Mechanism of the Removal of AuCl⁻₄ Ions from Aqueous Solution by Means of Peat Soil Humin

Abstract

Peat soil humin, obtained after the removal of fulvic and humic acid fractions according to the recommended procedure of the International Humic Substances Society (IHSS), has the ability to remove AuCl-4 ions from aqueous solution. The removal was greatest at medium acidity, which was equivalent to pH 2.0, and it was governed by two different processes, i.e. attachment onto un-ionized-COOH groups through hydrogen bonding and reduction to Au metal by the action of the-OH group. Purification of humin through repeated immersion and shaking in a mixed solution consisting of 0.1 M HCl and 0.05 M HF led to the abrupt increase in the-COOH contents from 386 mmol/kg to 2510 mmol/kg, while the content of total-OH functional groups remained relatively constant at values slightly higher than 3000 mmol/kg. The increase in-COOH content improved the removal of AuCl-4 ions from 23 mg/g to 27 mg/g, but it was unfortunately accompanied by a diminished reduction ability to Au metal. Hence, the role of the-COOH functional group in removing AuCl-4 ions through hydrogen bonding was more significant for the purified humin, while that of the-OH group in removing AuCl-4 ions through reduction to Au metal was better for the crude (unpurified) humin.

The Role of Carboxyl and Hydroxyl Groups of Humic Acid in Removing AuCl₄^- from Aqueous Solution

“…1230 and 1093 cm −1 after purification may be interpreted as the result of the removal of inorganic components such as SiO2 from HA. As observed for humin, SiO2 and Al2O3 were the main impurities of crude humin and these absorption bands were suppressed after purification using mixed solution consisting of HCl and HF [25]. After the purified humin interacted to AuCl4 -, the absorption band at 1093 cm -1 but not 1230 cm -1 was enhanced again.…”

“…The ability of this purified HA in removing AuCl4 -was higher than the sorption capacity of humin from peat soil of Amuntai, South Kalimantan, i.e. 27 mg/g (0.08 mmol/g) [25], but considerably lower than that of thiol-modified Fe3O4 (85 mg/g or 0.25 mmol/g) [51] and raw date pits (78 mg/g or 0.23 mmol/g) [52].…”

“…Accordingly, it is reasonable to assume that the removal of AuCl4 -by the -COOH group in HAs should proceed through the formation of hydrogen bridges (eq. 1) as occur for humin [25]. This assumption is surely more realistic than, for instance, a ligand-exchange mechanism that leads to the coordination of Au(III) onto ionized -COO -groups [3,37], since in acidic media (e.g.…”

“…The attached Au(III) may then be reduced to Au metal by the phenolic -OH group. For peat soil humin, the ability to attract and then reduce AuCl4 − was obviously observed [25]. It is believed that peat soil humin employs its -COOH group to attract AuCl4 -and subsequently, reduce the attached AuCl4 -to Au metal by the action of its -OH group.…”

Comparative Study on the Adsorption of [AuCl₄]^– onto Salicylic Acid and Gallic Acid Modified Magnetite Particles

“…Santosa et al [5] pointed out that humin had the ability to remove [AuCl4] -from the aqueous solution. The role of COOH functional group in removing [AuCl4] -through hydrogen bonding is more significant for the purified humin, while that of OH group in removing [AuCl4] -through reduction to Au metal is better for the crude (unpurified) humin.…”