Decomposition of Hydroxybenzoic and Humic Acids in Water by

University of Osaka Prefecture, Gakuen-cho 1-2, Sakai,. Osaka 593, Japan, and Department of Applied Materials. Science, College of Engineering, Univer...
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Environ. Sci. Technol. 1996, 30, 1133-1138

Decomposition of Hydroxybenzoic and Humic Acids in Water by Ultrasonic Irradiation Y O S H I O N A G A T A , * ,† K Y O Z O H I R A I , ‡ HIROSHI BANDOW,‡ AND YASUAKI MAEDA‡ Research Institute for Advanced Science and Technology, University of Osaka Prefecture, Gakuen-cho 1-2, Sakai, Osaka 593, Japan, and Department of Applied Materials Science, College of Engineering, University of Osaka Prefecture, Gakuen-cho 1-1, Sakai, Osaka 593, Japan

Sonochemical decomposition of a series of hydroxybenzoic acids such as monohydroxy-, 3,4-dihydroxy-, 3,4,5-trihydroxybenzoic acids, tannic acid, and reagent and prepared humic acids in water under argon or air atmosphere was investigated. The decomposition followed first-order kinetics at initial stage, and initial rates were in the range of 1.9-5.1 µM min-1 under argon and 1.9-16.4 µM min-1 under air. The rates of OH radical formation in the sonolysis of water were estimated to be 20 µM min-1 under argon and 15 µM min-1 under air from the yield of Fe(III) formed by the sonication of Fe(II) solution. The decomposition of 3-hydroxybenzoic acid was almost completely inhibited by the addition of 0.1 mM t-BuOH, which is an effective scavenger of OH radicals. It is suggested that, in sonolysis under argon, the main sonochemical decomposition of the substances employed in this study proceeds via reactions with OH radicals in the bulk solution and that the contribution of thermal decomposition in cavitation bubbles or the interfacial region (between the bubbles and bulk solution) is small. In the sonolysis under air atmosphere, the role of oxygen was small in monohydroxybenzoic acids but increased with increasing numbers of OH groups substituted on the aromatic ring, suggesting the occurrence of decomposition of polyhydroxybenzoic acids induced by oxygen molecules at the interface. The chloroform formation potentials of 3-hydroxybenzoic acid and humic acids decreased due to the sonication, but the reduction in the potential was less than the corresponding amounts of decomposition of the starting substances.

* Corresponding author telephone: 81-722-36-2221; fax: 81-72236-3876. † Research Institute for Advanced Science and Technology. ‡ College of Engineering.

0013-936X/96/0930-1133$12.00/0

 1996 American Chemical Society

Introduction Humic substances are among the most widely occurring natural products on the earth’s surface. They exist in soils, lakes, rivers, and the sea as water-soluble compounds (1, 2). These substances have been reported to form trihalomethanes when water was treated with chlorine for sterilization, and this has become a problem in drinking water treatment (3-6). On the other hand, hydroxybenzoic acids have been used in intermediary chemicals in a wide variety of industrial synthetic processes. They may be regarded as a sort of primitive model compound for humic acid (7, 8) considering their chemical structures and may also have trihalomethane formation potential. The chemical effects of ultrasound (9) are attributed to cavitation, which is accompanied by the conditions of high temperature and high pressure. In aqueous sonochemistry, three different reaction sites have been postulated: (1) Interiors of collapsing cavities where temperatures of several thousand degrees and pressures of hundreds of atmospheres have been reported to exist (10, 11). Water vapor is pyrolyzed to OH radicals and hydrogen atoms (12), and gas-phase pyrolysis and/or combustion reactions of volatile substances dissolved in water take place. (2) Interfacial regions between the cavitation bubbles and the bulk solution. Though the temperature is lower than in the bubbles, a high temperature with a high gradient is still present in this region (13). Locally condensed OH radicals in this region have been reported (14). (3) Bulk solution at ambient temperature where reactions of OH radicals or hydrogen atoms that survive migration from the interface may occur. Recently, the role of supercritical water during cavitation has been reported (15). Some studies have been done on the application of ultrasound to the degradation of contaminant substances in water, for example, polycyclic aromatic hydrocarbons (16), parathion (17), geosmin (18), diverse phenols (1921), hydrogen sulfide (22), chlorinated hydrocarbons (2326), and chlorofluorcarbons (27, 28) have been investigated. In the course of our studies on the chemical effects of ultrasound (18, 25, 26, 28, 29), we have found that chlorinated hydrocarbons and chlorofluorocarbons were readily decomposed by a sonochemical method, and we report here the results of sonochemical decomposition of monohydroxybenzoic acids (HBAs), 3,4-dihydroxybenzoic acid (3,4-DHBA), 3,4,5-trihydroxybenzoic acid (gallic acid) (GA), tannic acid (TA), and both reagent and prepared humic acids (HAs) in water with our interest focusing on the feasibility of sonochemical destruction of these nonvolatile substances of environmental concern. We also discuss the fundamental processes of aqueous sonochemical reactions of hydroxylated derivatives of benzoic acid.

Experimental Section All of the HBAs (2-, 3-, and 4-hydroxy isomers) obtained from Wako Chemicals and DHBAs (2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dihydroxy isomers) from Tokyo Kasei and GA from Wako Chemicals were purified by recrystallizaion from water-methanol mixtures. TA from Wako Chemicals was purified by washing with diethyl ether in a Soxhlet extractor. Reagent-grade Fe(II)SO4(NH4)2SO4, Na2HPO3, KH2PO3, Na2SO4, NaClO, and Na2SO3 were purchased from Wako

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Chemicals and used without further purification. Chloroform and t-BuOH from Wako Chemicals were purified by distillation. Five-nine-grade argon was purchased from Osaka Sanso. Water was treated in a Millipore system (MilliQ). Identification and determination of HBAs, DHBAs, GA, and TAs were performed by a HPLC (Shimadzu LC-6A) equipped with a photodetector on an ODS-18 column using Na2HPO3, KH2PO3, and Na2SO4 as eluents. A column (Asahi pack GS-510H) for GPC was used for measuring molecular weight distribution of HAs. Absorption spectra of starting materials and sonicated reaction mixtures were measured by a spectrophotometer (Shimadzu UV-3100S). Determination of chloroform and hydrogen was performed by a gas chromatograph (Hewlett Packard 5890A) equipped with an electron capture detector or a thermal conductivity detector. Preparation of HA was as follows: To about 5 L of soil collected from a field at Nagasone-cho, Sakai, Japan, 8 L of aqueous NaOH solution (pH 12) was added. After 3 h of stirring and then 1 day of standing, the mixture was filtered through 10-layered gauze and then through a no. 2 filter. About 5 L of dark brown solution was obtained by these procedures. This solution was centrifuged (Tomy Seiko RS-18P) at 2600g for 15 min, and the remaining soil matter was removed. The supernatant was acidified to pH 1 with concentrated HCl to precipitate humic acid. The precipitated crude humic acid was dissolved in a minimal amount of aqueous NaOH solution; the solution was then centrifuged (10000g, 40 min); and fine soil was removed as a precipitate. By this treatment, the humic acid solution could be kept from a prolonged alkaline state; otherwise, the deterioration of humic acid is encountered. To the resulting supernatant, concentrated HCl was again added to adjust to pH 1. After standing for 1 day, humic acid was obtained as a precipitate through centrifugation (10000g, 40 min). The precipitate was dissolved in a minimal amount of NaOH solution and was desalinated by treatment with cation (Dowex 50) and anion (Dowex 1) exchange resin. The prepared humic acid was obtained as a fine powder by freeze-drying the desalinated solution. Reagent HA from Wako Chemicals was purified in a procedure similar to that employed for the prepared HA except for the omission of the first step. The results of elemental analysis were C 48.72%, H 5.02%, and N 5.28% for prepared HA and C 55.6%, H 2.43%, and N 1.11% for reagent HA. A multiwave ultrasonic generator (Kaijyo 4021) and a barium titanate oscillator of 65 mm i.d. were used for ultrasonic irradiation and operated at 200 kHz with an input intensity of 200 W. A sample solution of 65 mL was sonicated in a cylindrical glass vessel of 50 mm i.d. with a total volume of 150 mL. The vessel had a side arm with a silicon rubber septum for gas bubbling or sample extracting without exposing the sample to the air. The bottom of the vessel was planar and made as thin as possible (1 mm) because transmission of ultrasonic waves increases with decreasing thickness of the bottom. The vessel was mounted at a constant position relative to a nodal plane of the sound wave (3.75 mm: λ/2 from the oscillator). During the irradiation, the vessel was closed. For evaluation of the intensity of ultrasound, pure water and 1-100 mM Fe(II)SO4(NH4)2SO4 solution were sonicated. The amount of hydrogen formed from pure water was measured by the gas chromatograph, and the amount of Fe(III) ions formed via oxidation of Fe(II) by OH radicals was spectrophoto-

