Ultrasonics Sonochemistry 23 (2015) 128–134
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Modeling the oxidation kinetics of sono-activated persulfate’s process on the degradation of humic acid Wang Songlin a,⇑, Zhou Ning a, Wu Si a,b, Zhang Qi a, Yang Zhi a a b
School of Environment Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Construction Quality Supervision Station of Navy, Beijing 100161, PR China
a r t i c l e
i n f o
Article history: Received 13 August 2014 Received in revised form 23 October 2014 Accepted 23 October 2014 Available online 30 October 2014 Keywords: Ultrasound Persulfate Humic acid Sulfate radical Mineralization Kinetic model
a b s t r a c t Ultrasound degradation of humic acid has been investigated in the presence of persulfate anions at ultrasonic frequency of 40 kHz. The effects of persulfate anion concentration, ultrasonic power input, humic acid concentration, reaction time, solution pH and temperature on humic acid removal efficiency were studied. It is found that up to 90% humic acid removal efficiency was achieved after 2 h reaction. In this system, sulfate radicals (SO 4 ) were considered to be the mainly oxidant to mineralize humic acid while persulfate anion can hardly react with humic acid directly. A novel kinetic model based on sulfate radicals (SO 4 ) oxidation was established to describe the humic acid mineralization process mathematically and chemically in sono-activated persulfate system. According to the new model, ultrasound power, persulfate dosage, solution pH and reaction temperature have great influence on humic acid degradation. Different initial concentration of persulfate anions and humic acid, ultrasonic power, initial pH and reaction temperature have been discussed to valid the effectiveness of the model, and the simulated data showed new model had good agreement with the experiments data. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Generally humic substances would be characterized as the most widely-occurring natural mixture of organic compounds, which play an important role in both pollutant control chemistry and biogeochemistry in global carbon biogeochemical cycle [1]. They often have complex structures and impart brown/yellow color to water, and may bind heavy metals by complexation [2]. The degradation or removal of these organic matters are major problems faced by the water treatment industry as they act as important precursors of disinfection by-products in water [3]. In common water treatment processes, the overall or partial removal of humic substances could be obtained by physical separation, such as coagulation, adsorption and filtration. However, the techniques could not eliminate the pollutants but only transfer them from water to another phase, and may return back to water environment to threaten human health [1]. Thus, alternative oxidation processes should be under investigation. Variety of refractory organics has been effectively degraded by the advanced oxidation processes (AOPs), because they can generate strong oxidizing oxygen species such as OH radicals [2–5]. ⇑ Corresponding author. Tel.: +86 27 87792155; fax: +86 27 87792101. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.ultsonch.2014.10.026 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Several studies have been conducted involving the degradation of humic acid (HA) in aqueous solution using advanced oxidation processes, including photo-catalysis [2], Fenton’s reaction [5] and ozonation [6], and the result showed that humic acid can be removed from water effectively. But the catalyst such as Fe2+ and TiO2 would be external contaminant and the high cost of ozone would be also the restriction reason of its application. Persulfate anions have strong oxidation–reduction potential (2.01 V) and could be applied to the degradation of organic matter [7,8]. During degradation process, persulfate could be catalyzed to generate sulfate radicals (SO 4 ) (2.6 V) [9,10], which were very likely to be attracted to the special function groups. Subsequently the sulfate radicals would be transferred to the organics. The presence of a free radical at the organics would weaken and break the carbon bond. Consequently, the organic was degraded into relatively small substances and eventually removed [11,12]. Persulfate could be activated by thermal method to generate sulfate radicals, while more active species (sulfate or hydroxyl radicals) may yield at higher temperatures resulting in faster target pollutants degradation [13,14]. Jing et al. found that 87.02% carbamazepine (CBZ) is removed using thermally activated persulfate method, while the corresponding mineralization is only (5.7 ± 1.3)% after 4 h reaction [13]. Therefore, there is a need for an alternative process for the activation of persulfate.
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As one kind of AOPs, ultrasonics (US) has been investigated to degrade non-biodegradable pollutants, because of the simplicity of the system and no production of toxic by-products [15,16]. While in most cases degradation efficiency are limited to low level, and energy use efficiency is very low due to energy losses during transfer processes. So it is estimated that degradation rate of organic pollutants in a conventional sonochemical process should be promoted by 10–100 times for its application in practical wastewater treatment [17]. But Sonication in aqueous solution causes rapid formation, growth, and violent collapse of cavitation bubbles, resulting in enormous local temperature and pressure rises. It is a beneficial condition in sonolytic degradation because sulfate radicals (SO 4 ) would be formed from the oxidant by thermolytic cleavage of persulfate [16]. Li et al. found 1,1,1-trichloroethane (TCA) removal efficiency is 20% using persulfate without activated and almost 100% when oxidizing agent persulfate is activated by sonolytic method [18]. Sono-activated persulfate processes provide a promising treatment for organic pollutant in water. Some literature investigated many kinds of pollutant degradation using sono-activated persulfate processes, such as tert-butyl (MTBE) and arsenic (III) [15,16]. Previous researchers discussed the degradation efficiency under different reaction conditions and tried to reveal the mechanism of the reaction processes [16,18]. Chen et al. found that sulfate radicals would be the mainly oxidant for degradation of dinitrotoluene (DNTs) [19]. TCA would be decomposed mainly through sulfate and hydroxyl radicals as well as ultrasonic pyrolysis [18]. Pseudo-first-order model was proposed to describe the MTBE degradation and denotes the overall pseudo-first-order rate constant as k [16]. However, even the pseudo-first-order simulation results may fit the experimental values well, but the mechanism explanation was not from chemical perspective but from mathematical perspective. It is necessary to build up a new model which could describe the sono-activated persulfate reaction kinetic from both perspectives. In this study, a novel kinetic model for humic acid degradation using combined persulfate and ultrasound method in aqueous solution was established mathematically and chemically to explain the mechanism of the reaction process. Ultrasonic power, persulfate anions dosages and initial humic acid concentration had been taken into account. Moreover, solution pH, reaction temperature and characteristics of humic acid were also discussed in this paper. The model was validated and evaluated by the experimental data.
