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Soil Science Society of America Journal 67:1127-1132 (2003)
© 2003 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

The Influence of pH, Electrolyte Type, and Surface Coating on Arsenic(V) Adsorption onto Kaolinites

Sophie Cornu*,a, Dominique Breezeb, Alain Saadab and Philippe Barangerb

a Unité de Sciences du Sol, INRA d'Orléans, av. de la Pomme de pin, BP 20619, Ardon, 45166 Olivet Cedex, France
b Bureau de Recherches Géologique et Minière (BRGM), 3 avenue Claude Guillemin-B.P. 6009–45060 Orléans Cedex 2, France

* Corresponding author (Sophie.Cornu{at}orleans.inra.fr)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
The adsorption of arsenate (As[V]) has been investigated through batch experiments using two solid phases: a purified kaolinite and a kaolinite coated with humic acid (HA). The influence of various chemical parameters on As(V) adsorption onto kaolinite are examined, namely initial As concentration, pH, electrolyte type (Ca and Na) and concentration, and the presence or absence of a HA coating on the kaolinite surface. In Na electrolyte, As(V) adsorption onto kaolinite and HA-coated kaolinite is pH-dependent. In Ca media, however, As(V) adsorption onto kaolinite is not pH-dependent, and pH has only a minor influence on As(V) adsorption onto kaolinite coated with HA. Depending on electrolyte type, the adsorption of As(V) respectively onto kaolinite and HA-coated kaolinite differs considerably. In NaNO3, As(V) adsorption is lower in the presence of HA regardless of the pH value, whereas in Ca(NO3)2, adsorption of As(V) onto HA-coated kaolinite is greater than onto kaolinite alone above pH 5. The effect of Ca on As(V) adsorption thus appears complex and concerns both As(V) speciation in solution and the kaolinite adsorption site level. In addition, a decrease in the Ca concentration of the electrolyte decreases As(V) adsorption onto both solids.

Abbreviations: DOC, dissolved organic C • HA, humic acid


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
ARSENIC TRANSFER in natural environments is governed by several chemical reactions, in particular sorption processes onto metal oxides, clays, and organic matter. The adsorption of As onto clays has already been largely investigated (Griffin et al., 1977; Goldberg and Glaubig, 1988; Manning and Goldberg, 1997), revealing the importance of pH and initial As(V) concentration. Arsenic (V) adsorption onto kaolinite reaches a maximum at pH 5, followed by an abrupt decrease above pH 6 (Griffin et al., 1977; Goldberg and Glaubig, 1988; Xu et al., 1988). Goldberg and Glaubig (1988) showed that As(V) adsorption increases again above pH 9. Griffin et al. (1977) believe that As(V) adsorption occurs according to an anion-exchange process involving H2AsO-4. Goldberg and Glaubig (1988) and Manning and Goldberg (1997) concluded that As(V) forms an inner sphere complex with acid-base groups (-SOH) present on the surface of clays. These complexes may be mono- or polynuclear.

Despite an extensive range of work in the literature describing the sorption of As(V) by metal hydrous oxides and clays (Gupta and Chen, 1978; Oscarson et al., 1983; Wilkie and Hering, 1996; Tossell, 1997; Griffin et al., 1977; Goldberg and Glaubig, 1988; Manning and Goldberg, 1997), the adsorption of As(V) onto organic matter and organomineral phases remains poorly studied (Xu et al., 1988; Cornu et al., 1999; Saada et al., 2003a). Previous work (Cornu et al., 1999) has shown, in Ca(NO3)2 media and at pH 7, that pre-adsorption of HA onto kaolinite increases As(V) adsorption. This result was confirmed by Saada et al. (2003a) and attributed to sorption reactions at protonated amine functional groups. It was already suspected that amine groups were responsible for As(V) adsorption by HA (Thanabalasingam and Pickering, 1986). However, these first results are nevertheless scarce and were obtained for a single pH value in a Ca medium, in contrast to most previous studies performed in a Na medium.

