Introduction

Hydrogen peroxide (H2O2) is a versatile chemical and widely exists in rain, ice, and surface water. The main industrial applications of H2O2 are bleaching of textiles and paper (Mounteer et al. 2007). It also plays an important role in Fenton and Fenton-like systems for removing organic pollutants from wastewater (Audino et al. 2018; Koltsakidou et al. 2017). However, H2O2 in water can also cause toxic to cells (Aydin et al. 2012). Consequently, there is a necessity for the rapid and accurate detection of H2O2 in water.

Until now, there are lots of methods available for the analysis of H2O2 concentration in water, including titration (Kieber and Helz 1986; Sully and Williams 1962), electrochemistry (Evans et al. 2002; Jia et al. 2009; Li et al. 2007; Razmi et al. 2010), fluorescence (Li and Townshend 1998; Sakuragawa et al. 1998), chemiluminescence (Hu et al. 2007; Tahirović et al. 2007), and spectrophotometry (Amelin et al. 2000; Hoshino et al. 2014; Sellers 1980; Zhang et al. 2000). As a classical titrimetric method, iodometric titration is usually used to measure H2O2 concentration (De Laat and Gallard 1999). Nevertheless, the iodometric method is quite cumbersome and time-consuming due to the titration steps. What’s more, due to its high detection limit (DL = 0.02 mM) (Steger and Mühlebach 1997), the iodometric method is unsuitable for the accurate measurement of low concentration of H2O2. Although electrochemistry, fluorescence, and chemiluminescence are very sensitive, the expensive apparatuses are required to measure the H2O2 concentration (Jia et al. 2009; Sakuragawa et al. 1998; Tahirović et al. 2007), which is not appropriate for routine analysis. Thus, spectrophotometry is considered to be a promising method for H2O2 determination due to its easy operation, fast analysis, and low cost.

Earlier, Bader et al. founded a spectrophotometry in which the colorless N, N’-diethyl-p-phenylenediamine (DPD) was oxidized by a peroxidase (POD)-catalyzed reaction and generated red-colored DPD•+ (Bader et al. 1988). The generated DPD•+ had a strong absorbance at 551 nm. However, as shown in Fig. 1a, when the concentration of H2O2 was in the range of 0–100 μM, the absorbance of the generated DPD•+ solution increased with the increase of H2O2 concentration, while then decreased from 100 μM, leading to one absorbance determined at 551 nm matches with two diverse H2O2 concentrations. Therefore, in order to accurately measure the H2O2 concentration by the POD-DPD method, it is necessary to dilute the water sample containing the high H2O2 concentration (Zou et al. 2019a). In addition, when there are some dyes (e.g., acid orange 7, methyl violet and rhodamine B), the absorbance measured by the POD-DPD method will increase significantly because of their strong absorption around 551 nm (Ding et al. 2011).

Fig. 1
figure 1

Three different spectrophotometric methods for the determination of H2O2 concentration (0–200 μM). a The POD-DPD method. Reaction conditions: [POD]0 = 0.01 mg L−1, [DPD]0 = 0.2 mM, pH = 6.0 (50 mM phosphate buffer), reaction time (t) = 30 s, and T = 24 ± 2 °C. b The POD-ABTS method. Reaction conditions: [POD]0 = 0.01 mg L−1, [ABTS]0 = 0.1 mM, pH = 6.0 (50 mM phosphate buffer), t = 30 s, and T = 24 ± 2 °C. c The Fenton-ABTS method. Reaction conditions: [ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C. Error bars represent the standard deviations of duplicate measurements

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On this basis, Cai et al. reported a multi-wavelength spectrophotometric method based on a POD-catalyzed reaction where the green-colored ABTS•+ was generated from the colorless 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) (Cai et al. 2018). The generated ABTS•+ had four characteristic peaks (i.e., 415 nm, 650 nm, 732 nm, and 820 nm), which could be measured by spectrophotometers. Thus, the POD-ABTS method could avoid the interference of colored coexisting substances, such as dyes. However, when the concentration of H2O2 was varying from 0 to 60 μM, the absorbance of the generated ABTS•+ solution increased with the increase of H2O2 concentration, while then slowly decreased from 60 μM, leading to one absorbance determined at 415 nm matches with two diverse H2O2 concentrations, as shown in Fig. 1b. In addition, the enzyme catalyst is too expensive for the routine assay. Besides, Luo et al. established a spectrophotometry to determine the H2O2 concentration, which was measured at 464 nm using hydroxyl radical (•OH) generated by Fenton reaction to decolor the methyl orange (MO) (Luo et al. 2008). Although the Fenton-MO method is easy to operate and inexpensive, the persistent dye pollutant is needed, resulting in the hazardous wastewater is produced after the determination of H2O2.

