A Modified Method for the Accurate Determination of Chemical Oxygen Demand ( COD ) in High Chloride Oilfield Wastewater

In this work, in order to find a method for the accurate determination of chemical oxygen demand (CODCr) in high chloride oilfield wastewater, the interferences of chloridion (Cl) on CODCr determination were first studied based on the Chinese national standard GB 11914-89. The results showed that the CODCr values and deviations determined by the classical potassium dichromate (K2Cr2O7) method increased gradually with the increase of Cl mass concentration in water samples. Then, theoretical and experimental studies on reducing interferences of Cl on CODCr determination were carried out. The results showed that different concentrations of K2Cr2O7 exhibited different oxidation capacities on Cl, while having little effects on the oxidation of organic compounds. More specifically, when the concentration of K2Cr2O7 was 0.025 mol·L and the mass ratio of HgSO4:Cl was 10:1, the interferences of Cl on CODCr determination were the slightest. Finally, a low concentration oxidant method was proposed for the accurate determination of CODCr in high chloride oilfield wastewaters. The performances of the proposed method were verified by the simulated and actual high chloride oilfield wastewater. The results showed that when Cl ≤ 20,000 mg·L and COD < 100 mg∙L, the relative prediction errors of CODCr provided by the proposed method were acceptable, which revealed that the low concentration oxidant method is feasible and reliable for the determination of CODCr in high chloride oilfield wastewater.


Introduction
Chemical oxygen demand (COD) is one of the most important indexes to assess the pollution of organic compounds in water systems [1].In China, potassium dichromate (K 2 Cr 2 O 7 ) method is legalized by the Chinese government to determine COD Cr in water samples [2] [3].In practical operation, the high level of chloridion (Cl − ) in water samples would cause serious interferences on COD Cr determination [4] [5].Therefore, the Chinese national standard method Water Quality-Determination of Chemical Oxygen Demand-Dichromate Method (GB 11914-89) regulated that when the content of Cl − is less than 1000 mg•L −1 , mercuric sulfate (HgSO 4 ) can be added to eliminate the interferences of Cl − [6].For the samples with Cl − content more than 1000 mg•L −1 , they should be firstly diluted quantitatively so as to reduce their Cl − contents to less than 1000 mg•L −1 and then determine their COD Cr according to the Chinese national standard GB 11914-89.Research revealed that, the COD values of water samples with high Cl − concentration and low COD content were still large after diluting, although HgSO 4 was used to mask Cl − , and the relative errors of COD Cr increased with the increase of Cl − concentration.However, during the process of monitoring and analysis, the high chloride wastewaters are often encountered, such as chemical wastewater, marine products processing wastewater, rare-earth mineral refining wastewater, oil produced wastewater.For example, Chen et al. [7] reported that the mass concentration of Cl − in wastewater of an oilfield was more than 120000 mg•L −1 , whose COD content was low, and the COD values would become smaller after treatment.But the local environmental protection department used the Chinese national standard GB 11914-89 to monitor its COD concentration with results fluctuating significantly and gave the actual measured COD Cr values ranging from several hundreds to 2500 mg•L −1 , which greatly exceeded the national requirements of wastewater discharge (COD < 100 mg•L −1 ) [8].This dilemma brings huge troubles to the wastewater disposal departments, local environmental monitoring and law enforcement departments as well as the water quality analysis workers.Therefore, how to eliminate the interferences of Cl − on the COD Cr determination has become a research hot for environmental monitoring workers [9].
At present, there are many methods for reducing the interferences of Cl − , such as chlorine correction method, low concentration oxidant method, silver salt precipitation method, sealed digestion method, mercury salt method and no external heating method [10] [11] [12] [13] [14], which have not yet been unified.However, in the practical operations of the environmental monitoring department, the national or industrial standards recommended by the Chinese Ministry of Environmental Protection are still used as the reference method.Among them, potassium dichromate method (GB 11914-89, abbreviated as the classical method) [6] is the mandatory national standard, whereas fast digestion-spectrophotometric method (HJ/T 399-2007) [15], chlorine correction method (HJ/T 70-2001) [16] and potassium iodide-alkaline potassium permanganate determination method (HJ/T 132-2003) [17] are the standards released by the National Environmental Protection Department, which are the supplements to the national standard, and whose legal status are naturally lower than that of the classical method (GB 11914-89) [6].Consequently, during the supervision of law enforcement, the COD determination of samples with high requirements such as discharge sample, assessment sample, arbitration analysis sample is preferred by the classical method (GB 11914-89).
Considering the current research situation mentioned above, the aim of the work is to develop a method that can meet the national standard, and can accurately determine the COD Cr in wastewaters with high content of Cl − .In this work, the interferences of Cl − on COD Cr determination were first investigated by experiments based on the Chinese national standard GB 11914-89, and then the mechanism of interferences was explained.After that, based on the theoretical analysis of electrode potential, a low concentration oxidant method was proposed for the determination of COD Cr in high chloride wastewaters.The performances of the proposed method were verified by simulated and actual high chloride oilfield wastewater.The results demonstrated that the proposed low concentration oxidant method based on GB 11914-89 was accurate and reliable, and thus it is a feasible method for the determination of COD Cr in high chloride oilfield wastewater.Instruments: JH-12 COD constant temperature heater (Laoshan Electronic General Factory Co., Ltd., Qingdao, China); ICS-2100 ion chromatography (Thermo Fisher Scientific, U.S.A ).

Experimental Methods
1) Classical K 2 Cr 2 O 7 method: this classical method was described in detailed in the Chinese national standard Water Quality-Determination of Chemical Oxygen Demand-Dichromate Method (GB 11914-89) [6].Before determination, the method was calibrated by the standard solution of KC 8 H 5 O 4 with a reference COD Cr of 500 mg•L −1 for 5 times, giving the relative errors of COD Cr less than 4%, which indicated that the operation technique, instrument and chemical reagents used in this work meet the requirements.
2) Low concentration oxidant method: this method was modified based on the classical standard method described above.More specifically, during the COD Cr determination, 0.1 mol•L −1 or 0.025 mol•L −1 of 1/6K 2 Cr 2 O 7 standard solution was used instead of 0.25 mol•L −1 of 1/6K 2 Cr 2 O 7 standard solution as the oxidant, whose residual content was then back titrated by corresponding 0.04 mol•L −1 and 0.01 mol•L −1 of [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution, respectively.As can be seen from Table 1, for the simulated high chloride oilfield wastewater samples with a theoretical COD Cr of 500 mg•L −1 and high level of Cl − , when the mass concentration of Cl − was less than 2000 mg•L −1 , the relative errors of COD Cr determined by the classical K 2 Cr 2 O 7 method were relatively small (< 5.7%), which almost accorded with the national requirements that the relative error of COD Cr determination should be within ±5% [18] [19].This is because the addition of 0.4 g HgSO 4 in 20 mL water samples was perfectly able to coordinate 2000 mg•L −1 Cl − in wastewater, and then the interferences of Cl − on COD Cr determination can be virtually eliminated.However, with the increase of Cl − mass concentration in water samples, the Cl − in water samples could not be completely coordinated by 0. As can be seen from Table 2, for the simulated high chloride oilfield wastewater samples with a theoretical COD Cr of 100 mg•L −1 and high level of Cl − , when the mass concentration of Cl − was less than 500 mg

Results and Discussion
In addition, the resulting AgCl precipitate can be further oxidized by K 2 Cr 2 O 7 , which also consumes some oxidants.Moreover, the white precipitate makes the solution color at the end of titration become grey and difficult to be titrated accurately.

Reducing Interferences of Cl − on CODCr Determination in High
Chloride Oilfield Wastewater

Theoretical Analysis of Reducing Cl − Interference
During the COD determination, the interference reaction of Cl − is as follows: Cr O 14H 6Cl 2Cr 7H O 3Cl In Equation ( 3), the electrode reaction of electron pair Similarly, the electrode potential of electron pair Cl 2 /Cl − can be calculated as: ).And the higher the oxidation potential is, the easier the Cl − will be oxidized, and consequently the greater the error introduced is.
As can be known from Equation ( 6), the concentration of Cr O − and Cr 3+ is one of the main factors that affect the electrode potential.Under certain conditions of COD Cr determination, the higher the oxidant concentration is, the more residual amount of oxidant will be after the reaction, which makes the ratio of Cr O Cr − + larger, and results in a higher oxidation potential.Theoretically speaking, the proper reduction of the oxidant concentration is helpful to reduce the oxidation potential of K 2 Cr 2 O 7 and thus reduce the interferences of Cl − on COD Cr determination.