We thus decided to study As(V) sorption onto an organomineral phase (kaolinite coated with HA) and compare its sorption capacities with those of the mineral alone (kaolinite). We focused in particular on the influence that electrolyte type and pH have on As(V) adsorption onto kaolinite and HA-coated kaolinite (henceforth termed kaolinite-HA) for different initial As(V) concentrations. As data on As(V) adsorption in Ca(NO3)2 media are rare, we concentrated on this medium rather than a Na medium.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Solids Used
The solids used in this work are the same as those used by Cornu et al. (1999), namely:

  1. Arvor 7A (France) kaolin, treated with hot hydrogen peroxide according to the technique of Robert and Tessier (1974) to reduce to a maximum its organic C content (residual C content of 0.006%; Saada et al., 2003b), followed by either Ca (three-fold exchange using 1 M CaCl2 with 1 g of solid for 10 mL) or Na exchange to obtain a solid with homo-ionic surfaces (Saada et al., 2003b). This kaolin consists of about 90% of kaolinite, the main accessory phase being illite. Its cation-exchange capacity (CEC) is 4.3 cmol kg-1 and its BET surface is 14.0 m2g-1 (Saada et al., 2003b).
  2. A kaolinite-HA phase, prepared by the adsorption of industrial HA (Fluka, lot no. 41968/1 594) onto purified and Ca-exchanged kaolinite. The HA was used without further purification and comprises 20% ash, mainly NaCl (Saada et al., 2003b), 43% C, 0.4% N (Saada et al., 2003a), 2 mol kg-1 of weak acid functional groups (mainly carboxylic acids) and 1.8 mol kg-1 of very weak acid functional groups (mainly phenol groups; Moreau-Kervévan, 1997). Humic acid adsorption was performed as described by Saada et al. (2003a)( 2003b): kaolinite was brought into contact with dissolved HA in a Ca medium for 48 h to promote HA adsorption. The obtained solid was then rinsed several times so that only the non-desorbing HA fraction remained. Final HA content of the coating is 0.12%.

Adsorption of As(V) onto Different Solid Phases
Adsorption of As(V) was performed according to Cornu et al. (1999) using a solid/liquid ratio of 1 g:40 mL. Calcium and Na electrolytes were considered: Ca(NO3)2 at 10-2 and 5 x 10-4 M, and NaNO3 at 10-2 M. Adsorption was performed using Ca-exchanged kaolinite in Ca(NO3)2 electrolyte and Na-exchanged kaolinite in NaNO3 electrolyte. Arsenic was added as a solution of As(V) in nitric acid. The pH was adjusted by adding NaOH or dilute nitric acid and measured at the end of the experiment. This last pH value was used for data interpretation. Contact times were set so that thermodynamic equilibrium was reached in each test; 24 h for kaolinite and 48 h for kaolinite-HA (Cornu et al., 1999). After each reaction time, the supernatant was filtered through a 0.45-µm pore diameter membrane and acidified with "Suprapur" HNO3. Dissolved As concentrations were determined by graphite furnace atomic absorption spectrometry. The lack of HA desorption during the As(V) sorption experiment was checked through dissolved organic C (DOC) measurements in the filtered solution. Dissolved organic C was analyzed at the end of the adsorption experiments using a TOC analyzer. Blank samples without solid phases were realized to control the initial As concentration and DOC concentration. After the experiment, the DOC concentrations of blank samples and kaolinite-HA samples were 0.8 ± 0.2 and 1.1 ± 0.2 M respectively, indicating that no HA desorption occurred during the experiment.

Influence of Initial As(V) Concentration
The influence of the initial As(V) concentration was determined in 10-2 M Ca(NO3)2 for both solids at pH 5 and 7, values classically encountered in natural soil. The tests were run with initial As(V) concentrations ranging from 0.05 to 2.5 mg L-1. A complementary experiment was performed in 5 x 10-4 M Ca(NO3)2 for both solids at pH 5 and 7 considering the initial As(V) concentrations of 1 and 2.5 mg L-1.

