Evaluation of oxychlorides as chemical oxygen demand interferents: The roles of the different oxidizing agents and the organic carbon source of wastewaters

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Gutiérrez-González, Ángel Fernández-Mohedano, Francisco Raposo Bejines This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4021635/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Chemical oxygen demand (COD) is considered to be the most useful analytical parameter to characterize wastewaters in terms of water quality, by providing their organic matter or pollution content. For COD determination, a few interferences have been reported but some of them have not been estimated in detail in scientific literature. Hence constituting a critical issue for COD analysis in wastewater samples. In this research work, the negative interference of oxychlorides in COD measurements has been evaluated at laboratory scale. Specifically, the role of oxychlorides as alternative oxidizing agents in competition with dichromate has been assessed. The COD reduction performance varied widely according to the particular oxidizing agent used and its concentration, as well as, the organic carbon source and amount present in the wastewater. The experimental values of COD removal performance should be considered as dual concentration dependent. On the one hand, for each oxidizing agent the COD reduction performance is directly proportional to the dosage used in the experiment. On the other hand, the influence of OM concentration on COD removal performance was inversely proportional. In addition, chlorate can be considered as the strongest oxidizing agent and the principal interferent responsible for the over-evaluation of the COD removal performance. Furthermore, the interference extent of oxychlorides on COD determination decreased in the order of: Phthalate > Hydrocarbons > Proteins. Chemical oxygen demand Interferent Organic matter Oxychlorides Oxidizing agents Wastewaters Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Chemical oxygen demand (COD) is defined, according to Standard Methods for the Examination of Water and Wastewater, as the quantity of a specified oxidant that reacts with the sample under controlled conditions (APHA 2017). The amount of oxidant expended is expressed in terms of its oxygen equivalence, commonly as the mass of oxygen consumed over volume of solution (Barbosa et al. 2023). Alternatively, COD can be described as a measure of the oxygen equivalent of the organic matter (OM) content of a sample that is susceptible to oxidation by a strong chemical oxidant (Raposo et al. 2008). Conventionally, potassium dichromate (K 2 Cr 2 O 7 ) is used as a chemical oxidant by a refluxing procedure at 150ºC during 2 hours. Furthermore, silver sulphate (AgSO 4 ) in sulphuric acid (H 2 SO 4 ) is added as a catalyst to increase the oxidation of relatively recalcitrant organic compounds. After the digestion, the reduced amount of K 2 Cr 2 O 7 can be considered proportional to the oxidizable OM present in the sample. Additionally, COD is considered the most used analytical parameter to characterize wastewaters in terms of water quality by providing an indirect measure of the pollutants present in a sample. It is important to note that COD parameter is used worldwide, particularly in the operation of wastewater treatment plants (WWTPs) because many environmental supervisory agencies impose strict regulations regarding the COD values of wastewater effluents before their discharge into the environment (Zhou et al. 2018). One objective of the WWTPs is to remove the maximum amount possible of pollutants because standard regulations fixed discharge limits between 100-125 mg O 2 /L (Meng et al. 2019). The suitability of judgements based on COD values depends on the quality of measurements (Viana da Silva et al. 2011). Therefore, reliable analysis of COD is required for accurately monitoring the OM content in the WWTPs (Ma 2017). However, a common problem relating to COD analysis is the interference effects of some inorganic substances potentially present in samples. This subject has been widely described in scientific literature, including APHA-Standard Methods. Firstly, the most common interfering element is the chloride ion (Cl - ). Under the COD digestion procedure, chloride and other halide ions react with dichromate oxidant producing a positive interference. The complications caused by the existence of chloride (until 2000 mg/L) can be overcome by complexing with mercuric sulphate (HgSO 4 ) before the refluxing procedure (Kayaalp et al 2010). Secondly, the effect of nitrite (NO 2 - ) as an interfering element has been quantified stochiometrically (1.1 mg O 2 /mg nitrite), but it is considered insignificant and is usually ignored. In any case, sulfamic acid can be used to eliminate a significant positive interference due to this element. Thirdly, other reduced inorganic species (Fe +2 , Mn +2 , sulphide) can be oxidized quantitatively under the test conditions. Appropriate corrections should be made according to the stoichiometric oxidation of these species. In spite of its treatment, it is possible that some COD values for wastewaters may not reach the discharge limit standard established. Therefore, further treatment methods such as physical and/or chemical are used to treat the remaining OM content present s in the wastewater. In this sense, advanced oxidation processes (AOP) refer to a set of chemical treatments planned to remove organic (and sometimes inorganic) materials considered as pollutants, by oxidation through reactions with hydroxyl radicals (Miklos et al. 2018). Unfortunately, some of the chemical species used in AOP have not been mentioned as interferent elements for COD determination in APHA-Standard Methods. However, the scientific literature has plenty of information related to this circumstance. In this way, the reducing effect of hydrogen peroxide (H 2 O 2 ) on K 2 Cr 2 O 7 was previously reported (Talinli and Anderson 1992). Later, Kang et al. (1999) proposed an equation to correct quantitatively the positive interference of H 2 O 2 on COD measurements. Similarly, potassium persulfate (K 2 S 2 O 8 ) could also cause a positive error in the COD determination. This interference can be eliminated based on sodium sulphite (Na 2 SO 3 ) addition and heating at 90ºC during 60 minutes (Yang et al. 2019) An alternative method to remove OM in industrial WWTPs has been reported over the last decade. This is a very simple system, corresponding to a dosing and mixing scheme, which involves adding in the wastewater a sodium chlorate-based product ambiguously labelled as “COD remover” (Meng et al. 2019; Htet & Zeng 2020). It has been reported as a very popular product for wastewater adulteration in some Chinese industries (Liu et al. 2022). This product can be added at intermediate or final stages of the WWTPs where samples are normally taken for compliance testing of COD levels. However, this “COD remover” product does not really react with the OM content; it just serves as an alternating oxidant versus dichromate during the analysis technique (Xiao et al. 2022). In fact, “COD remover” should be considered a negative interfering element in COD determination. Something that has not been specified in the APHA-Standard Methods. Moreover, new AOP based on the electrochemical technology have been developed more recently. This technology is so-called electrochemical advanced oxidation processes (EAOP) and it has been proposed as a way to prevent and remediate pollution problems (Sirés et al. 2014; Moreira et al. 2017). Unfortunately, it has been reported that some oxychloride compounds (ClO x - ) are formed during the application of this kind of electrochemical treatment systems of wastewater. They can be considered as interfering elements for COD determination and, similarly to the so-called “COD remover”, they are responsible for the false COD reduction (Zhu et al. 2022; Xiao et al. 2023a,b; Yan et al. 2023). In this mode, they would bring the wrong appearance about wastewater effluents meeting the COD discharge limit standard. On the other hand, depending on the wastewater itself, the overall chemical composition and the type of pollutant may vary (Sophonsiry and Morgenroth 2004). Most of the previously published studies regarding the negative interference of oxychlorides in COD determination, have been carried out by evaluating refractory organic matter in the form of benzene derived compounds such as benzohydroxamic acid (BHA) (Meng et al. 2019), phenol (Xiao et al. 2023b) and its degradation intermediate products (Zhu et al. 2022; Yan et al. 2023) such as benzoquinone, catechol and clorophenols. It is important to take into account that some of these compounds cannot be totally oxidised according to its relationship between experimental COD and theoretical oxygen demand (Baker et al. 1999). However, the interference effects on COD determination of some totally oxidizable organic components such as carbohydrates and proteins, commonly present as OM in the agro-industrial and the municipal wastewaters, have not been previously reported. The main objective of this research study was to evaluate more in detail the negative interference of oxychlorides in COD determination. This study was carried out from a novel viewpoint when compared to the current state of the art because not previously were assessed in depth the effects of interferent reagents, and neither the influence of different OM sources. Therefore, two main aims were established. Firstly, the influence of different oxidizing agents was evaluated according to phthalate as a conventional reference organic compound, and testing it individually at each concentration level and also through the complete calibration range. Secondly, the role of different oxidizing agents was assessed by considering the organic carbon source of synthetic wastewaters. Materials And Methods 2.1. Oxidizing agents Four oxychloride reagents were used in this research works: (i) Sodium hypochlorite (NaClO) was provided by Panreac AppliChem as 5% w/v solution (certified value of 5.5% w/v); (ii) Sodium chlorite (NaClO 2 ) was provided by Panreac AppliChem as 25% w/w solution (certified value of 31% w/w); (iii) Sodium Chlorate (NaClO 3 ) was provided by Roth in solid form (certified value of 100% w/w); (iv) Sodium perchlorate (NaClO 4 ) was provided by Alfa Aesar in solid form (certified value of 99.9% w/w). These concentrate reagents were used to prepare stock solutions at 2500, 5000, 7500, 10000, 15000 and 20000 mg/L. Finally, appropriate volumes of these solutions were taken to achieve the interferent level range of the study (500-2000 mg/L). 