of initial solution was adjusted using 0.1 M H2SO4 and 0.1 M NaOH before reaction. 2.3. Procedure The stock solution was prepared by dissolving 1 g humic acid in 1L 0.1 M NaOH, then the supernatant was further filtered through a 0.45 lm membrane filter. Then the sample was diluted to the starting concentration and adjusted to the required pH with H2SO4 or NaOH. The pH was measured by a model pHs-25 pH meter. After adding persulfate, the beaker was quickly sealed with the film (PTFE) and bundled with a rubber band, and then the sample was sonicated at the predetermined power intensity. At selected time interval, aliquot of 2 mL reaction mixture were taken and immediately cooled down to room temperature. The concentration of humic acid was measured using TOC analyzer (Analytik-Jena 5200). All experiments were repeated at least three times. Data were reported as averages. 3. Kinetic modeling It is generally believed that a typical sonochemical oxidation of pollutants in the persulfate anion reaction should involve two key reactions: (1) the generation of radicals from persulfate anions and water decomposed into sulfate and hydroxyl radicals (reaction (1) and (2)); (2) the degradation of organic substance by the radicals (reaction (3)). In the meantime, some reversed reactions and side reactions (reactions (4)–(7)) coexist along with the key reactions as summarized below [9,16]: S2 O2 8 þÞÞÞ ! 2SO4
k1
ðReaction 1Þ
H2 OþÞÞÞ ! OH þ H k2
ðReaction 2Þ
2 HA þ SO k3 4 ! SO4 þ products
ðReaction 3Þ
2 þ H2 O þ SO k4 4 ! H þ SO4 þ OH
ðReaction 4Þ
S2 O2 8 þ OH ! OH þ S2 O8 k5
ðReaction 5Þ
2 S2 O2 8 þ SO4 ! SO4 þ S2 O8 k6
ðReaction 6Þ
2. Materials and methods
2 SO 4 þ SO4 ! S2 O8
ðReaction 7Þ
2.1. Materials
To establish a new kinetic model for describing sono-activated persulfate reaction, it could be assumed that humic acid is primarily degraded by the SO 4 and the degradation rate of humic acid can be expressed by Eq. (1).
Commercial humic acid (CAS No. 308067-45-0) was purchased from Aldrich, USA. Analytical grade potassium persulfate (K2S2O8, CAS No. 7727-21-1), sodium hydroxide (NaOH, CAS No. 1310-73-2) and sulfuric acid(H2SO4, CAS No. 7664-93-9) were purchased from Shanghai Chemical Reagents Co. Ltd., China and were used as received. All other chemicals and solvents were analytical grade and used without further purification. 2.2. Experimental setup Experiment reactor was a 200 mL beaker, which was placed at certain position in ultrasonic bath (KQ5200DB, 300 240 150 mm) from Kunshan Ultrasonic Instruments Co. Ltd, China. The power input could be adjusted continuously from 80 to 200 W. A sample of 100 ml was sonicated in the covered beaker. The water level inside the ultrasonic bath was maintained by continuous circulation of cooling water, and subsequently the temperature was maintained constantly at intended temperature. The pH
rHA ¼
k7
d½HA ¼ k3 ½HA½SO 4 dt
ð1Þ
From the reactions (1), (3), (4), (6) and (7), the change of SO 4 concentration relies on its generation rate from perulfate anion activated by ultrasound, and its consumption rate reacting with humic acid, water, persulfate and sulfate radicals as shown below
d½SO 2 4 ¼ k1 ½S2 O2 8 k3 ½HA½SO4 k4 ½SO4 k6 ½S2 O8 ½SO4 dt k7 ½SO 4 ½SO4
ð2Þ
Similarly, the change of OH concentration can be shown as:
d½ OH 2 ¼ k2 þ k4 ½SO 4 k5 ½S2 O8 ½ OH dt
ð2aÞ
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Because concentration of SO 4 in solution is much lower than [HA] and [S2O2 8 ], so k7[SO4 ][SO4 ] can be neglected from Eq. (2). Since SO4 and OH are highly reactive free radical with a very short lifetime of microseconds [1], the concentration is normally considered to be at a constant low level and its change rate will approach zero at a certain time based on the pseudo steady state assumption.
½SO 4
¼
½ OH ¼
k1 ½S2 O2 8
ð3Þ
k3 ½HA þ k4 þ k6 ½S2 O2 8 k2 þ k4 ½SO 4
¼
d½HA ¼ k3 ½HA½SO 4 dt k1 k3 ½HA
k3 ½HA þ k4 þ k6 ½S2 O2 8
½S2 O2 8
ð4Þ
The consumption of S2O2 8 concentration could be expressed by Eq. (5).
d½S2 O2 2 2 8 ¼ k1 ½S2 O2 8 k5 ½S2 O8 ½ OH k6 ½S2 O8 ½SO4 dt
ð5Þ
Substituting Eq. (3a) into Eq. (5), [S2O2 8 ] could be eventually expressed as Eq. (6).
ðk1 þk6 ½SO4 Þt ½S2 O2 ½S2 O2 8 ¼ e 8 0
k2 þ k4 ½SO 4 ð1 eðk1 þk6 ½SO4 Þt Þ k1 þ k6 ½SO 4
ð6Þ Substituting Eq. (6) into Eq. (4), the humic acid concentration [HA] becomes a function of reaction time, decreasing from its initial concentration [HA]0 at the beginning of reaction gradually as described by Eq. (7).
k4 þ k6 ½S2 O2 1 ½HA0 8 0 ½HA0 ½HA þ ln k1 k1 k3 ½HA ðk1 þk6 ½SO Þ t 4 1e k2 þ k4 ½SO 2 4 O þ ½S ¼ 2 8 0 k1 þ k6 ½SO k1 þ k6 ½SO 4 4
4. Results and discussion A series experiments were carried out in aqueous humic acid solution to validate the new model for its application in sono-activated persulfate system by varying persulfate dosage, ultrasonic power and initial humic acid concentration, respectively. Each experiment lasted for up to 120 min.
ð3aÞ
k5 ½S2 O2 8
Substituting Eq. (3) into Eq. (1), humic acid degradation rate could be expressed as follows:
r HA ¼
Concentration of SO 4 radicals could be calculated using Eq. (3). At any selected time, concentration of persulfate and humic acid in solution could be simulated by Eqs. (6a) and (7a) when initial concentration of persulfate and humic acid were determined.
