[OLD] Where does the Palladium go?

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From: Brugger, Joel (SAM)

Subject: Where does the Palladium go?

I join a script file simulating reduction of a groundwater by carbon. The fluid contains Pd. According to GTplot, the concentration of Pd in the fluid decreases dramatically as reaction progresses. However, there is no Pd minerals among the minerals! Where does this Pd go?

Note that if I remove the flow-through option (flow-through off), life's good (or at least more confusing) and Pd appears among the minerals.

I'm using the LLNL database, and I can reproduce the problem with GWB versions 3 and 4.03 (just got it today!).

# Reduction of RC water by organic carbon

#

reset

suppress Uranopilite Rabejacite

suppress Pentlandite Violarite

suppress Delafossite

 

# precip off

# balance on Na+

 

T 23

 

# O2(aq) 3 mg/l

swap e- for O2(aq)

Eh -0.023

# swap O2(g) for O2(aq)

# log f O2(g) -50 # just above solubility of uraninite/coffinite

pH 6.92

 

# swap NO3- for NH3(aq)

# NO3- total mg/l = 45.73

Sb(OH)3(aq) total mg/l = 5.05e-5

VO++ total mg/l = 0.02201

 

H2O 1 kg solvent

 

Na+ total mg/l = 209

Mg++ total mg/l = 61

Ca++ total mg/l = 105

K+ total mg/l = 7

F- total mg/l = 3.67

Cl- total mg/l = 275.71

 

# Br- total mg/l = .94

 

SO4-- total mg/l = 125.69

HCO3- total mg/l = 589.98

SiO2(aq) total mg/l = 34.22

# Li+ total mg/l = .029

# Al+++ total mg/l = .00225

Ti(OH)4(aq) total mg/l = .015485

# CrO4-- total mg/l = .0161216

# Mn++ total mg/l = .02456

Co++ total mg/l = .00084

Ni++ total mg/l = .01626

Cu++ total mg/l = .00541

Zn++ total mg/l = .00689

H2AsO4- total mg/l = .03674

Sr++ total mg/l = .63367

# Y+++ total mg/l = 6e-4

# Zr(OH)2++ total mg/l = .000141

MoO4-- total mg/l = .09593

Pd++ total mg/l = .00118

Ag+ total mg/l = 2e-5

# Cd++ total mg/l = .00029

Sn++ total mg/l = 7e-5

# Cs+ total mg/l = .00029

Ba++ total mg/l = .06431

# Ce+++ total mg/l = 3e-5

WO4-- total mg/l = .0081

# Au+ total mg/l = .00031

# Tl+ total mg/l = 1e-5

Pb++ total mg/l = .00053

UO2++ total mg/l = .754058

 

# Ga+++ total mg/l = .00187

# Rb+ total mg/l = .01352

# Sc+++ total mg/l = .01197

# NbO3- total mg/l = 1.5e-5

# B(OH)3(aq) total mg/l = .571

# La+++ total mg/l = 2e-5

# Nd+++ total mg/l = 2e-5

# Sm+++ total mg/l = 1e-5

# Eu+++ total mg/l = 3e-5

# Gd+++ total mg/l = 2e-5

# Dy+++ total mg/l = 3e-5

# Ho+++ total mg/l = 1e-5

# Er+++ total mg/l = 3e-5

# Yb+++ total mg/l = 3e-5

 

suppress Quartz Tridymite Coesite Cristobalite(alpha) Cristobalite(beta)

suppress Chalcedony

# suppress SiO2(am)

# swap SiO2(am) for SiO2(aq)

# 100 free g SiO2(am)

0.01 molal SiO2(aq)

 

# swap Hematite for Fe++

# 1 free g Hematite

# Fe++ 1e-12 molal

 

go

 

pickup fluid

 

flow-through

react 0.07 g C

react 10000 g SiO2(aq)

delxi .05 log

dx_init 0.00000001

go

 

From: Craig Bethke

Subject: Re: Where does the Palladium go?

In a flow-through model, a reactant fluid displaces existing fluid from the system. Pb initially in solution in the system, therefore is lost over the course of the reaction path, as fluid is displaced.

 

From: Brugger, Joel (SAM)

Subject: Re: Where does the Palladium go?

I'm afraid that I'm still confused. According to the yellow manual, flow-through just prevents back-reaction between precipitated minerals and fluid, while flush dispaces the fluid (which i can't do with the present model as I haven't defined the displaced aquifer fluid). Also, if I plot the total composition of the system, the amount of Pd remains constant, as does the amount of H2O. So how can the concentration of Pd decrease in the fluid ?

 

From: Craig Bethke

Subject: RE: Where does the Palladium go?

Sorry, I misunderstood your question. Please ignore my last response. Where does the Pd go? It falls off the end of the computer chip. There is little Pd in your run, only about 10^-17 moles. Metallic Pd saturates over the course of your path and precipitates, but of course only an extremely small amount forms.

In a flow-through model, React transfers after each reaction step the minerals that formed over the step into a set of isolated minerals. A very small amount -- effectively a zero mass -- of the mineral must remain behind in the equilibrium system, to maintain the basis.

Your computer can carry a number to only about 12 or 13 significant digits. This means that the mass of Pd the program wants to leave behind for numerical reasons is more than the mass that has precipitated. So the Pd never makes it the list of isolated minerals, and therefore does not appear in your plotted output. But that very small mass is there, as far as React is concerned -- it is not lost and there is no mass balance problem. You just can't see it in your plots.

All that said, React is commonly a poor tool for modeling trace elements because it doesn't account for the trace element content of minerals that might form or dissolve. In your case, for example, you need to ask yourself whether Pd is likely to precipitate as a truly minuscule amount of metal, or whether it would more likely coprecipitate as impurities in the other minerals that form.