Effect of Oxidant Concentration on the Oxidation of Cl −
In order to investigate the effect of oxidant concentration on Cl − oxidation, a series of NaCl solutions with Cl − mass concentrations of 1000, 2000, 4000, 8000, 15,000 and 20,000 mg•L −1 were prepared by standard NaCl.20 mL of each water sample was transferred into a COD digestion tube, then appropriate amounts of Open Journal of Yangtze Gas and Oil  When the oxidant concentration was about 0.025 mol•L −1 , the oxidation rate of Cl − was the smallest.These results indicated that the oxidant concentration plays an very important role during the determination of COD in high chloride wastewater.Therefore, the proper reduction of oxidant concentration is helpful to reduce the oxidation rate of Cl − and improve the accuracy of COD Cr determination, which is highly consistent with our theoretical analysis in Section 3.2.1.

Effect of Oxidant Concentration on the Oxidation of Organic
Compounds In order to investigate the effect of oxidant concentration on the oxidation of organic compounds, a series of standard KC 8 H 5 O 4 solutions with theoretical COD Cr values of 50, 100, 300 and 600 mg•L −1 were prepared by standard KC 8 H 5 O 4 .After proper dilution, 20 mL of each water sample was transferred into a COD digestion tube, then 30 mL AgSO 4 -H 2 SO 4 was added to the sample so-lution, and finally the mixture of each water was oxidized with 10 mL of 0.25, 0.1 and 0.025 mol•L −1 K 2 Cr 2 O 7 standard solution, respectively.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by corresponding 0.1, 0.04 and 0.01 mol•L −1 [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution, respectively.The obtained COD Cr results in the prepared water samples are summarized in Table 3. Table 3.Effect of oxidant concentration on COD determination of standard KC 8 H 5 O 4 water samples.with the theoretical ones.In addition, with the increase of theoretical COD Cr value in the samples, the measured results by different concentrations of oxidants are almost the same, which implied that the concentration of oxidant has little effect on the oxidation of organic compounds as long as the amount of K 2 Cr 2 O 7 is enough.

Effect of the Amount of Masking Agent on CODCr Determination
In order to investigate the effect of the amount of masking agent on COD Cr determination, in this part, a series of standard water samples with a theoretical  4.
As can be seen from Table 4, the relative errors of COD Cr results decreased gradually with the increase of the mass ratio of HgSO 4 : Cl − .When the mass ratio of HgSO 4 : Cl − is 10:1, the relative errors are small (<±3%), and they are not significantly improved with the increase of the mass ratio of HgSO 4 : Cl − .However, the environmental pollution risk would increase if large amounts of HgSO 4 were used.Therefore, 10:1 was suggested to be the suitable mass ratio of HgSO 4 : Cl − for COD Cr determination in high chlorine wastewater.*The values in bracket are the relative errors and the same meanings in Table 5 and Table 6.  5.

Determination of CODCr in
Table 5. Determination results of high chlorine and low COD simulated oilfield wastewater using different concentrations of oxidants.

Sample
No. Table 5 shows that with the increase of Cl − mass concentration in wastewater samples, the relative errors of COD Cr determination increased gradually.However, the obtained results provided by low concentration oxidant method (0.025 mol•L −1 ) are much better than those determined by 0.25 mol•L −1 and 0.1 mol•L −1 oxidants, and the relative errors conform to the national requirements for COD Cr determination.These experimental results demonstrated that low concentration oxidant method is feasible for the determination of low COD Cr in high chlorine wastewater when Cl − concentration is not more than 20000 mg•L −1 and COD<100 mg•L −1 .