2.2. Synthetic wastewaters The utilization of synthetic wastewater provides an advantage to the work as using a fixed formulation avoids the inherent variability of real wastewaters (Kargol et al. 2023). However, different preparations simulating municipal wastewaters have been reported in the literature (O´Flaherty et al. 2013). In this study the most important characteristic is the organic content according to both, the COD concentration and the carbon source. On the contrary, the importance of macro and micronutrients was limited to the possible interference in COD removal performance (totally discarded). 2.2.1. Organic matter component The selected products were purchased in solid form: (i) Glucose (C 6 H 6 O 12 ) was provided by Panreac AppliChem; (ii) Meat extract (Lab-Lemco powder) was provided by Oxoid; Milk (Non-fat dried milk powder) was provided by Panreac AppliChem as a mixture of lactose (52% w/w), proteins (33.3% w/w) and fats (0.7% w/w). These products were characterized in terms of loss on drying and dry matter (105 ºC); and organic matter and ash (550 ºC). Table 1 summarizes their characterization. 2.2.2. Inorganic matter component The selected products were provided in solid form by Panreac Applichem. They were used to prepare individual stock solutions at different concentrations: (i) Sodium bicarbonate (NaHCO 3 ) - 25 g/L (ii) Ammonium chloride (NH 4 Cl) - 7 g/L; (iii) Potassium hydrogen phosphate (K 2 HPO 4 ) - 6.25 g/L; (iv) Magnesium sulphate (MgSO 4 ·7H 2 O) - 5 g/L; (v) Calcium Chloride (CaCl 2 ·2H 2 O) - 2.5 g/L. Later, a multicomponent stock solution was prepared by mixing appropriate volumes of each individual stock solution. Specifically, 100 mL/L were used for (i-iii), 50 mL/L for (iv) and 20 mL/L for (v). 2.2.3. Preparation of synthetic wastewaters The corresponding amount of glucose, meat extract and milk were weighed and mixed with 400 mL of distilled water in a 500 mL volumetric flask. Later, a volume of 50 mL of the multicomponent stock solution of nutrients was added and finally mixed appropriately with more deionized water to fill up the flask. The general composition of simulated wastewater was as follow: COD (500 mg O 2 /L); NaHCO 3 (250 mg/L); NH 4 Cl (70 mg/L); K 2 HPO 4 (62.5 mg/L); MgSO 4 ·7H 2 O (25 mg/L); CaCl 2 ·2H 2 O (5 mg/L). It should be pointed out that this COD content was selected considering the average value of published wastewater recipes (O´Flaherty et al. 2013) and also considering the intermediate level in relation to the COD method calibration range. 2.3. COD analysis The COD measurements throughout this research work were carried out according to APHA Standard Method-5220D (closed reflux and spectrophotometric method). This methodology is cost-effective according to both the chemicals used and the hazardous waste produced. For this purpose, 16 x 100 mm tubes were selected to add the three different components and achieving a total volume of 7.5 mL. Initially, 2.5 mL of sample were mixed with 1.5 mL of digestion solution (a mixture of K 2 Cr 2 O 7 , HgSO 4 and H 2 SO 4 ). Finally, a volume of 3.5 mL sulphuric acid reagent (a mixture of AgSO4 and H 2 SO 4 ) was carefully added and completely mixed with the rest of the components. 2.3.1. Potassium hydrogen phthalate (KHP) KPH was provided by Panreac Applichem as a powder product of high analytical purity. A KHP stock standard solution of 1000 mg O 2 /L was prepared, considering a theoretical value of 1.171 g O 2 / g KHP, by dissolving 0.425 g of dried (during 2 hours at 105 ºC) product in 500 mL of deionized water. 2.3.2. Preparation of COD calibration curve Seven standards of 50 mL were prepared from KHP stock standard solution through appropriate dilutions to provide COD equivalents of 0, 50, 100, 250, 500, 750 and 1000 mg O 2 /L. Duplicate absorbance measurements were done. A linear regression was obtained from absorbance readings versus COD concentration values. Statistical evaluation of this regression was carried out by using EXCEL-Data analysis. 2.3.3. Spectrophotometer The instrument Genesys 10S UV-Vis (Thermo Scientific) was utilized to measure directly the absorbance of tubes at 600 nm. Calculation of COD values was carried out according to the calibration curve. 2.4. Experimental procedure to evaluate oxychlorides interference on COD method 2.4.1. Using KHP as chemical standard For this purpose, the same concentration levels used for obtaining the COD calibration curve were this time set as mixtures including oxychloride reagents. Due to this reason, these standards were prepared from a more concentrate KHP standard solution of 2000 mg O 2 /L. Next, appropriate volumes of each oxychloride stock solutions (2500, 5000, 7500 and 10000 mg/L) were taken to achieve seven predefined interferent dosages such as 500, 750, 1000, 1250, 1500, 1750 and 2000 mg/L. After 10 minutes of mixing, the samples were taken for COD determination. The absorbance values obtained were used to obtain the corresponding COD results. The results were evaluated according to the different concentrations of standard analyte (KHP) and interferent oxychloride reagents. Moreover, to evaluate appropriately the full working range of KHP in terms of COD, a complementary test of calibration curves was obtained by using the COD absorbance values from solutions spiked with oxychloride reagents. These results were expressed as slope ratios between spiked and not spiked or original calibration curve. 2.4.2. Using synthetic wastewaters Three different organic carbon sources were considered for this study. The composition of the simulated wastewaters was similar with the only difference in the organic compound providing the wastewater COD value (500 mg O 2 /L). The working scheme was to mix 45 mL of synthetic wastewaters and 5 mL of oxychloride stock solutions (5000, 10000, 15000 and 20000 mg/L) to evaluate four interferent dosages such as 500, 1000, 1500 and 2000 mg/L. It is important to specify that after mixing, the synthetic wastewaters were diluted 10-fold. In addition, a control value (obtained without the effect of any interference in COD determination) was also included by replacing the volume of oxidizing agents with distilled water. After 10 minutes of mixing, the samples were taken for COD analyses. The COD ratio values of spiked versus not spiked (control) oxychloride reagents were used to calculate the COD removal performance according to the experimental conditions. 2.5. Experimental procedure to evaluate COD oxidation rate Synthetic wastewater samples were analysed for COD before heating them at 150 ºC and through the digestion time at 30, 60, 90 and 120 minutes. The experimental results were evaluated as % COD ratio in the form of specific digestion time versus 120 minutes’ results. Results And Discussion 3.1. Evaluation of oxychlorides interference in COD determination using KHP as chemical standard First of all, taking into consideration that sodium perchlorate is a strong oxidizing agent (only slightly weaker than dichromate), it was previously suggested that it could be used to reduce COD in fraudulent form as sodium chlorate (Meng et al. 2019). However, it is important to highlight the unusual lack of ClO 4 − reactivity observed in chemical systems (Urbansky et al. 2002). In fact, ClO 4 − was not effective at all as an oxidizing agent in the experimental conditions reported for COD determination. Therefore, in the working range evaluated of 500-2000 mg/L, ClO 4 − cannot be considered as a “COD remover” active agent. The same negative results were reported by Xiao et al. (2023a&b). They justified this fact according to the high activation energy necessary for ClO 4 − reduction (Clark et al. 2021). The effect of oxychlorides on the COD removal performance varied widely according to the experimental concentrations of both, the ClO x - interferent agent and the KHP chemical standard. Therefore, the experimental values of COD removal performance should be considered as dual concentration dependent. On the one hand, as expected, the COD removal performance observed was directly proportional to the ClO x - interferent concentration. As an example, figure 1 shows the ClO 3 − concentration