Ind. Eng. Chem. Res. 2008, 47, 2511-2517
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Industrial Coke as an Electrode Material for Environmental Remediation Jamie Haner, Keegan Rankin, Dorin Bejan, and Nigel J. Bunce* Electrochemical Technology Centre, Chemistry Department, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1
Industrial coke was evaluated as a low-cost electrode material for environmental remediation, using the dye Orange II as an example substrate. Coke was used as massive pieces in batch cells or in the ground form for use in a packed-bed reactor. The loss of Orange II was faster when the supporting electrolyte contained chloride ion, and under these conditions the reaction involved hypochlorination. In the batch reactor, the current efficiency for mineralization was only modest (4-14%). In the packed-bed reactor, the loss of both starting material and intermediates was fastest at high current and low flow rate, and a near-quantitative current efficiency was achieved. The high current efficiency was explained by the greater surface area of the electrodes in the packed-bed reactor compared with the batch reactor, and better contact between the solution to be remediated and the coke particles. A drawback to the use of coke electrodes for the remediation of aqueous wastes is their tendency to increase the total organic carbon content of an aqueous solution, especially under anodic polarization. Introduction
the dye Orange II (sodium 4-(2-hydroxy-1-naphthylazo)-benzenesulfonate) as an example substrate.18
Electrolysis has been extensively studied in recent years as a method of environmental remediation, especially for waste streams that are unsuited to conventional biological treatment, such as wastewater,1-5 tannery waste,6 munitions wastes,7 and the removal of herbicides from wastewater.8 Many of these studies have used novel electrodes such as boron-doped diamond (BDD), which affords reactive hydroxyl radicals upon anodic polarization.9-14 Because BDD is not yet available for commercial applications, there remains a need for other electrode materials that offer an appropriate tradeoff between low initial cost and longevity. For example, dimensionally stable anodes such as Ti/IrO2 are known for their ability to withstand the aggressive environment of chlorine production in membraneseparated chlor-alkali cells. Carbon anodes based on graphite, the conductive allotrope of carbon, can be produced inexpensively. They are used commercially in the older, flowingmercury chlor-alkali cells, and also in the electrolytic production of aluminum, in which the anode is sacrificially oxidized to CO2. Our laboratory is exploring the use of industrial coke as a low-cost electrode material for environmental remediation. Coke is a carbonaceous material produced by heating coal to drive off volatiles. Its electrical conductivity increases strongly as the coking temperature increases,15 and its open architecture provides a high surface-to-volume ratio. Coke can be cut to prepare massive electrodes for use in a batch reactor, or it can be ground to form an electrically conductive packing in a packed-bed reactor. Dye industry wastes are a significant source of environmental pollution: dyes are highly colored and are recalcitrant because they must resist chemical, biochemical, and photochemical change. The magnitude of the environmental problem is highlighted by estimates that up to 50 000 t yr-1 of dye wastes are discharged worldwide from dyeing installations,16 with aromatic azo dyes, which contain the -NdN- chromophore, comprising about two-thirds of the total.17 This study employed * To whom correspondence should be addressed. E-mail: nbunce@ uoguelph.ca. Tel.: 1-519-824-4120, Ext. 53962.