Determination of High CODCr in Simulated High Chloride Oilfield
Wastewater by Low Concentration Oxidant Method It can be inferred from the above experimental results that the determination of high COD Cr in high chlorine wastewater is also possible by low concentration oxidant method.In order to verify this inference, a series of simulated high chlorine oilfield wastewater samples with theoretical COD Cr values of 500, 1000 and 3000 mg•L −1 , and Cl − mass concentrations of 5000, 10,000, 50,000, 10,000 and 200,000 mg•L −1 were prepared by standard KC  As can be seen from Table 6, the low concentration oxidant method provides relatively small errors for the determination of high COD Cr in high chlorine wastewater, and the obtained results are accurate and reliable.The above experiments proved that it is feasible to determine COD Cr (including both low COD and high COD) in high chlorine wastewater by low concentration oxidant method.Therefore, in the next part, we intend to extend the low concentration

3. 1 . 3 . 1 . 1 .
Interferences of Cl − on CODCr Determination in Simulated High Chloride Oilfield Wastewater Determination of High CODCr in Simulated High Chloride Oilfield Wastewater by Classical K2Cr2O7 Method A series of standard water samples with a theoretical COD Cr value of 500 mg•L −1 and Cl − mass concentrations of 100, 500, 1000, 2000, 4000, 8000, 10,000, 15,000 and 20,000 mg•L −1 were prepared as simulated high chloride oilfield wastewater by using standard KC 8 H 5 O 4 and standard NaCl.According to the classical K 2 Cr 2 O 7 method, 20 mL of each water sample was transferred into a COD digestion tube, then 0.4 g HgSO 4 and 30 mL AgSO 4 -H 2 SO 4 were added to the sample solution, and finally the mixture of each water was oxidized with 10 mL of 0.25 mol•L −1 K 2 Cr 2 O 7 standard solution.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by corresponding 0.l mol•L −1

HgSO 4 [ 1 K 2
m(HgSO 4 ): m(Cl − )=10:1] were added to the sample solution, and finally the mixture of each water was oxidized with 10 mL of 0.25, 0.1 and 0.025 mol•L −Cr 2 O 7 standard solution, respectively.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by corresponding 0.1, 0.04 and 0.01 mol•L −1 [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution, respectively.The obtained COD Cr results in the prepared water samples are plotted in Figure 1.

Figure 1 .
Figure 1.Effect of oxidant concentration on the oxidation of Cl − .

COD
Cr value of 500 mg•L −1 and Cl − mass concentrations of 1000, 2000, 4000, 8000, 15,000 and 20,000 mg•L −1 were prepared by standard KC 8 H 5 O 4 and standard NaCl.After 10-folds dilution, 20 mL of each water sample was transferred into a COD digestion tube, then different amounts of HgSO 4 based on the mass ratio of HgSO 4 :Cl − together with 30 mL AgSO 4 -H 2 SO 4 were added to the sample solution, and finally the mixture of each water was oxidized with 10 mL of 0.25 mol•L −1 K 2 Cr 2 O 7 standard solution.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by corresponding 0.1 mol•L −1 [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution.The obtained COD Cr results in the prepared water samples are summarized in Table

High Chlorine Oilfield Wastewater by Low Concentration Oxidant Method 3 . 3 . 1 .
Determination of Low CODCr in Simulated High Chloride Oilfield Wastewater by Low Concentration Oxidant Method A series of simulated high chlorine oilfield wastewater samples with theoretical COD Cr values of 50 and 80 mg•L −1 , and Cl − mass concentrations of 1000, 2000, 4000, 8000, 15,000 and 20,000 mg•L −1 were prepared by standard KC 8 H 5 O 4 and standard NaCl.According to the classical K 2 Cr 2 O 7 method, 20 mL of each water sample was transferred into a COD digestion tube, then appropriate amounts of HgSO 4 [m(HgSO 4 ) : m(Cl − )=10:1] and 30 mL AgSO 4 -H 2 SO 4 were added to the sample solution, and finally the mixture of each wastewater was oxidized with 10 mL of 0.25 mol•L −1 , 0.1 mol•L −1 and 0.025 mol•L −1 K 2 Cr 2 O 7 standard solution, respectively.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by corresponding 0.1, 0.04 and 0.01 mol•L −1 [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution, respectively.The obtained COD Cr results in the simulated high chlorine oilfield wastewater samples are summarized in Table 8 H 5 O 4 and standard NaCl.After proper dilution, 20 mL of each water sample was transferred into a COD digestion tube, then appropriate amounts of HgSO 4 [m(HgSO 4 ): m(Cl − ) = 10:1] and 30 mL AgSO 4 -H 2 SO 4 were added to the sample solution, and finally the mixture of each wastewater was oxidized with 10 mL of 0.025 mol•L −1 K 2 Cr 2 O 7 standard solution.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by 0.01 mol•L −1 [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution.The obtained COD Cr results in the simulated high chlorine oilfield wastewater samples are summarized in Table6.

