[ RadSafe ] RE: Uranium solubility--RADSAFE Thank you

Tue May 13 23:32:31 CDT 2008

Dear Dr McCarn,

This discussion is very useful indeed. I remember in an International Conference on Natural Radiation Environment in India, there was a bitter controversy on results of uranium concentration measurements by a group of physicists from Brazil and another group from India. I do not remember  the outcome. A soft spoken Indian geochemist resolved the seemingly endless controversy by invoking concepts similar to what you described.

I guess the behaviour of some of these elements in sea water will also be unique to the microenvironment, though sea water may be more homogenous than soil.

Regards

K.S.Parthasarathy

----- Original Message ----
From: Dan W McCarn <hotgreenchile at gmail.com>
To: pottert at erols.com
Cc: radsafe at radlab.nl
Sent: Wednesday, 14 May, 2008 4:06:18 AM
Subject: [ RadSafe ] RE: Uranium solubility--RADSAFE

Dan W. McCarn, Geologist; 3118 Pebble Lake Drive; Sugar Land, TX 77479; USA 
HotGreenChile at gmail.com           UConcentrate at gmail.com

Hi Tom:

Since I've had a couple of questions about this, I'll post my answer.

1) Soils are complex in nature, and much depends not only on O2 and CO2
availability but also adsorption-desorption, cation exchange,
mineralization-demineralization, evapotranspirative and / or redox
mechanisms in the soils themselves.  Adsorption-desorption slows movement
down, and in some soils, mineralization processes such as in caliche-forming
soils (e.g. pedogenic calcretes, gypcretes & silcretes, also called
duracrusts or surficial deposits) uranium will concentrate as U(VI)
minerals. Organics also tend to hold onto uranium.

Remember that soils are not saturated media, and tend to have high
fluctuations in water content. Because of this, uranium movement and
mechanisms of movement through soils tends to be strongly influenced by this
feature / process (remembering the concept of FEPs - Features, Events &
Processes).

http://en.wikipedia.org/wiki/Features%2C_events%2C_and_processes

After reviewing data from desert soils in Kazakhstan (unpublished data)
subjected to infiltration of uranium-bearing waters for decades, the uranium
does appear to "zoom" through the upper soil zone reasonably quickly.  At
least it appears fairly uniform down to several meters (depth of
investigation) whereas radium tended to hang in the upper 0-30 cm (root
zone) probably adsorbed onto clays.  But again, that was a desert soil.  

2) Theoretical calculations about the ability of a water to solubilize
uranium minerals are just that: theoretical.  But if a source of uranium
suddenly appears, the capacity to move uranium is quite high.  Also, with
high-volume irrigation wells intercepting a redox front, the concentrations
could easily go to milligrams per liter, deposited directly onto a soil.
ISR/ISL mining involves injection of oxygen-enriched water near the redox
front and extracting uranium in a production well 30-50 feet across the
redox front with the head-grade of uranium in the 100s of milligrams per
liter.  This is common mining practice.

3) Uranium phosphate minerals form the most abundant class of uranium
minerals in nature with several dozen mineral species e.g. autunite. By
adding uranium-bearing phosphate fertilizer to a soil, there may be a
tendency for the uranium to mineralize, de-mineralize, and re-mineralize
(e.g. autunite, schoepite) depending on the available water, chemistry and
evapotranspirative characteristics of the soil, thus slowing-down
mass-transport below the root zone. Remember that this is controlled by the
solubility product of a mineral assuming that everything else is present for
a mineral to precipitate. Uranium minerals tend to dissolve and precipitate
quickly.  

http://en.wikipedia.org/wiki/Solubility_equilibrium

Because of this, uranium does not tend to be persistent in water such as is
the case for antimony (+3/+5 valent). Once antimony is dissolved in
oxidizing waters, even if the water becomes reducing, and everything is
available to re-precipitate, the antimony tends to remain mobile. Not only
redox, but authigenic, evaporative pedogenic and some non-pedogenic,
evaporative mechanisms can precipitate uranium (VI) very efficiently (e.g.
Australian & Namibian non-pedogenic calcrete uranium; Oxidized Jackpile
Deposit in the Grants Uranium Region).

