Environ. Sci. Technol. 1999, 33, 2645-2650
Ferrate(VI) Oxidation of Thiourea V I R E N D E R K . S H A R M A , * ,† WAYNE RIVERA,† VISHWAS N. JOSHI,† FRANK J. MILLERO,‡ AND DON O’CONNOR§ Department of Chemistry, Texas A&M UniversitysCorpus Christi, Corpus Christi, Texas 78412, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, and Center for Fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712
The rates of thiourea oxidation with Fe(VI) were determined as a function of pH (8.8-11.5) and temperature (10-35 °C). The rate law for the oxidation of thiourea was found to be -d[Fe(VI)]/dt ) k0{[H+]/[H+]+K9)}[Fe(VI)][thiourea] where k0 ) 5.9 × 104 M-1 s-1, pK9 ) 7.8. The activation energy, ∆E*, obtained was 32.2 ( 1.8 kJ mol-1. The rate constant for the reaction of Fe(VI) with the thiourea radical was determined by using a premix pulse radiolysis technique and was found to be 8.5 × 108 M-1 s-1 at pH ) 9.0. The stoichiometric ratio of Fe(VI) and thiourea was found to be 1:0.38 ( 0.02 at pH ) 9.0. Sulfate and urea were identified as the products of the reaction. Tests were conducted on the oxidative destruction of thiourea by Fe(VI) in synthetic boiler chemical cleaning wastes (BCCW) and are discussed.
8HFeO4- +
Introduction Thiourea (NH2CSNH2), a sulfur-containing organic compound, has many industrial applications. The reaction of thiourea with hydrogen peroxide under certain conditions produces a powerful reductive bleaching agent which is routinely used in the textile industry (1, 2). Thiourea and its derivatives are known corrosion inhibitors (3). Inner surfaces of industrial equipments, like boilers, develop scales due to corrosion (4). The corrosion products generally consist of metals and their oxides in small amounts. The slow buildup of corrosion products over a period adversely affects the performance of the boilers (5). Thiourea in dilute hydrochloric acid is used as a complexing agent for removing scales from boilers (6). The boiler chemical cleaning wastes (BCCWs) are then disposed of according to the guidelines established by environmental agencies. Thiourea is toxic (7) and a cancer support agent (8). High concentrations of thiourea in industrial wastes may not be acceptable due to high oxygen demand and organic nitrogen content. Organic sulfur compounds are common in anoxic environments of wastewater and sediments and are environmentally significant due to their offensive odor. These and other environmental concerns have promoted studies on the destruction of thiourea from BCCW (9, 10). Thiourea can be oxidized by a wide variety of oxidizing agents (11* Corresponding author e-mail:
[email protected]. Present address: Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901-6975. † Texas A&M UniversitysCorpus Christi. ‡ University of Miami. § University of Texas at Austin. 10.1021/es981083a CCC: $18.00 Published on Web 06/23/1999
17). The reaction pathways and the final products of the oxidation reaction depend on the pH and the condition of the reaction mixtures. Chlorine dioxide oxidation of thiourea at low pH and excess thiourea gave thiourea, sulfur, and cyanamide (11). However under excess chlorine dioxide, the formation of formamidine sulfanic acid occurred. When pH > 3, sulfate, as a byproduct, was detected in thiourea oxidation by chlorine dioxide. Oxidation of thiourea with sodium peroxydisulfate and hydrogen peroxide yields NH4+, sulfur, SO42-, and CO2 under acidic conditions and in excess of either of the oxidants (12, 14). But in excess thiourea, the formamidine disulfide was formed at pH < 1, and thiourea dioxide was produced under neutral conditions (1, 14, 15). A potential process for on-site treatment of BCCW involves alkali treatment to precipitate metals as hydroxides followed by oxidation of thiourea with hydrogen peroxide to urea and sulfate (9). The oxidative destruction of simulated BCCW by air (O2) at an activated charcoal surface has been examined in an alkaline medium (10). In this study, major products were dicyandiamide and thiosulfate; urea and sulfur were the minor products. We herein propose the use of hypervalent iron, ferrate(VI) (Fe(VI)), for the destruction of thiourea from BCCW. Fe(VI) has shown great promise as a multipurpose wastewater treatment chemical for disinfection, oxidation, and coagulation (18, 19). Investigation of removal of metals, nonmetals, and radionuclides by Fe(VI) has also been conducted (20, 21). Fe(VI) is a strong oxidizing agent (22) and oxidizes a wide variety of substances including those relevant to environmental pollution and toxicity (21-25). Recently (26, 27), we have reported the Fe(VI) oxidation of hydrogen sulfide and cyanide to less harmful products in the aquatic environment (eqs 1 and 2).
