6. condensation and cohesion

In freshwater, fine particles are stably dispersed by electrostatic repulsive force, but in liquids with high ionic strength, the electrostatic repulsive force decreases because the surface potential decreases, and the particles form aggregates and settle out. This phenomenon is called "condensation". Although increasing the ionic strength of the suspension allows the suspended particles to coagulate and precipitate, they are not suitable for actual separation operations because the van der Waals attraction is weak and they disintegrate easily. A familiar example is tofu, which is made by coagulating fine particles of ground soybeans with nigari.

Microorganisms (1 to several micrometers) have a high energy barrier at about 1 nm from the surface, which generates a large electrostatic repulsive force and allows them to disperse stably in water. When a substance with a positive charge opposite to that of the microorganism is added, it adsorbs on the surface of the microorganism, and if the size of the substance exceeds the energy barrier of the microorganism, it cross-links the microorganism and causes it to form an aggregate. Such a substance is called a "flocculant," and the solid-liquid separation method using a flocculant is called a "flocculation method". The aggregate of fine particles cross-linked by the flocculant is called flocculence (floc). In the case of flocculation, the size and physical strength of the aggregate are much larger than those of coagulation because the particles are strongly bonded and cross-linked by substances that have opposite charges to the particle surfaces. The flocculation method is widely used in water and wastewater treatment, as well as in wastewater treatment in factories and civil engineering works.