To our knowledge, there is not a rapid, sensitive, and low-cost spectrophotometry to detect the H2O2 concentration in water using non-toxic analytical reagents. Hence, developing a new method for detecting the H2O2 concentration in water is considerable important. Consequently, ABTS, a widely used and environmentally friendly indicator (Cai et al. 2018; Fan et al. 2017; Pinkernell et al. 1997; Pinkernell et al. 2000; Wang and Reckhow 2016; Lee et al. 2005; Zou et al. 2019b), was selected as the probe molecule for the generated •OH from Fe2+-activated H2O2 (Fenton reaction) then for the purpose of determination of H2O2 concentration. The purposes of this research were establishing a new multi-wavelength spectrophotometry to determine the concentration of H2O2 based on the Fenton reaction, optimizing operation parameters (i.e., reaction time, initial solution pH, and the initial concentrations of ABTS and Fe2+), determining the correction curves, investigating the stability of the generated ABTS•+, applying the proposed Fenton-ABTS method into the alkali-activated H2O2 system monitoring the variation of H2O2 concentration during the decolorization of RhB.

Materials and methods

Reagents and solutions

Hydrogen peroxide (H2O2, purity 30%), ferrous sulfate heptahydrate (FeSO4·7H2O, AR), rhodamine B (RhB, AR), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O, AR), disodium hydrogen phosphate (Na2HPO4, AR), sodium chloride (NaOH, AR), sodium sulfate (Na2SO4, AR), sodium bicarbonate (NaHCO3, AR), potassium nitrate (KNO3, AR), sodium hydroxide (NaOH, AR), and perchloric acid (HClO4 , AR) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). DPD (purity 98%), humic acid (purity 90%), ABTS (purity 98%), and POD (specific activity of 200 units mg−1) were purchased from Aladdin Bio-Chem Technology Co., Ltd (Shanghai, China).

The ABTS solution (10 mM) was freshly prepared by dissolving 0.1400 g of ABTS into 25 mL of ultrapure water. H2O2 concentration in 30% H2O2 solution was firstly titrated as 9.76 M with standard KMnO4 solution. H2O2 working solution (0.5 mM) was prepared from 30% H2O2 solution and stored under dark condition. POD solution (0.5 g L−1) and DPD solution (5 mM) were stored under dark condition and changed once a week. All the above solutions needed to be stored at 4 °C. FeSO4 solution (10 mM) was freshly prepared by dissolving 0.0695 g FeSO4·7H2O into 25 mL of ultrapure water. RhB solution (5 mM) was prepared by dissolving 0.1198 g of RhB into 50 mL of ultrapure water. 50 mM of phosphate buffers with pH 6.0 was prepared with NaH2PO4 solution (50 mM) and Na2HPO4 solution (50 mM). Throughout the experiments, the stock solutions of HClO4 and NaOH were employed for pH adjustment.

Experimental apparatus

The absorbance value in this study was recorded by a spectrophotometer (Persee TU-1901, China). The pH measurements and electrical conductivity were carried out with a PB-10 pH-meter (Sartorius, Germany). Dissolved organic carbon (DOC) and inorganic carbon (IC) were measured with a total organic carbon analyzer (TOC-V, Shimadzu, Japan). Concentrations of Cl−, NO3−, and SO42− were determined with an ion chromatography (930 Compact IC, China). Ultrapure water used in this study (18.2 MΩ cm) was produced with a laboratory ultrapure water system (Shanghai Hetai Instruments Co. Ltd, China).

