title: Effective treatment of alkaline Cr(VI) contaminated leachate using a novel Pd-bionanocatalyst: Impact of electron donor and aqueous geochemistry.
abstract: Palladium catalysts offer the potential for the effective treatment of a variety of priority reducible pollutants in natural waters. In this study, microbially synthesized magnetite nanoparticles were functionalized with Pd(0), creating a highly reactive, magnetically recoverable, nano-scale catalyst (Pd-BnM). This was then investigated for the treatment of model Cr(VI) contaminated solutions at a range of pH values, and also alkaline Cr(VI) contaminated leachates from chromite ore processing residue (COPR); a contaminant issue of global concern. The sample of COPR used in this study was obtained from a site in Glasgow, UK, where extensive Cr(VI) contamination has been reported. In initial experiments Pd-BnM was supplied with H2gas or formate as electron donors, and Cr(VI) removal from model synthetic solutions was quantified at various pH values (2-12). Effective removal was noted at neutral to environmentally relevant alkaline (pH 12) pH values, while the use of formate as an electron donor resulted in loss of performance under acidic conditions (pH 2). Reaction kinetics were then assessed with increasing Pd-BnM loading in both model pH 12 Cr(VI) solutions and the COPR leachate. When formate was used as the electron donor for Pd-BnM, to treat COPR leachate, there was significant inhibition of Cr(VI) removal. In contrast, a promotion of reaction rate, was observed when H2was employed. Upon sustained reaction with model Cr(VI) solutions, in the presence of excess electron donor (formate or H2), appreciable quantities of Cr(VI) were removed before eventual inactivation of the catalyst. Faster onset of inactivation was reported in the COPR leachates, removing 4% and 64% of Cr(VI) observed from model Cr(VI) solutions, when formate and H2were used as electron donors, respectively. XAS, TEM-EDX and XPS analysis of the catalysts that had been inactivated in the model solution, showed that the surface had an extensive covering of reduced Cr(III), most likely as a CrOOH phase. COPR reacted catalysts recorded a lower abundance of Cr(III) alongside a high abundance of the leachate components Ca and Si, implicating these elements in the faster onset of inactivation.
authors.affiliation: School of Earth, Atmospheric and Environmental Sciences, Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Manchester M13 9PL, UK.
authors.forename: Mathew P
authors.initials: MP
authors.lastname: Watts
authors.affiliation: School of Earth, Atmospheric and Environmental Sciences, Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Manchester M13 9PL, UK.
authors.forename: Victoria S
authors.initials: VS
authors.lastname: Coker
authors.affiliation: Diamond Light Source, Chilton, Didcot, Oxfordshire OX11 ODE, UK.
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Parry
authors.affiliation: Parsons Brinckerhoff, Queen Victoria House, Redland Hill, Bristol, UK.
authors.forename: Russell A P
authors.initials: RA
authors.lastname: Thomas
authors.affiliation: Department of Civil and Environmental Engineering, James Weir Building, University of Strathclyde, Glasgow G1 1XJ, UK.
authors.forename: Robert
authors.initials: R
authors.lastname: Kalin
authors.affiliation: School of Earth, Atmospheric and Environmental Sciences, Williamson Research Centre for Molecular Environmental Science, The University of Manchester, Manchester M13 9PL, UK.
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Lloyd
keywords: Alkaline; Cr(VI); Inactivation; Magnetite; Remediation