Mechanism of Gold Adsorption by Persimmon Tannin Gel

Abstract

IntroductionRecently, attention has been focused on the recovery of gold from aqueous solutions from the standpoints of gold recovery from industrial resources, such as electronics parts and plating materials. [1][2][3][4][5][6][7] Previously, one of the authors reported that persimmon tannin (PT) gel, a newly developed biosorbent, adsorbed large amounts of gold from a solution containing hydrogen tetrachloroaurate(III). 8 The gold adsorption by the gel was rapid, endothermic, and obeyed the Langmuir isotherm. On the other hand, the gel also adsorbed large amounts of uranium, 9 iron, 10 and vanadium. 11 The gel contained a number of adjacent hydroxyphenyl groups, which are suitable ligands for hard acids, such as uranyl, iron(III), and vanadyl ions, based on the HSAB idea. 12,13 Because Au(III) ion is softer than these cations, hydroxyphenyl groups in the gel are not suitable.Preliminary experiments revealed that colloidal Au(0) was formed in a HAuCl4 solution containing PT gel. In this note, therefore, further experiments were conducted to analyze the mechanism of gold adsorption. ExperimentalThe persimmon tannin (PT) solution used throughout this study was obtained from Tomiyama Shoten, Kyoto. PT gel was prepared as previously described. 8,11 Hydrogen tetrachloroaurate(III) tetrahydrate and other chemicals used for this study were obtained from Nacalai Tesque, Inc. (Kyoto, Japan). Gold adsorption experiments in a column system were conducted as follows: 1000 ml of a solution (pH 3) containing 0.1 mM HAuCl4 was passed through a column (diameter 8 mm, bed volume 8.9 ml) of PT gel at 150 and 300 ml/h, collecting 100 ml for each fraction. The pH of the gold solution was adjusted to 3 by 0.1 M HCl and 0.1 M NaOH. To avoid an optical reduction of Au(III) ion, adsorption experiments were conducted under dark conditions. The gold content in each fraction was determined by a inductively coupled plasma quantometer (Shimadzu ICPQ 1000II). A colloidal Au(0) solution was prepared by adding 20 µM of formaldehyde to a solution containing 0.05 mM HAuCl4. After adjusting the pH to the desired value by 0.1 M HCl and 0.1 M NaOH, the mixture solution was allowed to stand. The time when the gold solution became colored purplish red was recorded as the time for colloid formation. Fifty milliliters of the resulting colloidal Au(0) solution were passed through a column (diameter 8 mm, bed volume 5 ml) of PT gel at 100 ml/h. The gold adsorption ratio (%) was estimated from the gold content in each elute. The optical absorption spectra were measured using an absorption spectrophotometer (Japan Spectroscopic Co. Ltd., JASCO V-550). Powder X-ray diffraction (XRD) patterns of PT adsorbed gold were recorded with a powder X-ray diffractometer (Shimadzu XD-D1) using Cu Kα1 radiation (λ = 1.54056 Å) over a 2θ range from 1˚ to 80˚. Results and DiscussionThe effect of the flow rate on gold adsorption by PT gel was examined in a column system, the results of which are listed in Table 1. When the HAuCl4 solution (pH 3) was passed through the PT g...

Isolation of humic acid from peat soil and its application as an adsorbent for AuCl4− in solution

“…This maximum λ is similar to the maximum λ of AuCl 4 -stated in Paclawski and Sak [8] which is 314 nm.…”

“…The samples were interacted at room temperature for 24 h and filtered afterward. The AuCl 4 -remained in each solution was measured using UV-visible spectrophotometer (Shimadzu UV-1700 PharmaSpec).…”

“…Ten milligrams of HA was interacted with various concentrations of AuCl 4 -solutions: 30, 50, 150, 200, and 250 ppm at pH 2 for 24 h. The solutions then were filtered and analyzed using UV-Vis spectrophotometer. The effect of initial concentration data were analyzed to determine the adsorption isotherm model using Langmuir and Freundlich models.…”

“…Nakajima et al and Parajuli et al had successfully adsorbed and reduced gold ions using tannin from persimmon and persimmon peel gel. Tannin is a macromolecule rich in phenolic -OH group [4,5].…”

“…Similar to tannin of persimmon, HA is expected to be useful as an adsorbent for the recovery of gold. Therefore, this research aims at isolating the HA from peat soil in South Kalimantan, Indonesia and applying it as an adsorbent for AuCl 4 -ions in the solution.…”