Natural waters

Two different natural waters were adopted to evaluate the proposed Fenton-ABTS method: (1) Underground water sample collected from an industrial drinking water treatment plant in Longyan City (pH = 7.62, Electrical conductivity = 31.60 mV, DOC = 3.67 mg L−1, IC = 47.19 mg L−1, Total hardness = 230.58 mg L−1 as CaCO3, Cl− concentration = 35.59 mg/L, NO3− concentration = 25.40 mg/L, and SO42− concentration = 75.34 mg/L,); (2) Reservoir water sample collected from Lianban reservoir in Xiamen City (pH = 7.83, Electrical conductivity = 42.20 mV, DOC = 5.36 mg L−1, IC = 6.65 mg L−1, Total hardness = 49.85 mg L−1 as CaCO3, Cl− concentration = 21.39 mg/L, NO3− concentration = 7.51 mg/L, and SO42− concentration = 12.81 mg/L,). The natural water samples were filtered through 0.45 μm cellulose acetate membranes before the experiments and then stored at 4 °C.

Experimental procedures

General steps for determining H2O2 concentration in water samples with the Fenton-ABTS method were described as Scheme 1. The steps for determination of H2O2 concentration by the POD-DPD method and the POD-ABTS method were performed as reported earlier (Bader et al. 1988; Cai et al. 2018) (Scheme 1).

Scheme 1
scheme 1

Flowchart showing the analysis steps of the proposed Fenton-DPD method, the POD-DPD method, and the POD-ABTS method

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The steps for measuring the change of RhB concentration in alkali-activated hydrogen peroxide system were as follows: at the predetermined interval time, 1.2 mL of reaction solution was transferred into 1 cm quartz cell which had contained 1.3 mL of HClO4 stock solution (1.0 M) to suspend the reaction. Then, the change in absorbance from 200 to 800 nm was obtained by the UV-Vis spectrophotometer.

Thus, the H2O2 concentration in water was calculated from the measured absorbance change of ABTS•+ at 415 nm, 650 nm, 732 nm, or 820 nm by following relationship:

$$ {\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]}_{\mathrm{sample}}=\frac{\gamma \Delta {A}_l{V}_{\mathrm{final}}}{\varepsilon l{V}_{\mathrm{sample}}} $$

where

∆Al = absorbance at four characteristic wavelengths

l = path length of quartz cell

γ = stoichiometric factor of ABTS•+ generation (0.80, refer to the “Effect of reaction time” section for further details)

ɛ = molar absorptivity of ABTS•+

Vfinal = final volume of reaction solutions

Vsample = volume of original H2O2 samples

The molar absorptivity of ABTS•+ at four characteristic wavelengths were determined at pH 2.60 by using NaClO and ABTS to produce ABTS•+ in the presence of iodide (6 μM) as a catalyst. According to previous reports, 1 mol of NaClO and 2 mol of ABTS could generate 2 mol of ABTS•+ (Pinkernell et al. 2000; Lee et al. 2005). With this method, the molar absorptivity of ABTS•+ at these four characteristic wavelengths were respectively obtained to be 3.37 × 104 M−1 cm−1, 1.38 × 104 M−1 cm−1, 1.74 × 104 M−1 cm−1, and 1.55 × 104 M−1 cm−1. The calculated molar absorptivity of ABTS•+ at 415 nm was consistent with other reports (Cai et al. 2018; Fan et al. 2017; Tao and Reckhow 2016), while the other three molar absorptivity of ABTS•+ were higher than that at pH 6.0 reported by Cai et al., where the molar absorptivity of ABTS•+ were respectively reported to be 0.98 × 104 M−1 cm−1, 1.33 × 104 M−1 cm−1, and 1.04 × 104 M−1 cm−1 at 650 nm, 732 nm, and 820 nm (Cai et al. 2018). The cause of this phenomenon might be the difference in solution pH.

Results and discussion

Absorption spectra of ABTS•+

Figure 2 shows the absorption spectra of the generated ABTS•+ in Fenton-ABTS system. As could be seen, there were four easily distinguished peaks (i.e., 415 nm, 650 nm, 732 nm, and 820 nm) in the absorption spectra of ABTS•+, which was identical to other literatures (Fan et al. 2017; Ma et al. 2009; Pinkernell et al. 1997). As shown in Fig. 2, the measured absorbance increased with the increase of H2O2 concentration, while the H2O2 concentration showed no effect on the shape of the absorption curve and positions of these four characteristic peaks. The correlations between the H2O2 concentration and the absorbance of ABTS•+ at four characteristic wavelengths provide the possibility for developing a multi-wavelength spectrophotometric method for H2O2 determination.

