Copper heterogeneous nucleation on a palygorskitic clay: an XRD, EXAFS and molecular modeling study

Clay materials play a key role in determining the retention capacity of a soil, and are widely used in waste treatments. One of the most commonly used is palygorskite. The aim of this research is to determine the chemical species formed by copper when retained in a clay material, mainly consisting of palygorskite, and find the minimized energy reached by the species adsorbed by palygorskite. The clay material was put in contact with a copper(II) chloride solution, and after reaching the thermodynamic equilibrium, the dry residue was analysed by using XRD, X-ray fluorescence (XRF), and extended-X-ray absorption fine-structure (EXAFS) spectroscopy. Formation of copper hydroxychloride (atacamite, CuCl2.3Cu(OH)2) was studied. Computational studies, carried out with 890 atoms, showed the formation of an amorphous copper hydroxychloride. However, it seems probable that the whole precipitate would be firstly amorphous, later becoming crystalline (Ostwald's rule) in accordance with XRD and EXAFS data.