Alshawabkeh, Akram

Ottosen, Lisbeth M.

Paz-Garcia, Juan Manuel

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 19, 2017Mathematical Modeling of the Role of Chemical Speciation on Electrokinetic Transportof Chemicals in SoilPaz Garcia, Juan Manuel; Ottosen, Lisbeth M.; Alshawabkeh, AkramPublished in:Proceedings of EPA/TEI Sustainable Remediation Conference 2011Publication date:2011Link back to DTU OrbitCitation (APA):Paz-Garcia, J. M., Ottosen, L. M., & Alshawabkeh, A. (2011). Mathematical Modeling of the Role of ChemicalSpeciation on Electrokinetic Transport of Chemicals in Soil. In Proceedings of EPA/TEI Sustainable RemediationConference 2011Mathematical Modeling of the Role of Chemical Speciation on Electrokinetic Transport of Chemicals in Soil Juan Manuel Paz-Garcia, Lisbeth M. Ottosen, Technical University of Denmark Akram Alshawabkeh, Northeastern University  The efficiency of the electrokinetic remediation of heavy metal polluted soil is been demonstrated to be dependent upon the chemical reactivity of the target contaminant rather than the migration rate of the target ions. The contaminants have to be in a soluble species, preferably as ionic form, in order to be mobilized under the gradient of electrical potential induced in the system. In the present work, a mathematical model for chemical equilibrium calculation is developed and used for the analysis of the interactions between the species in the pore solution and the solid matrix in lead contaminated soil. The model is based on the simultaneous solution of a non-linear set of reversible aqueous and precipitation/dissolution chemical reactions. A line search Newton-Raphson method, iterating on the extent of each reaction, is used for the chemical equilibrium calculations.  Simulations are presented in order to show the behavior of the chemical system with respect to the buffering capacity of the soil and the pH changes of the pore solution induced by the electrode reactions in the electrochemical treatment. Additionally, the role of some extracting agents in the speciation of lead is discussed. Results from simulations shed light on the effect of the speciation of lead in soil in the transport process, and can be useful for a correct design of enhanced electrokinetic remediation treatments.   Juan Manuel Paz-Garcia obtained his M.Sc. in Chemical Engineering, at the University of Málaga, Spain. He is currently a Ph.D. student at the Civil Engineering Department of the Technical University of Denmark, participating in a Ph.D. project named "Chemical and Numerical Modeling of Electrokinetics". Recently, Juan Manuel is a guest visiting scholar at the Civil and Environmental Engineering department, Northeastern University, Boston.   

Mathematical Modeling of the Role of Chemical Speciation on Electrokinetic Transport of Chemicals in Soil

Alshawabkeh, Akram N.

