Biopolymer mixtures for the optimization of iron micro- and nanoparticle delivery in aquifer systems for groundwater remediation

The injection into the subsoil of micro- (nZVI) and nanoparticles (nZVI) of zerovalent iron as water-based slurries for groundwater remediation is hindered by the lack of stability and mobility of both micro- and nanoscale zerovalent iron particles in water suspensions. Green polymers, such as guar gum (GG) and xanthan gum (XG) are commonly used as stabilizing agents. Water based solutions containing high concentrations of these green polymers are shear thinning fluids, presenting high viscosity in static conditions and low viscosity in dynamic conditions. This shear thinning behaviour makes the storage of iron micro- and nanoparticles more effective, providing a reduction of sedimentation and aggregation phenomena, and promotes the injection of the particle in the subsoil, avoiding high injection pressures. In this master thesis an extensive study was carried out on the optimization of injection of mZVI and nZVI particles dispersed in GG-XG solutions for groundwater remediation applications. The mixture of different biopolymers, in fact, creates a viscoelastic gel with higher performance than the single biopolymer solution. Firstly, the preparation procedure of GG solutions was optimized to minimize the presence of undissolved particles and contain the clogging of the porous medium: the GG powder is dissolved in hot water, left to settle and filtered. This aim was achieved through several column filtration tests. The rheological characterization of the adopted biopolymer mixture was provided to prove its effectiveness. Rheological measurements and a multi-step filtration test were performed to verify that the rheological behaviour of the biopolymer solution in the bulk is similar to that in the porous medium. Secondly, the mobility in porous media of the iron particles dispersed in the biopolymer solution was explored through several column transport tests (1D geometry). The optimal concentration of the GG-XG mixtures was identified to improve the colloidal stability and the transport of mZVI and nZVI in the aquifer systems, maintaining at the same time low injection pressures. Furthermore, the flow velocity decreases hyperbolically with increasing distance from the injection well. Hence, the transport tests were carried out at different flow velocities, to investigate the effect of this important parameter on the mobility of iron particles during field injections. The results of laboratory column tests were analysed by inverse fitting using the MNMs 2018 software to determine the kinetic coefficients that govern the transport of iron particles in porous media. Finally, one-dimensional radial simulations of a hypothetical pilot injection were run to predict the radius of influence and the consequent final distribution of the particles around the injection well, verifying the effectiveness of the biopolymer mixture. The results of this study highlighted the necessity to find a compromise between the maximization of the colloidal mobility and the minimization of the injection pressure. However, this work showed that the GG-XG mixture even at lower biopolymer concentration provide a better stabilization than single biopolymer solution at higher concentrations. The solutions proposed are promising in the interest of a future application in the field and the study can be deepened and strengthened through full three-dimensional numerical simulations for the full-scale design of the injection of zerovalent iron particles.