Electroremediation of contaminated soil from a chlor-alkali factory

Pascal Suèr * and Bert Allard

Man – Technology – Environment Research Centre, Örebro University, S-70182 Örebro, Sweden

* Corresponding author: tel: +46 19 303546, fax: +46 19 303169, e-mail: pascal.suer@nat.oru.se

Abstract

An electric field was applied to contaminated soil from a chlor-alkali factory. Electroosmosis was kept to a minimum to study the effect of electromigration. Transport of heavy metals and macroelements occurred.

Most metals moved towards the cathode. The order of migration was calcium, magnesium, nickel, zinc manganese, copper, chromium. Strong hydrolysis led to retardation.

Lead formed an anionic complex which moved towards the anode. At the same time, cationic lead moved towards the cathode, leading to a double peak in the final profile.

Mercury moved towards the cathode, in spite of high chloride concentrations. Mercury near the cathode was transformed into volatile species.

Sequential extraction on the soil after 182 days showed extremely low metal availability where the metals concentrated. Risk of remobilisation is low.

Introduction

The application of an electrical field can move ionic and uncharged contaminants in polluted soil. This could be very valuable in the remediation of contaminated clayey soils, which are untreatable with current technology. A good review of electroremediation is given by Alshawabkeh et al. (1999).

There are indications that the treatment with an electric field might lead to a higher mobile metal fraction. Ribeiro (1988) and Ribeiro et al. (1998) have shown higher mobility for copper after exposure to the electric field, mainly due to accelerated weathering. If electroremediation causes a higher percentage of mobile ions in the soil, removal of the metals remaining after treatment would be facilitated. However, terminating the treatment process at an early stage would increase the short-term risk to the environment.

This study concerns the movement and mobility of mercury, lead, copper, zinc, nickel, chromium, cadmium, manganese, iron, calcium and magnesium during and after electroremediation. Metal concentrations in a transect between the electrodes are followed. Distribution and mobility is analysed by sequential extraction, or rather selective leaching, of soil samples after 6 months of electrokinetic treatment.

Methodology

Soil polluted with mercury, dioxins, lead and other contaminants was taken from the site of a choralkali factory. Several hundred kilograms were mixed on site. At the laboratory, the soil was sieved and the fraction smaller than 4 mm used for the experiment.

Electromigration experiments were conducted in a 47´10´10 cm³ plastic box. Graphite plate electrodes were placed at the ends of the box, separated from the soil by 10 cm of water and a geotextile (called electrode compartment). The total length of the soil column was 27 cm. The experiment was conducted with a constant charge of 30 Volt over the electrodes, and lasted 182 days.

The solutions in the electrode compartments consisted of 0.01M NaCl, and were stirred continuously. To hinder electroosmosis and drying of the soil, each electrode compartment was connected to a tank by a siphon. Due to evaporation, solution had to be added regularly to the tanks, resulting in a final chloride concentration of 0.08 M Cl at the anode and 0 M Cl at the cathode (analysed by AgNO₃-titration). Chlorine gas was detected at the anode. The anode and cathode solutions contained negligible amounts of metals.

Soil samples were taken with a plastic syringe at 4, 9, 14, 19 and 24 cm into the soil from the anode side. A fraction of the sample was centrifuged and the pore water was analysed by ICP-AES. Another fraction was dried for 24h at 40°C, and then subjected to a selective leaching procedure.

Selective leaching

The dried soil samples after 182 days were subjected to selective leaching according to the following procedure (adapted from Lifvergren et al. (2000a)): 1 gr of soil was agitated with 20 ml of respective leachant. Centrifugation and analysis of the supernatant followed.

For mercury, on top of the previous analyses, the centrifugation residue from the extractions on 4, 14 and 24 cm samples was digested and analysed, so that mercury recovery from the sequential extractions could be calculated.

The following leachants were used:

A. 1 M sodium chloride for 24h. Weakly adsorbed and readily mobilised metals are analysed in the supernatant

B. 1 M sodium hydroxide for 7d. The alkaline conditions solubilise e.g. humic matter.

C. 0,1 M hydrochloric acid for 5h. These metals are mainly associated with carbonates and iron- and manganese oxides. This fraction also includes weakly adsorbed metals according to A

D. 5 ml hydrogen peroxide and 15 ml concentrated nitric acid at 60 °C for 3 h. On top of metals mobilised in fraction A and C, the hydrogen peroxide oxidises all organic matter in the soil.

