Sequential precipitation of Cu and Fe using a three-stage sulfidogenic fluidized-bed reactor system Deniz Ucar a , Ozan K. Bekmezci a , Anna H. Kaksonen b , Erkan Sahinkaya a,⇑ a Harran University, Environmental Engineering Department, Osmanbey Campus, 63000 Sanliurfa, Turkey b CSIRO Land and Water, Underwood Avenue, Floreat, WA 6014, Australia article info Article history: Available online 26 February 2011 Keywords: Acid mine drainage Biotechnology Sulfide ores Reduction abstract The exposure of sulfides, such as pyrite (FeS 2 ) to water and air leads to the formation of acidic metal and sulfate containing waters, generally referred to as acid mine drainage (AMD). Under anaerobic conditions and in the presence of a suitable electron and carbon source, sulfate-reducing bacteria (SRB) can reduce sulfate to hydrogen sulfide which can precipitate metals as low-solubility sulfides. In the present study, a three-stage fluidized-bed reactor (FBR) system was operated at 35 °C with ethanol as an electron and car- bon source for SRB to sequentially precipitate Cu and Fe from synthetic AMD. The system consisted of two pre-settling tanks before a sulfidogenic FBR for the sequential precipitation of Cu and Fe with biogenic H 2 S gas and HS  containing effluent, respectively. Cu and Fe precipitation efficiencies were over 99% and sulfate and COD removals 60–90%. Biologically produced alkalinity increased the initial pH of the AMD from 3.0 to neutral values. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The commercial exploitation of sulfide minerals for valuable metals results in the oxidation of the exposed iron and sulfur with air and water (reaction 1). This often leads to a production of acidic metal and sulfate containing wastewaters generally referred as acid mine drainages or AMD (Foucher et al., 2001; Garcia et al., 2001; Tsukamoto and Miller, 1999; Jong and Parry, 2003) 4FeS 2 þ 14H 2 O þ 15O 2 ! 4FeðOHÞ 3 þ 8SO 2 4 þ 16H þ ð1Þ Other sulfide minerals are also oxidized in a similar way, releas- ing metals and sulfate in solution. This oxidation process forms AMD which may also contain several metals and metalloids such as Cu, Fe, Zn, Al, Pb, As, and Cd at high concentrations (Bechard et al., 1994; Szczepanska and Twardowska, 1999). Sulfate reducing bioreactors have become an economically via- ble alternative to conventional chemical processes for the treat- ment of acidic and metal containing wastewaters (Sahinkaya, 2009). Sulfate-reducing bacteria (SRB) have an ability to reduce sulfate to hydrogen sulfide, which produces stable precipitates upon reaction with heavy metals. Moreover, bicarbonate produced in the sulfidogenic oxidation of provided electron donors increases the pH of the wastewater (reactions 2 and 3). Hence, metals and sulfate can be concomitantly removed and pH increased from acidic to neutral or alkaline in a single reactor (Kaksonen et al., 2003 and Sahinkaya et al., 2009) SO 2 4 þ 2CH 2 O ! H 2 S þ 2HCO  3 ð2Þ H 2 S þ M 2þ ! MSðsÞþ 2H þ ð3Þ In one-stage biological reactors operated at neutral pHs, metals can be precipitated as metal-sulfides. However, this kind of appli- cation does not allow selective or separate metal precipitation, which is important to produce pure metal-sulfides that can directly be used in smelters to produce metals. In a recent study, Bijmans et al. (2009) showed selective recovery of nickel over iron in a gas-lift bioreactor operated at pH 5, which allowed precipitation of nickel as NiS while the iron remained solubilized. Hence, con- trolling the reactor pH at low levels (<5) allowed the selective me- tal precipitation from mixed metal streams. One key factor constraining the design and application of selective metal recovery in one-stage bioreactor systems is the sensitivity of sulfate- reducing bacteria (SRB) to even mild acidity (pH < 5) (Johnson et al., 2006), which may be due to increased toxicity of sulfide and acetate at low pHs. Although sulfate reduction by SRB has been observed even at a very acidic condition of pH 3.8 (Kimura et al., 2006), pH > 5 is still preferred to approach optimal rates in industrial-scale sulfate reduction processes (Bijmans et al., 2009). In the literature, although several studies has been conducted with one-stage reactor, limited information is available with two- and three-stage metal-recovering sulfidogenic bioreactors (see Kaksonen and Puhakka (2007) for a review). This study aimed 0892-6875/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2011.02.005 ⇑ Corresponding author. Tel.: +90 4143440020. E-mail address: erkansahinkaya@yahoo.com (E. Sahinkaya). Minerals Engineering 24 (2011) 1100–1105 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng