Continuous bioscorodite crystallization in CSTRs for arsenic removal and disposal Paula Gonza ´ lez-Contreras*, Jan Weijma, Cees J.N. Buisman Sub Department of Environmental Technology, Wageningen University, Bornse Weilanden 9, P.O. Box 17, 6708 WG Wageningen, The Netherlands article info Article history: Received 24 February 2012 Received in revised form 26 June 2012 Accepted 29 July 2012 Available online 11 August 2012 Keywords: Arsenic removal Biological crystallization Bioscorodite CSTR systems Ferrous oxidation Thermoacidophilic microorganisms abstract In CSTRs, ferrous iron was biologically oxidized followed by crystallization of scorodite (FeAsO 4 $2H 2 O) at pH 1.2 and 72  C. The CSTRs were fed with 2.8 g L 1 arsenate and 2.4 g L 1 ferrous and operated at an HRT of 40 h, without seed addition or crystal recirculation. Both oxidation and crystallization were stable for periods up to 200 days. The arsenic removal efficiency was higher than 99% at feed Fe/As molar ratios between 1 and 2, resulting in effluents with 29  18 mg As L 1 . Arsenic removal decreased to 40% at feed Fe/As molar ratios between 2 and 5. Microorganisms were not affected by arsenic concentrations up to 2.8 g As 5þ L 1 . The bioscorodite solid yield was 3.2 g/g arsenic removed. Bioscorodite crystals precipitated as aggregates, causing scaling on the glass wall of the reactor. The observed morphology through SE microscopy of the precipitates appeared amorphous but XRD analysis confirmed that these were crystalline scorodite. Arsenic leaching of bio- scorodite was 0.4 mg L 1 after 100 days under TCLP conditions, but when jarosite had been co-precipitated leaching was higher at 0.8 g L 1 . The robustness of the continuous process, the high removal efficiency and the very low arsenic leaching rates from bioscorodite sludge make the process very suitable for arsenic removal and disposal. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction As arsenic is present in over 320 minerals (Henke, 2009a), its presence in metallurgical operations cannot be avoided. Worldwide, there are over 150 locations recognized as signif- icant arsenic contamination sites as a result of ore deposit and mining (Henke, 2009b). Since 2000, worldwide arsenic production has increased from 40 to 60 thousand metric tons. In contrast, the consumption has decreased from 25 to 5 thousand metric tons since 2004 (Kelly and Matos, 2010). At many production sites, arsenic trioxide is accumulated and stored until a suitable technology for its disposal is developed. Arsenic is traditionally removed from industrial waste- waters from mineral processing and metallurgical operations by lime neutralization with co-precipitation of arsenic with ferric iron (Riveros et al., 2001; Twidwell and McCloskey, 2011). Arsenical ferrihydrite precipitation requires a high iron consumption with respect to arsenic, i.e. Fe/As >4 and thus large amounts of waste material are produced (Riveros and Dutrizac, 2001; Twidwell and McCloskey, 2011). Solideliquid separation of arsenical ferrihydrite is extremely inefficient. This gelatinous material contains no more than 6%wt arsenic, with a maximum solid content of 20e25%wt (Riveros and Dutrizac, 2001). The disposal and storage of these compounds is not entirely safe as they easily undergo physical and chemical changes with time, resulting in arsenic releases into the environment (Riveros and Dutrizac, 2001; Swash and Monhemius, 1998; pp 119e161). * Corresponding author. Tel.: þ31 317 483339. E-mail addresses: paulaa.gonzalezcontreras@wur.nl, paugonzalezcontreras@gmail.com (P. Gonza ´ lez-Contreras). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 5883 e5892 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.07.055