Fig. 2
figure 2

Absorption spectrum of the generated ABTS•+ in Fenton-ABTS system. Reaction conditions: [H2O2]0 = 0–40 μM, [ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C

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Effects of operation parameters on the determination of H2O2

To develop the multi-wavelength spectrophotometric method for detecting the concentration of H2O2 in water based on the Fenton oxidation of ABTS, investigating the effects of operation parameters (i.e., reaction time, initial solution pH, and the initial concentrations of ABTS and Fe2+) is of great significance.

Effect of reaction time

Figure 3a shows the influence of reaction time in Fenton-ABTS system, which is represented by absorbance change ∆A (∆A = A0 − At, where A0 and At are the absorbance of the ABTS solution at 415 nm before and after the reaction). ∆A increased quickly with the increase of reaction time initially, and then remained almost unchanged beyond 1 min for each given concentration of H2O2 in the range of 0–40 μM. The occurrence of the plateau could be attributed to the complete decomposition of H2O2 by the excess of Fe2+ in Fenton-ABTS system. Therefore, in our further experiments, reaction time of t = 1 min was chosen as the optimum reaction time for the measurement of H2O2.

Fig. 3
figure 3

Effects of reaction time (a), initial pH (b), initial ABTS concentration (c), and initial Fe2+ concentration (d) on the coloration extent of ABTS at 415 nm in Fenton-ABTS system. Reaction conditions: [ABTS]0 = 2.0 mM for (a), (b), and (d); [Fe2+ ]0 = 1.0 mM for (a), (b), and (c); pH0 = 2.60 ± 0.02 for (a), (c), and (d); t = 1 min for (b), (c) and (d); and T = 24 ± 2 °C. Error bars represent the standard deviations of duplicate measurements

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Effect of initial solution pH

In Fenton-ABTS system, solution pH is a crucial element. According to previous reports, the optimal pH for Fenton oxidation is near pH 3.0 (Pignatello et al. 2006). The precipitation of Fe3+ as ferric hydroxide is the major cause for the lower reactivity at higher pH (pH > 4.0), which inhibits the recycling of Fe2+ and Fe3+ (Georgi et al. 2007). For the sake of optimizing solution pH in Fenton-ABTS system, the effect of solution pH ranging from 1.5 to 3.5 on the coloration extent of ABTS was investigated. As Fig. 3b shows, at pH 1.5–2.0, ∆A increases with increasing pH, stabilizes at pH 2.0–3.0, and then gradually decreases beyond pH 3.0. Thus, pH 2.6 was chosen as the optimum initial solution pH to measure the H2O2 concentration in the present work.

Effect of initial ABTS concentration

Figure 3c illustrates the effect of initial ABTS concentration on the coloration extent of ABTS. Initially, ∆A increased with the increasing initial ABTS concentration, and then kept nearly unchanged beyond 2.0 mM ABTS for a given H2O2 concentration. Labrinea et al. have reported that ABTS•+ is more stable in the presence of excess ABTS, because excess ABTS will inhibit the disproportionation of ABTS•+ to produce 1 ABTS and azo salt under acidic pH conditions (Childs and Bardsley 1975; Labrinea and Georgiou 2004). Since our experiment was carried out under the acidic condition of pH 2.60, a slight excess ABTS was enough to achieve the purpose of stabilizing ABTS•+. Consequently, 2.0 mM ABTS was selected for further experiments.

Effect of initial Fe2+ concentration

In Fenton systems, in order to avoid the formation of a large amount of iron sludge, it is common to use Fe2+ in lower concentrations (Gomez-Herrero et al. 2018). Besides, as the concentration of Fe2+ increases, the scavenging effect of Fe2+ on •OH increases rapidly. So, it is of great significance to optimize the initial Fe2+ concentration in Fenton-ABTS system. The influence of the concentration of Fe2+ on the H2O2 measurement is shown in Fig. 3d. For each of the given H2O2 concentration, ∆A increased initially along with Fe2+ concentration in the range of 0.2–1.5 mM, then gradually decreased as Fe2+ concentration was beyond 1 mM. Thus, the initial Fe2+ concentration was chosen as 1 mM for further experiments.

