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topic 5927

Lead Removal from Wastewater


(2000)

I have a lead lined neutralization basin through which passes misc acids, reduced chrome waste, and oxidized cyanide waste. I have a sulfuric acid feed and a caustic soda feed. The pH is usually regulated at 9.5. When we have had problems with the pH control system, I have noticed lead leaching from the tank at both high and low pH levels.

I know that lead oxide is resistant to sulfuric acid and is therefore well suited to line a sulfuric acid tank, but lead oxide is affected by other acids, such as nitric or hydrochloric. When I have seen the pH at low levels, I find lead levels in excess of 50 ppm. This is very easily treated by raising the pH and precipitating the lead as a hydroxide.

I also know that lead will dissolve in a cold alkali solution. When we have experienced high pH levels in the tank, I have witnessed lead levels around 4 ppm. However, this lead is very difficult to remove from the water. Adjustment of the pH and polymer and DTC dosage just doesn't help me to get the level below 0.1 ppm. Is there a difference between lead that has dissolved in an alkali solution and lead that has been dissolved in an acidic solution? If so, what would help to remove the lead that has leached at a high pH? Thanks for all replies. P.S. - Am getting a new tank liner for Christmas!

Derek Burton
- Atlanta, GA USA


simultaneous (2000)

Had never thought about it, but yes there would probably be a difference. Acid forms lead sulfate or nitrate and etc. These tend to be nice salt like crystals.Caustic forms a lead hydroxide wich will tend to be a slime that takes quite a bit of agitation to get it completely dissolved. There could be a difference in the solubility of the two also which could slow the process down.

James Watts
- Navarre, Florida


(2000)

Derek,

Re: your lead problem, I can't help you, i,e., I am not a chemist and can't tell you what to do.
However, may I make one suggestion? Why, if you are having problems with lead, why don't you use a plastic tank?
I have supplied these in the past for the worst conglomeration of acids you never want to meet ... Assay acids.

If your neutralization tank isn't horribly huge, maybe you could consider a moulded rectangular Polyethylene tank. If exposed to sunlight, you can get it made using black Pe. If you are worried about excessive oxidization in the tank, then get the moulder to add more resin to his mould so as to give you a thicker tank, max. l/2" is the limitation, the max. thickness, for rotational moulding.

Your local plastics fabricator will, I'm sure, be happy to help you. If you have a heavy exothermic reaction, then lead is better as a Pe tank's limitation is 150 °F ... or go to CPVC:FRP for 2l0 or PVDF:frp for 280 etc. etc... supposing you want to spend more $$$ which I seriously doubt. Anyhow, a compatible sizewise Pe tank would cost much less than a lead or a lined lead one.

In short, you'd know in future that your pH tests would have nothing to do with the Pb content AS YOU GOT THE LEAD OUT ! Cheers

freeman newton portrait
Freeman Newton
White Rock, British Columbia, Canada

(It is our sad duty to
advise that Freeman passed away
April 21, 2012. R.I.P. old friend).




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