Hazardous Metal Cations Recovery Using Electrochemical Intercalation in the Chevrel Phases: A Combined Synchrotron X-Ray Powder Diffraction/Electrochemistry Study
Authors: Barbosa José , Hernandez Olivier , Prestipino Carmelo , Guyot Elodie , Guilloux-Viry Maryline , Boulanger Clotilde 
Published: February 2020

Industrial activities generate effluents containing metallic cations noxious for the environment, humans and wild life, motivating intensive research as those cations are considered as micro contaminants according to European standards [1]. It is important to detect and separate these cations in order to reduce pollution and also to collect the matter due to metals cost and ore depletion. Chalcogenide molybdenum clusters known as Chevrel phases [2] exhibit outstanding physical (superconductivity, thermoelectricity) [3] and chemical (cations intercalation, catalysis) [4] properties. The vacancies in the Chevrel Phases structure enable reversible redox reactions Mo6X8 + x Mn+ + x ne- ó MxMo6X8 [5] (X= S, Se) that lead to a room temperature ionic mobility exploited for selective recovery of metallic cations [6]; to amperometric detection heavy metals [7] and as electrode for magnesium battery [8].
Electrochemistry studies have been conducted on Mo6X8 in Cd2+, Ni2+, Co2+ and Zn2+ single-cationic solutions. Those cations were chosen so as to recreate effluents resulting from battery recycling. Multi-cationic electrolytes (namely Cd2+/Ni2+, Ni2+/Co2+, Cd2+/Co2+, Cd2+/Zn2+, Ni2+/Zn2+, Ni2+/Co2+/Cd2+, Ni2+/Co2+/Zn2+, Ni2+/Cd2+/Zn2+) were investigated with the aim to examine if selectivity occurs. Chronoamperometry and Chronopotentiometry were performed to obtain the fully intercalated phases. For few cationic solutions, further in-situ experiments were done at the ID22 beamline (European Synchrotron Radiation Facility) coupling electrochemistry and high resolution X-ray powder diffraction (XPRD). Complementary ex-situ XRPD experiments on CdxMo6S8 end NiyMo6S8 were also carried out.
It has been shown by in-situ experiments that Cd2+ is preferentially intercalated in presence of other cations due to its higher mobility in the channels of the host lattice. We also gathered information such as accurate cell parameters, intercalation kinetics and relative proportions of phases during the entire electrochemical processes. Ex-situ experiments performed in capillary at the ESRF allowed to determine a ternary phase never reported in the literature for the intercalation of Cd2+ into Mo6S8.
[1]http://eur-lex.europa.eu/legal-content/FR/ALL/?uri=CELEX:32008L0105http://eur-lex.europa.eu/legal-content/FR/ALL/?uri=CELEX:3200L0105, Directive 2008/105/CE, European Parliament and Union European Council (2008)
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