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This paper presents spectra of light absorption coefficient of oil-in-water emulsion, derived using the Mie theory. In order to achieve that concentration of oil, degree of oil dispersion in seawater as well as spectra of light absorption coefficient and refractive index of chosen oils must be known. A significant role of the size distribution of oil droplets has been revealed: light absorption coefficient of emulsion increases with the rate of dispersion.
©2007 Optical Society of America
1. Introduction
Various dissolved and suspended substances give various contributions to light absorption in the bulk of natural waters. They are responsible for the modification of spatial and spectral distribution of the radiance in seawater. Also spatial distribution of sun light reflectance, and consequently above water upward radiance field, depends on optical features of seawater constituents [1]. Numerous substances that absorb light in seawater were described in a monograph by Dera and Wozniak [2]. This monograph also presents the data on spectra of absorption coefficient of various oil substances, as well as describes selected studies on influence of oil pollution on marine optics.
Oil, after emulsification in the bulk of seawater, becomes one of light absorbents and participates in the light field formation in the marine environment. The question is if a spectrum of light absorption coefficient of fresh oil (before dispersion in a bulk of water) has a shape similar to the spectrum of light absorption coefficient of the same oil, but in oil-in-water emulsion form. Therefore, the theoretical analyses of shapes of absorption spectra of oil-in-water emulsion were carried out. In the derivation the Mie theory [3] was implemented using various oil droplets size distributions which were previously determined experimentally.
2. Size distributions and optical properties of oil droplets in oil-in-seawater emulsions
The problem regarding the type of the function which can correctly describe size distribution of oil droplets in natural sea environment still exists, and is difficult to solve due to lack of reliable data from direct measurements in the marine environment. Thus laboratory experiments are conducted with natural seawater (salinity 7 PSU) and with chosen crude oils. Two types of crude oils, which differ in optical features have been used. Those were: Romashkino and Petrobaltic [4]. Oil emulsion was obtained through oil in water steering for 1 hour (rotation rate of the agitator was 900 rpm), and further stored at a temperature of 10°C. Samples of emulsion were analyzed after various time-periods - namely size distribution of oil droplets was assessed, absorption coefficient and refractive index for various wavelengths were measured.
Microscopic picture of oil droplets altered with time – a number of great droplets got considerably smaller (Fig. 1).
Exemplary changes of oil droplet size distribution in the chart are shown in Fig. 2, where the curves depict parameterized functions of size distribution. Exponential-logarithmic function (1) as the function describing size distribution was used. This is a three-parameter function and is sometimes rather inappropriately called a lognormal distribution [5] (a lognormal distribution is similar to exponential-logarithmic one, but multiplied by a factor r-1).
where:
A - quantity parameter
ro - radius for maximum of distribution
σ - parameter of a shape of distribution (values of both ro and σ are in Tab. 1 listed)
Table 1. Parameters of size distribution of oil-in-water emulsion after various time-periods of ageing.
Mainly coagulation and flotation processes cause temporal changes in a shape of oil droplet size distribution, moreover oil clinging to suspended matter or even biodegradation processes can also take place. It is worth to notice, that relatively stable suspension of oil droplets in seawater occurs when parameter ro is less than 0.125 μm. One can expect that the intensity of the dynamic processes of sea surface also influence the size of oil droplets in the bulk of water. However, practical knowledge on the relations between the sea surface state and oil emulsification rate is poor. Chemicals and minerals implemented during oil spillage combat also play a significant role [6] in oil fragmentation, dispersion and spreading in a bulk of water. Values of size distribution for radiuses from 0.025 μm to 50 μm (that is 2,000 radiuses) were applied in computations (using the above mentioned Mie solution).
Fig. 2. Normalized size distributions of oil droplets in the oil-in-seawater emulsion. Curves reflect the expression (1) containing parameters ro and σ which in Tab. 1 are listed. Thick gray lines represent range of oil droplets radiuses measurable by microscopic method (to extract the data for size distribution parameterization).
