Heavy metals removal from wastewater. In recent years, the most widely used methods of wastewater treatment to remove chromium and heavy metals are chemical (reagent), ion-exchange and electrochemical.
Wastewater treatment with reagents
Reagent treatment of wastewater is based on the use of reducing agents for chromium purification and precipitation agents for heavy metals neutralization. The reagent treatment of chromium containing wastewater restores the hexavalent chromium to trivalent and precipitates it. The reducing agents are sulfur (sodium sulfite, sodium bisulfite) copperas, scrap steel and others.
In practice there are two methods of wastewater treatment to remove chrome:
- Reduction of Cr6+ to Cr3+ in acidic medium followed by precipitation of Cr3+, together with other heavy metals, in an alkaline environment, and neutralization of wastewater;
- Joint treatment of chromium containing and acid-alkaline wastewater in an alkaline environment with reduction of Cr6+ to Cr3+ with copperas and precipitation of heavy metals.
Assessing the opportunities and prospects of reagent treatment methods, their shortcomings should be considered: the insufficient purification from heavy metals, high salt content, which often prevents reuse of purified water in the cycle; high consumption of reagents, large amounts of sediment and its high humidity, complex and bulky metal equipment with large footprint.
Ion exchange methods for wastewater treatment
One of the promising methods of wastewater treatment is ion exchange. This method provides an almost complete removal of harmful impurities from wastewater and allows the water to be reused. But it has limitations in treating wastewater from salts of heavy metals. Ion exchange can be used for purification of wastewater with salt content of up to 2.3 g / dm3 and small amounts of metals and acids. The process is also complicated and the reuse of the concentrated solutions after regeneration is impossible too. The essence of ion exchange is the ability of cation and anion to exchange their ions for cations and anions and adsorb them from wastewater. At the same time, ion-exchangers must meet the following requirements: have a high exchange capacity, resistance to acids, alkali, oxidants, and perform as reducing agents, be insoluble in water and electrolyte solutions, have little change of volume.
This method is used in circular water systems at industrial facilities. It reduces freshwater consumption in the production process and in electroplating process, but it occupies a larger production area (1.5-2 times). The ion exchange method also has the following disadvantages: it uses large amounts of expensive reagents (3-4 times more than theoretically necessary), extra costs for regeneration of ion exchangers and decontamination of regeneration products, the costs of significant amounts of water, a large amount of mineral salts getting into the water bodies with neutralized regeneration products. The use of this method is also limited by the considerable costs, shortage of equipment and ion exchange resins, and it should only be used where there is a need for desalinated water.
Electrochemical methods of wastewater treatment
Recently, the electrochemical treatment of wastewater to remove chromium and other metals using steel electrodes has become very popular. Its essence lies in the chemical reduction of chromate ions due to electrochemical processes and electrolytic decomposition of water and oxygen and hydrogen evolution reaction. Simultaneously hydroxide and iron hydroxide, chromium and other metal hydroxide bond the OH groups during electrolysis.
The analysis of physico-chemical characteristics of the electrochemical cleaning method points to its complexity. This method has the following stages: chemical ionization and electrochemical dissolution of iron, the reduction of hexavalent chromium to trivalent chromium and iron, the formation of basic salts and hydroxides of metals and their precipitation. The processing rate of electrochemical treatment depends on pH, current density, treatment duration and other factors. At the same time, the recommendations and the parameters of treatment processes are quite diverse, which makes it difficult to design wastewater treatment plants.
We can recommend the equipment necessary to improve the efficiency of existing methods of wastewater treatment to remove chromium and heavy metals.