Gold adsorption on polyamine polymer

Keywords: Gold, recovery, adsorption, chelate resin, FTIR, XPS, flame atomic absorption spectrometry. In this study, the effect of linear alkyl amine length in the triazine polymer on the adsorption of Au(III) ions from chloride containing solutions was investigated. For this purpose, 1,3,5-triazine-triethylenetetramine (TETA), 1,3,5-triazine-pentaethylenehexaamine (PEHA) and 1,3,5-triazine-ethylenediamine (EDA) polymers was used in this study. Synthesis and characterization of TETA and PEHA was given in our previous studies. 1,3,5-triazine-ethylenediamine (EDA) polymer was newly synthesized in this study. The polymers were characterized by Fourier transform infrared (FTIR) spectroscopy, TGA for their thermal resistance, X-ray photoelectron spectroscopy (XPS) for investigation of the Au(III) adsorption mechanism, SEM and EDX for observation of changes on the polymer surfaces before and after Au(III) adsorption. The adsorption of Au(III) ions from chloride ion containing solutions by studying of effective factors including pH, pCl, contact time, temperature and initial concentration. After optimization of the adsorption procedure, kinetic, isotherm and thermodynamic calculations were made. Column studies were carried out to investigate reusage of the polymers by eluting adsorbed Au(III) ions with acidic thiourea solution. The synthesis of 1,3,5-triazine-ethylenediamine (EDA) polymer was performed under following conditions. Potassium carbonate, 10 g, was added to 50 mL of THF in a three-necked flask and the solution was cooled in an ice bath. In one neck of the flask of cyanuric chloride (7.5 g) in 50 mL THF was connected. In another neck of the flask ethylenediamine (9.0 g) in 50 mL THF was attached. The cyanuric chloride and ethylenediamine solutions were slowly added by dropping at flow rate of 1-2 mL min− 1 while the content of the flask was constantly agitated with a mechanical stirrer. The stirring of the reaction flask was continued to 24 h by cooling with an ice bath. After reaction period, THF was removed from the flask using rotary evaporator. The reaming polymer in the flask was washed with deionized water (3x100mL) and acetone (3x50 mL), respectively and then, the obtained 1,3,5-triazine-ethylenediamine (EDA) polymer dried in a vacuum oven at 60 °C for 24 h. The synthesis of TETA and PEHA polymers were given in our previous studies. Before using of the polymers for Au(III) adsorption from chloride containing solutions, all polymers were ground by mortar and then sieved. The particles between 150–500 μm were used in the studies. Au(III) adsorption capacity of the triazine polymers were tested by batch adsorption experiments. The effect of pH, pCl, contact time, temperature and initial concentration were studied. The amounts of EDA, TETA, and PEHA polymers in the batch studies were used to be 10 mg. The pH and pCl of Au(III) solutions at 25 and 50 mg/L were adjusted to the needed value and batch adsorption experiments were performed at a constant shaking rate using a orbital shaker or mechanical stirrer. The kinetic studies were carried outusing 1000 mL of the Au(III) solution and all the other experiments were done using 50 mL of the solution. All samples taken for the measurement of Au(III) level were centrifuged and filtered. Au(III) levels in the solutions were quantified using a Shimadzu 6711F flame atomic adsorption spectrometer (FAAS). The adsorbed Au(III) ions on the per gram of the polymers were calculated based on the mass-balance. In the column studies, 100 mg of the prepared EDA, TETA and PEHA polymers were filled to a glass column (10 cm in length and 0.7 cm in diameter). Au(III) solution (50 mL) of containing 1 mg/L of Au (III) at pH = 2 and pCl = 1 was passed through the column at definite flow rate 1 mL min-1. The retained Au(III) on the columns was eluted using 3% thiourea in 1 M HCl solution at flow rate 1 mL min-1 and then, Au(III) concentrations in the eluates was quantified using FAAS (n=5). The EDA and TETA polymers were characterized by SEM, EDS, BET and TG/DTA instruments and characterization of PEHA were given in our previous paper [3]. According to SEM images, EDA polymer has two different types of surface properties that are porous and less porous resulting low BET surface area of the EDA polymer. Nitrogen adsorption-desorption isotherms of EDA, fits type 1 physical adsorption (physisorption) isotherm of IUPAC classification. According to nitrogen adsorption and desorption of the TETA and PEHA sorbents, it can be concluded that type 1 isotherm shows H3 hysteresis. Brunauer–Emmett–Teller (BET) surface areas of the EDA, TETA and PEHA were found to be 3.204, 30.75, and 10.24 m2/g, respectively. The average pore size of the EDA, TETA and PEHA polymers were calculated to be 5.2, 31.81, and 15.93 nm, respectively. So, EDA, TETA and PEHA polymers have a mesoporous. The TETA polymer has a bigger pore the others and hence it expected that the TETA has higher capacity. According to EDS, elemental analysis of EDA polymer contains 49.9% C, 41.8% N, 4.4% O, 4.4% Cl, and 0.4% K. And TETA and PEHA polymers contains C and N elements. Although amine structures in the TETA and PEHA polymers have different lengths, the TETA polymer contains a higher percentage of N atoms according to EDS results. In order to test the stability of the polymers in high acidic conditions, EDA, TETA and PEHA polymers were treated with 5 M HCl for 24 h. Their mass loss was not observed. So, it can be concluded that polymers are stable at high HCl concentration at room temperature. According