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FIGURE 1. H2 formation from pure water and Fe(III) formation from Fe(II) solution during sonication under argon and air. (b) Fe3+ and (2) H2 under argon atmosphere; (O) Fe3+ and (4) H2 under air atmosphere.

metrically measured ( of Fe(III) ) 2194 M-1 cm-1, at λ ) 304 nm at 298 K). For comparative experiments of OH radical reactions, γ-irradiation was performed using 60Co 20000 Ci. A cylindrical glass vessel (35 mm i.d.) was used for irradiation. An N2O-saturated sample solution was irradiated at a dose rate of 1.5 × 103 Gy. Radiolysis of N2O-saturated water at this dose rate gave the rate of OH radical formation of 20 µM min-1, which is equal to the rate of formation of OH radicals in the sonolysis of water under argon atmosphere in the present work. The procedure for the determination of the chloroform formation potential was as follows: An aqueous solution of NaClO (6.7 mM, 5.56 mL) was added to a 50-mL sample of sonicated solution in a 100-mL flask, which had a side arm with a silicon rubber septum for head space sampling. The reaction mixture was shaken for 25 h at 25 °C in an incubator, and then an aqueous solution of Na2SO3 (20%, 150 µL) was added to the solution to remove residual chlorine. The amount of chloroform derived from the procedure was determined by the gas chromatograph using the head space method.

Results and Discussion Sonolysis of Water. The main primary chemical process in the sonolysis of water is the thermal dissociation of water to hydrogen atoms and hydroxyl radicals:

H2O f H• + •OH

(1)

In argon atmosphere, combination reactions would be predominant:

•

H• + H• f H2

(2)

OH + •OH f H2O2

(3)

H• + •OH f H2O

(4)

Figure 1 shows the formation of Fe(III) ions and hydrogen molecules during sonication of pure water and aqueous Fe(II) solution under argon or air. From the results, we can estimate the rate of formation of hydrogen molecules and hydrogen peroxide to be R(H2) ) 10.7 µM min-1 and R(H2O2) ) R(Fe3+)/2 ) 10.0 µM min-1 in the sonolysis of pure water under argon. These values suggest that reactions 2 and 3

proceed at nearly equal rate. The rate of OH radical formation (eq 1) is difficult to estimate because the fraction of reaction 4 in the combination reactions is unknown. The rate of Fe(III) formation in the sonolysis of Fe(II) solution under argon was 20 µM min-1 in a range of 1-100 mM Fe(II) concentration. In this system, all the OH radicals would eventually react with Fe(II) to form Fe(III) (eqs 5 and 6), except for reaction 4 in cavities where OH radicals cannot be scavenged by Fe(II) because Fe(II) ions do not have substantial vapor pressure:

2Fe2+ + H2O2 f 2Fe3+ + 2OH-

(5)

Fe2+ + •OH f Fe3+ + OH-

(6)

FIGURE 2. Yield of H2O2 as a function of the concentration of tertbutyl alcohol during sonication (10 min) of water under argon.

