AbstractTo enable the evaluation of water treatment efficiency of electrochemical advanced oxidation processes (EAOPs), an approach to remove H2O2 as a catalytic pretreatment was investigated to avoid interference in chemical oxygen demand (COD) measurements. Four wastewater types with COD and H2O2 concentrations up to 1,300  mg/L and 90  mmol/L, respectively, were investigated with a novel method. The method requires the addition of sodium bicarbonate (NaHCO3), which decomposes H2O2 at ambient temperature within 24 h without changing COD. For synthetic wastewater (SWW), this time is reduced to 1 h by heating at 70°C. A side-by-side comparison of NaHCO3 and Na2CO3 experiments confirmed H2O2 removal without changes in COD via NaHCO3, whereas a ∼20% decrease in original COD was observed using Na2CO3. The change in COD during catalytical H2O2 decomposition in Na2CO3 solution was highly correlated with the high pH value.IntroductionChemical oxygen demand (COD) remains one of the most important parameters in determining the degradation of organic matter (OM) during wastewater treatment and, accordingly, in evaluating treatment efficiency required by water regulations. The optical colorimetric method, via spectrophotometry (Barid et al. 2017), is used for COD analysis because it is rapid and simple and can be automated (Ying et al. 2006). In this method, potassium dichromate acts as an oxidizing agent in COD test cuvettes whereby organic substances of wastewater samples are mineralized to CO2 and H2O. Simultaneously, yellow dichromate (Cr2O72−) is reduced to the green chromic ion (Cr3+), changing the absorbance over a given wavelength. The measured absorbance is directly proportional to the amount of oxidized matter determined as COD (Weiner and Matthews 2003). The use of hydrogen peroxide (H2O2) for water and wastewater (WW) treatment and industrial water reuse by advanced oxidation processes (AOPs) to oxidize (recalcitrant) organic compounds (Kolyagin and Kornienko 2015; Magureanu et al. 2018; Sievers 2011; Von Sonntag 2008) often leads to residual H2O2 in treated water. This also affects EAOPs (Muddemann et al. 2019) due to the in situ generation and use of H2O2. Materials optimized for H2O2-generating gas diffusion electrodes may lead to H2O2 concentrations of up to 175  mmol/L at a low pH (Muddemann et al. 2020). These H2O2 residuals interfere in COD cuvette tests, causing overestimation of COD (Groele and Foster 2019) because H2O2 additionally reduces potassium dichromate according to the oxidation reaction shown in Eq. (1). Depending on the extent of interference, this can lead to misinterpretation of COD results (Zak 2008) (1) K2Cr2O7+3H2O2+4H2SO4→K2SO4+Cr2(SO4)3+7H2O+3O2Kang et al. (1999) theoretically derived Eq. (2) based on Eq. (1). The equation was successfully applied to determine COD interference for synthetic wastewater containing potassium hydrogen phthalate as a COD source (2) CODH2O2(mgL)=0.4591·[H2O2]−3.24·10−5[H2O2]2where CODH2O2 = calculated COD value of H2O2 (mg/L); and H2O2 = hydrogen peroxide concentration in wastewater (mg/L). The H2O2 concentration must be known or correctly measured. Differences in interferences, based on variable COD and H2O2 concentrations (Wu and Englehardt 2012) must be verified; alternatively, H2O2 can be removed from water samples before measuring COD. It is important to ensure, that the COD in water samples is kept constant during H2O2 removal because H2O2 may act as an oxidative species.Different methods have been proposed to remove H2O2 residuals in the last few decades. However, catalase enzymes (Liu et al. 2003), pH and temperature increases (Wu and Englehardt 2012), and chemical mineralization (USP Technologies 2020) are unaffordable and complicated procedures.The addition of sodium carbonate (Na2CO3) as a concentrated solution and subsequent heating at 90°C for one to several hours was proposed by Wu and Englehardt (2012). This method was applied and verified for municipal secondary effluents with COD values of ∼35  mg/L and H2O2 concentrations of 5–50  mmol/L.Yang et al. (2019) investigated the impact of bicarbonates on the transformation of organic contaminants using H2O2. Peroxymonocarbonate (HCO4−) was identified as an oxidative species by Carbon-13 nuclear