whether the Langmuir isotherm model is favorable for a given separation process (Eq. 5) Esteves et al.,2022): where C0 is the initial adsorbate concentration [mg/dm 3 ].
The parameter RL indicates the efficiency of the adsorption process. The isotherm is (i) unfavourable when RL > 1, (ii) linear when RL = 1, (iii) favorable when RL < 1, and (iv) irreversible when RL = 0 . Unfortunately, the obtained fitting data did not allow us to calculate the characteristic parameters for this isotherm model, which also confirms that this model is not suitable for the analysis of the sorption process of Ag(I) on resin I. The next investigated model was the Freundlich isotherm. This model is commonly adopted to describe non-ideal sorption on highly heterogeneous surfaces that provide adsorption sites of varying affinities (Esteves et al., 2022). The non-linear form of the model can be represented as described in equation (Eq. 6) ( Esteves et al., 2022): The Freundlich model is assumed as a power function relationship between qe and Ce and is easily applicable when the experimental data are plotted in log qe versus log Ce format (Eq. 7) (Cela-Pérez et al., 2011). The Freundlich isotherm is applicable to adsorption processes that occur on heterogonous surfaces. This isotherm gives an expression which defines the surface heterogeneity and the exponential distribution of active sites and their energies. The linear form of the Freundlich isotherm is as follows (Eq. 7) Ayawei et al., 2017): In this model, there are two fitting parameters a and 1/n that both yield a measure of physical binding. The a parameter is the constant related to the adsorption capacity. The 1/n parameter is known as the heterogeneity index. For homogeneous materials, 1/n would be equal to 1, when the adsorption is linear, the adsorption sites are homogeneous in energy, and no interactions occur between the adsorbed compounds. On the other hand, when the values of the 1/n parameter approach zero, they increase the heterogeneous character of the polymer studied. The constant n should have a value in the range of 1-10 for the adsorption to be classified as favourable (Cela-Pérez et al., 2011;Wolska and Bryjak, 2014;. The fitting of the experimental data by the Freundlich isotherm allowed to calculate the parameters a and 1/n, which also helped to determine whether the selected model is appropriate. The value of the calculated parameter a was 8.41•10 -8 for the synthesized resin. The values of n constant for the investigated resin were below 1 and it was 0.176 (therefore it can be assumed that the sorption of Ag(I) onto resin I is not favourable. Furthermore, by comparing the R 2 values for the Langmuir and Freudlich models, it can be seen that the second model is a better fit for Ag(I) adsorption onto the thiomorpholine modified resin. All results of the Langmiur and Freundlich analysis are given in Table 3.
The third investigated sorption model is the Dubinin-Radushkevich isotherm model, which helps to study the interaction between the sorbate and the sorbent (Cela-Pérez et al., 2011). This approach is generally used to distinguish the kind of sorption: the physical or chemical one dominates. The nonlinear form of this model can be represented according to Eq. 8 (Ulatowska, 2022): where qm is the maximum adsorption capacity of the material [mmol/g], KDR is the Dubinin-Radushkevich constant [kJ 2 /mol 2 ], ε is the Polanyi potential (Eq. 9): The linear form of this isotherm can be expressed by Eq. 10.
KDR is related to the free energy (E, [kJ/mol]) of adsorption per molecule of adsorbate when it is transferred to the surface of the solid from infinity (in the solution). The adsorption behavior could predict physical adsorption in the range of 1 -8 kJ/mol, where E is between 8 and 16 kJ/mol, the adsorption process followed by ion-exchange, and for more than 16 kJ/mol it is chemical adsorption. The free energy can be calculated by Eq. 11 (Cela-Pérez et al., 2011;Ulatowska, 2022).