k2 þ k4 ½SO 4 t k1 þ k6 ½SO 4
4.1. Effect of initial persulfate concentration on humic acid mineralization For the purpose of verifying the new model as well as finding the optimal concentration of persulfate anion, effect of initial perulfate concentration on humic acid mineralization was studied. Fig. 1 showed the effect of initial persulfate concentration on TOC removal efficiency of humic acid using the simulated values (SV) with the model and measured experimental data (MV) for different initial persulfate concentration when initial concentration of humic acid was 30 mg/L. These simulation data could be compared with experimental results. It is found that the measured values have the same trend with the model and present a good agreement with the simulated values. The observed TOC removal efficiency increases from 37.3% to 83.1% with the increase of initial persulfate concentration from 10 mM to 100 mM after 2 h. Fig. 2 showed the effect of initial persulfate concentration on total TOC removal quantity and removal quantity per unit persulfate. It could be seen that 0.42 mg/L TOC average mineralization per unit persulfate when initial concentration was 10 mM, while 0.24, 0.17 and 0.09 mg/L TOC mineralization when initial persulfate concentration were 25, 50 and 100 mM, respectively. The efficiency of persulfate declined when its initial concentration increased. This is probably because excess S2O2 8 in solution act as radical scavenger and thus inhibits humic acid mineralization, also radicals could annihilate themselves like reaction 7 [18]. Fig. 3 showed the effect of initial persulfate concentration on TOC removal rate. TOC removal rate was calculated during the
ð7Þ k þk ½S O2
To simplify Eqs. (6) and (7), let a ¼ k11 , b ¼ 4 6k1 k23 8 0 , c = k1 + k6[SO 4 ] and d = k2 + k4[SO4 ], the above equation could be rearranged in a simplified form as shown below.
d ct ct ½S2 O2 ½S2 O2 Þ 8 ¼ e 8 0 ð1 e c að½HA0 ½HAÞ þ b ln
ð6aÞ
½HA0 1 d þ ¼ ð1 ect Þ ½S2 O2 8 0 c c ½HA d t c
ð7aÞ
The above equations (Eqs. (7) and (7a)) are the main kinetic model for sono-activated persulfate reaction to describe the degradation of humic acid in aqueous solution against reaction time. The amount of free radicals generation would be related to applied ultrasonic power [15,19]. So if the conditions of ultrasonic irradiation is fixed (ultrasonic frequency = 40KHz, temperature = 40 °C, pH = 3), k1 as well as k2 would be functions of ultrasonic power. The parameters of k4, k5 and k6 would have certain values which have been obtained in previous researches [20].
Fig. 1. Effect of initial persulfate concentration on TOC removal efficiency of humic acid. ([HA]0 = 30 mg/L; initial pH = 3.0; ultrasonic power input = 200 W; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
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Babak et.al found the aromatic structural of humic acid was transformed to aliphatic groups by checking the values of UV, HPSEC and SUVA before and after irradiation [26]. In addition, the different mineralization rates may be concerned with oxidation mechanism of sulfate radicals. For many organic compounds, SO 4 radicals could react as an efficient oxidant because it is more selective for oxidation than OH radicals [27]. Sulfate radicals, having an unpaired electron, are prone to bind to humic acid molecular. But sulfate radicals are unable to form stabilized adduct with organic substrates, HA(SO 4 ) would decompose to form sulfate ion and humic acid [11]. Because different functional groups have different abilities to attract electrons, humic acid degradation by radicals would present a kind of multi-step degradation. Finally, increasing oxidant concentration could accomplish former stage more easily, but could not keep the high TOC removal rate constant. After 120 min reaction for 100 mM initial persulfate concentration, TOC removal rate was extremely low (0.0033 mg/(L min)).
Fig. 2. Effect of initial persulfate concentration on total TOC removal quantity and removal quantity per unit persulfate. ([HA]0 = 30 mg/L; initial pH = 3.0; ultrasonic power input = 200 W; temperature = 40 °C; reaction time = 2 h).
Fig. 3. Effect of initial persulfate concentration on TOC removal rate of humic acid. ([HA]0 = 30 mg/L; initial pH = 3.0; ultrasonic power input = 200 W; temperature = 40 °C).
reaction with different initial persulfate concentration ranging from 10 to 100 mM. As shown in Fig. 3, the removal rate declined generally along with reaction time. Initial concentration of persulfate 100 mM was taken as example, TOC removal rate declined during the beginning period of first 45 min, and then increased from 45 min to 60 min, and then dropped until the end of reaction time. There were similar tendency when initial persulfate concentration changed. The phenomenon could be explained by the complex composition and construction of humic acid. Humic substances may consist of a great variety of components with continuous polarity characteristics [21], and about 28% of these compounds were phenolic acids, 19% benzenecarboxylic acids, 13% alkanes and fatty acids, and 40% dialkyl phthalates [22]. Thus the difference of degradable ability among these components leads to the different removal rate of humic acid at different reaction time. Meanwhile, macromolecular components could be decomposed to small molecular substances, rather than totally mineralized immediately. Some researchers had already found that degradation of humic acid using photocatalysis and ozonation was multi-step reaction [23–25]. Refractory organic would transform into readily degradable substances, leading to the fact of removal rate goes up during the period from 45 min to 60 min.
4.2. Effect of ultrasonic power on humic acid mineralization Sono-activated persulfate oxidation of humic acid depends mainly on the amount of SO 4 radicals [18]. As a means of activation, ultrasonic plays a very important role in sono-activated degradation process [16]. Increased ultrasonic power is more favorable for humic acid mineralization. Fig. 4 showed the effect of ultrasonic power on TOC removal efficiency. As shown in Fig. 4, the efficiency of TOC removal increased from 58.6% to 83.1% while ultrasonic power improved from 80 to 200 W when initial persulfate concentration fixed at 100 mM after 120 min reaction. The actual power dissipated (Pdiss) was calculated using the same method given by previous literature [28]. The Pdiss in our experiment was 4.11, 3.41, 2.38 and 1.45 W, corresponding to power input 200, 160, 120 and 80 W, respectively. Higher ultrasonic power input would lead to a rapider generation of sulfate radicals and it was noted that reaction rate constant k1 had linear relationship with ultrasonic power in these experiments. In addition, reaction rate constant k1 increased from 5 105s1 to 1.25 104s1 when power input increased from 80 to 120 W. There are two aspects to explain increased removal efficiency with improved ultrasonic power. On the one hand, increased ultrasonic power generated more cavitation bubble, offering more energy to yield radical, so higher rate of radical
Fig. 4. Effect of ultrasonic power input on TOC removal efficiency of humic acid ([HA]0 = 30 mg/L; initial pH = 3.0; initial persulfate dosage = 100 mM; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
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was 200 W and the temperature was maintained constantly at 40 ± 2 °C. Fig. 6 showed the effect of initial humic acid concentration on TOC removal of humid acid. As Fig. 6 showed, mineralization of humic acid declined with decreased initial humic acid concentration when concentration ratio of persulfate and humic acid fixed. Simulated TOC removal efficiency values were not in good agreement with experimental data when concentration of humic acid and persulfate on lower level. There are two possibilities to explain the phenomenon. Firstly, some sulfate radicals are rapidly consumed by hydroxyl ions other than humic acid and converted to hydroxyl radicals under the low concentration conditions, as shown in reaction 4 [16]. Compared with SO 4 , hydroxyl radicals have too short lifetime to get close to humic acid molecules [29]. And secondly, effective collision between SO 4 radicals and humic acid decreases when concentration is at a lower level, thus reducing the chance of combining radicals with target pollutants and leading to decreasing degradation efficiency. Fig. 5. Effect of ultrasonic power input on TOC removal rate of humic acid. ([HA]0 = 30 mg/L; initial pH = 3.0; initial persulfate dosage = 100 mM; temperature = 40 °C).
