The influence of pH and phosphorus on the adsorption of chromate on smectites having synthetic hydroxy-Al interlayers.

Abstract: Chromium (Cr) is a transition metal commonly found in the environment because of human activity. In southeast Missouri, numerous Cr contaminated sites exist because of Aluminum (Al) metal fabrication and Cr plating of metals. Several of these areas are in populated regions and are registered as EPA superfund sites. The chromate species is of particular concern because of its appreciable migratory potential and the resultant threat to groundwater supplies. Recent research efforts have focused on describing chromate adsorption on a variety of absorbents, including soils, clay minerals and metal oxides. This investigation reports on efforts to describe the extent of chromate adsorption on smectite and synthetic hydroxy-Al interlayers associated with smectite (HIS) and to consider the influences of pH and phosphorus (P). The role of P is to act as a competing anion with Cr for surface adsorption sites and to assess the potential for Cr adsorption to be inhibited by P. Chromate adsorption is greater in a cidic smectite suspensions, thus adsorption is pH dependent. Phosphorus only marginally displaced chromate from smectite and its synthetic hydroxy-Al interlayer analog, as revealed by marginal to slightly greater Cr concentrations in the suspension's aqueous phase in the presence of P. Based on previous research by the authors, chromate adsorption on similar substrates is outer-sphere; therefore the small differences in Cr adsorption in the presence of P is likely attributable to a surplus of surface bonding sites and incremental differences in the activity coefficients for these sites.

Key Words: Chromium, Adsorption, Phosphorus, Smectite

Introduction

In the soil environment chromium is a transition metal capable of existing in two oxidation states: (i) the chromic species (Cr3+), and (ii) the chromate species (Cr(VI)). The chromate species is considered to be the more mobile and toxic Cr species (Bartlett and Kimble, 1976). Appreciable research involving soil Cr has been conducted on soil Cr contamination because of its relatively common occurrence and its human health consequences (Ainsworth et al., 1989; Bartlett and Kimble, 1976; James and Bartlett, 1983; Zachara et al., 1988 and 1989). Aide and Tibbs (1987) reported on Cr contamination at Malden, Missouri, where sandy soils contained up to 5000 ppm Cr and was deemed a threat to the local community. Subsequently, Aide (1994), using XANES spectroscopy, showed that the Cr was almost completely composed as [Cr(OH).sub.3] and its future migratory potential is low.

Employing kaolinite as a substrate, Zachara and others (1988) observed that [CrO.sub.4] = protonation was an important reaction affecting chromate adsorption, especially at lower pH intervals where Cr adsorption was observed to be greater. Using Al-substituted Feoxides, Ainsworth and others (1989) proposed that chromate adsorption was influenced by background electrolyte surface reactions and surface ionization reactions. Subsequently, Zachara and others (1989) observed that chromate adsorption increased in acidic media, especially if kaolinite or crystalline Fe-oxides were present. Using a kinetic approach, Makami and others (1983) proposed two chromium complexation products at an Al-oxide interface: = [AlOH.sub.2]-[HCrO.sub.4] and = [AlOH.sub.2]-[CrO.sub.4], where =AlOH is the surface site. Subsequently, Honeyman (1984), Zachara and others (1988, 1989) and Ainsworth and others (1989) each proposed = [AlOH.sub.2]-[HCrO.sub.4] and = [AlOH.sub.2]-[CrO.sub.4] as the surface complexation products for various Al oxide surfaces.

Interest is growing in quantitatively assessing competitive anion adsorption on phyllosilicate minerals and oxyhydroxides (Goldberg and Traina, 1987; Davis and Leckie, 1980; Zachara and others, 1988 and 1989).

Using kaolinite and goethite-hematite containing soils, Zachara and others (1988, 1989) confirmed that chromate adsorption was not affected because of competition with [SO.sub.4]= adsorption, suggesting the chromate and [SO.sub.4]= are competing for different surface sites. In their studies, chromate adsorption was dramatically reduced by an increase in the suspension's ionic strength, suggesting that the chromate - substrate bonding constituted an outer-sphere complex. Aide and Cummings (1997) showed that chromate adsorption on boehmite was outer-sphere and pH dependent, with Cr adsorption increasing in the order: pH 4.0 > pH 4.5> pH 5.0 > pH 6.0. These authors showed that low P concentrations did not inhibit Cr adsorption; however, at higher P concentrations Cr adsorption was reduced, suggesting that a specific degree of surface saturation by the competing anion was necessary to effectively inhibit Cr adsorption.

Presently, a significant need exists for laboratory data involving chromate adsorption on a variety of mineral surfaces where the external conditions of pH, ionic strength, oxidation-reduction potential, and temperature are closely monitored. Such data can form the basis to describe the migratory potential of soil chromate and to predict if chromate contamination poses a health risk. The primary purpose of this investigation was to describe the effects of pH and P on chromate adsorption on a reference smectite and a smectite having hydroxy-Al interlayers (HIS).

