TABLE OF CONTENTS Removal of heavy metals is most commonly achieved by precipitation of the metals as the hydroxide, carbonate, or sulfide forms. In some cases, co-precipitation is employed in which the target metal ion adsorbs to aluminum or iron hydroxide flocs that then settle and are removed from the waste stream.
Metal removal technologies are shown below:
Conventional precipitation
hydroxide
sulfide
carbonate
co-precipitation
Enhanced precipitation
dimethyl thio carbamate
diethyl thio carbamate
trimercapto-s-triazine, trisodium salt
Other methods
ion exchange
adsorption
Recovery opportunities
ion exchange
membrane technologies
electrochemical techniques
Different metals have different solubilities at different pH values. Copper and chromium have minimum solubilities (about 0.1 mg/l) at a pH range of between 8.0 and 10.0 units, for example. The designer will, through experimentation, determine the optimum pH needed for precipitation. The optimum dosage of coagulant aid for coagulation and sedimentation and the type and size of the precipitation unit are also determined through the experimental design approach.
Figure 6.11 summarizes the varying solubilities of certain metal hydroxide and sulfide precipitants over a range of pH values.
Every wastewater is somewhat different from others, even in industries making very similar products. Coagulation and precipitation reactions are very sensitive to interferences, and those that exist must be found and dealt with before design can proceed. Coagulation and precipitation processes that have operated successfully for years may fail when a change in process chemistry affects the chemical balance and characteristics of...
