Ind. Eng. Chem. Res.
1434
1991,30, 1434-1437
Effect of Simple Anions and Metal Ions on the Hydrolysis of Disulfate Ion Di-Xin Shen, David Littlejohn, and Shih-Ger Chang* Lawrence Berkeley Laboratory, Applied Science Division, Berkeley, California 94720
The influence of anions and cations commonly found in flue gas scrubber systems on the hydrolysis has been studied. The cations studied include Na+, K+, Ca2+,Mg2+, rate of disulfate ion (S2072-) and Mn2+. The anions studied include SO-,: SO-,: S2032-,S202-,HSO,, Cl-, H2P04-,and HP042-. Of the ions studied, Ca2+,Mg2+,S032-,and S202-were found to approximately double the rate of S207" hydrolysis. Adipic acid was found to have no effect on S20:- hydrolysis. Activation energies for the S2072-hydrolysis rate accelerated by Ca2+,Mg2+,and S032-,as well as for the unenhanced rate, were found to be in the range of 9-12 kcallmol. The oxidation of bisulfite ion (HS03-) is important in the chemistry of atmospheric aerosols and flue gas desulfurization systems. I t was recently discovered that disulfate ion is formed as an intermediate in the oxidation of bisulfite ion by oxygen in aqueous solutions (Chang et al., 1987; Littlejohn et al., 1988): O2
+ 2HS03-
-
S2072-
+ H20
(1)
The disulfate ion then hydrolyzes to form bisulfate ion and/or sulfate ion: S20T2- + H 2 0
-
HS04- + H+ + Sod2-
(2)
Several studies of the disulfate ion hydrolysis reaction have been reported in the literature. The activation energy (E, = 11.3 kcal/mol) has been established by several groups (Hofmeister and Von Wazer, 1962; Thilo and von Lampe, 1963; Ryss and Drabkina, 1973; Plathotnick and Drabkina, 1976, 1977). The hydrolysis rate of disulfate ion is influenced by other ions present in solution (Thilo and von Lampe,1963,1964,1967). To be able to accurately predict the disulfate hydrolysis rate, the cumulative effects of the ions present in solution must be determined. Thilo and von Lampe (1963,1964) studied the effect of cations on the hydrolysis reaction at 0 "C. They found that cations accelerate the hydrolysis and that the degree of acceleration is increased by increasing ionic charge and decreasing ionic radius of the cation. The enhancement of the hydrolysis reaction by metal ions is related to their stability constants for complex formation with EDTA, suggesting that metal ions form a complex with the disulfate ion to hasten the hydrolysis. In contrast to studies of cation effects, little work has been done on effects of anions on the disulfate hydrolysis rate, particularly those anions found in solutions used in flue gas desulfurization processes. Also, most work has been done at 0 "C, where the longer hydrolysis half-life makes it easier to study the reaction. Information is needed on the reaction at higher temperatures and on the effect of ions found in flue gas scrubbing solutions to be better able to model S2072-behavior in flue gas scrubbing solutions.
Experimental Section Sodium disulfate salt was prepared by heating sodium bisulfate to a temperature of about 180 "C for 3-5 days (Hofmeister and Van Wazer, 1962). The progress of the conversion was monitored by taking Raman spectra of samples of the heated solid. When spectra showed the absence of the NazHS04peak, the conversion to Na2S207
* Author to whom correspondence should be addressed.