Influence of pH
In 10-2 M Ca(NO3)2, the influence of pH was determined between 3 and 7 (steps every 0.5 pH) for an initial As(V) concentration of 0.5 mg L-1 for both substrates. No experiments were performed above pH 7 to avoid the risk of calcite precipitation. In 10-2 M NaNO3, the influence of pH was determined between 4 and 7 (steps every whole pH unit) for the same initial As(V) concentration (0.5 mg L-1).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
Influence of Initial As(V) Concentration in a 10-2 M Ca(NO3)2 Medium
For the two solids (kaolinite and kaolinite-HA) and two pH values (5 and 7) tested, the As(V) adsorption isotherm can be described by a Freundlich equation (Fig. 1A):

where A is the quantity of As adsorbed onto the surface of the solid (mg kg-1), Ceq is the equilibrium As concentration (µg L-1), n reflects the heterogeneity of the surface site, and Kf is the adsorption coefficient.



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  		Fig. 1. Influence of initial As(V) concentration on adsorption onto kaolinite and kaolinite-Humic acid (HA) at two pH values (5 and 7) in 10-2 M Ca(NO3)2. (A) Adsorption isotherms. Solid/liquid partition coefficient (Rd) as a function of initial As(V) concentration at (B) pH 7 and (C) pH 5.

 
Such a result was already observed for As(V) adsorption onto kaolinite in a Na medium (Griffin et al., 1977; Goldberg and Glaubig, 1988; Manning and Goldberg, 1997).

Comparison between the calculated values of Kf and n (Table 1) shows that the following sequence exists: Kf (Kaolinite; pH 5) << Kf (kaolinite-HA; pH 5); Kf (Kaolinite; pH 7) << Kf (kaolinite-HA; pH 7) indicating an increasing sequence of As(V) adsorption onto kaolinite in the presence of HA, and with increasing pH from 5 to 7.


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  		Table 1. Parameters of Freundlich's law for As(V) adsorption onto the two solids studied at pH 5 and 7 in 10-2 M Ca(NO3)2.

 
If the results are expressed in terms of the solid/liquid partition coefficient (Rd), the values of Rd decrease for both solids when the initial As(V) concentration increases (Fig. 1B and 1C). The decrease is less pronounced for the adsorption of As(V) onto kaolinite than onto kaolinite-HA (Fig. 1B and 1C). The solid/liquid partition coefficient (Rd) for kaolinite decreases by 23 and 56% at pH 7 and 5 respectively, whereas the error on the Rd value is <13% according to duplicate values. Thus, the Rd decrease for kaolinite is significant. Finally, for initial As(V) concentrations above 100 µg L-1, the quantities of As adsorbed at pH 7 are 10 to 30% and 23 to 44% greater than those absorbed at pH 5 (Fig. 1C) for kaolinite and kaolinite-HA.

Influence of pH
In Calcium Media
The quantities of adsorbed As increase as a function of pH (Fig. 2A) in Ca media. pH dependence is relatively low for kaolinite, and more pronounced for kaolinite-HA. The quantities adsorbed onto kaolinite-HA increase with pH in the pH 2 to 6 range, and stabilize between pH 6 and 7. Thus, the two solids exhibit considerably different behavior as a function of pH: adsorption is relatively pH insensitive for kaolinite and pronounced for kaolinite-HA.



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  		Fig. 2. Influence of pH on As(V) adsorption onto kaolinite and kaolinite-humic acid (HA) for an initial As(V) concentration of 0.5 mg L-1 and in two electrolyte types: (A) in 10-2 M Ca(NO3)2 and (B) in 10-2 M NaNO3.

 
At pH 5, the Rd value of As adsorption onto kaolinite decreases strongly as the initial As(V) concentration increases (Fig. 1C). Such a decrease is not recorded at pH 7 (Fig. 1B). For low initial As(V) concentrations, however, the Rd values at pH 5 and 7 are almost equivalent.

In Sodium Media
In Na media on the other hand, As(V) adsorption onto kaolinite-HA increases between pH 4 and 7, whereas adsorption onto kaolinite alone is more or less stable from pH 4 to 6, and decreases from pH 6 to 7 (Fig. 2B). Such behavior was already described by Griffin et al. (1977), Goldberg and Glaubig (1988), and Manning and Goldberg (1997). Thus, in Na media, the pH influence on As(V) adsorption onto two substrates differs from that recorded for these same substrates in Ca media. In addition, adsorption of As(V) is of the same order of magnitude as that of arsenate obtained by Manning and Goldberg (1997) for kaolinite.

