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Application of the enhanced sono-photo‑Fenton-like process in the presence of persulfate for the simultaneous removal of chromium and phenol from the aqueous solution
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Application of the enhanced sono-photo‑Fenton-like process in the presence of persulfate for the simultaneous removal of chromium and phenol from the aqueous solution Ahmadreza Yazdanbakhsha,b, Asma Aliyaric,*, Amir Sheikhmohammadid,*, Ehsan Aghayanie a
Workplace Health Promotion Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran c Research Committee, School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran d Department of Environmental Health Engineering, School of Public Health, Khoy University of Medical Sciences, Khoy, Iran e Department of Environmental Health Engineering, School of Public Health, Abadan University of Medical Sciences, Abadan, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Sono-photo-Fenton-like Persulfate Wastewater Energy consumption Kinetic
This study was conducted to investigate the enhanced sono-photo‑Fenton-like process (SPFP) in the presence of persulfate, as a method of Advanced Oxidation Processes (AOPs) to generate oxidative radicals (O2%−, %OH, SO4%−) for the simultaneous removal of chromium and phenol from aqueous solutions. The removal performance of chromium and phenol by UV alone, UV/H2O2 (UH), UV/H2O2/Fe0(UHF), UV/H2O2/Fe0/S2O8 (UHFS) and UV/H2O2/Fe0/S2O8/US (UHFSU) were estimated (12.5, 6.5 %), (35.1, 31 %), (52.5, 66.1 %), (74.9, 79.1 %) and (100, 100 %), respectively under selected conditions (phenol/chromium molar ratio, 2.0; pH, 3.0; H2O2/Fe0 molar ratio, 2.0; S2O8, 0.75 mM L−1 and reaction time, 4 min). Also, according to obtained results, robs value for UHFSU process in the removal of (chromium and phenol) was about (14.62, 30.84), (10, 4), (7.64, 2.47) and (3.2, 1.3) times than that of the UV alone, UH, UHF and UHFS, respectively. The study of the energy consumption for the various processes indicated the UHFSU process compared with other processes, has very low energy consumption (kWh m-3), which proves the process justification economically. The addition of the various scavengers (ammonium oxalate, 2-propanol and tert-butanol) was diminished the removal performance; it clearly proved oxidative radicals (O2%−, %OH, SO4%−) plays a noticeable role in the removal of chromium and phenol.
1. Introduction Heavy metals such as Cr(VI) and refractory organic matter like phenol are commonly detected in industrial wastewater such as leather tanning, textiles, wood preservation, electroplating, car manufacturing, petroleum refining and pharmaceuticals industries [1]. Entering these pollutants into aquatic environments without treatment with suitable methods, cause serious aesthetic and pollution problems, which will pose serious risks to the environment due to toxicity, persistence, as well as bioaccumulation. Phenolic compounds considered as a group of toxic priority pollutants by the US Environmental Protection Agency (EPA) and the National Pollutant Release Inventory (NPRI) of Canada [2]. Cr(VI) species are known highly toxic, carcinogenic, and high environmental mobility that can be converted to the less mobile and less toxic Cr(III) [3–5]. At present, coexistence of organic pollutants and heavy metals in
⁎
aquatic environments has been reported [6,7]. In recent years, the simultaneous removal of co-existing/multiple pollutants from contaminated waters have been received significant attention [8,9]. Advanced oxidation processes (AOPs) are one of the most effective and environmentally friendly processes that can improve the degradation/mineralization efficiency of contaminants, due to high performance, toxicity reduction, lack of selectivity and increasing biodegradability [10,11]. Photocatalytic technologies have also created great interest for the purification of contaminated water due to its ability to mineralize most of organic pollutants by using luminous energy [12–14]. Heterogeneous catalytic techniques (HCTs) have been examined by researchers, but some of the specific disadvantages including: long retention time and higher electric energy consumption, the need for excess catalyst [15,16] and oxidant and etc, have limited their potential in factual systems. Thus, for overcoming these limitations, production of
Corresponding authors. E-mail addresses: [email protected] (A. Aliyari), [email protected] (A. Sheikhmohammadi).
https://doi.org/10.1016/j.jwpe.2019.101080 Received 5 September 2019; Received in revised form 7 November 2019; Accepted 21 November 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ahmadreza Yazdanbakhsh, et al., Journal of Water Process Engineering, https://doi.org/10.1016/j.jwpe.2019.101080
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more radicals and faster degradation attaining better efficiencies, should be noted [17–20]. Researchers have been successfully used a broad range of coupling methods (such as ultraviolet (UV) light, heat, ultrasound (US) irradiation and electric current) in the removal of pollutants [21,22]. Amongst these processes, HCTs systems coupled to UV light and US, can be a good choice to the destruction of pollutants [23]. Ultrasound and UV irradiation processes are mainly responsible for the activation/decomposition of oxidant [24,25]. The use of ultrasound irradiation (sonolysis) for the degradation of pollutant is based on the formation of short-lived radical species generated in violent cavitational bubble collapses. These radicals can diffuse out of the bubble into the bulk fluid medium, where they would react with solute molecules [26,27]. To enhance degradation rate of target pollutants, two oxidants including peroxide hydrogen (H2O2), and persulfate (PS, S2O82−) were chosen, which expected to enhance generation rate of reactive species in system and consequently further degradation of pollutants [24,28]. Hereon, oxidants decomposition due to ultrasound (US) and ultraviolet (UV) irradiations (Eq. (1)) [29], and also occurrence of the reaction between oxidants and transition metal ions (Eqs. (2) and (3)) have been proposed as key routes to form further free radicals [30].
H2 O2 /S2 O82 − + UV or US→ •OH/SO•4−
(1)
2Fe 0 + Cr2 O72 − + 14H+ → 2Cr 3 + + 2Fe 2 + + 7H2 O
(2)
H2 O2 /S2 O82 − + Fe 2 + → •OH/SO•4−
(3)
(SO4%−,
Fig. 1. Schematic of experimental setup (1. Power supply; 2. UVC lamp; 3. Quartz Sleeve; 4. Photoreactor; 5.. Magnet; 6. Shaker; 7. Wastewater; 8. Ultrasound.
2.3. Analytical methods and performance indices Concentration of Cr (VI) was analyzed using a 1, 5-diphenylcarbazide (DPC) colorimetric method. A UV–vis spectrophotometer (Spectrophotometer- HACH) was used for measurements of absorbance at 540 nm in acidic solution. Phenol concentration was measured by spectrophotometer at 510 nm by the 4-aminoantipyrine method. The reactor was covered with a black cloth to prevent from any unnecessary light exposure. The extent of removal performance was monitored by determining the residual concentration of phenol and Cr (VI), based on standard methods. To quantify the determined extent, Eq. (4) was applied:
0
Sulfate free radicals E = 2.60 V) are very strong oxidizing agents with a high solubility and wide operative pH range. These radicals can be generated from persulfate (PS) and are an effective way for degradation of toxic, resistant and non-biodegradable contaminants [31,32]. For activation of persulfate and increase its oxidation potential, there are several ways like ultraviolet light irradiation, microwave, heat, transition metal and etc [32]. Also, in recent decades, nanoscale zero-valent iron (nZVI) owing to its advantages such as large specific surface area, high reactivity, high reductive capacity to reduce and stabilize different types of pollutants, has been considered [33]. These particles can be dispersed in aqueous slurries as stable suspensions for a time sufficient and thus directly injected into contaminated areas [34]. With above background, in this research we focused on combination of UV and US irradiations with a heterogeneous catalytic system for simultaneous removal of Cr(VI) and phenol.
where C0 and Ct refer to the initial concentration of phenol and Cr (VI) (mg L−1) and its concentration at time t, respectively. A pseudo firstorder kinetic (due to being impossible determination of the photogenerated oxidative species in the solution) was applied as a good indicator to evaluate the removal performance of phenol and Cr (VI) by SPFP (Eqs. (5)–(7)).
