Abstract proceedings for the 7th ICLRS, Sunderbyn, Sweden, June 25-27
Page 85
Geophysical monitoring of in situ redox processes
Konstantaki, L.A.,(1) l.a.konstantaki@tudelft.nl Heimovaara, T.J., (1) t.j.heimovaara@tudelft.nl
(1) Department of Geotechology, Delft University of Technology, Delft. Introduction
Hazards associated with municipal solid waste landfills have increased the interest of population for a development in the treatment of landfills. The main problems of landfills are gas and leachate emissions. Bioreactor landfills that use treatment methods, such as
recirculation of leachate and aeration for minimizing the production of leachate and the stabilization period of a landfill, are already in use [ITRC, 2006]. However, aftercare periods are still in the range of 30 years and there is no guarantee that the emissions will cease after this period. Therefore a method to quantify the leachate emissions and stabilization of a landfill is required [Heimovaara et al., 2012 submitted].
To be able to quantify these emissions it is necessary to understand the processes occurring inside a landfill. Our ambition is to be able to quantify the processes occurring by connecting the physical properties measured with geophysical measurements, with chemical properties. Geophysical measurements map the structure and provide information about the properties of the subsurface. Using geophysical measurements will allow us not only to map the leachate but to extract information about physical properties of the waste and leachate as well. An understanding of the physical and chemical properties, not only in stationary conditions, but also in time, allows us to improve our predictions of emission potential which will eventually improve emission potential reduction treatment of landfills.
This specific abstract focuses on the first step of our approach which includes the
understanding of the processes occurring by conducting laboratory experiments. In detail, a biobattery is set up is adopted to understand the coupling between redox gradients present in waste and geophysical measurements.
The method
We focus on the electrical properties measurable by a variety of geophysical measurements and aim to include those processes occuring within the landfill that affect the measurable signal; electro-kinetic (fluid flow), electro-thermal and electrochemical (junction and redox potential) signals. As a starting point we will use Self Potential (SP) measurements in order to connect the physical properties measured with chemical properties. SP are passive electrical methods that can measure the electrical signal obtained from the processes occurring inside the landfill. We focus on the electrochemical and the electrokinetic processes and aim to calculate the chemical and redox potential.
Studies by Revil [2004] have already shown that there is a coupling between fluid flow, junction potential and electric potential. However we aim to add the term of redox potential and try to uncouple these processes in order to estimate redox and chemical potential. The innovation of our method is the addition of this term. Equation 1 shows the suggested model.
Jjunction is the current density due to concentration differences, J is the electric current density,
U the Darcy velocity and Jredox the current density due to redox processes. The coefficients in the second matrix are the Onsager's coefficients that show the relation between the different processes. ı is the electrical conductivity, e the number of electrons, ı+ and ı- the ionic conductivities , C+ and C- the ionic concentrations and k and Ș the permeability and viscosity respectively. In the last matrix the gradients are shown with ȝ the chemical potential, ij the electrical potential, p the pressure and Ǽ the redox potential.
Abstract proceedings for the 7th ICLRS, Sunderbyn, Sweden, June 25-27. Page 86 (1)
The coefficients depicted with a question mark are the ones we aim to determine. These show the relation of the redox processes with fluid flow and with chemical, diffusion processes. In order to do that, we will conduct a number of experiments based on a biobattery set up. As shown in figure 1 the biobattery will consist of soil in a temperature controlled glass box, creating oxic (top) and anoxic (bottom) conditions. Redox sensitive (Ag/AgCl) electrodes will be connected to a voltmeter to measure the redox potential. By applying a fluid flow in our system the change of the redox potential due to fluid flow can be tested. In addition, adding concentration will change the chemical potential of the system and measuring again the redox can show us the connection between those two processes.
Figure 1: Biobattery set up. (Modified from [Keego, 2010]) Conclusions
The results will provide us with substantial information regarding the relation between the redox and the other processes occuring in the landfill, which we can use further on to develop our model and in the future for application on the field. Having the ability to calculate redox and chemical potential will allow us to make estimations regarding concentration gradients, Gibbs energy and pH, which will help us understand the system in detail and will allow for a development of a better treatment method.
References
ITRC (2006). Characterization, Design, Construction and Monitoring of Bioreactor Landfills. Heimovaara T.J., Konstantaki, L.A., van Turnhout, A.G., Bun, A., Baviskar, S.M. (2012 submitted).Reduction of the emission potential through leachate from sanitary landfills. Revil, A., Leroy, P. (2004). Constitutive equations for ionic transport in porous shales. Journal of Geophysical research, v.109
