**4. Abbreviations**

12 Will-be-set-by-IN-TECH

**Figure 9.** Relationship between grain contact angle and friction coefficient for colloid retention at the

AW*m*S contact line. But it had been refuted by Steenhuis et al [73].

**2.4. Attachment at the immobile zones**

**3. Conclusions and future directions**

in the chamber, drying of thin water film over the grain surface, and advaction of colloids to

In the partially saturated systems, colloids were found to be captured into stagnant/immobile zone. There was evidence of exchange of colloids between immobile and mobile zone due to long breakthrough curve tailing on colloid transport through unsaturated porous media [15, 25, 61, 68]. In a visualization study, it was found that colloids present in the immobile zone at the liquid-gas interface were not moved to mobile zone in steady flow, but the exchange of colloids between immobile and mobile zone occurred in varied flow rate [26]. The exchange of colloids between mobile and immobile zones were likely controlled by slow advection in addition to diffusion. The occurrence of larger quantity of colloids from unsaturated column studies were found in transient flow condition due to movement of colloids present in immobile zone [61, 66, 89]. All these studies indicated that the colloid can be attached in the immobile zone created by heterogeneity of the medium and by the presence of gaseous

The study of colloid fate and transport in important as there is strong affinity of radioactive contaminants to attach with the moving colloidal particles or radioactive elements can fall under colloidal size range. In subsurface systems (like soils and sediments) moving air-water interfaces are common, e.g., during infiltration and drainage of water, air and water displace each other in continuous cycles. Such moving air-water interfaces have a profound effect on detachment of colloids from surfaces. Several research efforts had been made to understand the mechanism of colloid retention and mobilization in unsaturated porous media. The

phase, which could be remobilized in the large rainfall and infiltration events.

AW*m*S interface.

AW*m*S: Air/water-meniscus/solid DLVO: Derjaguin, Landau, Verwey and Overbeek KBS: Kärnbränslesäkerhet
