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topic 3551

Excess lead in treated wastewater


Sun -

Can lead be removed from wastewater in the same manner that other common metals are? I generally treat my metals waste flow as follows: Chromium reduction or cyanide oxidation (if necessary), coagulant, DTC, pH adjustment, polymer dosage, and clarification. The only problems I have had are with lead, usually a concentration of about 1.7 mg/l.

I think this lead may possibly be coming from lead lined tanks. Could it be possible that some of it could come from a chrome etch clean process (tri to hex) if the electric current is passed through lead plates. Could treatment of the used catholyte solution contribute a significant amount of lead to a mixed acid batch tank?

Also, if a neutralization basin is lead lined, could excess sodium hydroxide feed cause the lead to leach out into the waste stream? Thanks in advance for your help.

D. Burton
Ellijay, Georgia


Ed. note: First of two simultaneous responses--

(1999)

Lead will leach out of alloys such as leaded brass at high pH creating soluble anionic plumbates. I would also expect it to leach out of lead liners at high pH.

The linings of anodizing or chrome plating tanks should be safe since the lead will form an insoluble (cationic) oxide, sulfate or chromate in these environments.

bill vins
Bill Vins
microwave & cable assemblies
Mesa (what a place-a), Arizona 


Ed. note: Second of two simultaneous responses--

(1999)

Lead will dissolve in concentrated nitric or, to a lesser extent, conc. hydrochloric acid. If you have either of these in a lead lined tank, that might be the source of your contamination. It is very unlikely that much lead would dissolve in a caustic soda solution.

I'm not sure I follow the business about the catholyte...you're running a trivalent Cr reoxidation cell? It's unlikely you'd get much Pb dissolution into a sulfuric acid electrolyte, unless it's very badly contaminated with chlorides, or something.

I would guess your trouble in waste treatment comes from Pb being colloidally suspended because of the type of treatment you're doing - DTC is very prone to this. I'd try 1) cutting back on DTC dosage 2) adding some sodium carbonate or bicarbonate after pH adjustment but *before* DTC addition or 3) using lime or mag hydroxide instead of caustic for pH adjustment.

Good luck, let me know how it comes out.

dave wichern
Dave Wichern
Consultant - The Bronx, New York



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