Correction curves for H2O2 determination

As shown in Fig. 4, under the optimized conditions (CABTS = 2.0 mM, CFe2+ = 1.0 mM, pH = 2.60 ± 0.02, and t = 1 min), the correction curves measured for H2O2 concentration ranging between 0 and 40 μM at four characteristic wavelengths were well linear (R2 > 0.999) and their slopes (k) at these four characteristic wavelengths were respectively calculated as high as 4.19 × 104 M−1 cm−1,1.73 × 104 M−1 cm−1, 2.18 × 104 M−1 cm−1, and 1.96 × 104 M−1 cm−1. Therefore, it provides the choice of analytical wavelength. Meanwhile, the detection limits (DL = 3σ/k, where σ represents the standard deviation of the blank samples (Luo et al. 2008)) of the proposed Fenton-ABTS method were respectively calculated to be 0.18 μM, 0.12 μM, 0.10 μM, and 0.11 μM at 415 nm, 650 nm, 732 nm, and 820 nm, which suggests that the proposed method has high sensitivity. The stoichiometric factor of the generated ABTS•+ (γ = ∆[H2O2]/∆[ABTS•+]) was calculated by dividing the molar absorptivity of ABTS•+ (ɛ = ∆A/∆[ABTS•+], which has been calculated in the “Experimental procedures” section) by the slope of the calibration curve (k = ∆A/∆[H2O2]). Hence, the stoichiometric factors of ABTS•+ at four characteristic wavelengths were all calculated to be 0.80 ± 0.01. Interestingly, the calculated stoichiometric factor of the generated ABTS•+ in Fenton-ABTS system was lower than 1. The phenomenon might be rationally interpreted by the fact that Fe3+ produced in Fenton-ABTS system would further react with the ABTS reagent to generate ABTS•+.

Fig. 4
figure 4

Correction curves of the Fenton-ABTS method for the measurement of H2O2 concentration in ultrapure water. Reaction conditions: [ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C. Error bars represent the standard deviations of six measurements

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Furthermore, the correction curves of four characteristic wavelengths for analyzing H2O2 in two types of practical samples (underground water and reservoir water) by the proposed Fenton-ABTS method were established. There were also well relationships between the H2O2 concentration and the coloration extent of ABTS, and the corresponding slopes of the calibration curves in underground water and reservoir water were nearly the same as that obtained in ultrapure water, as shown in Fig. 5. These results indicate that the proposed Fenton-ABTS method will not be greatly affected by the coexisting substances in natural waters. Furthermore, the experiments of recovery rate in ultrapure water, underground water, and reservoir water by the proposed Fenton-ABTS method were carried out. It was found that the recovery rates of H2O2 concentration spiked in these three water samples were all within (100 ± 5.00) % (Table 1).

Fig. 5
figure 5

Correction curves of the Fenton-ABTS method for the measurement of H2O2 concentration in underground water and reservoir water. Reaction conditions: [ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C. Error bars represent the standard deviations of six measurements

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Table 1 Recovery rate of H2O2 concentration with the proposed Fenton-ABTS method (n = 7)
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The influences of common coexisting foreign species on the determination of H2O2 by the proposed Fenton-ABTS method under the optimized reaction conditions ([ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C) were studied, which was shown in Fig. 6. The relative error for the determination of 25 μM H2O2 was no higher than 5% with the existence of 10 mM NaHCO3, 20 mM NaCl, 1 mM Na2SO4, 2 mM KNO3, or 5 mg L−1 humic acid, which indicates that the proposed Fenton-ABTS method well effective to tolerate the interferences of common coexisting foreign species in aqueous solutions.