During this analysis, the free energy values for resin I were calculated. For the investigated polymer, the value of E has a value greater than 16 kJ/mol, and it reached the value of 17.8 kJ/mol. This means that the chemical adsorption dominated. The linear coefficient of determination (R 2 ) value closest to 1 was obtained in the case of the Dubinin-Radushkevich isotherm model and therefore only on the basis of this model the maximum sorption capacity for resin I was calculated. The maximum sorption capacity of Ag(I) is 151.3 mg Ag/g dry resin, and this value is close to the experimental capacity obtained during the determination of the sorption isothrem. The parameters of the Dubinin-Radushkevich isotherm are given in Table 3. The maximum sorption capacity of AgCl4 3-obtained by the Dubinin-Radushkevich isotherm model is 120.4, 131.6, and 85.2 [mg Ag/g resin] for resins with 4-tert-butylpyridine, pyrrolidine, and 3-morpholinopropylamine ligands, respectively (Pilśniak-Rabiega and Wolska, 2021).
The OCS-ET-TU adsorbent based on a natural corn stalk modified by grafting epithiochlorohydrine and thiourea could selectively adsorb AgCl4 3-from the Ag(I) single and Ag(I)-Cu(II)-Ni(II) simulate nickel electrolyte system. The maximum adsorption capacity obtained from the Langmuir model was 107.9 mg/g in the single system and 79.94 mg/g in the ternary system .
Xiong (Xiong et al., 2016) investigated the adsorption of AgCl4 3-from a simulated nickel electrolyte using a corn stalk-based adsorbent modified by ammonia-thiosemicarbazide. The maximum adsorption capacity of the Ag(I) in single and Ag(I)-Cu(II)-Ni(II) ternary systems was obtained and calculated (Langmuir model) as 153.54 and 46.69 mg/g, respectively.
It could be found that thiomorpholine modified resin exhibited higher sorption capacity than most of the materials presented.

Effect of contact time
The effect of contact time on the removal of Ag(I) from the chloride solution on the thiomorpholine modified resin was studied at an initial concentration of 55.6 mg/dm 3 at room temperature. The dependence of sorption capacity and contact time is shown in Fig. 8. It can be seen that the removal of Ag(I) from the solution by the investigated polymer increased with the contact time until the equilibrium was attained. As shown in Fig. 8, the adsorption occurred more rapidly after 2 hours and after 5 hours gradually slowed down as it reached equilibrium. The experiments showed that the equilibrium was reached only after about 36 hours.
The results of the Ag(I) sorption rate on functional polymers presented in the literature are comparable to those obtained on the thiomorpholine-modified resin. The maximum AgCl4 3-adsorption on resins with 2-mercapto-1-methylimidazole and guanylthiourea ligands was reached within 48 hours (Pilśniak-Rabiega and Wolska, 2020). For the 2-mercaptobenzimidazole derivative of chitosan, the maximum Ag(I) adsorption was obtained after 28 hours (Elwakeel et al., 2021). The equilibrium uptake of AgCl4 3-was reached after 48 hours for functional polymers containing 4-tert-butylpyridine, pyrrolidine, and 3-morpholinopropylamine ligands (Pilśniak-Rabiega and Wolska, 2021). The rate-controlling step of the adsorption of Ag(I) ions was determined using five kinetic models. Sorption kinetics gives information on whether it occurs through diffusion, mass transfer, or chemical reaction. These studies explain the uptake of sorbate by the sorbent, which successively controls the time taken by the sorbate at the interface between the sorbent and the bulk solution (Haq et al., 2020).
To obtain all of this information, the sorption of Ag(I) in the first stage of the analysis was fitted to the diffusion model using the second Fick's law (Eq. 12 and 13), to find the rate (film diffusion or particle diffusion) determining steps for resin I (Juang et al., 2002;Kabay et al., 2007;Haq et al., 2020): where qt and qe represent the amount of adsorbed species (mg/g) at any time t and at equilibrium time, respectively, and ka represents the sorption rate constant (1/min). Sorption rate constant ka (1/min) can be calculated from the plot of − ( 1 − ) vs. time.