generation was achieved as Eqs. (3) and (3a) indicated [16]. On the other hand, the mass transfer resistances could be eliminated because of the turbulent flow achieved with ultrasonic irradiation. Observed TOC removal rate could be shown in Fig. 5 when ultrasonic power changed. Increased ultrasonic power leads to a higher TOC removal rate, the fastest degradation rate of humic acid was observed at 30 min and the rate increased from 0.09 to 0.18 mg/(L min) when ultrasonic power increased from 80 to 160 W. It is clearly observed that sono-activated persulfate mineralization process consists of different kinds of stages. During reaction, take 160 W as a example, TOC removal rate declined rapidly to 0.10 mg/(L min) at first 45 min, then increased slowly in next 15 min period and dropped slowly after 60 min until the end.
4.4. Effect of initial pH on humic acid mineralization Liang et al. found that thermally-activated persulfate reaction had different predominant radical when pH of the solution changed [20]. SO 4 radicals are the predominant radicals when pH is lower than 7; both SO 4 and OH are present at pH 9; OH is the predominant radical at a more basic pH (i.e., pH 12). So in alkaline solution, OH would be predominant radical because SO 4 react with OH and yield OH. 2 SO 4 þ OH ! OH þ SO4
k8
ðReaction 8Þ
Three different initial concentration of humic acid of 7.5 mg/L, 15 mg/L and 30 mg/L were studied to valid new model with the ratio between initial persulfate concentration and humic acid content keeps constant (weight ratio = 900:1). The ultrasonic power
The experiments of different pH of 3.0 and 11.0 were also performed that initial concentration of humic acid and persulfate were 30 mg/L and 100 mM, respectively, ultrasonic power was 200 W and temperature of solution was 40 °C. The mineralization efficiency increases from 50% to 83.1% as a consequence of the pH value decreasing from 11.0 to 3.0 within 120 min, as Fig. 7 indicated. The phenomenon can be explained from two aspects. On the one hand, in high pH solution OH is the predominant radical because of reaction process occurred (reaction 8). The shorter lifetime restricts the combination of OH with humic acid molecule since the lifetime of OH is about three times shorter than SO 4 , so leading to a lower mineralization of humic acid. On the other
Fig. 6. Effect of initial humic acid concentration on TOC removal of humid acid (ultrasonic power input = 200 W; initial pH = 3.0; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
Fig. 7. Effect of initial solution pH on TOC removal of humic acid ([HA]0 = 30 mg/L; initial persulfate dosage = 100 mM; ultrasonic power input = 200 W; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
4.3. Effect of initial humic acid concentration on TOC removal
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sulfate radical exist a temperature threshold [4]. And the main oxidant for destruction of humic acid could be supposed to be sulfate radicals other than persulfate anions according to the experimental data. 4.6. Kinetics of humic acid degradation by sono-activated persulfate method
Fig. 8. Effect of reaction temperature on TOC removal of humic acid ([HA]0 = 30 mg/ L; initial persulfate dosage = 100 mM; ultrasonic power input = 200 W; initial pH = 3.0; SV: simulated value; MV: measured experimental value).
The data of humic acid TOC removal by sono-activated persulfate method under different conditions or at different times was collected to valid the new model. Fig. 9 shows the simulated TOC concentration of humic acid by using the first-order model, the second-order model and the new model compared with measured TOC concentration of humic acid. The linear regressions for the first-order model, the second-order model and the new model have the slope of 1.0098, 0.9644 and 0.9994 with R2 of 0.9677, 0.9472 and 0.9778, respectively. The result indicated that the new model was reasonable to explain sono-activated persulfate reaction. From the above discussion, the whole degradation process of organic substance using sono-activated persulfate method in acid solution would be divided into two sections: (1) persulfate anions would be activated by ultrasound to generate sulfate radicals; (2) Organic substance would be decomposed by sulfate radicals and removed from solution. 5. Conclusions
Fig. 9. Relationship between measured values and simulated values of HA TOC by using the first-order, second-order and the new model (HA]0 = 30 mg/L; initial persulfate dosage = 10–100 mM; ultrasonic power input = 80–200 W; initial pH = 3.0, 11.0; temperature = 30–50 °C).
