Wastewater Treatment
ระบบบำบัดน้ำเสีย ระบบกำจัดไขมัน และน้ำมัน ระบบบำบัดน้ำเสียด้วยวิธีทางเคมี (Chemical Treatment System) ระบบบำบัดน้ำเสียด้วยวิธีทางชีวภาพ (Biological Treatment System)
Heavy Metal Removal
Heavy Metal Removal or Precipitation By pH
To convert dissolved (ionic) metals into insoluble particles, the traditional method is to adjust the pH of the solution using caustic, such as sodium hydroxide. The precipitating reaction forms metal hydroxides. The results obtained are effected by several conditions - one of which is the pH of the solution.
Every dissolved metal has a distinct pH at which the optimum hydroxide precipitation will occur. Here are a some examples:
- Cadmium pH 11.0
- Copper pH 8.1
- Chromium pH 7.5
- Nickel pH 10.8
- Zinc pH 10.1
Metal hydroxides are amphoteric, i.e., they are increasingly soluble at both low and high pH, and the point of minimum solubility (optimum pH for precipitation) occurs at a different pH value for every metal. At a pH at which the solubility of one metal hydroxide may be minimized, the solubility of another may be relatively high. Since metal hydroxides are quite soluble, many such hydroxides will start to go back into solution if the pH changes even slightly.
Wastewaters from industrial processes usually contain several metals. For example. the typical process wastewaters from printed circuit board manufacturing contain copper, tin, lead and nickel. Such mixed metals create a problem when using hydroxide precipitation since the ideal pH for one metal may put another metal back in solution.
In addition, chelators, sequestering agents, bath additives, cleaners and electroless formulations will interfere with the reaction of hydroxide precipitation. From a practical point of view, it is impossible to eliminate such components from a waste stream. Good treatment practices, such as segregation and pretreatment of some process waste streams, will minimize the anti-precipitant effects of such ingredients. Otherwise, an alternative precipitant may be required in conjunction with pH adjustment.
