Tutorial on Visual Minteq 2.30, adsorption. One of the main purposes for Minteq type of programs is to perform adsorption calculations, in addition to doing the straight forward acid-base-complexation- precipitation calculations in the presence and absence of gas with and without redox reactions. Often, the most important processes for trace elements are various adsorption reactions and these reactions are rarely treated in courses or text books, probably because they tend to be ill defined and there are not as many simplistic descriptions as with e.g. solubility constants. Solids in water generally form an oxide/hydroxide-water interface and are represented by for example, ≡SOH and ≡SO - , where the “≡S” represents the solid-surface interface. Adsorbed materials are written as complexes of these surface oxides, for example, ≡SOPb + , ≡SONa, or ≡SOAsO 3 - , etc. OH - Na + Cl - PbOH + H HCO 3 - + OH - Cl - Pb 2+ OH ≡SOH ≡SO - ≡SOPb + O - OPb + Solid Symbol Solution Example Reactions: ≡SOH → ≡SO - + H + ≡SOH + Pb 2+ → ≡SOPb + + H + The units used in adsorption reactions have several peculiarities and conventions that are not normal, but are in common use. At least four ideas need to be used to express concentrations with reactive solids: 1. the amount of solid in solution, mg/l, g/l, kg/l, etc.; 2. the concentration of reactive surface sites on the solid in solution, sites/m 2 , sites/nm 2 , etc; 3. the specific surface area of the solid, m 2 /g; and 4. the concentration of contaminant on the solid, mg/g, mg/kg, mol/kg. As will be seen these can be combined to express the concentration of reactive sites per liter of solution. In addition, there is no established set of symbols or conventions on how to represent these common concepts; Visual Minteq uses “moles of adsorbents per liter of solution” as the working unit. This simplifies the mathematics and allows all calculations to be performed in the exactly same manner as acid/base or complexation reations. 1. Concentration units in common use: Mason Tomson Visual Minteq Page 1 Adsorption 11/2/2004 1
Transcript
Tutorial on Visual Minteq 2.30, adsorption. One of the main purposes for Minteq type of programs is to perform adsorption calculations, in addition to doing the straight forward acid-base-complexation-precipitation calculations in the presence and absence of gas with and without redox reactions. Often, the most important processes for trace elements are various adsorption reactions and these reactions are rarely treated in courses or text books, probably because they tend to be ill defined and there are not as many simplistic descriptions as with e.g. solubility constants. Solids in water generally form an oxide/hydroxide-water interface and are represented by for example, ≡SOH and ≡SO-, where the “≡S” represents the solid-surface interface. Adsorbed materials are written as complexes of these surface oxides, for example, ≡SOPb+, ≡SONa, or ≡SOAsO3
-, etc.
OH-
Na+ Cl-
PbOH+
HHCO3
-
+ OH-
Cl-
Pb2+
OH ≡SOH ≡SO-
≡SOPb+O-
OPb+
Solid Symbol Solution
Example Reactions: ≡SOH → ≡SO- + H+
≡SOH + Pb2+ → ≡SOPb+ + H+
The units used in adsorption reactions have several peculiarities and conventions that are not normal, but are in common use. At least four ideas need to be used to express concentrations with reactive solids: 1. the amount of solid in solution, mg/l, g/l, kg/l, etc.; 2. the concentration of reactive surface sites on the solid in solution, sites/m2, sites/nm2, etc; 3. the specific surface area of the solid, m2/g; and 4. the concentration of contaminant on the solid, mg/g, mg/kg, mol/kg. As will be seen these can be combined to express the concentration of reactive sites per liter of solution. In addition, there is no established set of symbols or conventions on how to represent these common concepts; Visual Minteq uses “moles of adsorbents per liter of solution” as the working unit. This simplifies the mathematics and allows all calculations to be performed in the exactly same manner as acid/base or complexation reations.
