Advances in Arsenic Research - American Chemical Society

Chapter 2

Downloaded by FUDAN UNIV on February 13, 2017 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch002

Contrasting Sorption Behavior of Arsenic (III) and Arsenic(V) in Suspensions of Iron and Aluminum Oxyhydroxides 1

Janet G. Hering and Suvasis Dixit

1,2

1

2

Environmental Science and Engineering, California Institute of Technology, Pasadena, C A 91125-7800 Current address: Lawrence Livermore National Laboratory, Livermore, C A 94550

United Kingdom It has been widely observed that the sorption of arsenic (As) as As(III) and As(V) on iron (Fe) oxyhydroxides is comparable as is the sorption of As(V) on Fe and aluminum (Al) oxyhydroxides. However, quite different sorption behavior has been observed for As(III) on Fe and Al oxyhydroxides. Here, surface complexation modeling is used to show that recent reports of As(III) sorption onto Al oxyhydroxides are consistent with previous observations of negligible sorption under conditions relevant to water treatment. This modeling exercise also demonstrates that the level of protonation of surface species is not well constrained by commonly-reported pH edges and that the possibility of distinguishing between mononuclear and binuclear surface complexes depends on the level of surface coverage. These issues complicate the integration of macroscopic sorption studies with spectroscopic studies and molecular modeling.

8

© 2005 American Chemical Society

O'Day et al.; Advances in Arsenic Research ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

9


Introduction The sorption behavior of arsenic (As) is crucial to both its mobility in natural waters and the efficiency of its removal by many water treatment technologies. Iron and aluminum-based sorbents are ubiquitous in natural systems (e.g., iron oxides such as goethite and hematite, aluminum oxides such as gibbsite, and aluminosilicate clay minerals) and are used in water treatment technologies as packed bed media or formed in situ during coagulation with hydrolyzing metal salts such as ferric chloride and aluminum sulfate (alum). The sorption of arsenic onto iron and aluminum oxides and oxyhydroxides has therefore been extensively studied (for reviews see refs. [1,2]). It is a common observation in the water treatment literature [3,4] that alum is ineffective for the removal of As(III) from potable water but can efficiently remove As(V). Thus oxidation of As(III) to As(V), which can be accomplished by conventional disinfectants such as chlorine, is recommended if alum is to be used to treat source waters containing As(III) [J]. Similarly, activated alumina is not recommended as a packed bed filter media for As(III) removal [6], In contrast, removal of both As(III) and As(V) can be accomplished using ferric chloride as a coagulant [7,8] or granular ferric hydroxide as a packed bed media [9]. Under conditions relevant to water treatment, better removal efficiency is achieved with ferric chloride for As(V) than for As(III) [4,8,10]. Though it should be noted that, under conditions nearer to surface saturation, comparable sorption of As(III) and As(V) is observed at circumneutral pH and preferential sorption of As(III) at higher pH values [11-14]. Recent studies, however, have demonstrated sorption of As(III) onto aluminum oxyhydroxides [15-17]. The results of these studies appear to conflict with those obtained under conditions relevant to water treatment but the seeming inconsistency among these results may merely reflect differences in experimental conditions. Here, our objectives are: (i) to examine consistency of these recent studies with prior work, (ii) to identify differences between iron and aluminum oxyhydroxides as sorbents for arsenic in +III and +V oxidation states, (iii) to examine constraints on surface speciation provided by macroscopic sorption studies and surface complexation modeling (SCM), and (iv) to discuss the integration of macroscopic sorption studies and S C M with spectroscopic studies and molecular modeling (which is presented in detail by Kubicki, this volume).

Background Although a comprehensive review of the studies of As sorption/coprecipitation onto Fe and A l oxides is beyond the scope of this paper, it is worth highlighting selected evidence of the similarities and differences in the sorption behavior of As(III) and As(V) with Fe and A l oxides.


