Photo-oxidative Decolorization of Brilliant Blue with AgNPs as an Activator in the Presence of K2S2O8 and NaBH4

  • Abeer Saad Al-Shehri
    Abeer Saad Al-Shehri
    Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
  • Zoya Zaheer*
    Zoya Zaheer
    Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
    *Email: [email protected]; [email protected]
    More by Zoya Zaheer
  • Amell Musaid Alsudairi
    Amell Musaid Alsudairi
    Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
  • , and 
  • Samia A. Kosa
    Samia A. Kosa
    Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
Cite this: ACS Omega 2021, 6, 41, 27510–27526
Publication Date (Web):October 7, 2021
https://doi.org/10.1021/acsomega.1c04501
Copyright © 2021 The Authors. Published by American Chemical Society
ACS AuthorChoiceCC: Creative CommonsBY: Credit must be given to the creatorNC: Only noncommercial uses of the work are permittedND: No derivatives or adaptations of the work are permitted
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Abstract

The decolorization of brilliant blue (E133) in aqueous solution by K2S2O8 and NaBH4 with AgNPs as an activator was studied spectrophotometrically under normal laboratory conditions. Batch experiments were performed to investigate the effects of reaction time, initial dye concentration, activator concentration, solution pH, and temperature on the decolorization of E133. K2S2O8 and NaBH4 did not decolorize the dye E133 in the absence of AgNPs. The optimum dosage of AgNPs was 0.01 g/L, and 98% dye E133 degradation was observed with 3.75 mM K2S2O8 at 30 °C in ca. 60 min of reaction time. In the NaBH4/AgNPs system, only 60% dye degradation was observed for an identical reaction condition. The decolorization rate constant increases with the increase in concentrations of AgNPs, K2S2O8, NaBH4, and reaction temperature. The decolorization degree of the E133 responded linearly with K2S2O8 and NaBH4 concentrations. The existence of sulfate radicals (SO4·) and hydroxyl radicals (HO·) generated during the decolorization of E133 was identified by using ethanol and tertiary butyl alcohol as scavengers. Based on the E133 solution absorbance changes at 628 nm, the decolorization mechanism was proposed and discussed.

Introduction

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Triphenylmethane dyes are extensively used in various industries such as cosmetic, food, leather, paper, and textile for many purposes from many decades. (1−5) The expanding use of triphenylmethane dyes in textile industries for dying cotton, nylon, and wool may cause substantial ecological damage due to the release of different dyes into the environment. (6) Brilliant blue FCF is a water-soluble triphenylmethane dye, generally used as a water tracer agent. (7) Due to its nontoxic activities, brilliant blue has been used as a food colorant as well as to stain cell bacterial and fungal cells. (8,9) Weber et al. reported that brilliant blue shows an allergic reaction in some bronchial asthma patients. (10) The photocytotoxicity of triphenylmethane dye depends on the production of reactive oxygen species with regard to their photodynamic therapy. (11) The use of dyes as a food coloring agent has serious impact on human health and environment. (12)
Advanced oxidation technologies (AOTs) have been considered as important technologies for accelerating the oxidation and/or decolorization of a wide range of organic contaminants from polluted wastewater and air. (13) AOTs were based on in situ generation of highly reactive oxidizing radicals, such as HO· and SO4·, to oxidize organic pollutants using common oxidants such as hydrogen peroxide, (14) Fenton’s reagent, (15) potassium persulfate, (16) potassium periodate, (17) and potassium bromate. (18) Out of these, potassium persulfate will get an edge over the other oxidants due to its nonselective reactivity, stability, high aqueous solubility, ease of storage, relatively low cost, and higher reduction potential (E0 of S2O82–/SO42– = 2.01 V) in aqueous solution at neutral pH. Berlin (19) and Huag et al. (20) reported the generation of HO· (E0 = 2.8 V) and SO4· (E0 = 2.6 V) during the thermal decomposition of S2O82– ions in an aqueous phase. These radicals were also generated by the activation of S2O82– ions by UV irradiation, (21) transition metals, (22) and metal nanoparticles. (23)
Sodium borohydride is a better source of hydrogen and is generally used as a reducing agent (E0 = 1.24 V) for the synthesis of advanced transition metal nanoparticles. It is also used as a source of hydrogen to the reduction of metal ions and oxidative discoloration of various toxic nonbiodegradable organic dyes and other compounds in the presence of metal NPs. (24−26) The surface of NPs plays a significant role (electron relay effect) during the transfer of hydrogen from NaBH4 to the oxidizing agent. For example, Kaur and her co-workers reported the extraction of gold and silver nanoparticles from the aqueous phase by using water-soluble iron oxide nanoparticles and suggested that the degree of extraction depends on the strength of interactions between the AuNPs and AgNPs with Fe2O3 NPs. (27) Pal and his co-workers reported the degradation of aromatic nitro phenols by using AgNPs as a catalyst in the presence of NaBH4. (28) Ghaedi and his co-workers prepared various mono-, bi-, and tri-metallic nanoparticles (palladium, silver, bismuth–silver, copper–zinc–nickel, and nickel–cobalt–alumina) that were used as an adsorbent for the removal of Congo red. (29)
Synthetic dyes are toxic, nonbiodegradable, and carcinogenic and produce hazardous effects on the environment. (30) The decolorization of triphenylmethane dyes such as crystal violet, methyl violet, malachite green, Coomassie brilliant blue, and gentian violet has acquired increasing attention under various experimental conditions. (5,31,32) Brilliant FCF food dye (blue 1, E133; used as a food colorant) is a synthetic dark blue-colored dye of the triphenylmethane auxochrome category (Scheme 1).

