Maria Villen-Guzman

Effects of the buffering capacity of the soil on the mobilization of heavy metals. Equilibrium and kinetics.

Understanding the possible pH-buffering processes is of maximum importance for risk assessment and remediation feasibility studies of heavy-metal contaminated soils. This paper presents the results about the effect of the buffering capacity of a polluted soil, rich in carbonates, on the pH and on the leaching evolution of its main contaminant (lead) when a weak acid (acetic acid) or a strong one (nitric acid) are slowly added. In both cases, the behavior of lead dissolution could be predicted using available (scientifically verified freeware) models assuming equilibrium between the solid and the aqueous phase. However, the experimental results indicate that the dissolution of calcium and magnesium carbonates is kinetically controlled. These kinetic limitations affect the overall behavior, and should be considered to understand also the response of the metals under local equilibrium. The well-known BCR sequential extraction procedure was used before- and after-treatment, to fractionate the lead concentration in the soil according to its mobility. The BCR results were also in agreement with the predictions of the equilibrium model. This agreement allows new insights about the information that could be derived from the BCR fractionation analysis.

Acid enhanced electrokinetic remediation of a contaminated soil: Strong vs. weak acid

The most typical enhancement for electrokinetic remediation (EKR) involves the neutralization of the alkaline front generated at the cathode by the addition of an acid to the catholyte. Although the use of strong and weak acids can be found in the literature, there is still a clear lack of reliable comparisons between them. This paper presents a comparison of the results obtained for a real lead contaminated soil treated by EKR enhanced with nitric or with acetic acid. The divergences that are usually observed in those experimental results obtained when using constant voltage drop are avoided here by the use of EKR with constant current intensity and the disposition in series of the soil lab columns. Important differences are observed in the behavior of the system when the acid was changed. The time needed to achieve the same metal recovery is more than 3 times longer for the nitric acid. A standard sequential extraction procedure was used to fractionate the lead concentration in the soil according to its mobility in a before- and after-treatment way. Regardless of these important differences observed in the remediation time, the fractionation results after the experiments were almost the same for the two acid enhanced EKRs.

A Coupled Reactive-Transport Model for Electrokinetic Remediation

In this chapter, we present a model for the reactive-transport of chemical species through partially saturated porous media for electrokinetic remediation processes. A generalized theoretical model is presented, easily adaptable to specific remediation setups, target contaminants and supporting matrices; and we give detailed guidelines for the implementation of tailor-made numerical methods for the computer-aided solution. The model is subdivided into two coupled modules: one for reactive-transport, numerically solved by means of a non-linear finite element method; and another one for chemical equilibrium, solved using an enhanced Newton–Raphson method.

Specific Energy Requirements in Electrokinetic Remediation

Understanding the scaling-up process is essential for the application of the electrokinetic remediation at field scale, as the technique is site-specific and dependent on the target contaminants. In this paper, a generalized and easy-to-use model is presented for the prediction of the energy requirements at different scales. The mathematical model includes the definition of a specific energy parameter for each mobilized metal, obtained from the relationship between the amount of metal mobilized and the maximum that can be mobilized. The results indicate that the specific energy requirements for the removal of each target contaminant can be predicted as the process is scaled-up.

Feasibility Study of the Electrokinetic Remediation of a Mercury-Polluted Soil

This chapter is focused on the study of electroremediation of heavy metals from a real soil. Specifically, the case of the study was a soil from Almaden mining district, with a very high mercury concentration. The risk assessment of heavy metals depends on the mobility and bioavailability and not only on the total concentration. Therefore, this study evaluates the distribution of mercury into different fractions before and after the electrokinetic treatment.