effect over the COD removal performance at 500 mg O 2 /L. A similar trend, in decreasing COD values while interference concentration increased, was previously reported by Xiao et al. (2002). They used KHP water samples at 150 mg O 2 /L and ClO 3 − at a concentration range between 0-20 mM. Specifically, COD values dropped from 150 mg O 2 /L to 80 mg O 2 /L, 50 mg O 2 /L, 40 mg O 2 /L and 33 mg O 2 /L at interferent concentrations of 5, 10, 15 and 20 mM, respectively. That means 47%, 67%, 73% and 78% of COD removal performance, respectively. Similarly, it was reported that COD values dropped severely from 150 mg O 2 /L to below 100 mg O 2 /L at 5 mM of various ClO x - interferent agents and further decreased while interference concentration increased (Xiao et al. 2023a). On the other hand, the influence of KHP concentration on COD removal performance was inversely proportional. This effect has not been previously reported because ClO x - interference studies were carried out at single COD level where the only variable studied was the interfering concentration. As an example, figure 2 shows the influence of KHP concentration on COD removal performance at chlorate concentration of 1000 mg/L. As it can be seen, the COD removal performance ranged among 100-40% depending on the initial KHP concentration (50-1000 mg O 2 /L). The variable extent in these interference studies can be explained by considering the two oxidation reactions by K 2 Cr 2 O 7 and ClO x - interfering agents. Depending on the amounts of organic compounds in the samples, the effect of the alternative oxidizing agent varies. At low KHP concentration, there is an excess of ClO x - , so OM can be oxidized at a greater extent. At high KHP concentration, the amount of ClO x - is comparatively lower, meaning that the alternate oxidation reaction is significantly lower. It must be pointed out that similar results are obtained from COD removal when both concentrations are considered in the form of the same ratio value. In this line, the three concentration combinations (500, 750 and 1000 mg/L versus mg O 2 /L) giving a ratio value (ClO x - :COD) of one, provided the same COD removal performance of about 39(1) %. To try to simplify this dual concentration variability of the experimental results, the trial data was also assessed over the full calibration range through slope value ratios. For this purpose, slope values of testing calibration curves (spiked) were compared to the original calibration curve (not spiked). Experimental results provided a general trend where the absorbance values and their corresponding slopes decreased while oxychlorides concentration increased (Supplementary data). Therefore, the extent values of COD removal performance were confirmed as directly proportional to the dosage of oxidizing agents. Figure 3 summarizes the results obtained in terms of overall COD removal performance versus ClO x - interferent concentration. The COD removal extent of ClO − , ClO 2 − and ClO 3 − ranged from 7% to 36%, 15% to 44% and 21% to 62%, respectively. According to the overall slope value relationships obtained by these linear regressions, the COD removal extent for ClO 3 − was about 35-38% higher when compared to the rest of the oxidizing compounds. Therefore, the interference effect degree of oxychlorides on COD determination using KHP standard decreased in the order of: ClO 3 − > ClO 2 − > ClO − (while ClO 4 − showed no interference effect). 3.2. Evaluation of oxychlorides interference in COD determination using synthetic wastewaters Firstly, the COD removal performance of synthetic wastewaters for the oxychloride reagents was evaluated at different dosages (figure 4). For the simulated wastewaters containing about 450 mg O 2 /L, in the same way as the results from KHP standard, the COD removal performance values were directly proportional to the ClO x - interferent concentration. It is noteworthy to mention that ClO 3 − provided the maximum interference effect although the COD removal performance values varied widely. For example, ClO 3 − concentration effect over COD removal performance ranged among 13-59%, 15-68% and 21-78% for meat, milk and glucose respectively. On the contrary, ClO − and ClO 2 − interchanged their relative interfering effect. Therefore, the interference extent of oxychlorides on COD determination decreased in the order of: ClO 3 − > ClO − > ClO 2 − . Secondly, the COD removal performance for the oxychlorides reagents was evaluated from the organic carbon source viewpoint (figure 5). It is very interesting to highlight the great influence of the OM nature in the results obtained. In this sense, the interference extent of oxychlorides on COD determination decreased in the order of: KHP > Glucose > Milk > Meat. These results can be explained by taking into account the COD oxidation rate obtained by the different organic substrates evaluated. As it can be seen in figure 6, the % COD ratio at 2 h of digestion achieved maximum values in all cases, confirming the total oxidation of the selected organic substrates. In addition, some of them are more easily oxidised than others under the test conditions. For example, with the simple addition of the reagents and lacking of heat, the % COD ratio values were 92%, 85%, 75% and 49%, respectively. The higher oxidative rate of hydrocarbon versus protein like substrates has been demonstrated. In this study, the rate of COD oxidation varied in the decreasing order of: k KHP > k Glucose > k Milk > k Meat . Therefore, the faster the oxidation rate, the higher the positive interference provided by ClO x - agents and their corresponding COD removal performance. The influence of other organic carbon sources previously reported in the literature was considered as follows. BHA solutions were mixed with different dosages of NaClO 3 at 500, 1000 and 1500 mg/L (Meng et al. 2019). The COD values decreased from 220 mg O 2 /L to 100 mg O 2 /L, 60 mg O 2 /L and 22 mg O 2 /L, respectively. Therefore, the COD reduction performance was about 55%, 73% and 90%, respectively. These values were higher when compared to 40%, 60% and 82% obtained on this study using KHP at 250 mg O 2 /L. In other research work, the higher COD removal performance of ClO 3 − was confirmed (Zhu et al. 2022). In this sense, a 0.5 mM phenol solution was evaluated and the experimental COD values dropped from 127 mg O 2 /L to 78 mg O 2 /L and 105 mg O 2 /L using ClO 3 − (5 mM) and ClO − (15 mM), respectively. In addition, the COD values decreased severely from 90 mg O 2 /L to 63 mg O 2 /L, 104 mg O 2 /L to 72 mg O 2 /L and 182 mg O 2 /L to 125 mg O 2 /L in the solutions of benzoquinone, catechol and 2,4-dichlorophenol with the addition of ClO 3 − (5 mM). In contrast, the change of COD values for all phenol degradation intermediates were insignificant after the addition of different ClO − concentrations (3-15 mM). Conclusion In this study, the negative interference of oxychloride reagents in the COD determination has been evaluated. Based on the results and discussion made, the following conclusions can be described: The diverse ClO x - agents have different oxidizing capacity at the experimental conditions of temperature (150ºC) and time (120 minutes). However, ClO 4 − agent was not effective at all as a “COD remover”. In general, the experimental values of COD removal performance should be considered as dual concentration dependent. On the one hand, for each oxidizing agent the COD reduction performance is directly proportional to the dosage used in the experiment. On the other hand, the influence of OM concentration on COD removal performance was inversely proportional. 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ACS EST Engg 3: 283-294. https://doi.org/10.1021/acsestengg.2c00303 Xiao H, Xu F, Chen J, Hao Y, Guo Y, Zhu C, Luo S, Jiang B (2023b) Electrogenerated oxychlorides induced overlooked negative effects on electro-oxidation wastewater treatment in terms of over-evaluated COD removal efficiency and biotoxicity. J Hazard Mater 456: 131667. https://doi.org/10.1016/j.jhazmat.2023.131667 Yan W, Chen J, Wu J, Li Y, Liu Y, Yang Q, Tang Y, Jiang B (2023) Investigation on the adverse impacts of electrochemically produced ClOx- on assessing the treatment performance of dimensionally stable anode (DSA) for Cl−-containing wastewater. Chemosphere 310: 136848. https://doi.org/10.1016/j.chemosphere.2022.136848 Yang J, Liu Z, Zeng Z, Huang Z, Cui Y (2019) A method for removing persulfate interference in the analysis of the chemical oxygen demand in wastewater. Environ Chem Lett 17: 1085-1089. https://doi.org/10.1007/s10311-018-00832-2 Zhou Y, Duan N, Wu X, Fang H (2018) COD discharge limits for urban wastewater treatment plants in China based on statistical methods. Water 10(6):777 . https://doi.org/10.3390/w10060777 Zhu J, Ba X, Guo X, Zhang Q, Qi YF, Li Y, Wang J, Sun H, Jiang B (2022) Oxychlorides induced over-evaluation of electrochemical COD removal performance over dimensionally stable anode (DSA): The roles of cathode materials. Sep Purif Techno 303:122197. https://doi.org/10.1016/j.seppur.2022.122197 Declarations 6.1. Funding The authors wish to express their gratitude to the Ministry of Science and Innovation (Project number PID2020-114975RB-I00) for the financial support. 6.2. Competing interest The authors declare that they have no relevant financial or non-financial interest to disclose. 