Scheme 1

Scheme 1. Structure of Brilliant Blue (Blue 1)
The degradation studies of E133 and Coomassie brilliant blue by AgNPs have been reported under different experimental conditions, (33−35) but the kinetics of decolorization and mineralization of E133 with AgNPs under the UV/K2S2O8 system have been neglected. The main aim of this work purpose was to investigate the factors that influenced the photocatalytic and oxidative decolorization of E133 by K2S2O8 and NaBH4 with AgNPs as an activator and/or catalyst. The factor studies are the oxidant doses, initial E133 concentration, and effect of AgNPs concentration, pH, and temperature. We investigated the E133 decolorization followed by mineralization under the UV/K2S2O8 system. The kinetics of decolorization and mineralization were discussed and then a mechanism for the degradation was proposed for the first time.

Results and Discussion

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Preliminary Studies

In the first set of experiments, the UV–visible spectra of E133 were recorded for different dye concentrations. These results are given in Figure 1A, which shows that the E133 exhibits three absorption peaks at 309, 405, and 628 nm in an aqueous solution. The absorbance of the 628 nm peak was immense compared to those of others. At a higher E133 concentration (≥4.68 × 10–5 mol/L), the shape of the 628 nm peak has been changed entirely, which might be due to the self-aggregation of dye molecules. (36−38) A Beer–Lambert plot was drawn between the absorbance and concentrations of E133 to determine the molar extinction coefficient (ε). Interestingly, the absorbance–concentration plot deviated from the linearity at higher E133 concentrations (Figure 1B). Therefore, the value of ε was calculated from the linear part of the plot and found to be 4.8 × 104 L/mol/cm at 30 °C. The relation between the absorbance A628 and the concentration of commercial E133 is A628 = 4.8 × 104 × [E133] (mol/L), with R2 = 0.997. For the decolorization experiments, the lower E133 concentration (≤3.9 × 10–5 mol/L) was used in the entire study. The AgNPs were prepared by using hydrazine and CTAB as a reducing and stabilizing agent at 30 °C. In a typical experiment, the aqueous solution of AgNO3 (1.25 mM) was added into a reaction vessel containing hydrazine (5.0 mM) and CTAB (0.8 mM) and stirred with a magnetic stirrer. The UV–visible spectra of resulting orange-colored silver sols were recorded at different time intervals. The resulting AgNPs exhibited a sharp absorption peak at 440 nm, indicating the reduction of Ag+ ions into metallic Ag0, and the sols were stable for ca. 1 month (Figure 1C). (39) No precipitates and any type of turbidity appeared. To determine the effect of sodium thiosulfate on the stability of AgNPs, the UV–visible spectra of AgNPs were recorded for different Na2S2O3 concentrations. The absorbance and surface plasmon resonance (SPR) peak position of AgNPs remained unchanged at 440 nm even for ca. 24 h in the presence of Na2S2O3 (from 0.50 × 10–6 to 2.5 × 10–6 mol/L). The AgNPs were also stable for ca. 1 week with NaBH4 (20 × 10–5 mol/L). (40)Figure 1D shows a transmission electron microscopy (TEM) image of the AgNPs, which are spherical and polydispersed and have a diameter ranging from 5 to 80 nm. Inspection of the TEM image clearly indicates that the various small NPs are aggregated to each other, and large-size NPs are formed. The solid AgNPs were separated through centrifugation, washed with distilled water, dried at room temperature, stored in an amber glass-colored bottle to protect from light, and used as a catalyst and/or as an activator to the decolorization of E133 under different experimental conditions.

Figure 1

Figure 1. UV–visible spectra of E133 as a function of different concentrations (A), Beer–Lambert plot (B), UV–visible spectra of AgNPs (C), and its TEM image (D) at 30 °C. Reaction conditions: [AgNO3] = 1.25 mM, [hydrazine] = 5.0 mM, and [CTAB] = 0.8 mM for C.

Effect of UV Light on E133 Decolorization

It has been established that the S2O82– anion was activated with heat, UV light, transition metal ions, and metal NPs, producing a SO4· radical, which is a stronger oxidant than the S2O82– anion. Therefore, a control experiment was performed in the presence of UV light irradiation without K2S2O8. In a typical kinetic run, the 3.9 × 10–5 mol/L concentration of E133 was mixed with AgNPs (0.02 g/L) and irradiated under UV light, and the UV–visible spectra were recorded at different time intervals. Figure 2 shows that only UV light was not sufficient for the decolorization of E133. There was no observable color change with UV irradiation for ca. 100 min of reaction time (Figure 1, optical images). Surprisingly, the absorbance was increased at 409 nm with increasing reaction time (Figure 2, UV–visible spectra), and the intensity at 628 nm was slightly decreased (from 1.65 to 1.49). Under UV light irradiation, there was 9.6% decolorization of E133 at 628 nm with 100 min of reaction time. The increase in absorbance at 409 nm (from 0.38 to 0.68; Figure 2B) can be rationalized due to the adsorption of E133 onto the positive surface of AgNPs (Henglein reported the formation of Ag42+ as a stable species of AgNPs in an aqueous solution even in the absence of a stabilizing agent (41)) through the negative −SO3 of E133 (Scheme 2).

Figure 2

Figure 2. UV–visible spectra and optical images (A) and reaction–time profile (B) of decolorization of E133 by AgNPs without K2S2O8 under UV irradiation as a function of time at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 0.0 mol/L.

Scheme 2

Scheme 2. Adsorption of E133 on the Surface of AgNPs
As presented in Figure 1A,C, the spectrum of E133 shows the three absorption peaks at 309, 405, and 628 nm, whereas the AgNPs display an SPR band at 440 nm. Interestingly, a new band at around 409 nm in the absorption of AgNPs in complex with E133 (Figure 2) (38) might be due to the changes in surface plasmonic properties of AgNPs and not from the dye molecule. (42) On the other hand, no decolorization of E133 was observed with increasing K2S2O8 concentrations ranging from 0.75 × 10–3 to 5.0 × 10–3 mol/L under a similar E133 concentration, pH (5.5), and temperature (30 °C) in the absence of UV light.

