Bee-Min Goh

New insights into the adsorption of aurocyanide ion on activated carbon surface: Electron microscopy analysis and computational studies using fullerene-like models

Despite decades of concerted experimental studies dedicated to providing fundamental insights into the adsorption of aurocyanide ion, Au(CN)2(-), on activated carbon (AC) surface, such a mechanism is still poorly understood and remains a contentious issue. This adsorption process is an essential unit operation for extracting gold from ores using carbon-in-pulp (CIP) technology. We hereby attempt to shed more light on the subject by employing a range of transmission electron microscopy (TEM) associated techniques. Gold-based clusters on the AC surface are observed by Z-contrast scanning TEM imaging and energy-filtered TEM element mapping and are supported by X-ray microanalysis. Density functional theory (DFT) calculations are applied to investigate this adsorption process for the first time. Fullerene-like models incorporating convex, concave, or planar structure which mimic the eclectic porous structures on the AC surface are adopted. Pentagonal, hexagonal, and heptagonal arrangements of carbon rings are duly considered in the DFT study. By determining the favored adsorption sites in water environment, a general adsorption trend of Au(CN)2(-) adsorbed on AC surface is revealed whereby concave > convex ≈ planar. The results suggest a tendency for Au(CN)2(-) ion to adsorb on the carbon sheet defects or edges rather than on the basal plane. In addition, we show that the adsorption energy of Au(CN)2(-) is approximately 5 times higher than that of OH(-) in the alkaline environment (in negative ion form), compared to only about 2 times in acidic environment (in protonated form), indicating the Au extraction process is much favored in basic condition. The overall simulation results resolve certain ambiguities about the adsorption process for earlier studies. Our findings afford crucial information which could assist in enhancing our fundamental understanding of the CIP adsorption process.

Thermal Degradation of Rice Husks in Air and Nitrogen: Thermogravimetric and Kinetic Analyses

Abstract In this study, thermogravimetric and kinetic data of thermal degradation of Malaysian rice husks under air or nitrogen atmospheres are analyzed. Determination of elemental composition, ash content, and gross calorific value of Malaysian rice husks and their comparison with other biomass fuel are also conducted. It is found that Malaysian rice husks have a gross calorific value of approximately 15 MJ/kg. Thermogravimetric analysis shows that significant thermal degradation occurs within temperature ranges of 230–380°C (nitrogen) and 230–540°C (air), which is primarily attributed to decomposition of hemicellulose and cellulose. Linear regression analysis using LINEST function in Microsoft Excel indicates that the nitrogen atmosphere data (order of reaction = 0.44) fit the Arrhenius model better than the air data (order of reaction = 0.22). The data obtained from our study can be used for preliminary evaluation of rice husks either as a combustible biomass or source for a small-scale thermochemical conversion system.