6.3. Data availability All data used during the study are available from the corresponding author by request. 6.4. Ethical approval This is not a biomedical or biological research study and for that reason the ethical approval is not applicable. 6.5. Consent to participate This research study is not involving human subjects and then the informed consent to participate in the study is not applicable. 6.6. Consent to publish On the one hand, this research study is not involving human subjects and then the informed consent to publish the study is not applicable. On the other hand, all the co-authors consent to publish this research study. 6.7. Author contribution Julio A. Gutiérrez-González: Investigation, Writing-review & editing. Ángel Fernández-Mohedano: Supervision, Writing-review & editing. Francisco Raposo : Writing-original draft, Writing-review & editing, Investigation, Conceptualization. All authors read and approved the final version of the manuscript. Abbreviations AOP: Advanced oxidation processes; APHA: American public health association; BHA: Benzohydroxamic acid; ClO x - : Oxychloride agent; COD: Chemical oxygen demand; EAOP: Electrochemical advanced oxidation process; KHP: Potassium hydrogen phthalate; OM: Organic matter; w: Weight; WWTP: Wastewater treatment plant Tables Table 1. Characterization of the organic matter components in synthetic wastewater D-Glucose Meat Milk Loss of drying (%) 0.1 (0.0) 6.1 (0.2) 3.6 (0.1) Dry matter – DM (%) 99.9 (0.2) 93.9 (0.2) 96.4 (0.1) Organic matter (% DM) 100 (0.1) 85.6 (0.1) 89.7 (0.1) Ash (% DM) 0.0 (0.0) 8.3 (0.1) 6.8 (0.1) COD (g O 2 / g DM) 1.180 (50) 1.272 (45) 1.260 (50) Additional Declarations The authors declare no competing interests. Supplementary data not available with this version. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4021635","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":277328972,"identity":"80234097-9c55-4180-bf08-2926d401d540","order_by":0,"name":"Julio A. Gutiérrez-González","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Julio","middleName":"A.","lastName":"Gutiérrez-González","suffix":""},{"id":277329263,"identity":"78927fb3-4807-4a1d-bf61-d2eb9f229281","order_by":1,"name":"Ángel Fernández-Mohedano","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Ángel","middleName":"","lastName":"Fernández-Mohedano","suffix":""},{"id":277329264,"identity":"f2adbdfc-eb81-42a3-a8aa-c744694553a2","order_by":2,"name":"Francisco Raposo Bejines","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0UlEQVRIiWNgGAWjYBAC9gYILccGpgyI0MJzAEIbk64lsYFoh/FIH3724MMfm/Q+/gWMH38U2DHw8x8goIUvzdxwZltabpvEA2ZpHoNkBskZCfi12PMwmEnzNhwGajnAxsxgcIDB4AYhh/Gwf5Pm+XM4nQ2ohfEHUIv9eUIO4+Exk+ZhO5zAxt/AxsADsoWBgMOAWspBfjFsk2BsBvmFR+IGQS3s20AhJi/ff/jgxx9/7OT4+wk4DAggkcggAYkaHoLqEVoIRccoGAWjYBSMXAAAxFU3wyNyx3sAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-1458-8832","institution":"CSIC","correspondingAuthor":true,"prefix":"","firstName":"Francisco","middleName":"Raposo","lastName":"Bejines","suffix":""}],"badges":[],"createdAt":"2024-03-06 15:49:07","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-4021635/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4021635/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":53182308,"identity":"258340c1-5608-4298-8a7c-e8438ff9683e","added_by":"auto","created_at":"2024-03-21 15:49:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":17055,"visible":true,"origin":"","legend":"\u003cp\u003eCOD removal performance (%) according to NaClO\u003csub\u003e3\u003c/sub\u003e concentration for KHP as chemical standard at 500 mg O\u003csub\u003e2\u003c/sub\u003e/L.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/d32e820acd82af4a6a90aa1e.png"},{"id":53182309,"identity":"1194b405-1634-4b75-87de-aa9baa1e84dd","added_by":"auto","created_at":"2024-03-21 15:49:33","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":15898,"visible":true,"origin":"","legend":"\u003cp\u003eCOD removal performance (%) according to concentration of KHP as chemical standard for NaClO\u003csub\u003e3\u003c/sub\u003e concentration at 1000 mg/L\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/768f5023f7b38269864a449c.png"},{"id":53182310,"identity":"9bf2eb69-a9d7-4c77-9c7a-81ba7751fc04","added_by":"auto","created_at":"2024-03-21 15:49:33","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":38971,"visible":true,"origin":"","legend":"\u003cp\u003eCOD removal performance (%) according to oxychlorides concentration for KHP as chemical standard.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/103b3feadeffd0745a667a66.png"},{"id":53182708,"identity":"9d3aa1d6-b4ee-406d-b159-e9000c99c0e8","added_by":"auto","created_at":"2024-03-21 15:57:33","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":54705,"visible":true,"origin":"","legend":"\u003cp\u003eCOD removal performance (%) according to oxychlorides concentration for simulated wastewaters. Effect of oxidizing agent.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/c7e8ace64183199039dda8ca.png"},{"id":53182311,"identity":"6b422c4e-da82-49d1-a3a7-dbb3dffeb64b","added_by":"auto","created_at":"2024-03-21 15:49:33","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":51282,"visible":true,"origin":"","legend":"\u003cp\u003eCOD removal performance (%) according to oxychlorides concentration for simulated wastewaters. Effect of organic carbon source.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/4ceaaac39115f5102b21566c.png"},{"id":53182313,"identity":"e0e5f138-4292-41e2-8c14-74616742b2a5","added_by":"auto","created_at":"2024-03-21 15:49:33","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":17704,"visible":true,"origin":"","legend":"\u003cp\u003eCOD-oxidation rate for organic matter components of synthetic wastewater\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/165e5f8e3314fafd7884a82d.png"},{"id":53184459,"identity":"71bae7f4-fcb8-4497-994a-adf28a319fc8","added_by":"auto","created_at":"2024-03-21 16:05:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":535916,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4021635/v1/2a374a5d-877b-467d-b242-71d1dfea3813.pdf"}],"financialInterests":"\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003eSupplementary data not available with this version.\u003c/p\u003e","formattedTitle":"\u003cp\u003e\u003cstrong\u003eEvaluation of oxychlorides as chemical oxygen demand interferents: The roles of the different oxidizing agents and the organic carbon source of wastewaters\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eChemical oxygen demand (COD) is defined, according to Standard Methods for the Examination of Water and Wastewater, as the quantity of a specified oxidant that reacts with the sample under controlled conditions (APHA 2017). The amount of oxidant expended is expressed in terms of its oxygen equivalence, commonly as the mass of oxygen consumed over volume of solution (Barbosa et al. 2023). Alternatively, COD can be described as a measure of the oxygen equivalent of the organic matter (OM) content of a sample that is susceptible to oxidation by a strong chemical oxidant (Raposo et al. 2008). Conventionally, potassium dichromate (K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e) is used as a chemical oxidant by a refluxing procedure at 150\u0026ordm;C during 2 hours. Furthermore, silver sulphate (AgSO\u003csub\u003e4\u003c/sub\u003e) in sulphuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) is added as a catalyst to increase the oxidation of relatively recalcitrant organic compounds. After the digestion, the reduced amount of K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e can be considered proportional to the oxidizable OM present in the sample. Additionally, COD is considered the most used analytical parameter to characterize wastewaters in terms of water quality by providing an indirect measure of the pollutants present in a sample. It is important to note that COD parameter is used worldwide, particularly in the operation of wastewater treatment plants (WWTPs) because many environmental supervisory agencies impose strict regulations regarding the COD values of wastewater effluents before their discharge into the environment (Zhou et al. 2018). One objective of the WWTPs is to remove the maximum amount possible of pollutants because standard regulations fixed discharge limits between 100-125 mg O\u003csub\u003e2\u003c/sub\u003e/L (Meng et al. 2019). The suitability of judgements based on COD values depends on the quality of measurements (Viana da Silva et al. 2011). Therefore, reliable analysis of COD is required for accurately monitoring the OM content in the WWTPs (Ma 2017). However, a common problem relating to COD analysis is the interference effects of some inorganic substances potentially present in samples. This subject has been widely described in scientific literature, including APHA-Standard Methods. Firstly, the most common interfering element is the chloride ion (Cl\u003csup\u003e-\u003c/sup\u003e). Under the COD digestion procedure, chloride and other halide ions react with dichromate oxidant producing a positive interference. The complications caused by the existence of chloride (until 2000 mg/L) can be overcome by complexing with mercuric sulphate (HgSO\u003csub\u003e4\u003c/sub\u003e) before the refluxing procedure (Kayaalp et al 2010). Secondly, the effect of nitrite (NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) as an interfering element has been quantified stochiometrically (1.1 mg O\u003csub\u003e2\u003c/sub\u003e/mg nitrite), but it is considered insignificant and is usually ignored. In any case, sulfamic acid can be used to eliminate a significant positive interference due to this element. Thirdly, other reduced inorganic species (Fe\u003csup\u003e+2\u003c/sup\u003e, Mn\u003csup\u003e+2\u003c/sup\u003e, sulphide) can be oxidized quantitatively under the test conditions. Appropriate corrections should be made according to the stoichiometric oxidation of these species.