There are a number of parameters that can be measured in sites that have been polluted with oil spillage. This article is dedicated to teaching the knowledge of parameters that can be used to monitor and measure oil pollution.
This article however will not only address the pollution as a problem but will also teach the control of oil pollution as a preventive and corrective solution measure to oil pollution.
Monitored Parameters
The monitoring of oil pollution can be undertaken by the analyses for gross organic component of the water, sediment and soil of the environment where oil pollution has occurred. Additionally the individual organic compounds can be quantified.
Gross/Aggregate Organic Constituents
In this category of oil pollution monitoring, pollution issues considered and monitored include oxygen-demanding substances and organically bound elements.
Parameters like total organic carbon (TOC) and chemical oxygen demand (COD) are used to assess the total amount of organics present in the water, soil and sediment.
Alternatively, biochemical oxygen demand is used to evaluate the gross fractions of the organic matter (biodegradable organics present).
Oil and grease is useful in the evaluation of material extractable from the polluted sample using nonpolar solvents.
Others include dissolved organic halide (DOX) as a measure of organically bound halogens; total petroleum hydrocarbon as a gross measure of crude oil pollution and ultraviolet absorbing organic constituents for measure of UV active compounds in the polluted samples.
The relevance of these parameters to oil pollution and their method of analysis will be discussed.
Biochemical Oxygen Demand
Determination of BOD is an empirical test used to evaluate the relative oxygen requirements of wastewaters (including the wash water used in desalting of crude oil), effluents, and polluted waters.
BOD measures the molecular oxygen used in the biochemical degradation of organic material with a specified incubation period. The oxygen is used by the micro-organism in the processes of metabolising the organic pollutant thereby measuring the gross amount of the organic pollutant.
Apparatus used for the analysis are incubation bottles, air incubator or water bath. The reagents used are phosphate buffer solution (pH 7.2), Magnesium sulphate solution (22.5 g/L MgSO4⋅7H2O), Calcium chloride solution (27.5 g/L CaCl2), Ferric chloride solution (0.25 g/L FeCl3⋅6H2O), and acid and alkali solutions (1M) for neutralization of caustic or acidic water samples, freshly prepared Sodium sulphite solution (1.575 g/L Na2SO3).
Nitrification inhibitor (if needed) – 2-chloro-6- (trichloromethyl) pyridine, freshly prepared Glucose-glutamic acid solution (150 mg glucose and 150 mg glutamic acid dilute to 1 L in distilled water) and Ammonium chloride solution (1.15 g/L NH4Cl,adjust pH to 7.2 with NaOH solution).
Dilution water used for the BOD analysis is made from either of demineralized, distilled, tap, or natural water.
Procedure
The procedure for BOD determination is seeding and dilution procedures. The first step is to prepare dilutions of the sample with the dilution water either in graduated cylinders or volumetric glassware, and then transfer to BOD bottles or prepare directly in BOD bottles.
This is followed by seeding with microbes if it is necessary (i.e. if the microbes are sufficient to degrade the organic matter). The diluted samples are then filled into airtight bottles of the specified size until it overflowing.
This is followed by incubating it at the specified temperature for 5 days. Dissolved oxygen (DO) of the sample is measured at day 1 before incubation and at day 5 after incubation.
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The BOD is computed from the difference between initial and final DO. In case the required oxygen demand may be more that the DO, the sample is diluted with aerated water to ensure accurate result. It is also seeded to enhance microbial activity.
Chemical Oxygen Demand
Chemical oxygen demand (COD) is the amount of a specified oxidant (particularly potassium dichromate) that reacts with the sample under controlled conditions. The quantity of oxidant consumed is expressed in terms of its oxygen equivalence.
COD is used as a measurement of gross pollutants in wastewater and natural waters. It is related to other gross measurements like biochemical oxygen demand (BOD), total organic carbon (TOC), and total oxygen demand (TOD).
As earlier stated, BOD is a measure of oxygen consumed by microorganisms under specific conditions while TOC is a measure of organic carbon in a sample; TOD is a measure of the amount of oxygen consumed by all elements in a sample when complete (total) oxidation is achieved. Thus, it is possible to correlate the values of these parameters for a given sample.
