Environ. Sei. Technol. 1993, 27, 1871-1874
Oxidative Destruction of Thiourea in Boiler Chemical Cleaning Waste Solutions Jack
G. Frost
Completions Research and Engineering Department, Halliburton Services, Duncan, Oklahoma 73536-0439 Thiourea and its lower homologues are uniquely useful in an acidic process for dissolving utility power plant boiler scales containing iron oxides and copper. The oxidative destruction of thiourea in boiler chemical cleaning waste (BCCW) solutions can be easily accomplished by neutralizing the acid to an excess of the base, adding powdered charcoal and sparging with air or 0 2 . The major reaction path leads to dicyandiamide and sodium thiosulfate. A minor reaction path produces urea and sulfur. Using 0.52% powdered charcoal and an aeration rate of 7.5 mL/min-l 100 mL-’, thiourea is reduced from 2% (0.263 mol/L) to detection limits within 24 h or less.
Introduction Normal operation of a utility power plant steam generator results in the formation of corrosion products on the walls of all tubes in the boiler ( I ) . These corrosion products consist primarily of iron oxides (usually magnetite, FeFe204) and metallic copper. Also present are nickel, zinc, and much smaller amounts of chromium, probably all in the form of iron spinels. Over a period of 2-4 years, the slow buildup of these encrustations can adversely affect performance of the generator (2). They can reduce heat transfer, restrict flow, foster corrosion of the underlying metal, and may result in actual tube failure. One method of dissolving these encrustations involves a two-step process: one stage using a dilute, inhibited HC1 solution for iron oxide and spinel dissolution and a second stage consisting of ammonia and an oxidizingagent for dissolving metallic copper ( 3 , 4 ) . Another method is favored by some because it involves only a single cleaning stage. In this method, iron oxides and copper are dissolved simultaneously in dilute HC1 containing thiourea and/or lower homologues of thiourea (5-7). For this application, thiourea and its lower homologues are unique, because they are the only known ligands capable of forming copper complexes that are both appreciably soluble and highly stable toward disproportionation in dilute mineral acid (8).
There is sufficient evidence to conclude that thiourea is carcinogenic to laboratory animals (9, 10). Although there are no adequate data to evaluate the carcigenicity to humans, it seems prudent to deal with the potential problem by eliminating thiourea from these boiler chemical cleaning wastes (BCCWs). Thiourea will react with a wide variety of oxidizing agents (11-15). However, from a practical or an economical standpoint, most of these would not be considered useful in a large-scale application. At first glance, two excellent candidates as oxidizing agents for thiourea in BCCWs would appear to be sodium peroxydisulfate and hydrogen peroxide (16-21). Under controlled conditions, a variety of reaction products occur, but in acidic solution an excess of either oxidizing agent results in the formation of NH4+, sulfur, S04-2,and C02. In strongly basic solutions, H202 oxidizes thiourea to urea and sulfate ion (22). Since any 0013-936X/93/0927-1871$04.00/0
0 1993 American Chemical Society
Table I. Standard Electrode Potentials for S Z O ~ ~HaOz, -, 0 2 , and Cr(II1)
-+ + -
couple
+ 213-
2S042HzOz + 2H+ 2e- 2Hz0 02 + 2Hz0 4e- 40HCr3+ + 4Hz0 Cr04-2 + 8 H+ + 3eSzOs”
-
E’ (V) +2.0 +1.776 +0.401 -1.1
viable process for removing thiourea from BCCWs would require that all oxidation products be environmentally benign, these two oxidants would appear to be acceptable for use. This would be the case if not for the presence of chromium in BCCWs. Chromium is present as Cr(II1) and is easily precipitated from solution to levels considered satisfactory under National Pollutant Discharge Elimination System (NPDES) discharge requirements (23) simply by raising pH. Chromium present as Cr0d2-is not easily removed without greater expense and effort. The oxidation of Cr(II1) to Cr(V1) by either H202 or Na2S2Os is thermodynamically favored, as can be seen in Table I (24). Because of this potential side problem involving chromium, it seems prudent to examine other methods for eliminating thiourea from BCCW. Early attempts at oxidative destruction of thiourea simply involved sparging air into neutralized BCCW solutions (pH = 10-12). Those experiments resulted in only partial thiourea reaction over a 24-h period. When the experiments were modified, first to contain higher concentrations of OH- and then to include powdered, activated charcoal, a significant (essentially complete) loss in thiourea was observed over the same reaction period. In the present report, this oxidative degradation of thiourea using air ( 0 2 ) at an activated charcoal surface is examined.