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn spite of its treatment, it is possible that some COD values for wastewaters may not reach the discharge limit standard established. Therefore, further treatment methods such as physical and/or chemical are used to treat the remaining OM content present\u003cs\u003es\u003c/s\u003e in the wastewater. In this sense, advanced oxidation processes (AOP) refer to a set of chemical treatments planned to remove organic (and sometimes inorganic) materials considered as pollutants, by oxidation through reactions with hydroxyl radicals (Miklos et al. 2018). Unfortunately, some of the chemical species used in AOP have not been mentioned as interferent elements for COD determination in APHA-Standard Methods. However, the scientific literature has plenty of information related to this circumstance. In this way, the reducing effect of hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) on K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e was previously reported (Talinli and Anderson 1992). Later, Kang et al. (1999) proposed an equation to correct quantitatively the positive interference of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e on COD measurements. Similarly, potassium persulfate (K\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e) could also cause a positive error in the COD determination. This interference can be eliminated based on sodium sulphite (Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e3\u003c/sub\u003e) addition and heating at 90\u0026ordm;C during 60 minutes (Yang et al. 2019) \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAn alternative method to remove OM in industrial WWTPs has been reported over the last decade. This is a very simple system, corresponding to a dosing and mixing scheme, which involves adding in the wastewater a sodium chlorate-based product ambiguously labelled as \u0026ldquo;COD remover\u0026rdquo; (Meng et al. 2019; Htet \u0026amp; Zeng 2020). It has been reported as a very popular product for wastewater adulteration in some Chinese industries (Liu et al. 2022). This product can be added at intermediate or final stages of the WWTPs where samples are normally taken for compliance testing of COD levels. However, this \u0026ldquo;COD remover\u0026rdquo; product does not really react with the OM content; it just serves as an alternating oxidant versus dichromate during the analysis technique (Xiao et al. 2022). In fact, \u0026ldquo;COD remover\u0026rdquo; should be considered a negative interfering element in COD determination. Something that has not been specified in the APHA-Standard Methods. Moreover, new AOP based on the electrochemical technology have been developed more recently. This technology is so-called electrochemical advanced oxidation processes (EAOP) and it has been proposed as a way to prevent and remediate pollution problems (Sir\u0026eacute;s et al. 2014; Moreira et al. 2017). Unfortunately, it has been reported that some oxychloride compounds (ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) are formed during the application of this kind of electrochemical treatment systems of wastewater. They can be considered as interfering elements for COD determination and, similarly to the so-called \u0026ldquo;COD remover\u0026rdquo;, they are responsible for the false COD reduction (Zhu et al. 2022; Xiao et al. 2023a,b; Yan et al. 2023). In this mode, they would bring the wrong appearance about wastewater effluents meeting the COD discharge limit standard.\u003c/p\u003e\n\u003cp\u003eOn the other hand, depending on the wastewater itself, the overall chemical composition and the type of pollutant may vary (Sophonsiry and Morgenroth 2004). Most of the previously published studies regarding the negative interference of oxychlorides in COD determination, have been carried out by evaluating refractory organic matter in the form of benzene derived compounds such as benzohydroxamic acid (BHA) (Meng et al. 2019), phenol (Xiao et al. 2023b) and its degradation intermediate products (Zhu et al. 2022; Yan et al. 2023) such as benzoquinone, catechol and clorophenols. It is important to take into account that some of these compounds cannot be totally oxidised according to its relationship between experimental COD and theoretical oxygen demand (Baker et al. 1999). However, the interference effects on COD determination of some totally oxidizable organic components such as carbohydrates and proteins, commonly present as OM in the agro-industrial and the municipal wastewaters, have not been previously reported. \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe main objective of this research study was to evaluate more in detail the negative interference of oxychlorides in COD determination. This study was carried out from a novel viewpoint when compared to the current state of the art because not previously were assessed in depth the effects of interferent reagents, and neither the influence of different OM sources. Therefore, two main aims were established. Firstly, the influence of different oxidizing agents was evaluated according to phthalate as a conventional reference organic compound, and testing it individually at each concentration level and also through the complete calibration range. Secondly, the role of different oxidizing agents was assessed by considering the organic carbon source of synthetic wastewaters.\u003c/p\u003e"},{"header":"Materials And Methods","content":"\u003cp\u003e2.1. Oxidizing agents\u003c/p\u003e\n\u003cp\u003eFour oxychloride reagents were used in this research works: (i) Sodium hypochlorite (NaClO) was provided by Panreac AppliChem as 5% w/v solution (certified value of 5.5% w/v); (ii) Sodium chlorite (NaClO\u003csub\u003e2\u003c/sub\u003e) was provided by Panreac AppliChem as 25% w/w solution (certified value of 31% w/w); (iii) Sodium Chlorate (NaClO\u003csub\u003e3\u003c/sub\u003e) was provided by Roth in solid form (certified value of 100% w/w); (iv) Sodium perchlorate (NaClO\u003csub\u003e4\u003c/sub\u003e) was provided by Alfa Aesar in solid form (certified value of 99.9% w/w). These concentrate reagents were used to prepare stock solutions at 2500, 5000, 7500, 10000, 15000 and 20000 mg/L. Finally, appropriate volumes of these solutions were taken to achieve the interferent level range of the study (500-2000 mg/L).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.2. Synthetic wastewaters\u003c/p\u003e\n\u003cp\u003eThe utilization of synthetic wastewater provides an advantage to the work as using a fixed formulation avoids the inherent variability of real wastewaters (Kargol et al. 2023). However, different preparations simulating municipal wastewaters have been reported in the literature (O\u0026acute;Flaherty et al. 2013). In this study the most important characteristic is the organic content according to both, the COD concentration and the carbon source. On the contrary, the importance of macro and micronutrients was limited to the possible interference in COD removal performance (totally discarded). \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.2.1. Organic matter component\u003c/p\u003e\n\u003cp\u003eThe selected products were purchased in solid form: (i) Glucose (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e) was provided by Panreac AppliChem; (ii) Meat extract (Lab-Lemco powder) was provided by Oxoid; Milk (Non-fat dried milk powder) was provided by Panreac AppliChem as a mixture of lactose (52% w/w), proteins (33.3% w/w) and fats (0.7% w/w). These products were characterized in terms of loss on drying and dry matter (105 \u0026ordm;C); and organic matter and ash (550 \u0026ordm;C). Table 1 summarizes their characterization.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.2.2. Inorganic matter component\u003c/p\u003e\n\u003cp\u003eThe selected products were provided in solid form by Panreac Applichem. They were used to prepare individual stock solutions at different concentrations: (i) Sodium bicarbonate (NaHCO\u003csub\u003e3\u003c/sub\u003e) - 25 g/L (ii) Ammonium chloride (NH\u003csub\u003e4\u003c/sub\u003eCl) - 7 g/L; (iii) Potassium hydrogen phosphate (K\u003csub\u003e2\u003c/sub\u003eHPO\u003csub\u003e4\u003c/sub\u003e) - 6.25 g/L; (iv) Magnesium sulphate (MgSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO) - 5 g/L; (v) Calcium Chloride (CaCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO) - 2.5 g/L. Later, a multicomponent stock solution was prepared by mixing appropriate volumes of each individual stock solution. Specifically, 100 mL/L were used for (i-iii), 50 mL/L for (iv) and 20 mL/L for (v).