Effect of the UV/K2S2O8 System on E133 Degradation

Effect of K2S2O8
Figure 3A shows the optical images of a reaction mixture containing AgNPs and E133 as a function of time. We did not observe the appearance of any type of turbidity and/or precipitate for ca. 60 min, which suggests that the E133 was stable with AgNPs at room temperature. E133 was adsorbed on the surface of AgNPs with physical and chemical interactions. To gain insight into the combined effect of K2S2O8 and UV light, the UV–visible absorption spectrum changes in the AgNPs-E133 solution were observed, and corresponding spectral and optical images are shown in Figure 3B. Interestingly, the absorption intensity of E133 became weaker along with the reaction time. The blue color of E133 disappeared completely (Figure 3B, optical images), and 90% decolorization was observed in 30 min with the UV/K2S2O8 ([K2S2O8 = 3.75 mM]) treatment system, which indicates that UV/K2S2O8 was superior to UV alone in terms of the dye removal efficiency in the presence of an activator. A careful observation of Figure 3B revealed that, initially, the reaction has a time lag to observe any visible change in the absorbance value, i.e., the induction time. The absorbance of E133 decreases (from 1.67 to 0.03) at 628 nm within 60 min, and the blue color changes to almost colorless. The E133 degradation was calculated and found to be 98%. On the other hand, the absorbance changes from 0.4 to 0.35 with time at 408 nm (Figure 3B, inset), and no sharp SPR band of AgNPs appeared at 440 nm after the complete discoloration of E133 in 60 min. The degradation of E133 and oxidative dissolution of AgNPs occurred simultaneously under UV/K2S2O8 treatment due to the activation of persulfate by AgNPs. Kaur et al. also observed the color change of AgNPs and AuNPs after the addition of Fe2O3 NPs. (27) These NPs have strong complex-forming efficiency with Fe2O3 NPs.

Figure 3

Figure 3. Optical images of AgNPs and E133 (A) and UV–visible spectra and optical images of decolorization of E133 by K2S2O8 with AgNPs under UV irradiation as a function of time at 30 °C (B). Inset: Decay of absorbance at 408 and 628 nm with time. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 1.25 mM.

From plots of absorbance at 628 nm, A0At, and ln(A0/At) versus time (Figure 4A–C), it may be concluded that the decolorization has an induction period and then the rate increases with time. The extent of induction period depends on the K2S2O8 concentrations. (25) The decolorization follows excellent pseudo-first-order kinetics. The effect of reaction time on the decolorization extent was studied as shown in Figure 4B. The decolorization extent was presented by absorbance change (ΔA = A0At), where A0 and At are the absorbance of the initial E133 and at reaction time t. It was observed that the ΔA increases rapidly and then approaches an almost constant value after a certain time, which might be due to either complete decolorization of E133 or full consumption of K2S2O8. The saturation time was found to be ca. 30 and 60 min for 1.25 and 3.75 mM K2S2O8, respectively. The decolorization percentage also depended on the reaction time at different K2S2O8 concentrations, i.e., 90% E133 was decolorized within 30 min at 3.75 mM K2S2O8 (Figure 4D). Therefore, a SO4· radical was generated in the reaction mixture (E133 + S2O82– + Ag0) by the scission of a peroxide bond by UV light. (19) The resultant SO4· undergoes a series of chain reactions, and other oxidants such as HO· and peroxymonosulfate (HSO4) were generated (eqs 15).(1)(2)(3)(4)(5)

Figure 4

Figure 4. Plots of absorbance decay at 628 nm (A), A0At (B), ln(A0/At) (C), and percentage decolorization versus time to the decolorization of E133 by K2S2O8 with AgNPs at 30 °C (D). Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 1.25 mM.

