The application of thermally activated persulfate for degradation of Acid Blue 92 in aqueous solution
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Thermally activated persulfate (TAP) was applied for the degradation of Acid Blue 92 (AB92) dye in its aqueous solution. The effects of pH (3–11), temperature (298–333 K), contact time (15–75 min), sodium persulfate (SPS) concentration (0.05–0.5 mM) and initial AB92 concentration (50–400 mg/L) on the degradation of AB92 using TAP were examined. The initial and residual AB92 concentrations were determined spectrophotometrically at the wavelength of 260 nm and the dye mineralization was examined via the total organic carbon analysis. In addition, the chemical oxygen demand was also measured. The activation energy (Ea) of AB92 degradation was calculated as 17.38 kJ mol−1 based on the Arrhenius equation. Maximum degradation efficiency of 86.47% was reached after 75 min of treatment at a pH of 5, AB92 concentration of 200 mg/L, SPS concentration of 0.5 mM and temperature of 333 K. The degradation efficiency declined with the addition of different sodium chloride concentrations and organic radical scavengers. AB92 degradation was reduced from 86.5 to 74%, 65, and 59.1% using ethylenediaminetetraacetic acid, tert-butanol, and ethanol, respectively. A kinetic model was also developed to estimate the pseudo-first-order constants as a function of the main operational parameters (initial dye concentration and TAP concentration). Decolorization rate constants (k) of 0.0009, 0.001, 0.0012, 0.0014, and 0.0018 min−1 were obtained at 303, 308, 313, 328, and 333 K, respectively, using the Langmuir–Hinshelwood kinetic model. The results obtained indicate that the TAP degradation process has great potential for the reduction of azo dyes in aqueous environments.
KeywordsAcid Blue 92 Thermally activated persulfate Degradation efficiency Total organic carbon Chemical oxygen demand
The textile industry is considered as a prominent dye production sector . The utilization of various types of colors in addition to chemical substances in dyeing processes generates wastewater with unique characteristics such as pH, color, and composition . The disposal of colored wastewater into the aquatic ecosystem significantly hinders the penetration of light into the deep waters [3, 4]. It may also disturb the process of photosynthesis; this can also lead to the obliteration of aquatic plants . In addition, colored dye effluents are significantly hazardous to the environment even at lower concentrations . Moreover, the majority of dyes employed by textile industries are of organic origin; they are produced from phthalocyanine, diazo and anthraquinone salts which contain benzene rings that are highly carcinogenic and toxic in nature [6, 7]. An example of such dye is the C.I. Acid Blue 92(AB92), which is utilized on a regular basis by textile industries. Many researchers have proven that dyes are not completely removed during biological treatment, and they enter into water resources via wastewater effluents originating from treatment plants . These compounds are not eliminated effectively through traditional wastewater removal procedures since they are non-biodegradable.
Several techniques have been employed for the elimination of dyes from polluted waters including coagulation–flocculation [9, 10], chemical treatment , oxidation [12, 13], adsorption [14, 15, 16, 17, 18, 19, 20, 21, 22] and photocatalytic degradation [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. Adsorption is the most widely used because of its simplicity, low cost and adsorption recovery properties in removing contaminants . Adsorption is usually done with an adsorbent such as activated carbon to eliminate dyes, but this process only transfers pollutants from one phase to another .
SO 4 ·− is comparably more stable to oxidize organic contaminants, thus providing the possibility for greater dispersion distance and improved mineralization in water [55, 56]. SO 4 ·− has been proven to possess great potential for methylparaben degradation via UV-activated persulfate compared to other activators . Cai et al.  proved that the bimetallic Fe–Co/GAC catalyst may be utilized to heterogeneously activate SPS oxidation for Acid Orange 7 degradation, which has also been proven in other studies. Also, very toxic persistent organic pollutants (POPs) can be decontaminated with persulfates .
Thus, in this research, the impact of heat activation on SPS for the elimination of AB92 from its aqueous solution was examined. The impact of different operating parameters such as pH, contact time, sodium persulfate concentration, and initial AB92 concentration on the degradation process was also examined. No prior studies on AB92 dye removal using the TAP process have been observed in the literature. The effect of different radical scavengers including ethylenediaminetetraacetic acid (EDTA), tert-butanol (TBA), and ethanol (EtOH) on the removal of AB92 was examined. The impact of sodium chloride concentrations was also considered.
Materials and methods
Physical and chemical properties of AB92
Molecular weight (g mol−1)
Acid Blue 92
Experimental procedure and analysis
The effects of different parameters such as pH (3, 5, 7, 9, and 11), contact time (10, 20, 40, 60, and 80 min), dye concentration (50, 75, 100, 150, and 200 mg/L), SPS concentration (0.05, 0.07, 0.09, 0.3, and 0.5 mM), and temperature (313, 323, 333, 343 and 353 K) on AB92 degradation were studied.