\u003c/p\u003e\n\u003cp\u003e2.2.3. Preparation of synthetic wastewaters\u003c/p\u003e\n\u003cp\u003eThe corresponding amount of glucose, meat extract and milk were weighed and mixed with 400 mL of distilled water in a 500 mL volumetric flask. Later, a volume of 50 mL of the multicomponent stock solution of nutrients was added and finally mixed appropriately with more deionized water to fill up the flask. The general composition of simulated wastewater was as follow: COD (500 mg O\u003csub\u003e2\u003c/sub\u003e/L); NaHCO\u003csub\u003e3\u0026nbsp;\u003c/sub\u003e(250 mg/L); NH\u003csub\u003e4\u003c/sub\u003eCl (70 mg/L); K\u003csub\u003e2\u003c/sub\u003eHPO\u003csub\u003e4\u003c/sub\u003e (62.5 mg/L); MgSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;7H\u003csub\u003e2\u003c/sub\u003eO (25 mg/L); CaCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO (5 mg/L). It should be pointed out that this COD content was selected considering the average value of published wastewater recipes (O\u0026acute;Flaherty et al. 2013) and also considering the intermediate level in relation to the COD method calibration range.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.3. COD analysis\u003c/p\u003e\n\u003cp\u003eThe COD measurements throughout this research work were carried out according to APHA Standard Method-5220D (closed reflux and spectrophotometric method). This methodology is cost-effective according to both the chemicals used and the hazardous waste produced. For this purpose, 16 x 100 mm tubes were selected to add the three different components and achieving a total volume of 7.5 mL. Initially, 2.5 mL of sample were mixed with 1.5 mL of digestion solution (a mixture of K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, HgSO\u003csub\u003e4\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e). Finally, a volume of 3.5 mL sulphuric acid reagent (a mixture of AgSO4 and H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) was carefully added and completely mixed with the rest of the components.\u003c/p\u003e\n\u003cp\u003e2.3.1. Potassium hydrogen phthalate (KHP)\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eKPH was provided by Panreac Applichem as a powder product of high analytical purity. A KHP stock standard solution of 1000 mg O\u003csub\u003e2\u003c/sub\u003e/L was prepared, considering a theoretical value of 1.171 g O\u003csub\u003e2\u003c/sub\u003e/ g KHP, by dissolving 0.425 g of dried (during 2 hours at 105 \u0026ordm;C) product in 500 mL of deionized water.\u003c/p\u003e\n\u003cp\u003e2.3.2. Preparation of COD calibration curve\u003c/p\u003e\n\u003cp\u003eSeven standards of 50 mL were prepared from KHP stock standard solution through appropriate dilutions to provide COD equivalents of 0, 50, 100, 250, 500, 750 and 1000 mg O\u003csub\u003e2\u003c/sub\u003e/L. Duplicate absorbance measurements were done. A linear regression was obtained from absorbance readings versus COD concentration values. Statistical evaluation of this regression was carried out by using EXCEL-Data analysis.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.3.3. Spectrophotometer\u003c/p\u003e\n\u003cp\u003eThe instrument Genesys 10S UV-Vis (Thermo Scientific) was utilized to measure directly the absorbance of tubes at 600 nm. Calculation of COD values was carried out according to the calibration curve.\u003c/p\u003e\n\u003cp\u003e2.4. Experimental procedure to evaluate oxychlorides interference on COD method\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.4.1. Using KHP as chemical standard\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFor this purpose, the same concentration levels used for obtaining the COD calibration curve were this time set as mixtures including oxychloride reagents. Due to this reason, these standards were prepared from a more concentrate KHP standard solution of 2000 mg O\u003csub\u003e2\u003c/sub\u003e/L. Next, appropriate volumes of each oxychloride stock solutions (2500, 5000, 7500 and 10000 mg/L) were taken to achieve seven predefined interferent dosages such as 500, 750, 1000, 1250, 1500, 1750 and 2000 mg/L. After 10 minutes of mixing, the samples were taken for COD determination. The absorbance values obtained were used to obtain the corresponding COD results. The results were evaluated according to the different concentrations of standard analyte (KHP) and interferent oxychloride reagents. Moreover, to evaluate appropriately the full working range of KHP in terms of COD, a complementary test of calibration curves was obtained by using the COD absorbance values from solutions spiked with oxychloride reagents. These results were expressed as slope ratios between spiked and not spiked or original calibration curve.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.4.2. Using synthetic wastewaters\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThree different organic carbon sources were considered for this study. The composition of the simulated wastewaters was similar with the only difference in the organic compound providing the wastewater COD value (500 mg O\u003csub\u003e2\u003c/sub\u003e/L). The working scheme was to mix 45 mL of synthetic wastewaters and 5 mL of oxychloride stock solutions (5000, 10000, 15000 and 20000 mg/L) to evaluate four interferent dosages such as 500, 1000, 1500 and 2000 mg/L. It is important to specify that after mixing, the synthetic wastewaters were diluted 10-fold. In addition, a control value (obtained without the effect of any interference in COD determination) was also included by replacing the volume of oxidizing agents with distilled water. After 10 minutes of mixing, the samples were taken for COD analyses. The COD ratio values of spiked versus not spiked (control) oxychloride reagents were used to calculate the COD removal performance according to the experimental conditions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2.5. Experimental procedure to evaluate COD oxidation rate\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSynthetic wastewater samples were analysed for COD before heating them at 150 \u0026ordm;C and through the digestion time at 30, 60, 90 and 120 minutes. The experimental results were evaluated as % COD ratio in the form of specific digestion time versus 120 minutes\u0026rsquo; results.\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003e3.1. Evaluation of oxychlorides interference in COD determination using KHP as chemical standard\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFirst of all, taking into consideration that sodium perchlorate is a strong oxidizing agent (only slightly weaker than dichromate), it was previously suggested that it could be used to reduce COD in fraudulent form as sodium chlorate (Meng et al. 2019). However, it is important to highlight the unusual lack of ClO\u003cem\u003e\u003csub\u003e4\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003ereactivity observed in chemical systems (Urbansky et al. 2002). In fact, ClO\u003cem\u003e\u003csub\u003e4\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003ewas not effective at all as an oxidizing agent in the experimental conditions reported for COD determination. Therefore, in the working range evaluated of 500-2000 mg/L, ClO\u003cem\u003e\u003csub\u003e4\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003ecannot be considered as a \u0026ldquo;COD remover\u0026rdquo; active agent. The same negative results were reported by Xiao et al. (2023a\u0026amp;b). They justified this fact according to the high activation energy necessary for ClO\u003cem\u003e\u003csub\u003e4\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction (Clark et al. 2021).\u003c/p\u003e\n\u003cp\u003eThe effect of oxychlorides on the\u003csup\u003e\u0026nbsp;\u003c/sup\u003eCOD removal performance varied widely according to the experimental concentrations of both, the ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterferent agent and the KHP chemical standard. Therefore, the experimental values of COD removal performance should be considered as dual concentration dependent. On the one hand, as expected, the COD removal performance observed was directly proportional to the ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterferent concentration. As an example, figure 1 shows the ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003econcentration effect\u003csup\u003e\u0026nbsp;\u003c/sup\u003eover the COD\u0026nbsp;removal performance at 500 mg O\u003csub\u003e2\u003c/sub\u003e/L. A similar trend, in decreasing COD values while interference concentration increased, was previously reported by Xiao et al. (2002). They used KHP water samples at 150 mg O\u003csub\u003e2\u003c/sub\u003e/L and ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003eat a concentration range between 0-20 mM. Specifically, COD values dropped from 150 mg O\u003csub\u003e2\u003c/sub\u003e/L to 80 mg O\u003csub\u003e2\u003c/sub\u003e/L, 50 mg O\u003csub\u003e2\u003c/sub\u003e/L, 40 mg O\u003csub\u003e2\u003c/sub\u003e/L and 33 mg O\u003csub\u003e2\u003c/sub\u003e/L at interferent concentrations of 5, 10, 15 and 20 mM, respectively. That means 47%, 67%, 73% and 78% of COD removal performance, respectively. Similarly, it was reported that COD values dropped severely from 150 mg O\u003csub\u003e2\u003c/sub\u003e/L to below 100 mg O\u003csub\u003e2\u003c/sub\u003e/L at 5 mM of various ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterferent agents and further decreased while interference concentration increased (Xiao et al. 2023a). On the other hand, the influence of KHP concentration on COD removal performance was inversely proportional. This effect has not been previously reported because ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterference studies were carried out at single COD level where the only variable studied was the interfering concentration. As an example, figure 2 shows the influence of KHP concentration on COD removal performance at chlorate concentration of 1000 mg/L. \u0026nbsp;As it can be seen, the COD removal performance ranged among 100-40% depending on the initial KHP concentration (50-1000 mg O\u003csub\u003e2\u003c/sub\u003e/L).