Dr. Svetlana Yefremova

Abstract:

In sorption processes, one of the characteristics of the interaction between a carbon sorbent and an extracted compound is the distribution of the latter on the sorbent surface. The aim of this work was to study the distribution of noble and rare metal ions on the surface of carbon sorbents using scanning electron microscopy (SEM) with energy dispersive (EDS) and wavelength dispersive (WDS) spectroscopy. Activated carbon products obtained from special coke fines (CBCS) and rice husk (RH_p-850VA) were selected as objects of study. 

CBCS was produced according to the method written elsewhere [1]. Fines of special coke (+2-5 mm) were elutriated in water with slow stirring for 15 min. The wet carbon-containing material was activated with water vapor at 850 °C for 30 min. The furnace heating was turned off at the end of the activation process while water vapor was continued to be supplied to the reactor for another 30-40 min to cool the activated product to 500 °C. Then the vapor supply was stopped and the prepared material was kept in the reactor until cooling to a temperature of 60 °C. 

To produce RH_p-850VA, rice husk was washed with water, dried, pyrolyzed at 450 °С for 30 min, activated with water vapor at 850 °С for 30 min, and boiled with 70 g dm^-3 sodium hydroxide solution at a Solid (g) : Liquid (cm³) (S:L) ratio of 1:10 for 90 min. Then it was washed with distilled water until the wash water was neutral and dried at 150 °С for 1 h. 

Realistic production solutions obtained in gold-bearing ore (2.5 mg dm^-3 of Au ions) and lead production cakes (640 mg dm^-3 of Re ions) processing were used. Gold and rhenium adsorption to load sorbents was carried out under dynamic conditions. Gold- and rhenium-containing solutions were passed through 20 cm³ columns filled with CBCS and RH_p-850VA, respectively. The experiments were continued until the concentrations of metal ions in the filtrates became equal to their concentrations in the initial solutions.

The sorbents before and after the sorption processes were tested using SEM and EDS, WDS mappingby elements. It was determined that both sorbents differ each other by their structures. Although the main element of the studied materials was carbon, carbon particles had different shapes and structures. CBCS was a porous material with a developed porous structure. The pores are predominantly round and/or oval in shape and up to 20 microns in size. As shown in [1], nano-sized pores were not detected. The RH_p-850VA sample had a microfibrous structure. The pore space was formed by pores located between carbon fibers.

A correspondence in the distribution of such elements as carbon, oxygen, sulfur and gold was recorded by element EDS and WDS mapping of the CBCS sample after contact with a gold-containing solution. A similar distribution patterns of carbon and oxygen can be explained by the presence of C_xO_y complexes formed on the sorbent surface during its activation [2]. The identical nature of the distribution of sulfur and gold may indicate the sorption of gold in the form of [Au(S)₂]⁻ complexes. The latter were most likely formed during the leaching of gold from ore, since the sulfur concentration in the special coke fines-based sorbent is insignificant [1], and it cannot affect the sorption process [2].

According to results of EDS and WDS mapping by elements of the RHp-850VA sample after contact with production rhenium-containing solution, rhenium ions were on the carbon fiber surface. Element EDS mapping showed a great compliance between distribution patterns of oxygen and rhenium and iodine and rhenium. The distribution of other elements (chlorine, sulfur) was not associated with the carbon surface. They were present on carbon surface and filled the pore space as well. But element WDS mapping, as more sensitive method, confirmed an absolute accordance between distribution patterns of oxygen and rhenium. This fact suggests that rhenium is distributed on the carbon fiber surface in combination with oxygen obviously in the form of ReO₄^-1 although Re₂I₈^-2 clusters can be present as well. 

The obtained results on the distribution of gold and rhenium ions on the surface of special coke fines- and rice husk-based sorbents indicate the interaction between adsorbates and functional groups active in relation to them. Ion exchange processes may occur. These findings are required to be confirmed by other research methods.

This study is funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (grant number AP 19677767).