Materials and Methods Preparation of a Synthetic Smectite

A reference smectite was used because the smectite could be easily extracted and it was relatively free of iron oxide and organic matter coatings. Soil clays from Missouri are always mixtures of specific clay minerals and contain appreciable quantities of Fe oxide and organic matter. Thus, a reference clay was used to provide a "clean" system so that fewer assumptions would be involved in the data analysis. The smectite is a bentonite obtained from the Amory Mine operated by the American Colloid Company. The clay fraction (less than 2 [mu]m) was separated by centrifuge fractionation after Na saturation and carbonate, organic matter and Fe-oxide removal (Kunze and Dixon, 1986). Smectite stock suspensions were prepared having exactly 5% smectite dispersed in distilled water. X-ray diffraction was performed to authenticate the presence of smectite.

Preparation of hydroxy-Al interlayered smectites (HIS)

Al-clay suspensions were prepared by the slow addition of 0.01 mole NaOH * [1.sup.-1] to [Al([NO.sub.3]).sub.3] in the presence of smectite. Specifically, known volumes of 0.01 mole [Al([NO.sub.3]).sub.3] * [1.sup.-1] stock solution (pH 3.0) were slowly added to 2.5 g of smectite as a suspension. Total Al additions were equal to either 0 or 100% of the suspension's capacity for cation exchange. The pH levels were immediately adjusted to 4.0, 4.5, 5.0, 6.0 or 7.0 with additions of either 0.1 mole [HNO.sub.3] * [1.sup.-1] or 0.1 mole KOH * [1.sup.-1] at a rate of approximately 5[mu]L [s.sup.-1]. Subsequent pH readjustments were at 40 and 100 hr. A few drops of Toluene were added as a microbial suppressant.

The aging period for the Al-treated smectite suspension was approximately 2 months. All suspensions were diluted to 1% (w/w) and the ionic strength adjusted to 0.05 mole * [1.sup.-1] using 1 mole [KNO.sub.3] * [1.sup.-1]. The Al-treated suspensions were routinely monitored for mole KCl * [1.sup.-1] exchangeable Al, Al solubility, pH, and cation exchange capacity by Mg saturation (Carter, 1993).

Preparation of Chromate Adsorption Isotherms

Chromate equilibrating solutions were prepared, using reagent grade [K.sub.2][Cr.sub.2][O.sub.7], in concentrations of 0, 0.02, 0.04, 0.08, 0.12, 0.15, and 0.20 mmole chromate * [1.sup.-1] and buffered to the following pH levels: 4.0, 4.5, 5.0, 6.0, and 7.0 with either 0.01 mole HCl or KOH * [1.sup.-1]. To test the effect of phosphorus as a competing anion, a second set of Cr-bearing solutions were prepared having a concentration of 0 or 0.25 mmole P * [1.sup.-1] using [K.sub.2][HPO.sub.4]. ionic strength of all solutions was 0.05 mole * [1.sup.-1] using [KNO.sub.3].

Chromate isotherms were prepared using a batch technique. A predetermined amount of clay suspension was added to weighed screw-capped vials to provide 0.1 g smectite. The vials were centrifuged, decanted and re-weighed to determine the volume of residual solution. Chromate bearing solutions were transferred and the vials again weighed to determine the amount of equilibrating solution, approximately 0.01 1. The equilibration period was three days. Separate samples were assessed at longer time intervals and no significant chromate concentration differences were observed. Chromate analysis was by the s-diphenylcarbazide method (Bartlett and Kimble, 1976) and the Cr adsorption was calculated using the initial and equilibrium solution concentrations and the total suspension volume. Phosphorus concentrations were analyzed using the Murphy and Riley molybdenum blue method (1962).

The Cr isotherms were described by linear regression after a natural logarithm transformation of the independent variable and assessed for significant difference (Neter and Wasserman, 1974). The U.S. Environmental Protection Agency's software package MINTEQA2 (Allison, 1991) was used to simulate Cr(VI) speciation.

Results and Discussion Chromate isotherms

The adsorption isotherms (Fig. 1) show a dramatic pH dependency. For both untreated smectite and HIS, Cr adsorption is greatest at pH 4.0 and decreases steadily as the pH increases from 4.0 to 5.0 (Fig. 1 and 2). Chromate isotherms prepared at pH 6 and 7 are experimentally equivalent and show little to no Cr adsorption (Data not shown). Chromate adsorption in the presence of HIS was greater than equivalent comparisons to untreated smectite (Fig. 2). As an example, at a Cr concentration of 60 [micro]mole * [1.sup.-1] , Cr adsorption at pH 4.0 for untreated smectite has a Cr adsorption of approximately 10 mmole Cr * [1.sup.-1] , whereas HIS has a Cr adsorption of approximately 12 mmole Cr * [1.sup.-1].

MINTEQA2 Simulation of Cr(VI) Speciation.