E. Total content. 1 gram of dried soil was digested by open focused microwave extraction with aqua regia (Lifvergren et al., 2000b).

Each leaching was performed on a fresh soil sample (in parallel).

Mercury was analysed by CV-AAS; copper, zinc, lead, nickel, chromium, cadmium, manganese, iron, calcium and magnesium were analysed by ICP-AES.

Results

The pH, voltage gradient and total content of metals in the soil were measured several times during the electromigration experiment. Hydrogen ions produced at the anode, and hydroxyl ions produced at the cathode, migrated into the soil, resulting in a steep pH gradient. On the anodic side of the pH shift, pH is below 2, while the cathodic side has pH of ca 10. Almost all the soil is acidic after 6 months (fig 1).

Calcium, magnesium, nickel, zinc, manganese, copper and chromium are removed from the soil as pH in the respective soil location decreases, and concentrate in the high pH zone near the cathode. Lead however, has moved both towards the cathode and the anode, resulting in a double peak (fig 1).

The mercury content near the anode does not decrease, despite high chloride concentration and low pH, which should lead to formation of mercurychloride complexes (Barrow and Cox, 1992). Mercury moved from the middle zone to 19 cm, where concentrations are highest (fig 1).

The iron in the soil is redistributed, resulting in higher concentrations near the cathode (not shown).

Selective leaching

Near the anode, where removal takes place, total concentrations are low. Bearing that in mind, the mobilisation potential is relatively low, except for nickel (fig 2). Copper is also associated with low molecular weight organic material, 40-50% of the total copper is extracted by leachant B (not shown).

The mercury in the untreated soil was strongly sorbed to insoluble organic matter (50%, leachant D), or in forms even less available (Lifvergren et al., 2000a). After the electrokinetic treatment, mercury distribution near the anode is similar to the original distribution (fig 2).

Fig 1: pH and some cations during the electromigration experiment

Fig 2: Selective leaching after 6 months, 4 cm and 24 cm from the anode filter

Many metals have accumulated at 24 cm from the anode filter. Here, around 60% of nickel, zinc, and manganese, and 20 % of lead and copper are mobilised by acidification (leachant C, fig 2), solutions A and B extract none. The remainder of the metals is only mobilised by microwave digestion with aqua regia.

At the mercury peak near the cathode, no mercury was mobilised by leachant D. The mercury was extremely insoluble. However, the mass balance of the sequential extractions shows only 45% mercury recovery. This leads us to believe that mercury is volatile.

Discussion

Calcium, magnesium, nickel, zinc, manganese, copper, and chromium are transported towards the cathode and are immobilised when pH rises. The rate of peak transport follows the hydrolysis constant for the elements, i.e. Ca > Mg >Ni » Zn » Mn > Cu > Cr(III) (Allard, 1995; Appelo and Postma, 1996). High hydrolysis leads to retardation, so that calcium is removed first and chromium last.

The double peak observed for lead indicates the presence of two dominating species. One fraction is transported towards the cathode, in analogy with the other cationic metals species. A significant fraction is moving towards the anode, which indicates the formation of an anionic lead species.

Mercury immobilisation near the cathode is not an effect of pH, since pH is still below 3 at the peak location. Reductive conditions imposed by the cathode could result in the formation of elemental mercury. Formation of a volatile mercury form is supported by the results from the sequential extractions. Transformation of volatile mercury species to non-volatile near the anode has previously been detected by Alliger (1997). Here the opposite would be occurring.

In the alkaline zone, the metals are difficult to mobilise. No metals are found in the pore water, and surprisingly little is mobilised under acidic conditions. The high amounts of metals in fraction E compared to the starting values for the metals prove that fresh metals are found in this fraction. The elements are strongly bound to the soil in spite of their recent arrival, and risk for remobilisation is low. Thus, in this system, electroremediation seems to be feasible for nickel, zinc, copper and possibly lead. Mercury exhibits only minor mobility in the soil, but volatile species might be formed.

Acknowledgements

The authors are much beholden to K Gitye and H Ericsen at the Swedish hazardous waste treatment plant (SAKAB) for ICP and chloride analysis respectively. This work was financially supported by MISTRA, as a part of the COLDREM program, and by the KK- foundation.

References

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