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Comparability of As(III) and As(V) sorption onto Fe oxides Numerous studies have compared the sorption of As(III) and As(V) onto Fe(III) oxides such as hydrous ferric oxide (HFO), goethite, and hematite [//14,18,19]. It has been widely observed that the relative extent of As(III) and As(V) sorption is a function of pH and of the relative and absolute concentrations of the sorbent and sorbate. Since As(III) sorption is independent of pH over a broad range and As(V) sorption decreases with increasing pH, a crossover pH can be observed such that As(V) is sorbed to a greater extent than As(III) below that p H value and to a lesser extent above it (Figure 1). It should be noted, however, that, at total As and Fe concentrations where complete sorption of As(V) is observed over some p H range, the sorption of As(III) is not complete at any pH value, which indicates a greater affinity of the surface Fe sites for As(V) than for As(III). This effect becomes less significant at increasing total As concentrations. 100

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ρΗ Figure 1. %As sorbed vs. pHfor As (111) (closed symbols) and As(V) (open symbols) on HFO with [As] =10 μΜ (circles) and50 μΜ(squares), 1 = 0.01, 30 mg/L HFO. Data from ref [11J. T

In water treatment studies, As removal has been examined in coagulation experiments where As removal occurs concurrently with the formation of hydrous ferric oxides upon addition of a coagulant, such as ferric chloride, to test solutions. Conditions of these experiments are generally chosen to be relevant to water treatment (i.e., at low equivalent sorbent concentrations). Under these conditions, As removal (by sorptton/eo-precipitation) is generally observed to be more efficient for As(V) than for As(III), particularly at lower coagulant doses [3,8]. Similar results were observed for adsorption of As(III)


11 and As(V) onto pre-formed H F O at comparable sorbate concentrations [19],

and sorbent

Comparability of As(V) sorption onto Fe and Al oxides


There are apparently significant inconsistencies in the literature with regard to the affinity of As(III) for Al-based sorbents and coagulants. Several studies have reported negligible or poor removal of As(III) by alum [3,5,7]. A companion adsorption study [20] conducted under similar conditions of total sorbate and sorbent concentrations illustrated the striking difference between sorption of As(III) and As(V) (Figure 2).

Figure 2. Sorption of As(III) and As(V) on amorphous Al hydroxide at varying [AlJr with [As] = 1.33 μΜ, pH = 6,1 = 0.01. NM = not measured. Data from réf. [20]. T

In adsorption studies with higher concentrations (i.e., 1-4 g/L) of activated alumina [21] or amorphous A l hydroxide [15,16,22], adsorption of As(III) has been reported with maximum adsorption at pH 8. The extent of As(III) adsorption at this pH is less than that of As(V) and the pH range over which As(III) is adsorbed onto amorphous A l hydroxide is significantly narrower than that for As(V) [15,16,22]. This behavior will be examined in greater detail below. Weak adsorption of As(III) on gibbsite, exhibiting some dependence on ionic strength but nearly none on pH, has been reported [23].

Surface species invoked in modeling macroscopic sorption experiments In many macroscopic sorption studies, As adsorption and its effects (e.g., on electrophoretic mobility) have been interpreted in terms of chemisorption, the formation of inner-sphere complexes between As species and surface metal



12 centers. Such surface complexation models (SCMs) may use different formalisms (e.g., constant capacitance, diffuse double layer, triple layer, etc.) to describe electrostatic interactions at the oxide surface yet have common representations of adsorbed As species. If surface species are defined merely to describe the results of macroscopic adsorption experiments, it is often sufficient to consider mononuclear complexes (i.e., As species bound to a single surface metal center). The level of protonation of adsorbed As species is constrained by the pH dependence of adsorption or (less often) by the observed stoichiometry of proton exchange concurrent with As adsorption [24]. The stoichiometry of these surface species may correspond to monodentate or bidentate complexes (i.e., binding the surface metal center through one or two coordinating ligand moieties) but this distinction is not robustly supported on the basis of only macroscopic adsorption behavior. On the basis of spectroscopic studies, binuclear complexes (i.e., As species bound to two adjacent surface metal centers) have also been invoked to describe macroscopic adsorption behavior. For adsorption of As(V) and As(III) on amorphous A l hydroxide, comparable descriptions of macroscopic sorption data were obtained with only mononuclear complexes or by including binuclear complexes [16]. Since a unique set of surface species cannot be defined based on macroscopic adsorption data, surface species are chosen to obtain a minimum number of species [25] or to apply the same set of species for different conditions or solids [//]. For As(III), outer sphere as well as inner sphere complexes have been used in modeling. In an outer sphere complex, the As(III) anion is farther from the oxide surface due to the presence of intervening water molecules and experiences less electrostatic interaction with the surface. The triple layer model is generally used to accommodate this type of surface species. Outer sphere complexes are often invoked when a strong effect of ionic strength on adsorption is observed. However, the model fit to data using outer sphere complexes may be very poorly constrained, particularly if the constants for outer sphere complexes of major cations and anions (e.g., N a and CI") are used as adjustable fitting parameters. The lack of uniqueness of surface species used to describe the adsorption of As(V) and As(III) on Fe and A l oxyhydroxides is evident in Tables I and II. +