2. Materials and methods
dCPollutant = −kCPollutant Crs dt
(5)
2.1. Chemicals
Ct = C0 × e−k obs× t
(6)
The removal performance (VI) = (C0 − Ct)/C0
percentage
−1
phenol
and
Cr (4)
(7)
robs = −k obsCPollutant
The prepared nano zero-valent iron (nZVI) used in this study was purchased from US Research Nanomaterials (US-Nano), Inc. Potassium dichromate (K2Cr2O7), potassium persulfate (K2S2O8) and other chemicals obtained from the Merck company, Germany. All chemicals were of analytical grade and used as received without further purification. All solutions were prepared with double-distilled water.
of
−1
Here, kobs (min ), Crs and robs (mg L min) are the second order rate constant, concentration of reactive species in the solution and the observed rate of phenol and Cr (VI) removal by the SPFP, respectively. Also C0 and Ct are influent and effluent concentrations of phenol and Cr (VI) in the reactor, respectively.
2.2. Sono-photo reactor
2.4. Experimental procedure
A photoreactor equipped with a UVC lamp (11 W power and 254 nm wavelength) and a quartz sleeve enclosure placed at the center of the reactor was applied to do the tests. The intensity of the UV light radiation was measured by the UVC 512 light meter. The reaction vessel (photoreactor) was placed in an ultrasound bath (at a frequency of 35 kHz, and the output power was set between 60 and 960 W).A water jacket (water circulation) was applied to prevention from the temperature influence on the process performance. Schematic view of the photoreactor placed in an ultrasound bath portrayed in Fig. 1.
In a typical procedure, all batch experiments were done in duplicate in 300-mL batch reactor containing 150 mL solution of Cr (VI) and phenol with molar ratios of phenol/Cr(VI) (0.5:1,1:1,2:1,3:1,4:1). Then pH (3–11) was adjusted (to adjust pH levels, 0.1 M HCl and NaOH solutions were used) and subsequently, H2O2/Fe0 (0.5:1, 1:1, 2:1, 4:1) and S2O8 (0.25–1 mM) were added to the solution. Then solution was exposed to and radiations for a preset time. The SPFP process initiated by switching on the power supply (UV (253.7 nm)) and US (35 kHz). Next, the reactor was mixed with a shaker with a rotation speed of 2
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Table 1a Kinetic information for the removal of Cr (VI) using different processes. Process
R2
Kobs (min−1)
robs (mg L−1.min)
Ratio robs
UV(A) UH (B) UHF (C) UHFS(D) UHFSU(E)
0.83 0.98 0.92 0.92 0.97
0.031 0.045 0.059 0.14 0.45
0.8 1.17 1.53 3.64 11.7
E/A = 14.62 E/B = 10 E/C = 7.64 E/D = 3.2 –
Table 1b Kinetic information for the removal of phenol using different processes. Process
R2
Kobs (min−1)
robs (mg L−1.min)
Ratio robs
UV (A) UH (B) UHF (C) UHFS (D) UHFSU (E)
0.98 0.96 0.97 0.96 0.97
0.007 0.052 0.085 0.16 0.21
0.32 2.44 3.99 7.52 9.87
E/A = 30.84 E/B = 4 E/C = 2.47 E/D = 1.3 –
The little removal performance of the only UV can be because of low Cr (VI) and phenol inclination in UV absorption with wavelength 254.3 nm [36]. Kinetic studies were also performed to compare the efficiency of different processes, and the kinetic information was tabulated in Tables 1a and 1b. According to Tables 1a and 1b, the kinetic information obtained also proves the superiority of UHFSU process than other processes [37,38] (values of robs for UHFSU in the removal of Cr (VI) and phenol were 11.7 and 9.87 mg L−1 min, respectively), so that value of robs for UHFSU process in the removal of Cr (VI) and phenol was (14.62, 30.84), (10, 4), (7.67, 2.47) and (3.2, 1.3) times than that of the UV alone, UH, UHF and UHFS, respectively. The presented results in above indicate that among the tested techniques, the UHFSU treatment due to the very high generation rate of free radicals, the higher values of kobs and robs and also very low energy consumption [1,39], which is economically justified (complete removal of pollutants in less time) is the most appropriate method for simultaneous removal of chromium and phenol from aqueous solutions.
Fig. 2. Influence of the various processes on the removal of Cr (VI) (a) and phenol (b) (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; H2O2/Fe0 molar ratio, 2:1; S2O8, 0.75 mM L−1; time, 1–80 min).
150 rpm at atmospheric pressure for a preset time. Finally a sample was taken from the solution, centrifuged at 10,000 rpm for 10 min to separate the particles and analyzed by spectrophotometer. Also, for comparison, control experiments were done for UV alone, UH, UHF, UHFS and UHFSU under similar conditions. The quenching experimental (quenchers of the oxidative agents including ammonium oxalate, 2-propanol and tert-butanol) were done in order to investigate of the involvement of each specific reactive agent in the removal of Cr (VI) and phenol, and results were compared with condition without quencher.
3.2. Role of pH in the simultaneous removal of chromium and phenol by the UHFSU The batch experiments were conducted in order to evaluate the influence of pH (in the over range from 3.0–11) on the simultaneous removal of chromium and phenol (phenol/Cr(VI) molar ratio, 2: 1) by UHFSU from aqueous solutions and results were presented in Fig. 3. As seen in Fig. 3, pH is one of the decisive factors on the UHFSU performance [24,40,41], as the complete removal of chromium and phenol
3. Results and discussion 3.1. Degradation efficiency In order to compare the efficiency of different processes in simultaneous reduction of chromium and phenol from aqueous solutions, the rate of simultaneous removal of these pollutants from aqueous solutions by UV alone, UH, UHF, UHFS and UHFSU was investigated and the results are portrayed in Fig. 2. In this comparison, the performance of different processes in the simultaneous removal of two pollutants (with a phenol/Cr(VI) molar ratio of 2:1) within 1−80 min and under the same conditions (pH, 3.0; H2O2/Fe0 molar ratio 2 and S2O8, 0.75 mM L−1) was investigated. As shown in Fig. 2, UHFSU process (with removal performance of Cr (VI) and phenol of 100 % and 100 %, respectively) showed higher performance than the other processes within 4 min, as the removal performances of Cr (VI) and phenol for UV alone, UH, UHF and UHFS were (12.5 % and 6.5 %), (35.1 % and 31 %), (52.5 % and 66.1 %) and (74.9 % 79.1 %) at the same time, respectively. The very low performance of processes in the absence of US can be due to two reasons: 1) very low generation rate of free radicals in the absence of US 2) reduction of synergistic effect UV light coupled with US (in the absence of US) [35].