COD determination is based on the principle that most types of organic matter can be oxidized by boiling it in a mixture of potassium dichromate and sulphuric acids.
Thus, a polluted sample (with crude oil spill or other industrial effluent) is refluxed (for 2-h) in strongly acid solution with a known excess of potassium dichromate (K2Cr2O7).
The excess unreacted dichromate is titrated with ferrous ammonium sulphate to determine the amount of dichromate consumed as a measure of the oxidisable matter which is calculated in terms of oxygen equivalent. A blank analysis is carried out and this analysis is carried in replicate to yield reliable data.
The apparatus needed for the analysis are a refluxing system and apparatus needed for titration. The reagents are 0.04167 M standard potassium dichromate solution (12.259 g/L of dried K2Cr2O7); sulphuric acid reagent (add 0.55 g Ag2SO4 to 100g conc.
H2SO4and let it stand for 1 to 2 d to dissolve); Ferroin indicator solution (dissolve 1.485 g 1,10- phenanthroline monohydrate and 695mg FeSO4⋅7H2O in distilled water and dilute to 100 mL); freshly prepared standard ferrous ammonium sulphate (FAS) titrant, about 0.25 M (dissolve 98 g Fe(NH4)2(SO4)2⋅6H2O in distilled water, add 20 mL conc. H2SO4, cool, and dilute to 1 L and standardize this solution against standard K2Cr2O7 solution to determine the exact concentration); Mercuric sulphate, HgSO4, crystals or powder.
Procedure
Homogenize the sample if necessary and pipet 50.00 mL into a 500-mL refluxing flask. Add 1 g HgSO4, some glass beads, and very slowly add 5.0 mL sulphuric acid reagent, with mixing to dissolve HgSO4. Cool while mixing to avoid possible loss of volatile materials. Add 25.00 mL 0.04167 M K2Cr2O7 solution and mix. Attach flask to condenser and turn on cooling water.
Add remaining sulphuric acid reagent (70 mL) through open end of condenser. Continue swirling and mixing while adding sulphuric acid reagent. Cover open end of condenser with a small beaker to prevent foreign material from entering refluxing mixture and reflux for 2 h. Cool and wash down condenser with distilled water.
Disconnect reflux condenser and dilute mixture to about twice its volume with distilled water. Cool to room temperature and titrate excess K2Cr2O7 with FAS, using 0.10 to 0.15 mL (2 to 3 drops) ferroin indicator.
Take as the end point of the titration the first sharp colour change from blue-green to reddish brown that persists for 1 min or longer.
Total Organic Carbon
Total organic carbon (TOC) is another gross measurement of organic pollution which can be determined in water samples or solid samples such as soil and sediment.
It is a more convenient and direct expression of total organic content than either BOD or COD earlier discussed.
It however does not provide the same kind of information as BOD and COD. TOC, unlike BOD or COD, is independent of the oxidation state of the organic matter.
It measures only carbon and does not measure other organically bound elements, such as nitrogen and hydrogen, and inorganics that can contribute to the oxygen demand measured by BOD and COD.
Thus, TOC measurement does not replace BOD or COD testing. TOC measurement is vital to the operation of water treatment and wastewater treatment plants.
It is used in assessment of drinking water (TOCs range <100 µg/L – 25,000 µg/L maximum).Wastewater may contain very high levels of organic compounds (TOC >100 mg/L).
It can therefore be an important parameter in oil pollution assessment. TOC determination principle is based on catalytic heating reaction of the total organic carbon in a sample in a chamber packed with an oxidative catalyst such as cobalt oxide, platinum group metals, or barium chromate.
The organic carbon is oxidized to CO2 and H2O. The CO2 from oxidation of organic and inorganic carbon is quantified.
The CO2 quantification can be done instrumentally in a TOC analyser by transporting it in the carrier-gas streams and is measured by means of a non-dispersive infrared analyzer. It may also be titrated coulometrically.
The apparatus needed for the analysis include: Total organic carbon analyser (using combustion techniques); Sample injection and sample preparation accessories; Sample blender or homogenizer; Magnetic stirrer and TFE-coated stirring bars; Filtering apparatus and 0.45-µm- pore filters (rinse filters before use and monitor filter blanks).