Spectroscopic evidence for surface speciation Spectroscopic techniques, including Raman, Fourier Transform Infrared (FTIR), and Extended X-ray Absorption (EXAFS) spectroscopy, have been used


Table I. Sets of As(V) Surface Species for Metal Oxyhydroxides pH range

surface species

4-10

=MH As0 =MHAs0 ~ 2

4

solid

reference [25]

HFO

4

3

=MOHAs0 " 4

4-10 4-10 4-10 3-10 2-10 2-12 3-11 4-10 5-8

H F O , goethite

[//]

HFO

[261

HFO

[15] [27] [28]

4-11 3-11

am-Al oxide am-Al oxide

[16] [22]

"

2-11

HFO

[22]

^MAs0 " sM As0

4-11

am-Al oxide

[16]

=MH As0 2

4

HMHAS0 " 4

2

sMAs0 ~


4

E=MHAS0 ~ 4

2

^MAS0 ~ 4

^MAS0

2 4

2

4

2

goethite goethite am-Al oxide gibbsite gibbsite am-Al oxide

[15] [28] [27] [27]

4

Table II. Sets of As(III) Surface Species for Metal Oxyhydroxides pH range

surface species =MH As0

3

4-10

HMH AS0

3

4-10

2

solid

reference

HFO

[25] [11] [15] [22]

3-11

HFO, goethite HFO HFO am-Al oxide

^MHAsO-f =MAs0

4-11

am-Al oxide

[16]

sM HAs0 sM As0 ~

3-11

2

=MHAs0 ~

3-11

3

2-11

[22J.

2 _

3

2

2

3

goethite

[12]

3

2

=MAI0 " =M HAs0 =M As0 ~ 3

2

2

4-11

am-Al oxide

[12]

2-10

am-Al oxide*

[15]

3

3

H(MOH

+ 2

) -HAS0 2

+

=(MOH ) -As0 2

2

2 3

~

3_ 3

* outer-sphere complex



14 to interrogate the structure of surface complexes. Direct comparison with macroscopic sorption studies is complicated by differences in experimental conditions; spectroscopic studies generally require higher concentrations of both sorbate and sorbent and also higher surface coverage than are typical for macroscopic adsorption studies. Some spectroscopic studies have also been conducted with dried solids or crystalline salts rather than with suspensions. Nonetheless, the results of spectroscopic studies have been used to constrain interpretations of adsorption studies [16]. Vibrational spectra of adsorbed As(V) support the assignment of innersphere complexes on both Fe and A l oxyhydroxides. Both Raman and IR spectra show distinct differences in frequencies associated with the arsenate anion for As(V) in solution, adsorbed to Fe or A l oxyhydroxides, and in crystalline As(V) salts [15,29-32]. Absorbances associated with the surface hydroxyl groups of goethite were also found to be influenced by adsorption of As(III) and As(V) [30,31]. Features attributable to inner sphere complexes of As(III) have been reported for Fe oxyhydroxides [15,30-32] but were not apparent in the case of A l oxyhydroxides [15]. Numerous studies have used E X A F S to probe the coordination environment of As(V) adsorbed onto various Fe oxyhydroxides, such as ferrihydrite, goethite, akaganeite, and lepidocrocite [33-36]. In E X A F S studies, surface structures are distinguished primarily on the basis of the observed distance between As and Fe atoms. While there is some disagreement regarding the occurrence of various surface structures, it is generally agreed that As(V) adsorbs in large part as a binuclear, bidentate complex in which As(V) is bound to apical oxygens of two adjacent, edge-sharing Fe octahedra [33-36]. However, other structures, specifically a mononuclear, monodentate complex [34-36] and mononuclear, bidentate complex [36], have also been proposed. The contributions of the various complexes vary with surface coverage [34,36]. A binuclear, bidentate complex has also been proposed for As(III) adsorbed onto goethite [12]. Protonation of the surface complex with decreasing pH was proposed but this would not change the As-Fe distance nor were spectra obtained below pH 6.4. A mononuclear, monodentate complex was considered but not supported by the spectroscopic evidence. E X A F S spectra for As(V) and As(III) adsorbed onto y-Al 0 (s) have also been interpreted as consistent with binuclear, bidentate complexes [17]. However, the X-ray Absorption Near Edge (XANES) spectrum for adsorbed As(III) indicated a contribution of an outer sphere as well as the inner sphere complex. 2

