**5. Remediation of polluted soil with mercury**

The most commonly used techniques for the remediation of mercury contaminated soils have been classified as either excavation techniques or containment techniques, and are grouped as follows (Hinton and Veiga, 2001): excavation and *ex-situ* treatments, containment and *in situ* chemical treatment (Figure 4).

**Figure 4.** Representation of the techniques for the remediation of mercury contaminated soils: (A) excavation and *exsitu* treatments, (B) containment and (C) *in situ* chemical treatment.

#### **5.1. Excavation and** *ex situ* **treatments**

They will be treated off-site soil contaminated when removing soil or contaminated soil-like materials to a place outside the place in which they are located, for submission to authorized treatment fixtures. (Hinton and Veiga, 2001):


#### **5.2. Containment**

or dynamic systems as the reactive barrier material, by the high mercury capture coal are able

The most commonly used techniques for the remediation of mercury contaminated soils have been classified as either excavation techniques or containment techniques, and are grouped as follows (Hinton and Veiga, 2001): excavation and *ex-situ* treatments, containment and *in situ*

**Figure 4.** Representation of the techniques for the remediation of mercury contaminated soils: (A) excavation and *ex-*

They will be treated off-site soil contaminated when removing soil or contaminated soil-like materials to a place outside the place in which they are located, for submission to authorized

**2.** *Thermal treatment.* The volatility of mercury increases with increasing temperature; therefore, a heat treatment technique of excavated soil is a potentially effective technique

**1.** *Physical separation.* Mercury has affinity for the smallest particles of soil.

to retard or remove the aquatic mercury contamination (Lakatos et al,1999).

**5. Remediation of polluted soil with mercury**

*situ* treatments, (B) containment and (C) *in situ* chemical treatment.

**5.1. Excavation and** *ex situ* **treatments**

treatment fixtures. (Hinton and Veiga, 2001):

for the removal of mercury in soils.

chemical treatment (Figure 4).

840 Environmental Risk Assessment of Soil Contamination

In the containment treatment the soils are treated on one side of the contaminated site, where the processing is performed on an area adjacent to the contaminated site or an area within the contaminated site upon removal of soil or soil-like materials. In this classification are (Hinton and Veiga, 2001):


#### **5.3.** *In situ* **chemical treatment**

Another option is the *in situ* chemical treatment option, which is the name of all treatments that involve the injection of a chemical reagent into an aquifer source upstream of the conta‐ minated site. This chemical agent reacts with the contaminant, transforming it into an innoc‐ uous form; eventually, it can pump through a given volume of water which can later be recycled for injection. The following actions must be considered:


#### **5.4. Electroremediation of polluted soil with mercury**

Electroremediation has been successfully applied in a variety of soil restoration studies, this methodology having the advantage of exhibiting simultaneous chemical, hydraulic and electrical gradients. Indeed, for efficient mercury removal from a saturated soil with electro‐ remediation, application of either an electric field or direct current through two electrodes (anode and cathode) is required. These are usually inserted in wells containing a supporting electrolyte made from inert salts, leading to improved electric field conductive properties (Rajeshwar et al, 1994; Huang et al, 2001; Acar and Alshawabkeh, 1993).

Furthermore, since electroremediation is a physicochemical technique based on ion transport, it is an excellent tool for the removal of inorganic species, such as Hg+2 (Rajeshwar et al, 1994; Bustos, 2013). The main advantages of electroremediation, as compared with other soil treatment procedures, are (Huang et al, 2001; Acar and Alshawabkeh, 1993; Ibañez et al, 1998; Segall and Bruell, 1992; Cabrera – Guzmán et al, 1990): (1) electroosmotic flow is not dependent on either pore or particle size, (2) hydraulic gradient is enhanced by electromigration, (3) treatment can be applied *in situ*, (4) it can be applied to low permeability soils, (5) there is minimal disruption of normal activities at the site, (6) the required investment is usually lower than that for other conventional treatments, and (7) it can be applied in conjunction with techniques such as pumping, vacuum extraction or bioremediation.

