Study of copper fixation mechanisms on Bayer Process Electrostatic precipitator Microparticles (BPEM) using ¹H and ²⁷Al NMR spectroscopy

The removal of heavy metals from natural, mining, or industrial ponds or water flows is a problem for metallurgical industries. The reason is that heavy metals represent a risk for the health of people living close to these industries. One of these heavy metals is copper, whose removal was widely studied using different aluminous materials (clays, zeolites, smectites or power plant ashes). In this manuscript, a different aluminous material is proposed to remove copper from water, the Bayer Process Electrostatic precipitator Microparticles (BPEM). The BPEM mainly consists of alpha alumina (min. 6.5 % and max. 53 %), aluminum hydrates (min. 12 % and max. 72 %) and intermediate or gamma alumina. The adsorption of Cu2+ by different BPEMs was studied using Nuclear Magnetic Resonance (NMR) technique. The NMR H-1 and Al-27 spectra were tested to identify the presence of oxides and hydroxides and their bonding structures, and how the spectra varied once the copper had been captured by the BPEMs. A huge attenuation of the H-1 spectrum was found in all the studied BPEMs when copper is adsorbed. There does not seem to be any Cu2+ adsorbed related to Al-27, although some spectra show a slight attenuation of the corresponding peak, but not comparable with that observed in the case of the H-1. Thus, Cu fixation is produced by replacement of hydrogen bonding, present on the surface of the hydrated alumina, by the Cu cations.