Fig. 3. Influence of the pH on the simultaneous removal of Cr (VI) and phenol (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0–11; H2O2/Fe0 molar ratio, 2:1; S2O8; S2O8, 0.75 mM L−1; Time, 4 min). 3
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was obtained in acidic pH after 4 min of reaction. Increase of pH from 4.0–11, resulted to decrease by 15 % in the removal efficiency of Cr(VI), although was no observed significant difference between acidic and alkaline conditions for the removal phenol by the UHFSU. It reveals the UHFSU process has the better performance in the acidic condition. It is well known influence of pH in controlling the catalytic activity, stability of H2O2 and activity of iron species [42,43]. As the previous conducted researches [43,44]; in the acidic pH, ferrous ions are produced from reaction between zero valent iron and H2O2 as the following equation.
Fe0 + H2 O2 +2H+ → Fe2+ + H2 O
(8) H2O2/S2O8−
as Eq. (3) The ferrous ions produced can react with and resulted to generation of %OH/SO%4−radicals; thus, the availability of the free radicals for the degradation of the pollutants is enhanced. Also in the acidic pH, zero valent iron can reduces Cr2O72- to Cr3+ as Eq. (2) and produced ferrous ions that in turn can reacts withH2O2/ S2O8− as Eq. (3) [24,45]. With increase of pH above 5, the ferrous ions produced in the Eqs. (2) and (4) react with H2O2and is resulted to production of weaker oxidants such as ferryl ions (e.g., FeO2+) as Eq. (9) that are more selective than %OH/SO4-% radicals.
Fe2+ + H2 O2 → Fe(IV)(e.g., FeO2+) + H2 O
(9)
Also decrease in oxidation performance of UHFSU in the pH above 5 can be related to decomposition of H2O2 and the deactivation of catalyst due to the precipitation of ferric oxyhydroxide (the formationof precipitates of Fe(OH)2 (s) and Fe(OH)3 (s)). In the acidic condition, SO4−% could be formed during the catalyzation and accelerate the removal of pollutants. At the lower pH, through the reaction between hydrogen ions and copper Fe+ is generated as Eq. (10) that in turn reacts with S2O8- for production of SO4−% radicals as Eq. (11) [24].
2Fe0+2H2+ → 2Fe+ + H2
(10)
2Fe0 + S2 O82 − → 2Fe2++SO−4 •+SO24−
(11)
Fig. 4. Influence of the H2O2/Fe0 molar ratio on the removal of Cr (VI) (a) and phenol (b) (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; H2O2/Fe0 molar ratio, 0.5–4; S2O8, 0.75 mM L−1; time, 1–4 min).
H2O2to water and oxygen as Eqs. (17) and (18). So, H2O2 should be added at an optimum amount.
3.3. Role of H2O2/Fe0 molar ratio in the simultaneous removal of chromium and phenol by the UHFSU
H2 O2 + O2 → 2H++2e−
(17)
2H2 O2 → 2H2 O+O2
(18)
0
As it can be seen from Fig. 4, by increasing the H2O2/Fe molar ratio from 0.5 to 2.0 in constant condition (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; S2O8, 0.75 mM L−1; time, 1–4 min), the simultaneous removal percentage of Cr (VI) and phenol by UHFSU(because of the produced more %OH radicals)based on the following equations (Eqs. (12)–(15)) [46] was enhanced, as the complete removal of the both Cr (VI) and phenol pollutants, were achieved in the H2O2/Fe0 molar ratio of 2.0, so the efficiency was improved [47,48].
(14)
Many studies show that by increasing the iron concentration, the rate of removal of pollutants (due to reaction between Fe2+ with H2O2 and formation of more hydroxyl radicals) is increased (Eq. (8)). Thus, an optimum value for Fe0 is suggested (this concentration is dependence on the molar proportion of H2O2/Fe0). The reason is that in the extra values of Fe0 (more than the optimum value), the solution was saturated with iron and thus, ferrous iron reacted with hydroxyl radical; and finally, ferric ions was formed that in turn reacted with hydrogen peroxide, and was produced HO2% with a less oxidizing ability (Eqs. (19) and (20)) [11,51,52].
(15)
Fe2 + +• OH → Fe3 ++OH−
(19)
Fe3+ + H2 O2 → Fe2++HO•2 + H+
(20)
H2 O2 +Fe2+ → •OH+OH−+Fe3+
(12)
•
(13)
OH+Fe2+ → OH−+Fe3+ → FeOH2+ )))
H2 O2 → 2•OH •
H2 O2 + hv → 2 OH 0
However, by increasing the H2O2/Fe molar ratio more than the optimum value (from 2 to 4), the progress in the removal performance was not notable or even it was decreased. Because in the extra concentrations of hydrogen peroxide, H2O2 acts as the consumer of hydroxyl radical(the hydrogen peroxide acts as free-radical scavenger) and produces hydroperoxyl radicals (HO%2) according to the following reaction (Eq. (16)) that have a less oxidizing ability [45,49,50].
H2 O2 + •O H→ HO2 •+H2 O
Also, it is possibility the creation of the brown turbidity in the solution at high dosages of ferrous ion that reduces the absorption of the UV light by solution resulting the recombination of %OH radicals (Eq. (16)). The concentration of Fe2+ ions in solution was also determined using phenanthroline method (according to the standard methods) and it was found that during the different reaction times, 93–97 % of zerovalent iron is converted to Fe2+ ions. Therefore, it was proved the presence of Fe2+ ions in the solution in throughout the reaction [53].
(16)
In addition, due to the completely dependence of produced hydroxyl ions to the molar proportion of H2O2/Fe0, so increase of H2O2 more than the optimum value, leads to the auto decomposition of 4
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Fig. 6. Influence of phenol/Cr(VI) molar ratio in the simultaneous removal of chromium and phenol by the UHFSU) pH, 3.0; H2O2/Fe0 molar ratio, 2; time, 4 min).
3.5. Role of phenol/Cr(VI) molar ratio in the simultaneous removal of chromium and phenol by the UHFSU Investigation of the efficiency of the process in industrial applications is an important issue inrelation to evaluate of process. To this purpose, the UHFSU performance was evaluated in the different phenol/Cr(VI) molar ratios (0.5–4) within 4 min. Fig. 6 portraits the rate of the potential performance of UHFSU process in the simultaneous removal of chromium and phenolin the different molar ratios. As shown in Fig. 6, with increasing phenol/Cr (VI) molar ratio from 0.5 to 2, the removal efficiency increased, as the complete removal of phenol and Cr(VI) was observed in the phenol/Cr (VI) molar ratio of 2 after 4 min of the reaction. Of course, complete removal of phenol was achieved at lower molar ratios, but the simultaneous and complete removal of both pollutants was achieved at molar ratio 2. It was also observed that by increasing the molar ratio above 2, it caused a decrease in process performance. This indicates that the phenol/Cr(VI) molar ratio in the aqueous solution plays a major role in the process efficiency.
Fig. 5. Influence of the S2O8 concentrationon the removal of Cr (VI) (a) and phenol (b) (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; H2O2/Fe0 molar ratio, 2:1; time, 1–4 min).
3.4. Role of S2O8 concentration in the simultaneous removal of chromium and phenol by the UHFSU Fig. 5 portraits the influence of S2O8 concentration on the simultaneous removal of Cr (VI) and phenol. As can be seen in Fig. 5, in the higher initial concentration of S2O8, removal performance increases dramatically. That way with changing the initial S2O8 concentration from 0.25 mM L−1 to 0.75 Mm L−1, the complete removal of Cr (VI) and phenol was obtained within 4 min. With increasing the concentration to 1 mM was no observed the significance influence in the removal performance. This phenomenon can be related to the large amounts of generated radicals (SO4%−) due to a high dosage of S2O8, which was enough to complete removal of both pollutants target existing in aqueous solutions (Eqs. (1), (3) and (7)). The generated radicals can be reacted with more selectively via electron transfer. Many studies have shown that in the excessive amounts of persulfate, efficiency of the reaction will be reduced in which can be due to some reverse reactions, side reactions, and excessive oxidant (Eqs. (21)–(24)) resulting to a scavenging effect on SO4%−. However, no inhibition was observed in the present study since the oxidant dosage was below the inhibition point [54–56].
