4.9: Coagulation
Colloidal solids are solid particles suspended in solution. They are usually negatively charged, attracting a compact primary layer of positively charged ions, which attract more counterions to form an electrical double layer. Electrostatic repulsion between the charged double layers prevents the particles from colliding, stabilizing the colloids. These solids are often undesirable because they can contain toxins that are difficult to remove. Coagulation is a technique that helps aggregate and remove colloids, and it is applicable to wastewater purification.
Metal coagulants are one type of coagulants that can be used in this purification process. When metal coagulants, such as aluminum or ferric salts, are added to the colloidal solution, they react with the bicarbonates in wastewater to yield the respective hydroxides. These hydroxides provide highly charged counterions that can be adsorbed on the suspended colloids, neutralizing their charge. Rapid mixing ensures the dispersal of these highly charged ions and therefore the frequency of formation of neutralized colloids. At the same time, mixing promotes collisions between the neutral colloids, leading to their coalescence as a result of the hydrophobic effect. Upon continued mixing, the particles come together and form slightly larger but still microscopic particles called microflocs. When microflocs reach optimal size and weight and become macroflocs, they can be removed by sedimentation or filtration to obtain pure water.
