surface complexation

[Francis Adu]

Dear All,
I will appreciate your help and thought about my surface complexation model.

I am simulating the adsorption of uranium by an adsorbent with two active sites (aluminium and silicon). The surface master species and surface species are defined as;

[PHASES 1
fix_pH
    H+ = H+
    log_k     0
SURFACE_MASTER_SPECIES 1
       Surf_al  Surf_alOH
       Surf_si  Surf_siOH

SURFACE_SPECIES 1

Surf_alOH = Surf_alOH
log_k 0.0   

Surf_alOH + H+ = Surf_alOH2+
log_k 12.3

Surf_alOH = Surf_alO- + H+
log_k -13.16

Surf_alOH + UO2+2 = Surf_alOUO2+ + H+
log_k 7.51

Surf_alOH + 2UO2+2 + CO3-2 + 3H2O = Surf_alO(UO2)2CO3(OH)3-2 + 4H+
 log_k 0.99

Surf_siOH = Surf_siOH
 log_k 0.0

 Surf_siOH + H+ = Surf_siOH2+
 log_k -0.95

Surf_siOH = Surf_siO- + H+
 log_k -6.95

Surf_siOH + UO2+2 = Surf_siOUO2+ + H+
log_k 0.99

Surf_siOH + UO2OH+ = Surf_siOUO2OH + H+
 log_k 4.06

Surf_siOH + UO2(CO3)3-4 = Surf_siOHUO2(CO3)3-4
 log_k 8.0

Surf_siOH + UO2(OH)3- = Surf_siOHUO2(OH)3-
 log_k  6.9
SURFACE 1
Surf_siOH  94.9e-6  484  0.15

Surf_alOH  90.13e-6 16  0.15

SOLUTION 1
    temp      20
    pH        2
    pe        4
    redox     pe
    units     mmol/l
    density   1
    C(4)      1
    Cl        10
    Na        11
    U         250 ug/L
    -water    1 # kg
END

SELECTED_OUTPUT 1
    -file                 new.prn
    -high_precision       true
    -reset                false
 
USER_PUNCH 1
    -headings
    -start
10 q_sorb = mol("Surf_alO(UO2)2CO3(OH)3-2")+mol("Surf_alOUO2+")+mol("Surf_siOHUO2(OH)3-")+mol("Surf_siOUO2OH")+mol("Surf_siOHUO2(CO3)3-4")+mol("Surf_siOUO2+")
15 punch q_sorb/1.051e-06*100
    -end
 USER_GRAPH 1
    -axis_titles            "pH" "% Removal" ""
    -chart_title            "Removal of U"
    -initial_solutions      true
    -connect_simulations    true
   
#-plot_concentration_vs  x
  -start
10 q_sorb = mol("Surf_alO(UO2)2CO3(OH)3-2")+mol("Surf_alOUO2+")+mol("Surf_siOHUO2(OH)3-")+mol("Surf_siOUO2OH")+mol("Surf_siOHUO2(CO3)3-4")+mol("Surf_siOUO2+")+ mol("Surf_feOUO2+")
15 PLOT_XY -LA("H+"),q_sorb/1.051e-06*100, symbol = star, color = blue, symbol_size = 7
  -end
    -active                 true
 

USE solution 1
USE surface 1
#USE surface 2
EQUILIBRIUM_PHASES 1   
    fix_pH    -2.0 NaOH      10
END


USE solution 1
USE surface 1
#USE surface 2
EQUILIBRIUM_PHASES 1   
    fix_pH    -4.0 NaOH      10
END
 
USE solution 1
USE surface 1
#USE surface 2
EQUILIBRIUM_PHASES 1
       fix_pH    -6.0 NaOH      10
END
 
USE solution 1
USE surface 1
#USE surface 2
EQUILIBRIUM_PHASES 1
       fix_pH    -8.0 NaOH      10
END
 
USE solution 1
USE surface 1
#USE surface 2
EQUILIBRIUM_PHASES 1
     fix_pH    -10.0 NaOH      10
END

1. After running the simulation, the simulation is only considering the surface area of the aluminium surface (Surf_alOH  90.13e-6 16  0.15 ) and not that of the silicon surface. I would appreciate it if someone could explain the reason to me.
Even if I define the silicon surface as 1 and the aluminium surface as 2, the first simulation uses the aluminium surface and beyond that simulation, the silicon surface is used. The idea was to consider both surfaces in each simulation but the output is giving something different.