**1.** *Pump and treat*. With certain contaminants or systems, pollution removal is not possible and it is necessary to protect hydraulic content. When the contaminant mass remains in

**2.** *Impermeable barriers (sealed surfaces and drainage)*. Mud barriers are slightly permeable barriers made of bentonite or cement-bentonite mixtures. Generally, these barriers are between 0.5 and 2 m thick and have a maximum depth of 50 m. There are other types of barriers that are constructed by injection molding or by vibratory forces. On the other hand, surface seals and drainage are used to controlling filtration and limit pollutant

**3.** *Stabilization and solidification.* Stabilization and solidification techniques use both *in situ* or *ex situ* conditions by mixing impacted sites. Stabilization attaches contaminants to the soil structure, which usually decreases soil permeability. Moreover, solidification improves the physical characteristics of materials such as mudor sediments; they can be excavated

**4.** *Sediment covering. In situ* covering involves placing an insulating layer over the contami‐

Another option is the *in situ* chemical treatment option, which is the name of all treatments that involve the injection of a chemical reagent into an aquifer source upstream of the conta‐ minated site. This chemical agent reacts with the contaminant, transforming it into an innoc‐ uous form; eventually, it can pump through a given volume of water which can later be

**1.** Increase the output rate of the ground water through the contaminated zone by increasing

Electroremediation has been successfully applied in a variety of soil restoration studies, this methodology having the advantage of exhibiting simultaneous chemical, hydraulic and electrical gradients. Indeed, for efficient mercury removal from a saturated soil with electro‐ remediation, application of either an electric field or direct current through two electrodes (anode and cathode) is required. These are usually inserted in wells containing a supporting electrolyte made from inert salts, leading to improved electric field conductive properties

Furthermore, since electroremediation is a physicochemical technique based on ion transport, it is an excellent tool for the removal of inorganic species, such as Hg+2 (Rajeshwar et al, 1994;

recycled for injection. The following actions must be considered:

the hydraulic gradient through injection and extraction.

**5.4. Electroremediation of polluted soil with mercury**

**2.** Transform the contaminant using chemical reaction within the aquifer.

(Rajeshwar et al, 1994; Huang et al, 2001; Acar and Alshawabkeh, 1993).

the subsurface, pump and treat systems can prevent site contamination.

movement towards groundwater.

842 Environmental Risk Assessment of Soil Contamination

and transported more easily.

nated material.

**5.3.** *In situ* **chemical treatment**

The processes taking place during electroremediation can be classified into two main catego‐ ries: (a) processes occurring as a consequence of the applied electric potential. These processes include electromigration (ion transport), electroosmosis (mass transport), and electrophoresis (charged particle transport); (b) processes occurring in the absence of an electric potential. This includes concentration induced processes like diffusion, sorption, complexation, precipitation and acid - base reactions (Reed et al, 1995; Bustos, 2013).

Specifically, for mercury polluted soil electroremediation, the use of complexing agents like ethylendiaminetetraacetic acid (EDTA), KI, and NaCl under a constant potential gradient has been reported (Reddy et al, 2003). Based on the above precedents, the electroremediation was developed aided by extracting agents for mercury removal from San Joaquin's Sierra Gorda soil samples (Figure 5, Robles et al, 2012).

Electroremediation of mercury polluted soil, facilitated by the use of complexing agents, proved to be an attractive alternative treatment for the removal of mercury from polluted soil in mining areas located at Sierra Gorda in Queretaro, Mexico (Figure 5A and 5B). Implemen‐ tation of this remediation protocol is expected to improve the living conditions and general health of the population in the Mine "El Rincón" in San Joaquin (Figure 5C). Experimental observations suggest that it is possible to remove up to 75 % of metal contaminants in mercury polluted soil samples by wetting them with 0.1 M EDTA, placing them in an experimental cell equipped with Ti electrodes, and then applying a 5 V electric field for 6 hours (Figure 5D, Robles et al, 2012). When we followed the electrochemical removal of mercury in a batch reactor (Figure 6A), it was removed around 87 % of Hg2+ in a time of 9 hours close to the anode side by the presence of EDTA (Figure 6B). The pH remains nearly constant at 4 and conductivity showed values close to 10 mS cm-1 by the ionic species.

The efficient removal of mercury contaminants observed under these conditions is attributed to electromigration of the coordination complexes that form between the terminal hydroxyl groups in EDTA and divalent mercury (Hg+2), which is probably strengthened by supramo‐ lecular interactions between unshared electrons at EDTA's tertiary amino nitrogens and Hg +2. These interactions are particularly effective with the presence of potassium ions. This observation is supported by molecular modeling of several possible interactions in the proposed complex using the Density Functional Theory method (B3LYP LANL2DZ, Robles et al, 2012, Figure 7).

**Figure 5.** Localization of Queretaro in Mexico (A) with satellite image from San Joaquin's Sierra Gorda, Queretaro (B) where there is the Mine "El Rincón" (C) with high concentration of Hg2+, which was removed with electroremediation process in continues flow in presence of EDTA (D).

**Figure 6.** Electroremediation process in batch reactor assisted by EDTA (A), and its corresponding removal percentage of Hg2+ followed during 13 h of treatment, close to anode and cathode.

**Figure 7.** Optimized conformation and molecular structure of the proposed 2 Hg+2 / 2 EDTA / 4 Na+ complexes (B3LYP LANL2DZ, Robles et al, 2012).
