Synergistic Removal of Cr(VI) with Stable Nanozerovalent Iron Particles and Ultrasound Assistance in the Presence of Organic Additives

Commercial stable zerovalent iron nanoparticles (nZVI) (NSTAR, not activated) combined with ultrasound (US) were tested for Cr(VI) reductive removal (0.3 mM, pH 3, Fe/Cr molar ratio of 3:1) in the presence of the carboxylic acids (CAs) ethylenediaminetetraacetic acid (EDTA, 1 mM) or citric acid (Cit, 2 mM), in a system open to the air. No Cr(VI) decay up to 180 min was observed under US when only NSTAR nanoparticles were used, while other previously tested commercial nZVI (N25) showed about 40% decay in a few minutes but without further Cr(VI) removal. The addition of EDTA and Cit enabled Cr(VI) removal with NSTAR in the absence of US without prior activation of the particles. A pseudo zero-order kinetics was followed, yielding 32 and 49% removal with EDTA and Cit, respectively. When US was applied, these values almost doubled, reaching 59 and 88% for EDTA and Cit, respectively. A mechanism for Cr(VI) decay was proposed. The present results indicate that the simultaneous use of US and CAs allows for a synergistic Cr(VI) removal by NSTAR avoiding the need of an activation step of the nanoparticles, with a more effective result of Cit compared with EDTA.