Ind. Eng. Chem. Res. 2010, 49, 6261–6266
6261
Electrochemical Oxidation of the Sulfide Ion in Synthetic Geothermal Brines in Batch Cells Using Coke Electrodes Keegan Rankin, Dorin Bejan, and Nigel J. Bunce* Electrochemical Technology Centre, Chemistry Department, UniVersity of Guelph, 50, Stone Road East, Guelph, Ontario, Canada N1G 2W1
The oxidation of the sulfide ion occurs efficiently in batch cells at massive coke electrodes. At all currents studied, the products included a low yield of elemental sulfur, which deposited on the anode; the yields of sulfate were also low, except at the highest current. The remaining products were soluble organosulfur species, indicating that the coke anodes acted sacrificially. The reaction displayed unusual kinetic behavior with respect to the disappearance of sulfide: a two-stage reaction was observed in which the loss of sulfide was faster in the early stages of reaction, while elemental sulfur deposited on the anode. A subsequent slower currentcontrolled reaction was associated with the formation of the remaining products. Introduction Geothermal brines that accompany oil and gas extraction are commonly contaminated with sulfide ions, sometimes at concentrations up to 60 mM. Hydrogen sulfide, which has pKa values of 7.04 and 11.96, is volatile at a pH ∼ 8, typical of these brines, causing diverse problems of odor, toxicity, and metal corrosion.1-5 Electrochemical oxidation is of interest as a possible remediation technology for sour brines because chemical treatment methods6,7 involve costly chemicals and sludge disposal. Work in our laboratory initially focused on the oxidation of the sulfide ion at a boron-doped diamond (BDD) anode.8 The sulfate ion was formed almost quantitatively and with near quantitative current efficiency in both the absence and presence of chloride ion. We were prompted to investigate other anodes for the oxidation of sulfidic brines because BDD anodes are not available in a large format and because the eight-electron oxidation of sulfide to sulfate is 4 times as energy intensive as the two-electron oxidation to elemental sulfur, suggesting the desirability of forming elemental sulfur as a major oxidation product. It has also been suggested high concentrations of sulfate may cause precipitation of calcium sulfate when the treated sour brine is pumped into a reinjection well.8 Moderate yields of sulfur were formed at a low conversion of sulfide at a dimensionally stable anode of Ti/IrO2-Ta2O5; otherwise, sulfate was again the chief product. In addition, we observed poisoning of the noble metal-based anode by sulfide,9 a problem also encountered at platinum and gold10-13 and in the catalytic oxidation of the sulfide ion at a microporous anode based on a cobalt phthalocyanine network polymer.14 Ateya and co-workers have studied the oxidation of sulfide in salt water at carbon-based anodessgraphite and carbon cloth. Sulfur was deposited when graphite was used as the anode,1,15-17 as confirmed by X-ray photoelectron spectroscopy (XPS).16,17 The authors observed no products of further oxidation sorbed to the anode but did not report a material balance for sulfur. The oxidation of sulfide at a graphite anode is strongly accelerated in superheated brines and gives sulfate rather than elemental sulfur according to XPS analysis.18 Our laboratory recently introduced commercial coke as an electrode material.19 Coke, which is made by pyrolysis of coal, * To whom correspondence should be addressed. Tel.: 1-519-8244120, Ext. 53962. E-mail:
[email protected].
is a porous carbonaceous material that can be used either as massive electrodes in a batch reactor or ground for use in a packed bed electrochemical reactor. In this work, we report the oxidation of sulfide under batch conditions at coke anodes, with emphasis on the rates and products of the reaction at ambient temperature and pressure. Materials and Methods Chemicals. Sodium sulfide nonahydrate (ACS reagent, 98%), sodium sulfate (anhydrous, 99.9%), and sodium thiosulfate for iodometry were supplied by Sigma-Aldrich (Oakville, ON, Canada). Corn starch for use as an indicator was supplied by Dominic No Frills (Guelph, ON, Canada). Chloroform (reagent grade), sodium chloride, sodium hydroxide, barium chloride, and hydrochloric acid were supplied by Fisher Scientific (Toronto, ON, Canada). Naphthenic acids were supplied by Acros Organics (Geel, Belgium, Canada). Iodine (sublimed) was supplied by Caledon Laboratories (Georgetown, ON, Canada). Potassium hexacyano-ferrate(II) trihydrate (99.9+%) was supplied by Sigma-Aldrich (St. Louis, MO). Electrode Materials. The coke material was generously supplied by Dofasco Inc. and Stelco Inc. (now U.S. Steel Canada), both located in Hamilton, Ontario, Canada. Coke pieces for use as electrodes were selected on the basis of their electrical conductivity (80%) of the initial sulfur species remained in solution (Table 5), despite the poor material balance based on residual sulfide, elemental sulfur, and sulfate (see Figure 4). Earlier work had shown that coke anodes release soluble carbon during electrolysis.19 In this work, TOC analysis revealed that new anodes released more soluble carbon than used anodes. The TOC increased with the applied current, and the solutions became very dark in color, especially when sulfide was present (Table 5). We hypothesized that the unidentified material might be an organic sulfonate (eq 2, where ′CtH represents a carbon atom at the surface of the coke), which would be compatible with the eight-electron oxidation suggested by cyclic voltammetry. tC-H(s) + S2-(aq) + 3H2O(l) f tC-SO3 (aq) + 7H+(aq) + 8e- (2) Because high yields of sulfate were formed at high current density (20 mA cm-2), we attempted to convert the putative sulfonate to sulfate. Initial electrolyses were carried out at 10 mA cm-2; then, one-half of the solution was used to analyze for sulfide, sulfate, and sulfur, and the other was further electrolyzed at 20 mA cm-2 using either a coke anode or a BDD anode. The coke anode failed to promote the formation of additional sulfate, showing that the soluble sulfur species is not an intermediate en route to sulfate, but sulfate was formed quantitatively when the second electrolysis was carried out at BDD (Table 6). Mineralization of aryl sulfonate detergents has previously been reported at BDD anodes,28,29 although neither the formation nor the yield of sulfate as a product could be verified from that work because Na2SO4 was used as the supporting electrolyte. Sulfate was indeed formed upon oxida-
Table 5. Final Concentrations of Total Sulfur in Solution by ICP in Relation to the Total Organic Carbona entry number
nature of anode
calculated initial concentration of sulfur (µg/g)
final ICP concentration of sulfur (µg/g)
TOC (mg/L)
1 2 3 4 5 6
new new used new used used
0 0 0 975 1036 955