Materials and Methods Chemicals. Orange II, certified [Acid Orange 7, C.I. 15510, sodium 4-(2-hydroxy-1-naphthylazo)benzenesulfonate], technical (90%) 1-amino-2-naphthol hydrochloride (stored in Schlenk flask under argon), sulfanilic acid (H2NC6H4SO3H; minimum 99%), 1,2-naphthoquinone, 97% (stored in refrigerator) were supplied by Sigma-Aldrich (Oakville, ON). Ammonium acetate, potassium chloride and sulfuric acid were provided by Fisher Scientific (Toronto, ON). Sodium sulfate and HPLC grade acetonitrile were supplied by Caledon Laboratories Ltd. (Georgetown, ON). Solutions for electrolysis were prepared in water having resistivity not less than 11 MΩ cm from a Millipore Milli-Q reagent water system. Electrode Materials. Coke was obtained from Dofasco, Hamilton, ON and Stelco, Hamilton, ON. It was cut into flat electrodes or ground into granular particles 1-3 mm in diameter. Hollow Ebonex cylinders (100 mm long, 18 mm external diameter, and 2 mm thick) were obtained from Vector Construction Group (Stoney Creek, ON); they were cut in half lengthwise for use as electrodes. Graphite rods 6.3 mm in diameter were obtained from Alfa Aesar, Ward Hill, MA), stainless steel from the Chemistry Department machine shop, University of Guelph, and dimensionally stabilized anodes made of titanium coated with IrO2 from ELTECH Systems Corp., Painesville, OH. Reactors. The electrochemical reactors were constructed in the machine shop at the University of Guelph. In batch-cell experiments, the electrodes were connected by means of alligator clips to an Amel Instruments Model 2049 potentiostat/galvanostat, which was operated in amperostatic mode. The undivided batch cell was constructed from 5 mm thick Plexiglas and had dimensions 5 × 5 cm width × 7.5 cm height. The electrodes were placed ∼1 cm from the bottom of the cell and were supported by clamps. The divided batch cell was constructed from two blocks of Plexiglas, having wall thicknesses of ∼2.5 cm, with inside dimensions 5 × 4 cm width × 5 cm height for the
10.1021/ie0716464 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/15/2008
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Metals’ analyses were carried out by Laboratory Services Division, University of Guelph, using inductively coupled plasma spectrometry (ICP). Total organic carbon (TOC) analyses were carried out by Ms. Joanne Ryks, using a Shimadzu model TOC-VCSH analyzer.
Results and Discussion
Figure 1. Packed-bed reactor without and with a gap between the electrodes, allowing solutions to be analyzed at the gap.
total cell volume. The two components were screwed together, locking a Nafion 424 cation exchange membrane in place. The potential difference between the anode and cathode was monitored with a Wavetek model DM5XL voltmeter. The packed-bed reactor was constructed from 2 mm thick Plexiglas; it had in/out ports, and outer dimensions 10.4 cm length × 2.4 cm width × 10.2 cm height (see Figure 1). The ground coke was sieved through a 0.93 mm diameter mesh. Plastic 1 mm mesh was used to prevent electrical contact between coke particles in the anode and cathode compartments. The solution was pumped through the cell using a Masterflex peristaltic pump, using flow rates of 1-5 mL min-1. The configurations cathode first (CA) or anode first (AC) allowed samples to be taken only after passing through both compartments. In this configuration, both compartments had dimensions 4.7 cm length in the direction of the flow × width 2.0 cm × height 5.8 cm and contained 36 g of coke. A modified configuration AC allowed samples also to be taken at the gap following the anode, to allow analysis of the oxidation products; this reduced the length dimension of the second compartment to 2.9 cm and the mass of coke to 22 g. Electrical connection to the packed-bed electrodes was achieved by placing graphite rods of 0.63 cm diameter and length 9 cm into the coke bed; alligator clips were connected to the exposed parts of the rods. Analytical. HPLC analysis involved a Waters 600E system equipped with a Model 486 UV-visible detector set at 256 nm, using 1:1 acetonitrile/0.1 M ammonium acetate as eluent and a flow rate of 1 mL min-1. Samples (250 µL) were injected into a 10 µL sample loop prior to analysis on a 250 × 4.6 mm Phenomenex Spherisorb 5 ODS (1) column, equipped with a precolumn Delta-Pak C18 5 µm 100 Å (Waters, Mississagua, ON). The chromatograms recorded 10 min of retention time (Orange II 3.4 min, sulfanilic acid 2.6 min, 1,2 naphthoquinone 5.8 min).
The samples of coke were highly variable with respect to their electrical conductance; only pieces having resistance