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 19, 2017Mathematical Modeling of the Role of Chemical Speciation on Electrokinetic Transportof Chemicals in SoilPaz Garcia, Juan Manuel; Alshawabkeh, Akram N.; Ottosen, Lisbeth M.Publication date:2011Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Paz-Garcia, J. M., Alshawabkeh, A. N., & Ottosen, L. M. (2011). Mathematical Modeling of the Role of ChemicalSpeciation on Electrokinetic Transport of Chemicals in Soil. Poster session presented at EPA/TEI SustainableRemediation Conference 2011, Amherst, MA, United States.  Mathematical Modeling of the Role of Chemical Speciation on Electrokinetic Transport of Chemicals in SoilJuan Manuel Paz-Garcia1, 2, Akram N. Alshawabkeh2, Lisbeth M. Ottosen11: Civil Engineering Department, Technical University of Denmark; 2 Civil and Environmental Engineering; Northeastern University IntroductionElectrokinetic (EK) transport of pollutants from soil consists of the mobilization of species by the application of an external electric DC field. In EK processes (Fig. 1) the electric current is transported by the ions in the pore solution, which move towards the electrode of different charge (electromigration). Non-ionic species are also mobilized due to electroosmotic advection, i.e. the transport of the pore solution itself under the gradient of electrical potential. In certain conditions, the chemical diffusion can play an important role in the transport process. Figure 1: Electrokinetic transport process. The efficiency of the EK soil remediation treatments was demonstrated to be dependent upon the chemical reactivity of the target pollutants rather than their migration rates. The contaminants have to be in a soluble species, preferably as ionic form, in order to be mobilized under the gradient of electrical potential induced in the system. Heavy metals easily precipitate as insoluble minerals (Fig.  2). In those conditions, the removal rate decreases while the energy consumption increases. Figure 2: Feasible precipitation/dissolution equilibria for lead minerals during electrokinetic treatments. Materials and MethodsIn the present work, a mathematical model for chemical equilibrium calculation is developed and used for the analysis of the interactions between the species in the pore solution and the solid matrix in lead contaminated soil. The model is based on the simultaneous solution of a set of non-linear equations describing the reversible aqueous and precipitation/dissolution chemical reactions. A tailor-made line-search Newton-Raphson method, iterating on the extent of each reaction, is used to  minimize the residual function; defined as the distance to the equilibrium state in the mass action equation of each chemical reaction included in the system. Table 1: Solubility products of some lead mineralsStoichiometric	  reaction	   log10(KSP)*-­‐14,8412,628,15-­‐4,81PbCO3(s)! Pb+2 +CO3!2 -­‐13,29Table 2: Equilibrium constants of some lead aqueous compounds log10(Keq)*	  as	  function	  of	  x1 2 3 4-­‐7,51 -­‐16,95 -­‐27,20 -­‐38,907,00 10,13 -­‐ -­‐13,20 -­‐ -­‐ -­‐1,06 1,48 -­‐ -­‐2,78 4,08 3,59 3,407,27 6,53 -­‐ -­‐1,44 2,00 1,69 1,40The electric current applied is transformed to an ionic current by means of the electrochemical reactions at the electrodes. Electrolysis of water reactions are always likely to happen. Consequently, sharp pH profiles are expected in EK treatments.Figure 3: Typical experimental setup for a lab-scale acid-enhanced electrokinetic remediation treatment of a soil sample in horizontal column.  Figure 4: pH profiles from simulations with a Poisson-Nernst-Planck transport model.  Constant current 1 mA. 12 hours treatment, one profile per hour. a) non-calcareous soil b) Acid enhanced treatment c) calcareous soil.   I. The acid front that is developing at the anode promotes the desorption and mobilization of most heavy metals during the remediation process.II. Hindering the basic front developed in the cathode assures acid media in the whole domain and promotes a better mobilization of the contaminants. III. The buffer capacity of the soil, typically due to  the carbonate content, significantly affects the advance of the acid front. Additionally, the formation of insoluble carbonates, such as PbCO3 may be an important type of retention of the heavy metals in calcareous soils.Extracting agents are used to increase the solubility of the target contaminant, by balancing the equilibrium to the formation of aqueous complexes. The technique can be combined with the acid-enhanced procedures for an efficient and more selective removal of contaminants.  Different extracting agents have been tested to enhance the electrokinetic removal of lead, such as EDTA, acetic acid, citric acid and nitric acid (Table 2).ResultsSimulations presented (Table 3) show the behavior of the chemical system with respect to the buffering capacity of the soil and the pH changes of the pore solution induced by the electrode reactions. Additionally, the role of some extracting agents in the speciation of lead is discussed. Simulations consist of computing the concentration of aqueous species by considering the equilibrium state of pure water (or the specified electrolyte at 0.01 M) and a solid matrix containing enough  mineral, that dissolves until over-saturated equilibrium conditions. Table 3: Simulation results. a) Non-calcareous soil. b) calcareous soil. pH Pb+2	  [mol/L] Lead	  compounds	  [mol/L]Ionics Non-­‐ionicsa) PbS 7.00 1.4783	  ×	  10-­‐11 1.9347	  ×	  10-­‐11 1.6568	  ×	  10-­‐14PbS	  +	  HAc 468 2.4619	  ×	  10-­‐9 2.5024	  ×	  10-­‐9 1.2887	  ×	  10-­‐14PbO 7.48 9.2789	  ×	  10-­‐8 1.8020	  ×	  10-­‐7 9.6887	  ×	  10-­‐10PbO	  +	  KCl 7.38 1.0076	  ×	  10-­‐7 1.0076	  ×	  10-­‐7 6.2621	  ×	  10-­‐19PbO	  +	  HAcetate 5.82 9.5024	  ×	  10-­‐6 9.8358	  ×	  10-­‐6 9.3012	  ×	  10-­‐11PbO	  +	  H3Citrate 5.92 1.0059	  ×	  10-­‐5 3.0733	  ×	  10-­‐5 7.7083	  ×	  10-­‐11b) PbS 9.90 4.4489	  ×	  10-­‐13 4.4489	  ×	  10-­‐13 2.4307	  ×	  10-­‐19PbS	  +	  HAc 5.68 2.1850	  ×	  10-­‐11 2.1850	  ×	  10-­‐11 2.4307	  ×	  10-­‐19PbCO3 8.14 2.2960	  ×	  10-­‐9 1.2141	  ×	  10-­‐8 6.9923	  ×	  10-­‐10PbCO3	  	  +	  KCl 8.12 8.8100	  ×	  10-­‐9 5.3683	  ×	  10-­‐8 5.1436	  ×	  10-­‐7PbCO3	  	  +	  HAcetate 8.12 8.9228	  ×	  10-­‐9 5.4088	  ×	  10-­‐8 5.1436	  ×	  10-­‐7PbCO3	  	  +	  H3Citrate 6.35 4.4241	  ×	  10-­‐8 5.6458	  ×	  10-­‐6 8.4370	  ×	  10-­‐10Results from simulations shown that acid enhanced methods, combined with the used of some extracting agents will increase the efficiency of the treatment. In calcareous soil, the buffer capacity limits the acid enhanced applicability. ConclusionIn the present study a mathematical model for the solution of a set of simultaneous reactions in a multicomponent-multiphase chemical system under the assumption of chemical equilibrium is described. The model used here can be included in a more general model for electrokinetic transport accounting for the chemical reactivity.Simulations shown the effect of the pH, the initial speciation of the target contaminant, and the influence of the different extracting agents and the soil buffering capacity on the  aqueous concentration of the contaminant in the pore solution, which determine the efficiency of the treatment (in terms of the removal rate and the energy consumption).Acknowledgments This program is supported by Award Number P42ES017198 from the National Institute of Environmental Health Sciences, USA. Juan Manuel Paz-Garcia acknowledges the Danish Agency of Science Technology and Innovation, for the economical support in the project 274-08-0386 “Fundamentals of Electrokinetics in In-homogeneous Matrices”.                                                                                                                                                                                                                                                                                                                                                               

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Mathematical Modeling of the Role of Chemical Speciation on Electrokinetic Transport of Chemicals in Soil

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