SO4· (E0 = 2.6 V) can oxidize E133 via a one-electron transfer step, leading to decolorization of the dyes. (43,44) The values of rate constants were calculated for different K2S2O8 concentrations at a fixed E133 concentration, AgNP concentration, pH, and temperature. Table 1 shows that the rate constant of E133 decolorization was increased with K2S2O8 from 0.75 to 5.0 mM. However, a further increase in K2S2O8 above certain limits decreased the decolorization rate constants. This is mainly because K2S2O8 has two different roles during the dye degradation. (45,46) At lower concentrations, the generated SO4· will be available to attack on E133 molecules. As a result, the reaction rate increases. On the other hand, S2O82– acts as a SO4· radical scavenger instead of a free radical generator at higher concentrations (eqs 6 and 7).(6)(7)
Table 1. Effects of [E133], pH, [K2S2O8], and AgNPs on the Decolorization of E133 with the UV/K2S2O8 System at 30 °C
105[E133] (mol/L)[K2S2O8] (mM)[AgNPs] (g/L)pH104kobs (s–1)
3.10.00.015.50.0
3.13.750.015.539.7 ± 4.4
3.93.750.015.536.1 ± 4.4
4.63.750.015.533.6 ± 5.2
5.43.750.015.529.8 ± 5.1
3.90.750.015.510.5 ± 4.3
3.91.250.015.522.9 ± 4.5
3.92.50.015.530.2 ± 4.4
3.95.00.015.542.1 ± 3.4
3.95.00.015.542.1 ± 4.7
3.93.750.025.540.2 ± 4.5
3.93.750.035.544.6 ± 4.6
3.93.750.045.546.5 ± 5.1
3.93.750.055.548.2 ± 4.6
3.93.750.013.540.2 ± 4.8
3.93.750.018.525.2 ± 4.4
3.93.750.0110.518.5 ± 4.6
There is a competition between S2O82– and E133 to react with a SO4· free radical in excess of K2S2O8. The S2O82– reactivity was higher toward SO4·, decreasing the free radical concentration in solution, which in turn decreases the rate of decolorization at higher K2S2O8 concentrations. (47)
Effect of Dye and pH
The effect of initial E133 concentration on the rate of decolorization efficiency in the UV/K2S2O8 system with AgNPs was monitored in the range of 3.1 × 10–5 to 5.4 × 10–5 mol/L while maintaining the other parameter constant (Table 1). It can be seen that the rate of decolorization decreases as the initial dye concentration increases at a constant K2S2O8 concentration, which might be due to the fact that the high concentration of E133 would consume more SO4·. On the other hand, the penetration of photons entering into the reaction mixture decreases due to the high concentration of E133. As a result, the solution became more impermeable to the UV radiation due to the inter-filter effect. Finally, the SO4· concentration decreases, which in turn decreases the rate of decolorization. The solution pH is an important parameter and has significant impact on the dye degradation. Therefore, the effect of pH (3.5, 5.5, 8.5, and 10.5) was investigated on the decolorization of E133. The results of pH dependency on the rate of decolorization are summarized in Table 1, which shows that the decolorization of E133 decreased with the increase in the pH value from 5.5 to 10.5. It was observed that the E133 was completely removed after 25 min at pH 3.5. The percentage decolorization values of E133 were 90, 93, 55, and 45% at pH 3.5, 5.5, 8.5, and 10.5, respectively. At pH 5.5, the percentage degradation was higher than that at pH 3.5, indicating the scavenging of SO4· in acidic media. (48) The generation of SO4· was increased in acidic solution (i.e., pH = 3.5) due to acid catalyzation (eqs 12 and 13), which leads to increases in the concentration of SO4· in solution, and scavenging of SO4· occurs simultaneously via eqs 8 and 9.(8)(9)
In alkaline solution (pH ≥ 8.5), SO4· was unstable, started to decompose, and transformed rapidly into HO· due to the base activation mechanism of persulfate for the generation of SO4· in the aqueous, neutral, and alkaline media (eq 10). (49,50)(10)
SO4· reacts with HO and immediately generates HO· in basic conditions (eq 11), which has little reduction potential than SO4·. The reactivity of HO· was lower in solution due to the existence of SO42–. (51) As a result, the percentage degradation and decolorization rate of E133 were decreased at pH ≥ 8.5 due to the recombination of both reactive species, SO4· and HO· (eqs 11 and 12).(11)(12)
Our results are in good agreement with the observations of Hussain et al., (23) Xu and Li, (44) and Furman et al. (50) regarding the degradation of toxic dyes and water pollutants with persulfate.
Effect of AgNPs and Temperature
Using 3.7 mM K2S2O8, different concentrations of AgNPs (0.01 to 0.05 g/L) were used to investigate their effect on E133 decolorization (3.75 × 10–5 mol/L) at pH = 5.5 and 30 °C. It was observed that the AgNPs and/or K2S2O8 only do not provide any decolorization effect in the absence of UV light as shown by the control experiment in Figure 4A. The decolorization rate increased with increasing AgNPs (Table 1). The higher concentration of AgNPs provides more sites for SO4·, thereby increasing the reaction rate. Henglein reported the formation of Ag42+ of AgNPs in an aqueous solution (eqs 13 and 14). (41)(13)(14)When the AgNP concentrations were varied at 0.01, 0.02, 0.03, 0.04, and 0.05 g/L, the decolorization rates were 36.1 × 10–4, 40.2 × 10–4, 44.6 × 10–4, 46.5 × 10–4, and 48.2 × 10–4 s–1, respectively, which might be due to the higher production of SO4·. Thus, S2O82– ions were adsorbed on the surface of AgNPs through electrostatic interactions and acted as an activator for the generation of SO4· (eqs 15 and 16). (52−54)(15)(16)
Generally, heat has been used as another important parameter to the activation of S2O82–. (55) Therefore, the effect of temperature (30, 40, 50, 60, and 70 °C) was studied on the decolorization of E133 by keeping other parameters constant. In the absence of AgNPs, the E133 decolorization was not observed at 30 and 40 °C for the K2S2O8/E133 system. As the temperature increases from 50 to 70 °C, the rate significantly increases (Table 2) due to the activation of S2O82– by heat. On the other hand, the decolorization was observed at 30 °C for the AgNPs/K2S2O8 system, indicating the combined effect of heat and AgNPs to the activation of S2O82–. The rate constants of E133 decolorization were 12.4 × 10–4, 24.4 × 10–4, 40.2 × 10–4, and 76.0 × 10–4 s–1 at 30, 40, 50, and 60 °C, respectively. The E133 removal efficiency by AgNPs/heat-activated S2O82– was higher than that without AgNPs (Table 2). The activation energy (Ea), enthalpy of activation (ΔH#), and entropy of activation (ΔS#) for the reaction were calculated by using Arrhenius (eq 17) and Eyring (eq 18) equations.(17)(18)where kobs, A, R, T, and kb are the rate constant, Arrhenius frequency factor, gas constant, temperature (in Kelvin), and Boltzmann constant, respectively. A good linear relationship was observed between the ln kobs versus 1/T (Figure 5A). The Ea values for the reaction were 57.4 and 30.6 kJ/mol, respectively, for heat and AgNPs/heat-activated S2O82–. The decrease in the Ea in the presence of AgNPs relative to that in the absence of AgNPs proves their catalytic effect (Table 2).

Figure 5

Figure 5. Arrhenius plot (A) and effects of scavenger on the decolorization of E133 by K2S2O8 with AgNPs at 30 °C (B). Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 1.25 mM.