Influence of Humic Acid Pre-Adsorbed onto Kaolinite
In Calcium Media
In Ca media, the Kf values obtained for kaolinite-HA are higher than those for kaolinite (Table 1). This reflects the lower affinity of As(V) for kaolinite compared with kaolinite-HA. In addition, the Rd calculated for the kaolinite-HA phases is higher than that for kaolinite at both pH 5 and 7 (Fig. 1B and 1C). This is only observed for initial As(V) concentrations below 1.5 mg L-1; above this concentration, the Rd for both solids tends toward the same plateau.

When comparing the quantities of As adsorbed, as a function of pH and for both substrates (Fig. 2A), it is seen that the presence of HA on the kaolinite surface reduces As(V) adsorption at pH values below 4. Above pH 4, no significant influence of HA on As(V) adsorption is observed.

In Sodium Media
In Na media, adsorption of As(V) onto kaolinite is generally some 30% greater than onto kaolinite-HA, regardless of the pH conditions (Fig. 2B). At pH 7, however, only a small difference is observed between As(V) adsorption onto the two substrates (14% more As(V) adsorbed onto kaolinite than onto kaolinite-HA).

Influence of Electrolyte Type and Concentration
As already mentioned above, As(V) adsorption differs markedly in Na and Ca media.

  1. In Ca media, HA pre-adsorbed onto kaolinite does not affect As(V) adsorption above pH 4, whereas in Na media it markedly decreases As(V) adsorption regardless of the pH value.
  2. In Ca media, As(V) adsorption onto kaolinite-HA is more pH-sensitive (adsorption increases by 24% between pH 4 and 7), than adsorption onto kaolinite (no significant adsorption change recorded within this pH range). In Na media, adsorption onto kaolinite decreases by 10% between pH 4 and 7, whereas adsorption onto kaolinite-HA increases by 30%.

Calcium thus has a strong influence on As(V) adsorption onto kaolinite and kaolinite-HA. In addition, at pH 7, a decrease in Ca electrolyte concentration (5 x 10-4 instead of 10-2 M) causes a marked decrease in As(V) adsorption onto kaolinite, regardless of the initial As(V) concentration (Table 2). Adsorption onto kaolinite-HA also decreases. At pH 5, no change in As(V) adsorption onto both solids is recorded with a decrease in Ca concentration (Table 2).


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  		Table 2. Quantities (mg kg-1) of As(V) adsorbed onto kaolinite and kaolinite-HA in 10-2 and 5.10-4 M Ca(NO3)2, for two initial As(V) concentrations and two pH values (5 and 7).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
Electrolyte Type
As presented above, electrolyte type has a strong influence on As(V) adsorption onto kaolinite. This can be interpreted as an effect of either cation type or ionic strength. Indeed, the 10-2 M NaNO3 electrolyte corresponds to an ionic strength of 0.01 M, whereas the 10-2 M Ca(NO3)2 electrolyte corresponds an ionic strength of 0.03 M. However, Manning and Goldberg (1997) demonstrated that ionic strength has only a minor influence on As adsorption onto kaolinite, even for larger ionic strength variations (between 0.005 and 0.1 M of NaCl). Goldberg and Johnston (2001) also noted a minor influence of ionic strength on As(V) adsorption onto Al and Fe oxides. Cation type thus seems the most probable interpretation.

Arsenic(V) adsorption onto kaolinite decreases in Ca media, this adsorption being more reduced at 5 x 10-4 than at 10-2 M Ca(NO3)2. The influence of calcium on As(V) adsorption thus appears complex. Indeed, Ca ions can participate in As(V) adsorption onto kaolinite at several levels: (i) by the formation of Ca bridges, described in the literature especially for phosphates (McBride, 1994); (ii) by the precipitation of calcium arsenates; (iii) by the formation of soluble complexes such as Ca2, CaAsO-4, and CaH2AsO+4 (Whiting, 1992).

In the experimental conditions of the present work (initial As(V) concentration below 2.5 M), a speciation calculation performed with the Eq. [3] code (Wolery, 1992) shows that the solution remains undersaturated with respect to calcium arsenate. The precipitation of calcium arsenate can be excluded.

