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Electrokinetic Injection in Highly Organic Soil—A Review

Hossein Moayedi

PhD Candidate, Department of Civil Engineering, University Putra Malaysia, Serdang, Selangor, Malaysia

e-mail: hossein.moayedi@gmail.com

Bujang B. K. Huat Professor, Department of Civil Engineering, University Putra Malaysia, Serdang,

Selangor, Malaysia; e-mail: bujang@eng.upm.edu.my

Thamer Ahmad Mohammad Ali Shokoufeh Ansari Moghaddam

Pezhman Taherei Ghazvinei Department of Civil Engineering, University Putra Malaysia, Serdang, Selangor,

Malaysia

ABSTRACT Peats soil which named after highly organic matter soil does not include any kind of minerals except mineral from clay content which may increase strength parameters of based peat. An overview of the principals of the electrokinetic bioremediation technique in peats is presented. The current status of the electrokinetic remediation technique and recent development in ion migration transport induced undrained shear strength of soft clay soils are discussed through a review of the bench-scale and pilot-scale tests. The main objective of this review is more on the soft soil stabilization and finding easier way to achieving isoelectric point during EK processes. Keywords: Zeta potential, Electrokinetic, Ion migration, Tropical Peat, Isoelectric point.

INTRODUCTION The behavior of soils that consist predominantly of organic matter or non-crystalline materials is so intimately

related to the soil composition that for them the soil taxonomy replaces mineralogy with other indicators of soil behavior. In organic soils, the differences in soil behavior arise in part from differences in the degree of decomposition of the organic residue. These differences in the degree of decomposition are accommodated at the suborder level in the soil taxonomy. For soils dominated by non-crystalline materials, the differences among soils are accounted for by particle-sized modifiers specifically designed for these soils. In fact mineralogy plays such an important role in forming the character of a soil that the key features employed to differentiate soils at the highest level depend on mineralogy.

Peats soil which named after highly organic matter soil does not include any kind of minerals except mineral from clay content which may increase strength parameters of based peat. On the one hand, they have high water holding capacity, high negative charge and high cation exchange capacity. Construction on these soils are often

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affected by stability and settlement problems. Building constructed over soft organic soil commonly affected from two main problems which are high compressibility as well as high water content. Also, low permeability of peat soils make it tough to be stabilized by injecting grout materials. While conventional soil grouting methods are able to stabilize soft soils with high permeability, appropriate remedies should be considered regarding injecting stabilizers through low permeability soils. Several methods are commonly can be used namely; induced consolidation, artificial ground freezing and hydro-fracture grouting, either by applying electric fields or surcharging the soil. All of these methods shows considerable degree of ground movement, and have other drawbacks. With the increase in construction in mechanically substandard soils regarding urban developing, new methods are needed to improve soft soil with low permeability.

The electrokinetic (EK) technique is defined as a physicochemical transport of charge, action of charged particles, and effects of applied electric potentials on formation and fluid transport in porous media [1-2]. Ionic species in the pore fluid are transported across the soil mass both by electromigration and also by electroosmotic transport [3]. Recent improvements in understanding the principles of electrokinetic remediation offer the opportunity to employ chemical species transport mechanisms under electrical fields to inject chemical species for in-situ immobilization/stabilization of chemicals and porous media; to inject nutrients and process additives for enhancement of in-situ bioremediation [4-8]. A good example of such media could be soft and compressible soils which are most susceptible between porous media such as soil with highly organic matter. On the other hand, chemical reagents which mostly used as stabilizer significantly effect on charge which surrounded particles. They cause colloidal particles to flocculate enhanced their strength properties. The charged surfaces hold ions, and ions are held preferentially depending upon relative abundance. Also, the quantity of exchangeable cations in the zone adjacent to the charge surface that can be exchangeable for other cations is termed the cation exchange capacity (CEC). In a word, CEC is the ability of the soils to supply cations [9] The ease of cation replacement depends on the valence, ion size, and ion relative amount. The range of soil colloid CEC is from a mean minimum about 4 for Al or Fe oxides to a mean maximum about 200 cmol+/kg of colloid for humus [10]. Thus humus has the highest CEC in comparison with minerals like kaolinite, montmorillonite, smectite and even vermiculite. Unlike clay minerals, organic matter does not have a fixed capacity for binding exchangeable cations and exchange capacity increases markedly with increasing pH. This paper aims to provide a review of the fundamentals of electrokinetics, probable transport in tropical peat soils, and the recent development of ion migration in electrokinetic phenomena enhanced chemical species flow as injection material through soft soils with a very low hydraulic conductivity.