Table 2. Effect of Temperature on the Decolorization of E133 under Various Reaction Conditions
reaction systemtemperature (°C)104kobs (s–1)Ea (kJ/mol)ΔH# (kJ/mol)ΔS# (J/K/mol)
E133/K2S2O830no reaction   
40no reaction   
502.5 ± 5.057.4 ± 4.454.8 ± 4.6–139.1 ± 4.6
604.6 ± 4.0   
709.2 ± 5.0   
E133/AgNPs30no reaction   
40no reaction   
E133/AgNPs/K2S2O8308.7 ± 4.030.6 ± 4.728.1 ± 4.8–128.8 ± 4.8
4016.2 ± 5.0   
5032.4 ± 5.0   
6064.4 ± 4.0   
Effect of Scavengers
To identify the dominating role of radical species (SO4· and HO·) in the AgNPs/K2S2O8 system, ethanol, isopropyl alcohol (IPA), and tertiary butyl alcohol (TBA) were used as quenchers. (56−58) The quenching experiments were carried out with the addition of these quenchers (0.01 mol/L) under the similar conditions as those described previously (Figure 4). The results clearly shows that the E133 decolorization was decreased by the addition of all alcohols (Figure 5B). For example, decolorization was 92% in the absence of any scavenger. However, 54, 47, and 22% decolorization was observed in the presence of IPA, TBA, and ethanol, respectively. The reaction was completely quenched by TBA, which is a more effective quenching agent for SO4· (quenching rate = 1.6 × 107 to 7.7 × 107 mol–1 s–1). (57) It reacts much slower with HO· (quenching rate = 1.2 × 105 to 2.8 × 105 mol–1 s–1). (58) These results indicate that the SO4· radicals are indeed the primary species generated by the AgNPs/K2S2O8 system. However, the role of HO· cannot be ruled out completely because ethanol also quenched the decolorization of E133. (59)
Total Organic Carbon Disappearance
The decolorization and mineralization are the two different paths of the complete dye degradation into CO2 and water. The various short- and/or long-lived intermediate(s) were formed during the decolorization of dye, which may be more toxic than the original dye. Colonna et al. (60) and Aleboyeh et al. (61) suggested that the final step toward the mineralization of dye starts only when decolorization is nearly complete. The kinetic experiments were also performed to evaluate the removal of the TOC ratio of the reaction mixture at a fixed initial E133 concentration of 3.75 × 10–5 mol/L, AgNP concentration of 0.01 g/L, and pH 5.5 at 1.25 and 3.75 mM K2S2O8. Figure 6 shows the corresponding change in TOC with time. TOC versus time plots clearly show that after an initial time of constant (induction time), a relatively fast decrease occurs (auto-acceleration). During the first 10, 20, and 40 min, ca. 3 to 12% TOC degradation was observed in solution for both K2S2O8 concentrations, indicating that the TOC removal is very slow in the colored solution (Figure 6A). The TOC removal increased with increasing time, and 80% TOC was removed after 100 min reaction time for 3.75 mM K2S2O8. Our results are in accordance to the observations of Colonna et al. (60) and Aleboyeh et al. (61) regarding the TOC of dye degradation. The TOC complete removal time depends on the concentration of K2S2O8.

Figure 6

Figure 6. Plot of mineralization % (A), TOC/TOC0 (B), and ln(TOC/TOC0) versus time (C) for the degradation of E133 with AgNPs at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L and [dye] = 3.9 × 10–5 mol/L.

Figure 6B shows the TOC ratio (TOC/TOC0) in E133 solutions as a function of time for two different K2S2O8 concentrations. The TOC decay follows the apparent first-order kinetics after the decolorization time (eq 19).(19)
The values of kTOC are calculated from the slope of the final parts of the ln(TOC/TOC0) versus time and are summarized in Table 3, which indicates that the mineralization reaction rate was very sensitive to the initial concentration of K2S2O8. The kTOC rate constants are much lower than those of kobs (rate of decolorization), suggesting that the initial reactive radical attack on the carbon of the E133 molecule leads to the formation of an intermediate (Scheme 3), which maintains the basic aromatic triphenylmethane structure of E133. Further reactions with SO4· and/or HO· lead to the cleavage of benzene rings into the smaller organic molecules. The constant level of TOC in Figure 6C after extended irradiation of time suggests that the E133 does not completely mineralize into CO2 and water under our reaction conditions.

Scheme 3

Scheme 3. Decolorization and Mineralization of E133 by AgNPs under UV/K2S2O8 Treatment
Table 3. Kinetic Parameters of the Decolorization and Mineralization of E133 under the UV/K2S2O8 System
parameter[K2S2O8] (1.25 mM)[K2S2O8] (3.75 mM)
kobs (s–1)22.9 × 10–4 ± 4.536.1 × 10–4 ± 4.4
R20.9940.996
kTOC1.0 × 10–4 ± 4.75.5 × 10–4 ± 4.9
R20.9300.938
Mechanism of E133 Degradation
Figure 3B shows the decolorization of E133 under treatment with the AgNPs/K2S2O8/UV system at 30 °C. The blue color and absorbance intensity of E133 became weaker along with the reaction time, the entirety of the spectra in the UV–visible decreased, and the blue color disappeared completely. The decrease in absorbance in the visible region at 628 nm may be responsible for the oxidative degradation of E133 by the generated reactive oxidants (SO4· and HO·). Inspection of Figure 3B indicates that the UV–visible spectra became featureless within ca. 60 min. In addition, no new absorption peaks occurred near the original maximum. When we see the UV–visible spectrum, an isosbestic point was observed at 542 nm, which clearly indicates the equilibrium between the parent E133 and its degraded products. Surprisingly, the peak position (wavelength maximum) remains constant at 628 nm during the dye degradation for ca. 44 min and 8 min of the reaction time, respectively, for Figure 3B, and no any shift (red and blue) was observed. On the other hand, the polyaromatic rings were also completely destroyed at the initial stage of the process.
Behnajady et al. observed a blue shift in the degradation of malachite green, which is an N-demethylation process. (31) Rayaroth et al. reported the degradation of Coomassie brilliant blue under sonochemical treatment in the presence of K2S2O8. (32) They identified the formation of 13 transformed products as an intermediate during the 10 min of the process and suggested that most of the products were formed mainly due to the attack of HO· via two different mechanisms. Gosetti et al. (62) reported the degradation of E133 by using different K2S2O8/dye molar ratios under natural sun light irradiation and suggested the formation of various intermediates under different ratios of dye/persulfate. Li and his co-workers reported the photodegradation of cationic triarylmethane dyes (crystal violet and fuchsin basic) under visible irradiation in TiO2. (6) They suggested that the dyes were photodegraded via two competitive mechanisms: N-demethylation and the destruction of the conjugated structure. E133 (triphenylmethane dye) is a highly aromatic compound and it has many attacking sites. Therefore, the addition of HO· is more favorable than the N-demethylation process during the initial stages of the degradation. (32) From Figure 3B, we conclude that the AgNP-activated photocatalytic decolorization of E133 by K2S2O8 occurs via destruction of the whole conjugated structure of the dye. (6) The disappearance of absorbance peaks at 628 and 409 nm might be due to the simultaneous decolorization of E133 and dissolution of AgNPs under UV/K2S2O8 treatment. On the basis of these results and data available in the literature, Schemes 3 and 4 are proposed for the degradation of E133 (decolorization and mineralization) and dissolution of AgNPs, respectively.

