A sorption isotherm is determined by comparing the sorbed concentration of contaminants to the concentration in solution. More specifically, it describes the ability of dissolved pollutants to adsorb or absorb themselves onto the solid particles (soil or particulates). Because the sorbing behavior of pollutants is affected by temperature, it is measured at a constant temperature. Sorption exerts a significant effect on the fate of contaminant, especially in the groundwater system because it can be used as one of the parameters to predict the lateral extent of contamination in a given time.  For this experiment, a soil sample was collected at the Palmer stadium and air-dried for three days. To simulate the behavior of a pesticide in the sediment, Eosin B (a biodegradable dye) solutions of known concentrations were prepared.

 

5 samples were prepared using 5 grams of sand and various concentrations of Eosin B solution (total volume of 10 ml).  The mixture was agitated, allowed to settle, and the supernatent was tested. In theory, through the agitation process, the Eosin B contaminant should bond with the soil particles and thereby come out of solution. The change in Eosin B solution concentration implied the sorptive quality of the soil sample. This was quantified by the change in optical absorption of the Eosin samples using a HACH spectrophotometer.

The results of the experiment are shown in the table below.

The sorbed concentration of pollutant in the soil is expressed as mg solute/kg solid.  In this case, the solid is 0.005 kg sand.  A table of sorbed concentration as a function of dissolved solution is shown below and graphed accordingly. 

 From the equation of the line, y = 0.6867x, the slope of the line is the partition coefficient (K_p).  The K_p value of 0.6867 indicates that the contaminant used in our experiment is moderately sorbing onto soil grains. 

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