S2 O82 − + •O H→ OH− + S2 O•8−
(21)
S2 O82 −+SO•4− → SO24− + S2 O•8−
(22)
SO•4−+SO•4− → S2 O82 −
(23)
S2 O82 −+2•O H→ 2HSO5−
(24)
3.6. Electrical energy consumption and the total cost of the system One of the important parameter that affected performance of AOPs processes is the amount of required electrical energy (kW) to diminish the content of a pollutant by 90 % in 1 m3 of the contaminated solution and also the total cost of the system that is calculated by electric energy per mass removed (EEM) (in kWh/kg) as the following formula [1]:
EEM =
38.4 × p V × kobs
(25)
where P is electrical power applied(kW) for the system (UV lamp) and V is the volume of pollutant in inside reactor (L). Also Eq. (26) was applied to calculate the total cost of the system (TCS) ($ m−3) [57,58].
TCS = 1.45 × (EE /O (kWhm−3) × power cos t ($/ kWh)) + oxidant cos t ($g −1)
(26)
The values of EEM and TCS for the various processes were calculated and tabulated in Tables 2a and 2b. As tabulated in Tables 2a and 2b, UHFSU process due to energy saving and the less total cost rather than other processes (because of reduction of energy consumption) can be a good cost-effective alternative and can be more efficient process in the simultaneous removal of Cr (VI) and phenol from aqueous solutions. 5
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analyzed according to the standard methods [53] and were determined 140, 120, 15, 56, 72 and 1.5–2.5 mg L−1 respectively. As mentioned in the previous sections, the removal efficiency of both phenol and chromium contaminants in distilled water and under the selected conditions, was 100 %. The anions present in the tap water indicated the negligible influence on the reaction efficiency, as removal efficiency for phenol and chromium reached from 100 % at absence of anions (distillated water) to 96.3 % and 94.8 %, respectively (at the presence of different anions (tap water)). However, it is important to note, that this slight decrease in efficiency may be due to the low amounts of anions present in the tap water, and at higher levels of these anions, the decrease in efficiency will be more significant. The positive influence of anions present in the tap water on the simultaneous removal of Cr (VI) and phenol can be explained on the basis of the salting-out effect and thus the solute was pushed towards the highly reactive interface region of the cavitation bubble to facilitate the removal [1]. Also, some humic acid (15 mg L−1) was added to the tap water and the reaction efficiency was re-evaluated. The reaction efficiency decreased further in the presence of humic acid and reached from 100 % to 88.3 % and 91.2 % for chromium and phenol, respectively. This dramatic decrease in the removal efficiency in the presence of humic acid and anions (simultaneously) can be due to the complex structure of humic acid, which can consumes hydroxyl radicals and sulfate radicals and affect the reaction efficiency [59].
Table 2a Values of EEO and the total cost for the removal of Cr (VI) using different processes. Process
Kobs (min−1)
EEO (kWh m−3)
Total cost ($ m−3)
UV UH UHF UHFS UHFSU
0.031 0.045 0.059 0.14 0.45
90.8 62.22 47.45 20 6.22
2.6 2.18 2.05 1.55 1.15
Table 2b Values of EEO and the total cost for the removal of phenolusing different processes. Process
Kobs (min−1)
EEO (kWh m−3)
Total cost ($ m−3)
UV UH UHF UHFS UHFSU
0.007 0.052 0.085 0.16 0.21
400 53.8 32.9 17.5 13.33
11.6 1.94 1.62 1.47 1.35
3.7. Effect of scavengers and possible degradation mechanism of UHFSU system To approve involvement of free radical species in the simultaneous removal of chromium and phenol by the UHFSU, various scavengers (0.1 mM) including ammonium oxalate, 2-propanol and tert-butanol, were added to the reactor and results were portrayed in Fig. 7. As portrayed in Fig. 7, upon addition of the scavengers (specifically, addition of tert-butanol as a strong scavenger %OH radical), simultaneous removal performance of chromium and phenol reduced. It can be concluded that process efficiency is highly dependent on the generated reaction radicals, and oxidative radicals play a key role in the removal of both pollutants. The reason for slightly decrease of UHFSU performance with addition of scavengers can be due to the low amount of scavengers added in solution (0.1 mM), as it possibility the higher decrease of UHFSU performance in the higher amounts of scavengers [1,24].
4. Conclusion In the present study the performance of sono-photo‑Fenton-like process (SPFP)in the presence of persulfate, to generate oxidative radicals (O2%−, %OH, SO4%−) for the simultaneous removal of chromium and phenol from aqueous solutions. The UHFSU process showed higher performance than the other processes (UV alone, UH, UHF and UHFS) within 4 min. The kinetic information obtained also proves the superiority of UHFSU process than other processes. It reveals the UHFSU process has the better performance in the acidic condition. The complete removal of the both Cr (VI) and phenol pollutants, were achieved in the H2O2/Fe0 molar ratio of 2.0, so the efficiency was improved. Although, it is notable, in the extra concentrations of hydrogen peroxide and Fe0, they acts as the scavenger of hydroxyl radical. It was observed 0.75 mM L−1 of S2O8was enough to complete removal of both pollutants target existing in aqueous solutions. The results indicated that the phenol/Cr(VI) molar ratio in the aqueous solution plays a major role in the process efficiency as the complete removal of the both pollutants was obtained in the phenol/Cr(VI) molar ratio equal to 2 and by increasing the molar ratio above 2, it caused a decrease in process performance. It was resulted, UHFSU process due to energy saving and the less total cost rather than other processes (because of reduction of energy consumption) can be a good cost-effective alternative and can be more efficient process in the simultaneous removal of Cr (VI) and phenol from aqueous solutions. Upon addition of the scavengers, simultaneous removal performance of chromium and phenol reduced. It can be concluded that process efficiency is highly dependent on the generated reaction radicals, and oxidative radicals play a key role in the removal of both pollutants. Finally, it was found that the UHFSU process improved the simultaneous removal of Cr (VI) and phenol from aqueous solutions
3.8. Influence of humic acid and coexisting inorganic anions By introducing inorganic anions such nitrate, carbonate, bicarbonate, sulfate, chloride and fluoride in aquatic systems, we could study how inorganic anions influence on the simultaneous removal of Cr (VI) and phenol by UHFSU process. The tap water was used to this purpose. The inlet phenol/Cr(VI) molar ratio was 2:1 and reaction time set at 4 min (pH, 3.0; H2O2/Fe0 molar ratio, 2:1). The concentrations of carbonate, bicarbonate, nitrate, sulfate, chloride and fluoride at tap water
Authors contribution All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
Fig. 7. The effect of various quenchers in the simultaneous removal of chromium and phenol by the UHFSU. 6
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Acknowledgements [22]
The authors would like to thank Shahid Beheshti University of Medical Sciences, Tehran, Iran, for financial supporting of current research project (Grant number 14164 and ethical code: IR.SBMU.PHNS.1397.133).
