The flocculation process of released clay particles and its effect on the permeability of porous media

In the process of artificial recharge to remediate seawater intrusion, the disturbances that impact hydro-geochemistry and hydrodynamics lead to an abrupt change of permeability. When freshwater drives seawater, there is a critical salt concentration (CSC) and a critical flocculation concentration (CFC). When the salinity is lower than the CSC, the particles will release from the pore surfaces. When the salinity is higher than the CFC, flocculation will occur between the released clay particles. The CSC depends on the interaction force between the clay particles and the pore surface, and the CFC depends on the interaction force between the released clay particles. This study found the distinction between CSC and CFC through batch experiments and sand column experiments. The CFC of clay particles for a NaCl solution and a seawater/freshwater mixed solution were determined as 40 mmol/L (4,485 mu S/cm) and 4,800 mu S/cm, respectively, both lower than the CSC (60 mmol/L and 6,800 mu S/cm, respectively), which shows that the released clay particles can be re-flocculated. The result was consistent with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theoretical calculation. In this experiment, the flocculation of the released clay particles had a significant effect, by reducing the permeability. When the electrical conductivity was higher than the CFC, the clay particles concentration in the solution fell to about 20% of the initial concentration, and the permeability reduced with the decrease of electrical conductivity during the whole process. When the electrical conductivity was lower than the CFC (4,800 mu S/cm), the permeability reduction of the porous medium was not obvious.