Determination of oil in wastewater, 1. What are the methods? Similarities and differences and applicable conditions?

I. Methodological principles

Principle of gravimetric method (CJ/T5 1-2004): Acidify the sample with sulfuric acid, extract the oil in the sample with petroleum ether, remove the petroleum ether by evaporation, and then weigh.

The principle of infrared spectrophotometry (GB/T 16488- 1996) is to extract the oil in water with carbon tetrachloride, determine the total extract, then adsorb the extract with magnesium silicate, and determine the oil after removing polar substances such as animal and vegetable oil. The contents of total extract and petroleum are at wave numbers of A2930 cm-1(stretching vibration of C-H bond in CH2 group), A2960 cm-1(stretching vibration of C-H bond in CH3 group) and A3030 cm- 1 (stretching vibration of C-H bond in aromatic ring) respectively. Animal and vegetable oil content is calculated according to the difference between total extract and oil content.

From the principles of the above two methods, it can be seen that the gravimetric method determines the total amount of substances that can be extracted by petroleum ether in the acidified sample and are not volatile during the test. In the process of desolventizing, some light oil will volatilize with it, and there will be obvious losses. Because petroleum ether selectively dissolves petroleum, heavier components in petroleum may contain substances that are not extracted by solvents. Therefore, the petroleum ether extraction gravimetric method is often incomplete and the determination result is low. Moreover, the gravimetric method only determines the total amount of substances that can be extracted by petroleum ether in water, and cannot accurately determine the contents of petroleum and animal and vegetable oils in samples. Infrared spectrophotometry is not affected by the composition and structure of oil. In the infrared absorption spectrum, not only the C-H bond in methylene CH2 group and the C-H bond in methyl CH3 group are considered, but also the C-H bond in aromatic ring, so the judgment of oil products is relatively complete. Moreover, this method uses carbon tetrachloride solvent, which only contains C-Cl bonds, so it will not affect the infrared absorption of the above three C-H bonds. Petroleum and animal and vegetable oils can be accurately determined by this method. It can be seen that infrared spectrophotometry is more suitable for the analysis and determination of oil substances in water than gravimetric method, which is also an improvement of analytical methods.

Two. Scope of application and emission standards of this method

Gravimetric method (CJ/T5 1-2004) is only suitable for the determination of petroleum in urban sewage, and its application scope is narrow. Infrared spectrophotometry (GB/T 16488- 1996) is suitable for the determination of petroleum and animal and vegetable oils in surface water, groundwater, domestic sewage and industrial wastewater. In addition, it can also be used for the determination of kitchen fume in catering industry in environmental monitoring, and its application range is quite wide. In the Integrated Wastewater Discharge Standard of People's Republic of China (PRC) (GB8978- 1996), infrared spectrophotometry is adopted as the standard method to detect oil substances. In China People's * * and National Urban Construction Industry Standard "Water Quality Standard for Sewage Discharged into Urban Sewers" (CJ3082- 1999), gravimetric method and infrared spectrophotometry are respectively used as standard methods for detecting oil substances.

Different methods for determining oil substances have different emission standards. See table 1 for emission standards.

Table 1 emission standard

Discharge standard number of pollutants

Discharge standard value (mg/L)

CJ 3082- 1999

oil

100

mineral oil

GB8978- 1996

pollutant

Reference object

Sub-benchmark

Three-level standard

petroleum

Animal and vegetable oil

100

Three. Extraction solvent

Petroleum ether solvent is used for gravimetric extraction. The boiling range of this solvent is 30℃-60℃, which is very volatile and flammable, and its vapor can form an explosive mixture with air. Therefore, this method can not be used when the temperature exceeds 30℃, which brings great limitations to the monitoring of urban sewage. The solvent used in infrared spectrophotometry extraction is carbon tetrachloride, which is an excellent solvent for oil and fat, with a boiling point of 76.5℃, and will not be limited by external temperature when used. Infrared spectrophotometry requires high purity of carbon tetrachloride, and sometimes the blank values of carbon tetrachloride in different batches are quite different. Therefore, when there are many samples in the same batch, several bottles of carbon tetrachloride should be mixed to reduce the influence of the change of carbon tetrachloride blank value on the final determination results. However, it must be noted that carbon tetrachloride is a toxic solvent, long-term use will affect the health of operators, and excessive inhalation will cause poisoning, so it must be operated in a well-ventilated environment.

Four. operating procedure

When determining samples by gravimetric method, the operation time is long, the method is complex, and the determination error of samples with low oil content is large, but its determination cost is relatively low. The determination of samples by infrared spectrophotometry is simple, rapid and mature. At present, there are many infrared oil meters with high degree of automation at home and abroad, with simple operation, high analysis efficiency and high accuracy.

Verb (abbreviation for verb) detection range

The detection limit of gravimetric method is 5 mg/L, and the error of samples less than 5mg/L is large. The detection limit of infrared spectrophotometry can reach 0. 1mg/L, which is the most remarkable advantage of infrared spectrophotometry.

Accuracy of intransitive verbs

For gravimetric method (CJ/T5 1-2004), there is no special reference material to determine its recovery rate in China. The method also does not clearly indicate the accuracy that can be achieved. For infrared spectrophotometry, the mineral oil standard specially developed by the Institute of Standard Samples of the State Environmental Protection Administration can be used to determine the accuracy of the test results by measuring the recovery rate of standard samples and the recovery rate of standard samples.

The following is the determination of standard recovery rate and standard recovery rate of mineral oil. See Table 2 and Table 3 for the determination results.

Table 2 Determination of recovery rate

Determination sequence 1

three

four

five

Standard value (mg/L)

5.55

10.4

29.8

56.9

74.5

Measured value (mg/L)