Experimental Section Reagents. The thiourea used in these experiments was Baker Analyzed Reagent (99.1% ), purified by recrystallization from warm 0.5% HC1. Activated charcoal was obtained from Norit Co., grade SX-1, with 050:090:010 = 20.1:64.1:4.1 pm and specific surface area of 0.638 m2/mL. This was used without further treatment. All other, common chemicals were reagent grade, used without further purification. Test Procedure. All test samples were prepared to contain approximately 2% thiourea, 5000 mg/L Fez+,and 300 mg/L Cul+ in 5% HCl. The samples were treated with sufficient NaOH to neutralize the acid and provide an excess of OH-. Experiments were conducted in a threeneck, 250-mL round-bottom flask equipped with a condenser, an air sparge, and a temperature-control probe. Airflow during the experiments was controlled with a Matheson Model 8270 flow controller. Constant mechanical agitation was done magnetically. Throughout the tests, small samples were removed from the flask, immediately filtered through a 0.45-pm filter membrane, cooled, and serially diluted 3:lO 000 for analyses. All glassware Environ. Scl. Technol., Vol. 27, No. 9,1993 1871
Table 11. Oxidation of Thiourea on Charcoal” time [NaOHl [thiourea] lapse [NaOHl [thiourea] consumed consumed (h) (mol/L) (mol/L)* (mol/L) (mol/L) 0.0 0.5 1.0 2.0 3.0 4.0 5.0 6.0
0.169 0.158 0.150 0.133 0.112 0.097 0.085 0.078
0.000 0.011 0.019 0.036 0.057 0.072 0.084 0.091
0.244 0.233 0.223 0.202 0.180 0.170 0.149 0.148
molar ratio
0.000 0.011 0.0.21 0.042 0.064 0.074 0.095 0.096
1.00 0.90 0.86 0.89 0.97 0.88 0.95
mean 0.92 The followingconditions were used: temperature, 25 “C; NaOH, 1.35 g/200 mL; charcoal, 4 g/200 mL; 02, 16 mL/min. Thiourea is
*
partially absorbed by activated charcoal, so the original bulk concentration of 0.256 mol/L was reduced to 0.244 mol/L after equilibrating with charcoal.
used for dilution was cleaned with chromic acid cleaning solution prior to use. Analysis Procedure. Thiourea is known to be UVactive with an absorption maximum reported at X = 23542 nm and a molar extinction coefficient oft = 4.1 (25,26). Thus, all quantitative analyses for thiourea were done by UV spectrophotometry. Samples were analyzed using a Beckman Model DU-7 spectrophotometer.
Results A thiourea solution was prepared and analyzed by ultraviolet spectrophotometry at 235 nm to contain 0.256 mol/L thiourea. To 200 mL of this solution were added 1.35g of NaOH and 4 g of activated charcoal. The solution was maintained at ambient temperature and sparged with pure oxygen at a rate of 16 mL/min. Samples were taken throughout the experiment and analyzed for NaOH and thiourea content. Data in Table I1 show that the molar ratio of OHconsumption to thiourea consumption is very nearly 1:l. This result conflicts with an early report by Freundlich (27)on the air oxidation of thiourea at a charcoal surface. In his work, Freundlich conducted his aeration experiments in unbuffered, neutral solutions. On the basis of a limited amount of analysis data, he concluded that the following reaction takes place: 2CS(NH,),
+ 0,
-
C,H,N,S
Envlron. Scl. Technol., Vol. 27, No. 9, 1993
data. Further, subsequent infrared and X-ray spectra of a known sample of dicyandiamide exactly matched spectra of the alcohol-soluble reaction products. All of the above information support the following as the predominant reaction:
2H20
+
2H2N-C-NH2
+ 2NaOH + 202
-
NH
II
+S
(1) Perhaps eq 1 is the reaction path for air oxidation of thiourea at a charcoal surface in neutral solution. No attempt was made to reproduce Freundlich’s unbuffered experiments. Experiments conducted early in this work revealed that air oxidation of thiourea on charcoal in neutral-to-weakly basic solution is too slow to be commercially viable. Indeed, those experiments confirmed Freundlich’s earlier observation: “The reaction proceeds relatively slowly; at 25 “C, for example, the thiourea concentration decreases from about 0.05 M to half that value in approximately 27 h.” Whatever may be the case in neutral solution, the experiment summarized in Table I gives convincing evidence that the reaction path is different at high pH from that of eq 1. Next, 400 mL of a solution was prepared containing 0.256 mol/L thiourea and 0.256 mol/L NaOH, to which 8.0 g of activated charcoal was added. This slurry was aerated with 0 2 at a rate of 35 mL/min for 16 h at 60 “C. At the end of this period solution pH was measured to be 9.7, and 1872
UV analysis showed no detectable thiourea. The charcoal was then removed by filtration, and the filtrate was freezedried. Total weight of white solids = 11.96g. This residue was extracted with ethanol overnight in a Soxhlet extractor, and the alcohol solution was evaporated to dryness. X-ray diffraction analysis showed that the alcohol-insoluble solids were primarily NazSz0~5Hz0,and the alcoholsoluble solids contained an unidentified component (major) and urea (small). Wet analysis of the total freezedried solids showed them to contain 59.7 % as anhydrous NazSz03 (7.14 9). The charcoal was stirred overnight in CS2, after which the CSz was evaporated to dryness. This yielded 0.22 g of yellow solids, which was analyzed by X-ray diffraction analysis to be elemental sulfur. Solids from the alcohol extract were further examined by infrared spectroscopy,CHN, and sulfur analyses. There was less than 0.5% S in the alcohol-soluble solids, confirming the virtual absence of Freundlich‘s proposed product. Further elemental analysis showed [C:H:Nl = 27.67:4.75:64.09% ,values that can be reduced to a molar ratio of 1.00:2.05:1.99. Infrared spectra showed strong absorption peaks at 2131-2207 and 3209-3251 cm-1, regions indicative of C=N and N-H stretching, respectively. Additionally, there was a strong absorption peak at 1635 cm-l, indicative of double bond stretching. The C:H:N ratio, coupled with the infrared data suggested the presence of cyanamide, H~NCEN, but the strong double bond stretch was not consistent with cyanamide. Noller (28) states, “Cyanamide is stable in aqueous solutions of pH