Initially, the initial AB92 dye concentration, SPS concentration, pH, and temperature were set to be constant at 200 mg/L, 0.07 mM, 3, and 333 K, respectively. The effect of each parameter on the removal percentage of AB92 dye was evaluated. AB92 dye and SPS stock solutions were prepared using double-distilled water. The reactor was filled with 250 mL of the prepared AB92 solution. The initial and residual AB92 dye concentrations were determined using a UV–visible spectrophotometer (Shimadzu Model: CE-1021-UK). The COD concentration was determined using a spectrophotometer (COD VAQIO). The total organic carbon (TOC) was determined through the TOC analyzer (ANATOC Series II). Before adding SPS to the solution, the solution temperature was set using a shaker incubator (Froilabo EC180). To ensure the complete mixing of the mixture during the experiment, the shaker speed was set at 150 rpm.
Results and discussion
Effect of pH
Although the hydroxyl radicals showed high oxidation potential (E0 = 2.8 V), they were attracted by non-target species or converted to hydrogen peroxide (E0 = 1.8 V) because of their non-selective reaction [43, 45]. Sulfate radical showed selective reactions too. The difference in reaction mechanisms could differentiate the dye decomposition rate in alkaline and acidic conditions because electron transfer is the main mechanism of sulfate radicals’ reaction . Ji et al.  showed that the maximum rate of trichloroethylene (atrazine) removal using persulfate occurred at a pH of 5.
Effect of initial dye concentration
The effect of SPS concentration
Yang et al.  proved that when the initial persulfate concentration exceeded a particular point, the azo dye, Acid Orange 7 decomposition rate is marginally slowed down.
Effect of temperature
The reaction time is another parameter of importance in SPS oxidation with heat . After some time, the production of sulfate radicals increased parallel to the rate of pollutants’ removal.
Ghauch and Tuqan  proved that bisoprolol decomposition via heated persulfate/H2O requires an activation energy of 119.8 kJ mol−1.
Effect of chloride ion
The SO 4 ·− and Cl− reaction at neutral pH creates ·OH. Thus, Cl− is not regarded as a competing solute, since the result of this reaction is another profound oxidant that can cause a reaction with the contaminants. The Cl− is regarded as a radical scavenger, that is, a computing solute at pH < 5 since the reaction result was not a secondary radical applicable to oxidizing the contaminants. Furthermore, at this pH, the reaction creates ClO3− which is possibly a toxic compound.
Effect of inorganic and organic radical scavengers
In addition, EDTA is an organic molecule that can react with ·OH and SO 4 ·− . Therefore, the addition of an excess amount of EDTA should be avoided to prevent the EDTA from competing with the contaminant for the reactive radical species.
Comparison with other AOPs for the degradation of AB92
The summary of the AOP degradation processes applied in the treatment of the AB92 dye
Method of analysis
Initial dye concentration = 10 mg/L
Catalyst dosage (Sm-doped ZnO) = 1 g/L
Dopant percentage = 6%
Ultrasonic power = 150 W
Reaction time = 150 min
Frequency = 36 kHz
UV–Vis spectrophotometer (WPA Light wave S2000, England)
DE = 90.10%
Initial dye concentration = 40 mg/L
pH = 10
Time = 20 min
UV–vis spectrophotometer (PG, T80)
DE = 96.30%
Initial dye concentration = 40 mg/L
H2O2 concentration = 6 mg/L
pH = 10
Time = 30 min
UV–Vis spectrophotometer(PG, T80)
DE = 97.8%
O3/activated carbon (AC)
Initial dye concentration = 40 mg/L
Carbon concentration = 1 g/L (for DE)
Carbon concentration = 2 g/L (for COD)
pH = 4
Time = 5 min
UV–Vis spectrophotometer (PG, T80) and Open reflux method (ORM)
DE = 98.2%; COD = 100%
pH = 8
Laccase activity of 2.5 U/mL
Dye concentration = 75 mg/mL
UVD 2950, Labomed, Culver City, USA
DE = 94.1%
Thermal activation process
Initial dye concentration = 200 mg/L
Persulfate concentration = 0.5 mM
pH = 5
Time = 75 min
Temperature = 333 K
UV–visible spectrophotometer (Shimadzu Model: CE-1021-UK) and COD VAQIO
DE = 86.47%; COD = 74.92%
The applicability of thermally activated sodium persulfate (SPS) for the degradation of Acid Blue 92 (AB92) from its aqueous solution was investigated. The effects of pH (3–11), temperature (298–333 K), contact time (15–75 min), SPS concentration (0.05–0.5 mM), and initial AB92 concentrations (50– 400 mg/L) on the degradation of AB92 were examined. The AB92 disintegration kinetics was studied. The degradation of AB92 by SPS was fitted into the Langmuir–Hinshelwood model equation. The effect of various organic radical scavengers (EDTA, tert-butanol, and ethanol) and chloride ion concentration on AB92 removal was examined. Optimum conditions of pH 5, SPS concentration of 0.5 mM, reaction time of 75 min, and temperature of 333 K and initial AB92 concentration of 200 mg/L were obtained for the AB92 removal using the TAP process, which gave a removal efficiency of 86.74%. AB19 degradation efficiency declined with the addition of radical scavengers and chloride ion. The TAP degradation process was employed efficiently for the treatment of azo dyes in aqueous environments.
The authors are grateful to the Zabol University of Medical Sciences for their financial support in this study (Project No. 1396.329).
SA and CAI Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper. Contributed reagents, materials, analysis tools or data. SR Performed the experiments; Analyzed and interpreted the data.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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