magnetic resonance (C13-NMR) analysis; the proposed decomposition reaction of H2O2 by bicarbonate is shown in Eq. (3) (3) HCO3−+H2O2↔HCO4−+H2OC13-NMR analysis of the chemical oxidation pathway indicated that the species HCO4– potentially contributes to selective reactions with electron-rich compounds. A reaction scheme for HCO4− under varying concentrations of reagents was proposed and is shown in Fig. 1.Following the reaction scheme shown in Fig. 1, we propose to promote the competing reaction pathways (B and C) by adding high amounts of HCO3− for proper removal of H2O2 by the reaction given in Eq. (3) (Truzzi et al. 2019). A higher H2O2 concentration promotes HCO4− decomposition through the reaction pathway (C) along with the self-decomposition pathway (B), which both generate oxygen gas as a final product (Yang et al. 2019).The approach just described is different from the method of Wu and Englehardt (2012) because pH may play an important role in H2O2 decomposition and COD oxidation, whereby the use of sodium bicarbonate instead of sodium carbonate changes the pH of the water sample, as CO32− (pKa=10.32) is a stronger conjugate base than HCO3− (pKa=6.37) (UCSB 2021).In alkaline solutions, H2O2 is dissociated to its conjugate base HO2−, according to Eq. (4) (Hvitved-Jacobsen et al. 2013). HO2− is a strong nucleophile and may contribute to the oxidation of COD (4) According to the Henderson-Hasselbach equation [Eq. (5)] (Satyanarayana and Chakrpani 2017), the dissociation constant pKa(H2O2/HOO−)=11.62 and pH-dependent HO2– formation through H2O2 dissociation is lower for the bicarbonate approach than for the carbonate approach (5) pH=pKa+log[HO2−]/[H2O2]Therefore, using bicarbonate instead of carbonate for H2O2 decomposition could be of interest. The reduction of the oxidation pathway (A) by bicarbonate is important; bicarbonate has also been identified as a catalyst for oxidation and bleaching via H2O2 activation.Jawad et al. (2016) determined that bicarbonate acts as a catalyst to generate reactive species for COD oxidation, which are effectively inhibited at pH ≥ 9. This is due to the deprotonation of highly reactive HCO4– to form CO42– (Rothbart 2012). Zhao et al. (2018) reported that HCO3– under alkaline conditions was deleterious for COD oxidation.A strong overstoichiometric use of bicarbonate causes a strong shift to bicarbonate equilibrium at pH ∼9. This facilitates both a minimum of CO2 concentration to decrease COD oxidation and a maximum of HCO3– concentration to promote the decomposition pathways (B and C; Fig. 1). Conversely, higher pH (∼11) from the addition of overstoichiometric carbonate (Wu and Englehardt 2012) may support an increased dissociation of the strong nucleophile oxidative species HOO–. To compare these effects, the overstoichiometric use of bicarbonate and carbonate to remove analytical interferences was investigated in this study.The validity of Eq. (2) was evaluated by different wastewater samples because the difference between the measured COD subject to interference by H2O2 and the correct COD may be useful for rapid approximation of H2O2 concentration by COD measurement. The bicarbonate approach was optimized to the highest H2O2 concentration detected in the EAOP reactor effluent (Haupt et al. 2019) in terms of NaHCO3 catalyst concentration and temperature.Chemicals and MethodsThe applicability of H2O2 decomposition was tested using four wastewater types. All types were filtered (0.45-μm glass fiber prefilters; Macherey-Nagel, Germany) and characterized as shown in Table 1. Synthetic wastewater (SWW) was diluted from its original solution (COD: 4,450  mg/L). This was done in cooperation with a vacuum toilet manufacturer to simulate vacuum toilet wastewater (Haupt et al. 2019). Landfill leachate and municipal primary effluent (MPE) from a German landfill site and wastewater plant (WWP), respectively, were also investigated. The final wastewater type was pretreated car industry discharge (CID) industrial wastewater collected from the production site of a German car manufacturer.Table 1. Analyses of tested wastewater samplesTable 1. Analyses of tested wastewater samplesWastewaterCODorg (mg/L)pHConductivity (ms/cm)H2O2 (mg/L)SWW1,194±77.08±0.062.50Leachate1,328±28.18±0.24130MPE180±108.18±0.211.260CID45±27.35±0.280.680The