where kb is the sorption rate constant (1/min), qe and qt are the amount of adsorbed species (mg/g) at equilibrium and at time t. Sorption rate constant kb (1/min) can be calculated from the plot of − ( 1 − ( ) 2 ) vs. time. Table 4 gives the slope values, the linear coefficients of determination and the calculated values of ka and kb. Analyzes of ka and kb can show which of the processes of sorption is faster for the synthetized polymer. The analysis of the coefficients of determination can show what kind of diffusion mostly controlled the process. In the case of the investigated resin, the film diffusion controls the process mostly, but the differences in the R 2 values are not so significant; because of that, it was decided to use a different model that would also allow one to determine which stage of the sorption process is the decisive one. The limiting step of the Ag(I) adsorption process on resin I was also determined using the Weber-Morris interparticle diffusion model (Ulatowska, 2022). The equation of this model is described by Eq. 14. The kinetics of intraparticle diffusion is illustrated in Fig. 9 as the relationship of qt=f(t 0.5 ).
where kWM is the intraparticle diffusion rate constant (mg/g min 0.5 ) calculated from the plot qt=f(t 0.5 ). If adsorption occurred solely by intraparticle diffusion, then this relation (qt=f(t 0.5 )) would be rectilinear throughout, and the curve would pass through the origin of the coordinate system. The lack of linearity as in our case (see Fig. 9 and the broken line on the graph) indicates that several processes are involved in the adsorption process and not only intraparticle diffusion. The first section of this curve corresponds to the adsorption on the external surface of the adsorbent grain or the immediate adsorption stage. The second section corresponds to a gentle and gradual adsorption stage, where intraparticle diffusion is the stage that controls the rate of the entire adsorption process (Ulatowska, 2022). As shown in Fig. 9, none of the curves pass through the origin of the coordinate system, suggesting that the intraparticle diffusion is not the only limiting step in the process of Ag(I) adsorption from the solution to resin I. Furthermore, the dependence of qt on t 0.5 over the whole time range was not rectilinear, clearly indicating that the adsorption rate depends not only on the intraparticle diffusion. The calculated kWM values for individual stages are given in Table 4. The analysis of the results obtained suggests that the limiting stage for the Ag(I) adsorption process on the synthetized resin is the first stage, the kWM1 is the lowest and indicates that this stage affects the whole process of sorption. This intraparticle diffusion model also confirmed the previous considerations that the adsorption on the external surface of grains is this decisive stage and influences the effectiveness of the sorption process.  To predict the mechanism involved in the sorption process, several different kinetic models were applied. Among them, the sorption kinetics is usually described by simple kinetic models: by pseudofirst or pseudo-second-order models (Kabay et al., 2007, Santander et al., 2014. The pseudo-first-order model describes the relationship between the sorption rate and the equilibrium time. This model can be described by Eq. 15 (Kabay et al., 2007): where k1 represents the sorption rate constant (1/min), qt and qe represent the amount of adsorbed species (mg/g) at any time t and at the equilibrium time, respectively. Integrating Eq. 15 with respect to the boundary conditions q=0 at t=0, and q=qt at t=t, one obtains Eq. 16: Sorption rate constant k1 (1/min) can be calculated from the plot of log (qe -qt) vs. time.
The kinetic data can also be analyzed by means of pseudo-second-order kinetics. According to this model, the sorption behaviour is controlled by the chemisorption process occurring either electronic sharing or electronic exchange. The non-linear form of this model is represented by Eq. 17 (Kabay et al., 2007;Haq et al., 2020): where k2 is the pseudo-second-order rate constant (g/mg min), qe and qt are the amount of adsorbed species (mg/g) at equilibrium and at time t, respectively. Varying variables in Eq. 17, one gets from (Eq. 18): and integrating Eq. 18 for the boundary conditions q=0 at t=0, and q=qt at t=t, one obtains the final form (Eq. 19): A plot t/q versus t gives the value of the constants k2 (g/mg min). During these analyzes of both models, it is also possible to calculate qe (mg/g). The experimental data were fitted with Eq. 16 and 19 that served for the calculation of the characteristic parameters for each model. Linear regression analysis (coefficient of determination (R 2 )) was used to analyze the linear form of the kinetic models. The kinetic constants k1 and k2 were determined using the slope and intercept values of the linear plots (see Table 5) which presented pseudo-first-order and pseudo-second-order rate constants, calculated and experimental qe values for Ag(I) sorption on resin I. The results of the kinetic analysis are given in Table 5. Based on the high values of the coefficient of determination (R 2 ), it can be concluded that the experimental data obtained during the study of the kinetic of Ag(I) adsorption on the thiomorpholine modified resin are better described by the pseudo-first-order model than by the pseudo-second-order model. Additionally, the calculated values of the adsorption capacity from the pseudo-first-order model (q1) are closer to the experimental values (qexp) than from the pseudo-second-order model (qmax2), thus indicating the physical nature of the Ag(I) sorption process in the studied systems rather than the chemical one (see Table 5).
