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Mercury is one of the most strictly regulated elements, often restricted to less than 1 ug/l, microgram per liter, or less.
Mercury is often found in landfill leachate and incinerator scrubber water. It may also be found in research and development laboratory wastewater.
Mercury can be precipitated to low levels using carbonate, phosphate or sulfide.
When mercury is precipitated as the sulfide and high mercury residuals are often observed. This effect is due to the reduction of the mercury to the metallic mercury by the sulfide. Once in the metallic form, the mercury cannot form the insoluble sulfide. Metallic mercury is soluble in water at about 25 ug/l, which is above the regulatory limits. It may be visible as a lake floating on the surface of the reactor during the settling step. The residual mercury in the treated water must by oxidized to mercury 2 and then retreated to achieve low residual concentrations.
When treating mercury to form mercury phosphate, the oxidation step should be done prior to the precipitation step.
Following the initial precipitation step, the residual phosphate must be precipitated by the addition of calcium ion.
Mercury can also be removed by ion exchange. Again an oxidation step is required ahead of the ion exchange column and either a chelating resin or a mercury specific resin should be used. Once the resin is spent it is virtually impossible to regenerate, so the resin must be disposed of as a hazardous waste.
Mercury can be reduced to low concentrations by a reducing agents.
Granulated carbon is often used to polish treated mercury solutions with varying success.
The equipment used for the treatment of mercury is the same as described for treating copper.
A multi-step process is required to reduce mercury concentrations to very low levels.
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