ELECTROKINETIC REMEDIATION MERITS The presence of the diffuse double layer gives rise to several electrokinetic phenomena in soil, which may result

from either the movement of different phases with respect to each other including transport of charge, or the movement of different phases relative to each other due to the application of an electric field [3, 11]. The point of zero net charge (PZNC) is the pH at which there is equal adsorption of cations and anions from an indifferent electrolyte of a sample containing a mixture of variable and permanent charge minerals.

While the acid generated at the anode advances through the soil toward the cathode by ionic migration and electro-osmosis, base developed at the cathode initially advances toward the anode by diffusion and ionic migration. However, the counter flow due to electroosmosis retards the back diffusion and migration of the base front. The advance of this front is slower than the advance of the acid front because of the counteracting electro-osmotic flow and also because the ionic mobility of H+ is higher than OH- [12-14]. Geotechnical reactions in the soil pores significantly impact electrokinetic phenomena and can enhance or retard the process. Geomechanical reaction including precipitation, dissolution, sorption, and complexation reactions are highly dependent upon the pH condition generated by the process [2].

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A major advantage of electrokinetic remediation is that flow migration by electroosmosis or ion transportation by means of electro-migration is remarkable in fine-grained soils with low permeability. Another advantage is the possibility of developing a relatively uniform transport in heterogeneous deposits [15]. Although ion migration rates are higher in sand than in clays, the differences are not as significant when compared with transport by hydraulic gradient, where the hydraulic conductivity of sandy soils is orders of magnitude higher than clayey soils. Thus hydraulic gradients are ineffective in removing contaminants from heterogeneous soils because of the limited transport in clays [15]. Figure 1 shows comparisons between transport under hydraulic and electrical gradients in sands and clay. As can be seen a relatively uniform transport can be produced in heterogeneous soils by ionic migration.

Figure 1: Comparison of transport rates by hydraulic and electrical gradients in sand and clay [15]

ELECTRO GROUT INJECTION PROCESS Electro-kinetic injection technique, form an electric field due to insertion of both anode and cathode electrodes

and applying low direct current through them. Injected ions are transported through ion migration by electric potential gradient, electro-osmotic flow, advection by hydraulic gradient, and diffusion of chemical gradient under electric field [16-17]. It should be mentioned that electro-osmosis mobilizes the pore fluid, carrying the ions toward the cathode, while, ionic migration effectively transports negative anions to the positive anode and positive cations to the negative cathode. The important difference is that this mode of chemical transport can occur without any fluid flow [17-20].

Gray (1970) studied electrochemical hardening of soft soils by electro-osmotic injection of aluminum, used because it is a product of electrolysis reactions in aluminum anodes. The results show the liquid limit of montmorillonite dropping to about half its initial value, while the liquid limit for illite was not significantly changed. Their results clearly indicate that it is possible to alter the clay properties and cause mineralization by electrochemical methods [19]. Acar and Alshawabkeh (1993) have demonstrated the electroosmotic component of transport may disappear in coarse sands and high plasticity clays at low water contents. Also they mentioned that, that mass transport by migration may be up to 300 times more than mass transport by electroosmosis in experiments carried out by placing electrodes in a pore fluid chamber on both sides of a kaolinite specimen [1]. Acar et al. (1997) suggested a both-direction injection system, in which ammonium hydroxide and sulfuric acid were added in anode and cathode reservoirs, respectively. Hydrogen ions generated from anode by electrolysis neutralized the hydroxyl of ammonium hydroxide injected to control the reservoir via pH regulation pump. In addition, hydroxyl ions generated from cathode by electrolysis were neutralized by sulfuric acid. Through these processes, ammonium ions were injected into the anode and sulfate ions were injected into the cathode [4].