“Total contaminant per gram” is what is measured, e.g., “mg of Pb/g of solid” or “mole of Pb/g of solid.” This is what is measured in most laboratories by filtering an amount of solid phase and acid digesting the lead from the solid and measuring by ICP, AA, titrations…
b. ⎟⎠⎞
⎜⎝⎛⋅⎟⎟
⎠
⎞⎜⎜⎝
⎛=⎥⎦
⎤⎢⎣⎡
Solution ofLiter solid of kg
Solid of kgPb mol
solution ofLiter solidon lead of molesConc.
This concentration of contaminant on the solid phase is used in Visual Minteq to do calculations wherein the concentration on the solid are in terms of M, just like all other concentrations. For example, if Pb is 100 mg/kg (essentially 100 ppm), which is a common concentration of lead in soil, sediments, etc. If the solid content is 10 mg/l, as with a slightly turbid lake, then the concentration would be 10-3 (mg of Pb)/(Liter of solution), which is of little toxicity concern. On the other hand, if the same solids concentration is 1 kg/l, as in sediment slurry, the total possible lead would be 100 mg/l –a deadly poison. c. Conc. in terms of Tot≡SOH
⎟⎠⎞
⎜⎝⎛⋅⎟⎟
⎠
⎞⎜⎜⎝
⎛⋅⎟⎟⎠
⎞⎜⎜⎝
⎛⋅⎟⎠⎞
⎜⎝⎛=⎥⎦
⎤⎢⎣⎡
# sAv.'mole 1
msites #
kgm
Solution ofLiter Solid of kg
solution of litersites reactive of moles
2
2
This the form of the concentration that is used in Visual Minteq, represented as, for example, Tot≡SOH (mol/l) component. Notice that this is combination of solid concentration, surface area, site density on the surface, and Avagadro’s number.
2. {≡SOPb+} = [≡SOPb+], i.e., the solid phase activity and concentration are taken to be the same, the activity coefficients of the solid phase species are always taken to be 1.00. This is a rather controversial assumption and many have challenged it, but it is the assumption used in nearly all software programs. –Many books have written on this topic, but for the sake of simplicity we will not cover all these alternative arguments in this discussion.
3. Solid-solution equilibrium constants are typically written in terms solution phase activities, {Pb2+} = [Pb2+]·γPb2+, of individual species. The activity coefficient in solution is composed of two parts, the first related to normal ion-ion interactions in solution from the other ions in solution (treated by the Davies or the Debye-Huckel equation) and secondly a contribution from the charged solid-phase surface, see next item.
4. As an ion approaches the solid surface, the electrostatic potential of the solid surface can greatly affect the activity of the of the solution ion. This effect is expressed as a Boltzmann factor, , where ψRT/Foe ψ−
o(V) is the potential of the surface due to electrostatic potential from the charged solid surface.
Overall objective of adsorption speciation. Regardless of the specific mathematical formulation of the adsorption, partitioning, complexing, etc., the overall objective in terms of environmental fate and transport of contaminants in the environment is generally to arrive at a single distribution coefficient, Kd(L of solution/kg of solid) that describes the amount of the contaminant on the solid phase versus the amount in the mobile solution phase:
)solution of L/mol(]forms allin TotCont[)solid of kg/mol(]forms allin on concentrai TotCont[)kg/L(K
solution aqueousin
solid on thed =
Often these measured Kd values are used to report data. These Kd values are tabulated for specific solids and waters, e.g., sea water or lake water, etc., and as long as the overall conditions of pH, redox, complexing agents, etc., are reasonably constant these Kd values can be used to calculate the overall concentration in solution from the solid phase value, or vice versa, and thereby bio-availability to organisms: TotConton solid(mol/kg) = Kd(L/kg)⋅TotContin solution(mol/L) These values of Kd that are required to mathematically describe the transport of contaminants of all kinds, regardless or the functional form used to arrive at the ratio of adsorbed to aqueous concentrations. That is, the species that are in the aqueous phase are assumed to be mobile and the species adsorbed to the solid are assumed to be immobile, or not available. Minteq and Visual Minteq authors were sensitive to this end use when they formulated the program for the EPA in the 1970’s and later. Notice that the output screen always lists Total of Component in Solution and the Total of Component Adsorbed, which is what is needed to know, the total that is available to organisms, etc. After the various adsorption models are introduced, the Kd values can be examined. Solid metal oxides and other surfaces often are strong adsorbers of aqueous species, such as Pb2+, PO4
3-, AsO43-, etc. There are many types of options for adsorption reactions.
