Chinese
Principle of Heavy Metal Wastewater Treatment:
Heavy metals in wastewater cannot be decomposed or destroyed by common treatment methods. Instead, treatment processes mainly aim to transfer their location or convert them into different physical and chemical forms. After treatment, heavy metal wastewater generally produces two types of outputs: one is treated water with most heavy metals removed, and the other is a concentrated product containing the heavy metals.
Methods for Heavy Metal Wastewater Treatment:
Treatment methods for heavy metal wastewater can generally be divided into two categories. The first category converts dissolved heavy metals in wastewater into insoluble heavy metal compounds or elemental forms, which can then be removed through sedimentation or flotation. Methods in this category include neutralization precipitation, sulfide precipitation, flotation separation, ion flotation, electrolytic precipitation or electrolytic flotation, and diaphragm electrolysis.
The second category concentrates and separates heavy metals from wastewater without changing their chemical forms. Methods used in this category include reverse osmosis, electrodialysis, evaporation, and ion exchange. The principles of several common heavy metal wastewater treatment methods are described below:
(1) Electrolysis Method:
This method is widely used in the treatment of heavy metal wastewater containing cyanide. Electrolytic oxidation is used to decompose cyanide while simultaneously converting heavy metals into hydroxide precipitates for removal. Electrolysis can also be used to process mercury sulfide waste residue, enabling efficient recovery of pure mercury or mercury compounds.
(2) Flotation Method:
Heavy metal hydroxides and sulfides in wastewater can be removed by aeration flotation. Among these methods, pressurized dissolved air flotation is particularly effective. Electrolytic flotation can also efficiently treat various types of heavy metal wastewater, especially wastewater containing heavy metal complexes.
(3) Ion Flotation Method:
Anionic surfactants such as sodium xanthate, sodium dodecylbenzene sulfonate, and gelatin are added to heavy metal wastewater. These agents form surface-active complexes or chelates with heavy metal ions. Different surfactants have selective complexation properties for different metal ions, and the same surfactant may selectively bind different metal ions under varying pH conditions. This selectivity allows heavy metals in wastewater to be separated through flotation.
(4) Membrane Method:
The main membrane technologies used are electrodialysis and reverse osmosis. Electrodialysis is characterized by a limited concentration ratio and typically requires multi-stage treatment to concentrate useful substances in wastewater to a reusable level. Reverse osmosis is commonly used for treating electroplating rinse wastewater, including wastewater from nickel plating, copper plating, zinc plating, and cadmium plating processes.
If the treated heavy metal wastewater still does not meet discharge standards, a heavy metal trapping agent can be added directly for further treatment. These agents react quickly and provide excellent treatment performance. They are suitable for final-stage treatment of heavy metal wastewater and serve as an effective auxiliary treatment chemical.
