Thesis title: Nanoparticles Application in Groundwater Remediation: Lab and Numerical approaches
Nanoremediation represents a remediation technique through which aqueous suspensions of reactive nanoparticles are injected into the soil with the aim to degrade, transform and immobilize in situ pollutants. Zerovalent iron nanoparticle (nZVI) technology has been found to be promising and effective for the remediation of soils or groundwater. In fact, they have been applied for in situ remediation of a wide range of contaminants, both organic and inorganic. One of the main benefits of nZVI is linked to the dimensions, extremely small (1-100 nm), giving to them a considerable specific surface area. It determines a high reactivity, and therefore a greater tendency to interact with numerous contaminants. However, once injected into the aquifer, the nanoparticles have a low stability, caused by the high physical-chemical interactions and magnetic attractions between the particles and particles - porous medium. These phenomena lead to the formation of higher aggregates, causing their settling and deposition. However, this behavior not only limits the remediation of contaminated areas of an aquifer, but also makes unknown the distribution of the nanoparticles when they are injected in situ. Therefore, the Ph.D. thesis aims to study the behavior and mobility of nZVI in saturated porous media, in order to assess their suitability as an alternative remediation strategy for contaminated areas. In the first phase of the Ph.D. activities, the aim has been to study the treatment efficacy of water-contaminated by arsenic (As). In fact, several batch tests have been carried out in the laboratory, with water or sediment contaminated by arsenic, changing the i) mass of nZVI injected, ii) chemical composition of water and iii) the interaction times. The results of experimental activities have been shown very high As-nZVI reaction rates, resulting in a progressive reduction of the As concentrations in solution. So, these results prove the effectiveness of nZVI for the remediation of water-contaminated by arsenic (As); however, it also determines the need for continuous monitoring on nZVI-injected quantities in the field. The rapid As-nZVI reactions lead easily to non-active nZVI, which have no use for arsenic removal. This causes a decrease in the active sites on the nanoparticles, making nanoparticles become no-activity. In fact, as the As-nZVI interaction time increases, there is a decrease in the available sites for arsenic immobilization, and so the As concentrations in solution tend to become constant. Once this condition is reached, it is not easy to treat the residual arsenic. This behavior is associated with the progressive saturation of the available sites for arsenic immobilization on the nZVI surface. Therefore, these results underline the need to monitor the As concentrations during applications in field, in order to verify the need or not to inject active nanoparticles for arsenic removal. On the contrary, the second phase of Ph.D. activities focused on the assessment of mobility and dispersion of nZVI in a saturated porous medium. In fact, several laboratory experiments have been carried out with groundwater flow in a two-dimensional laboratory-scale tank to assess the nZVI behavior. The experiments have been involved the use of glass beads as the saturated porous medium. Into the saturated porous medium, nZVI, the same used during the first phase of the Ph.D. activities, have been injected with different concentrations and diverse injection methods, in order to simulate several field conditions. Moreover, the laboratory set up included the use of a digital camera for the acquisition of images. Therefore, image analysis procedures, processed ad hoc, have been used to assess the behavior of nZVI plume. The results highlighted that the nanoparticles have small dispersive effects, and the nZVI mobility is strongly influenced by the higher weight of them with respect to the water. In fact, the results have shown that nZVI have an own motion in part influenced by the fluid flow but more determined from the injection phase and gravity. Therefore, the use of the classical dispersion equation cannot provide a reliable result because it is based on the classical hypothesis in which the behavior of the receiving fluid governs the main parameters of transport and dispersion. Moreover, the experimental data have been used for the calibration of a numerical model, developed by the Finite Difference Method (FDM). The proposed numerical model has been implemented in order to simulate the nZVI distribution in a saturated porous medium. The nZVI transport in porous media can be represented by a modified advection-dispersion partial differential equation, including a term to define the interaction between particles (nZVI) in the liquid (water) and solid (glass beads) phase. However, according to experimental results, the proposed numerical model has been corrected to consider any aggregation phenomena for nanoparticles, that can influence the sedimentation velocities of the nanoparticles. In fact, as shown by the experimental results, the nZVI does not behave as a solute dissolved in water, but as a mass. Their mobility does not appear only subject to the fluid motion but also characterized by its own mobility due to the aggregation phenomena. Moreover, the proposed numerical model shows that, after crossing through the saturated porous medium (glass beads), a portion of nanoparticles, i.e., the mobile nZVI, are still mobile through the porous medium and can react with contaminants, where they are present. On the contrary, other portion of nanoparticles, i.e., the deposited nZVI, are linked to the grains of saturated porous medium. The results obtained by the numerical model, joined to experimental evidence, have allowed deriving useful considerations that can provide valid aid for the application of numerical models in the design of remediation activities based upon the nZVI approach.