The reagents needed are: Reagent water for preparation of blanks and standard solutions; phosphoric acid or sulphuric acid; Organic carbon standard stock solution (prepared by dissolving 2.1254 g anhydrous primary-standard-grade potassium biphthalate, C8H5KO4, in carbon-free water and dilute to 1000 mL; 1.00 mL = 1.00 mg carbon and preserve by acidifying with phosphoric or sulphuric acid to pH ≤2, and store at 4°C); inorganic standard stock solution (prepared by dissolving 4.4122 g anhydrous sodium carbonate, Na2CO3, in water, add 3.497 g anhydrous sodium bicarbonate, NaHCO3, and dilute to 1000 mL; 1.00 mL = 1.00 mg carbon, keep tightly stoppered and do not acidify); Carrier gas: Purified oxygen or air, CO2-free and containing less than 1 ppm hydrocarbon (as methane); Purging gas: Any gas free of CO2 and hydrocarbons.
Procedure
Homogenize samples containing gross solids or insoluble matter. Calibrate, optimize combustion temperature of the TOC instrument and monitor temperature to insure instrument stability.
Withdraw a portion of prepared sample or standards using a syringe fitted. Select appropriate sample volume and inject samples or standards into analyzer according to ROC instrument’s instructional manual and record response.
Carry out replicate analysis until consecutive measurements are obtained that are reproducible to within ±10% relative standard deviation (RSD).
Prepare standard curve of organic and inorganic carbon series by diluting stock solutions to cover the expected range in samples within the linear range of the instrument.
Plot carbon concentration in mg/L against corrected peak height or area and use the curve for the determination of the unknown TOC in the samples.
Oil and Grease
Determination of oil and grease in different sample matrix (water and soil) of crude oil polluted area is another key parameter.
In this investigation, rather than measure an absolute quantity of a specific substance a groups of substances with similar physical characteristics are determined quantitatively on the basis of their common solubility in an organic extracting solvent.
Oil and grease is any material recovered as a substance soluble in organic solvent. This test includes other non- volatile materials like biological lipids extracted by the solvent from the sample. Solvents used include petroleum ether or n-hexane.
The conduct of this test is important because some constituents measured by the oil and grease analysis do influence wastewater treatment systems’ operation and effectiveness.
Excessive amounts of these components may interfere with aerobic and anaerobic biological processes and decreased the efficiency of the wastewater treatment.
The discharged of oil and grease into the environment may cause surface films and shoreline deposits leading to environmental degradation.
Investigating the quantity of oil and grease in and environment especially crude oil exploration ones is helpful in proper management, design and operation of wastewater treatment systems and also may call attention to certain treatment difficulties.
If specially modified industrial products are absence, oil and grease is often composed primarily of fatty matter from animal and vegetable sources and from hydrocarbons of petroleum origin.
The apparatus needed for oil and grease analysis are separating funnel; distilling flask; glass liquid funnel; Filter paper; Centrifuge with centrifuge tubes; thermostated water bath; vacuum pump; Ice bath; Desiccator.
Procedure
If the sample is solid, it can be subjected to ultrasonic bath solvent extraction or extraction in Soxhlet extractor. If the sample is liquid, use a funnel to transfer known volume of the sample to a separating funnel.
Carefully rinse sample bottle with 30 mL extracting solvent (either 100% n-hexane, or solvent mixture) and add solvent washings to separating funnel. Shake vigorously for 2 min. Let layers separate.
Drain aqueous layer and small amount of organic layer into original sample container. Drain solvent layer through a funnel containing a filter paper and 10 g Na2SO4, both of which have been solvent-rinsed, into a clean, pre-weighed distilling flask.
Extract two or more times with 30 mL solvent each time, but first rinse sample container with each solvent portion. Distil solvent from flask in a water bath at 85°C for either solvent system. When visible solvent condensation stops, remove flask from water bath.
Cover water bath and dry flasks on top of cover, with water bath still at 85°C, for 15 min. Cool in desiccator for at least 30 min and weigh. Determine the oil and grease by difference from the initial weight.
