Effects Of Electrolyte Characteristics On Electrokinetic Remediation Of Lead-contaminated Soil

In China, soil contamination is a critical problem that is imperative to be restored.In this paper, the effects of different electrolytes in anolyte and catholyte upon theprocess and efficiency of electrokinetic remediation (EKR) were investigated, using thelead-contamination soil from a factory site.The cations from the anolyte and anions from catholyte migrate into soil under theelectric field and affect the process and efficiency of EKR. This study investigated theinfluence of the encounter of Ac-ions from widely-adopted catholyte of NaAc and H+(or Fe2+) ions from anode reactions on the EKR process. H+ions were generated fromthe electrolysis reaction at Pt-anode, and Fe2+ions were from Fe-anode.Fe2+ions did not promote the lead desorption in the soil; whereas, H+enhanced thelead desorption significantly. In the Pt-anode tests, H+ions migrated into the soil in thedirection of lead migrating resulting in significant change of soil pH. Furthermore, apotential gradient peak (PGP) appeared in the soil. It featured excessively unevendistribution of potential gradient in the soil. The potential applied on the soil mainlyfocused on one soil segment resulting in far higher potential gradient in the segmentthan others. The migration velocity of lead ions in the PGP segment was much higherthan in other segments. The PGP, like a brush, forced the lead ions in the PGP segmentto move from the anode to the cathode. The lead ions in the other segments, however,migrated slowly owing to the relatively low potential gradient. Thus, lead ions focusedbefore the PGP segment forming a lead concentration peak (lead focusing phenomenon).This concentration peak moved towards the cathode driven by the PGP. The EKR willfail to meet the expectation if this concentration peak remains in the soil. By contrast,this phenomenon did not occur in the Fe-anode tests.The potential gradient variance in the soil is related to the soil solution conductivity.These two values are in inverse proportion. The dramatic change of soil conductivitycan cause PGP in some segments. The soil solution conductivity value reached theminimum at pH=3.7in the conductometric titration test using acetic acid as the titrationagent to the soil/NaAc solution system. In contrast, the similar phenomenon did notemerge in the pure soil solution conductometric titration test. Based on these twotitration tests, it can be supposed that the generation of non-charged acetic acidmolecule, due to the complex reaction between H+ions from the anode and Ac-ions from the cathode, results in the lowest conductivity at pH=3.7, and furthermore, causesthe highest potential gradient.In carbonate-free experiment, the PGP emerged in one soil segment in the case ofusing HAc/NaAc solution as the catholyte. When HAc/NaAc solution was replaced byKNO3solution, the PGP disappeared quickly This test clearly demonstrated that thegeneration of non-charged acetic acid molecule was the primary reason of the PGP. Soilcarbonate is not the origin of the PGP though it may influence the shape and migrationof the PGP.