Mixing electrolytes during electroreclamation of marine sediment for optimized reagent consumption

We studied the effect of catholyte-anolyte mixing during electrokinetic remediation of marine sediment for the optimization of reagent consumption during operation. We developed a model which simulates the effect of recirculation as a function of the flow rate. Laboratory experiments with real dredged sediment were performed in laboratory in order to validate the model. Reagent consumption is particularly relevant when electrokinetic remediation technique is applied to field scale. A large part of the total cost is covered by reagents for electrolyte conditioning, especially when the buffering capacity of the material subjected to remediation is high, which is a frequent condition for marine sediments due to their high carbonate content. In order to achieve successful removal of contaminants (e.g. heavy metals) it is often necessary to acidify the sediments. Due to the high buffering capacity, longer remediation duration is required and the amount of acid used to reach the target pH can be significantly high. We performed electrolyte mixing by pumping the anolyte into the catholyte. The anolyte is acid because of the H+ production at the anode, instead the catholyte tends to become alkaline due to the production of OH-. In this way the acidity produced at the anode is used directly to buffer the alkalinity at the cathode. The developed model is composed of a series of continuous flow stirred-tank reactors used to simulate the pH in all the points of the system. To verify the model performance we carried out laboratory experiments at different recirculation flow rates. The model is able to reproduce the experimental data with very good accuracy. Futhermore, the model allowed us to identify the precise behavior of the system. We observed that the acid consumption is reduced even with very low recirculation flow rates. The system response can be divided into a transient state, whose duration depends upon the value of the recirculation flow rate and a steady state which is independent of the flow rate. We found that the anolyte pH can be maintained to a certain value by making an appropriate choice of the recirculation flow rate. This allows to keep a certain target pH value in the anolyte without further chemical addition. In conclusion, the experimental and model results allowed to identify specific operating conditions which can be adopted to design of a full scale remediation plant with optimized reagent consumption.