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

摘要

In this work, in order to find a method for the accurate determination of chemical oxygen demand (COD_(Cr)) in high chloride oilfield wastewater, the interferences of chloridion (Cl~(-)) on COD_(Cr) determination were first studied based on the Chinese national standard GB 11914-89. The results showed that the COD_(Cr) values and deviations determined by the classical potassium dichromate (K_(2)Cr_(2)O_(7)) method increased gradually with the increase of Cl~(-) mass concentration in water samples. Then, theoretical and experimental studies on reducing interferences of Cl~(-) on COD_(Cr) determination were carried out. The results showed that different concentrations of K_(2)Cr_(2)O_(7) exhibited different oxidation capacities on Cl~(-), while having little effects on the oxidation of organic compounds. More specifically, when the concentration of K_(2)Cr_(2)O_(7) was 0.025 mol·L~(-1) and the mass ratio of HgSO_(4):Cl~(-) was 10:1, the interferences of Cl~(-) on COD_(Cr) determination were the slightest. Finally, a low concentration oxidant method was proposed for the accurate determination of COD_(Cr) 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~(-1) and COD < 100 mg·L~(-1), the relative prediction errors of COD_(Cr) provided by the proposed method were acceptable, which revealed that the low concentration oxidant method is feasible and reliable for the determination of COD_(Cr) in high chloride oilfield wastewater.