The adsorption of Ag(I) from an aqueous solution on the chelating polymer derived from 3-amino-1,2,4-triazole-5-thiol and glutaraldehyde was also perfectly fit the pseudo-first-order model (Abd El-Ghaffar et al., 2009).
The recovery of Ag(I) from chloride solution was studied using functional polymers containing 2-mercapto-1-methylimidazole and guanylthiourea ligands. Kinetic studies indicated that the adsorption reaction follows the pseudo-first-order kinetics (Pilśniak-Rabiega and Wolska, 2020).

Desorption of Ag(I) from the resin and resin's stability in cycles of sorption/desorption
For the thiomorpholine modified resin, desorption and resin's stability in cycles of sorption/desorption were studied. In this work, the Ag(I) was desorbed by different eluents: 0.50% sodium thiosulphate solution, 0.50% thiourea solution, 1.0% potassium cyanide solution, 1.0% potassium cyanide solution in 0.50% hydrogen peroxide solution, 0.50 M sodium hydroxide solution and ammonium buffer (2.86 M (NH4)2SO4, 0.380 M NH3·H2O). This process was carried out at room temperature (23±2 o C) and at elevated temperature (50 o C). The results are given in Table 6. The eluents more effectively desorbed Ag(I) from resin I at elevated temperature, especially 1.0% potassium cyanide solution in 0.50% hydrogen peroxide solution. It seemed probable that during the desorption of Ag(I), cyanide anions formed Ag(CN)4 3-complexes by the ligand exchange mechanism. The silver-cyanide complex, Ag(CN)4 3-, is the most stable complex ion formed by Ag(I), and the logarithm of the stability constant, log β4, was estimated to be 22.3, for AgCl4 3-the value of log β4 corresponds to 5.30 (Högfeldt, 1982). The values of log β4 for cyanide and chlorine complexes may suggest that chlorine ligands, Cl -, were substituted for cyanide anions, CN -, in Ag(I) complexes, during Ag(I) desorption from the synthesized resin. On the other hand, Ag can be present on the surface of resin I as highly disintegrated metallic silver, which is weakly connected to the polymer and can be better eluted by 1.0% KCN under oxidizing conditions.
To verify the stability of the resin, five cycles of Ag(I) sorption of chloride solution (60.0 mg Ag/dm 3 , 4.0 M NaCl, 0.10 M HCl) and desorption with 1.0% KCN in 0.50% H2O2 were performed on the modified resin thiomorpholine. It could be concluded from these results (see Fig. 10), that the Ag(I) sorption ability increased, the sorption efficiency was 67.3% (13.9 mg/g) in the first cycle, in the case of 2-5 cycles it was 85.2-97.3% (17.6-20.1 mg/g). The desorption efficiency was 35.6-36.2% in 1-5 cycles.
