What forces pull particles together during coagulation and flocculation?

In the process of coagulation and flocculation, particles in water treatment come together primarily due to Van der Waals forces. These forces are weak, attractive interactions that occur between molecules or particles that are close to each other. Although they are relatively weak compared to other types of bonding, such as covalent bonds, they play a significant role in the aggregation of colloidal particles.

During coagulation, chemicals known as coagulants are added to disrupt the stability of colloidal particles, which can result from factors like electrostatic repulsion. As these particles come close during flocculation, Van der Waals forces assist in pulling them together, promoting the formation of larger aggregates or flocs. This is crucial in the water treatment process as the larger flocs can then be more easily removed from the water through sedimentation or filtration.

The other forces mentioned in the choices, such as hydrophobic interactions, while they may occur in some contexts, are not the main forces responsible for particle attraction during coagulation. Electrostatic repulsion generally counters attraction, making particles less likely to come together unless effective coagulation occurs. Covalent bonding, on the other hand, involves strong chemical bonds that would not typically apply in this context, as the focus is