Experimental Studies To Determine the Fate of Petroleum Hydrocarbons from Refinery Effluent on an Estuarine System Anthony H. Knap"
Bermuda Biological Station, St. Georges West 1-15, Bermuda Peter J. LeB. Wllliams
Department of Oceanography, The University, Southampton SO9 5NH, England Laboratory experiments were undertaken to investigate the biological and physical fates of refinery effluent once discharged to Southampton Water estuary. Refinery effluent was incubated in glass containers and processes such as biodegradation, evaporation and bubble ejection, and adsorption of hydrocarbons by estuarine sediments were investigated. The most important removal mechanism within the time scale of the estuarine system appears to be adsorption of petroleum hydrocarbons to estuarine sediments. In the experimental systems, 70% of the hydrocarbons originally in the water column were found in the sediments after 1 h. The rate-determining factor appears to be initial concentration of the starting material. Compounds of high water solubility are adsorbed to a lesser extent than the less soluble compounds.
Introduction The occurrence of fossil fuel hydrocarbons in the marine environment has been well documented (1). They enter the environment as a consequence of off-shore production, transportation, urban runoff, atmospheric fallout, rivers, and coastal refineries. Although much of the earlier work has dealt with major oil spills, (2, 3) chronic continuous inputs, such as municipal and industrial effluents to estuaries and near-shore areas have received increasing attention (4,5). Estuaries have traditionally been receptacles for industrial and municipal wastes, therefore processes that act on these waste materials are important to the overall health of these important ecosystems. Southampton Water is a major estuary in the United Kingdom. I t has an area of 20 km2,a mean depth of 6 m, and a fresh water flushing time in the region of 7 days. It handles about 3.0 X lo7 tons of shipping annually, twothirds of which is bearing oil. It is, in addition, part of the principal water sport recreation area in the United Kingdom. It is also the site of the Esso petroleum refinery at Fawley, which has the largest refining capacity in the U.K. In previous work designed to improve our understanding of the carbon cycle in the estuary, it was concluded that much of the organic material in the estuary over the winter months comes from industrial discharges, the largest of which is from the refinery (6). The refinery discharges about 4.5 X lo8 L of seawater waste per day derived from cooling and processing. Although the API separators 0013-936X/82/0916-0001$0i.25/0
Table I. Identification Criteria for TLC Analyses' TLC major compd fraction R f value color (example)
1 2 3 4
5
0.78-1.00 none alkanes, cycloalkanes 0.30-0.77 quenching olefins, 1- and 2-ring aromatics (naphthalenes) 0.15-0.29 purple 3-ring aromatics (phenanthrenes) 0.05-0.14 light blue 4-ring aromatics (chrysene) 0.00-0.04 blue 5-ring and greater aromatics (3,4-benzopyrene)
a TLC conditions were 0.25-mm silica gel glass plates, developed in isooctane.
within the refinery complex remove much of the heavy and floating oils in the wastewater, some dissolved, dispersed, and particulate adsorbed material is discharged to the estuary. In 1973, the estimated daily oil discharge from the refinery was about 20 tons (7). The nonvolatile content of the effluent, in a recent study (8),was found to be about 2 tons. In order to understand the impact of this discharge to the estuary, we felt it was necessary to gain insight into some processes that would govern the fate of hydrocarbons in the effluent once it entered the estuary. This report deals with a series of laboratory experiments designed to put into perspective the various biological, chemical, and physical processes that would act on the refinery effluent close to the point of discharge.
Methods The methods used were designed to give information on compound types rather than individual components. These methods are discussed in detail elsewhere (9). Briefly, water samples were extracted with an organic solvent (pentane) and sediment samples subjected to alkaline digestion. Hydrocarbons were isolated by column chromatography (10) by elution with pentane:benzene (4:l). Thin-layer chromatography (TLC) was used as the major separation step with visualization of five major fractions at 254 and 366 nm (Table I). Identification of the fractions was based on the behavior of known stand-
0 1981 American Chemical Society
Envlron. Scl. Technol., Vol. 16, No. i, I982
1
Table 11. Biodegradation and Evaporation Experiments with Refinery Effluenta 40-day sample TLC 0 4 7 40 as % of fraction day day day day control control 1 420 265 225 200 265 75 2 208 138 66 74 155 48 3 101 68 70 68 70 97 4 35 30 34 15 25 60 5 18 15 18 15 18 83 total 782 516 413 376 533 70 a Static system without aeration. Control was 40-day sample that had been initially poisoned with HgC1,. All values as Gg of C/liter. amt degradation