MIINTEQA2 simulation of Cr(VI) speciation, with inclusion of the ion pair [KcrO.sub.4.sup.-] and adjusted to an ionic strength of 0.05 mole * [1.sup.-1], reveals that [HCrO.sub.4.sup.-] is the dominant species in acidic media, with [HCrO.sub.4.sup.-] and CrO4=having approximately equal concentrations at pH 6.5. A similar result may be obtained by using Baes and Mesmer's (1976) thermodynamic data, where the pH dependent concentration of [HCrO.sub.4.sup.-] in dilute solutions may be approximated as:

Total Cr = [[HCrO.sub.4.sup.-]] { 1 + [10.sup.-651]/[H]} 1.

Equation #1 predicts that the [HCrO.sub.4.sup.-] and CrO4=species are present in equivalent concentrations at pH 6.5 and [HCrO.sub.4.sup.-] becomes increasingly abundant at lower pH levels. Chromate adsorption isotherms show an enhanced adsorptivity in acidic media, suggesting that the presence of the [HCrO.sub.4.sup.-] species favors the formation of the Cr - clay complex.

The Charge Distribution of the Smectite and HIS

Protonation of Al sites may be described as:

= [AlOH.sub.2.sup+.sub.-] = AlOH + [H.sup.+] 2a.

= [AlOH.sub._] =[A10.sup.-] + [H.sup.+] 2b.

where =[AlOH.sub.2.sup.+], =AlOH and =[A10.sup.-] are surface sites. Using a log K value of -6.3 (Table 1) for equation 2a shows that the majority of the surface sites should be extensively protonated at pH levels more acidic than pH 6. Thus the Al-sites of the smectite surface should be extensively protonated in the pH interval where significant Cr adsorption differences are evident. HIS has a greater reservoir of Al sites than the untreated smectite and should provide a greater Cr adsorptivity. Thus Cr(VI) adsorption on HIS involves the pH-dependent speciation of HCrO4- in the aqueous phase and the protonation of Al-sites on smectite and HIS.

Chromate Isotherms in the Presence of Phosphorus

Chromate isotherms, prepared with phosphorus as a competing co-solute, demonstrated slightly decreased Cr absorptivity (Fig. 3 and 4). Chromate isotherms, at each pH level, show that P suppresses Cr adsorption, with slightly greater Cr suppression at the highest phosphorus concentrations. Phosphorus is commonly recognized as having a strong affinity for oxyhydroxide surfaces (Aide and Cummings, 1997). Acid dissociation constants for P (Table 1) suggest that the [H.sub.2][PO.sub.4.sup.-] anion is the dominant species in the pH interval 4.0 to 6. Thus [H.sub.2][PO.sub.4.sup.-] and [HcrO.sub.4.sup.-] appear to compete for similar surface sites; however, the extent of anion competition between untreated smectite and HIS is insignificant. The lack of differential competitive differences suggests that Cr adsorption is not dramatically affected by the blocking of adsorption sites by P. Rather, the Cr adsorption differences are attributed more to incremental changes in the activity coefficients of the charged surfac e complexes or to a surplus of available surface sites. These findings are similar to those of Zachara and others (1988 and 1989) and Aide and Cummings (1997).

Conclusions

Chromate adsorption on untreated smectite and HIS is pH dependent, with more acidic pH levels being favorable for chromate adsorption. Phosphorus did not dramatically interfere with chromate adsortion, because the systems adsorptive capacity was not exceeded. It is proposed that HCrO4- interacts with = AlOH2+ to form =AlOH2-HCrO4 as the surface complex. The strength of the complex is appears to be an outer-sphere complex because of the ease of Cr displacement when the system is swamped with a competing species.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Table 1.

Reported dissociation and surface complexation constants.

Reaction                                log K

=[[AlOH.sub.2.sup.+]_=[AlOH+H.sup.+]    -7.4 Stuum et al.,
                                        1980
                                        -8.2 Cambier et al.,
                                        1991
                                        -6.3 Ainsworth et al.,
                                        1989

=AlOH_=[AlO.sup-]+[H.sup.+]             -10.0 Stuum et al.,
                                        1980
                                        -10.4 Cambier et al.,
                                        1991
                                        -12.0 Ainsworth et al.,
                                        1989

[H.sub.3][PO.sub.4]_[H.sub.2][PO.sup.-  -2.2 Stuum et al.,
.sub.4] + [H.sup.+]                     1981

[H.sub.2][PO.sup.-.sub.4]_[HPO.sub.4]=  -7.2 Stuum et al.,
+ [H.sup.+]                             1981

=AlOH+_[CrO.sub.4.sup.2-]+[H.sup.+]_    +11.3 Ainsworth et al.,
[AlOH.sub.2.sup.+-[[CrO.sub.4.sup.2-].  1989
sup-]

=AlOH+_[CrO.sub.4.sup.2-]+[2H.sup.+]_   +18.1 Ainsworth et al.,
[AlOH.sub.2.sup.+-[[HCrO.sub.4.sup.-].  1989
sup.[degrees]]

where =[AlO.sup.-],=AlOH, and = =[[AlOH.sub.2.sup.+]_=[AlOH+H.sup.+] are
surface sites

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