Removal of organic contaminants from soils by an electrokinetic process: The case of molinate and bentazone. Experimental and modeling

Abstract

The herbicides molinate and bentazone are used in paddy rice fields. Their behaviour in soils when submitted to an electric field was studied and the applicability of the electrokinetic (EK) process in soil remediation was evaluated. Three soils were used and nine EK experiments were carried out at a laboratory scale. A one-dimensional model is developed for simulating the EK treatment of soils containing the herbicides. The model reproduces satisfactorily the experimental data of cumulative volumes removed towards the anode and the cathode compartments, as well as the difference between catholyte and anolyte cumulative volumes, showing that the electroosmotic flow (EOF) is very sensible to soil pH. The experimental results show that the EK process is able to mobilize molinate and bentazone from soils, and remove them from soil solution. Molinate seems to be mobilized preferentially to the cathode compartment. The model also satisfactorily reproduces the experimental data. The highest quantities of molinate move towards the catholyte, particularly when the EOF is kept constant, allowing molinate to be constantly swept towards it. Bentazone also shows a high decay from soil, being mobilized towards both electrode compartments. The results point towards a relationship between the differential pH in the catholyte and anolyte, and the bentazone decay. The model shows that bentazone is mobilized towards the anolyte in higher current intensity conditions, because there is a predominant electromigration (EM) movement of enolate towards the anode compartment, against the electroosmotic transport of bentazone towards cathode, whereas in lower intensity conditions the opposite occurs. This happens because the EOF grows less than EM, when current intensity is duplicated.