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Application of the enhanced sono-photo‑Fenton-like process in the presence of persulfate for the simultaneous removal of chromium and phenol from the aqueous solution Ahmadreza Yazdanbakhsha,b, Asma Aliyaric,*, Amir Sheikhmohammadid,*, Ehsan Aghayanie a
Workplace Health Promotion Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran c Research Committee, School of Public Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, Iran d Department of Environmental Health Engineering, School of Public Health, Khoy University of Medical Sciences, Khoy, Iran e Department of Environmental Health Engineering, School of Public Health, Abadan University of Medical Sciences, Abadan, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Sono-photo-Fenton-like Persulfate Wastewater Energy consumption Kinetic
This study was conducted to investigate the enhanced sono-photo‑Fenton-like process (SPFP) in the presence of persulfate, as a method of Advanced Oxidation Processes (AOPs) to generate oxidative radicals (O2%−, %OH, SO4%−) for the simultaneous removal of chromium and phenol from aqueous solutions. The removal performance of chromium and phenol by UV alone, UV/H2O2 (UH), UV/H2O2/Fe0(UHF), UV/H2O2/Fe0/S2O8 (UHFS) and UV/H2O2/Fe0/S2O8/US (UHFSU) were estimated (12.5, 6.5 %), (35.1, 31 %), (52.5, 66.1 %), (74.9, 79.1 %) and (100, 100 %), respectively under selected conditions (phenol/chromium molar ratio, 2.0; pH, 3.0; H2O2/Fe0 molar ratio, 2.0; S2O8, 0.75 mM L−1 and reaction time, 4 min). Also, according to obtained results, robs value for UHFSU process in the removal of (chromium and phenol) was about (14.62, 30.84), (10, 4), (7.64, 2.47) and (3.2, 1.3) times than that of the UV alone, UH, UHF and UHFS, respectively. The study of the energy consumption for the various processes indicated the UHFSU process compared with other processes, has very low energy consumption (kWh m-3), which proves the process justification economically. The addition of the various scavengers (ammonium oxalate, 2-propanol and tert-butanol) was diminished the removal performance; it clearly proved oxidative radicals (O2%−, %OH, SO4%−) plays a noticeable role in the removal of chromium and phenol.
1. Introduction Heavy metals such as Cr(VI) and refractory organic matter like phenol are commonly detected in industrial wastewater such as leather tanning, textiles, wood preservation, electroplating, car manufacturing, petroleum refining and pharmaceuticals industries [1]. Entering these pollutants into aquatic environments without treatment with suitable methods, cause serious aesthetic and pollution problems, which will pose serious risks to the environment due to toxicity, persistence, as well as bioaccumulation. Phenolic compounds considered as a group of toxic priority pollutants by the US Environmental Protection Agency (EPA) and the National Pollutant Release Inventory (NPRI) of Canada [2]. Cr(VI) species are known highly toxic, carcinogenic, and high environmental mobility that can be converted to the less mobile and less toxic Cr(III) [3–5]. At present, coexistence of organic pollutants and heavy metals in
⁎
aquatic environments has been reported [6,7]. In recent years, the simultaneous removal of co-existing/multiple pollutants from contaminated waters have been received significant attention [8,9]. Advanced oxidation processes (AOPs) are one of the most effective and environmentally friendly processes that can improve the degradation/mineralization efficiency of contaminants, due to high performance, toxicity reduction, lack of selectivity and increasing biodegradability [10,11]. Photocatalytic technologies have also created great interest for the purification of contaminated water due to its ability to mineralize most of organic pollutants by using luminous energy [12–14]. Heterogeneous catalytic techniques (HCTs) have been examined by researchers, but some of the specific disadvantages including: long retention time and higher electric energy consumption, the need for excess catalyst [15,16] and oxidant and etc, have limited their potential in factual systems. Thus, for overcoming these limitations, production of
Corresponding authors. E-mail addresses: [email protected] (A. Aliyari), [email protected] (A. Sheikhmohammadi).
https://doi.org/10.1016/j.jwpe.2019.101080 Received 5 September 2019; Received in revised form 7 November 2019; Accepted 21 November 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ahmadreza Yazdanbakhsh, et al., Journal of Water Process Engineering, https://doi.org/10.1016/j.jwpe.2019.101080
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more radicals and faster degradation attaining better efficiencies, should be noted [17–20]. Researchers have been successfully used a broad range of coupling methods (such as ultraviolet (UV) light, heat, ultrasound (US) irradiation and electric current) in the removal of pollutants [21,22]. Amongst these processes, HCTs systems coupled to UV light and US, can be a good choice to the destruction of pollutants [23]. Ultrasound and UV irradiation processes are mainly responsible for the activation/decomposition of oxidant [24,25]. The use of ultrasound irradiation (sonolysis) for the degradation of pollutant is based on the formation of short-lived radical species generated in violent cavitational bubble collapses. These radicals can diffuse out of the bubble into the bulk fluid medium, where they would react with solute molecules [26,27]. To enhance degradation rate of target pollutants, two oxidants including peroxide hydrogen (H2O2), and persulfate (PS, S2O82−) were chosen, which expected to enhance generation rate of reactive species in system and consequently further degradation of pollutants [24,28]. Hereon, oxidants decomposition due to ultrasound (US) and ultraviolet (UV) irradiations (Eq. (1)) [29], and also occurrence of the reaction between oxidants and transition metal ions (Eqs. (2) and (3)) have been proposed as key routes to form further free radicals [30].
H2 O2 /S2 O82 − + UV or US→ •OH/SO•4−
(1)
2Fe 0 + Cr2 O72 − + 14H+ → 2Cr 3 + + 2Fe 2 + + 7H2 O
(2)
H2 O2 /S2 O82 − + Fe 2 + → •OH/SO•4−
(3)
(SO4%−,
Fig. 1. Schematic of experimental setup (1. Power supply; 2. UVC lamp; 3. Quartz Sleeve; 4. Photoreactor; 5.. Magnet; 6. Shaker; 7. Wastewater; 8. Ultrasound.
2.3. Analytical methods and performance indices Concentration of Cr (VI) was analyzed using a 1, 5-diphenylcarbazide (DPC) colorimetric method. A UV–vis spectrophotometer (Spectrophotometer- HACH) was used for measurements of absorbance at 540 nm in acidic solution. Phenol concentration was measured by spectrophotometer at 510 nm by the 4-aminoantipyrine method. The reactor was covered with a black cloth to prevent from any unnecessary light exposure. The extent of removal performance was monitored by determining the residual concentration of phenol and Cr (VI), based on standard methods. To quantify the determined extent, Eq. (4) was applied:
0
Sulfate free radicals E = 2.60 V) are very strong oxidizing agents with a high solubility and wide operative pH range. These radicals can be generated from persulfate (PS) and are an effective way for degradation of toxic, resistant and non-biodegradable contaminants [31,32]. For activation of persulfate and increase its oxidation potential, there are several ways like ultraviolet light irradiation, microwave, heat, transition metal and etc [32]. Also, in recent decades, nanoscale zero-valent iron (nZVI) owing to its advantages such as large specific surface area, high reactivity, high reductive capacity to reduce and stabilize different types of pollutants, has been considered [33]. These particles can be dispersed in aqueous slurries as stable suspensions for a time sufficient and thus directly injected into contaminated areas [34]. With above background, in this research we focused on combination of UV and US irradiations with a heterogeneous catalytic system for simultaneous removal of Cr(VI) and phenol.
where C0 and Ct refer to the initial concentration of phenol and Cr (VI) (mg L−1) and its concentration at time t, respectively. A pseudo firstorder kinetic (due to being impossible determination of the photogenerated oxidative species in the solution) was applied as a good indicator to evaluate the removal performance of phenol and Cr (VI) by SPFP (Eqs. (5)–(7)).