In the HA mineralization process by sono-activated persulfate method, the addition of S2O2 anions and the improvement of 8 ultrasonic power could advantageously promote the removal of humic acid in aqueous solutions as a result of production of the 2 SO activated by ultra4 radicals from the decompose of S2O8 sound. Higher initial concentration of S2O2 anions is beneficial 8 for the degradation of humic acid. In alkaline solution and lower temperature condition the removal efficiency of humic acid was lower than in acid solution and high temperature condition. Relatively high mineralization of humic acid can be achieved under the optimal conditions of initial pH 3.0, ultrasonic power 200 W and frequency 40 kHz, temperature 40 °C. It is found that up to 90% humic acid removal efficiency was achieved after 2 h reaction. A new kinetic model based on sulfate radicals for the degradation of humic acid using sono-activated persulfate method was established. New model could simulate the concentration of humic acid well when initial concentration of persulfate and ultrasonic power was changed. However the new model was only validated by humic acid mineralization so far. Further studies to apply this kinetic model in degradation of other organics become necessary. References
hand, the oxidation potential of OH decreases at higher pH. OH radicals have a standard reduction potential of 2.7 V in acidic solution and 1.8 V in neutral solution [30]. Therefore, the higher solution pH is, the weaker oxidation capacity is, so the lower degradation rate was observed at basic pH. 4.5. Effect of temperature on humic acid mineralization Generally, temperature is a very important parameter in all reaction [19]. Fig. 8 shows the effect of reaction temperature on TOC removal of humic acid. As can be seen in Fig. 8, only 10% mineralization efficiency of 30 mg/L humic acid was achieved when temperature lower than 30 °C after 120 min reaction. When reaction temperature was improved to 40 °C, 83.1% TOC removal was observed. When temperature was raised up to 50 °C, TOC removal was increased to 91%. The possible explanation is that the yield of
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Modeling the oxidation kinetics of sono-activated persulfate’s process on the degradation of humic acid Wang Songlin a,⇑, Zhou Ning a, Wu Si a,b, Zhang Qi a, Yang Zhi a a b
School of Environment Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Construction Quality Supervision Station of Navy, Beijing 100161, PR China
a r t i c l e
i n f o
Article history: Received 13 August 2014 Received in revised form 23 October 2014 Accepted 23 October 2014 Available online 30 October 2014 Keywords: Ultrasound Persulfate Humic acid Sulfate radical Mineralization Kinetic model
a b s t r a c t Ultrasound degradation of humic acid has been investigated in the presence of persulfate anions at ultrasonic frequency of 40 kHz. The effects of persulfate anion concentration, ultrasonic power input, humic acid concentration, reaction time, solution pH and temperature on humic acid removal efficiency were studied. It is found that up to 90% humic acid removal efficiency was achieved after 2 h reaction. In this system, sulfate radicals (SO 4 ) were considered to be the mainly oxidant to mineralize humic acid while persulfate anion can hardly react with humic acid directly. A novel kinetic model based on sulfate radicals (SO 4 ) oxidation was established to describe the humic acid mineralization process mathematically and chemically in sono-activated persulfate system. According to the new model, ultrasound power, persulfate dosage, solution pH and reaction temperature have great influence on humic acid degradation. Different initial concentration of persulfate anions and humic acid, ultrasonic power, initial pH and reaction temperature have been discussed to valid the effectiveness of the model, and the simulated data showed new model had good agreement with the experiments data. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Generally humic substances would be characterized as the most widely-occurring natural mixture of organic compounds, which play an important role in both pollutant control chemistry and biogeochemistry in global carbon biogeochemical cycle [1]. They often have complex structures and impart brown/yellow color to water, and may bind heavy metals by complexation [2]. The degradation or removal of these organic matters are major problems faced by the water treatment industry as they act as important precursors of disinfection by-products in water [3]. In common water treatment processes, the overall or partial removal of humic substances could be obtained by physical separation, such as coagulation, adsorption and filtration. However, the techniques could not eliminate the pollutants but only transfer them from water to another phase, and may return back to water environment to threaten human health [1]. Thus, alternative oxidation processes should be under investigation. Variety of refractory organics has been effectively degraded by the advanced oxidation processes (AOPs), because they can generate strong oxidizing oxygen species such as OH radicals [2–5]. ⇑ Corresponding author. Tel.: +86 27 87792155; fax: +86 27 87792101. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.ultsonch.2014.10.026 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Several studies have been conducted involving the degradation of humic acid (HA) in aqueous solution using advanced oxidation processes, including photo-catalysis [2], Fenton’s reaction [5] and ozonation [6], and the result showed that humic acid can be removed from water effectively. But the catalyst such as Fe2+ and TiO2 would be external contaminant and the high cost of ozone would be also the restriction reason of its application. Persulfate anions have strong oxidation–reduction potential (2.01 V) and could be applied to the degradation of organic matter [7,8]. During degradation process, persulfate could be catalyzed to generate sulfate radicals (SO 4 ) (2.6 V) [9,10], which were very likely to be attracted to the special function groups. Subsequently the sulfate radicals would be transferred to the organics. The presence of a free radical at the organics would weaken and break the carbon bond. Consequently, the organic was degraded into relatively small substances and eventually removed [11,12]. Persulfate could be activated by thermal method to generate sulfate radicals, while more active species (sulfate or hydroxyl radicals) may yield at higher temperatures resulting in faster target pollutants degradation [13,14]. Jing et al. found that 87.02% carbamazepine (CBZ) is removed using thermally activated persulfate method, while the corresponding mineralization is only (5.7 ± 1.3)% after 4 h reaction [13]. Therefore, there is a need for an alternative process for the activation of persulfate.
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As one kind of AOPs, ultrasonics (US) has been investigated to degrade non-biodegradable pollutants, because of the simplicity of the system and no production of toxic by-products [15,16]. While in most cases degradation efficiency are limited to low level, and energy use efficiency is very low due to energy losses during transfer processes. So it is estimated that degradation rate of organic pollutants in a conventional sonochemical process should be promoted by 10–100 times for its application in practical wastewater treatment [17]. But Sonication in aqueous solution causes rapid formation, growth, and violent collapse of cavitation bubbles, resulting in enormous local temperature and pressure rises. It is a beneficial condition in sonolytic degradation because sulfate radicals (SO 4 ) would be formed from the oxidant by thermolytic cleavage of persulfate [16]. Li et al. found 1,1,1-trichloroethane (TCA) removal efficiency is 20% using persulfate without activated and almost 100% when oxidizing agent persulfate is activated by sonolytic method [18]. Sono-activated persulfate processes provide a promising treatment for organic pollutant in water. Some literature investigated many kinds of pollutant degradation using sono-activated persulfate processes, such as tert-butyl (MTBE) and arsenic (III) [15,16]. Previous researchers discussed the degradation efficiency under different reaction conditions and tried to reveal the mechanism of the reaction processes [16,18]. Chen et al. found that sulfate radicals would be the mainly oxidant for degradation of dinitrotoluene (DNTs) [19]. TCA would be decomposed mainly through sulfate and hydroxyl radicals as well as ultrasonic pyrolysis [18]. Pseudo-first-order model was proposed to describe the MTBE degradation and denotes the overall pseudo-first-order rate constant as k [16]. However, even the pseudo-first-order simulation results may fit the experimental values well, but the mechanism explanation was not from chemical perspective but from mathematical perspective. It is necessary to build up a new model which could describe the sono-activated persulfate reaction kinetic from both perspectives. In this study, a novel kinetic model for humic acid degradation using combined persulfate and ultrasound method in aqueous solution was established mathematically and chemically to explain the mechanism of the reaction process. Ultrasonic power, persulfate anions dosages and initial humic acid concentration had been taken into account. Moreover, solution pH, reaction temperature and characteristics of humic acid were also discussed in this paper. The model was validated and evaluated by the experimental data.
