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Heavy metals form a significant part of environmental pollution. They occur in water and in the soil in the form of their dissolved or relatively easily soluble compounds. Mercury, arsenic, lead, cadmium, copper, zinc, etc. are toxic and must therefore be removed from the water cycle and the soil.
Theoretically, there are several ways to separate heavy metals from wastewater. These are e.g. precipitation, ion exchange, electrolysis or ultrafiltration. However, only precipitation is suitable for the treatment of dirty waste water, in which case the heavy metals are made insoluble by adding a precipitating agent to the water and are thus precipitated.
The precipitated sludge can then be removed from the water by settling and filtration. Depending on which precipitating agent is added to the waste water, the precipitation can in turn be carried out by hydroxide, carbonate or sulfide methods.
In practice, hydroxide precipitation is mainly used because it is cheaper than alternative methods. The mostly acidic heavy metal waters are neutralized by adding alkaline solutions, whereby the heavy metals precipitate as hydroxides, i.e. bound to hydroxide ions. The neutralizing agents such as caustic soda (sodium hydroxide) or milk of lime (calcium hydroxide) are also the precipitating agents in this process.
Part of the heavy metals can already be precipitated in the acidic waste water as sulphides. Another part with sulfide only falls when the pH value increases, i.e. after neutralization. To do this, precipitating and neutralizing agents must be added to the water. The residual metal content in the water after precipitation determines which type of precipitation should be used. Like most chemical reactions, the precipitations are not complete. Even after precipitation has been carried out under optimal conditions, small amounts of the dissolved metal remain in the water.
Compared to hydroxide precipitation, sulfidic precipitation leads to far lower residual metal values in the waste water. After the sulfide precipitation, these are several powers of ten (10,000 to 10 million times) lower than after the hydroxide precipitation.
Despite the better results of sulphide precipitation, this has not been able to establish itself to this day. The reasons for this are the risk of hydrogen sulphide gas escaping, which can occur during the sulphidic treatment of acidic waste water, and the retention of toxic sulphides in the cleaned water.
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