Mostly, they divide into three groups. 1. Simple isotherms, illustrated for aqueous cadmium ion activity, {Cd2+}, adsorption
to a surface oxide site, ≡SOH, to for form ≡SOCd+ + {H+}:
a. Linear isotherm: [≡SOCd+]adsorbed = Kads.{Cd2+}
with Kads(L of solution/kg of solid). These Kads. constants are often supplied by a local curve fitting of the adsorption of some metal onto the overall bulk solid phase. Visual Minteq permits you to add any Kads. value you wish and it will then incorporate that Kads. value into an overall speciation with redox, precipitation, etc., scheme and model the amount of trace metal adsorbed. This is a very practical and useful option for real site data to account for the effect of e.g., added complexing agents or changes in pH in the site and still use the measured Kads.
with the constant, KLang.(L/mol), note that when {Cd2+} is small the isotherm is linear in {Cd2+} and when {Cd2+} is large the isotherm equals a constant,
. Numerous tables and books report these KMaximumadsorbed]SOCd[ +≡ Lang constants
(see below) and even values of for numerous different soils or sediments. Also, you can make only one or two measurements to determine K
Maximumadsorbed]SOCd[ +≡
Lang and as constants for your soil or sediment and then use Visual Minteq to calculate the effects of solution conditions on the adsorption or fate of the trace heavy metal or contaminant.
Maximumadsorbed]SOCd[ +≡
c. Freundlich isotherm: [≡SOCd+]adsorbed = KF{Cd2+}m
with m = constant and normally m ≈ 0.5 to 1.0. Again, values of KF and m are often tabulated for different soils and sediments.
d. Various combination isotherms and numerous other isotherms are listed in Visual Minteq.
For example, the following tables of data are taken from Bobek, et al., “Environmental Inorganic Chemistry: Properties, Processes, and Estimation methods,” Pergamon Press, 1988. This text and numerous others like it list literally thousands of constants and sets of constants for all kinds of isotherms and conditions. These tabulations are commonly used to estimate the aqueous or solid concentrations by matching, as closely as possible, one’s needs against what has already been measured. First, in the following table is listed a few constants for Cadmium adsorption to a few materials:
Second for Arsenic(III), the much more toxic oxidation state of arsenic:
2. Ion exchange adsorption of ions, for example for Cd2+ exchanging with Ca2+:
[≡SOCa+]adsorbed + Cd2+ [≡SOCd+]adsorbed + Ca2+,
or:
}Cd{]SOCa[}Ca{]SOCd[
K 2adsorbed
2adsorbed
exchangeion ++
++
⋅≡
⋅≡= ,
with these dimensionless Kion exchange constants sometimes called “selectivity coefficients.” The soil, sediment, clay, or organic matter are also characterized by a total cation exchange capacity, CEC (mequivalents of exchangeable ions/g of solid) There are literally thousands of such constants in texts, such as the one referenced above and references therein.