All the compounds employed in the present investigation was nonvolatile, and the reaction of these compounds in cavitation bubbles may be excluded as in the case of Fe(II); therefore, the rate of the supply of OH radicals that can contribute to the decomposition of the solutes in the interfacial region and in the bulk solution may be estimated to be 20 µM min-1 under argon. In oxygen or air atmosphere, hydrogen atoms would be captured by oxygen molecules:

•

H• + O2 f •HO2

(7)

HO2 + •HO2 f O2 + H2O2

(8)

Under the present experimental conditions, no hydrogen molecules were observed in the sonolysis of water under air atmosphere, i.e., all of the hydrogen atoms would react with oxygen molecules (eq 7). The rate of formation of Fe(III) ions in an Fe(II) solution under air was 30 µM min-1; therefore, one can estimate the rate of formation of OH radicals R(•OH) ) R(Fe3+)/2 ) 15 µM min-1 under air atmosphere assuming R(•H) ) R(•OH). However, some of the radicals might be consumed by reaction 9, so the net value of R(•OH) under air may be •

HO2 + •OH f H2O + O2

FIGURE 3. 2-HBA (100 µmol/L) decomposition, products formation, and pH change during sonication: (-×-) pH; 2-HBA under air (O) and under argon (b); 2,3-DHBA under air (4) and under argon (2); 2,5-DHBA unde air (0) and under argon (9).

(9)

somewhat larger than that estimated above. In brief, the rates of OH radical formation were estimated to be 20 µM min-1 under argon and 15 µM min-1 under air and that of hydrogen formation was estimated to be 20 µM min-1 under argon in the sonolysis of pure water. The temperature in a collapsing cavity was defined (10) as Tfin ) Tin(Pfin(γ - 1)/Pin) where Tfin and Pfin are the final temperature and pressure; Tin and Pin are the initial temperature and pressure in the cavities; γ ) Cp/Cv is the ratio of the specific heat at constant pressure to the specific heat at constant volume of the gas in the bubble. Tfin is higher under argon than under air because of the higher γ value of argon (1.67) than that of air (1.40); therefore, the rate of OH radical formation would be faster under argon than under air. Scavenging of OH Radicals by t-BuOH. Figure 2 shows the yield of H2O2 as a function of the concentration of t-BuOH in the sonolysis of water. The formation of H2O2 was completely scavenged at a concentration of 1 mM t-BuOH; the concentration C1/2, at which t-BuOH depresses the H2O2 yield by 1/2, was 5 × 10-2 mM. These results are in fair agreement with those of the literature (30) despite

FIGURE 4. Decomposition of 3-HBA, 4-HBA, 3,4-DHBA, GA, and TA under argon: (2) 3-HBA, (4) 4-HBA, (O) 3,4-DHBA, (b) GA, (0) TA.

the different irradiation conditions (frequency, input power, and irradiation vessel) and suggest that t-BuOH is an effective OH radical scavenger in sonolysis of water. Sonolysis of Hydroxybenzoic Acids. Figure 3 shows the decomposition and the product formation and pH change during sonolysis of air- or argon-saturated aqueous solutions of 2-HBA. Figures 4 and 5 show the decomposition plofiles of sonolysis of 3-HBA, 4-HBA, 3,4-DHBA, GA, and TA under argon and air, respectively. In sonolysis of HBA, the decomposition at the initial stage obeyed a firstorder decay, and the rate did not differ very much for each

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FIGURE 5. Decomposition of 3-HBA, 4-HBA, 3,4-DHBA, GA, and TA under air: (2) 3-HBA, (4) 4-HBA, (O) 3,4-DHBA, (b) GA, (0) TA. TABLE 1

Pseudo-First-Order Rate Constants for Decomposition of Hydroxybenzoic Acids 102 × k′ (min-1) compd

Ar

2-HBA 3-HBA 4-HBA 3,4-DHBA GA TA

3.0 4.9 5.1 1.9 2.6 6.4

a

(R

2)a

(0.9981) (0.9997) (0.9999) (0.9760) (0.9875) (0.9997)

air

(R 2)

