Flocculation is a physicochemical process during which colloidal particles in suspension agglomerate and form bigger particles called flocs. Thanks to their large size, the flocs are characterized by an appreciable settlement rate, which allows their separation from the liquid phase. This method is used in many applications, such as water treatment. However, this process is limited by the relatively slow decantation of the flocs under gravity, due to their low density. Our work consists in studying the replacement of conventional flocculating agents used in water treatment, by magnetic nanoparticles (NPs), to facilitate the decantation step. The aim of the study is to treat model colloidal aqueous suspensions of Beidellite platelets by Maghemite NPs, the obtained magnetic flocs being recovered using a Nd-B-Fe magnet. Our work has been carried out according to the following procedure. First, magnetic maghemite (γ-Fe2O3) NPs were synthesized, sorted into different sizes, and dispersed in slightly acidic solutions. Those dilutions were then added to suspensions of Beidellite platelets. Those platelets belong to the smectite family with a substitution in the tetrahedral sheet. The flocculation was immediately observed due to the strong electrostatic interactions between the positively charged NPs and the negatively charged platelets. After 1 min of stirring, the flocs were left to settle in presence of a Nd-B-Fe magnet. In comparison to gravity, the settlement rate of the flocs resulting from the magnetic field gradient was up to 100 times faster.
First, we will describe our results about the physico-chemical conditions allowing the formation of the magnetic flocs. As a result of this preliminary study, we were able to build a phase diagram giving an overview of the optimal area for flocculation. We will also give an estimation of the settlement rates of the flocs, depending on the way of decantation (magnet or gravity), and on the main parameters varied during the flocculation. Finally, we will present our first attempts to determine the structure of the flocs. Thanks to cryo-TEM and SAXS experiments, we obtained a first description of the organization at the nanometer scale. But the complexity of the system led us to use SANS and to recompute the Small Angle Scattering curves by image analysis of the optical micrographs of the flocs. In order to apply this procedure to real systems, which need a neutral or a slightly basic pH, we also studied flocculation with citrated γ-Fe2O3 NPs. Since these NPs are negatively charged, no flocculation occurs when the clay platelets are added. It is then necessary to add a polymer to trigger the flocculation. Afterwards, we attempted to apply the method to natural surface waters. Instead of a clay suspension, we worked with water from the Seine River. We chose to evaluate the evolution of turbidity, iron concentration and total organic carbon after addition of the magnetic NPs and settlement using a Nd-B-Fe magnet.