Electrostatic double layer models (dlm), overview. These models will be the primary focus of the remainder of this Visual Minteq Tutorial. Minteq can be used to describe most heterogeneous reactions; the sample test problems that are listed in the File input menu are excellent points of departure and can be used as “seed problems” that you can modify for your particular needs. Adsorption data is not nearly as systematic and well accepted as for acid/base and complexation. This is probably because not enough effort has been spent to organize and establish the measurements in a manner that they can be reliably used in all circumstances, i.e., this is still a developing field of science and applied practice. Therefore, most of the adsorption data entries in Visual Minteq are such that the values can be readily changed. Hohl and Westall in 1980 published a critical paper (Adv. Coll. Interface Sci., 12,265) in which they compared five common adsorption models and proved that you could not normally justify picking one as clearly superior to the others. That is, it doesn’t matter too much which model you use as long as the sorption constants have been curve fitted to that model. If you pick another model and use the corresponding sorption constants, both will typically fit the experimental data equally well. Therefore, it is probably only necessary to describe only one model and that is what will be done herein. This discussion will focus on the model for which there is the most data, the simple two pKa double layer model and will focus on adsorption to what has been called “hydrous ferric oxide” which can be used as a model adsorbents for a wide range of conditions and for which a Dzomback and Morel have published a large amount of data. The basic problem will be to describe the adsorption of arsenic, AsO4
3-, and cadmium, Cd2+, to “hydrous ferric oxide, Fe(III)Oxide⋅H2O, versus pH and pe and in the presence of various competing ions, etc., but first the acid/base chemistry of simple hydrous ferric oxide. We will use 1 g/L at surface area of 100 m2/g and a site density of active ≡FeOH sites of 1site/nm2 –this is a reasonable, but low sorption condition that might correspond to ferric oxide in sediment or in water treatment by adding ferric chloride to induce “hydrous ferric oxide” precipitation and adsorption for arsenic removal. A graphic representation is as follows from Stumm and Morgan, “Aquatic Chemistry 3rd ed.,” Wiley, 1996:
The surfaces of most solids accumulate a net charge, σo, and this charge is counter balanced by an equal and opposite charge in the diffuse water layer around and near the solid surface, σd, i.e.: σo + σd = 0.00 Almost every text seems to use a different set of units to express surface charge and related quantities, but Visual Minteq uses the following: TSurface charge (mol/L) = Ns(sites/m2)⋅SA(m2/g)⋅Cs(g/L)/NA(#sites/mol) The Ns input is in units of (sites/nm2), which is commonly about 1 to 2 site/nm2, and then the program converts to m2, internally. This expresses surface charge in moles of surface charge per liter of solution –see example below. Ordinarily, it would be expected to treat the charge on the surface just like is done with activity coefficients and ionic strength and use some simple expression to correct for the surface charge as is done using the Davies or the Debye-Huckel equations. The problem with this reasonable approach is that practically solids accumulate so much charge on the surface under some circumstances of pH and ionic strength the impact on the adsorption or ions can be orders of magnitude in value, but at other ionic strengths or pH’s it may not too important. All these circumstances are treated with the electrostatic double layer models used in Visual Minteq (from the Minteq tutorial on the EPA web site):
The iron, manganese, silica, etc., metal oxide surface accumulates charge similar to the surface of the pH electrode, wherein the ≡SOH groups can either add a H+ at lower pHs values to form a net ≡SOH2
+ or at higher pH values loss of a H+ to form a net negatively charged surface, ≡SO-: ≡SOH + H+ ≡SOH2
+ and ≡SOH ≡SO- + H+