chemicals used in this study were NaHCO3 (Th. Geyer GmbH & Co. KG, Germany) and Na2CO3 (AppliChem GmbH, Germany), H2O2 solution (30% weight by weight ratio; Th. Geyer GmbH & Co. KG, Germany), NaOH (Merck, Germany), and deionized water. H2O2 was added to each wastewater sample as a 30%-by-weight solution to a concentration of 44.5  mmol/L per sample.The experimental scheme is summarized in Fig. 2. The interfered-with COD was directly measured after H2O2 addition (Route 2). The difference in COD obtained by Routes 1 (Table 1) and 2 quantified H2O2 interference. Using Route 2, possible COD removal by H2O2 was also evaluated. Further reference tests were carried out with the single addition of carbonate or bicarbonate (Route 3). Routes 4 and 5 determined H2O2 decomposition with the addition of carbonate and bicarbonate catalysts, respectively. Additionally, a reference H2O2 decomposition test was carried out in a deionized water sample and an aqueous solution of 1 M NaHCO3 (44.5  mmol/LH2O2).The treatment temperatures for H2O2 decomposition were 20°C, 70°C, 90°C, and 148°C. H2O2 was measured at different reaction times but always at the beginning and end of the reaction period. The appropriate dose of NaHCO3 for complete decomposition of H2O2 was determined by the NaHCO3 concentrations 0.5, 1, and 2  mol/L. The concentration calculated from the added H2O2 amount was compared with the measured H2O2 concentration to verify the measurement method.COD was measured by cuvette tests (Macherey-Nagel, Germany) and spectrophotometry (NANOCOLOR UV/VIS; Macherey-Nagel, Germany) according to DIN 38 409­ H41­1 (DIN 2005). Iodometric titration was conducted to quantify the H2O2 concentration (Hilt and Rinze 2015) after rapid predetermination by an H2O2 strip test (Macherey-Nagel, Germany). Nitro blue tetrazolium (NBT; Merck, Germany) was used as a selective substance to qualitatively investigate superoxide (O2–) during H2O2 decomposition. Two samples (SWW +H2O2+NaHCO3 and SWW +H2O2+Na2CO3) were shaken with NBT for 24 h at ambient temperature (∼20°C).A shaking device (Edmund Bühler GmbH, Germany) was used for samples pretreated with NaHCO3 for 24 h at ambient temperature, whereas dissolved oxygen was measured by the Portavo 907 Multi device (Knick GmbH, Germany). Other reaction temperatures were adjusted by a Thermoblock Nanocolor Vario 3 (Macherey-Nagel, Germany). A pH and conductivity meter (Windaus-Labortechnik GmbH & Co. KG, Germany) was also used in the analysis.Results and DiscussionValidation of Calculation Method and Carbonate Approach with Real COD ValuesThe difference between measured COD (CODmeas after H2O2 addition) and original COD (CODorg before H2O2 addition) is the excess in COD caused by H2O2, as a ratio in mgCOD/mmolH2O2. Table S1 lists the ratios determined in this work and in the literature for different wastewaters, whereby interference ratios are in the range 10.2−19.72  mgCOD/mmolH2O2. These results demonstrate the need for a uniform experimental approach to remove H2O2 from wastewater samples without changing COD.To overcome the difficulties in determining COD values in effluents containing H2O2, different methods were investigated. Eq. (2) was first used to calculate a CODH2O2 of 620  mg/L for all tested wastewaters based on the H2O2 added to each sample (44.5  mmolH2O2/L). CODmeas, measured in the presence of H2O2, was corrected by CODH2O2 to CODcal and compared with CODorg of the original wastewater (Table 1). Results of the comparison are summarized in Table 2. The difference (ΔCOD) between CODcal and CODorg indicated the predictive quality of the COD calculation based on measured or known H2O2 concentrations for different wastewater types.Table 2. CODmeas, CODcal, and CODorg of wastewater samplesTable 2. CODmeas, CODcal, and CODorg of wastewater samplesWastewaterCODmeas (mg/L)CODcal (mg/L)CODorg (mg/L)ΔCOD (mg/L)Relative error (%)SWW1,942±91,3221,194±71289.7Leachate1,822±111,2021,328±2−126−10.5MPE912±4292180±1011238.3CID812±719245±214776.6ΔCOD ranged from −126 to +147  mg/L, where negative ΔCOD indicated that predicted COD was lower than original (measured) COD. The relative error was between −10.5% (for leachate) and 76.6% (for CID) and increased with decreasing