Many secondary, oxidized U(VI), minerals are "meta-stable" including the
uranyl vanadates carnotite [K2UO2VO4] and tyuyamunite [CaUO2VO4], or
phosphates such as autunite [Ca(UO2)2(PO4)2]. If outcrops containing these
minerals are exposed to meteoric conditions, they tend to be driven back
into the rock or soil.

4) Availability of uranium is ALWAYS the controlling factor for
concentrations found in most surface and ground waters.

Hope that helps!

Dan ii

-----Original Message-----
From: THOMAS POTTER [mailto:pottert at starpower.net] 
Sent: Tuesday, May 13, 2008 2:15 PM
To: Dan W McCarn
Subject: Uranium solubility--RADSAFE offline

Dan,

Sending this offline since it isn't exactly on point.

Salsman, Ben There, and all aliases aside, my reading leads me to agree 100%
with everything you posted about uranium solubility in the environment.  

But I have never gotten a satisfactory answer to one gnawing question.  How
is all of this consistent with what is observed regarding concentrations of
uranium in topsoils and surface waters?  If all of the thermodynamics is
right, uranium should be leaching out of the topmost soil layers, which are
in equilibrium with oxygen and carbon dioxide in the atmosphere.
Simultaneously, concentrations of uranium in surface waters should be much
higher than the observed levels--in the range of micrograms per liter rather
than milligrams per liter predicted on the basis of thermodynamics.  But
this is not the case.  Something else more important to uranium solubility
must be going on in the general environment.

Any ideas?

By the way, I believe that uranium applications to agricultural lands are
through the fertilizer.  Minable uranium deposits are found near
(geologically speaking) phosphate deposits.

Regards,
Tom Potter 

Date: Mon, 12 May 2008 12:54:25 -0500 
From: Dan W McCarn <hotgreenchile at gmail.com> 
Subject: [ RadSafe ] RE: UO{2,3} dissolved: which increases? 
To: "'radsafelist'" <radsafe at radlab.nl> 
Message-ID: <005c01c8b459$385d3260$a9179720$@com> 
Content-Type: text/plain; charset="utf-8" 

Dan W. McCarn, Geologist; 3118 Pebble Lake Drive; Sugar Land, TX 77479; USA 
HotGreenChile at gmail.com          UConcentrate at gmail.com 

1) May I also suggest Stumm and Morgan, "Aquatic Chemistry: Chemical
Equilibria and Rates in Natural Waters" ISBN: 0-471-51185-4.  They have a
long section on carbonate speciation in waters as does Langmuir.  Amazon
offers both Langmuir's and Stumm & Morgan's books together for a discount. 

http://www.amazon.com/Aquatic-Chemistry-Werner-Stumm/dp/0471511854 

2) Typically groundwater contains CO2 from 10^-2.5 to 10^-2 atmospheres
pressure. Normal air is about 10^-3.5 atm CO2.  See Freeze & Cherry,
Groundwater: 

http://www.amazon.com/Groundwater-Alan-R-Freeze/dp/0133653129/ 

3) Carbonate does not "oxidize" uranium, but does provide a complexing ion
to form an aqueous species. Uranium (IV) will rapidly convert to uranium
(VI) in the presence of oxidants such as free oxygen and be transported
until a reducing environment is encountered.  There, it will promptly
precipitate.  This natural mechanism gives rise to redox-controlled
sandstone deposits and several other interesting uranium-concentrating
mechanisms. 

4) I would say that virtually all irrigation and other natural waters used
for domestic and livestock purposes contains concentrations of bicarbonate
(HCO3-) suitable to transport mg/L concentrations of uranium if available
and depending on the availability of oxygen.  These concentrations of
bicarbonate range from 10-100 mg/L. 

5) Much depends on the availability of uranium.  Uranium can be "weathered"
from uranium-enriched granites. Oxidizing meteoric waters infiltrating and
recharging a basin from such a source can, in geologic time, produce uranium
deposits such as is the case for the Wyoming Basins uranium and many other
sandstone uranium deposits. Is uranium in GW or fertilizer being directly
placed on crops? Or is it somewhere sequestered with the "bottom ash" of a
power plant?  The fate and distribution of uranium in these cases depends on
local mechanisms in soils and geologic media as well as the chemical
characteristics of water passing through uranium-bearing materials.

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