The charge of kaolinite is null around pH 4 to 5 (Sposito, 1989). At values below pH 4, the kaolinite surface is positively charged and it will tend to bind anions, whereas above pH 5, its negative charge will tend to bind cations. Calcium can bind to the kaolinite aluminol and silanol surface sites (-SO-) either as -SOCa+ or as bidendate species (-SO)2Ca, as is classically described for divalent cations (McBride, 1994).

Calcium may thus have a complex influence on As(V) adsorption onto kaolinite:

  1. It probably decreases As(V) adsorption through the formation of (-SO)2Ca neutral sites, which are unable to adsorb As(V); in Na media, kaolinite surface sites are of the cationic type that favor As(V) adsorption.
  2. Calcium-2+ may form bridges between kaolinite negative surface sites and As(V) anions, which would increase As(V) adsorption onto kaolinite as Ca concentration in the medium increases.
  3. Calcium may form CaHAsO4 neutral species, poorly adsorbed onto kaolinite.

Competition among these three mechanisms could possibly explain the complex behavior of As(V) adsorption onto kaolinite obtained in this study. Sorption modeling is in progress to verify this hypothesis.

Humic Acids
In Na media, As(V) adsorption onto kaolinite coated with HA decreases drastically, which is consistent with the reduction of surface sites (Cornu et al., 1999; Saada et al., 2003a).

In Ca media, the presence of HA adsorbed onto the kaolinite surface reduces As(V) adsorption at pH values below 4. This is consistent with the reduction in the number of -SOH sites when HA is present on the kaolinite surface, as demonstrated by Cornu et al. (1999) using titration curves. At pH values above 5, however, adsorption is favored by the presence of HA on the kaolinite surface. This could be due to complex phenomena, not fully understood as yet, involving competition between a reduction in the number of -SOH sites on the kaolinite surface and the presence of organic sites such as -COOH, -COOCa+, -NH+3, etc. At pH 7, the Rd of kaolinite-HA decreases toward the Rd value of kaolinite when the As(V) concentration increases (Fig. 1B). This confirms the presence of binding sites because of the HA coating, which are few in number as they do not influence As(V) sorption onto kaolinite for initial As(V) concentrations above 1 mg L-1. These organic sites are absent from the kaolinite surface. Since these organic sites are saturated before those of kaolinite, they are apparently more reactive than the latter. Cornu et al. (1999) assumed that N groups of HA coating contributed to greater adsorption of As(V) onto the kaolinite-HA phase. Results obtained by Saada et al. (2003a) during their experiments performed with a HA coating extracted from peat are consistent with this hypothesis. This peat HA, which is characterized by a higher N content than the Fluka HA used in the present study, seems to favor As(V) adsorption.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
The isotherms of As(V) adsorption onto the two solids can be described by Freundlich's equation. In the presence of Ca, As(V) adsorption increases with increasing pH from 3 to 7. This trend is less pronounced for As(V) adsorption onto kaolinite than onto kaolinite coated with HA. Hypotheses for the main mechanisms of As(V) adsorption onto kaolinite are discussed.

The role of the major dissolved cations on adsorption is crucial in terms of environmental consequences. In agricultural soils encountered in Europe, the major cations in solution are most commonly Ca2+ or Mg2+, but rarely Na+ found in the specific case of saline soils. To be able to extrapolate experimental laboratory results into the field, it would be necessary to perform adsorption experiments using both electrolyte types.

In the presence of a Na electrolyte, a HA coating causes a decrease in As(V) adsorption regardless of the pH value. However, in Ca media, As(V) adsorption decreases in the presence of a HA coating compared with kaolinite alone at pH values below 4. At pH values above 4, no significant difference in As(V) adsorption onto the two solids is observed. This is related notably to the N groups of HA on the surface of the kaolinite-HA phase. The environmental consequences of these results are important inasmuch as the pH of agricultural soils is usually above 6, and intimate and relatively stable associations exist between clayey phases and organic matter. In temperate soils, however, kaolinite is rarely the dominant clay. Similar work should thus be performed on other types of clay to complement these results.


   ACKNOWLEDGMENTS
 
The present study was carried out as part of the BRGM research project entitled "Cycling of metals in the environment".

Received for publication April 8, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 




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