1999 American Chemical Society
3H2S + 6H2O f 8Fe(OH)3 + 3SO42- + 2OH- (1)
2HFeO4- + 2HCN + 5/2O2 + H2O +
2OH- f 2Fe(OH)3 + 2HCO3- + 2NO2- (2)
In the present research, the rates of oxidation of thiourea by Fe(VI) were measured as a function of pH (8.8-11.5) and temperature (10-35 °C). The reactivity of Fe(VI) with a thiourea radical, a likely intermediate of the reaction, has also been determined by using premix pulse radiolysis techniques to understand the mechanism of the thiourea oxidation. Finally, experiments were conducted to test the Fe(VI) efficiency in removing thiourea from synthetic BCCW.
Experimental Section All chemicals used (Sigma, Aldrich) were of reagent grade and were used without further purification. Solutions were prepared with water that had been distilled and then passed through an 18 M Ω Milli-Q water purification system. Potassium ferrate (K2FeO4) of high purity (98% plus) was prepared by the method of Thompson et al. (28). The Fe(VI) solutions were prepared by addition of solid samples of K2FeO4 to 0.005 M Na2HPO4/0.001 M borate, pH 9.0, where solutions are most stable (19, 29). Phosphate serves as a complexing agent for Fe(III), which otherwise precipitates rapidly as a hydroxide that instantly interferes with the optical monitoring of the reaction and also accelerates the spontaneous decomposition of Fe(VI) (19). A molar absorption coefficient 510 nm ) 1150 M-1 cm-1 was used for the calculation of [FeO42-] at pH ) 9.0 (29). VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The stoichiometry of the Fe(VI) oxidation of thiourea was examined by product analysis after complete reaction of excess Fe(VI) with thiourea or vice versa. The experiments were done at pH ) 9.0 where Fe(VI) is most stable and the effect of its spontaneous decomposition is insignificant (19). A high performance liquid chromatography (HPLC) technique was used for determining the concentration of thiourea (30). Briefly, in this technique, a separating column, NovaPak C18 (3.9 × 150 mm), was connected to a Waters 996 photodiode array detector. The eluent was a mixture of methanol and 2.5 × 10-3 M tetrabutylammonium hydrogen sulfate (5:95). The flow rate was 1 cm3 min-1, and the detection wavelength was 214 nm. In the product analysis experiments, HPLC methods were used for analysis of sulfate and urea (26, 31). Kinetic studies were carried out by using a Rapid Kinetic Accessory (Applied Photophysics, UK), attached to a UV/vis diode array spectrophotometer (Hewlett-Packard model 8452A) (31). Kinetic runs were acquired and interpreted using OLIS Diode Array software (On-Line Instrument Systems Inc.) which had been interfaced with the diode array spectrophotometer. The temperature of the reactions was controlled within (0.1 °C by an isothermal circular bath (Fisher Scientific). Experiments were carried out under pseudo-firstorder conditions. The concentration of thiourea was kept in excess. Fe(VI) concentrations ranged from 100 to 125 µM. Thiourea solutions were prepared in 0.01 M phosphate buffers to obtain desired pH of the reaction mixtures. Reactions were monitored at 510 nm wavelength at which there is an absorption maximum for Fe(VI) (29). Rate constants represent the average of nine experimental runs for each thiourea concentration. With our kinetic system, we were able to determine the rates of the reaction only in the pH range 8.80-11.70. The reactions were too rapid at pH lower than 8.80. The reaction of Fe(VI) with thiourea radical (TU•) was performed on a premix pulse radiolysis setup. The following reaction scheme was used. N2O
H2O ' OH•
(3)
TU + OH• f TU•
(4)
•
Fe(VI) + TU f Fe(V) + product
(5)
The hydroxyl radicals (
