Chapter 21
Constant-Capacitance Surface Complexation Model Adsorption in Silica—Iron Binary Oxide Suspensions 1
Downloaded by COLUMBIA UNIV on October 1, 2014 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch021
Paul R. Anderson and Mark M. Benjamin Department of Civil Engineering, University of Washington, Seattle, WA 98195
A conceptual and mechanistic model of particle interactions in silica-iron binary oxide suspensions is described. The model is consistent with a process involving partial SiO2 dissolution and sorption of silicate onto Fe(OH) . The constant capacitance model is used to test the mechanistic model and estimate the effect of particle interactions on adsorbate distribution. The model results, in agreement with experimental results, indicate that the presence of soluble silica interferes with the adsorption of anionic adsorbates but has little effect on cationic adsorbates. 3
A number of researchers have attempted to model trace element distribution among multicomponent solids. For example, Balistrieri et al. (1) modeled trace metal scavenging by heterogeneous, deep ocean particulate matter; Oakley et al. (2) and Davies-Colley et al. (3) modeled trace metal partitioning in marine sediments and estuarine sediments, respectively; and Goldberg and Sposito (4) modeled phosphorus adsorption onto soils with the constant capacitance model. Common to all these modeling efforts was the simplifying assumption that the multi-component system could be represented either by some average collective property for the group or as a collection of discrete pure solid phases, a concept which Honeyman (5) called "adsorptive additivity". Honeyman tested this concept and demonstrated in several experiments that particle interactions in binary suspensions of oxides can lead to significant deviations from the adsorptive additivity concept. This paper presents an alternative approach which can account for some of these deviations. The model used to evaluate surface chemistry in these systems is the constant capacitance surface complexation model. This model has been used to describe the adsorption of cations (6,7) and anions (4,8) onto oxides similar to those used in our experiments. A significant difference between those studies and the present study is that we have adapted the model to simulate some of the interactions that might occur between particles in a binary suspension. In a previous paper (Anderson and Benjamin; accepted for publication in Environmental Science and Technology), surface and bulk characteristics of amorphous oxides of silica, aluminum, and iron, both singly and in binary mixtures were described. The solids were characterized with an array of complementary analytical and experimental techniques, including scanning electron microscopy, particle size distribution, x-ray photoelectron spectroscopy (XPS), Current address: Pritzker Department of Environmental Engineering, Illinois Institute of Technology, Chicago, IL 60616 0097-6156/90/0416-0272S06.00/0 © 1990 American Chemical Society
In Chemical Modeling of Aqueous Systems II; Melchior, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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Downloaded by COLUMBIA UNIV on October 1, 2014 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch021
and measurement of the pH of the zero point of charge (PZC) and of the specific surface area (N2-BET). Also, batch adsorption experiments were used to characterize the adsorption behavior of Ag, Cd, PO4, Se03, and Zn. The experiments revealed a number of physical and chemical differences in the particles' properties in the binary versus the single oxide suspensions. These changes resulted from interactions such as aggregation, disaggregation, dissolution, and precipitation among the suspended particles. Details on the adsorbent preparation and experimental and analytical techniques are presented elsewhere (9). This paper briefly reviews the experimental results for the Fe(OH)3 and S1O2 suspensions and describes a conceptual and mechanistic model for particle interactions which is qualitatively consistent with the experimental observations. Similar results were obtained for binary Al(OH)3 and S1O2 suspensions (9). The constant capacitance surface complexation model is then used to test the mechanistic model and estimate the quantitative influence of the particle-particle interactions on adsorbate distribution. CHARACTERISTICS OF T H E BINARY SUSPENSIONS The binary systems were synthesized using two different processes, and the bulk and surface characteristics of suspensions synthesized by these two processes were indistinguishable. One, the mixed suspension, was prepared by mixing an Fe oxide suspension with a suspension of S1O2. In the other method, Fe(OH)3 was precipitated from a solution in which amorphous S1O2 particles were suspended. The particle size distributions (PSD) for the binary suspensions were much more like that of the pure S1O2 suspension than that of the Fe oxide suspension (Figure 1). The smaller and intermediate size particles from the Fe(OH)3 suspensions were not present as discrete particles in the mixed systems, while the size distribution of the larger (apparently S1O2) particles remained about the same. Other characteristics of the S1O2 binary oxides and the component solids are summarized in Table I. Most of these properties suggest that the surfaces of particles in the binary S1O2 suspensions were dominated by Fe oxides. The most likely process by which this might have occurred is heterocoagulation between the negatively charged S1O2 and positively charged Fe oxide particles. Coverage of the S1O2 particles by the smaller Fe colloidal particles is consistent with the surface charge of the resulting particles being dominated by Fe(OH)3, and the specific surface areas being nearly those expected for a collection of discrete solids. However, the S1O2 was not completely masked because the XPS analyses indicate the presence of S1O2 at the particle surface. The adsorbent properties of the solids in the binary suspensions were also characterized. Adsorption of Cd (Figure 2), Ag, and Zn, in systems containing Fe(OH)3 was nearly the same whether S1O2 was present or not. In contrast, anion removal was inhibited in the binary systems relative to the pure Fe(OH)3 system. For both Se03 and PO4, the pH region of the adsorption edge shifted in the acid direction about 1 pH unit when S1O2 was present compared to the corresponding Si02-free systems. The data for P O 4 are shown in Figure 3. A M O D E L OF INTERACTIONS IN T H E BINARY S I 0 SYSTEMS 2
One mechanism that is consistent with the observed properties of the particles in these suspensions involves the dissolution of amorphous S1O2 and adsorption of soluble silicate on the Fe(OH)3 surface. This process could occur in parallel with the heterocoagulation mentioned earlier. Soluble silicate species might then compete with Se03 or P O 4 for surface sites as suggested by Goldberg (8) for the P04/silicate/goethite system. Sorption of silicate species onto Fe(OH)3 need not affect cationic adsorbates. Benjamin and Bloom (10) demonstrated that adsorption of cations is often minimally affected by anion adsorption even under conditions where anion-anion competition is severe (11). The computer programs FITEQL (12) and MICROQL (13) were used to model chemical speciation in this study. FITEQL uses a non-linear, least squares optimization technique to calculate equilibrium constants from chemical data. The program was used here to select
In Chemical Modeling of Aqueous Systems II; Melchior, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by COLUMBIA UNIV on October 1, 2014 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch021
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CHEMICAL MODELING OF AQUEOUS SYSTEMS II
Diameter (um)
Diameter (um)
Figure 1. Histograms of the particle size distribution in Fe(OH)^, S1O2, and the binary suspensions.
Cd (1.0x10" M) Removal
+
6
Fractional Removal
Of
0
+ 0 + 0
+
Fe(OH)
0
Fe - Si Mix
3
PH
Figure 2. Fractional removal of Cd onto Fe(OH)^ and an Fe-Si binary suspension. Solids concentrations are given in Table I.
In Chemical Modeling of Aqueous Systems II; Melchior, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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Surface Complexation Model
Table I. Bulk and Surface Characteristics of Reference Oxides and Binary Solid
Suspension
Property
Fe(OH)
3
Si0
0
Fe after Si
Downloaded by COLUMBIA UNIV on October 1, 2014 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch021
Total Cone. A d d e d ^ (mol/L) 0.001
0.001
Fe Si
0.008
0.008
105
144
Specific Surface Area (m /g) 2
Measured
186-201
120
Expecte 7.2(SA.)
PZCP)