Both refractive index and absorption coefficient of oil (from oil-in-water emulsion removed by centrifuge method) were relatively time-stable (values of that magnitudes for various wavelengths are listed in Tab. 2). The first one - refractive index n - was measured using the Abbe refractometer in various wavelengths from 420 nm to 680 nm (then extrapolated up to the range 350 nm – 750 nm), whereas second one - absorption coefficient a – was measured with the UV-VIS spectrophotometer in 1 cm quartz cuvette for various concentrations of oil in hexane solutions. This facilitated the extrapolation of the obtained values to 100% of oil concentration.
Table 2. Optical parameters of two types of oil: Petrobaltic and Romashkino
3. Results
The contribution of oil droplets in a total light absorption coefficient of seawater is presented in Tab. 3. The table includes the values of absorption coefficient for two types of crude oil (Romashkino and Petrobaltic) obtained for various wavelengths.
Table 3. Values of absorption coefficient of oil-in-water emulsions in seawater for volume concentration equal to 1 ppm. Parameters of size distribution (ro and σ from expression 1), are listed in Tab. 1.
The content of the Tab. 3 is visualized in Fig. 3 as 3D charts, for both Romashkino and Petrobaltic crude oils combined in the same coordinates. A space visible between the upper and the lower “chart-surfaces” relates to the greatest types of oil substances that are transported in seas and oceans, or are in use as exploitive materials by merchant or navy ships.
Fig. 3. Absorption coefficient of oil-in-water emulsion (1 ppm) as a function of two variables: wavelength and time. Flow of time is a factor changing parameters of size distribution of oil droplets as in Tab. 2.
4. Discussion
Spectra of light absorption coefficient of oil-in-water emulsion depend, above all, on light absorption spectrum of oil (from which emulsion was made), but also on oil droplet size distribution. This has been revealed in this paper. There is a general tendency which involves the increase of absorption coefficient when maximum of size distribution moves to small droplet radiuses. This phenomenon is very clear for short light wavelengths in which oils manifest high absorption coefficients. Analogous phenomenon is indicated in the ocean optics as a packing effect - for example when pigments in phytoplankton cells, or resuspended sediment are considered [2, 7].
The shapes of spectra of absorption coefficient of oil in a form of emulsion differ in comparison with the spectra for oil, which would be hypothetically completely dissolved in the water to the same concentration as the oil emulsion. This phenomenon is presented in Fig. 4, where spectra of absorption coefficient of crude oil - in both forms (dispersed and hypothetically dissolved) are shown, both for oil concentration of 1 ppm. If relatively transparent oil (Petrobaltic) is considered, emulsification results in an increase of absorption coefficient by several tens of percent, whereas for relatively weakly transparent oil (Romashkino) – the absorption coefficient of emulsion is smaller in comparison with absorption coefficient for hypothetically dissolved oil, but for relatively fresh emulsion only, because appears higher for the aged emulsion (when large droplets disappear).
Fig. 4. Spectra of light absorption coefficient of two types of oil dispersed in seawater (gray areas) and hypothetically dissolved in seawater (broken and dotted lines).
Absorption coefficient of oil-in-water emulsion for defined size distribution depends linearly on oil concentration. Some auxiliary tests revealed that the impact of oil droplet size distribution on light absorption is controlled by refractive indexes of both oil and water. However, this is not a serious effect and thus it can be neglected.
The results presented refer to seawater contaminated with a relatively stable oil emulsion. However, when the sea surface is rough, directly after oil discharge great droplets dominate. Therefore, in the future, once the knowledge of the size distribution of such emulsions is improved, the spectra of absorption coefficient could be derived in the way described in this paper.
Acknowledgments
This paper was prepared as a part of the Gdynia Maritime University grant No. 682/BW/GU/2007.
References and links
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2. B. Wozniak and J. Dera, Light Absorption in Sea Water, (Springer, 2007).
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