Scheme 4

Scheme 4. Dissolution of AgNPs by HO· Generated by K2S2O8
Effect of NaBH4
NaBH4 is a strong reducing agent and is used for the oxidative degradation of aromatic nitro compounds, (28) methyl orange, eosin yellow, (63) and bromothymol blue (59) in the presence of AgNPs and [email protected] as catalysts. Therefore, to gain insight into the decolorization of E133, a series of kinetic experiments were performed with NaBH4 = 0.75 mM to 5.0 mM at fixed concentrations of E133 (3.75 × 10–5 mol/L) and AgNPs (0.01 g/L) at 30 °C. The representative results are summarized in Figure 7 as the UV–visible spectra of dye discoloration and their optical images. The UV–visible spectrum of E133 shows three absorption peaks at 309, 405, and 628 nm in an aqueous solution (Figure 7, black line). The peak position and intensity remain the same after the addition of NaBH4 for a couple of days. No decolorization of dye was observed with NaBH4. Surprisingly, the dye absorbance decreased very fast after the addition of AgNPs, and the blue color of the reaction mixture became green (Figure 7, optical images). On the other hand, the appearance of a SPR peak at ca. 440 nm due to AgNPs was not noticed. The SPR band position is highly sensitive to the adsorption nucleophile onto the particle surface. AgNPs and E133 formed a complex, and the resulting complex showed two major absorption peaks at 409 and 628 nm. The SPR peak of AgNPs at 440 nm remains masked under the E133 peak. Addition of AgNPs to the reaction solution containing E133 and NaBH4 caused the fading of the blue color of E133. Figure 7 shows that the absorbance at 628 nm decreases, and the blue solution of E133 turns blue-green. The whole UV–visible spectrum changes entirely with increasing time. The successive decrease in the peak height at 628 nm is accompanied by an increase in absorbance at 400 nm with no isosbestic point, indicating the degradation of E133 in the presence of AgNPs. We did not observe the appearance of any new peaks in the UV–visible spectra or an isosbestic point, which ruled out the partial degradation of E133 (N-demethylation and deamination). On the basis of these observations, Scheme 5 is proposed for the interaction of E133 with AgNPs in the presence of NaBH4.

Figure 7

Figure 7. UV–visible spectra and optical images of decolorization of E133 by NaBH4 with AgNPs as a function of time at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [NaBH4] = 1.25 mM.

Scheme 5

Scheme 5. Interaction of E133 with AgNPs in the Presence of NaBH4
In Scheme 5, BH4 and E133 were adsorbed onto the AgNPs. The hydrogen was transferred from BH4 to E133 via the surface of AgNPs due to the electron relay effect. (25)
Figure 8 shows the effect of [NaBH4] on the decolorization of E133 in the presence of AgNPs. The decay of absorbance was increased with increasing concentration of NaBH4, and the reaction followed a pseudo-first-order kinetics. The decay was fast initially, and no decolorization was observed after ca. 30 min of the reaction time (Figure 8A). The ΔA (A0At) versus time plots indicate that 30 min is an optimum time for the decolorization of dye (Figure 8B). The 60% decolorization occurred with 3.75 mM NaBH4 (Figure 8D). For a given NaBH4 concentration, the absorption peak at 404 nm was sensitive for the reaction time, and the absorbance increased initially, then decreased when the time is beyond 30 min, and became constant for ca. 140 min (Figure 9A). The peak was also blue-shifted (from 409 to 394 nm) and red-shifted (from 394 to 404 nm) during the course of the reaction (Figure 9B), which can be explained due to the strong AgNP surface interactions with the nucleophile. (60) The reappearance of the SPR peak at ca. 404 nm with reaction time indicates the catalytic role of AgNPs in the decolorization of E133 by NaBH4. Actually, NaBH4 serves two main purposes, viz, fast reduction of E133 and slow dissolution of AgNPs simultaneously. (25,40) The rate of decolorization was increased with NaBH4 (Figure 9C), and the double log–log plot kobs and [NaBH4 or K2S2O8] were linear with slope = 0.64 and 0.79, indicating the fractional-order first-order kinetics with NaBH4 and K2S2O8, respectively.

Figure 8

Figure 8. Plots of absorbance decay at 628 nm (A), A0At (B), ln(A0/At) (C), and percentage decolorization versus time to the decolorization of E133 by NaBH4 with AgNPs at 30 °C (D). Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [NaBH4] = 1.25 mM.

Figure 9

Figure 9. Plot of absorbance decay at 404 nm versus time (A), change in wavelength with time (B), rate constants versus [K2S2O8 or NaBH4] (C), and A0At versus [K2S2O8 or NaBH4] (D) for the decolorization of E133 with AgNPs at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [NaBH4] = 1.25 mM.