wherein the ≡SOH refers to the surface metal hydroxide, such as ≡FeOH with hydrous ferric oxide or ≡SiOH with silica. The effect of the surface charge, ≡SO- or ≡SOH2
+, on adsorption is given by a Boltzmann-type factor for activities and in Visual Minteq it is written as follows:
where z = charge on the ion including the sign, = activity of the ion immediately next to the surface, = activity of the ion is solution far enough
away to not be influenced by the surface, = Boltzmann factor, (Volts) surface potential relative to bulk solution and may be positive or negative, F = Faraday constant = 96,500 C/mol, R = gas constant = 8.31 V⋅C/(mol⋅
}X{ zs
}X{ z
)TR/(Foe ⋅⋅ψ−oψ
oK) = 8.31 J/(mol⋅oK), and T = oK. Note for reference that the value of the exponential in the Boltzmann factor, , is equal to 1.00 for the following common conditions of 20
)TR/(Foe ⋅⋅ψ−oC and a common surface potential of 25 mV (from Stumm and Morgan), -note
As the surface becomes protonated, ionizes, or adsorbs charged ions, it obtains some calculated surface charge, , as will be seen below. For a diffuse layer of ions next to a solid the theoretical relationship between surface charge and surface potential is given from electrostatic theory by:
oTσ
)TR2/Zsinh(I1174.0T o
2/1o
⋅⋅ψ⋅⋅=σ at 25 oC with I (M) ionic strength and Z = 1.00. The value of oψ (V)can be either plus or minus. The sinh(x) function looks like the following:
2 1 0 1 25
0
53.627
3.627−
sinh x( )
22− x At small values of “±x”, ≈ 2.3⋅I(M)
oTσ
1/2⋅ oψ (V). For example, for an iron oxide surface with no other adsorbing ions the surface charge is calculated from the surface speciation:
oTσ = [≡FeOH2
+] - [≡FeO-] Net surface charge. The mass balance for all surface sites, Tot≡FeOH, would be:
In Visual Minteq the reference component for surface sites is always the neutral species, [≡FeOH], generally written as [≡SOH]. The equilibrium constant for the formation of [≡FeOH2
+] and [≡FeO-] are as follows in Visual Minteq terminology in terms of components, : 1RT/F ]e[ and }H{ and ]FeOH[ o +ψ−+≡ ≡FeOH + H+ ≡FeOH2
+
1o
29.7
1RT/F29.7s
29.72
]1Psi}[H{]FeOH[10
]e}[H{]FeOH[10}H{]FeOH[10]FeOH[ o
++
+ψ−+++
⋅≡⋅=
⋅≡⋅=⋅≡⋅=≡
as seen in the following screen from Visual Minteq. It should be noted that the {H+} term refers to the normal aqueous hydrogen ion activity which is the concentration times the activity coefficient, {H+} = [H+]⋅ , which the program calculates. For mathematical simplicity the Boltzmann factor term is treated as an explicit component and given a name, Psi
+γH
o1, in Visual Minteq. This is illustrated in Visual Minteq (details of how to open this screen will be listed below):
In summary, surface ionization and complexation is treated exactly like any other acid/base or complex species in solution, except that the Boltzmann correction term must be solved simultaneously with the sinh(…) term, similar to the activity coefficient in solution and the Davies equation. Even though this complicates the algebra, it adds nothing new to the formulation of a problem. ≡FeOH ≡FeO- + H+ , or 1
Expressed as the corresponding dibasic acid with two ionization constants, ≡FeOH2+,
would have pK1 = 7.29 for ≡FeOH2+ ≡FeOH + H+ and pK2 = 8.93 for the reaction
≡FeOH ≡FeO- + H+, in an exactly analogous manner to a dibasic acid, H2A. Illustration of how to run Visual Minteq 2.30: Adsorption 1 HFO.VDA Acid/base reactions of hydrous ferric oxide (HFA).
a. Open Visual Minteq b. Ionic strength = 0.01 M c. Select Adsorption and Surface complexation reactions
e. Fill in the following, starting with “1” Specify number… box and then select the drop-down button and select “2-pK DLM” option from the rather long list of options. Fill in the numbers 1 g/l, 100 m2/g, and 1 site/nm2.
f. Click the X! button with the Site Conc. space empty and the program calculates the Site conc. (low affinity; mmol of sites/l) = 0.166 mM low affinity sites. This means that with this solid concentration the surface is the same as having 0.166x10-3 M in solution for titration, etc. We normally just use the low affinity sites, the high affinity sites are given used to model some systems and are designated, ≡FehOH, etc. It is important to note that for the X! button to work, the Site Conc. field must be blank!