Fig. 6
figure 6

The influences of common coexisting foreign species on the determination of H2O2 by the proposed Fenton-ABTS method. Reaction conditions: [ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, [H2O2]0 = 25 μM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C. Error bars represent the standard deviations of three measurements

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Interestingly, when detecting the H2O2 concentration in the range of 0–200 μM with the proposed Fenton-ABTS method, the measured absorbance increased continuously with H2O2 concentration (Fig. 1c), implying that one absorbance obtained at 415 nm matches with one specific H2O2 concentration. Nevertheless, as shown in Fig. 1a, b, the absorbance of the generated DPD•+/ABTS•+ solution initially increased with H2O2 concentration and then decreased, when the POD-DPD method and the POD-ABTS method were employed for detecting the H2O2 concentration in the range of 0–200 μM. In this respect, our proposed Fenton-ABTS method is superior to both of the POD-DPD method and the POD-ABTS method previously reported.

Stability of the generated ABTS•+

In order to assess the stability of ABTS•+ generated in Fenton-ABTS system, the absorbance change of the ABTS•+ in ultrapure water, underground water, and reservoir water was measured at 415 nm, as shown in Fig. 7. The absorbance of ABTS•+ in these three water samples was quite stable and decreased 0.83%, 1.03%, and 0.00% within 0.5 h, respectively. Therefore, it had enough time to accurately measure the concentration of H2O2 in natural waters and ultrapure water using the proposed Fenton-ABTS method. However, it should be noted that the proposed Fenton-ABTS method might be unsuitable for detecting the H2O2 concentration in aqueous samples which contain strong reducing substances, because of the antioxidant activity of the generated ABTS•+ (Lee et al. 2014; Re et al. 1999; Song et al. 2015; Gu et al. 2019).

Fig. 7
figure 7

Stability of ABTS•+ generated by Fenton oxidation of ABTS in ultrapure water and natural waters. Reaction conditions: [ABTS]0 = 2.0 mM, [Fe2+]0 = 1.0 mM, pH = 2.60 ± 0.02, t = 1 min, and T = 24 ± 2 °C

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Variation of H2O2 concentration in alkali-activated H2O2 system

The alkali-activated H2O2 system had been widely employed for the treatment of dyeing wastewater (Li et al. 2018; Long et al. 2012; Wang et al. 2018). The inset in Fig. 8 showed the decolorization of RhB by alkali-activated H2O2. It was found that the strong absorption of RhB near 551 nm was gradually decreased within 120 min. Meanwhile, the proposed Fenton-ABTS method and the previously reported POD-ABTS method were used to monitor the variation of H2O2 concentration during the decolorization of RhB in alkali-activated H2O2 system, as shown in Fig. 8. As could be seen, H2O2 was gradually decomposed over time, and the variation of H2O2 concentration monitored with the proposed Fenton-ABTS method was well consistent with that measured by the previously reported POD-ABTS method. These results further suggested that the Fenton-ABTS method proposed in this study was highly accurate for the determination of H2O2 concentration. Meanwhile, the proposed Fenton-ABTS method might be superior to the previously reported POD-ABTS method for the routine analysis because Fe2+ was much cheaper than POD. Additionally, it should be noted that the traditional POD-DPD method should be unsuitable to measure the H2O2 concentration when water samples have strong absorption near 551 nm.

Fig. 8
figure 8

Variations of H2O2 concentration during the oxidative decolorization of RhB in alkali-activated H2O2 system. Reaction conditions: [H2O2]0 = 36 mM, [RhB]0 = 0.2 mM, and pH 0 = 11.50. Error bars represent the standard deviations of duplicate measurements. The inset shows the decolorization of RhB over time

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Conclusions

A new multi-wavelength spectrophotometric method was presented. This method depended on the oxidative coloration of ABTS via Fenton reaction. The major features of the proposed Fenton-ABTS method were as follows:

  1. a.

    Without tedious titrimetric procedures and expensive materials, the proposed Fenton-ABTS method was fast to detect the H2O2 concentration within 1 min. This method was quite sensitive (4.19 × 104 M−1 cm−1 at 415 nm) and the detection limit was as low as 0.18 μM.

  2. b.

    The product ABTS•+ was very stable which permits enough time to accurately measure the H2O2 concentration in water.

  3. c.

    The Fenton-ABTS method well monitored the variation of H2O2 concentration during the oxidative decolorization of RhB by alkali-activated H2O2.