ARTICLE pubs.acs.org/IECR
Synthesis and Characterization of Acicular Iron Oxide Particles Obtained from Acid Mine Drainage and Their Catalytic Properties in Toluene Oxidation Silvia L. F. Andersen,* Rubia G. Flores, Vivian S. Madeira, Humberto J. Jose, and Regina F. P. M. Moreira Department of Chemical and Food Engineering, Federal University of Santa Catarina, Campus Universitario Trindade, 88040-900, Florianopolis-SC, Brazil ABSTRACT: Acid mine drainage (AMD) has long been a significant environmental problem resulting from the microbial oxidation of iron pyrite in the presence of water and air. This Article describes a method to produce acicular goethite particles (AGNs) from AMD and their application as a catalyst for the combustion of volatile organic compounds. Ferric ions were recovered from AMD as AGNs with relatively high purity via an oxidation-selective precipitation process during AMD treatment. Hematite materials (PNs) were produced when the AGNs were treated at 450 °C. These materials were characterized by XRD, SEM, TGA, and N2 adsorption/desorption. The PNs were tested as a combustion catalyst and shown to be useful for the oxidation of toluene. AGNs impregnated with manganese were used to prepare the PN_Mn5 catalyst. The catalysts selectively produced CO2 and water, representing a good alternative to commercially available catalysts in terms of origin and availability.
1. INTRODUCTION Acid mine drainage (AMD) is a water pollution problem of great concern in the mining industry around the world.1,2 It is characterized as a low pH, high acidity effluent containing various dissolved metals (such as iron, aluminum, manganese, zinc, copper, nickel, calcium, magnesium, and chromium) and sulfate.3,4 AMD can be difficult and costly to treat.5 Treatment is usually accomplished with passive systems for low-flow abandoned discharges, and active processes for regulated discharges.6 Active AMD treatment involves the addition of neutralizing agents to raise the pH of the AMD water and precipitate metals, producing a chemical sludge with a high content of iron and other metal hydroxides.7,8 According to Madeira (2010),4 at one active coal mine alone, in Brazil, around 60 m3 h1 of AMD is treated, which generates a sludge containing approximately 80% w/w of iron oxides and hydroxides. Because of the high content of iron, the recovery of iron oxides from AMD residues is of interest for the production of pigments, catalysts, etc. In a few cases, the iron oxides have been recovered for use as pigments, magnetic materials, and catalysts.6,912 However, the resulting sludge can contain manganese, aluminum, and other metals, making resource recovery problematic if pure iron oxides are required for a specific application.5,6 Recently, Wei and Viadero Jr. (2007) have reported the synthesis of magnetite nanoparticles from acid mine drainage and reagentgrade ferrous sulfate, through coprecipitation at room temperature. The authors note that the characteristics of the nanosized magnetite can provide future opportunities for the application of nanosized iron oxides as catalyst or adsorbents in environmental engineering. Cheng et al. (2011) have demonstrated that it is also possible to use the soluble iron solutions of AMD in fuel cell-based technologies to create spherical nanoparticles of iron oxide (ferrihydrite), which are transformed to goethite (α-FeOOH) upon drying. However, reports on the recovery of acicular goethite in nanoscale from AMD treatment sludge appeared in the literature only recently. r 2011 American Chemical Society
The use of transition metal oxides for the catalytic oxidation of volatile organic compounds (VOCs) has been widely studied in the literature with a view to replacing expensive noble metal catalysts.13,14 Iron, being a transition metal, would be a good option, and the fact that the oxide form is produced from AMD enhanced the motivation for this study. Some previous studies have addressed the use of iron oxides as catalysts for VOC oxidation.1517 However, in relation to the application of iron oxides for VOC removal, most publications refer to their use as a noble metal catalyst support,18,19 or in mixtures with other transition metal oxides, such as manganese, nickel, and cerium.16,17,20 Nanoscale iron oxides also show high performance as an oxidation catalyst.21 The small particle size and the goethite component in the catalyst contribute to its high performance as an oxidation catalyst. In this study, acicular goethite particles (AGNs) were prepared from AMD active treatment sludge through a process of sequential precipitation. We also investigated the oxidation of a representative VOC (toluene) on acicular hematite particles (PNs) thermally produced from AGNs and their activity when impregnated with manganese. No references that describe the synthesis of acicular goethite from AMD treatment sludge and its use as a precursor for the preparation of VOC abatement catalysts could be found in the literature.
2. MATERIAL AND METHODS 2.1. Acicular Goethite Preparation. Raw AMD was collected from an underground coal mine (Carbonífera Criciuma S.A) in Santa Catarina State, Brazil, and sealed in high-density polyethylene Received: June 14, 2011 Accepted: December 2, 2011 Revised: November 20, 2011 Published: December 02, 2011 767
dx.doi.org/10.1021/ie201269y | Ind. Eng. Chem. Res. 2012, 51, 767–774
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Chemical Characterization of the AMD (pH = 3.1) and Supernatant 1 (pH = 4.3)a acidity Fe2+ Feb Al3+ Mn SO42 Cu2 Zn2 AMD [g L1]
9.35
supernatant 1 [g L1] 5.33
3.4
3.4
0.459 N.I. 12.03
2.96 2.96 0.11
N.I.
9.5
N.I.
N.I.
N.I.
N.I.
a
N.I.: Below the detection limit by colorimetric methods (APHA, 1998). b Total iron. The thermal behavior of the AGNs and PNs was evaluated by thermogravimetric analysis (TGA) using a Shimadzu DTG60/ 60H analyzer, under air (99.995%) flow (50 mL min1). The analysis conditions were: heating rate = 10 °C min1 up to 900 °C. X-ray diffraction (XRD) patterns of the powdered samples were obtained using a Philips X’Pert Multi-Purpose diffractometer at a scan rate of 0.038/s with Cu Kα radiation. The crystalline phases were identified with reference to powder diffraction data (JCPDS 1993). The N2 adsorption/desorption isotherms were measured at liquid nitrogen temperature using an Autosorb 1C (Quantachrome, USC) nitrogen adsorption instrument. The samples were degassed at 100 °C for 4 h prior to the analysis. Particle size and morphology were examined under a scanning electron microscopy (SEM) using a JEOL JSM-6390LV scanning electron microscope. The composition of the AGN sample was determined by X-ray fluorescence (XRF) on a Philips PW 2400 X-ray fluorescence spectrometer. The calibration was carried out using the appropriate patterns. 2.4. Catalytic Activity Measurements. Catalytic experiments with toluene were carried out in a fixed-bed stainless steel reactor at atmospheric pressure (Figure 2). In each run, approximately 0.1 g of catalyst (