2.7 3.4 3.1 1.9 5.5 16.4

(0.9875) (0.9972) (0.9988) (0.9835) (0.9955) (0.9869)

R is the correlation coefficient.

starting compound. The rate was faster in argon than in air as already seen in pure water sonolysis. The ratio of the mean initial decomposition rate of HBAs under argon R(HBA)Ar to that under air R(HBA)air ) (3.0 + 4.9 + 5.1)/(2.7 + 3.4 + 3.1) ) 1.4 (see Table 1). The ratio of the rate of OH radical formation under argon to that under air was estimated to be R(•OH)Ar/R(•OH)air ) 20/15 ) 1.3 and is close to the value of R(HBA)Ar/R(HBA)air. Hence we assume that the decomposition of HBAs is mainly caused by OH radicals, and oxygen molecules have little effect on the decomposition. The noticeable intermediary products were hydroxylated compounds in all cases. The sites of hydroxylation show ortho and para orientation to the OH group in the parent compounds as generally observed in OH radical addition to aromatic compounds due to the electrophilic character of OH radicals, and ortho orientation was predominant over para orientation to some extent. No meta-hydroxylated compound was observed. All of the hydroxylated compounds, as seen in Figure 3, were subjected to further decomposition by subsequent ultrasonic irradiation. In sonolysis of polyhydroxybenzoic acids, the decomposition shows a first-order decay mode in a fashion similar to those of HBAs. However, the effect of atmospheric gas on the decomposition of polyhydroxybenzoic acids was different from that with HBAs. In 3,4-DHBA decomposition, the rates were nearly equal under air and argon atmosphere, and in GA the rates under air were faster than those under argon. This trend was more obvious in TA, i.e., decomposition rates in air increased with an increasing number of OH groups bound to the aromatic ring. The oxidation of phenolic compounds with oxygen molecules generally tends to be facilitated by increasing numbers of OH groups

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FIGURE 6. Formation potential of CHCl3 by sonolysis of 3-HBA and humic acid under air: (4) 3-HBA, (9) prepared HA, (O) reagent HA.

attached to the aromatic ring. We assumed that oxygen molecules contribute to the oxidation of polyhydroxybenzoic acids at the interface. The contribution of oxygen would be small in the case of HBAs but would become larger with increasing numbers of attached OH groups and would become important in the sonolyses of polyhydroxybenzoic acids, such as GA and TA, under air. Sonolysis and Radiolysis of Humic Acid. Sonolysis of both reagent HA and prepared HA resulted in the lowering of the molecular weight, e.g., 4800 f 3400 in reagent HA, and 200 000 f 100 000 and 10 500 f 6400 in prepared HA. On the other hand, γ-irradiation of N2O-saturated aqueous prepared HA, in which the reaction with OH radicals may be predominant:

H2O f H• + •OH + eaq

(10)

eaq + N2O + H2O f •OH + N2 + OH-

(11)

resulted in an increase in molecular weight of the high molecular weight part (200 000 f 250 000) and a decrease in molecular weight of the low molecular weight part (10 500 f 7000). The lowering of molecular weight of the lower part would be caused by OH radical decomposition both in radiolysis and sonolysis. The increase in molecular weight of the higher part in radiolysis may take place due to the condensation of radicals induced by OH radicals while ultrasound-induced lowering of molecular weight of the higher part would be caused by shock wave or hydrodynamic shear force during the collapse of the bubbles, which has been postulated in the general depolymerization mechanism of polymers by ultrasound (31, 32). Depolymerization by these physical forces, which would have no influence on low molecular compounds, may proceed through the condensation induced by OH radicals. Effect of Sonolysis on Chloroform Formation Potential of Humic Acids and 3-HBA. Figure 6 shows the change in the chloroform formation potential by sonolysis of 3-HBA and HAs. The effect of ultrasound on the potential differs in each substance: prepared HA > 3-HBA > reagent HA. In reagent HA, the effect was small. There were appreciable amounts of the water-insoluble part in the reagent HA, and the molecular weight of the soluble part was considerably lower than that of prepared HA. The different behavior of this reagent HA from that of naturally occurring HA was reported (33), and the choice

