Determination of oil content in wastewater by ultraviolet spectrophotometry

Zhan Huiying
(College of Chemical Engineering, Gansu Lianhe University, Lanzhou, 730000, China)

AbstractIn this paper, the analytical method of measurement oil content in waste water using UV-spectrum was introduced. When the detection wavelength was selected as 255nm, we got a linear regression equation. The standard deviation is 1.91% and the detection limit is 0.043mg/L, so the precision and sensitivity are high and the detection limit is relatively low. The petroleum ether was used solvents, because its solvency is strong, its source is relatively wide and its toxicity is small. It showed that the method of measurement oil content in waste water using UV-spectrum is simple, feasible and reliable.
Keywords UV-spectrum ; oil in waste water; Measurement

In water quality monitoring, oils are an important item, because oils in water are easy to form films which float in the water surface, obstruct the exchange between the oxygen in water and atmosphere. Scattering oil, emulsifying oil ,soluble oil and oil-solid resulted from the absorption of elaioplast to the surface of suspended solids in the water were easy to oxidative decomposition by microorganisms and thus deteriorated water quality. Determination of oils in wastewater was complicated, due to their complex components and no single standard to refer, besides oil sorts varied in wastewater imposed by different regions and industries. Therefor, determination of oils in wastewater was an old, cardinal and tough issue in environmental analytical chemistry. Currently, routine analysis included gravimetric method, ultraviolet spectrophotometry, undispersed infrared method, fluorophotometry and etc. Gravimetric method is a traditional determination with complicated performance and less sensitivity, and the precision varies greatly with the operating condition and proficiency. Oils in the wastewater determined by ultraviolet spectrophotometry is easy to perform with a good precision and sensitivity, a broad measurement range as well.

1.1 Experimental principle
Determination of oils in wastewater by ultraviolet spectrophotometry was based on characteristic absorption of aromatic compounds with conjugated bond and benzene ring in oils in ultraviolet region. Oil content in wastewater was obtained by determination of such substances with conjugated double bonds.
Theoretical basis of the present method was Lambert – the Bill law as follows:
A =bC
Where A is absorbance;is absorption coefficient; C is concentration; b is optical path.
1.2 Reagents and apparatus
UV-3400 UV-VIS Spectrophotometer(Japan); THZ-C desktop constant temperature oscillator(Jiangshu, China); automatical analysis balance(Shanghai, China).
Dearomatization petroleum ether(boiling point from 60~ 90℃, analytical pure, Beijing, China); anhydrous sodium sulfate(Dried at 300℃); sulfuric acid(9mol/L); NaCl solution(8% volume ratio).
Standard oil: petroleum ether distilled at 30-60℃was redistilled by dearomatization, oils in the samples were extracted by the resultant petroleum ether, and then dehydrated by anhydrous sodium sulfate and filtrated. Filtrates were steamed at the boiling water bath at 65±5℃to eliminate petroleum ether, further steamed in an incubator at 65±5℃, and then standard oil was obtained. Due to the difficulty of extraction of oils from wastewater, in the present paper, we used 15# engine oil as standard oil.
Standard oil stock solution(1.662mg/mL): accurate 83.10mg standard oil was dissolved by petroleum ether until a constant volume of 50ml.
Water samples were taken from wastewater in small west lake section of Yellow River.
1.3 Experimental methods
25ml accurate three parallel water samples were added to 100ml conical flasks with the addition of 1.0 ml standard solution, acidified by 0.5mL 9mol/L sulfuric acid, and then agitated completely in constant temperature oscillator for 10min with the addition of several drops of NaCl and 25ml petroleum ether. Resultant solutions were transferred into separating funnel, and extracted until layered. Aqueous phase repeated twice, once 10ml. Organic phase was merged and transferred into sand separating funnel spreaded by 5mm anhydrous sodium sulfate and collected in 50ml volumetric flask. Funnel was rinsed by petroleum ether until attaining a constant volume of 50mL. Absorbency was measured at 255nm, after blank value calibration, concentration was obtained using marker fitting equation. Oil content in water samples was calculated by quantitative relationship.

2 Results and discussions
2.1 Wavelength selection
Oils and its products have characteristic absorption in ultraviolet region. Primary absorption wavelength of aromatic compounds with conjugated bond and benzene ring is 250-260nm and 215-230nm, respectively. Two absorption wavelengths of ordinary crude oils are 225nm and 254nm. Due to different samples with different absorbency, incident wavelength should be carefully selected. Spectra was obtained by ultraviolet scanning(Figure 1) and maximum absorption peak was found. As seen from figure 1, detected samples have obvious absorption at the wavelength of 255nm, considering the principle of larger absorption and less interference, and thus 255nm wavelength was selected to further investigation.


Figure 1. Ultraviolet absorption spectra of the measured oil

2.2 Standard curve
Standard oil solutions ranged from 0 to 2.0ml at increments of 0.2 ml was added to 11 25ml colorimetric tubes respectively, diluted by petroleum ether until attaining a constant volume of 10mL, and then measured the absorbency at 255nm using 1cm quartz absorption cell and petroleum ether as control. Standard curve was depicted as Figure 2. Its linear fitting equation and correlation coefficient was as follows:
A = bC + a = 1.3296C + 0.0017, r = 0.9970.


Figure 2. Standard curve

2.3 Water sample treatment
Oil content in water samples was calculated by quantitative relationship, and results were listed in Table 1. As seen from Table 1, relative error of three parallel water samples was 0.24%,0.49% and 0.24%, respectively with a great precision.

Table 1 Oil content in water

absorbency (A) 0.2030 0.2024 0.2032
concentration in extraction (mg/mL) 0.0537 0.0536 0.0538
concentration in water (mg/L) 40.9 40.8 41.1

2.4 Precision and detection limit
8 blank solutions were measured at 225nm and relative standard deviation was 1.91 %. Detection limit was calculated as 3 times standard deviation of reagent blank. Through calculation, least detection limit was 0.043mg/mL.

In the present paper, oils in the wastewater were determined by ultraviolet spectrophotometry with the properties of easy operation, good precision and sensitivity, abroad measurement range and lower detection limit. Solvent used was petroleum ether which has strong solubility, extensive source and less toxicity. However, standard oil was tough to obtain, and thus data had less comparability, which was unlike other monitoring projects with uniform standard substances prepared for control measuring.