Methods for Analyzing Organic Matter in Water - Mineral Oil

Mineral oil in water originates from industrial wastewater and domestic sewage. Petroleum-based pollutants (mixtures of various hydrocarbons) in industrial wastewater mainly come from crude oil extraction, processing, and various refining processes. Mineral oil floats on the water surface, affecting oxygen exchange at the air-water interface; oil dispersed in water can be oxidized and decomposed by microorganisms, consuming dissolved oxygen and deteriorating water quality. Mineral oil also contains highly toxic aromatic hydrocarbons.

Methods for determining mineral oil include gravimetric methods, non-dispersive infrared methods, ultraviolet spectrophotometry, fluorescence methods, and turbidimetric methods.

Gravimetric Method

The gravimetric method is commonly used and is not limited by the type of oil, but it is cumbersome to operate, has low sensitivity, and is only suitable for determining oily water samples larger than 10 ml. The principle of the method is to acidify the water sample with sulfuric acid, extract the mineral oil with petroleum ether, then evaporate the petroleum ether to remove it, weigh the residue, and calculate the mineral oil content.

This method refers to the total amount of substances in water that can be extracted by petroleum ether; it may contain heavier petroleum components that cannot be extracted. Evaporation to remove the solvent also results in the loss of lighter oils.

Non-dispersive Infrared Method

This method utilizes the characteristic absorption of methyl (-CH₂) and methylene (-H₂O₄H₁₂) groups in the near-infrared region (3.4 μm) of petroleum substances as the basis for determining the oil content in water samples. The standard oil can be petroleum ether extract from water from contaminated sites. Based on the characteristics of crude oil composition in my country, a mixed petroleum hydrocarbon mixture can also be used as the standard oil; its composition is: hexadecane: isooctane: benzene 2 65:25:10 (y/y).

During the determination, the water sample is first acidified with sulfuric acid, then demulsified with sodium chloride, and then extracted with trichlorotrifluoroethane. The extract is filtered through an anhydrous sodium sulfate layer, diluted to volume, and injected into an infrared analyzer to determine its content.

All organic substances containing methyl and methylene groups will cause interference. If animal or vegetable oils and fatty acids are present in the water sample, they should be separated beforehand. Furthermore, some heavier components in petroleum are not melt-soluble in trichlorotrifluoroethane, leading to lower measurement results.

Ultraviolet Spectrophotometry

Petroleum and its products exhibit characteristic absorption in the ultraviolet region. Aromatic compounds with benzene rings have a main absorption wavelength of 250-260 nm; compounds with conjugated double bonds have a main absorption wavelength of 215-230 nm. Generally, crude oil has two absorption peak wavelengths of 225 nm and 254 nm; for light oils and refinery products, 225 nm can be selected.

Water samples are acidified with sulfuric acid, demulsified with sodium chloride, then extracted with petroleum ether, dehydrated, and diluted to volume before measurement. Standard oils are obtained from petroleum ether extracts of water samples from contaminated sites. Different oil products have different characteristic absorption peaks. If it is difficult to determine the measurement wavelength, the absorption spectrum of a standard oil sample between 215-300 nm can be used, and the position of its maximum absorption peak can be adopted. Generally, it is between 220-225 nm.