of initial solution was adjusted using 0.1 M H2SO4 and 0.1 M NaOH before reaction. 2.3. Procedure The stock solution was prepared by dissolving 1 g humic acid in 1L 0.1 M NaOH, then the supernatant was further filtered through a 0.45 lm membrane filter. Then the sample was diluted to the starting concentration and adjusted to the required pH with H2SO4 or NaOH. The pH was measured by a model pHs-25 pH meter. After adding persulfate, the beaker was quickly sealed with the film (PTFE) and bundled with a rubber band, and then the sample was sonicated at the predetermined power intensity. At selected time interval, aliquot of 2 mL reaction mixture were taken and immediately cooled down to room temperature. The concentration of humic acid was measured using TOC analyzer (Analytik-Jena 5200). All experiments were repeated at least three times. Data were reported as averages. 3. Kinetic modeling It is generally believed that a typical sonochemical oxidation of pollutants in the persulfate anion reaction should involve two key reactions: (1) the generation of radicals from persulfate anions and water decomposed into sulfate and hydroxyl radicals (reaction (1) and (2)); (2) the degradation of organic substance by the radicals (reaction (3)). In the meantime, some reversed reactions and side reactions (reactions (4)–(7)) coexist along with the key reactions as summarized below [9,16]: S2 O2 8 þÞÞÞ ! 2SO4
k1
ðReaction 1Þ
H2 OþÞÞÞ ! OH þ H k2
ðReaction 2Þ
2 HA þ SO k3 4 ! SO4 þ products
ðReaction 3Þ
2 þ H2 O þ SO k4 4 ! H þ SO4 þ OH
ðReaction 4Þ
S2 O2 8 þ OH ! OH þ S2 O8 k5
ðReaction 5Þ
2 S2 O2 8 þ SO4 ! SO4 þ S2 O8 k6
ðReaction 6Þ
2. Materials and methods
2 SO 4 þ SO4 ! S2 O8
ðReaction 7Þ
2.1. Materials
To establish a new kinetic model for describing sono-activated persulfate reaction, it could be assumed that humic acid is primarily degraded by the SO 4 and the degradation rate of humic acid can be expressed by Eq. (1).
Commercial humic acid (CAS No. 308067-45-0) was purchased from Aldrich, USA. Analytical grade potassium persulfate (K2S2O8, CAS No. 7727-21-1), sodium hydroxide (NaOH, CAS No. 1310-73-2) and sulfuric acid(H2SO4, CAS No. 7664-93-9) were purchased from Shanghai Chemical Reagents Co. Ltd., China and were used as received. All other chemicals and solvents were analytical grade and used without further purification. 2.2. Experimental setup Experiment reactor was a 200 mL beaker, which was placed at certain position in ultrasonic bath (KQ5200DB, 300 240 150 mm) from Kunshan Ultrasonic Instruments Co. Ltd, China. The power input could be adjusted continuously from 80 to 200 W. A sample of 100 ml was sonicated in the covered beaker. The water level inside the ultrasonic bath was maintained by continuous circulation of cooling water, and subsequently the temperature was maintained constantly at intended temperature. The pH
rHA ¼
k7
d½HA ¼ k3 ½HA½SO 4 dt
ð1Þ
From the reactions (1), (3), (4), (6) and (7), the change of SO 4 concentration relies on its generation rate from perulfate anion activated by ultrasound, and its consumption rate reacting with humic acid, water, persulfate and sulfate radicals as shown below
d½SO 2 4 ¼ k1 ½S2 O2 8 k3 ½HA½SO4 k4 ½SO4 k6 ½S2 O8 ½SO4 dt k7 ½SO 4 ½SO4
ð2Þ
Similarly, the change of OH concentration can be shown as:
d½ OH 2 ¼ k2 þ k4 ½SO 4 k5 ½S2 O8 ½ OH dt
ð2aÞ
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Because concentration of SO 4 in solution is much lower than [HA] and [S2O2 8 ], so k7[SO4 ][SO4 ] can be neglected from Eq. (2). Since SO4 and OH are highly reactive free radical with a very short lifetime of microseconds [1], the concentration is normally considered to be at a constant low level and its change rate will approach zero at a certain time based on the pseudo steady state assumption.
½SO 4
¼
½ OH ¼
k1 ½S2 O2 8
ð3Þ
k3 ½HA þ k4 þ k6 ½S2 O2 8 k2 þ k4 ½SO 4
¼
d½HA ¼ k3 ½HA½SO 4 dt k1 k3 ½HA
k3 ½HA þ k4 þ k6 ½S2 O2 8
½S2 O2 8
ð4Þ
The consumption of S2O2 8 concentration could be expressed by Eq. (5).
d½S2 O2 2 2 8 ¼ k1 ½S2 O2 8 k5 ½S2 O8 ½ OH k6 ½S2 O8 ½SO4 dt
ð5Þ
Substituting Eq. (3a) into Eq. (5), [S2O2 8 ] could be eventually expressed as Eq. (6).
ðk1 þk6 ½SO4 Þt ½S2 O2 ½S2 O2 8 ¼ e 8 0
k2 þ k4 ½SO 4 ð1 eðk1 þk6 ½SO4 Þt Þ k1 þ k6 ½SO 4
ð6Þ Substituting Eq. (6) into Eq. (4), the humic acid concentration [HA] becomes a function of reaction time, decreasing from its initial concentration [HA]0 at the beginning of reaction gradually as described by Eq. (7).
k4 þ k6 ½S2 O2 1 ½HA0 8 0 ½HA0 ½HA þ ln k1 k1 k3 ½HA ðk1 þk6 ½SO Þ t 4 1e k2 þ k4 ½SO 2 4 O þ ½S ¼ 2 8 0 k1 þ k6 ½SO k1 þ k6 ½SO 4 4
4. Results and discussion A series experiments were carried out in aqueous humic acid solution to validate the new model for its application in sono-activated persulfate system by varying persulfate dosage, ultrasonic power and initial humic acid concentration, respectively. Each experiment lasted for up to 120 min.
ð3aÞ
k5 ½S2 O2 8
Substituting Eq. (3) into Eq. (1), humic acid degradation rate could be expressed as follows:
r HA ¼
Concentration of SO 4 radicals could be calculated using Eq. (3). At any selected time, concentration of persulfate and humic acid in solution could be simulated by Eqs. (6a) and (7a) when initial concentration of persulfate and humic acid were determined.