g. Click on the “Go!” button and the “Read Reactions from…” button:
h. The following will appear and double click on the file, feo-dlm.mdb:
This tutorial will use this data base, feo-dlm.mdb, to illustrate the methods, but the reader is encouraged to open and examine the various other data bases and examine the Minteq tutorials for explanations of the various details and assumptions.
i. Click on Edit sorption … button and scroll down and click on ≡FeOCd+ species and the following will appear:
This illustrates what the values of the various constants are and it is not normally necessary to do anything if you want to use the recommended constants, but you can use this template to change log K values and to enter new constants for species not listed. We will use these values in all calculations, so just click on Quit. This also illustrates the way the component Psio1 is defined; the equilibrium constant for Cd2+ adsorption is: ≡FeOH + Cd2+ ≡FeOCd+ + H+
11o
121eq
1o
22o
2
1o
eq
}H{]1Psi[}Cd{]FeOH[K]FeOCd[
or]1Psi}[Cd{]FeOH[
}H{]FeOCd[]1Psi}[Cd{]FeOH[]1Psi}[H{]FeOCd[K
−++++++
+
++
+
++
⋅≡⋅=≡
⋅≡
⋅≡=
⋅≡
⋅≡=
with Keq = 10-2.9 this is the expression represented in the above screen image for the species ≡FeOCd+. Again, unless it is necessary to change these values or to add equilibrium values not listed in the data base, it is not necessary to do anything; when you choose a component from the list on the Main Menu or you add solid phase, etc., all equilibrium related to that component and solids will be automatically included. Click on Quit and then Save and Back to Main Menu and the adsorption data base will be automatically included in all calculations. Back at the Main Menu, click on “View/add edit” and the following will appear:
j. In the Main Menu, click on Run Minteq and then See calculated… and the following will appear, click on See calculated adsorption parameters:
This Output screen says that 1 g/l of hydrous ferric oxide with 100 m2/g surface area and 1 site/nm2 on the surface when added to water with 0.01 M ionic strength will have a pH = 8.015, a value of Psio1 = 0.0048 and surface charge of 0.00111 C/m2.
Adsorption 2 HFO low ionic strength.VDA Alternatively, if you had selected the Calculate Ionic Strength option on the Main Menu, the pH = 7.117, etc…
c. Set up a pH titration and output and Click on Save and Back to Main Menu:
Notice that the species ≡FeOH(1) is represented as ≡SOH(1) in Visual Minteq.
d. Set ionic strength to 0.001 M and click on Run Minteq, selected sweep results on the output menu, print to Excel and after a little plot formatting:
Notice that the point of zero surface charge, where
curve for weak a[≡FeOH], the neuSince the correspmiddle species, [surface potential the solution H+ c[≡FeOH]) to be a
Log C vs. pH for 10 g FeOx/L at 100 m^2/g and 1 site/nm^2
1.E-061.E-051.E-041.E-031.E-02
0Site
con
cent
ratio
n, M
>FeO
Mason Tomson
at 0.001 M Ionic strength
[≡FeOH2
+] = [≡FeO-] is at about 8.1 pH, as expected from other sources such zeta potential, and from (pK1+pK2)/2 = (7.29+8.93)/2 = 8.11. The unusual thing to note about this curve compared to the corresponding log C vs. pH
cids with the same pK-values is the exceptionally wide range over which tral species, is the predominant species –from about 4 pH to 12 pH. onding dibasic acid would have had pK1 = 7.29 and pK2 = 8.93, the ≡FeOH], would be predominant from pH = 7.29 to pH = 8.93. The is the difference: at low pH values the positively charged surface repels ausing the apparent ionization constant (i.e., where [≡FeOH2
+] = bout pKapparent,1 ≈ 4.0 versus 7.29. At higher pH’s the opposite happens,
1 2 3 4 5 6 7 8 9 10 11 12 13 14pH
H2+ (1) >SOH(1) >FeO- (1)
Visual Minteq Page 19 Adsorption 11/2/2004 19
the negatively charged surface holds the H+ on the surface making the pKapparent,2 ≈ 12, where [≡FeOH] = [≡FeO-]. Adsorption 4 HFO vs pH at 100 mM IS.VDS Repeating the above calculations, but at 0.1 M ionic strength produces the following data. Notice that the range that [≡FeOH] is
the predominant species is now only from about 6 to10 pH vs. from 4 to 12 pH when I = 0.001 M. The ionic strength has altered the solid phase predominant species by two orders of magnitude at the high and lower pH values. This can be shown to have an enormous impact on anion and cation adsorptions.