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On the one hand, Ozkan et al. (1999) investigated the use of electrokinetics to inject aluminum and phosphate ions to stabilize kaolinite and also proved that there was an increase in the undrained shear strength of up to 500–600% in the treated samples [21]. Lefebvre and Burnotte (2002) showed that a chemical treatment to increase the soil conductivity at the soil-electrode contact can double the electrical potential effectively transmitted to the soil and improve the performance of electro-osmotic consolidation [22]. Otsuki et al. (2007) injected several anolyte and catholyte solutions into kaolininte and proved that there was an increase of shear strength up to 300 kPa [7]. Effects of the injection of chemical solutions vary from one chemical to another and are contingent upon the test material. It could be due to different reaction between colloids and cations that come from stabilizers reagents

A NEW NOVEL TECHNIQUE ENHANCED ELECTROKINETIC INJECTION IN PEATS

In electro-osmotic dewatering, the frictional drag is produced by the movement of hydrated ions. The quantity

of these ions depends on the soil CEC. The CEC range of humus is from 100 to 300 kgcmol /+ , which is the highest among colloids. The mineral fractions of many tropical regions are dominated by kaolinite, aluminium

oxides, and iron oxides. The CEC range of kaolinite and Al, Fe oxides is 5-10 and 2-6 kgcmol /+ of soil respectively. Thus, humus is the key component to create a potential ability for a water momentum in electro-osmotic phenomena. Since cations adsorption in peat is high pH dependent, therefore, precise measure of CEC at soil pH is highly recommended for electrokinetic experiments.

The charge on humus, kaolinite, and Fe and Al oxides is affected by pH. As the pH goes up, the H+ dissociates and produces a net negative charge [23-24]. As the pH drops, there is less and less negative charge [2]. In all

Al and Fe oxides, as well as some silicate clays, exposed −OH groups in moderate to acid conditions experience

protonation. This occurs as an +H attaches to the −OH . Since organic matter is generally negatively charged, thus, the occurrence of a water flow from anode to cathode is fully expected in electro-osmotic phenomena.

Increasing the pH leads to greater inter-particle repulsion (and thus smaller particles) while low-pH conditions favor aggregation (and thus larger particles). As the pH increases, the grain size distribution becomes skewed toward a finer distribution. Lower pH at anode may cause decreasing of diffuse double layer thickness and flocculation of particles results in an increase in undrained shear strength, PL, and LL, while higher pH at cathode may cause more negative ζ, thickening of diffuse double layer, and dispersion of particles which results in a decrease in LL and undrained shear strength at anode. As in peat soils, the charge is fully pH dependent, thus, the sensitivity of peat is more than minerals due to mentioned reactions. In addition, the peat soil CEC is high, therefore, the movement of ions away from the anode may cause thickening of diffuse layer at the cathode. Since humified peat is more sensitive to pH than fibrous peat, therefore, humified peat sounds make many geoenvironmental and geotechnical surprises under utilization of electrokinetic techniques. That means by adjusting pH of soil after injecting chemicals close to the pH at isoelectric point, results will be more remarkable since maximum flocculation occurs in this pH. On the other hand, the water holding capacity of a fibrous peat is higher than a humified peat, however, the higher CEC and higher pH of the humified peat due to higher charge, may make a better condition for electro-osmotic phenomena especially by applying new novel EK injection technique. Based on the soil physicochemical properties (i.e. soil pH), stabilizers will be selected to lead zeta potential close to zero since this is the point that maximum flocculation occur.

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CONCLUSIONS Electrokinetic injection of nutrients and process additives could become an essential tool for the in-situ

bioremediation of heterogeneous subsurface deposits with widely varying hydraulic conductivities. Multi-component species transport under electric fields is an area that is getting more attention [15]. Species transport mechanism under electric fields are envisaged to be employed in remediating soils from inorganic and organic species (electrokinetic remediation), injection of electron acceptors and nutrients in in-situ bioremediation, injection of grouts in soil stabilization and waste contaminant. In such cases, optimization of electrokinetic behavior could lead to higher peat soil resistivity and low energy expenditure on the processing, however, peat itself has a remarkable resistivity. Also, such optimization is based on pH controlling, suitable electrolyte type and appropriate concentration. Moreover, these parameters could act as boundary condition that effect on physicochemical parameters of colloidal soil such as peat. Implementing such idea can utilized in different problems which regularly occurred in geotechnical application [25-27].

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[25] Moayedi, H., Huat, B.K., Thamer, A.M.A., Torabihaghighi, A and Asadi, A., 2010., "Analysis of Longitudinal Cracks in Crest of Doroodzan Dam" Electronic Journal of Geotechnical Engineering, USA.

[26] Moayedi, H., Huat, B.K., Kazemian, S. and Asadi, A., 2010, "Optimization of Shear Behavior of Reinforcement through the Reinforced Slope" Electronic Journal of Geotechnical Engineering, USA.

[27] Moayedi, H., Kazemian, S. Parasad, A. and Huat, B.K., 2009, "Effect of Geogrid Reinforcement Location in Paved Road Improvement" Electronic Journal of Geotechnical Engineering, USA.

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