Effect of Ammonium Sulfate Concentration on Hydraulic Conductivity of Sedimented Na-Clay
M.MIZOGUCHI* and T.SHIBATA, Mie Univ., Mie 514, Japan
Email:mizo@bio.mie-u.ac.jp
http://www.bio.mie-u.ac.jp/doboku/user1/mizo/mizohomee.html
ABSTRACT
Hydraulic conductivity (HC) of a soil generally depends on electrolyte concentration and the HC decreases due to clay swelling and dispersion. However, the effect of concentration in the percolating solution on HC of clay soils has scarcely been investigated. In this study, the HC of sedimented Na-clays has been measured with a percolating ammonium sulfate solution. The HC of bentonite, which contains 65% montmorillonite, increased with increasing concentration (0-0.5N) of the solution, whereas the HC of kaolinite was constant. The dependence of the HC of bentonite on the concentration may be related to a variation in electric double layer resulting from ammonium fixation by 2:1 type clay.
INTRODUCTION
Control of nitrogen outflow from agricultural areas , especially paddy fields, is a major problem in Japan. For appropriate control it is necessary to know the movement of nitrogen through saturated clayey soil. Since the nitrogen applied to a paddy field is dissolved in water as a form of ammonium, the effect of concentration in ammonium on soil permeability is very important in order to predict the movement of the nitrogen in the field.
The hydraulic conductivity (HC) of a soil consisting of clay and sand has been studied by many investigators(Pupisky and Shainberg, 1979; Keren and Singer, 1988). As a result, it is now accepted that permeability of a soil generally decreases with increasing exchangeable sodium percentage and decreasing electrolyte concentration due to clay swelling and/or dispersion. However, few studies have been made for the permeability of a clay soil through which solution percolates (Iwata and Tabuchi, 1995).
The objective of the present study is to investigate the effect of concentration in the percolating ammonium sulfate solution on hydraulic conductivity of clay soils.
EXPERIMENTAL
(1)Materials
Two kinds of clay, bentonite and kaolinite, were used for the measurement of hydraulic conductivity. Bentonite contains 65% montmorillonite. Each clay was saturated with sodium; 1.5 g dry clay was washed three times with 1 N NaCl solution and then rinsed three times with 80 % methanol.
(2)Apparatus
Figure 1 shows an apparatus for the measurement of hydraulic conductivity. The apparatus consists of a Mariotte tube (300 ml), a sample tube (30 mm inside diameter) and a collector tube. A 5-mm thick clay is settled on a glass beads (0.2-0.29 mm) layer.
(3)Experimental procedure
(i) A 50-ml mixture of Na-clay and 80 % methanol, which contains 1.5 g dry clay, was poured into a sample tube to settle the clay.
(ii) To obtain a homogeneous clay sample, 80 % methanol was percolated continuously through the sample for 10 hours.
(iii) To start the measurement of hydraulic conductivity, the methanol was removed and then ammonium sulfate solution was added instead of the methanol.
(iv) Outflow solution was collected and weighed at each time interval.
(v) Ion concentration in the outflow solution was then analyzed.
RESULTS
(1)Cumulative outflow weight as a function of time (Fig. 2)
The gradient of cumulative outflow weight increased with concentration in the percolating solution for bentonite whereas it did not change for kaolinite.
(2)Hydraulic conductivity (HC) as a function of time (Fig. 3)
HC was calculated based on Darcy's law. The HC of bentonite increased gradually with time and became constant for the concentration below 0.3 N whereas it decreased for 0.5 N. Nevertheless, the HC of kaolinite depended on neither time nor the concentration.
(3)Hydraulic conductivity versus concentration in the percolating solution (Fig. 4)
The log(HC) of bentonite was proportional to concentration in the percolating solution at the range of 0 to 0.5 N whereas that of kaolinite was constant.
(4)Relative outflow concentrations versus time for 0.2 N ammonium sulfate solution percolating through bentonite (Fig. 5)
NH4+ passed through the clay slower than SO42-. The slower outflow of NH4+ can be attributed to cation exchange between Na+ and NH4+.
DISCUSSION
DLVO theory needs charge density or electric potential on surface as a boundary condition to calculate kinetic properties in electric double layer. It is difficult, however, to determine the boundary condition theoretically because a little specific adsorption on a clay particle can bring a significant change in the charge density on clay surface.
Ammonium ion is just the right size to fit into "cavities" between the crystal units of a silica sheet, thereby becoming fixed (Ammonium fixation). Moreover, the ammonium fixation is greater by montmorillonite than by kaolinite. During the percolation of ammonium sulfate solution through a clay the ammonium ions will be fixed to clay minerals, with the rate of fixation dependent on the concentration in the percolating solution. The ammonium fixation in the bentonite may cause the decrease in electrical density on clay surface to change the kinetic properties of the electric double layer formed around clay surfaces(Fig. 6). Thus, the HC of bentonite containing more montmorillonite is presumed to increase with increasing concentration in the solution as shown in Fig. 4.
CONCLUSION
The HC of bentonite, which contains 65% montmorillonite, increased with increasing concentration (0-0.5N) of the solution, whereas the HC of kaolinite was constant. An explanation by ammonium fixation model has been proposed for the dependence of HC of bentonite on the concentration.
REFERENCES
Iwata, S and T. Tabuchi. 1995. Soil-water Interactions (Second Edition). Dekker.194-218.
Keren, R and M.J.Singer. 1988. Effect of low electrolyte concentration on hydraulic conductivity of sodium/calcium-montmorillonite-sand system. Soil Sci. Soc. Am. J. 52:368-378.
Pupisky, H and J. Shainberg. 1979. Salt effects on the hydraulic conductivity of a sandy soil. Soil Sci. Soc. Am. J. 43:429-433.