Sciencemadness Discussion Board - Geochemistry > Zn, Fe Precipitation

Geochemistry > Zn, Fe Precipitation

demiurgent - 6-2-2013 at 13:02

Hi all,
I've got a question regarding reduction, precipitation, and potential co-precipitation.

Basically we have an old mining environment loaded with Fe, Zn, and some SO4. The groundwater is anoxic and pH ranges from 6.2-6.8.

Because of Iron staining we've put in two different system... one was a blower system that injected O2, reducing Fe into iron oxides which precipitated out, achieving our goal. The second was an anoxic limestone system intended to raise the pH and promote iron precipitation.

However, people are also still concerned about zinc.

Empirical evidence suggests that there seems to be a little bit of zinc concentration reduction taking place through the systems as well, which doesn't make a whole lot of sense to me... it could be chalked up to environmental variability, but I was wondering if the real chemists out there might be able to suggest a mechanism for that to be taking place?

Appreciate the time.
-demi

DraconicAcid - 6-2-2013 at 13:17

Zinc carbonate is less soluble than calcium carbonate, so maybe the limestone is taking some of it out. Or possibly the increase in pH from the limestone is convincing some of the zinc to precipitate as the hydroxide.

[Edited on 6-2-2013 by DraconicAcid]

bfesser - 6-2-2013 at 13:36

Are you attempting contaminated groundwater remediation? Where is this? Are you qualified?

demiurgent - 6-2-2013 at 13:42

Quote: Originally posted by bfesser

Are you attempting contaminated groundwater remediation? Where is this? Are you qualified?

In a sense. As I said, the remediation systems were installed years ago and now it's more about the evaluation of those remediation systems. The primary driver for the systems was Iron/Rust removal but the regulators have since become interested in their ability to remove zinc, which is not what the systems were designed for. I actually became involved with the project after it had already been operating for several years and well into the closure process.

At a site of ours.

I'm an Environmental Engineering P.E.

chemrox - 6-2-2013 at 17:11

He shouldn't have asked if you were qualified. None of his business! Or ours. This is part of my bailiwick. Can you obtain an elemental analysis of the gw? If you have one, p2p me and I'll send back my email address. After that we can follow up with a post for the good of the order. Sounds like fun. Even better, if you like, post the analytical results here. In situ pH would be worthwhile. Could you grab a sample from the wells and measure it in the field? Lab pH are about worthless for gw work.

demiurgent - 7-2-2013 at 08:28

Ha, it did seem a little weird.

I actually sample the wells quarterly, but the analyte suite for the quarterly monitoring is very limited... only diss zn and fe, total zn and fe, and total alkalinity.

I also take field conductivity, pH, and temperature readings quarterly. As stated in the OP, the GW pH varies from 6.2-6.8 depending on the well... or even specific to a well, the pH will range by +/- .3 over the course of a year due to wet season, dry season, etc. To make things even more chaotic, you end up with subterranean preferential pathways which the groundwater uses and our upgradient > downgradient wells may well not even be reporting the same water the way we'd like to see under ideal conditions.

I did manage to dig up some historic data when the characterization was being performed... here's a typical upgradient well's chemistry. One thing we never do is elemental species breakdowns.... drives me a little bit crazy.

Parameter Qualifier Value Units
Bicarbonate as CaCO3 -- 37 mg/L
Calcium, dissolved -- 122 mg/L
Calcium, total -- 122 mg/L
Carbonate as CaCO3 U <2 mg/L
Chloride -- 3 mg/L
Fe, dissolved -- 19.5 mg/L
Fe, Total -- 22.4 mg/L
Hydroxide as CaCO3 U <2 mg/L
Sodium, dissolved -- 4.1 mg/L
Sodium, total -- 4.1 mg/L
Sulfate -- 500 mg/L
Total Alkalinity -- 37 mg/L
Zn, dissolved -- 11.3 mg/L
Zn, total -- 9.78 mg/L

I can say there's likely some arsenic and lead in there as well. Some of the ferro-arseno complexes seem like they might be possible.... zinc wiggle its way in there anywhere?

blogfast25 - 7-2-2013 at 10:22

Quote: Originally posted by DraconicAcid

Zinc carbonate is less soluble than calcium carbonate, so maybe the limestone is taking some of it out.
[Edited on 6-2-2013 by DraconicAcid]

You have evidence for this? The solubility products suggest otherwise: Ks, CaCO3 about 4 x 10-9, Ks, ZnCO3 about 1.5 x 10-10.