To compare the reactivity of the K2S2O8/AgNPs system, the degradation % and rate constants of different dyes with H2O2 and other metal NPs are summarized in Table 4. It is well known that the experimental conditions such as the type of irradiation and metal NPs have significant impact on the degradation of dyes. (64−67) K2S2O8/AgNPs is found to be more active to the E133 degradation compared to others. AgNPs activate the peroxide bond of S2O82– and generate the oxygen reactive species (SO4· and HO·), which might be responsible for the degradation of toxic wastewater pollutants.
Table 4. Discoloration Efficiency of E133 under Various Reductants
dyereaction conditionsdegradationkobs (min–1)ref
Coomassie brilliant blueultrasonic degradation; pH = 5.790%0.133 (32)
Coomassie brilliant blueultrasonic degradation; H2O297%0.195 (32)
brilliant blue FCFNaBH4; AgNPs97%0.186 (33)
Coomassie brilliant blueNaBH4; AgNPs92%  (34)
brilliant blue FCFK2S2O8; zero-valent iron98.8%  (64)
brilliant blue FCFphotocatalytic degradation; tungsten-doped TiO293%0.075 (65)
Coomassie brilliant blueUV radiation; H2O222%0.041 (66)
brilliant blueultraviolet light-emitting diodes; H2O2100%  (67)
brilliant blue FCFUV/K2S2O8/AgNPs98%0.253present work
brilliant blue FCFNaBH4/AgNPs60%0.052present work

Detection Limit of K2S2O8 and NaBH4

To determine the detection limit for K2S2O8 and NaBH4, the calibration curves were constructed by using the optimize reaction conditions for E133 at 30 °C. A good linear correlation was obtained between ΔA (A0At) and concentrations of both K2S2O8 and NaBH4 (Figure 9D). The following relation was used for the evaluation of the detection limit (DL) (eq 20).(20)where σ and s are the standard deviation of the blank sample and the slope. These values were calculated from the slopes of Figure 9D and found to be 293.42 and 119.58 for NaBH4 and K2S2O8, respectively, and DL values were calculated as 2.42 × 10–5 and 1.65 × 10–5 mol/L with linear correlation coefficients (R2) = 0.994 and 0.998, respectively.

Concluding Remarks

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Persulfate can readily degrade and decolorize E133 food colorant dye, adding AgNPs as an activator under thermal and UV irradiation in an aqueous solution. The E133 was completely degraded in the AgNPs/persulfate system within 30 min at a temperature of 30 °C under UV light. The results showed that high AgNP dosages, low pH values (nearly 4 to 5), and low E133 concentrations were more favorable for complete decolorization and mineralization of dye. The decolorization excellently followed the Arrhenius equation with activation energies of ca. 57.4 and 30.6 kJ/mol for E133/AgNPs and E133/AgNPs/K2S2O8, respectively, with UV light. Radical scavengers, IPA, TBA, and ethanol demonstrated the responsibility of SO4· and HO· radicals in the decolorization of E133. The decolorization of the E133 is of pseudo-first order with respect to the dye concentration. The kTOC reaction rate is much slower than kobs. AgNPs/sodium borohydride is an effective system for the decolorization of E133. The decolorization was fast and incomplete with AgNPs/NaBH4. The AgNPs activated the persulfate and BH4 ions toward the degradation of dye. The detection limits, 1.65 × 10–5 K2S2O8 and 2.4 × 10–5 NaBH4, were determined by using a spectrophotometric method based on the decolorization of E133 food colorant. In the presence of UV/K2S2O8, the added AgNPs were fully decomposed along with the decolorization and mineralization of E133, leaving no environmental risk.

Experimental Section

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Chemicals

Brilliant blue (C37H34N2Na2O9S3; color index = 42,090, 99%) was purchased from Sigma-Aldrich and used as received. K2S2O8 (potassium persulfate, 99%), NaBH4 (99%), silver nitrate (AgNO3, ≥99%), cetyltrimethylammonium bromide (C19H42BrN, 99%), sodium hydroxide (NaOH, ≥98%), and hydrochloric acid (HCl, 37%) were of analytical grade. Standard HCl (0.1 mol/L) and NaOH (0.1 mol/L) were used for pH adjustment, and all experiments were conducted at 30 °C, unless specified elsewhere, by using double-distilled water as a solvent. The diluted solutions of E133 (ranging from 0.78 × 10–5 mol/L to 7.8110–5 mol/L) were prepared from an initial concentration (0.01 mol/L), and a calibration plot was constructed for E133 absorbance at 628 nm and concentrations of E133 to calculate the molar extinction coefficient as well as to verify if the commercial dye obeys Beer–Lambert law.

Preparation and Characterization of AgNPs

The chemical reduction method was used for the preparation of AgNPs with slight modification. (26,68) In the first set of experiments, AgNO3 solution (1.25 mM) was added into a reaction vessel containing cetyltrimethylammonium bromide (0.8 mM) and equilibrated at 30 °C for 20 min. The freshly prepared hydrazine solution (5.0 mM) was added dropwise into the reaction vessel (total volume, 50 mL). The colorless reaction mixture became dark yellow to orange (λmax = 440 nm), indicating the reduction of Ag+ ions into the metallic Ag0. The resulting color was stable for ca. 1 month and any type of turbidity and precipitate appeared. In the second set of experiments, hydrazine (1.0 mM), Ag+ (1.0 mM), and CTAB (0.8 mM) were used for the preparation of AgNPs. The resulting silver sols were analyzed with a Varian Carry 50-UV visible spectrophotometer. The absorbance of silver sols was recorded in the wavelength spectra ranging from 300 to 600 nm. The morphology (size, shape, and size distribution) of the AgNPs was determined using a transmission electron microscope (JEM-1400) equipped with an energy-dispersed X-ray detector, operating at a beam energy of 100 keV. For measurements, samples were prepared by adding droplets of resulting silver sols on the copper–carbon-coated grids (300 meshes) and allowing them to dry at room temperature in a dry box. An X-ray diffractometer (Rigaku, Japan) was used to determine the crystalline nature of the AgNPs. The stability and zeta potential of AgNPs were determined by using the Smoluchowski equation.

