Comparative study on the adsorption of cyanide gold complexes onto different carbonaceous samples: Measurement of the reversibility of the process and assessment of the active surface inferred by flow microcalorimetry

In this paper, adsorption of potassium gold cyanide from water onto three a ctivated carbon samples, four graphites, and two carbon blacks (Graphon and Vulcan) is compared. The present work collects the results of flow microca lorimetric studies for each carbonaceous adsorbent so as to obtain estimate s of their polar and accessible apolar graphitic basal plane surface areas. The relative polar and graphitic nature of the carbons is evaluated and re lated where possible to their capacity of adsorption for gold complexes. Fi nally, the thermodynamic reversibility of gold adsorption on the carbonaceo us adsorbents from an aqueous solution of potassium aurocyanide at room tem perature has also been studied. A flow adsorption microcalorimetry method w as used to measure the amounts of aurocyanide adsorbed and desorbed in one adsorption-desorption cycle, as well as the corresponding enthalpic changes upon adsorption and desorption. On the basis of the adsorption and enthalp y values obtained in adsorption and desorption cycles, estimates could be m ade of the reversible and irreversible contributions to the total adsorptio n capacity and the integral molar enthalpy of adsorption. The degree of ads orption irreversibility ranges from 25% for Graphon to 54% for G212. The ir reversible and reversible enthalpy components have respective values of abo ut -50 and -25 kJ mol(-1). A large part of the microporous structure in act ivated carbons is not accessible to the adsorption of gold complexes. One p art of the aurocyanide is believed to irreversibly adsorb as an unpaired an ion Au(CN)(2)(-) through electrostatic interactions on the very active surf ace sites having polar character. The less active sites are occupied by the ion-paired neutral molecular species (KAu(CN)(2)) through the action of va n der Waals forces and account for the reversible adsorption. In reversible and irreversible processes, the active sites are thus expected to be assoc iated with the graphitic-like structure containing nonideal aromatic rings and polar structures located at the edge defects in the graphite structure.