was considered complete. The sodium disulfate salt is hygroscopic and was stored in a desiccator. There are other methods for preparing disulfate salts (Thilo and von Lampe, 1963). The other compounds used in this study were reagent grade chemicals. Doubly deionized water was used for preparing solutions. Initial studies of the disulfate hydrolysis reaction S207'-+ H 2 0 2HS04- 2S042-+ 2H+ (3) were done using Raman spectroscopy. Disulfate ion and its hydrolysis products sulfate ion and bisulfate ion have fairly strong Raman lines at 1090, 981, and 1050 cm-', respectively (Millen, 1950; Littlejohn et al., 1988), which can be used to obtain quantitative information when a reference compound is added to the solution. The hydrolysis reaction was initially studied by rapidly dissolving the disulfate salt in water or aqueous solution of the desired composition and inserting a sample in the Raman spectrometer, which monitored the signal intensities. The decay or buildup of the signal was used to determine the hydrolysis rate constant. Since Raman spectroscopy is not a very sensitive technique, it is not usable for disulfate ion concentrations below about 0.1 M. However, it did allow unambiguous determination of the reaction products and indicated no significant side reactions were occurring, No products other than sulfate and bisulfate ions were observed. For studies involving lower concentrations of disulfate ion, a pH meter monitored the change in the acidity of the solution during the hydrolysis of disulfate ion. The majority of the measurements were made using thistechnique. The signal from the pH meter was sent to a chart recorder for graphical output of the pH change vs time. The combined response of the pH electrode, meter, and recorder was tested by rapidly switching the pH probe between stirred, buffered solutions of acid and base. The testa indicated that greater than 95% of the signal change occured in 1s. For unbuffered solutions, the initial pH of the solution was adjusted to alkaline conditions (about pH 12) before the addition of the disulfate salt. Since OHwas found not to enhance the hydrolysis, the pH adjustment did not influence the results. A measured quantity of the disulfate salt was then added to the rapidly stirred solution containing the pH probe. Dissolution occurred more quickly than hydrolysis. The pH of the solution was generally followed to pH 3-4. At lower pH conditions, acceleration of the hydrolysis by H+ could occur, and corrections for the formation of HS04- would have been needed. Measurements by Thilo and von Lampe indicated that H+ enhancement of disulfate hydrolysis occurs only below pH 3. This may be due to the formation of the species HS207-,which could hydrolyze more quickly than
-
0888-5885/91/ 2630-1434$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 1436 Table I. Effect of Cations on the Hydrolysis Rate Constant of s*o,a10% 8-1 this study Thilo and von L a m p salt (0.010 M) (at 20 "C) (at 0 "C) chlorides Nat 8.0 f 1.6 2.2 Ca2+ 19.0 f 1.0 3.55 nitrates K+ 7.5 f 0.7 2.2 Mg2+ 18.0f 1.5 3.5 Ca2+ 19.0 f 1.2 3.55 S2072-alone" 9.7 f 0.2 2.2 ~~
-l."
I
8
0
10
.
8
20
30
40
50
60
Time (seconds)
Figure 1. Hydrolysis of S2071- at 20 "C.
S2O7*. The recorded signal was used to determine the H+
production as a function of time. This was used, in turn, to calculate the concentration of S20T2- vs time by using eq 3. Plots of In ([S2072-],/[S2072-]o) vs time were generated, in which the slope equals -kbd, the hydrolysis rate constant. A typical plot is shown in Figure 1. The time indicated in the figure is relative to the start of the measurements, which lagged behind the time of addition of the salt to the solution due to mixing and starting the recorder. The data a t longer times were generally not included to avoid the need to correct for the formation of HS04-. The slope was obtained by a least-squares fit to the data. Several runs were done at each condition and averaged together. The individual runs generally agreed well with each other. In cases where compounds that could act as buffers were used, such as HS03-, SOg", H2P04-, and HPOt-, the pH measurements were made over a smaller range where corrections due to buffering were not significant. For example, SO3* was studied by using a pH range of about 12-8. The majority of the pH change measurements were done with 0.010 M sodium disulfate solutions at a temperature of 20 "C. For temperature-dependent studies, runs were done from 0 to 30 "C. At temperatures above 30 "C, the hydrolysis was too rapid to obtain accurate measurements with the system used. The concentrations of the added salts tested for accelerating effects generally ranged from 0.01 to 1.0 M. The activation energies for the hydrolysis reaction in the presence of different ions was determined from Arrhenius plots in which In (khg)was plotted against 1/T for the values of khYdat 0, 10, 20, and 30 "C.
