Treatment and Pollution Control Technologies ENVM 644: New Technologies in Environmental Management Dr. Robert Beauchamp

CHEMICAL PRECIPITATION n A SOLUBLE SUBSTANCE IS CONVERTED TO AN INSOLUBLE SUBSTANCE. n CAN BE REMOVED BY SETTLING OR BY FILTERING. n METALS OF CONCERN: –ARSENIC,BARIUM,CADMIUM,CHROMIUM, COPPER, LEAD, MERCURY, SILVER, ZINC. n SOURCES OF METALS: –METAL PLATING AND POLISHING, STEEL & ELECTRONICS INDUSTRY. n PROCESS: –A PRECIPITANT IS ADDED - HYDROXIDE (LIME) I.E. CALCIUM HYDROXIDE. –REMOVES METALS AS: METAL HYDROXIDE.

METALS PRECIPITATED AS SULFIDES & CARBONATES n DEPENDS ON: –CONCENTRATION, PH –PRESENCE OF OTHER MATERIALS. –PH OF IS OPTIMAL n CAN REDUCE TO = OR < 1.0 MG/L n NOTE: METAL HYDROXIDES ARE AMPHOTERIC HIGH AND LOW PH).

SULFIDE PRECIPITATION n SULFIDES ARE NOT AMPHOTERIC n THEY HAVE VERY LOW SOLUBILITIES n SOURCES OF SULFIDE INCLUDE IRON SULFIDE & SODIUM SULFIDE. n PROBLEM - PH NEEDS TO BE > 8 TO PREVENT FORMATION OF HYDROGEN SULFIDE (TOXIC GAS)

CARBONATE PRECIPITATION n NEED A LOWER PH (LOWER THAN THE HYDROXIDE METHOD)

SODIUM BOROHYDRIDE PRECIPITATION n A REDUCING AGENT n PH GOOD FOR LEAD, MERCURY, NICKEL, COPPER, GOLD.

ELECTROLYTIC RECOVERY TECHNIQUE n WORKS BEST FOR PROCESS STREAMS WITH HIGH CONCENTRATION OF METALS. REQUIRES ANODE & CATHODE ELECTRODES n CATHODES ARE IMMERSED IN A CHEMICAL MEDIUM UNDER AN APPLIED POTENTIAL. METAL IONS ARE REDUCED TO ELEMENTAL FORM AT THE CATHODE.

ELECTROLYTIC RECOVERY TECHNIQUE (cont) n ADVANTAGES: –METALS RECOVERED FOR REUSE. –TOXIC DISCHARGE IS REDUCED. –INITIAL INVESTMENT IS LOW. –OPERATION EXPENSES ARE LOW. –MINIMAL MAINTENANCE. n DISADVANTAGES: –ONLY GOOD FOR STREAMS WITH HIGH CONCENTRATIONS. –DEPLETION OF METALS IN WASTE STREAM OFFER RESISTANCE TO CONTINUOUS METAL MIGRATION TO THE CATHODE. –GAS CATHODES (H & CL).