___
ards, as well as scanning UV and gas chromatography/ mass spectrometry (GC/MS) criteria (11). The GC/MS analyses were carried out on a Finnegan 32003-6110 MS with Grob injector and capillary OV 1 column. Hydrocarbons were identified by examination of their mass spectra and by mass fragmentography. The silica gel was removed from the TLC plates by suction into microcolumns, and the hydrocarbons were eluted with benzene. Subsamples were used for GC and carbon analysis (9). Gas chromatography was performed on all samples by means of temperature-programmed gas chromatography with packed columns. All quantification was carried out in a carbon analyzer, therefore all results are in terms of extractable hydrocarbon as carbon in each TLC fraction (Pg of C/L). The experiments were carried out in 5-L glass containers at 15 "C in the dark. The refinery effluent was sampled in bulk, allowed to equilibrate in a sealed vessel for 8 h, and then subsampled by siphon. The siphoning procedure enabled a sample to be collected without the inclusion of either sedimentary material or surface film. At each time period of the experiment, the contents of the beaker were extracted including the removal of material that may have been adsorbed to the sides of the vessels. Three replicates gave a coefficient of variation of 4%. In some cases microbiological activity was arrested in the controls by the addition of 1 mL of saturated mercuric chloride (HgClJ per liter. Bacteriological plate counts ensured that this procedure was effective. Results and Discussion Different experimental conditions were used to determine the differences between biodegradation, evaporation, and adsorption of petroleum hydrocarbons to sediment. These will be dealt with separately. Biodegradation. An experiment was carried out to investigate the possible role of biodegradation on the hydrocarbons present in the estuary as microbial modification of petroleum hydrocarbons is a well-documented fate (12). It was run over a 40-day period and the petroleum hydrocarbon content measured at zero time, 4,7, and 40 days. A sterile control was used in order to offset losses due to evaporation. The results of this experiment (Table 11) indicate that bacterial degradation of refinery hydrocarbons occurs rapidly. The hydrocarbon levels were reduced to 70% of the control after 40 days. The rates in our experiment are somewhat higher than rates found by others (13). This may be due to favorable conditions for microbial growth present in the estuary and hence in out experimental containers. The nutrients and oxygen are abundant in the estuary (8). Also hydrocarbon 2
Environ. Sci. Technol., Vol. 16, No. 1, 1982
Table 111. Removal of Petroleum Hydrocarbons by Evaporation vs. Aerationa amt removed aerated as % loss over stirred, aerated, TLC static, 24 h stirred tank fraction Oh 24 h 1 2 3 4 5 total
292 135 47 24 26 544
226 121 43 20 25 443
93 43 11 16 12 191
59 65 77 20 52 57
a Static system without aeration or stirring. Stirred tank stirred at 300 rpm with magnetic stirrer. Aerated tank bubbled with air at 200 mL/min through a glass sinter. All samples were treated with HgCl,. Values as pg CIL.
degraders are more likely to be found in high numbers in areas that have been subjected to previous instances of oil pollution (14). In polluted coastal waters 50% of the isolated bacteria have been able to oxidize oil (15). Therefore, it is quite possible that an estuary such as Southampton Water, which has been exposed to a continuous oil input from the refinery for the past 50 years, would have a substantial population of oil degraders present. Our work bears this out and most of the degradation takes place in the lower molecular weight aliphatic fractions. This is consistent with the observation that n-alkanes are degraded in preference to other compounds (2,16,17). The gas chromatography showed no preference for degradation of n-alkanes being odd or even chained. There was also no evidence to suggest that there was any extensive microbial decomposition of the nonvolatile aromatic fraction of the effluent. The breakdown of aromatic compounds is known to occur at a slower rate (18); our results bear this out. Although microbial degradation appears to be important, in the long term it has less bearing on petroleum hydrocarbons in Southampton Water estuary. The residence time of water near the refinery is about 1-2 days, therefore most of the refinery effluent would be flushed to sea prior to extensive degradation. However, we do have evidence that biodegradation is important in modification of sediment-adsorbed hydrocarbons (19) and have suggested that it is an important process for the removal of low molecular weight hydrocarbons (8). Evaporation. In order to investigate the process of evaporation, two experiments were carried out with sterile seawater. One was a long-term control of one of the biodegradation experiments over 40 days (Table 11). The other was carried out over 24 h, taking into account the time scale of the estuary at the point of discharge. The second experiment also took into account the difference between normal evaporation using stirred refinery effluent and bubble ejection using aerated refinery effluent through a glass sinter at 200 mL/min for the same time period. The difference in the two processes is illustrated in Table 111. After 24 h 15% of the initial hydrocarbon material is removed by evaporation whereas aeration is responsible for a 60% decrease in the hydrocarbon concentration. Analysis of individual n-alkanes by gas chromatography indicated that there was a specificity of removal due to molecular weight. Evaporation was responsible for the removal of n-alkanes below carbon number 18 but little detectable loss of the higher molecular weight compounds, whereas the bubble ejection process stripped hydrocarbons from the seawater irrespective of molecular weight.
Table IV. Removal of Low and High Concentrations of Petroleum Hydrocarbons of Refinery Effluent by Estuarine Sedimentsa a. Low Concentration 0 1.0 4.5 18.0 4.80 incubation time, h matrix water water water water water 250 151 59 nd total 1364 hydrocarbon content b. High Concentration incubation 0 0 1.0 1.0 48.0 48.0 time, 11 matrix water sedmt water sedmt water sedmt total 3231 249 206 2515 39 2511 hydrocarbon content ' All values as pg C/L for water and pg of C/gram dry weight for sediment. Sediment stirred a t 300 rpm, water poisoned with HgC1,. nd denotes not detectable within experimental limits (
