Application Of Flow-injection Combined With Ultrasonic Technique For Rapid Determination Of COD In Complex Wastewater

The novel analysis method based on the flow injection combined with ultrasonic technique has been widespread applied in environmental monitoring field.The core component of this technique is its flow injection analysis.However due to the complication of the water samples some problems including lower detected value and jammed line of flow injection analyzer often occurred during the contaminant determination in the wastewater.The particle and oil in the complicated water samples can be broken up and emulsified by ultrasonic technique before entering into the flow injection analyzer.So the processing steps was not only relatively simple,the accuracy of the experimental results was also greatly improved.In this study,the determination of COD in the sample after external and internal ultrasonic treatment for the complex matrix sample using ultrasonic technique is studied.The basic research contents:(1)Determination of COD in the water by the national standard method: COD value in water samples was detected by the classic national standard method,at the same time,precision and the spiked standard recovery experiments was carried out.The detected value using national standard method was regarded as the reference value of determination by the instrument.The results were accurate and good reproducible with the relative standard deviation of 1.53%,the spiked standard recovery was in the range of 97.0%~109.5%.However the national standard method was time-consuming and high consumption of agentia and energy,which was difficult to achieve batch analysis.(2)Determination of COD in the water by continuous flow-injection method: While COD value was determined by continuous flow-injection analyzer,the precision and spiked standard recovery experiments were also carried out.Comparing with the national standard method,this method can effectively reduce human error,and improve the precision and addition recovery with the relative standard deviation of 0.08 % and the recovery range of 96.6%~104.9%.Therefore,the determination of COD in water samples using flow injection method was conducted with simple treatment,rapid detection,a small amount of water samples and reagents and higher measurement accuracy.But the detected value was lower than the actual value due to the short heating time and incomplete digestion.Moreover,the syringer could not reach the bottom of water samples,which would cause large error once precipitate appeared.(3)COD in the water samples was measured using the flow-injection analyzer after ultrasonic shock in ultrasonic cleaner and subsequent ultrasonic emulsification in automatic sampler.The power of ultrasonic cleaner was set at 300 W,and ultrasound 5 min with the ultrasonic frequency of 45,80 and 100 KHz,respectively(15min),then ultrasound emulsification 8 s through automatic sampler with power of 200 w and ultrasonic frequency of 200 KHz.Then the sample was introduced into the continuous flow-injection analyzer.The detected value was basically the same as the national standard methods in all samples if not indicated.In comparison,relative error of COD value was in the range from-1.88% to 2.08%,from-6.75% to 1.54% and from-2.91% to 5.07% in sewage,industrial wastewater and surface water,respectively.The results indicated that this novel approach possessed simple treatment,higher precision,repeatability and batch analysis within a short time.Finally,the method was successfully applied for rapid determination of COD in complex matrix water.