COD values. Eq. (2) may therefore be useful to estimate possible interference with an error of ±130–150  mg/L in COD at an H2O2 concentration of 44.5  mmol/L. However, this error is often not acceptable for evaluations of COD removal efficiency. These results and the interferences shown in Table S1 confirm that the interference ratio is influenced by general wastewater characteristics and not only by H2O2 concentration [as stated by Kang et al. (1999)] in Eq. (2).The carbonate approach was then tested for SWW containing 90  mmolH2O2 under the conditions proposed by Wu and Englehardt (2012) [90°C and high Na2CO3 (38  mmolNa2CO3/mmolH2O2)], which resulted in a pH of 11. Samples were taken during the 60-min reaction of SWW at 90°C to determine COD and H2O2 concentrations at different reaction times. The standardized values (CODt/CODorg) are shown in Fig. S1. During the first 5 min, 90  mmol/L of H2O2 was removed but COD also decreased by ∼10%. COD was reduced by ∼16% in a reaction time of more than 5 min, indicating a competing reaction of COD oxidation caused by oxidative species, as well as H2O2 decomposition. Therefore, the role of carbonate/bicarbonate species and their pH effect were studied in further detail.H2O2 Decomposition in NaHCO3To study the effect of NaHCO3 on H2O2 decomposition, the decrease in H2O2 in two samples of deionized water and 1 M NaHCO3 was observed at ambient temperature. After shaking both samples for 24 h, H2O2 concentration decreased from 44.5  mmolH2O2/L to 43.2 and 7.2  mmolH2O2/L in deionized water and 1 M NaHCO3 solution, respectively. Iodometric measurement confirmed this result, which was directly visible by the different color densities for the deionized water and the NaHCO3 solution, as shown in Fig. 3(a). The calculated H2O2 decomposition rate was approximately 1.55  mmol/L/h in 1 M NaHCO3 solution, while in deionized water H2O2 was almost stable for one day.During shaking, the generation and decomposition of HCO4– was expected according to Reaction 3 followed by Pathways B and C, generating the final species: HCO3–, H2O, and O2. The generation of oxygen was confirmed by the formation of oxygen bubbles observed in samples containing NaHCO3. The bubbles started to grow on the wall of sample tubes 20 min after NaHCO3 was added [Fig. 3(b)]. Dissolved oxygen was measured at ambient temperature (∼ 20°C) in both samples, at 20.52  mg/L for the NaHCO3 solution and 9.63  mg/L for the deionized water, indicating higher saturation with pure oxygen generated by NaHCO3 in solution.NaHCO3 Optimization for Different Wastewater TypesTable 3 summarizes the progress of H2O2 decomposition and pH to determine the appropriate dose of NaHCO3 to completely remove H2O2. The addition of NaHCO3 shifted the pH value of SWW from almost neutral (pH 7.1) to a basic range (pH∼9). H2O2 concentration decreased at ambient temperature nonlinearly with NaHCO3 concentration over 24 h. In the first 5 h, only 2.1  mmol/L  H2O2 decomposed at an NaHCO3 concentration of 0.5  mol/L (Sample 1), whereas ∼8 and 20  mmol/L  H2O2 decomposed when the NaHCO3 concentration was 1 and 2  mol/L, respectively. After 24 h, 6  mmol/L of H2O2 was still found in Sample 1, whereas H2O2 was completely removed in Samples 2 and 3. The minimum dose of NaHCO3 for complete H2O2 decomposition of the samples with residual H2O2 concentration of up to 44.5  mmol/L was calculated as 22.5mmolNaHCO3/mmolH2O2 (with 1 M NaHCO3 solution).Table 3. Influence of NaHCO3 on H2O2 removal in SWW samplesTable 3. Influence of NaHCO3 on H2O2 removal in SWW samplesSampleCOD (mol/L)NaHCO3 (mol/L)pH0524SWW1,175±3507.08±0.06———11,195±100.58.84±0.0544.542.4±1.56±0.242—18.82±0.0144.536.2±0.903—28.72±0.0644.523.5±0.8041,200±918.75±0.0490—1.2±0.55—28.71±0.0890—0To clarify the applicability of the proposed method at higher H2O2 concentrations, the last run was repeated with an H2O2 concentration of 90  mmol/L. As shown in Table 3, after 24 h shaking of SWW samples with 1 M NaHCO3 solution, an H2O2 residual of 1.2  mmol/L was still found (Sample 4). A 2-mol/L increase in NaHCO3 ensured the complete decomposition of H2O2 after 24 h (Sample 5), which met the ratio of 22.2  mmolNaHCO3/mmolH2O2. Calculation of the concentration ratio (NaHCO3:H2O2) for samples with complete