Removal of dissolved Zn(II) using coal mine drainage sludge: Implications for acidic wastewater treatment Mingcan Cui a, b , Min Jang c, * , Fred S. Cannon d , Seunmin Na a , Jeehyeong Khim a, ** , Jae Kwang Park e a School of Civil Environmental and Architectural Engineering, Korea University, 5 Anam-dong, Seoul 136-701, Republic of Korea b Jilin Institute of Chemical Technology, 5 Cheng De Jie, Jilin 132022, China c Department of Civil Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia d Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA 16802, USA e Department of Civil and Environmental Engineering, University of WisconsineMadison, Madison, WI 53706, USA article info Article history: Received 11 May 2012 Received in revised form 10 December 2012 Accepted 18 December 2012 Available online 5 January 2013 Keywords: Zeta potential Coal mine drainage sludge Goethite Hematite Complexation Fourier transform infrared spectroscopy abstract The mechanism for the removal of Zn(II) by using coal mine drainage sludge (CMDS) was investigated by spectroscopic analysis and observations of batch tests using model materials. Zeta potential analysis showed that CMDS 25 (dried at 25  C) and CMDS 550 (dried at 550  C) had a much lower isoelectric point of pH (pH IEP ) than either goethite or calcite, which are the main constituents of CMDS. This indicates that the negatively charged anion (sulfate) was incorporated into the structural networks and adsorbed on the surface of CMDS via outer-sphere complexation. The removal of Zn(II) by CMDS was thought to be primarily caused by sulfate-complexed iron (oxy)hydroxide and calcite. In particular, the electrostatic attraction of the negatively charged functional group, FeOHeSO 4 2 , to the dissolved Zn(II) could provide high removal efficiencies over a wide pH range. Thermodynamic modeling and Fourier transform infrared spectroscopy (FT-IR) demonstrated that ZnSO 4 is the dominant species in the pH range 3e7 as the sulfate complexes with the hydroxyl groups, whereas the precipitation of Zn(II) as ZnCO 3 or Zn 5 (CO 3 ) 2 (OH) 6 through the dissolution of calcite is the dominant mechanism in the pH range 7e9.6. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The occurrence of acid mine drainage (AMD) at both operating or abandoned coal mines is generally characterized by a low pH, with high concentrations of dissolved metals such as Fe, Al, and Mn often present, while as there are low concentrations of heavy metals such as Zn, Cu, Cd, and Pb (Cheng et al., 2009). Dissolved metals in AMD at coal mines are typically removed by oxidation, precipitation, and separation processes. However, one of the most significant challenges faced in AMD treatment is the large volume of sludge involved. This is because the low economic value, diffi- culties in dewatering, and the high cost of off-site hauling of sludge leads to inherent difficulties in the disposal of the sludge (Dempsey et al., 2001; Viadero et al., 2006). The sludge is composed mostly of amorphous micrometer- and submicrometer-sized iron oxide/ hydroxide particles that contain sulfate. Furthermore, its high surface area and numerous functional groups make it suitable for use in removing traces of dissolved metals through adsorption and co-precipitation (Webster et al., 1998; Kirby et al., 1999; Cornell and Schwertmann, 2003). As one of the main constituents of such sludge, iron sulfate (oxy)hydroxides can possess amphoteric qual- ities that can remove heavy metals (e.g., Cd, Cu, Pb, and Zn) and anionic metalloids (e.g., As and Se) in an aqueous phase (Smith, 2007). To date, however, very little work has been reported on the beneficial use of the sludge for removing heavy metals or on the clarification of the removal mechanism. Two approaches have primarily been used for elucidating the surface reaction and removal mechanism of heavy metals: empir- ical (Langmuir and Freundlich models) and surface complexation modeling (Erdemoglu and Sarikaya, 2006). Empirical models are the simplest and most commonly used, while the surface complexation model can be used for describing specific surface reactions (Lutzenkirchen, 2005; Erdemoglu and Sarikaya, 2006). However, the requirement of multiple parameters may make these models rather impractical and inconvenient. Meanwhile, Juang and Wu (2002) proposed that a zeta potential (ZP) analysis may be used for predicting the adsorption and precipitation behaviors because it is a function of the surface coverage by charged species at a given * Corresponding author. Tel.: þ60 3 7967 7649; fax: þ60 3 7967 5318. ** Corresponding author. Tel.: þ82 2 3290 3318; fax: þ82 2 928 7656. E-mail addresses: minjang@um.edu.my, heejaejang@gmail.com (M. Jang), hyeong@korea.ac.kr (J. Khim). Contents lists available at SciVerse ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman 0301-4797/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2012.12.013 Journal of Environmental Management 116 (2013) 107e112