2. I would also want to know some information about the binding site. I have read a lot of papers and I think the term is used interchangeably.
a) I would like to know the true definition of the number of binding sites
b) is there any relationship between the number of binding sites and surface or site density?
c) How can I theoretically determine the number of binding sites of a surface?
d) In what unit is the number of the binding site reported? In some papers is reported as site/mol, site mol/ L, and others just moles. This is really creating a lot of confusion for me and I would appreciate it if someone could clarify these things for me.

Thank you and looking forward to hearing from you all.

[dlparkhurst]

1. When you define Surf_al and Surf_si, you are defining two types of sites on the same surface--Surf. The underscore is used to differentiate different sites on the same surface. In phreeqc.dat Hfo_w and Hfo_s indicate weak and strong sites on the  Hfo (hydrous ferric oxide) surface.

If you want two different surfaces, you must define something like Surfal and Surfsi, in which case you must define two surface areas, one for each surface. If needed, you can define multiple site types for each surface by using the underscore, "_", notation.


Only one SURFACE definition can be used in a simulation, but a SURFACE may have multiple surfaces (for example, both Surfal and Surfsi), each optionally with multiple site types.

2. The number of binding sites is used in a mole balance calculation. For example, the sum of the Hfo_w species (accounting for stoichiometry if a surface species uses multiple sites) will be equal to the number of Hfo_w sites defined in SURFACE.

There are several ways to define the number of sites in SURFACE. You can either define the number of surface sites for each site type as a number of moles. You can define a site density for each site type as sites per nanometer squared, in which case, the surface area (calculated by grams times m^2/g) is used to determine the number of moles of a site type. You can define a surface to be related to either an EQUILIBRIUM_PHASES phase or a KINETICS reaction, in which case the number of sites per mole of phase or reaction is input and multiplied by the number of moles of the phase or reaction to get the number of sites. In any of these cases, a number of moles of sites is calculated (not mol/L or mol/kgw); so mol/kgw is a derived quantity dependent on the number of sites in SURFACE, and the kg water in the solution. The number of kg water can be adjusted by -water in SOLUTION, by reactions, or by MIX.

The number of binding sites per nm^2 can be estimated by experimental data (see Dzombak and Morel for how this was done for HFO), or sometimes it is simply fit to sorption data.

In PHREEQC, the various SURFACE input options determine a number of moles of binding sites. Using the proportionality option with EQUILIBRIUM_PHASES or KINETICS, the moles of sites will vary during reactions as the moles of phase or reactant vary.

[Francis Adu]

Dear dlparkhurst,

Thank you very much for taking the time to explain my questions to me. I really appreciate your timely effort. However, there are still some pieces of stuff that are unclear to me and I would need your help.
1. As suggested by you, I defined the two surfaces with different notations accompanied by their specific surface areas(m2/g) and dosage (g)
[SURFACE 1
SurfsiOH  94.9e-6  484  0.15
SURFACE 2
SurfalOH  90.13e-6 16  0.15 ]. After the simulation, only one surface is recognized for the first simulation and the other in the subsequent simulations. Pardon me for that, maybe I am missing some key concepts. If one surface is recognized at a time, then what is the essence of defining two or more surfaces and calling them to be used in the simulation?
2. I would also want to know in the case of HfO where there are two sites involved (HfO_s and HfO_w), can you denote two separate specific surface areas and dosage for instance (HfO_s 5e-6 600. 0.09 and HfO_w 2e-4 200 0.09) for the same surface?

Thank you very much in advance

[dlparkhurst]

Put both definitions in the same SURFACE definition.

Code: [Select]

SURFACE 1
SurfsiOH  94.9e-6  484  0.15
SurfalOH  90.13e-6 16  0.15

There is only one surface area for Hfo, even though there are two site types--Hfo_s and Hfo_w. Site types are identified by the characters following an "_", whereas a surface is defined by the name preceding the underscore (or a name without an underscore).

If you wanted additional site types for Surfsi and Surfal, the following would be a legal example:

Code: [Select]

SURFACE 1
Surfsi_wOH  94.9e-6  484  0.15
Surfsi_sOH   1e-7
Surfal_wOH  90.13e-6 16  0.15
Surfal_sOH  1e-7


[Francis Adu]

Dear dlparkhurst,

Thank you very much. I now understand it. Much appreciate