Discussion

The Kdvalues calculated from retardation factors for MA-80 and Triton X-100 were 0.077 L/kg and 0.084 L/kg respectively in Column 1, while the Kdvalues at 1.0% MA-80 and Triton X-100 predicted from the batch study were significantly greater at 0.207 L/kg for MA- 80 and 0.184 L/kg for Triton X-100. Kdvalues from Column 2 were 0.125 L/kg for MA-80 and 0.114 L/kg for Triton X-100, which are slightly less than the Kdvalues estimated for a solution containing 1.425% MA-80 and 1.575% Triton X-100 from the batch sorption experiment of 0.160 L/kg for MA-80 and 0.125 L/kg for Triton X-100. It is expected that

sorption coefficients predicted from column experiments should be somewhat lower than batch experiments due to kinetic and surface area limitations (Adeel and Luthy 1995). It should also be noted that due to the complex sorption behavior exhibited by both surfactants in these systems, the Kdvalues calculated from both column experiments may vary

drastically for various influent surfactant concentrations.

However, the difference between Column 1 Kdvalues and batch sorption values were inconsistent as Column 1 sorption coefficients are lower than those calculated for Column 2 yet the batch experiment predicts higher sorption for the surfactant concentrations used in Column 1. The retardation factors calculated for Columns 1 and 2 were also surprising, for the degree of retardation should decrease for increasing surfactant concentrations (Adeel and Luthy 1995), and the opposite is seen here. There are a few possible explanations. First, initial breakthrough was earlier than expected in both columns, but it was earliest in Column 2. This possibly signifies more non-Fickian behavior exhibited in Column 2 that could lead to retention of surfactants beyond what was caused by sorption alone. In addition, the length of Column 2 was roughly half that of Column 1, which increased the likelihood of non- Fickian flow, and is likely a partial explanation for the increase in apparent retardation. Still, this does not appear to be the case in this situation as Kdvalues calculated from Column 2 more closely resemble those calculated in the batch study. Another major difference between the surfactant solutions used in Column 1 and 2 aside from the surfactant concentrations is that a higher amount of CaCl2was present in the surfactant solution injected into Column 1.

Though it seems logical that an increase in salt concentration would shift Kdmore towards solid phase sorption, it might also increase surfactant micellization, which would decrease the surfactant available for sorption. Also, surfactant breakthrough was not complete in

Column 1, and had the experiment been allowed to attain complete breakthrough, the Kdand Rfwould increase. Thus, Column 1 Kdand Rf estimates were lower than the actual values.

Retardation factors calculated from desorption in Column 2 were lower than the values derived from sorption, which is again slightly misleading because relatively large amounts of MA-80 and Triton X-100 remained on the column at the end of the experiment. Therefore, Rfvalues estimated from desorption are too low. The retention of surfactant could be partially due to non-Fickian behavior, but the fact that 3 times more Triton X-100 than MA-80 is retained leads to the belief that sorption plays a major role in these extensive retention times. Although the relative oligomer concentrations of Triton X-100 were not monitored, it is possible that partitioning of hydrophobic, short ethoxy chain Triton oligomers into organic matter may be the cause of Triton X-100 recalcitrance during the desorption phase. Such interactions are more strongly binding than hydrogen bonds and Van der Waals interactions, which are the primary factors governing MA-80 and long ethoxy chain Triton sorption (Yeh and Lin 2003). Long retention times have been reported for Triton X-100 in past studies also (Adeel and Luthy 1995, Smith 1997, Yeh and Lin 2003).

In batch study samples, MA-80 sorption consistently increased with concentration, but began tailing off dramatically around 1,500 mg/L. This was in excess of its CMC of 890 mg/L (Franzetti 2006) where maximum MA-80 sorption has previously been determined (Cho 2004). Also, it did not appear that significant MA-80 sorption occurs below 300 mg/L. This finding does not agree with sorption isotherms produced in another study (Franzetti 2006) yet the Kdof 0.17 L/kg given in the same study is very similar to the Kdcalculated at high concentrations from batch samples. However, all prior studies examined MA-80 sorption characteristics in the absence of Triton X-100 and other solution components. It

appears that Triton X-100 sorption was strongly preferred over MA-80 at low concentrations, but due to the presence of large amounts of metal oxides, MA-80 sorption persisted even at very high concentrations (Cho 2004). In addition, MA-80 sorption at low concentrations could have been slowed by the existence of previously sorbed MA-80 from past studies.

The discovery that Triton X-100 sorption followed the Freundlich model near CMC levels is well supported in previous studies (Smith 1997, John 2000, Zhang 2002).

Furthermore, Triton X-100 sorption has formerly exhibited a plateau in sorbed concentrations at about 200 mg/L (Zhu 2003), which is similar to what was observed in the batch

experiment. Although individual oligomers were not quantified throughout the field study, the distribution of Triton oligomer distribution seen in effluent samples shifted towards the more hydrophobic, short ethoxy chain oligomers when compared to the distribution observed in the original surfactant solution. This shift is realized by comparing the oligomer

distribution of the injected surfactant in Figure 5.7 to the distribution in a typical effluent sample in Figure 5.8. Notice here that the more hydrophobic oligomers increase in relative concentration in the effluent sample. Therefore, the dominant sorption mechanisms within the field experiment were likely hydrogen bonding and Van der Waals interactions, which primarily affected the sorption of Triton oligomers with long ethoxy chains.

Figure 5.7: Triton Oligomer Distribution in Surfactant Solution Prior to Injection.

AU 0.00 0.05 0.10 0.15 Minutes 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 S07-SURF4 MA-80 - 8.390 4-BFB - 20.781

Figure 5.8: Triton Oligomer Distribution in a Typical Effluent Sample. AU 0.000 0.010 0.020 0.030 0.040 0.050 Minutes 2.00 4.00 6.00 8.00 10.00 12.00 14.0 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.000 S08-COMP25 MA-80 - 8.228 4-BFB - 20.290 PCE - 25.525