Guar gum solutions for improved delivery of iron particles in porous media (Part 2): Iron transport tests and modeling in radial geometry

Abstract

In the present work column transport tests were performed in order to study the mobility of guar-gum suspensions of microscale zero-valent iron particles (MZVI) in porous media. The results were analyzed with the purpose of implementing a radial model for the design of full scale interventions. The transport tests were performed using several concentrations of shear thinning guar gum solutions as stabilizer (1.5, 3 and 4g/l) and applying different flow rates (Darcy velocity in the range 1·10−4 to 2·10−3m/s), representative of different distances from the injection point in the radial domain. Empirical relationships, expressing the dependence of the deposition and release parameters on the flow velocity, were derived by inverse fitting of the column transport tests using a modified version of E-MNM1D (Tosco and Sethi, 2010) and the user interface MNMs (www.polito.it/groundwater/software). They were used to develop a comprehensive transport model of MZVI suspensions in radial coordinates, called E-MNM1R, which takes into account the non Newtonian (shear thinning) rheological properties of the dispersant fluid and the porous medium clogging associated with filtration and sedimentation in the porous medium of both MZVI and guar gum residual undissolved particles. The radial model was run in forward mode to simulate the injection of MZVI dispersed in guar gum in conditions similar to those applied in the column transport tests. In a second stage, we demonstrated how the model can be used as a valid tool for the design and the optimization of a full scale intervention. The simulation results indicated that several concurrent aspects are to be taken into account for the design of a successful delivery of MZVI/guar gum slurries via permeation injection, and a compromise is necessary between maximizing the radius of influence of the injection and minimizing the injection pressure, to guarantee a sufficiently homogeneous distribution of the particles around the injection point and to prevent preferential flow paths.