to thermal analysis results, mass loss of the EDA polymer (5.3%) up to heating at 449 K (176 °C) arises from the loss of moisture in the pores. TETA polymer mass was decreased 7.1% at 395 K (122 °C). The main decomposition of the EDA and TETA polymer structures starts at 596 K (323 °C) and 514 K (241 °C), respectively. PEHA polymer was not decomposed until 604 K (331 °C). In the effect of pH studies, pH values of the samples containing 25 or 50 mg/L Au(III) were adjusted between 0.0 and 5.0 using dilute HNO3 and NaOH solutions. According to results, it can be concluded that the increasing the pH affects adsorption positively. Au(III) adsorption decreased lower pH than 1.0. The Au(III) adsorption was nearly equal between pH 2.0 and 5.0. Hence, pH = 2.0 was selected to be optimum value for Au(III) adsorption on the EDA, TETA and PEHA polymers. In order to investigate the effect of chloride ions on the Au(III) adsorption, the experiments were carried out at pH = 2.0 and the level of chloride level was changed between 0.1 and 1.0 M. The increase in chloride concentration caused a decreasing in Au(III) adsorption capacity of three polymers. Among the polymers, EDA is the most affected by chloride ions. For all polymers, 0.1 M chloride concentration was determined to be optimum. According to the results of the experiments on the Au (III) adsorption kinetics by EDAand PEHA, 97% adsorption of Au (III) onto EDA and PEHA was completed in 10 h.The contact time of 24 h were enough for TETA. So, 24 h was defined as the optimum time due to practical reason for adsorption of Au(III) on EDA, TETA and PEHA polymers. Pseudo first-order, pseudo second-order, and intraparticle diffusion equation models were used in the study to simulate the Au(III) adsorption kinetics by1,3,5-triazine-triethylenetetramine (TETA), 1,3,5-triazine-pentaethylenehexaamine (PEHA) and 1,3,5-triazine-ethylenediamine (EDA) polymers. The results showed that the EDA polymer has revealed different kinetic properties from TETA and PEHA polymers during adsorption of Au(III) ions and intraparticle diffusion equation with r2of 0.9722 presents the adsorption is fitted by pseudo second-order. This shows that the calculated theoretical qe value is close to the experimental qe value. The Au(III) adsorption kinetics by 1,3,5-triazine-triethylenetetramine (TETA) and 1,3,5-triazine-pentaethylenehexaamine (PEHA) polymers is the best represented by pseudo-second-order. In order to search, the impact of the initial concentration of Au(III) on the Au(III) adsorption using 1,3,5-triazine-triethylenetetramine (TETA), 1,3,5-triazine-pentaethylenehexaamine (PEHA) and 1,3,5-triazine-ethylenediamine (EDA) polymers, the experiments were performed at different initial Au(III) concentrations between 50 to 400 mg/L with shaking time of 24 h. Au(III) adsorption percentages of the 1,3,5-triazine-triethylenetetramine (TETA), 1,3,5-triazine- pentaethylene hexaamine (PEHA) and 1,3,5-triazine-ethylenediamine (EDA) polymers were found to be between 18–66%, 48–90% and 52–94%, respectively. The Au(III) adsorption percentage was the highest in then solutions containing of 50 mg/L Au(III) and was found to be at 94% by 1,3,5-triazine-pentaethylenehexaamine (PEHA)polymer. The results revealed that the 1,3,5-triazine-pentaethylenehexaamine (PEHA) polymer has the highest Au(III) adsorption capacity. Langmuir, Freundlich and Redlich-Peterson (R-P) isotherm equations were used to investigate Au(III) adsorption equilibrium by 1,3,5-triazine-triethylenetetramine (TETA), 1,3,5-triazine-pentaethylenehexaamine (PEHA) and 1,3,5-triazine-ethylenediamine (EDA) polymers. Langmuir isotherm is more acceptable for Au(III) adsorption isotherm by the all polymers based on the correlation coefficients. The maximum Langmuir monolayer Au(III) adsorption capacities of 1,3,5-triazine-ethylenediamine (EDA), 1,3,5-triazine-triethylenetetramine (TETA) and 1,3,5-triazine-pentaethylenehexaamine (PEHA) polymers are 548.2, 1002.8 and 1086.4 mg/g, respectively. To investigate the effect of temperature on the Au(III) adsorption using EDA, TETA and PEHA polymers, 10 mg of the polymers were added 50 mL of solutions containing 200 mg/L Au(III) and the suspensions were shaken for 24 h at 298, 308, 318 and 328 K (25, 35, 45 and 55 °C) temperatures. Au(III) adsorption onto EDA polymer was exothermic, and by using TETA and PEHA polymer Au(III) adsorption was found to be endothermic,may be due to the chemical interaction types. The reusability of sorbents as an adsorbent is of critical importance as it will reduce the cost in the industry. In order to investigate the reusage of EDA, TETA and PEHA polymers, the adsorption and stripping efficiencies were performed. It was found that be seen that all Au(III) ions adsorbed on all polymers could be completely separated from the polymers again. In addition, there is no decrease in cycle efficiency even after 5 cycles. Additionally, the acidic thiourea solutions as most popular eluent for the stripping Au(III) ions was used in the present study. It was concluded that Au(III) ions adsorbed on EDA, TETA and PEHA polymers could be successfully stripped with 3% thiourea solution in 1 M HCl. The high Au(III) adsorption capacity of newly synthesized 1,3,5-triazine-ethylenediamine (EDA), 1,3,5-triazine-triethylenetetramine (TETA) and 1,3,5-triazine-pentaethylenehexaamine (PEHA) polymers and their reusability, their high resistance to heat and their easily regeneration with acidic thiourea solutions means that they could easily be used as efficient and high capacity adsorbents in the recycling of Au from wastes.