Apparatus and Measurements

Stock solutions of AgNO3 (0.01 mol/L), E133 (0.01 mol/L), NaBH4 (0.01 mol/L), and K2S2O8 (0.01 mol/L) were prepared in double-distilled water prior to each batch experiment. All of the reactions were carried out in a cylindrical Pyrex reaction vessel (250 mL capacity) with a total reaction volume of 50 mL. For the K2S2O8/AgNPs system, the reaction mixture was prepared by mixing a required amount of dye and water on a rotatory shaker at 110 rpm and heated in a water bath for 20 min at 30 °C to attain equilibrium. The decolorization reaction was initiated by mixing a required volume of K2S2O8 solution to the mixture solution, which was per-equilibrated at the same temperature followed by the addition of the requited amount of AgNPs. The degradation batch experiments were performed under a UV–visible lamp (12 W, G8 T5 Philips). Solution samples were taken from the reaction vessel at definite time intervals and filtered through a 0.45 μm pore size filter membrane (VWR, USA). Sodium thiosulfate solution was added into the filtrate immediately to quench the oxidation reaction, and the remaining E133 dye was determined by measuring the absorbance at 628 nm (wavelength maximum of dye) on a Varian Carry 50-UV visible spectrophotometer. On the basis of observed results, a linear correlation was established between the K2S2O8 concentration and the extent of E133 dye. The same experiments were performed with the required concentration of the NaBH4/AgNPs system to determine the detection limit of NaBH4. For all measurements, duplicate reactions were performed simultaneously. The total organic carbon (TOC) of E133 solution by the catalytic oxidation was determined with a Shimadzu TOC 5000 apparatus.
To determine the order with respect to the K2S2O8 concentration, a kinetic study was performed under large excess of different concentrations of K2S2O8 (0.75, 1.2, 2.5, 3.7, and 5.0 × 10–3 mol/L) with a fixed concentration of E133 (3.9 × 10–5 mol/L), pH (5.5), and temperature (30 °C). The effect of AgNP dosages on the dye decolorization was studied at 0.01, 0.02, 0.03, 0.04, and 0.05 g/L AgNPs with a fixed concentration of E133, pH, and temperature. To determine the role of temperature on E133 decolorization, the effect of temperature was studied at 30, 40, and 50 °C at other fixed parameters. The effect of E133 concentration (ranging from 3.1 × 10–5 to 5.4 × 10–5 mol/L) was also studied to determine the order with respect to the dye concentration. The pseudo-first-order rate constants were calculated by using the pseudo-first-order rate law (eq 21).(21)
The rate law for the decolorization reaction is:(22)where kobs is the rate constant determined experimentally. The n and m, respectively, are the order of the reaction with respect to E133 and K2S2O8 (or NaBH4). The absorbance is directly proportional to the concentration of dye used in the entire studies. Therefore, kobs will be equivalent to the decolorization rate constant kapp, (69) according to the Langmuir–Hinshelwood pseudo-first-order kinetic rate law. The percentages of E133 decolorization and mineralization (degradation of an organic compound to its mineral components, i.e., CO2, H2O, and other products) were calculated with the following relations (eqs 23 and 24).(23)(24)

Author Information

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  • Corresponding Author
  • Authors
    • Abeer Saad Al-Shehri - Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
    • Amell Musaid Alsudairi - Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
    • Samia A. Kosa - Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This research work was funded by Institutional Fund Projects under grant number IFPHI-064-247-2020. The authors therefore gratefully acknowledge technical and financial support from the Ministry of Education and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.

References

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  • Abstract

    Scheme 1

    Scheme 1. Structure of Brilliant Blue (Blue 1)

    Figure 1

    Figure 1. UV–visible spectra of E133 as a function of different concentrations (A), Beer–Lambert plot (B), UV–visible spectra of AgNPs (C), and its TEM image (D) at 30 °C. Reaction conditions: [AgNO3] = 1.25 mM, [hydrazine] = 5.0 mM, and [CTAB] = 0.8 mM for C.

    Figure 2

    Figure 2. UV–visible spectra and optical images (A) and reaction–time profile (B) of decolorization of E133 by AgNPs without K2S2O8 under UV irradiation as a function of time at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 0.0 mol/L.

    Scheme 2

    Scheme 2. Adsorption of E133 on the Surface of AgNPs

    Figure 3

    Figure 3. Optical images of AgNPs and E133 (A) and UV–visible spectra and optical images of decolorization of E133 by K2S2O8 with AgNPs under UV irradiation as a function of time at 30 °C (B). Inset: Decay of absorbance at 408 and 628 nm with time. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 1.25 mM.

    Figure 4

    Figure 4. Plots of absorbance decay at 628 nm (A), A0At (B), ln(A0/At) (C), and percentage decolorization versus time to the decolorization of E133 by K2S2O8 with AgNPs at 30 °C (D). Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 1.25 mM.

    Figure 5

    Figure 5. Arrhenius plot (A) and effects of scavenger on the decolorization of E133 by K2S2O8 with AgNPs at 30 °C (B). Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [K2S2O8] = 1.25 mM.

    Figure 6

    Figure 6. Plot of mineralization % (A), TOC/TOC0 (B), and ln(TOC/TOC0) versus time (C) for the degradation of E133 with AgNPs at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L and [dye] = 3.9 × 10–5 mol/L.

    Scheme 3

    Scheme 3. Decolorization and Mineralization of E133 by AgNPs under UV/K2S2O8 Treatment

    Scheme 4

    Scheme 4. Dissolution of AgNPs by HO· Generated by K2S2O8

    Figure 7

    Figure 7. UV–visible spectra and optical images of decolorization of E133 by NaBH4 with AgNPs as a function of time at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [NaBH4] = 1.25 mM.

    Scheme 5

    Scheme 5. Interaction of E133 with AgNPs in the Presence of NaBH4

    Figure 8

    Figure 8. Plots of absorbance decay at 628 nm (A), A0At (B), ln(A0/At) (C), and percentage decolorization versus time to the decolorization of E133 by NaBH4 with AgNPs at 30 °C (D). Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [NaBH4] = 1.25 mM.

    Figure 9

    Figure 9. Plot of absorbance decay at 404 nm versus time (A), change in wavelength with time (B), rate constants versus [K2S2O8 or NaBH4] (C), and A0At versus [K2S2O8 or NaBH4] (D) for the decolorization of E133 with AgNPs at 30 °C. Reaction conditions: [AgNPs] = 0.01 g/L, [dye] = 3.9 × 10–5 mol/L, and [NaBH4] = 1.25 mM.

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