Effectiveness of Chemical Dispersants
The effectiveness of a dispersant is dependent upon the properties of the dispersants, concentration of salt and other constituents of sea water, pH, and the ratio of dispersant to oil. We are constantly exploring these relationships and attempting to determine the best dispersant under the best conditions. The experiments conducted by this research group consist of bottles of 120 mL of artificial sea water. Oil is then added to these bottles, followed by a specified amount of dispersant depending upon the trial being performed. The bottles are then shaken for 30 minutes. After the liquid settles in separatory funnels for 15-20 minutes, the concentration of oil is measured using a spectrophotometer. Each of the parameters were varied within one trial. For pH and saltwater concentration experiments, a DOR of 1:50 was chosen.
There are three different parameters that can be used to quantify how well the dispersant works. The most telling of them being the concentration of dispersed oil in the sample, measured using a spectrometer. The relationship between the light absorbance and oil concentration can be determined by measuring the absorbance of a known concentrations of oil. After the oil, dispersant, and water mixture has settled, the solution is divided into three portions, top, bottom, and middle. The concentration of these portions are measured individually and the amount of dispersed oil in each section is determined. This helps researchers observe how dispersed oil will settle within the water column of the ocean. Measuring surface tension can also help determine which dispersant is best since part of the role of a dispersant is to lower interfacial tension (the tension that exists at the oil-water interface). Methods to measure this parameter have not yet been determined. The last way in which we can determine the effectiveness of a dispersant is to measure the size of the oil droplets within the sample. The smaller the droplets are, the better the dispersant is working. To measure the droplet size, a droplet of the sample is placed on a microscope slide and the droplets are measured manually using pictures taken through the microscope. |
Dispersant Properties
For the hyperbranched polymers, their effectiveness as a dispersant depends heavily upon the molecular weight of the polymer. When discussing polymers, the molecular weight is used to indicate the number of branches contained within a single molecule, which is the property that is actually affecting the ability of the polymer to disperse oil. There seems to be a parabolic relationship between molecular weight and the amount of oil that the polymer is able to disperse. The effectiveness of HDPEI peaks at 10 kDa, then declines at higher weights. The graph below shows this relationship. After the oil has settled in the seperatory funnel, it is evaluated in four different volumes in order to observe the way that the oil will be distributed within the water column. These are the volumes represented on the x-axis in the graph below, 0-30 mL being the bottom of the column and 90-120 mL being the top fraction. The absorbance represents the fraction of oil dispersed, the more oil that exists within the sample, the higher the absorbance is.
Salt Water Concentration
It is important to understand the effects of sea water on the ability of the polymers and other compounds to disperse oil because of the many different environments that exist in the ocean. Not only does the oil contaminate the open ocean, but it can also have an effect on salt marshes and estuaries. These areas do not contain the same concentration of compounds such as sodium chloride and bicarbonate as the rest of the ocean does. Due to the delicacy of these ecosystems, it is important to understand the manners with which to effectively remediate oil spills affecting them. The graphs below indicate the relationship between the concentrations of salt and sobium bicarbonate and the dispersing capabilities of the polymers, as well as corexit. The y-axis represents the total fraction of oil dispersed over the entire water column.
pH
The pH of a liquid is a measure of it's acidity/basicity determined by the concentration of hydrogen ions present in the liquid. This measurement is based on a log scale, which means that the more hydrogen ions that are present, the lower the pH is and the more acidic the liquid is. pH is highly variable within the ocean due to the carbonate cycle and other natural phenomena. The pH does not have a large effect on corexit, but HDPEI is. It is theorized that this due to the positive amine groups gain or lose protons based upon the pH of the surrounding medium.
Dispersant to Oil Ratio
When a dispersant is applied, it is essential to understand how much needs to be applied based on the amount of oil present and the disperant being used. Depending upon the dispersant, there is a specific ratio of dispersant to oil (DOR) at which each compound works best. For the polymers that have been tested, 1 parts dispersant to 50 parts oil seems to be the most effective DOR. The following graph depicts the relationship between dispersant to oil ration and total fraction dispersed within the water column.