Electrokinetic-Enhanced Bioremediation of Organic Contaminants: a Review of Processes And

Appendix F

FT - Fish tank experiments

Observe electromigration of the fluorescent tracer, fluorescein under pH controlled conditions.

The first electrokinetic experiment was conducted in a disused fish tank. Before operation, perforated acrylic dividers were inserted into the tank to separate the electrode and sediment chambers (Figure 1). A recirculation method to neutralise the pH changes at the electrodes was applied (FT-1) (Figure 2) this was in-order to prevent precipitation of fluorescein at low pH. The method could not maintain an equal hydraulic head between electrode chambers because different pumps were used to control the inflow and out flow from the system. To prevent advection of fluorescein result from hydraulic head differences the levels were controlled manually by modifying pumping rates, however this did restrict the duration a fish tank experiment could run for as well as reduce the effectiveness of the pH control method. Experiments were run over 3 days for 8 hours each day.

1 Figure 1. Digital image showing the experimental setup for the fish tank experiment. Recirculation pump 1 and reservoir tank not shown.

Figure 2 Schematic showing the fish tank recirculation method (FT-1).

Materials and methods:

Results

Fluorescein appears to move into the system and stop at around 12 cm into the glass beads (0.3 normalised distance from cathode) (Figure 3-A) (this is also seen in FT-2, data not shown). Originally this initial movement was attributed electromigration as the dominant transport mechanism. However comparison between the calculated and observed values shows that the extent of electromigration is limited over the short

2 timescales of the experiment (9 hours). Calculated values predict that 48 hours of

constant running is required to see any appreciable increase in fluorescein

concentration. Therefore, any movement in the observed system is most likely due to

advection caused by the recirculation system. Limited electromigration is due to the low

ionic mobility of fluorescein (3.6x10-6 cm2/V-s compared to 7.4x10-4 cm2/V-s for nitrate), a

function of its high atomic mass (332 g mol-1).

A. Observed Values B. Calculated Values Figure 3. Fluorescein profile for FT-3 Day 1. 0-7 hours; A. Observed and B. Calculated values for FT-3 and predicted fluorescein distribution after 48 and 168 hours.

Results in Figure 4 suggest a flux of fluorescein from the anode. This could be due to

electroosmosis. To confirm this, the zeta potential ( ) was obtained by isolating the

electroosmotic pore fluid flux () from observed data and applying the following equation

derived from Kaya & Yukselen (2005):

where is the electrolyte viscosity, is the electrical permittivity, the tortuosity factor,

porosity, electric field strength and is area. This equation returned an unrealistic value

of -1341 mV for the glass beads used in the experiment compared to a range of -30 mV

for a fine sand (Mitchell, 1993) and -130 mV for clay (Kaya & Yukselen 2005). Again, this

3 indicates advection is having the greatest influence on fluorescein distribution in these experiments.

Figure 4 fluorescein concentrations from day 3 of fish tank experiments.

The pH changes observed during the experiment were minimal (Figure 5). This is due to the short experiment duration. It is anticipated that any pH front would generate after at least 12 hours of constant operation (Lohner et al., 2008) in the absence of efforts to control pH values.

Figure 5 pH values from pore fluid samples taken from the fish tank experiments. Only data from day 3 shown.

Conclusions and recommendations

 Fluorescein migration was difficult to discern over the influence of advection from

the recirculation system.

 Any further experiments in the fish tank must be conducted without re-circulating

the electrode chamber fluid to extend running time.