Log C vs. pH for 10 g FeOx/L at 100 m^2/g and 1 site/nm^2 at 0.10 M ionic strenght.
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14pH
Site
con
cent
ratio
n, M
>FeOH2+ (1) >SOH(1) >FeO- (1)
Adsorption 5 Cd on HFO vs pH at 0.01 M IS.VDA Adsorption of Cadmium to hydrous ferric oxide versus pH. Produce the following screen and select save and return to main menu:
Set the Cd2+ to 1e-7 M, ionic strength to 0.01 M, and do a pH sweep from 0 to 14 pH and plot the amount of TotCd(aqueous) vs. pH:
TotCd(aqueous), M
1.E-12
1.E-10
1.E-08
1.E-06
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Notice that the cadmium is adsorbed but only above about 7 to 8 pH. At higher pH values, the hydrolysis of Cd2+ to form hydroxide complexes increases the solubility.
Adsorption 6 Cd on HFO with CdCO3.VDA Impact of carbonate on adsorption. Set TotCd = 1E-5 M, TotCO3 = 0.01 M and I = 0.01 M and permit Otivite (CdCO3,ppt): Series 2 is without Otivite possible precipitation.
TotCd, aq, M; Series 2 is without CdCO3 ppt.
1.E-091.E-081.E-071.E-061.E-051.E-04
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cd+2 Series2
Adsorption 7 As(III) As(V) on HFO vs pH.VDA Impact of adsorption on arsenic in solution: Adsorption or arsenic (III) and (V) to 1 g/l FeOx, I = 0.01, TotAs = 1e-6, in each case.
Notice that nearly all of the arsenic remains in solution as the toxic form, As(III) till about 8.5 pH, but at this pH, the surface is negatively charged and therefore the As(V) is repelled from adsorption. This is apparently what happens as the system goes anaerobic at pE = -3, a common value.
1.E-201.E-181.E-161.E-141.E-121.E-101.E-081.E-06
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
AsO4-3 H3AsO3
Adsorption 9 As(III) As(V) Fe(II) Fe(III) on HFO precipitation.VDA Same as above but with AsO4-3 and H3AsO3 = 1e-6, Fe(II) and Fe(III) = 1E-4 M, pE = -3, also permitting ferrihydrate (Fe(OH)3) and FeAsO4, precipitation.
b. Select the redox menu on the main menu and select both arsenic and iron redox.
c. Permit both ferrihydrate and FeAsO4 solid formation as possible solids:
d. Select the pH sweep from 0 to 14 pH and the output of Total Dissolved for each component in Excel will look something like this after some rearrangements. First for specific pH = 9.0 and then for pH sweep.
Apparently, the minimum Total As conc. occurs at about 8.5 pH where from the above plot output screen TotAs(III) ≈ TotAs(V) ≈ 3E-9 M ≈ 0.2 ppb TotAs dissolved in solution. The present EPA standard is 50 ppb and that will be reduced to 10 ppb in 2006. Alternatively, if the pH is 8.0 instead of 9.0, the TotAs conc. is about the same, but the predominant form is As(III) which is by far the more toxic of the two forms. The following is the Component outputs at 8 and then 9 pH. At 8 pH:
