Electrical Resistivity of Tropical Peat Afshin Asadi Department of Civil Engineering, University Putra Malaysia Serdang, Selangor 43400 Afshin.Asadi@yahoo.com Bujang B.K. Huat Department of Civil Engineering, University Putra Malaysia Serdang, Selangor 43400 bujang@eng.upm.edu.my ABSTRACT Peat represents the extreme form of soft soil. Using electro-osmotic techniques to improve the peat entails evaluating electrical properties of the soil. Electrical resistivity and conductivity of peat soil regarding to its pore fluid, organic content, temperature, and degree of humification were investigated to conceptualize electro-osmotic phenomena. The results of the study revealed that the electrical resistivity of the peat decreased as the water content or the temperature increased. The resistivity of the humified peat was lower than the unhumified peat, meaning a higher degree of peat decomposition resulted in a lower peat resistivity. The study showed that the resistivity of peat increased as the organic content increased. The electrical conductivity of the humified peat was higher than the electrical conductivity of the unhumified peat. The electrical conductivity of peat decreased as the organic content increased. The study thus far showed that a highly decomposed peat could be a better candidate for using electro-osmotic techniques in comparison with unhumified peat. KEYWORDS: Peat, conductivity, electro-osmosis, humification, resistivity. INTRODUCTION Soil conductivity is essential to understand the mechanism of electro-osmotic (EO) phenomena in soils. In view of better control of geotechnical, civil, and environmental engineering applications of electro-osmotic (EO) phenomena, fundamental research studies have been done in mineral soils dewatering, consolidation, and stabilization (Casagrande, 1949, Barker et al., 2004); and contaminant removal (Castillo et al., 2008; Fernandez et al., 2009). Although the principle of EO phenomena is simple, physicochemical interactions that occur simultaneously at soil-fluid interface are not well understood (Asadi et al. 2007). Complications arise in that the changes in physicochemical properties at soil-liquid interface are ph-, moisture content-, and resistivity- dependent. Application of direct current through a soil specimen induces
Vol. 14, Bund. P 2 three mechanisms including redox, water decomposition, and ion migration. Such mechanisms could change the physicochemical properties at the soil-liquid interface and would alter the electrical properties of the soils (Liaki et al., 2008). Soil resistivity is one of the important EO properties of soil colloids. Electrical conductivity phenomena arise from the movement of ions or electrons through a conducting system under the influence of an electric field. Electricity conduction in soils takes place through the moisturefilled pores that occur between individual soils particles (Hamed and Bahdra, 1997). Resistivity of a soil depends on the surface conductivity of the colloids (i.e. clay or/and humus), presence of ions, moisture content, and temperature; and is determined according to the Ohm s law. Resistance is that property of a conductor which opposes electrical current when a voltage is applied across the two ends. Resistance is a ratio of the applied voltage to the resulting current flow. The resistance of a conductor depends on the atomic structure of the material or its resistivity, which is that property of a material that measures its ability to conduct electricity. The resistivity measured in Ohm-m, and can be derived from the resistance, length, and cross sectional area of the conductor. Electrical conductivity (EC) of a soil is also an ability of a soil to conduct electrical current. EC is measured in siemens per distance between the measurement points in a medium point. EC rises from a very low value (around 1 µs/cm) for deionized distilled water to several orders of magnitude higher for a fluid containing electrolytes, depending on type and concentration (Hamed and Bahdra, 1997). Peats have noticeable qualities (saturated mass, net negative surface charge, high cation exchange capacity, and high specific surface area) to make a suitable environment for utilization of electrokinetic techniques (Asadi et al. 2009; Huat et al. 2009). Therefore, study of electrical properties of peat would be important. The resistivity of humus colloidal material regarding to electro-osmosis has not investigated in detail yet. The humus colloids are the most chemically fractions of peat soils. Based on the American Standard for Testing and Material (ASTM), peat is a soil material having organic content of more than 75%. Peat is formed under anaerobic conditions through the action of bacteria, fungi, and chemical compounds on plant remains (Huat, 2004). The variations in peat arise from the variety of plants whose residues contribute to peat formation, and from the environmental conditions in which humification takes place (Edil and Fox, 2000). The first researcher to classify peat on physical properties was Von Post (1922), who developed a field method to indicate stages of decomposition. There are 10 degrees of humification (H1 to H10) in the Von Post system, which are determined based on the appearance of the peat water that is extruded when the soil is squeezed by hand (Huat 2004). Based on the ASTM D 1997, peat is classified according to the fiber content. The brownish, fibrous, and partially decomposed peat is termed fibric and semi-fibric; and highly humified, black, and powdery peat is termed amorphous. The total tropical peat land in the world amounts to about 30 million hectares. In Malaysia some 3 million hectares of the country s land area are covered with peat. These soils are extremely soft and geotechnically problematic (Huat, 2004).
Vol. 14, Bund. P 3 The resistivity of peat soils regarding to some soil properties such as pore fluid, organic content, temperature, degree of humification, and CEC has not been reported yet and provide an excellent context for this study. MATERIALS AND METHODS Materials Representative peat samples of a very slightly and a highly decomposed peat were selected using Van Post humification scale and collected in accordance to the British Standard Institution (BSI) methods of test for soils from two different locations of Klang, Selangor, Malaysia. Clay soil samples from a layer of the soil on the surface were also collected from same area. Some holes were dug up to collect the respective pore peat fluids. Figure 1 shows the sample collection of the peat, clay soil, and pore fluid of the peat. (a) (b) (c) Figure 1: (a) Peat Collection, (b) Clay Soil Collection, and (c) Peat Pore Fluid Collection Laboratory Resistivity Cell A diagram of the resistivity cell used in this study is presented in Figure 2. The resistivity cell consisted of an acrylic box which was 170 mm in length, 8 mm in width, and 120 mm in depth, where the soil was moulded, connected at both ends to titanium electrodes. The electrodes had a central pin that went out of the box and were connected to a power supply. An oscilloscope and digital multimeter allowed the signal voltages and current to be viewed, respectively. Figure 2: Schematic Setup of Resistivity Cell
Vol. 14, Bund. P 4 Physicochemical Properties The soil samples were prepared in accordance with BSI methods of test for soils 1377-1: 1990. The liquid limit (LL), particle density, and organic content were determined according to BSI 1377-2-4-3: 1990, 1377-2-8-3: 1990, 1377-3-4: 1990, respectively. To measure the electro-negativity of the soils, the cation exchange capacity (CEC) of the peat soils was measured at ph 7 with ammonium acetate method (Chapman, 1965). X-ray diffraction (XRD) method was used to identify the crystalline mineral species presented in the clay fraction (Brindley and Brown, 1980). Electrical Test Procedures The resistivities of samples were determined according to the Ohm s law. The resistivity cell was fabricated to be compatible with EO laboratory apparatus. In order to make a wider difference between two representative samples, the H7 peat was mixed in 10 % passing the No. 100 sieve of the peat. The soil specimens were prepared by mixing 1 kg of the last representative peat samples with different amount of the clay soil and peat pore fluid to bring the soils to the desired specimens in a process of trial and error. After sample preparation, the organic content and water content of the specimens were measured again and the last measured values were recorded as the soil properties. The temperature of each specimen was adjusted using an incubator. Each specimen was then gently placed in the resistivity cell and the titanium electrodes were inserted into the soil at the both ends of the resistivity cell. In order to increase the degree of accuracy, different constant electrical potentials of 40, 70, and 90 V were applied across the specimen. The currents were recorded to calculate the average resistivity of the soil. At the end of testing the current was terminated and the resistivity setup was dismantled. A conductivity meter was also used to measure the electrical conductivity of the different samples in the process. RESULTS AND DISCUSSION Physicochemical Properties Table 1 shows the basic properties of the representative samples. The main mineral fraction of the clay soil was dominated by kaolinite. The organic content and liquid limit of the very slightly decomposed peat was higher than the highly decomposed peat. The CEC of the highly decomposed peat was higher than the CEC of very slightly decomposed peat. Organic content is the loss of ignition as a percentage of the oven dried mass. Humification process means the loss of organic matter either as gas or in solution and the end products of humification are carbon dioxide and water (Huat, 2004). Therefore, there were two possibilities for difference between organic contents of the peats, (i) the mass of ignition loss decreased during time due to humification process, and (ii) since the age of a humified peat is higher than the age of an unhumufied peat, thus there was a higher chance to be mixed with mineral materials. However, the loss of ignition test is easy to make clear organic content of a peat.
Vol. 14, Bund. P 5 Table 1: Physicochemical properties of representative samples Parameter Squeezed pore fluid Particle density, Mg/m³ ASTM classification system Liquid limit, % H2 peat Yellowish 1.13 Fibric 235 H7 peat Very dark 1.2 Amorphous 155 Soil ph Organic content, % CEC, meq/1000 g soil 4.5 94 43 6.50 76 92 Clay - 2.63-75 5.1 16 12 Electrical Properties of Peats The resistivity of the specimens was affected by water content and temperaturee as depicted in Figures 3 and 4. The results of the study showed that the resistivity of the peat decreased as the water content or the temperature increased. Despite the fact that the water content of the very slightly decomposed peat was higher than the water content of the highly decomposed peat, interestingly, the resistivity of the highly decomposed peat was lower than the very slightly decomposed peat, meaning a higher degree of peat humification results in a lower peat resistivity. The study revealed that the resistivity of both very slightly decomposed and highly decomposed peat increased as the organic content increased. Figure 3: Resistivity and Water Content (Very Slightly Decomposed Peat)
Vol. 14, Bund. P 6 Figure 4: Resistivity and Water Content (Highly Decomposed Peat) The study also showed thatt the EC of the highly decomposed peat was higher than the EC of the very slightly decomposed peat. The EC of both highly- and very slightly- decomposed peat decreased as the organic conten increased (Figure 5). Figure 5: Electrical Conductivity A good understanding of the humus as a most chemically active fraction of the peat colloids could make clear the underlying reasons for the electricall behavior of tropical peat. The peat have a net negative charge because of the negative charges on the organic matter (Asadi et al., 2009). The negative charge of humus is generally believed to be due to the dissociation of H + from functional groups. Colloids (humus) are the most chemically active fractions of the peat soils.
Vol. 14, Bund. P 7 They are dynamic and very active in charge. The source of ions and source of electro-negativity) are chemical properties, and the large surface area per unit of mass and the consistency are physical properties imparted to the peat by humus, respectively (Stevenson, 1994). Since the highly decomposed peat have a higher CEC and higher negative charge; and have higher quality and quantity of chargeable colloidal particles compared to the very slightly decomposed peat, resulted in a lower electrical resistivity and higher electrical conductivity. The humification processes of peat soils are chemical, biological, and enzymes (Yule and Gomez, 2008). The bacteria, soil micro flora, and fungi are responsible for the breakdown of the plants. The higher degree of humification results in the higher contribution of humus to the soil surface charge. Therefore, two conditions would affect the soil matrix due to humification processes: (i) the changes in soil particle size and tends to the finer particles and (ii) the complex mechanisms of humification that increase the quality of surface charge of the fine particles. Since in order to bring the representative peat to the highly decomposed peat, the sieving process has been used, thus, the second condition of humification has not been considered well, meaning the classification of peat just based on particle size could not be enough for electro-osmotic studies of peat. However, since the humification processes could not only increase the quality of the surface charge of particle, but also the quantity of the humus particles, thus, the higher degree of humification would decrease the resistivity of a peat soil due to both conditions. Since the average temperature in Malaysia ranges from 23 to 32 C, the temperatures were adjusted in such ranges. The results showed that the effect of the temperature on the soil resistivity was around 20%, meaning the temperature could have pronounced effect on the EO phenomena. The economics of electro-osmosis are governed by the energy expenditures. In electroosmostic process, the energy expenditure is governed by the resistivity of the soil (Acar et al., 1994; Asavadorndeja and Glawe, 2005). As the resistivity of the soil decreases, the energy required for the process increases proportionally (Mohamedelhassan and Shang, 2002). Therefore, the peat with a high degree of humification would have higher energy expenditure in comparison with unhumified peat. CONCLUSIONS The results of the study showed that the resistivity of the peat decreased as the water content or the temperature increased. The resistivity of the highly decomposed peat was lower than the very slightly decomposed peat, meaning a higher degree of peat humification resulted in a lower peat resistivity. The study revealed that the resistivity of both very slightly decomposed and highly decomposed peat increased as the organic content increased. The electrical conductivity of the highly decomposed peat was higher than the electrical conductivity of the very slightly decomposed peat. The electrical conductivity of both highly- and very slightly- decomposed peat decreased as the organic content increased. The results thus far showed that a highly decomposed peat may make higher electro-osmotic conductivity in comparison with very slightly decomposed peat; and degree of peat humification would be an important factor in electro-osmotic efficiency. A good understanding of the colloidal fractions of peat could make clear the underlying reasons of the behavior.
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