k2 þ k4 ½SO 4 t k1 þ k6 ½SO 4
4.1. Effect of initial persulfate concentration on humic acid mineralization For the purpose of verifying the new model as well as finding the optimal concentration of persulfate anion, effect of initial perulfate concentration on humic acid mineralization was studied. Fig. 1 showed the effect of initial persulfate concentration on TOC removal efficiency of humic acid using the simulated values (SV) with the model and measured experimental data (MV) for different initial persulfate concentration when initial concentration of humic acid was 30 mg/L. These simulation data could be compared with experimental results. It is found that the measured values have the same trend with the model and present a good agreement with the simulated values. The observed TOC removal efficiency increases from 37.3% to 83.1% with the increase of initial persulfate concentration from 10 mM to 100 mM after 2 h. Fig. 2 showed the effect of initial persulfate concentration on total TOC removal quantity and removal quantity per unit persulfate. It could be seen that 0.42 mg/L TOC average mineralization per unit persulfate when initial concentration was 10 mM, while 0.24, 0.17 and 0.09 mg/L TOC mineralization when initial persulfate concentration were 25, 50 and 100 mM, respectively. The efficiency of persulfate declined when its initial concentration increased. This is probably because excess S2O2 8 in solution act as radical scavenger and thus inhibits humic acid mineralization, also radicals could annihilate themselves like reaction 7 [18]. Fig. 3 showed the effect of initial persulfate concentration on TOC removal rate. TOC removal rate was calculated during the
ð7Þ k þk ½S O2
To simplify Eqs. (6) and (7), let a ¼ k11 , b ¼ 4 6k1 k23 8 0 , c = k1 + k6[SO 4 ] and d = k2 + k4[SO4 ], the above equation could be rearranged in a simplified form as shown below.
d ct ct ½S2 O2 ½S2 O2 Þ 8 ¼ e 8 0 ð1 e c að½HA0 ½HAÞ þ b ln
ð6aÞ
½HA0 1 d þ ¼ ð1 ect Þ ½S2 O2 8 0 c c ½HA d t c
ð7aÞ
The above equations (Eqs. (7) and (7a)) are the main kinetic model for sono-activated persulfate reaction to describe the degradation of humic acid in aqueous solution against reaction time. The amount of free radicals generation would be related to applied ultrasonic power [15,19]. So if the conditions of ultrasonic irradiation is fixed (ultrasonic frequency = 40KHz, temperature = 40 °C, pH = 3), k1 as well as k2 would be functions of ultrasonic power. The parameters of k4, k5 and k6 would have certain values which have been obtained in previous researches [20].
Fig. 1. Effect of initial persulfate concentration on TOC removal efficiency of humic acid. ([HA]0 = 30 mg/L; initial pH = 3.0; ultrasonic power input = 200 W; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
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Babak et.al found the aromatic structural of humic acid was transformed to aliphatic groups by checking the values of UV, HPSEC and SUVA before and after irradiation [26]. In addition, the different mineralization rates may be concerned with oxidation mechanism of sulfate radicals. For many organic compounds, SO 4 radicals could react as an efficient oxidant because it is more selective for oxidation than OH radicals [27]. Sulfate radicals, having an unpaired electron, are prone to bind to humic acid molecular. But sulfate radicals are unable to form stabilized adduct with organic substrates, HA(SO 4 ) would decompose to form sulfate ion and humic acid [11]. Because different functional groups have different abilities to attract electrons, humic acid degradation by radicals would present a kind of multi-step degradation. Finally, increasing oxidant concentration could accomplish former stage more easily, but could not keep the high TOC removal rate constant. After 120 min reaction for 100 mM initial persulfate concentration, TOC removal rate was extremely low (0.0033 mg/(L min)).
Fig. 2. Effect of initial persulfate concentration on total TOC removal quantity and removal quantity per unit persulfate. ([HA]0 = 30 mg/L; initial pH = 3.0; ultrasonic power input = 200 W; temperature = 40 °C; reaction time = 2 h).
Fig. 3. Effect of initial persulfate concentration on TOC removal rate of humic acid. ([HA]0 = 30 mg/L; initial pH = 3.0; ultrasonic power input = 200 W; temperature = 40 °C).
reaction with different initial persulfate concentration ranging from 10 to 100 mM. As shown in Fig. 3, the removal rate declined generally along with reaction time. Initial concentration of persulfate 100 mM was taken as example, TOC removal rate declined during the beginning period of first 45 min, and then increased from 45 min to 60 min, and then dropped until the end of reaction time. There were similar tendency when initial persulfate concentration changed. The phenomenon could be explained by the complex composition and construction of humic acid. Humic substances may consist of a great variety of components with continuous polarity characteristics [21], and about 28% of these compounds were phenolic acids, 19% benzenecarboxylic acids, 13% alkanes and fatty acids, and 40% dialkyl phthalates [22]. Thus the difference of degradable ability among these components leads to the different removal rate of humic acid at different reaction time. Meanwhile, macromolecular components could be decomposed to small molecular substances, rather than totally mineralized immediately. Some researchers had already found that degradation of humic acid using photocatalysis and ozonation was multi-step reaction [23–25]. Refractory organic would transform into readily degradable substances, leading to the fact of removal rate goes up during the period from 45 min to 60 min.
4.2. Effect of ultrasonic power on humic acid mineralization Sono-activated persulfate oxidation of humic acid depends mainly on the amount of SO 4 radicals [18]. As a means of activation, ultrasonic plays a very important role in sono-activated degradation process [16]. Increased ultrasonic power is more favorable for humic acid mineralization. Fig. 4 showed the effect of ultrasonic power on TOC removal efficiency. As shown in Fig. 4, the efficiency of TOC removal increased from 58.6% to 83.1% while ultrasonic power improved from 80 to 200 W when initial persulfate concentration fixed at 100 mM after 120 min reaction. The actual power dissipated (Pdiss) was calculated using the same method given by previous literature [28]. The Pdiss in our experiment was 4.11, 3.41, 2.38 and 1.45 W, corresponding to power input 200, 160, 120 and 80 W, respectively. Higher ultrasonic power input would lead to a rapider generation of sulfate radicals and it was noted that reaction rate constant k1 had linear relationship with ultrasonic power in these experiments. In addition, reaction rate constant k1 increased from 5 105s1 to 1.25 104s1 when power input increased from 80 to 120 W. There are two aspects to explain increased removal efficiency with improved ultrasonic power. On the one hand, increased ultrasonic power generated more cavitation bubble, offering more energy to yield radical, so higher rate of radical
Fig. 4. Effect of ultrasonic power input on TOC removal efficiency of humic acid ([HA]0 = 30 mg/L; initial pH = 3.0; initial persulfate dosage = 100 mM; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
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was 200 W and the temperature was maintained constantly at 40 ± 2 °C. Fig. 6 showed the effect of initial humic acid concentration on TOC removal of humid acid. As Fig. 6 showed, mineralization of humic acid declined with decreased initial humic acid concentration when concentration ratio of persulfate and humic acid fixed. Simulated TOC removal efficiency values were not in good agreement with experimental data when concentration of humic acid and persulfate on lower level. There are two possibilities to explain the phenomenon. Firstly, some sulfate radicals are rapidly consumed by hydroxyl ions other than humic acid and converted to hydroxyl radicals under the low concentration conditions, as shown in reaction 4 [16]. Compared with SO 4 , hydroxyl radicals have too short lifetime to get close to humic acid molecules [29]. And secondly, effective collision between SO 4 radicals and humic acid decreases when concentration is at a lower level, thus reducing the chance of combining radicals with target pollutants and leading to decreasing degradation efficiency. Fig. 5. Effect of ultrasonic power input on TOC removal rate of humic acid. ([HA]0 = 30 mg/L; initial pH = 3.0; initial persulfate dosage = 100 mM; temperature = 40 °C).