2. Materials and methods
dCPollutant = −kCPollutant Crs dt
(5)
2.1. Chemicals
Ct = C0 × e−k obs× t
(6)
The removal performance (VI) = (C0 − Ct)/C0
percentage
−1
phenol
and
Cr (4)
(7)
robs = −k obsCPollutant
The prepared nano zero-valent iron (nZVI) used in this study was purchased from US Research Nanomaterials (US-Nano), Inc. Potassium dichromate (K2Cr2O7), potassium persulfate (K2S2O8) and other chemicals obtained from the Merck company, Germany. All chemicals were of analytical grade and used as received without further purification. All solutions were prepared with double-distilled water.
of
−1
Here, kobs (min ), Crs and robs (mg L min) are the second order rate constant, concentration of reactive species in the solution and the observed rate of phenol and Cr (VI) removal by the SPFP, respectively. Also C0 and Ct are influent and effluent concentrations of phenol and Cr (VI) in the reactor, respectively.
2.2. Sono-photo reactor
2.4. Experimental procedure
A photoreactor equipped with a UVC lamp (11 W power and 254 nm wavelength) and a quartz sleeve enclosure placed at the center of the reactor was applied to do the tests. The intensity of the UV light radiation was measured by the UVC 512 light meter. The reaction vessel (photoreactor) was placed in an ultrasound bath (at a frequency of 35 kHz, and the output power was set between 60 and 960 W).A water jacket (water circulation) was applied to prevention from the temperature influence on the process performance. Schematic view of the photoreactor placed in an ultrasound bath portrayed in Fig. 1.
In a typical procedure, all batch experiments were done in duplicate in 300-mL batch reactor containing 150 mL solution of Cr (VI) and phenol with molar ratios of phenol/Cr(VI) (0.5:1,1:1,2:1,3:1,4:1). Then pH (3–11) was adjusted (to adjust pH levels, 0.1 M HCl and NaOH solutions were used) and subsequently, H2O2/Fe0 (0.5:1, 1:1, 2:1, 4:1) and S2O8 (0.25–1 mM) were added to the solution. Then solution was exposed to and radiations for a preset time. The SPFP process initiated by switching on the power supply (UV (253.7 nm)) and US (35 kHz). Next, the reactor was mixed with a shaker with a rotation speed of 2
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Table 1a Kinetic information for the removal of Cr (VI) using different processes. Process
R2
Kobs (min−1)
robs (mg L−1.min)
Ratio robs
UV(A) UH (B) UHF (C) UHFS(D) UHFSU(E)
0.83 0.98 0.92 0.92 0.97
0.031 0.045 0.059 0.14 0.45
0.8 1.17 1.53 3.64 11.7
E/A = 14.62 E/B = 10 E/C = 7.64 E/D = 3.2 –
Table 1b Kinetic information for the removal of phenol using different processes. Process
R2
Kobs (min−1)
robs (mg L−1.min)
Ratio robs
UV (A) UH (B) UHF (C) UHFS (D) UHFSU (E)
0.98 0.96 0.97 0.96 0.97
0.007 0.052 0.085 0.16 0.21
0.32 2.44 3.99 7.52 9.87
E/A = 30.84 E/B = 4 E/C = 2.47 E/D = 1.3 –
The little removal performance of the only UV can be because of low Cr (VI) and phenol inclination in UV absorption with wavelength 254.3 nm [36]. Kinetic studies were also performed to compare the efficiency of different processes, and the kinetic information was tabulated in Tables 1a and 1b. According to Tables 1a and 1b, the kinetic information obtained also proves the superiority of UHFSU process than other processes [37,38] (values of robs for UHFSU in the removal of Cr (VI) and phenol were 11.7 and 9.87 mg L−1 min, respectively), so that value of robs for UHFSU process in the removal of Cr (VI) and phenol was (14.62, 30.84), (10, 4), (7.67, 2.47) and (3.2, 1.3) times than that of the UV alone, UH, UHF and UHFS, respectively. The presented results in above indicate that among the tested techniques, the UHFSU treatment due to the very high generation rate of free radicals, the higher values of kobs and robs and also very low energy consumption [1,39], which is economically justified (complete removal of pollutants in less time) is the most appropriate method for simultaneous removal of chromium and phenol from aqueous solutions.
Fig. 2. Influence of the various processes on the removal of Cr (VI) (a) and phenol (b) (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; H2O2/Fe0 molar ratio, 2:1; S2O8, 0.75 mM L−1; time, 1–80 min).
150 rpm at atmospheric pressure for a preset time. Finally a sample was taken from the solution, centrifuged at 10,000 rpm for 10 min to separate the particles and analyzed by spectrophotometer. Also, for comparison, control experiments were done for UV alone, UH, UHF, UHFS and UHFSU under similar conditions. The quenching experimental (quenchers of the oxidative agents including ammonium oxalate, 2-propanol and tert-butanol) were done in order to investigate of the involvement of each specific reactive agent in the removal of Cr (VI) and phenol, and results were compared with condition without quencher.
3.2. Role of pH in the simultaneous removal of chromium and phenol by the UHFSU The batch experiments were conducted in order to evaluate the influence of pH (in the over range from 3.0–11) on the simultaneous removal of chromium and phenol (phenol/Cr(VI) molar ratio, 2: 1) by UHFSU from aqueous solutions and results were presented in Fig. 3. As seen in Fig. 3, pH is one of the decisive factors on the UHFSU performance [24,40,41], as the complete removal of chromium and phenol
3. Results and discussion 3.1. Degradation efficiency In order to compare the efficiency of different processes in simultaneous reduction of chromium and phenol from aqueous solutions, the rate of simultaneous removal of these pollutants from aqueous solutions by UV alone, UH, UHF, UHFS and UHFSU was investigated and the results are portrayed in Fig. 2. In this comparison, the performance of different processes in the simultaneous removal of two pollutants (with a phenol/Cr(VI) molar ratio of 2:1) within 1−80 min and under the same conditions (pH, 3.0; H2O2/Fe0 molar ratio 2 and S2O8, 0.75 mM L−1) was investigated. As shown in Fig. 2, UHFSU process (with removal performance of Cr (VI) and phenol of 100 % and 100 %, respectively) showed higher performance than the other processes within 4 min, as the removal performances of Cr (VI) and phenol for UV alone, UH, UHF and UHFS were (12.5 % and 6.5 %), (35.1 % and 31 %), (52.5 % and 66.1 %) and (74.9 % 79.1 %) at the same time, respectively. The very low performance of processes in the absence of US can be due to two reasons: 1) very low generation rate of free radicals in the absence of US 2) reduction of synergistic effect UV light coupled with US (in the absence of US) [35].