Results and Discussion Cations. A series of hydrolysis runs at 20 "C were done by using the pH change method to determine if the observations of the cation effect at 0 "C made by Thilo and von Lampe (1963,1964) persisted at higher temperatures. Na2S207was added to 0.010 M solutions of the cations under study to create an initial disulfate ion concentration of 0.010 M. The runs were analyzed as described above. The salts and the averaged rate constants from the experiments at 20 "C are listed in Table I along with the values obtained by Thilo and von Lampe (1964) at 0 "C. Chloride and nitrate salts were used because we found these anions to have essentially no accelerating effect on disulfate ion hydrolysis. The 2+ cations continue to cause an enhancement of the hydrolysis at 20 "C. Measurements were made at 0, 10, 20, and 30 "C. At 0 OC, cation concentrations were varied from 0.01 to 1.0 M. Our measurements at 0 "C agreed well with those of Thilo and von Lampe. The effect of metal ion concentration on the hy-
[Na2S2O7Io= 0.010 M. Table 11. Effect of Anions of the Hydrolysis Rate Constant of S.02- at 20 "C anion rate const &, anion rate const k, (0.010 M) s-l X lo9 (0.010M) 8-1 x 103 c18.0 f 1.6 SO? 32.0 f 2.0 so?8.2 f 1.1 H2POi 9.2 f 3.0 520929.7 f 0.2 HPOt9.4 & 1.8 S200" 28.4 f 3.8 S2072-alone 9.7 f 0.2 HSO; 9.0 0.3 ~~
~
~
*
I
0.012 -
-
-
"."
0.0
0.2
0.4
0.6
0.8
1 .o
Ion Concentration (M) Figure 2. Effect of concentration on the disulfate ion hydrolysis rate constant at 0 "C for Mg2+,Mn*+,and SO8%-.
drolysis rate constant at 0 "C for Mg2+ and Mn2+from our measurements is shown in Figure 2. The curves shown all start from the same value at zero ion concentration. Anions. In their studies of the disulfate hydrolysis reaction, Thilo and von Lampe (1967) did only a cursory study of anion effects. They observed that Na+ and Li+ salts of C1-, NO3-, C104-, and SO4* had very similar, small effects on disulfate ion hydrolysis, except at high concentrations. We did a systematic study of the effect on the rate of disulfate ion hydrolysis by anions commonly present in flue gas scrubbing solutions. Sodium salts were used in this part of the study because sodium ions were found to have no effect on the S20$- hydrolysis. Sodium salts of sulfite (SO3*), sulfate (SO,"), thiosulfate (S20S2-), dithionate (S20e2-),monohydrogen phosphate (HPOd2-), bisulfite (HSO;), chloride (Cl-), and dihydrogen phosphate (H2P04-) were used to study their effects on the disulfate ion hydrolysis rate at 20 "C at anion concentrations of 0.010 M. The HSOf, SO3%,H2PO;, and HP04% salts were studied over restricted pH ranges to minimize problems associated with buffer effects. Na2S207was added to the solutions to obtain an initial S20T2- concentration of 0.010 M. The results are compiled in Table 11. The effect of SOZ- concentration on the S207* hydrolysis rate constant at 0 "C is shown in Figure 2. The change in hydrolysis rate constant is initially much larger than that for the cations. However, at higher concentrations the influence of SOS* and the cations is similar. Sulfite and dithionate are the
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 Table 111. Activation Energies for Disulfate ion Hydrolysis" solution activation energy, kcal/mol water 11.6 i 0.6 11.9 1.3 0.01 M Mg(N03)z 8.4 0.7 0.01 M Ca(NO,), 9.0 i 1.0 0.01 M NaZSO3
[S03(2-)]=0.010 M
0.01
:
OO
**
[NazS2O7]= 0.010 M.
,
I
1
0.002
0.004
0.006
Magnesium ion concentration (M)
[Mg(2+)]=0.010 M
ob
1
, 0.002
0.004
0 006
Sulfite ion concentration (M)
Figure 4. Effect of SO?- concentration on the disulfate ion hydrolysis rate constant at 20 "C in the presence of 0.010 M Mg2+.
only anions of those tested that exhibit significant hydrolysis enhancement at these concentrations. Apparently, SO$- and S2062-interact with S2072- to enhance the hydrolysis rate. However, the mechanism by which this occurs is unclear. It is interesting to note that S032-has a much larger effect on the disulfate ion hydrolysis than HS03-. At conditions where HS03- predominates (