generation was achieved as Eqs. (3) and (3a) indicated [16]. On the other hand, the mass transfer resistances could be eliminated because of the turbulent flow achieved with ultrasonic irradiation. Observed TOC removal rate could be shown in Fig. 5 when ultrasonic power changed. Increased ultrasonic power leads to a higher TOC removal rate, the fastest degradation rate of humic acid was observed at 30 min and the rate increased from 0.09 to 0.18 mg/(L min) when ultrasonic power increased from 80 to 160 W. It is clearly observed that sono-activated persulfate mineralization process consists of different kinds of stages. During reaction, take 160 W as a example, TOC removal rate declined rapidly to 0.10 mg/(L min) at first 45 min, then increased slowly in next 15 min period and dropped slowly after 60 min until the end.
4.4. Effect of initial pH on humic acid mineralization Liang et al. found that thermally-activated persulfate reaction had different predominant radical when pH of the solution changed [20]. SO 4 radicals are the predominant radicals when pH is lower than 7; both SO 4 and OH are present at pH 9; OH is the predominant radical at a more basic pH (i.e., pH 12). So in alkaline solution, OH would be predominant radical because SO 4 react with OH and yield OH. 2 SO 4 þ OH ! OH þ SO4
k8
ðReaction 8Þ
Three different initial concentration of humic acid of 7.5 mg/L, 15 mg/L and 30 mg/L were studied to valid new model with the ratio between initial persulfate concentration and humic acid content keeps constant (weight ratio = 900:1). The ultrasonic power
The experiments of different pH of 3.0 and 11.0 were also performed that initial concentration of humic acid and persulfate were 30 mg/L and 100 mM, respectively, ultrasonic power was 200 W and temperature of solution was 40 °C. The mineralization efficiency increases from 50% to 83.1% as a consequence of the pH value decreasing from 11.0 to 3.0 within 120 min, as Fig. 7 indicated. The phenomenon can be explained from two aspects. On the one hand, in high pH solution OH is the predominant radical because of reaction process occurred (reaction 8). The shorter lifetime restricts the combination of OH with humic acid molecule since the lifetime of OH is about three times shorter than SO 4 , so leading to a lower mineralization of humic acid. On the other
Fig. 6. Effect of initial humic acid concentration on TOC removal of humid acid (ultrasonic power input = 200 W; initial pH = 3.0; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
Fig. 7. Effect of initial solution pH on TOC removal of humic acid ([HA]0 = 30 mg/L; initial persulfate dosage = 100 mM; ultrasonic power input = 200 W; temperature = 40 °C; SV: simulated value; MV: measured experimental value).
4.3. Effect of initial humic acid concentration on TOC removal
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sulfate radical exist a temperature threshold [4]. And the main oxidant for destruction of humic acid could be supposed to be sulfate radicals other than persulfate anions according to the experimental data. 4.6. Kinetics of humic acid degradation by sono-activated persulfate method
Fig. 8. Effect of reaction temperature on TOC removal of humic acid ([HA]0 = 30 mg/ L; initial persulfate dosage = 100 mM; ultrasonic power input = 200 W; initial pH = 3.0; SV: simulated value; MV: measured experimental value).
The data of humic acid TOC removal by sono-activated persulfate method under different conditions or at different times was collected to valid the new model. Fig. 9 shows the simulated TOC concentration of humic acid by using the first-order model, the second-order model and the new model compared with measured TOC concentration of humic acid. The linear regressions for the first-order model, the second-order model and the new model have the slope of 1.0098, 0.9644 and 0.9994 with R2 of 0.9677, 0.9472 and 0.9778, respectively. The result indicated that the new model was reasonable to explain sono-activated persulfate reaction. From the above discussion, the whole degradation process of organic substance using sono-activated persulfate method in acid solution would be divided into two sections: (1) persulfate anions would be activated by ultrasound to generate sulfate radicals; (2) Organic substance would be decomposed by sulfate radicals and removed from solution. 5. Conclusions
Fig. 9. Relationship between measured values and simulated values of HA TOC by using the first-order, second-order and the new model (HA]0 = 30 mg/L; initial persulfate dosage = 10–100 mM; ultrasonic power input = 80–200 W; initial pH = 3.0, 11.0; temperature = 30–50 °C).
In the HA mineralization process by sono-activated persulfate method, the addition of S2O2 anions and the improvement of 8 ultrasonic power could advantageously promote the removal of humic acid in aqueous solutions as a result of production of the 2 SO activated by ultra4 radicals from the decompose of S2O8 sound. Higher initial concentration of S2O2 anions is beneficial 8 for the degradation of humic acid. In alkaline solution and lower temperature condition the removal efficiency of humic acid was lower than in acid solution and high temperature condition. Relatively high mineralization of humic acid can be achieved under the optimal conditions of initial pH 3.0, ultrasonic power 200 W and frequency 40 kHz, temperature 40 °C. It is found that up to 90% humic acid removal efficiency was achieved after 2 h reaction. A new kinetic model based on sulfate radicals for the degradation of humic acid using sono-activated persulfate method was established. New model could simulate the concentration of humic acid well when initial concentration of persulfate and ultrasonic power was changed. However the new model was only validated by humic acid mineralization so far. Further studies to apply this kinetic model in degradation of other organics become necessary. References
hand, the oxidation potential of OH decreases at higher pH. OH radicals have a standard reduction potential of 2.7 V in acidic solution and 1.8 V in neutral solution [30]. Therefore, the higher solution pH is, the weaker oxidation capacity is, so the lower degradation rate was observed at basic pH. 4.5. Effect of temperature on humic acid mineralization Generally, temperature is a very important parameter in all reaction [19]. Fig. 8 shows the effect of reaction temperature on TOC removal of humic acid. As can be seen in Fig. 8, only 10% mineralization efficiency of 30 mg/L humic acid was achieved when temperature lower than 30 °C after 120 min reaction. When reaction temperature was improved to 40 °C, 83.1% TOC removal was observed. When temperature was raised up to 50 °C, TOC removal was increased to 91%. The possible explanation is that the yield of
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