Considering complications with CO2 forming bicarbonates and zinc also forming a basic carbonate, there wouldn't appear to be much difference between them, in terms of overall solubility. Ironically, CO2 is the reason why you're finding quite a bit of Ca: as Ca(HCO3)2...

You [demiurgent] might also want to take into account that zinc is amphoteric though: at higher alkalinity it's solubility should be much increased...

Your measure of blowing O2 through the water to oxidise all Fe to Fe (III) is probably not needed: it's probably already as ferric iron because your water is old.

[Edited on 7-2-2013 by blogfast25]

[Edited on 7-2-2013 by blogfast25]

DraconicAcid - 7-2-2013 at 11:43

Quote: Originally posted by blogfast25

Quote: Originally posted by DraconicAcid

Zinc carbonate is less soluble than calcium carbonate, so maybe the limestone is taking some of it out.
[Edited on 6-2-2013 by DraconicAcid]

You have evidence for this? The solubility products suggest otherwise: Ks, CaCO3 about 4 x 10-9, Ks, ZnCO3 about 1.5 x 10-10.

Smaller Ksp means less soluble. I should have added the word "slightly".

Quote:

Considering complications with CO2 forming bicarbonates and zinc also forming a basic carbonate, there wouldn't appear to be much difference between them, in terms of overall solubility. Ironically, CO2 is the reason why you're finding quite a bit of Ca: as Ca(HCO3)2...

You [demiurgent] might also want to take into account that zinc is amphoteric though: at higher alkalinity it's solubility should be much increased...

If the solution's being made basic with calcium carbonate, it's not going to be basic enough to form [Zn(OH)₄]^2-. Kf = 4.6e+17; Ksp for Zn(OH)2 is 3e-17 (from a textbook referencing Lange's Handbook of Chemistry), so the K for the reaction of Zn(OH)2 + 2 OH- = [Zn(OH)₄]^2- is only 14.

14 = [Zn(OH)₄]^2-/[OH-]^2, so [Zn(OH)₄]^2- = 14 x [OH-]^2. Even at a pH of 9, a solution saturated with zinc will contain the complex ion on the order of 1e-10 mol/L

[Edited on 7-2-2013 by DraconicAcid]

demiurgent - 7-2-2013 at 11:49

Quote: Originally posted by blogfast25

Quote: Originally posted by DraconicAcid

Zinc carbonate is less soluble than calcium carbonate, so maybe the limestone is taking some of it out.
[Edited on 6-2-2013 by DraconicAcid]

The water is mostly precip and snowmelt in a shallow glacial alluvium basin.... we know for sure that it wasn't ferric because of a few on the ground circumstances, it was oxidizing as soon as it daylighted in a seep and causing staining.

By blowing O2 we managed to take it from dissolved into suspended phase and remove the staining as the precipitate locked itself up in the soil pore spaces.

Really appreciate all the input from everyone! Great stuff!

Very solid point on the amphoteric nature of zinc, and it's fun to get back into reaction rates and equilibrium ks.... been too long.

demiurgent - 7-2-2013 at 12:04

It's really the zinc hydroxide that's amphoteric though, correct?

My Eh-pH curves seem to suggest I'm a bit too low on the pH end to be getting into the zinc hydroxide range....

blogfast25 - 8-2-2013 at 05:47

Quote: Originally posted by demiurgent

It's really the zinc hydroxide that's amphoteric though, correct?

Yes. The point remains that 'nominal' solubilities can be greatly affected by 'secondary' parameters like CO2 and pH.