Total Petroleum Hydrocarbons
Total petroleum hydrocarbons (TPHs) are mixture of short and long chain aliphatic hydrocarbons (C10 – C36 compounds) and aromatic hydrocarbons. They are derived from crude oil and suitable monitoring parameter of oil pollution.
TPHs are monitored in soil, sediment and water of oil spills areas. Information about THPs is need to adequately plan for remediation and clean-up of oil spills. There are a number of methods for the determination of TPH.
There are a number of spectroscopic and non-spectroscopic methods of analysis of TPHs developed over the years.
Some of these are infrared (IR) spectroscopy, Raman spectroscopy, and fluorescence spectroscopy and the non- spectroscopic methods are general gravimetry, immunoassay (IMA), gas chromatography (GC) with flame ionization detection (GC-FID) or mass spectrometry (GC-MS) detector.
For GC-FID or GC-MS analysis, the steps involved include the quantitative extraction of the TPH from the environmental media of interest using various extraction methods.
The extract is then cleaned up and concentrated before the use of GC-FID for the TPH-determination. The details of the GC-FID operations will also be discussed in Unit 8. Another method is the use of FTIR spectroscopic method.
The method is quick, simple, and inexpensive with common detection limits of approximately 10 mg/kg in soil.
Procedure
This method uses the spectra of the stretching and bending vibration of hydrocarbon derivatives mainly from the combinations of C-H stretching modes of saturated CH2 and terminal -CH3 or aromatic C-H functional groups being the common constituent of hydrocarbons.
These IR wavenumbers occur within the range of 3000 to 2900 cm−1 or at the specific wavenumber of 2930 cm−1. Samples are first extracted with suitable solvent (containing no C-H bonds e.g. CCl4) and extraction technique and the eluate is cleaned up before being subjected to IR spectrometry.
The absorbance of the eluate is then measured at within the range of 3000 to 2900 cm−1 or at the specific wavenumber of 2930 cm−1 and compared against the calibration curve developed for the instrument. The instrument calibration standard usually is a petroleum hydrocarbon of known TPH concentration.
Ultraviolet Absorbing Organic Constituents
Crude oil contains various aromatic compounds which are ultraviolet (UV) radiation absorb substances. Therefore, UV absorption is a useful surrogate measure of oil pollution in water system or any other environmental media where oil pollution is being monitored.
More often than none, there are strong correlations between UV absorption and organic carbon content of oil polluted area.
UV absorption may also be used to monitor industrial wastewater effluents; to evaluate organic removal by coagulation; to evaluate carbon adsorption; for monitoring other water treatment processes among others.
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Specific absorption, the ratio of UV absorption to organic carbon concentration is also very useful in the characterization of natural organic matter in pollution studies.
As we shall discuss further under detection of certain individual organic contaminants, UV absorption is useful in quantification of individual organic pollutants after separation with HPLC provided the pollutant is UV active.
The UV-absorbing organic constituents describe here is however intended to be used to provide an indication of the aggregate concentration of oil pollution.
The apparatus and reagents needed are: Spectrophotometer, wavelength 200 to 400 nm with quartz cuvette; Organic-free water and filter; 0.1 M HCl (optional) and 0.1 M NaOH (optional) for pH adjustment, Phosphate buffer (optional) by dissolving 4.08 g dried anhydrous KH2PO4 and 2.84 g dried anhydrous Na2HPO4 in 800 mL organic-free water.
Verify that pH is 7.0 and dilute to 1 L with organic-free water. Store the buffer in brown glass bottle at 4°C. Prepare fresh weekly to avoid microbial growth.
Procedure
Take a 50-mL sample filter by passing it through an organic water pre- cleaned filter. Do a sample pH adjustment with HCl or NaOH or buffered as appropriate but avoid precipitate formation during pH adjustment. Report sample pH value used with recorded absorbance.
Prepare organic-free water blank and the sample in the same manner. To carry out the spectrophotometric measurement equilibrate the instrument set wavelength to 253.7 nm and adjust spectrophotometer to read zero absorbance with the organic-free water blank.