Fig. 3. Influence of the pH on the simultaneous removal of Cr (VI) and phenol (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0–11; H2O2/Fe0 molar ratio, 2:1; S2O8; S2O8, 0.75 mM L−1; Time, 4 min). 3
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was obtained in acidic pH after 4 min of reaction. Increase of pH from 4.0–11, resulted to decrease by 15 % in the removal efficiency of Cr(VI), although was no observed significant difference between acidic and alkaline conditions for the removal phenol by the UHFSU. It reveals the UHFSU process has the better performance in the acidic condition. It is well known influence of pH in controlling the catalytic activity, stability of H2O2 and activity of iron species [42,43]. As the previous conducted researches [43,44]; in the acidic pH, ferrous ions are produced from reaction between zero valent iron and H2O2 as the following equation.
Fe0 + H2 O2 +2H+ → Fe2+ + H2 O
(8) H2O2/S2O8−
as Eq. (3) The ferrous ions produced can react with and resulted to generation of %OH/SO%4−radicals; thus, the availability of the free radicals for the degradation of the pollutants is enhanced. Also in the acidic pH, zero valent iron can reduces Cr2O72- to Cr3+ as Eq. (2) and produced ferrous ions that in turn can reacts withH2O2/ S2O8− as Eq. (3) [24,45]. With increase of pH above 5, the ferrous ions produced in the Eqs. (2) and (4) react with H2O2and is resulted to production of weaker oxidants such as ferryl ions (e.g., FeO2+) as Eq. (9) that are more selective than %OH/SO4-% radicals.
Fe2+ + H2 O2 → Fe(IV)(e.g., FeO2+) + H2 O
(9)
Also decrease in oxidation performance of UHFSU in the pH above 5 can be related to decomposition of H2O2 and the deactivation of catalyst due to the precipitation of ferric oxyhydroxide (the formationof precipitates of Fe(OH)2 (s) and Fe(OH)3 (s)). In the acidic condition, SO4−% could be formed during the catalyzation and accelerate the removal of pollutants. At the lower pH, through the reaction between hydrogen ions and copper Fe+ is generated as Eq. (10) that in turn reacts with S2O8- for production of SO4−% radicals as Eq. (11) [24].
2Fe0+2H2+ → 2Fe+ + H2
(10)
2Fe0 + S2 O82 − → 2Fe2++SO−4 •+SO24−
(11)
Fig. 4. Influence of the H2O2/Fe0 molar ratio on the removal of Cr (VI) (a) and phenol (b) (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; H2O2/Fe0 molar ratio, 0.5–4; S2O8, 0.75 mM L−1; time, 1–4 min).
H2O2to water and oxygen as Eqs. (17) and (18). So, H2O2 should be added at an optimum amount.
3.3. Role of H2O2/Fe0 molar ratio in the simultaneous removal of chromium and phenol by the UHFSU
H2 O2 + O2 → 2H++2e−
(17)
2H2 O2 → 2H2 O+O2
(18)
0
As it can be seen from Fig. 4, by increasing the H2O2/Fe molar ratio from 0.5 to 2.0 in constant condition (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; S2O8, 0.75 mM L−1; time, 1–4 min), the simultaneous removal percentage of Cr (VI) and phenol by UHFSU(because of the produced more %OH radicals)based on the following equations (Eqs. (12)–(15)) [46] was enhanced, as the complete removal of the both Cr (VI) and phenol pollutants, were achieved in the H2O2/Fe0 molar ratio of 2.0, so the efficiency was improved [47,48].
(14)
Many studies show that by increasing the iron concentration, the rate of removal of pollutants (due to reaction between Fe2+ with H2O2 and formation of more hydroxyl radicals) is increased (Eq. (8)). Thus, an optimum value for Fe0 is suggested (this concentration is dependence on the molar proportion of H2O2/Fe0). The reason is that in the extra values of Fe0 (more than the optimum value), the solution was saturated with iron and thus, ferrous iron reacted with hydroxyl radical; and finally, ferric ions was formed that in turn reacted with hydrogen peroxide, and was produced HO2% with a less oxidizing ability (Eqs. (19) and (20)) [11,51,52].
(15)
Fe2 + +• OH → Fe3 ++OH−
(19)
Fe3+ + H2 O2 → Fe2++HO•2 + H+
(20)
H2 O2 +Fe2+ → •OH+OH−+Fe3+
(12)
•
(13)
OH+Fe2+ → OH−+Fe3+ → FeOH2+ )))
H2 O2 → 2•OH •
H2 O2 + hv → 2 OH 0
However, by increasing the H2O2/Fe molar ratio more than the optimum value (from 2 to 4), the progress in the removal performance was not notable or even it was decreased. Because in the extra concentrations of hydrogen peroxide, H2O2 acts as the consumer of hydroxyl radical(the hydrogen peroxide acts as free-radical scavenger) and produces hydroperoxyl radicals (HO%2) according to the following reaction (Eq. (16)) that have a less oxidizing ability [45,49,50].
H2 O2 + •O H→ HO2 •+H2 O
Also, it is possibility the creation of the brown turbidity in the solution at high dosages of ferrous ion that reduces the absorption of the UV light by solution resulting the recombination of %OH radicals (Eq. (16)). The concentration of Fe2+ ions in solution was also determined using phenanthroline method (according to the standard methods) and it was found that during the different reaction times, 93–97 % of zerovalent iron is converted to Fe2+ ions. Therefore, it was proved the presence of Fe2+ ions in the solution in throughout the reaction [53].
(16)
In addition, due to the completely dependence of produced hydroxyl ions to the molar proportion of H2O2/Fe0, so increase of H2O2 more than the optimum value, leads to the auto decomposition of 4
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Fig. 6. Influence of phenol/Cr(VI) molar ratio in the simultaneous removal of chromium and phenol by the UHFSU) pH, 3.0; H2O2/Fe0 molar ratio, 2; time, 4 min).
3.5. Role of phenol/Cr(VI) molar ratio in the simultaneous removal of chromium and phenol by the UHFSU Investigation of the efficiency of the process in industrial applications is an important issue inrelation to evaluate of process. To this purpose, the UHFSU performance was evaluated in the different phenol/Cr(VI) molar ratios (0.5–4) within 4 min. Fig. 6 portraits the rate of the potential performance of UHFSU process in the simultaneous removal of chromium and phenolin the different molar ratios. As shown in Fig. 6, with increasing phenol/Cr (VI) molar ratio from 0.5 to 2, the removal efficiency increased, as the complete removal of phenol and Cr(VI) was observed in the phenol/Cr (VI) molar ratio of 2 after 4 min of the reaction. Of course, complete removal of phenol was achieved at lower molar ratios, but the simultaneous and complete removal of both pollutants was achieved at molar ratio 2. It was also observed that by increasing the molar ratio above 2, it caused a decrease in process performance. This indicates that the phenol/Cr(VI) molar ratio in the aqueous solution plays a major role in the process efficiency.
Fig. 5. Influence of the S2O8 concentrationon the removal of Cr (VI) (a) and phenol (b) (phenol/Cr(VI) molar ratio, 2: 1; pH, 3.0; H2O2/Fe0 molar ratio, 2:1; time, 1–4 min).
3.4. Role of S2O8 concentration in the simultaneous removal of chromium and phenol by the UHFSU Fig. 5 portraits the influence of S2O8 concentration on the simultaneous removal of Cr (VI) and phenol. As can be seen in Fig. 5, in the higher initial concentration of S2O8, removal performance increases dramatically. That way with changing the initial S2O8 concentration from 0.25 mM L−1 to 0.75 Mm L−1, the complete removal of Cr (VI) and phenol was obtained within 4 min. With increasing the concentration to 1 mM was no observed the significance influence in the removal performance. This phenomenon can be related to the large amounts of generated radicals (SO4%−) due to a high dosage of S2O8, which was enough to complete removal of both pollutants target existing in aqueous solutions (Eqs. (1), (3) and (7)). The generated radicals can be reacted with more selectively via electron transfer. Many studies have shown that in the excessive amounts of persulfate, efficiency of the reaction will be reduced in which can be due to some reverse reactions, side reactions, and excessive oxidant (Eqs. (21)–(24)) resulting to a scavenging effect on SO4%−. However, no inhibition was observed in the present study since the oxidant dosage was below the inhibition point [54–56].
