How does the 'zeta potential' relate to the stability of colloidal particles in water and how is it manipulated during coagulation?

Zeta potential is a measure of the electrical potential difference at the slipping plane of a colloidal particle, and it directly relates to the stability of these particles in water. Colloidal particles are very small particles that remain suspended in water due to their surface charge, which is typically negative. This surface charge attracts a layer of oppositely charged ions from the surrounding water, forming an electrical double layer. The zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. A high zeta potential, whether positive or negative, indicates a strong electrostatic repulsion between the particles, preventing them from coming close enough to aggregate and settle. This high repulsion results in a stable colloidal suspension. Coagulation aims to destabilize these colloidal particles so that they can clump together and be removed. This is achieved by manipulating the zeta potential. Coagulants, such as alum or ferric chloride, are added to the water to reduce the zeta potential, essentially neutralizing the negative charges on the particles. By reducing the zeta potential to near zero, the electrostatic repulsion is minimized, allowing the particles to come close together and aggregate through van der Waals forces and other attractive forces. The particles then form larger flocs that can be removed by sedimentation or filtration. Therefore, the goal of coagulation is to reduce the zeta potential to a point where the particles are no longer stable and can readily aggregate, facilitating their removal from the water. Jar tests are often performed to determine the optimal coagulant dose needed to achieve this desired zeta potential reduction.