H2O2 decomposition gave a minimum value of 22.5  mmolNaHCO3/mmolH2O2 (Sample 2) for 44.5  mmol/L  H2O2, and 22.2  mmolNaHCO3/mmolH2O2 for 90  mmol/L  H2O2. Therefore, we propose a value of 22.5  mmolNaHCO3/mmolH2O2 as a minimum ratio to ensure complete H2O2 decomposition regardless of H2O2 concentration.The proposed method was then applied to the four wastewaters (Table 1) containing H2O2 at two concentrations (44.5 and 90  mmol/L). For each wastewater, four samples were prepared by mixing wastewater with NaHCO3 solutions of 1 (Samples A and B) and 2  mol/L (Samples C and D) (1∶1 volume per volume). The 22.5  mmolNaHCO3/mmolH2O2 ratio was almost met in Samples B and D (44.5  mmolH2O2/L and 90  mmolH2O2/L, respectively). Samples A and C were run as blanks without H2O2. After 24-h shaking, no H2O2 was detected in Samples B and D for all wastewater types, as shown in Table 4, demonstrating that H2O2 was effectively decomposed by NaHCO3·Additionally, the comparison of blank and treated samples indicated minimal changes considering the standard deviation between original COD and COD after NaHCO3 pretreatment.Table 4. H2O2 decomposition by NaHCO3 during 24 h shakingTable 4. H2O2 decomposition by NaHCO3 during 24 h shakingWastewaterSampleNaHCO3 (mol/L)pHH2O2 before shaking (mmol/L)H2O2 after shaking (mmol/L)COD (mg/L)SD (mg/L)SWW (CODorg=1,194±7  mg/L)A-118.82 (±0.01)——1,190 (±14)±7B-11—44.501,180 (±11)—C-12———1,194 (±10)±17D-12—9001,170 (±7)—Leachate (CODorg=1,328±2  mg/L)A-218.65 (±0.03)——1,324 (±10)±38B-21—44.501,270 (±11)—C-22———1,328 (±9)±45D-22—9001,264 (±10)—MPE (CODorg=180±10  mg/L)A-318.71 (±0.01)——180 (±4)±11B-31—44.50164 (±8)—C-32———180 (±8)±14D-32—900160 (±7)—CID (CODorg=45±2  mg/L)A-418.62 (±0.01)——45 (±4)±5B-41—44.5052 (±5)—C-42———42 (±4)±3D-42—90046 (±3)—Under the experimental conditions, there appeared to be more than one factor influencing the decomposition of H2O2 into its final products (O2,H2O, and HCO3–). The first factor was the high NaHCO3 concentration versus the low H2O2 concentration (22.5  mmolNaHCO3/mmolH2O2). The high concentration of NaHCO3 activated the decomposition reaction (Reaction 3) forming HCO4– (Bakhmutova-Albert et al. 2010), which was subsequently decomposed through the preferred pathways (B and C). The second factor was the basic pH value of all samples (Table 4). A basic pH of 8.62–8.82 increased the abundance of HCO3– species according to lime-carbonic acid equilibrium (Zeebe and Wolf-Gladrow 2005); however, it also supported the self-decomposition of H2O2 according to Reaction 4 (Kolyagin and Kornienko 2003).The extrapolation of COD results in Table 4 was supported by the formation of oxygen bubbles observed in Samples B and D (active H2O2 decomposition by HCO3–), as shown in Fig. 3(b). This observation confirmed the dominance of O2-generating reactions in Pathways B and C until H2O2 decomposition was complete.A comparison of Sample A with Sample B and Sample C with Sample D for each wastewater showed very close COD values, with a deviation of 4  mg/L (9.5% relative to 42  mg/L) to 64  mg/L (4.8% relative to 1.328  mg/L). Additionally, Samples A and C (Table 4) confirmed the original COD values for all wastewater samples (Table 1) with absolute deviations between 0 and 4  mg/L. These results indicated that the addition of NaHCO3≤2  mol/L did not interfere with the COD measurements (see Runs A and C) and that the decomposition of H2O2 was complete (see Runs B and D), while the original sample values were verified after H2O2 decomposition (Table 4).Additionally, the calculated standard deviation (SD) of the COD analysis (Table 4) relating to the accuracy of the spectrophotometer (1%) and sample preparation was approximately 1%–11% relative to the original COD. Therefore, the deviations derived by the proposed approach were in the same range as those derived by the standard method. Indeed, Samples B and D, with NaHCO3 solution, showed a small decrease in COD (Table 4) compared with the COD of the original sample (CODorg in Table 1). This small decrease may indicate that the competing reaction pathway (A in Fig. 1) made only a small contribution to COD oxidation, as proposed by Yang et al. (2019), because HCO4– reacts with electron-rich compounds, such as organic sulfides (Richardson et al. 2000), amines (Balagam and Richardson 2008), and alkenes (Yao and Richardson 2000), which may exist in untreated wastewater. In untreated leachate, this fraction seemed to be higher than in the other wastewaters examined, resulting in the highest SD, which could be neglected, however, due to the much higher COD value of the leachate.In treated wastewaters, especially those treated by AOPs including the addition or generation of H2O2, organic electron-rich compounds is not expected due to their oxidation by AOPs. Therefore, the competition reaction (A in Fig. 1) is expected to be less relevant for treated wastewater and proposed pretreatments with NaHCO3. This approach would be more useful for correct COD measurements, which are essential to determine the required limit of COD effluent or to evaluate the COD removal effeciency by AOPs. In summary, the proposed approach at ambient temperature removed H2O2 without any significant change in the COD of original samples for the four wastewaters.Catalytic H2O2 Decomposition Acceleration by Increased TemperatureThe 24-h pretreatment described is sometimes unacceptable due to delays such as for process evaluation. A typical method to accelerate chemical reactions is increasing the temperature. A simple approach for the described method of H2O2 decomposition is to use the same COD standard test equipment for heating and decomposition. The thermal heat block system commonly includes preset temperatures (e.g., 70°C, 90°C, 100°C, 148°C) for the reaction of organic compounds in the standard COD test. Here, for the mixture of SWW and NaHCO3 as well as for subsequent heating and reactions at a fixed standard temperature, empty cuvettes appropriate for the thermal heat block system were used.The SWW/NaHCO3 mixture, with the proposed ratio of 22.5  mmolNaHCO3/mmolH2O2 (pH∼9) was heated at 70°C, 100°C, and 148°C to accelerate the decomposition of H2O2. Each experiment was stopped when the H2O2 was no longer present. Fig. 4 shows that decomposition was complete after 5 min at 148°C. In contrast to the results at ambient temperature (∼20°C), ∼22% COD removal occurred, indicating oxidation of the original COD.At 100°C and 5 min reaction time, 6 mmol H2O2 remained in the samples: a 12% increase in COD compared with the original tested wastewater. Further heating at 100°C for 30 min caused an additional decrease in COD, to the same concentration as at 148°C. The results for 100°C and 148°C indicated oxidation of a constant fraction of COD in the SWW by NaHCO3-activated H2O2 following the oxidation pathway (A in Fig. 1). Parallel to these experiments, blank samples of SWW/NaHCO3 without H2O2 were heated at 70°C, 100°C, and 148°C for 60, 30, and 20 min, respectively. No COD removal was detected. A decrease in COD at ≥100°C is expected to be catalytic oxidation by the NaHCO3/H2O2 mixture.At 70°C, H2O2 decreased from 90 to 6 and 2  mmol/L after 20 and 30 min, respectively. The measured COD after this pretreatment was approximately equal to CODorg before treatment. Therefore, 30-min pretreatment at 70°C seems to accelerate H2O2 decomposition without COD removal. The optimal temperature for a wide range of water and wastewater types is recommended for further study.To further elucidate the effect of pH on H2O2 decomposition, experiments were repeated four times at 70°C and side by side for NaHCO3 and Na2CO3 as H2O2 decomposition catalysts. This resulted in pHs of ∼9 and ∼11, respectively. Fig. 5 summarizes the results with mean values and SD (details of each experiment are provided in Table S2). The carbonate catalyst decomposed H2O2 approximately 4–5 times faster than the bicarbonate catalyst. Additionally, the COD of the original SWW sample decreased by 211  mg/L with the carbonate catalyst (from 1,195 to 984  mg/L) after 1 h heating, whereas COD removal was four times lower (55  mg/L) during H2O2 decomposition by the bicarbonate catalyst. The main difference between the carbonate and the bicarbonate was pH.To confirm the importance of pH, additional experiments were carried out using sodium bicarbonate, in which pH was increased from ∼9 to ∼10.7 by NaOH addition, followed by H2O2 addition and a 30-min reaction at 70°C. According to lime-carbonic acid equilibrium (Zeebe