S2 O82 − + •O H→ OH− + S2 O•8−
(21)
S2 O82 −+SO•4− → SO24− + S2 O•8−
(22)
SO•4−+SO•4− → S2 O82 −
(23)
S2 O82 −+2•O H→ 2HSO5−
(24)
3.6. Electrical energy consumption and the total cost of the system One of the important parameter that affected performance of AOPs processes is the amount of required electrical energy (kW) to diminish the content of a pollutant by 90 % in 1 m3 of the contaminated solution and also the total cost of the system that is calculated by electric energy per mass removed (EEM) (in kWh/kg) as the following formula [1]:
EEM =
38.4 × p V × kobs
(25)
where P is electrical power applied(kW) for the system (UV lamp) and V is the volume of pollutant in inside reactor (L). Also Eq. (26) was applied to calculate the total cost of the system (TCS) ($ m−3) [57,58].
TCS = 1.45 × (EE /O (kWhm−3) × power cos t ($/ kWh)) + oxidant cos t ($g −1)
(26)
The values of EEM and TCS for the various processes were calculated and tabulated in Tables 2a and 2b. As tabulated in Tables 2a and 2b, UHFSU process due to energy saving and the less total cost rather than other processes (because of reduction of energy consumption) can be a good cost-effective alternative and can be more efficient process in the simultaneous removal of Cr (VI) and phenol from aqueous solutions. 5
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analyzed according to the standard methods [53] and were determined 140, 120, 15, 56, 72 and 1.5–2.5 mg L−1 respectively. As mentioned in the previous sections, the removal efficiency of both phenol and chromium contaminants in distilled water and under the selected conditions, was 100 %. The anions present in the tap water indicated the negligible influence on the reaction efficiency, as removal efficiency for phenol and chromium reached from 100 % at absence of anions (distillated water) to 96.3 % and 94.8 %, respectively (at the presence of different anions (tap water)). However, it is important to note, that this slight decrease in efficiency may be due to the low amounts of anions present in the tap water, and at higher levels of these anions, the decrease in efficiency will be more significant. The positive influence of anions present in the tap water on the simultaneous removal of Cr (VI) and phenol can be explained on the basis of the salting-out effect and thus the solute was pushed towards the highly reactive interface region of the cavitation bubble to facilitate the removal [1]. Also, some humic acid (15 mg L−1) was added to the tap water and the reaction efficiency was re-evaluated. The reaction efficiency decreased further in the presence of humic acid and reached from 100 % to 88.3 % and 91.2 % for chromium and phenol, respectively. This dramatic decrease in the removal efficiency in the presence of humic acid and anions (simultaneously) can be due to the complex structure of humic acid, which can consumes hydroxyl radicals and sulfate radicals and affect the reaction efficiency [59].
Table 2a Values of EEO and the total cost for the removal of Cr (VI) using different processes. Process
Kobs (min−1)
EEO (kWh m−3)
Total cost ($ m−3)
UV UH UHF UHFS UHFSU
0.031 0.045 0.059 0.14 0.45
90.8 62.22 47.45 20 6.22
2.6 2.18 2.05 1.55 1.15
Table 2b Values of EEO and the total cost for the removal of phenolusing different processes. Process
Kobs (min−1)
EEO (kWh m−3)
Total cost ($ m−3)
UV UH UHF UHFS UHFSU
0.007 0.052 0.085 0.16 0.21
400 53.8 32.9 17.5 13.33
11.6 1.94 1.62 1.47 1.35
3.7. Effect of scavengers and possible degradation mechanism of UHFSU system To approve involvement of free radical species in the simultaneous removal of chromium and phenol by the UHFSU, various scavengers (0.1 mM) including ammonium oxalate, 2-propanol and tert-butanol, were added to the reactor and results were portrayed in Fig. 7. As portrayed in Fig. 7, upon addition of the scavengers (specifically, addition of tert-butanol as a strong scavenger %OH radical), simultaneous removal performance of chromium and phenol reduced. It can be concluded that process efficiency is highly dependent on the generated reaction radicals, and oxidative radicals play a key role in the removal of both pollutants. The reason for slightly decrease of UHFSU performance with addition of scavengers can be due to the low amount of scavengers added in solution (0.1 mM), as it possibility the higher decrease of UHFSU performance in the higher amounts of scavengers [1,24].
4. Conclusion In the present study the performance of sono-photo‑Fenton-like process (SPFP)in the presence of persulfate, to generate oxidative radicals (O2%−, %OH, SO4%−) for the simultaneous removal of chromium and phenol from aqueous solutions. The UHFSU process showed higher performance than the other processes (UV alone, UH, UHF and UHFS) within 4 min. The kinetic information obtained also proves the superiority of UHFSU process than other processes. It reveals the UHFSU process has the better performance in the acidic condition. The complete removal of the both Cr (VI) and phenol pollutants, were achieved in the H2O2/Fe0 molar ratio of 2.0, so the efficiency was improved. Although, it is notable, in the extra concentrations of hydrogen peroxide and Fe0, they acts as the scavenger of hydroxyl radical. It was observed 0.75 mM L−1 of S2O8was enough to complete removal of both pollutants target existing in aqueous solutions. The results indicated that the phenol/Cr(VI) molar ratio in the aqueous solution plays a major role in the process efficiency as the complete removal of the both pollutants was obtained in the phenol/Cr(VI) molar ratio equal to 2 and by increasing the molar ratio above 2, it caused a decrease in process performance. It was resulted, UHFSU process due to energy saving and the less total cost rather than other processes (because of reduction of energy consumption) can be a good cost-effective alternative and can be more efficient process in the simultaneous removal of Cr (VI) and phenol from aqueous solutions. Upon addition of the scavengers, simultaneous removal performance of chromium and phenol reduced. It can be concluded that process efficiency is highly dependent on the generated reaction radicals, and oxidative radicals play a key role in the removal of both pollutants. Finally, it was found that the UHFSU process improved the simultaneous removal of Cr (VI) and phenol from aqueous solutions
3.8. Influence of humic acid and coexisting inorganic anions By introducing inorganic anions such nitrate, carbonate, bicarbonate, sulfate, chloride and fluoride in aquatic systems, we could study how inorganic anions influence on the simultaneous removal of Cr (VI) and phenol by UHFSU process. The tap water was used to this purpose. The inlet phenol/Cr(VI) molar ratio was 2:1 and reaction time set at 4 min (pH, 3.0; H2O2/Fe0 molar ratio, 2:1). The concentrations of carbonate, bicarbonate, nitrate, sulfate, chloride and fluoride at tap water
Authors contribution All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
Fig. 7. The effect of various quenchers in the simultaneous removal of chromium and phenol by the UHFSU. 6
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Acknowledgements [22]
The authors would like to thank Shahid Beheshti University of Medical Sciences, Tehran, Iran, for financial supporting of current research project (Grant number 14164 and ethical code: IR.SBMU.PHNS.1397.133).
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