Selective Adsorption Mechanism of Gold by Grape Peel AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY22040035

Chinese Library Classification Number:TF111

Citation Information:

Among the biosorbents for precious metal recovery，polyphenols have always been a research hotspot. Among them，tannins containing phenolic substances on the surface of persimmon skin have always been a research hotspot，but tannins soluble in water are not conducive to solid-liquid separation，so loading persimmon tannins on the surface of resin has always been the focus of research. However，the high cost of loading modification is not conducive to its popularization and application in industrial production. Grape skin waste generated during wine preparation accounts for 25%~35% of the total consumption of grapes and contains a large number of phenolic substances. Currently，there are few studies on the absorption and recovery of gold using grape skins. This paper proposed the selective adsorption of gold using grape skins containing polyphenols. The grape peel was prepared by a hydrothermally treatment of the mixture of raw grape peel particle and water at 120 ℃ for 12 h with the solution with modifier. Then the above-mentioned suspension was filtrated and the peel was washed 3 times with distilled water and then dried at 80 ℃ for 12 h. A series of adsorption experiments were carried out and showed that only 30 min could reach a high adsorption efficiency and adsorption capacity of gold. With the prolonging of adsorption time，the adsorption efficiency of gold would gradually approach to a platform. With the increasing of the temperature，the adsorption efficiency and the adsorption capacity of gold would slightly be increasing. Above experimental data were used for fitting by pseudo-first-order and pseudo-second-order kinetic equations，and it could be found that they fit the latter model well. By comparing the adsorption capacity of the theoretical calculation and the experimental result，it could be found that the results obtained based on the pseudo-second-order model were more identical，meaning that this model was more appropriate to describe the adsorption process. The kinetic parameters of adsorption of grape peel from the pseudo-second-order model were used for fitting by Arrhenius equation，and it could be found that they fit the model well. The adsorption reaction activation energy on grape peel for gold was 2.1925 kJ·mol⁻¹. With HCl concentration from 0.5 to 6 mol·L⁻¹，the adsorption efficiency of gold was kept at 98.01% to 97.13%，and the adsorption of Fe，Zn and Cu was kept at 0 to 2.31%，0 to 16.05%，1.05% to 12.57%，showing a gradual increasing trend when HCl concentration increased. The concentration of HCl had a litter impact on the adsorption of Fe，Zn and Cu. By comparison，grape peel adsorbent had great selective adsorption for gold，even in the condition of high HCl concentration. The maximum adsorption capacity for gold in 0.1 mol·L⁻¹ HCl media was assessed to be 34 g_Au·kg⁻¹ at room temperature. Langmuir and Freundlich models were used to fit the isotherm adsorption data，and it was found that the Langmuir model could better describe the adsorption process. Oxidation and reduction potential （ORP） value of the solution during the adsorption process would decrease drastically，which provided another proof of the reduction occurring in the adsorption of gold ions towards the grape peel adsorbent. The higher temperature，the faster decrease of ORP value. Scanning electron microscopy （SEM） images demonstrated that grape peel surface was slightly plush，after adsorption of gold，it turned to the surface of the adsorbent with a number of tiny gold particles. According to X-ray diffraction （XRD） and polarizing microscope，the polyphenols in frape peel successfully reduced the gold ions in the solution to obtain solid gold particles. According to Fourier transform infrared spectroscopy （FTIR），it was easy to see that the intensity of C=C bond assigned for the stretching decreased，thus demonstrating the oxidation of phenolic groups to carbonyls. On the other hand，it was evident to observe that C=O bond also decreased after adsorption of gold. The above results indicated that the polyphenols on the surface of grape peel reduced the gold ions into the adsorption solution. According to X-ray photoelectron spectroscopy （XPS），it could be confirmed that after protonation in the acidic solutions，OH bond of polyphenol in grape peel was positively charged and adsorbed the negatively charged ${\text{AuCl}}_{\text{4}}^{\text{-}}$ from the solution. Afterwards，C=C bond from polyphenol contained the large π bonds would donate rich electrons to reduce the adsorbed ${\text{AuCl}}_{\text{4}}^{\text{-}}$ into the gold element. More than 99.9% gold ions would be reduced to be the gold element after adsorption，and the large π bonds in the aromatic structure of polyphenol were believed to be the dominant electron donator responsible for the reduction of gold，and it showed that the adsorption mechanism of gold adsorption on grape peel was the electrostatic attraction coupled with reduction reaction. Therefore，the adsorbent could selectively adsorb gold and had a large adsorption capacity for gold，and this research put bio sorbents into industrial production to provide guidance research.