The reduction of chromate ions by Fe(II) layered hydroxides

International audienceThe reduction of chromate ions by Fe(OH)2 and the iron (II)-iron (III) hydroxysulphate green rust, GR(SO42-), was studied to evaluate whether such synthetic layered hydroxides and the corresponding natural green rust mineral could be involved in the natural attenuation of contaminated environments. The resulting Cr (III) bearing phases, which would govern the subsequent behaviour of chromium, were clearly characterised. Both compounds proved to be very reactive and oxidised instantaneously while chromate ions were reduced to Cr (III) as evidenced by X-ray photoelectron spectroscopy. Mass balance (ICP-AES) demonstrated that the Fe/Cr ratio inside the solid end product was equal to the initial Fe/Cr ratio. The solid phases, analysed by X-ray diffraction, Raman and Mossbauer spectroscopies were identified as Cr-substituted poorly crystallised iron (III) oxyhydroxides in both cases, more precisely d-FeOOH when starting with Fe(OH)2 and ferrihydrite when starting with GR(SO42-)

Manganese and cobalt redox cycling in laterites; Biogeochemical and bioprocessing implications

This research was developed during the PhD studies of Agustín Solano Arguedas in the University of Manchester, United Kingdom. PhD scholarship was funded by the Ministerio de Ciencia, Tecnología y Telecomunicaciones (MICITT) of the Government of Costa Rica and the Universidad de Costa Rica (UCR). Agustín Solano Arguedas is a researcher at the Unidad de Recursos Forestales (Reforesta, Unit of Forest Resources) of the Instituto de Investigaciones en Ingeniería (INII, Engineering Research Institute), UCR.Cobalt is essential for the modern technology that underpins the decarbonisation of our economies, but its supply is limited leading to its designation as a critical metal. Cobalt biogeochemistry is poorly understood, yet knowledge of how biogeochemical cycling impacts cobalt behaviour could assist the development of new techniques to recover cobalt from ores, and so improve the security of supply. Laterites are an important source of cobalt, they are primarily processed for nickel using energy or chemical intensive processes, with cobalt recovered as a by-product. Metal-reducing conditions were stimulated in laterite sediment microcosms by the addition of simple and cheaply available organic substrates (acetate or glucose). At the end of the experiment the amount of easily recoverable cobalt (aqueous or extractable with acetic acid) increased from < 1% to up to 64%, which closely mirrored the behaviour of manganese, while only a small proportion of iron was transformed into an easily recoverable phase. Sequencing of the microbial community showed that the addition of organic substrates stimulated the growth of indigenous prokaryotes closely related to known manganese(IV)/iron(III)-reducers, particularly from the Clostridiales, and that fungi assigned to Penicillium, known to produce organic acids beneficial for leaching cobalt and nickel from laterites, were identified. Overall, the results indicate that the environmental behaviour of cobalt in laterites is likely to be controlled by manganese biogeochemical cycling by microorganisms. These results are compelling given that similar behaviour was observed in four laterites (Acoje, Çaldağ, Piauí and Shevchenko) from different continents. A new bioprocessing strategy is proposed whereby laterites are treated with an organic substrate to generate metal-reducing conditions, then rinsed with acetic acid to remove the cobalt. Not only are organic substrates environmentally-friendly and potentially sourced from waste carbon substrates, a minimal amount of iron oxides was mobilised and consequently less waste generated.Natural Environment Research Council/[CoG3 NE/M011518/1]/NERC/Reino UnidoDiamond Light Source/[SP16735]//Reino UnidoDiamond Light Source/[SP17313]//Reino UnidoOffice of Science User Facility/[DE-AC02-05CH11231]//Estados UnidosUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ingeniería::Instituto Investigaciones en Ingeniería (INII

Effective Cr(VI) removal from simulated groundwater through the hydrotalcite-derived adsorbent

We investigated the feasibility of using calcined hydrotalcite (CHT) as the adsorbent of chromate to treat Cr(VI)-contaminated water through column tests under varied conditions. The column tests reveal that CHT can take up 34.3-44.7 mg(Cr)/g when the Cr(VI) concentration in the influent varies over a range of 50-200 mg/L (e.g., 0.96-3.85 mM) with pH 6-7 at 298 K. This uptake capacity is only reduced to 29.1 mg(Cr)/g when HCO-3 (1.0 mM) and Cl- (1.0 mM) coexist in the influent. We note that the treated water is of high quality and is free of Cr(VI), with Mg and Al concentrations of <5-10 mg/L, and a pH of 6.5-7.0. The quick desorption of Cr(VI) from the adsorbent (CHT) has enabled us to recover Cr(VI) from the contaminated water and regenerate the adsorbent. All these findings promise CHT as an effective regenerable adsorbent for the remediation of Cr(VI)-contaminated groundwater. © 2010 American Chemical Society.Yunfeng Xu, Jia Zhang, Guangren Qian, Zhong Ren, Zhi Ping Xu, Yueying Wu, Qiang Liu, and Shizhang Qia