Adsorption of heavy metal ion species from aqueous solution in activated carbon

Abstract

Activated carbons are used widely for the adsorption of environmentally unfriendly species from both liquid and gas phases, the separation of gases and adsorption of species from aqueous solution. Examples of the processes are the adsorption of Au(CN)2 and Ag(CN)2 from aqueous solution for the recovery of precious metals, the treatment of waste water containing organic chemicals and toxic metal species and the preparation of metal catalysts supported on carbon. This investigation has involved the study of the influence of porous structure and surface functional groups on the adsorption of both anionic and cationic metal species in order to understand the mechanism of adsorption of these species on active carbon from aqueous solution. Various types of oxygen functional groups were introduced onto the surface of coconut shell derived activated carbon through oxidation using nitric acid. Fourier transform infrared spectroscopy (FTIR), temperature programmed desorption (TPD) and selective neutralisation were used to characterise the surface oxygen functional groups. The oxidised carbons were also heat treated to provide a suite of carbons where the oxygen functional groups of various thermal stability were varied progressively. It was found that acidic oxygen functional groups mainly as carboxylic acid groups were incorporated into activated carbon by HNO3 oxidation. The phenol and quinone groups were also introduced by the oxidation process while the lactone groups were formed during heat treatment. The oxygen functional groups had a range of thermal stabilities with carboxylic acid groups being the least stable. A coconut shell derived active carbon was treated with ammonia and nitric acid followed by ammonia to incorporate nitrogen functional groups into the carbon. Active carbon with high nitrogen content was also prepared from nitrogen-rich precursor polyacrylonitrile (PAN). X-ray absorption near edge structure spectroscopy (XANES) and FTIR were used to investigate the structures of the nitrogen functional groups in carbons. The possible nitrogen functional groups present on carbon surface were pyridinic, pyrrolic (or indolic), pyridonic and aromatic amine-like structures. The adsorption characteristics of gold and silver cyanide anionic species on a suite of active carbons derived from coconut shell, polyacrylonitrile and chemical modification of the coconut shell carbon were investigated. The gold and silver cyanide adsorption capacities for coconut shell derived carbons correlate with total pore volume. Nitric acid oxidation treatment of the carbon was detrimental to gold adsorption in spite of the incorporation of oxygen content of carbon. The influence of nitrogen functional groups in the carbon structure on gold and silver adsorption was investigated using carbons derived from polyacrylonitrile. The addition of ethanol and butanol to the solution had an adverse effect on gold adsorption. Adsorption of silver cyanide ionic species on the active carbon was suppressed in the presence of excess free cyanide ions in solution whereas gold cyanide adsorption was not greatly affected at room temperature. The adsorption of gold cyanide was suppressed by the excess free cyanide and sodium sulphide at 70 °C. The adsorption of cadmium ions was enhanced dramatically by oxidation of the carbon. The ratio of released proton to adsorbed cadmium on oxidised carbon was approximately 2 indicating cation exchange was involved in the adsorption process. Na+ exchange studies with the oxidised carbon gave a similar ratio. After heat treatment of the oxidised carbons to remove oxygen functional groups, the ratio of H+/Cd2+ decreased significantly as well as the adsorption capacity. Both reversible and irreversible adsorption were involved in the process of cadmium adsorption with reversible adsorption having higher enthalpy. The irreversible adsorption resulted from cation exchange with carboxylic acid groups whereas the reversible adsorption probably involved physisorption of the partially hydrated cadmium ion. The nitrogen functional groups may act as ligands which can coordinate with transition metal cations. The adsorption of transition metal cations such as Cd 2+, Ni2+ and Cu2+ on active carbon was appreciably increased by the nitrogen functional groups present on carbon surface whereas ammonia treatment of the carbon showed little effect on the adsorption of alkali earth metal cation Cat+. There is little difference in the adsorption capacities of cadmium ions on coconut shell derived carbon at pH 4.1 and pH 7 whereas the adsorption of cadmium ions was significantly enhanced with increasing pH for the carbons with high nitrogen content. The nitrogen rich carbons show selectivity towards various transition metal cations reflected by adsorbing more Cu 2+ than Cd2+. This is consistent with the fact that the coordination compound of Cu 2+ with pyridine has higher stability constant than that of cadmium.