Table 1 .
[(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution.The obtained COD Cr results in the prepared water samples are summarized in Table 1.Results of determination of high chlorine and high COD (500 mg•L −1 ) standard water samples by classical K 2 Cr 2 O 7 method.

Determination of Low CODCr in Simulated High Chloride Oilfield Wastewater by Classical K2Cr2O7 Method
4 g HgSO 4 , resulting in the obtained concentration of COD Cr in water samples increased gradually, and the differences between the experimental values and theoretical ones also increased gradually.All the above results indicated that pseudo COD Cr was introduced during the determination of COD Cr in high chloride wastewater by the classical K 2 Cr 2 O 7 method.As a result, if the interferences of Cl − cannot be effectively eliminated, the obtained inaccurate COD Cr results will cause wrong expert conclusion for water quality.A series of standard water samples with a theoretical COD Cr value of 100 mg•L −1 and Cl − mass concentrations of 100, 500, 1000, 2000, 4000, 8000, 10,000, 15,000 and 20,000 mg•L −1 were prepared as simulated high chloride oilfield wastewater by using standard KC 8 H 5 O 4 and standard NaCl.According to the classical K 2 Cr 2 O 7 method, 20 mL of each water sample was transferred into a COD digestion tube, then 0.4 g HgSO 4 and 30 mL AgSO 4 -H 2 SO 4 were added to the sample solution, and finally the mixture of each water was oxidized with 10 mL of 0.25 mol•L −1 K 2 Cr 2 O 7 standard solution.After heating reflux for about 2 h, the residual content of K 2 Cr 2 O 7 was back titrated by corresponding 0.1 mol•L −1 [(NH 4 ) 2 Fe(SO 4 ) 2 •6H 2 O] standard solution.The obtained COD Cr results in the prepared water samples are summarized in Table2.

Table 2 .
Results of determination of high chlorine and low COD (100 mg•L −1 ) standard water samples by classical K 2 Cr 2 O 7 method.
− mass concentration in water samples, not only the COD Cr values determined by the classical K 2 Cr 2 O 7 method increased gradually, but also the differences between the experimental values and theoretical ones increased gradually.And the relative errors obtained in low COD water samples are more remarkable than those of high COD water samples as described in Section 3.1.1,whichis mainly due to the higher relative concentration of Cl − in low COD water samples than those in high COD water samples under the same mass concentration of Cl − .Therefore, it is very necessary for pollutant discharging companies, environmental monitoring and legislation departments to develop a method for the accurate determination of COD in high chloride wastewater.At present, the classical K 2 Cr 2 O 7 method (GB 11914-89) is used to determine COD in water sample by concentrated H 2 SO 4 -assisted heating reflux.During the reaction process, not only organic compounds but also Cl − in water sample were oxidized by K 2 Cr 2 O 7 .The interferences of Cl − are mainly attributed to the fol-Regarding water sample containing Cl − , apart from the reaction between organic compounds and K 2 Cr 2 O 7 , the Cl − can be also oxidized by K 2 Cr 2 O 7 and thus consumes some oxidants, which will affect the accuracy of COD determination.The corresponding reaction equation about Cl − in the water sample can be ex-In the mixture system of H 2 SO 4 -K 2 Cr 2 O 7 , acidified K 2 Cr 2 O 7 can just oxidize most of the organic compounds in the water sample.Only after adding catalyst Ag 2 SO 4 , can the straight chain aliphatic compounds be completely oxidized, which increases the oxidation rate of organic compounds and makes the oxidation reaction proceed more completely.However, if the water sample contains •L −1 , the relative errors of COD Cr determined by the classical K 2 Cr 2 O 7 method almost conformed to the allowable error range required by the Chinese government.But with the increase Open Journal of Yangtze Gas and Oil of Cl

Table 3
revealed that under the same condition of COD Cr determination, different concentrations of oxidants exhibit almost the same oxidation ability on the standard KC 8 H 5 O 4 , and the measured COD Cr values are nearly consistent

Table 4 .
Effect of mass ratio of HgSO 4 :Cl − on COD Cr determination in high chlorine water samples.

Table 6 .
Determination results of high chlorine and high COD simulated oilfield wastewater by low concentration oxidant method.