While the industrially developed regions of the country suffer from lack of clean water, electroplating consumes enormous amounts of water: more than 50 million cubic meter per year. This industry is one of the most hazardous, with harmful work conditions, large amounts of wastewater contaminated with highly toxic chemicals. The main components of electroplating wastewater are inorganic substances, such as heavy metals: chrome, zinc, nickel, copper, cadmium etc. Hexavalent chrome compounds are especially hazardous.
To toxicity of Сr6+ is in the suppression of growth, retardation of metabolic processes in the form of genetic, gonadotropic, embryotropic changes, while chrome compounds are highly carcinogenic. One of the priorities is implementation of modern decontamination processes to obtain waste with the concentration of contaminants which does not exceed regulatory limits.
When selecting the method of decontamination, it is necessary to consider the methods of handling solid waste. This is due to the fact the solid waste extracted from wastewater poses a great danger to groundwater if stockpiled in landfills.
Among the many existing methods of chromic wastewater decontamination, such as chemical, ion-exchange, adsorption, membrane, biochemical and electrochemical, the chemical method is used the most. The reduction agent of hexavalent chrome can be iron sulfide, calcium sulfide, hydrogen peroxide, formaldehyde, powder aluminum, hydrazine, barium salts, lead compounds, metal scrap etc. For reduction of one part of Cr6+ up to 16 parts of reduction agent are required. In the end, the process results in large amounts of solid waste(10.0 – 12.5 thousand tons per year), which contain large amounts of hydroxides, carbonates and heavy metal salts.
The membrane methods of wastewater decontamination are the reverse osmosis and ultra-filtration. Membrane methods offer high degree of purification, allow to return the water back into the production and regeneration of the solved substances, all with rather low energy costs. However, ultra-filtration units accumulate the captured substances on the surface of membranes, reducing permeability and selectiveness. Low chemical resistance in aggressive media and high cost allow membrane application only where the media is not aggressive and the concentration of metal ions exceeds the limits by a small amount. Due to the above, membrane technology has not come into wide use for local decontamination of electroplating wastewater.
The sorption methods of electroplating are adsorption and ion-exchange. Natural and porous materials are used in this process. Industrial application of this process is limited due to the complicated regeneration and reuse of the sorbent.
Ion exchange method is widely used. Chrome is extracted and reduced by two columns, filled with highly acidic cationite and highly basic anionite.
Another possible method is electrochemical coagulation with soluble steel anodes. In this method, bichromate and chromate ions are reduced by iron ions Fe (II), which form during electrolytic dissociation and due to cathode reactions.