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe variable extent in these interference studies can be explained by considering the two oxidation reactions by K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e and ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterfering agents. Depending on the amounts of organic compounds in the samples, the effect of the alternative oxidizing agent varies. At low KHP concentration, there is an excess of ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, so OM can be oxidized at a greater extent. At high KHP concentration, the amount of ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eis comparatively lower, meaning that the alternate oxidation reaction is significantly lower. It must be pointed out that similar results are obtained from COD removal when both concentrations are considered in the form of the same ratio value. In this line, the three concentration combinations (500, 750 and 1000 mg/L versus mg O\u003csub\u003e2\u003c/sub\u003e/L) giving a ratio value (ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e:COD) of one, provided the same COD removal performance of about 39(1) %.\u003c/p\u003e\n\u003cp\u003eTo try to simplify this dual concentration variability of the experimental results, the trial data was also assessed over the full calibration range through slope value ratios. For this purpose, slope values of testing calibration curves (spiked) were compared to the original calibration curve (not spiked). Experimental results provided a general trend where the absorbance values and their corresponding slopes decreased while oxychlorides concentration increased (Supplementary data). Therefore, the extent values of COD removal performance were confirmed as directly proportional to the dosage of oxidizing agents. Figure 3 summarizes the results obtained in terms of overall COD removal performance versus ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterferent concentration. The COD removal extent of ClO\u003csup\u003e\u0026minus;\u003c/sup\u003e, ClO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e ranged from 7% to 36%, 15% to 44% and 21% to 62%, respectively. According to the overall slope value relationships obtained by these linear regressions, the COD removal extent for ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was about 35-38% higher when compared to the rest of the oxidizing compounds. Therefore, the interference effect degree of\u0026nbsp;oxychlorides on COD determination using KHP standard decreased in the order of: ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026gt; ClO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e\u0026gt;\u003csup\u003e\u0026nbsp;\u003c/sup\u003eClO\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e(while ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003eshowed no interference effect).\u003c/p\u003e\n\u003cp\u003e3.2. Evaluation of oxychlorides interference in COD determination using synthetic wastewaters\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFirstly, the COD removal performance of synthetic wastewaters for the oxychloride reagents was evaluated at different dosages (figure 4). For the simulated wastewaters containing about 450 mg O\u003csub\u003e2\u003c/sub\u003e/L, in the same way as the results from KHP standard, the COD removal performance values were directly proportional to the ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003einterferent concentration. It is noteworthy to mention that ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003eprovided the\u003csup\u003e\u0026nbsp;\u003c/sup\u003emaximum interference effect although the COD removal performance values varied widely. For example, ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003econcentration\u003csup\u003e\u0026nbsp;\u003c/sup\u003eeffect over COD removal performance ranged among 13-59%, 15-68% and 21-78% for meat, milk and glucose respectively. On the contrary, ClO\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003eand ClO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003einterchanged their relative interfering effect. Therefore, the interference extent of\u0026nbsp;oxychlorides on COD determination decreased in the order of: ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026gt; ClO\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e\u0026gt;\u003csup\u003e\u0026nbsp;\u003c/sup\u003eClO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSecondly, the COD removal performance for the oxychlorides reagents was evaluated from the organic carbon source viewpoint (figure 5). It is very interesting to highlight the great influence of the OM nature in the results obtained. In this sense, the interference extent of\u0026nbsp;oxychlorides on COD determination decreased in the order of: KHP \u0026gt; Glucose \u0026gt; Milk \u0026gt; Meat. These results can be explained by taking into account the COD oxidation rate obtained by the different organic substrates evaluated. As it can be seen in figure 6, the % COD ratio at 2 h of digestion achieved maximum values in all cases, confirming the total oxidation of the selected organic substrates. In addition, some of them are more easily oxidised than others under the test conditions. For example, with the simple addition of the reagents and lacking of heat, the % COD ratio values were 92%, 85%, 75% and 49%, respectively. The higher oxidative rate of hydrocarbon versus protein like substrates has been demonstrated. In this study, the rate of COD oxidation varied in the decreasing order of: \u003cem\u003ek\u003c/em\u003e \u003csub\u003eKHP\u003c/sub\u003e \u0026gt; \u003cem\u003ek\u003c/em\u003e \u003csub\u003eGlucose\u003c/sub\u003e \u0026gt; \u003cem\u003ek\u003c/em\u003e \u003csub\u003eMilk\u003c/sub\u003e \u0026gt; \u003cem\u003ek\u003c/em\u003e \u003csub\u003eMeat\u003c/sub\u003e. Therefore, the faster the oxidation rate, the higher the positive interference provided by ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eagents\u003csup\u003e\u0026nbsp;\u003c/sup\u003eand their corresponding COD removal performance.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe influence of other organic carbon sources previously reported in the literature was considered as follows. BHA solutions were mixed with different dosages of NaClO\u003cem\u003e\u003csub\u003e3\u0026nbsp;\u003c/sub\u003e\u003c/em\u003eat 500, 1000 and 1500 mg/L (Meng et al. 2019). The COD values decreased from 220 mg O\u003csub\u003e2\u003c/sub\u003e/L to 100 mg O\u003csub\u003e2\u003c/sub\u003e/L, 60 mg O\u003csub\u003e2\u003c/sub\u003e/L and 22 mg O\u003csub\u003e2\u003c/sub\u003e/L, respectively. Therefore, the COD reduction performance was about 55%, 73% and 90%, respectively. These values were higher when compared to 40%, 60% and 82% obtained on this study using KHP at 250 mg O\u003csub\u003e2\u003c/sub\u003e/L. In other research work, the higher COD removal performance of ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was confirmed\u0026nbsp;(Zhu et al. 2022). In this sense, a\u0026nbsp;0.5 mM phenol solution was evaluated and the experimental COD values dropped from 127 mg O\u003csub\u003e2\u003c/sub\u003e/L to 78 mg O\u003csub\u003e2\u003c/sub\u003e/L and 105 mg O\u003csub\u003e2\u003c/sub\u003e/L using ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (5 mM) and ClO\u003csup\u003e\u0026minus;\u003c/sup\u003e (15 mM), respectively. In addition, the COD values decreased severely from 90 mg O\u003csub\u003e2\u003c/sub\u003e/L to 63 mg O\u003csub\u003e2\u003c/sub\u003e/L, 104 mg O\u003csub\u003e2\u003c/sub\u003e/L to 72 mg O\u003csub\u003e2\u003c/sub\u003e/L and 182 mg O\u003csub\u003e2\u003c/sub\u003e/L to 125 mg O\u003csub\u003e2\u003c/sub\u003e/L in the solutions of benzoquinone, catechol and 2,4-dichlorophenol with the addition of ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e(5 mM). In contrast, the change of COD values for all phenol degradation intermediates were insignificant after the addition of different ClO\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003econcentrations (3-15 mM).\u0026nbsp;\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this study, the negative interference of oxychloride reagents in the COD determination has been evaluated. Based on the results and discussion made, the following conclusions can be described:\u0026nbsp;\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003eThe diverse ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eagents\u003csup\u003e\u0026nbsp;\u003c/sup\u003ehave different oxidizing capacity at the experimental conditions of temperature (150\u0026ordm;C) and time (120 minutes). However, ClO\u003cem\u003e\u003csub\u003e4\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003eagent\u003csup\u003e\u0026nbsp;\u003c/sup\u003ewas not effective at all as a \u0026ldquo;COD remover\u0026rdquo;.