Measure the UV absorbance of sample at 253.7 nm at room temperature in triplicate. Dilute the samples of the absorbance are too high and calculate the UV- absorbing species in the sample using the expression:
Where UV is the mean UV absorption, cm−1 (subscript denotes wavelength used, nm, and superscript denotes pH used if other than 7.0), b= cell path length, cm, Ᾱ = mean absorbance measured, and D = dilution factor (if sample is diluted) resulting from pH adjustment and/or dilution with organic-free water.
Metals and Radioactive Compounds
The fact that crude oil is associated with rock make is probable that there may be trace level of metals and radioactive compounds in crude oil and its waste discharged into the environment.
It is therefore essential that environmental monitoring for crude oil pollution involve determination of metals and radioactive compounds in samples from monitoring area and samples of interest.
Metals are often determined by a variety of methods. The choice of method is often dependent on the precision and sensitivity required by the study.
Methods that can be used include spectrophotometry, atomic absorption spectrometry (flame or electrothermal (furnace) or hydride and/or cold vapour techniques); flame photometry; inductively coupled plasma (ICP) emission spectrometry; inductively coupled plasma mass spectrometry, and anodic stripping voltammetry.
Flame atomic absorption methods are the most generally applied and will be the one discussed in the section.
Procedure
Because of the tendency of interference by organic matter present in the crude oil and to convert some of the metals associated with particulates into free metal digestion of the samples in whatever matrix it is present (water, soil, sediment etc.) is carried out.
This must be done before the metal content can be determined by atomic absorption spectrometry or inductively-coupled plasma spectroscopy.
Digestion may be carried out with any of the following acid combination depending on the sample matrix and on the aim of the analysis: HNO3 alone which is adequate if the samples are clean or can be easily oxidized; HNO3+H2SO4 or HNO3+HCl digestion is if there is large quantity of readily oxidizable organic matter; HNO3+HClO4 or HNO3+HClO4+HF digestion if the sample is difficult-to-oxidize organic matter or minerals containing silicates.
It will be observed that in all acid combination HNO3 is constantly involved. This is because all metal nitrate salts are soluble and the aim of digestion is to get the metals into solution.
The digested samples can then be read in Flame atomic absorption spectrophotometer of with inductively coupled plasma mass spectrometry. The radioactive metals can be detected with ICP or with counting instruments like the Gas-flow proportional counters or alpha scintillation counter.
Control of Oil Pollution
As important as monitoring of crude oil pollution is, the control of the pollution is much more important. The control may be preventive or corrective/remediation approach.
The preventive measures are often regarded as being better because it is less costly from all angles. This section discusses the preventive and remediation measures towards controlling oil pollutions (Michel and Fingas, 2015).
Preventive Approach
The use of cleaner production technology, the regular routine maintenance of equipment, introduction of best and safe practice and adoption of total quality management procedures, the regular training of worker are some of the key components toward prevention of oil spill.
Beside these, there must be strong regulation, environmental protection laws, and strong institutions to enforce the laws against crude oil pollution.
The best and safe practices should include structured plans to regularly risk analysis for and to identify potential oil spill point(s); studying previous spills as predictive model for identifying potential ones; acting promptly on danger signal(s); ensuring resources are available to remove possibility of discharge; training workers on Health, Safety and Environment (HSE) Systems; other training to continuous improve oil worker’s skills; testing, regular routine maintenance and ensuring that the regular routine maintenance are carried out; carrying out unannounced drills on emergency response; using of remote sensing to monitor crude oil installation and pipes; use of pipes and tankers that are double layer/hull to prevent spill since the oil must penetrate both layers before being released; investing in research and development aimed and technology development and anticorrosion studies.
Government’s institution should also be strengthened to fight the illegal activities that have been implicated to contribute significantly to oil pollution especially in Nigeria. These among others are preventive measure that will reduce oil pollution.
Remediation
There are a number of remediation methods. The first we will discuss is the chemical agent’s treatment methods. There are numerous chemicals formulated that can be used to interact with oil spill for treatment and to assist in clean-up of oil.
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This approach should only be used after approval of necessary authorities because of the potential toxic effects of chemicals on aquatic and other wildlife. A typical and common chemical agent is the surfactants/dispersants.
Surfactants have lipophilic and hydrophilic parts which make them useful chemicals in spill-treating. They promote the formation of small droplets from the oil slicks and the oil droplets are dispersed by these dispersant/surfactants from the top layer slick into the water column by with the aids of wave action and currents.
