Environ. Sci. Technol. 2001, 35, 2151-2155
Major Ion and Electrical Potential Distribution in Soil under Electrokinetic Remediation SHIN-ICHIRO WADA* AND YUKI UMEGAKI Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan
To assess the effect of pore water chemistry on the contaminant removal rate, we monitored major ion concentrations in the pore water and the electrical potential distribution of a soil during electrokinetic remediation treatment. On application of the voltage, the soil near the anode became acidic due to the electrolytic generation of H+, and the acid region gradually spread toward the cathode. The major cation in the acid region was, however, not H+ but Al3+ that arose from the acid-induced dissolution of soil minerals, and it migrated very slowly toward the cathode. The measured pH and accompanying ion concentrations indicated that the anomalously slow migration of Al3+ was due to its precipitation-dissolution reaction at the acid front. The stagnancy of Al3+ increased the ionic concentration, flattened the electrical potential profile, and in turn, diminished electromigration in the acid region. This seems to be one of the causes of the relatively low removal rate of cationic and anionic contaminants in electrokinetic treatments.
niques (3, 7-9), however, it took more than a week to remove heavy metals from 10- to 20-cm columns of soils and clays with an applied voltage of 20-30 V, suggesting that the migration velocity was much less than reported values (10). Major causes for this would be the slow kinetics of desorption and dissolution (4) of heavy metal cations and reprecipitation as hydroxide (3, 7). In addition, local flattening of the electrical potential profile might have contributed to the observed slow migration. Segall and Bruell (11), Hamed and Bhadra (12), and West et al. (13) reported that a region of very low potential gradient developed from the anode end toward the anode. The migration velocity of a contaminant trapped in this region would be much slower than that expected from the average potential gradient calculated from the length of a soil column and the potential difference applied to the electrodes. If the potential difference between the electrodes is kept constant, the potential profile in a soil is determined practically by the ionic distribution in pore water. Thus, the major ion distribution in a soil would be one of the important factors affecting the efficiency of electrokinetic remediation. Dzenitis (14) found that the cation exchange and release of Al from soil minerals in acidic region are the key reactions to be included in a simulation model for prediction of electroosmotic flow rate. To date, however, other comprehensive experimental data on the pore water chemistry during electrokinetic remediation and its effect on the electric potential profile in the soil have not been presented. The present study was conducted, therefore, to obtain basic information of how electrode reactions, soil mineral dissolution, electroosmosis, and electromigration contribute to the ion transport and electric potential profile in a soil under electrokinetic remediation. A constant voltage was applied to three equivalent test samples of a noncontaminated natural soil for different periods of time. The samples were sliced into several segments, and they were analyzed for pH and major ion concentration of the pore water.
Introduction
Experimental Section
Electrokinetic remediation is recognized as one of the hopeful technologies that has been developed to remove contaminants from soils and sediments. Some advantages of the method over other technologies are that it can be practiced in situ, in principle, and there is less difficulty in implementing in fine textured soils having low hydraulic conductivity. Among the two important transport mechanisms, i.e., electromigration and electroosmosis, the former is more important for the removal of ionic contaminants (1). The observed migration of anionic contaminants (2-4) demonstrates that the contribution of the electromigration is larger than that of the electroosmosis. For successful removal, therefore, it is necessary to convert the precipitated and adsorbed contaminants to dissolved ionic forms. Fortunately, electrolytic hydrogen ion production at the anode and the subsequent acidification of the soil induce the release of adsorbed heavy metal cations into pore water (1, 5, 6) and dissolution of metal hydroxides. Injection of inorganic or organic acids and supply of chelating agents are often employed to enhance acidification and solublization (1, 3, 4, 7-9). Baraud et al. (10) measured velocity of some cations in a kaolinite bed under a uniform electric potential gradient and found that Na+, K+, and Ca2+ traveled at 50-60 cm days-1 V-1. In many remediation studies with enhancement tech-
Soil Sample. A surface soil was collected at a depth of 0-20 cm from the experimental farm of Kyushu University. According to US Soil Taxonomy (15) the soil was classified as Dystric Eutrochrept. The collected soil was air-dried at room temperature, gently ground with a wooden pestle, and passed through a 2-mm screen. Table 1 lists some chemical and mineralogical properties. For particle size analysis, a 10 g portion of the air-dried sample was digested with hot 70 g kg-1 H2O2 to remove soil organic matter and subjected to 19.5 kHz ultrasonic treatment at a power of 200 W for 15 min followed by pH adjustment to 9 with NaOH. From the resulting stable suspension, the clay fraction (