\u003c/li\u003e\n \u003cli\u003eIn general, the experimental values of COD removal performance should be considered as dual concentration dependent. On the one hand, for each oxidizing agent the COD reduction performance is directly proportional to the dosage used in the experiment. On the other hand, the influence of OM concentration on COD removal performance was inversely proportional.\u003c/li\u003e\n \u003cli\u003eWhen FHP is used as an organic carbon source, the interference extent of ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eon COD determination through the full calibration range decreased in the order of: ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026gt; ClO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e\u0026gt;\u003csup\u003e\u0026nbsp;\u003c/sup\u003eClO\u003csup\u003e\u0026minus;\u003c/sup\u003e.\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eUsing simulated wastewaters at a concentration of about 450 mg O\u003csub\u003e2\u003c/sub\u003e/L, the COD reduction performance varied widely according to:\u0026nbsp;\u003cul\u003e\n \u003cli\u003ethe oxidizing agent: the interference extent of ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eagents\u003csup\u003e\u0026nbsp;\u003c/sup\u003eon COD measurements decreased in the order of: ClO\u003cem\u003e\u003csub\u003e3\u003c/sub\u003e\u003c/em\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026gt; ClO\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e\u0026gt;\u003csup\u003e\u0026nbsp;\u003c/sup\u003eClO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e.\u003c/li\u003e\n \u003cli\u003ethe organic carbon source: the interference extent of ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eagents on COD measurements decreased in the order of: KHP \u0026gt; Glucose \u0026gt; Milk \u0026gt; Meat.\u003c/li\u003e\n \u003c/ul\u003e\n \u003c/li\u003e\n\u003c/ul\u003e"},{"header":"References","content":"\u003cp\u003eAmerican Public Health Association, American Water Works Association, Water Environment Federation (2017). 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Funding\u003c/p\u003e\n\u003cp\u003eThe authors wish to express their gratitude to the Ministry of Science and Innovation (Project number PID2020-114975RB-I00) for the financial support.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6.2. Competing interest\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no relevant financial or non-financial interest to disclose.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6.3. Data availability\u003c/p\u003e\n\u003cp\u003eAll data used during the study are available from the corresponding author by request.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6.4. Ethical approval\u003c/p\u003e\n\u003cp\u003eThis is not a biomedical or biological research study and for that reason the ethical approval is not applicable.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6.5. Consent to participate\u003c/p\u003e\n\u003cp\u003eThis research study is not involving human subjects and then the informed consent to participate in the study is not applicable.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6.6. Consent to publish\u003c/p\u003e\n\u003cp\u003eOn the one hand, this research study is not involving human subjects and then the informed consent to publish the study is not applicable. On the other hand, all the co-authors consent to publish this research study.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6.7. Author contribution\u003c/p\u003e\n\u003cp\u003eJulio A. Guti\u0026eacute;rrez-Gonz\u0026aacute;lez:\u0026nbsp;Investigation, Writing-review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u0026Aacute;ngel Fern\u0026aacute;ndez-Mohedano:\u0026nbsp;Supervision, Writing-review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003eFrancisco Raposo\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003eWriting-original draft, Writing-review \u0026amp; editing, Investigation, Conceptualization.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eAll authors read and approved the final version of the manuscript.\u003c/em\u003e\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eAOP: Advanced oxidation processes; APHA: American public health association; BHA: Benzohydroxamic acid; ClO\u003csub\u003ex\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e: Oxychloride agent; COD: Chemical oxygen demand; EAOP: Electrochemical advanced oxidation process; KHP: Potassium hydrogen phthalate; OM: Organic matter; w: Weight; WWTP: Wastewater treatment plant\u003c/p\u003e"},{"header":"Tables","content":"\u003cp style='margin-top:0in;margin-right:0in;margin-bottom:8.0pt;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;'\u003e\u003cspan style='font-size:16px;font-family:\"Arial\",sans-serif;'\u003eTable 1. Characterization of the organic matter components in synthetic wastewater\u003c/span\u003e\u003c/p\u003e\n\u003cdiv align=\"center\" style='margin-top:0in;margin-right:0in;margin-bottom:8.0pt;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;'\u003e\n \u003ctable style=\"border-collapse:collapse;border:none;\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:141.5pt;border:solid windowtext 1.0pt;padding:0in 5.4pt 0in 5.4pt;height:22.7pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;line-height:normal;'\u003e\u003cspan style='font-size:16px;font-family: \"Arial\",sans-serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:103.8pt;border:solid windowtext 1.0pt;border-left: none;padding:0in 5.4pt 0in 5.4pt;height:22.7pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;text-align:center;line-height:normal;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family:\"Arial\",sans-serif;'\u003eD-Glucose\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:103.85pt;border:solid windowtext 1.0pt;border-left: none;padding:0in 5.4pt 0in 5.4pt;height:22.7pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;text-align:center;line-height:normal;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family:\"Arial\",sans-serif;'\u003eMeat\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:103.85pt;border:solid windowtext 1.0pt;border-left: none;padding:0in 5.4pt 0in 5.4pt;height:22.7pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;text-align:center;line-height:normal;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family:\"Arial\",sans-serif;'\u003eMilk\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:141.5pt;border:solid windowtext 1.0pt;border-top: none;padding:0in 5.4pt 0in 5.4pt;height:22.7pt;\"\u003e\n \u003cp style='margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:0in;font-size:11.0pt;font-family:\"Calibri\",sans-serif;line-height:normal;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family:\"Arial\",sans-serif;'\u003eLoss of drying (%)\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:103.8pt;border-top:none;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0in 5.4pt 0in 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Científicas","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Chemical oxygen demand, Interferent, Organic matter, Oxychlorides, Oxidizing agents, Wastewaters","lastPublishedDoi":"10.21203/rs.3.rs-4021635/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4021635/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eChemical oxygen demand (COD) is considered to be the most useful analytical parameter to characterize wastewaters in terms of water quality, by providing their organic matter or pollution content. For COD determination, a few interferences have been reported but some of them have not been estimated in detail in scientific literature. Hence constituting a critical issue for COD analysis in wastewater samples. In this research work, the negative interference of oxychlorides in COD measurements has been evaluated at laboratory scale. Specifically, the role of oxychlorides as alternative oxidizing agents in competition with dichromate has been assessed. The COD reduction performance varied widely according to the particular oxidizing agent used and its concentration, as well as, the organic carbon source and amount present in the wastewater. The experimental values of COD removal performance should be considered as dual concentration dependent. On the one hand, for each oxidizing agent the COD reduction performance is directly proportional to the dosage used in the experiment. On the other hand, the influence of OM concentration on COD removal performance was inversely proportional. In addition, chlorate\u003csup\u003e \u003c/sup\u003ecan be considered as the strongest oxidizing agent and the principal interferent responsible for the over-evaluation of the COD removal performance. Furthermore, the interference extent of oxychlorides on COD determination decreased in the order of: Phthalate \u0026gt; Hydrocarbons \u0026gt; Proteins.\u003c/p\u003e","manuscriptTitle":"Evaluation of oxychlorides as chemical oxygen demand interferents: The roles of the different oxidizing agents and the organic carbon source of wastewaters","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-21 15:49:28","doi":"10.21203/rs.3.rs-4021635/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0f285339-0ade-4558-9403-e8f85ec3e938","owner":[],"postedDate":"March 21st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-03-21T15:49:28+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-21 15:49:28","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4021635","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4021635","identity":"rs-4021635","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}