Unfortunately, these dispersants are not perfectly effective in the treatment because the oil is still there but only dispersed and may resurface into slicks. The oil becomes less dispersible with change in weathers, reduction in tidal current force which aids the dispersion, increase in viscosity among others.
As a result of this, dispersants have narrow effectiveness window. Moreover, the use of dispersants has been a contentious and controversial one. Typically, use of dispersant in freshwater and in land applications is not acceptable. The use of chemical dispersant is a trade-off.
Another remediation method is the use of skimmers. Skimmers are mechanical devices designed for the purpose of removing floating oil only in water systems. They are of varying sizes, applications, capacities and recovery efficiencies.
Skimmers are classified according to the area of application such as: inshore or offshore, shallow water, rivers or deep water. They can also be differentiated by the viscosity of the oil they are used on.
They can also be classified according to their basic operating principles such as oleophilic surface skimmers; weir skimmers; suction skimmers or vacuum devices; elevating skimmers; and submersion skimmers.
The efficiency of skimmers is enhanced in high viscosity oils slick (thick oil slick). To achieve this, the oil is collected in booms and the skimmer is then placed where the oil is most concentrated to recover as much oil as possible before and to effectively clean the oil spill.
Factors that affect the efficiency of skimmers are: weather conditions at a spill site, wave’s height (>1 m), wave’s currents (>0.5 m/s), water with ice or debris (plant branches, floating wastes, seaweeds etc.) and very viscous oils, tar balls, or oiled debris can clog the intake or entrance of skimmers.
Some skimmers are designed to have screens to prevent debris from entering, conveyors to remove debris, and cutters to deal with sea weed.
Another method is the use of sorbents. Sorbents are materials that are used to recover oil either by adsorption or by absorption. Sorbents can be either absorbent or adsorbent; they can be natural or synthetic.
Natural sorbents can be organic or inorganic. Organic sorbents include: biomass such as agricultural waste (wood products), peat moss, sea weeds, activated charcoal or carbon from biomass among others. Inorganic sorbents may be clays, synthetic materials such as zeolite among others.
Sorbents can be applied in different forms such as: in granules, cubes, powder, chunks and can be packaged into pads, rolls, blankets, and pillows, bags, nets, or socks for application.
They can be applied to either clean up the final traces of oil spills in water systems or on land after the initial use of the methods described above; used as backup to other means of clean ups; used as a primary means of recovery if the oil spill is of low volume among others.
Another promising remediation method is the use of micro-organismsfor oil spill clean-up. A number of studies have been carried out to use microbes to degrade oil in crude oil polluted areas.
Gammaproteobacterial which including representatives of genera with known oil degraders Alcanivorax, Marinobacter, Pseudomonas, and Acinetobacter has been applied successfully for this purpose (Kostka etal., 2011; Dubinsky et al., 2013). The only drawback of this method is the slow rate of degradation.
In-situ burning is another oil spill clean-up technique. It involves controlled burning of the oil at or near the spill site. It is unfortunately a destructive remediation method because the oil cannot be recovered for useful purpose but it is a final solution, it has the potential for removing large oil quantities over an extensive area, it can be operated in less or approximately same time than as other techniques. The technique is most suitable for oil spill on land.
In summary,the empirical evidences of oil pollution are the collection and analysis of samples from oil polluted area for parameters that are indicators of pollution. This evidences will give credit to the control and remediation efforts of such pollution.
Relevance and procedure for determination of parameters like BOD, COD, TOC, Oil and Grease among others were discussed in detail.
Preventive approaches to oil pollution control like cleaner production technology, the regular routine maintenance, best and safe practice, total quality management procedures, the regular training of worker among others were presented. The use of dispersant, skimmer, microbial degradation, insitu burning among other methods for the remediation was also taught.
Oil pollution monitoring is very essential for evidence based understanding of extent of pollution and policy formulation for preventive and remediation purpose. The application of gross measure of oil pollution provides such evidence based understanding.
More importantly, there is the need to continuously enforce prevention of oil pollution and remediation of polluted environment because of the adverse effects of oil pollution.