The better Ag(I) sorption on the synthesized resin in 2-5 cycles than in the first cycle can be explained by the presence of cyanide anions on the polymer surface after the first desorption and the formation of Ag(CN)4 3-complexes by ligand exchange mechanism. Cyano complexes of Ag(I) are more stable than chloro complexes of Ag(I). The thiomorpholine modified resin was not fully regenerated, nonetheless sorption of Ag(I) was effective. The sorbents with heterocyclic ligands, i.e. 4-tertbutylpyridine, pyrrolidine, and 3-morpholinopropylamine, were stable in subsequent sorption and desorption processes of Ag (I) ions from their surface (Pilśniak-Rabiega and Wolska, 2021). Sorption capacity of functional polymers for Ag(I) ions increased significantly after the first adsorption-desorption cycle (by 49-64% for the second cycle). Desorption efficiency in 1-5 cycles was 62-97%. The 2-MBI-chitosan sorbent showed sorption and desorption stability in five consecutive recycling cycles. Desorption was highly efficient (greater than 98%) and stable along a minimum of five cycles, while sorption slightly decreases with recycling (about 13-16% at the fifth cycle) (Elwakeel et al., 2021).

Ag(I) sorption from synthetic chloride solution
To determine the selectivity of the thiomorpholine modified resin for Ag(I) and Cu(II), static sorption tests were carried out from a synthetic chloride solution (4.0 M NaCl, 0.10 M HCl) containing Ag(I) and Cu(II) anions. The concentrations of Ag(I) and Cu(II) were 60.0 and 32.1 (0.556, and 0.505 mmol/dm 3 ), respectively. The selectivity coefficient (αsel.) was calculated as the quotient of the distribution coefficients calculated for Ag(I) and Cu(II). The results are presented in Table 9. It could be seen that the featured resin showed preference for AgCl4 3-complexes over CuCl4 2-anions. The value of the selectivity coefficient, αsel., was 194. Quantitative separation of the two ions is achieved, when the corresponding values of the selectivity coefficients comprise 10-30 at least (Marhol, 1982). Thus, the thiomorpholine modified resin is a special material for the selective removal of Ag(I) ions from the Ag(I)-Cu(II) binary system (see Table 7).
The high affinity of the thiomorpholine modified resin to Ag (I) could be explained as a result of the presence of ligands with S-and N-donor centres, which had a high affinity for "soft" Ag, but a minor affinity towards Cu, which is known as a borderline metal and is "harder" than Ag. On the other hand, it could be caused by the fact that resin I prefer complexes, which have the highest density of negative charge, what should increase their hydration requirements and facilitate their transport through the VBC/DVB matrices of resin. The Ag(I) and Cu(II) chloro complexes have a tetrahedral structure. The complex of AgCl4 3-has a higher density of negative charge (three charges are carried over by five atoms) than the CuCl4 2-complex.  The resins with the 2-mercapto-1-methylimidazole and guanylthiourea groups showed good selectivity for Ag(I) ions in the Ag(I)-Cu(II) binary system (Pilśniak-Rabiega et al., 2020). The values of the selectivity coefficient (αsel.) were 17.3 and 35.4, respectively.
Functional polymers with imidazole ligands showed preferential sorption of Ag(I) in the presence of Cu(II) from synthetic chloride solution (Pilśniak-Rabiega et al., 2019). The chloro complexes of Cu(II) were not sorbed by these sorbents from a solution containing Ag(I) and Cu(II) anions, each of concentration 0.56 mmol/dm 3 .
The melamine resin modified with thiourea (R1) showed high selectivity towards Ag(I) from a binary mixture with Cu(II) (Abd El-Ghaffar et al., 2009). The value of the selectivity coefficient for the adsorption of Ag(I) over Cu(II) is 19.68.

Ag(I) sorption from real chloride leach solution
The thiomorpholine modified resin is characterized by a high affinity to Ag(I) from synthetic solutions. Therefore, this functional polymer was used to recover Ag(I) from the real solution.
The 2-mercaptobenzimidazole-chitosan sorbent has been used for the recovery and separation of precious metals (Ag, Au, and Pd) from base metals (iron, copper, aluminum, and tin) in acidic leaching solutions. The results showed the high potential of this material for Ag (and Au, Pd) even at low concentrations of highly complex metal solutions with large base metal collection (in large molars excess, ~67 times) (Elwakeel et al., 2021).