and Wolf-Gladrow 2005), this action is supposed to force most NaHCO3 to change into Na2CO3 prior to the activation reaction of H2O2. Fig. S2 shows that H2O2 decomposition increased in the first 20 min while COD degraded to below the original concentration after 30 min, similar to COD degradation with Na2CO3 (Fig. 5). The difference between the effect of carbonate and that of bicarbonate on COD removal is most probably related to the quantity and quality of oxidative species formed at different pH values. Other than HCO4– and HO2– as oxidative species, singlet oxygen (O21) was probably generated through the recombination of O2– formed during H2O2 decomposition, as observed by Bokare and Choi (2015) for alkaline solutions. Moreover, the copresence of O2– and H2O2 could lead to the slow generation of hydroxyl radicals via the Haber-Weis mechanism (Bokare and Choi 2015). Using NBT (1.5 mM) to detect O2– caused the occurrence of deep-blue formazan pigment precipitates after just 1 min in the carbonate sample, while the bicarbonate was still light yellow (Fig. S3). The dark color of the carbonate sample indicated the generation of O2– radicals produced via the catalytical decomposition of H2O2 (by carbonate). Continuous shaking for 24 h caused the reaction between O2– and NBT in both samples to become much darker, with more precipitates in the carbonate sample than in the bicarbonate sample, which was light blue with few precipitates in [Figs. S3(b and c)]. This confirmed that NaHCO3 enabled much less O2– species generation than Na2CO3, which was responsible for the original COD that remained unchanged. This suggests that NaHCO3 is beneficial for H2O2 decomposition to avoid interference in spectrophotometric COD tests. A quantitative determination of those oxidative species will be a topic for future research.ConclusionsThe decomposition of H2O2 residuals in wastewater samples by carbonate and bicarbonate catalysts was investigated to avoid interference in spectrophotometric COD measurements for evaluation of treatment processes such as EAOPs, which sometimes generate high residual H2O2 concentrations (>45  mmol/L).An easy-to-implement bicarbonate-based method was identified and confirmed as suitable for four wastewater types, using a 1∶1 ratio of 1 or 2  mol/LNaHCO3 solution in water containing 45 or 90 mmol H2O2, respectively. Subsequent H2O2 decomposition occurred at ambient temperature or 70°C. At ambient temperature (∼20°C), a reaction (shaking) time of 24 h was needed for complete selective H2O2 decomposition, while 70°C was identified as a good alternative for efficient H2O2 decomposition within 60 min.A side-by-side comparison of carbonate and bicarbonate catalysts revealed that bicarbonate maintained the COD content of wastewater (≤5% change) during H2O2 decomposition, while carbonate led to a ∼ 20% decrease in COD. The difference between the two is due to pH, as demonstrated by a control experiment in which COD content was significantly decreased by increasing pH in the bicarbonate to 10.7, approximating that of carbonate. The pH dependency of competitive COD oxidation by carbonate and/ bicarbonate during H2O2 decomposition may depend on the contribution of HO2– due to its pKa value, in addition to the contribution of HCO4–. Furthermore, the presence of O2– confirmed by the NBT test, indicated another COD oxidation pathway by O2– species (and/or O21 and/or OH), as higher O2– radical generation was confirmed for carbonate compared with bicarbonate. The proposed bicarbonate method seems to be a promising approach to decompose H2O2 in water samples to ensure reliable COD measurement.Data Availability StatementAll data, models, and code generated or used during the study appear in the published article.AcknowledgmentsThis study was supported by the German Federal Ministry of Education and Research Bundesministerium für Bildung und Forschung (BMBF) (Grant No. 03XP0107E). Niedersächsisches Ministerium für Wissenschaft und Kultur” (Lower Saxony Ministry for Science and Culture), Hannover, Germany, is acknowledged for the financial support received by Mohammad Issa via the scholarship Wissenschaft. Niedersachsen. Weltoffen.References Afsane, C., R. Aezam, and A. G. 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