3

Using surface complexation modeling to evaluate surface speciation and consistency of experimental observations Despite the issue of non-uniqueness discussed above, it may be instructive to use surface complexation modeling as a tool to examine the macroscopic


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implications of surface speciation and to integrate structural information obtained from other (e.g., spectroscopic) methods. For example, E X A F S data provide a compelling rationale for the inclusion of binuclear, bidentate complexes in modeling macroscopic adsorption data [16] yet this information must be integrated with constraints on surface speciation derived from macroscopic data. SCMs can also be used, with appropriate caution, to compare macroscopic data obtained under varying experimental conditions. Here, we use surface complexation modeling to compare and contrast the sorption behavior of As(III) and As(V) on A l oxyhydroxide as reported by Goldberg and Johnston [15].

Extraction of constants for surface complexation modeling Equilibrium constants for inner-sphere As(III) and As(V) surface complexes on amorphous A l oxide were obtained for data reported in ref. [15] using FITEQL [37] to optimize values for the constants. Electrostatic corrections for surface species were made using the diffuse layer model (DLM). The values for total concentrations of surface sites were not optimized but rather were fixed based on the solids concentration, the reported B E T surface area (209 m /g), and a value for surface site density of 2.31 sites/nm as recommended by Davis and Kent [38]. The sorption data used in the fitting were obtained for [As] = 1 m M , 0.5 or 4.0 g/L solid, and I = 0.01 [15]. The values of the constants as optimized by FITEQL are not adjusted for ionic strength and are valid for I = 0.01. The values for the sorption constants reported in Table III have been adjusted to zero ionic strength using the Davies equation to obtain activity coefficients for dissolved species. Predictions based on these constants were made using MINEQL+, v. 4.5 [39]. Activity coefficients of all dissolved species (including those participating in sorption reactions) were adjusted for ionic strength in MINEQL+ using the Davies equation. Table III lists the reactions included in the fitting and modeling exercises with the corresponding mass law expressions and intrinsic equilibrium constants. The constants for protonation and deprotonation of the surface hydroxyl groups (Table III, Rxns. la,b) are the same as those used in ref. [15], in which the constant capacitance model (CCM) was used to make electrostatic corrections. Although we applied the diffuse layer rather than the constant capacitance model, we felt that modification of the values of these constants was not necessary. As discussed by Westall and Hohl [40], the values of these 2

2

T


16 constants are similar whether the electrostatic term is obtained with the C C M or D L M . For binuclear complexes, the mass law expressions in Table III include a squared term for the concentration of the free surface hydroxyl groups. Although there is some disagreement in the literature as to the appropriate mass law expression for binuclear complexes, Benjamin [41] has argued that the squared term should be included and this convention has been followed by several other researchers [12,15,16].


Modeling sorption of As(IH) on Al oxyhydroxide Surface complexation modeling was used, in the case of As(III) sorption onto A l oxyhydroxide, to examine two issues: the level of protonation of surface complexes and the formation of binuclear vs. mononuclear surface complexes.

Protonation of surface complexes Model fits to the sorption data for As(III) on A l oxyhydroxide using surface species with varying levels of protonation were compared for binuclear surface complexes. As previously discussed by Manning [16], binuclear surface complexes were selected based on spectroscopic information obtained for As(III) surface complexes on Fe(III) oxyhydroxides. As can be seen in Figure 3, the fit obtained using only the charged surface species is only marginally improved i f the uncharged surface species in included. (See Table III, Rxns. 3a,b, for stoichiometry of the surface species and corresponding equilibrium constants for surface complex formation.) When both charged and uncharged species are included in the fitting exercise,

Advances in Arsenic Research - American Chemical Society

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