FIGURE 7. Change in absorption spectra of 2-HBA on sonolysis (60 min): (a) before irradiation; (b) under air; (c) under argon.

of the reagent HA may not be suitable for the experiment on chloroform formation potential. The potentials decreased with irradiation in all substances, but the decreased amounts were less than the corresponding amount of the starting material decomposed. The intermediary decomposition products having chloroform formation potential are assumed to be formed. A longer irradiation time than that for complete destruction of starting materials would then be required to remove the entire potential. Decomposition Mechanism and Reaction Site. Figure 7 shows the change in the UV absorption spectra of 60-min sonicated 2-HBA solution under argon and air. The absorption around 240 and 280 nm, both assigned to aromatic rings, decreased with irradiation. Similar results were obtained in 3- and 4-HBA both under air and argon atmosphere. Figure 8 shows the change in the absorption spectra of TA due to the sonolysis under argon and air atmosphere. As observed for HBAs, absorption attributed to aromatic rings decreased with the irradiation. Similar profiles were observed in the sonolyses of GA. The absorbance of humic acid around 240-360 nm also decreased with ultrasonic irradiation. These results suggests that, in all of the substances employed in the present experiments, the aromatic rings are cleaved by ultrasonic irradiation. Figure 9 shows the inhibitory effect of t-BuOH on sonolysis of 3-HBA. The decomposition of 3-HBA was almost completely quenched at the concentration of 0.1 mM t-BuOH. These results suggest that the sonolytic degradation of HBAs mainly proceeds via reaction with OH radicals, and thermal decomposition in cavitation bubbles or interfacial regions may be excluded. Table 1 shows pseudo-first-rate constants for the decomposition of the substances examined in the present work. The rates under argon were only 1/10-1/4 of the rates of OH radical formation from sonolysis of water, suggesting that the recombination of OH radicals may occur to a fairly large extent. The rate constants for the reaction of the substances presently examined with OH radicals in water at ambient temperature may be on the order of 109 M-1 s-1 (rate constants of OH radical addition to aromatic rings), which is the same order of OH radical recombination. It is reasonably assumed that in the bulk solution the concentration of the substances is far higher than that of transient OH radicals, therefore, the recombination of OH radicals in bulk solution would hardly occur. Because

FIGURE 8. Change in absorption spectra of TA (ppm) on sonolysis.

FIGURE 9. Effect of t-BuOH on sonolysis of 3-HBA: (2) in 0.1 mM t-BuOH; (b) in pure water.

hydroxybenzoic acids have appreciable hydrophilicity, local condensation of these substances at the interface would not occur, and the difference in concentration between the interfacial region and the bulk solution may be small. Accordingly, a fair amount of locally condensed OH radicals (15) may recombine at the interface. We suggest in conclusion that sonochemical decomposition of hydroxybenzoic acid derivatives under argon atmosphere mainly proceeds via reactions with OH radicals in the bulk solution, and under air, oxygen molecules participate in the decomposition of polyhydroxybenzoic acids at the interface. Thermal decomposition in cavitation bubbles and the interfacial region, which play an important role in the degradation of volatile and hydrophobic substances, is small under both atmospheres.

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To improve the decomposition yield, more effective ultilization of OH radicals is desirable. We have found that sonolytic decomposition of chlorophenols in water was enhanced in the presence of Fe(II) and assumed the following reactions that are similar to the Fenton reaction (34) to occur:

•

H2O2 + Fe(II) f Fe(III) + OH- + •OH

(12)

OH + chlorophenols f decomposition

(13)

In analogy, it may be possible that an appropriate amount of Fe(II) addition will increase the efficiency of decomposition of HBAs by ultrasound. An experiment in line with this expectation is now in preparation.

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Received for review May 16, 1995. Revised manuscript received November 3, 1995. Accepted November 8, 1995.X ES950336M X

Abstract published in Advance ACS Abstracts, February 1, 1996.