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Journal of Environmental Quality 31:557-563 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Heavy Metals in the Environment

Chemistry of Inorganic Arsenic in Soils

II. Effect of Phosphorus, Sodium, and Calcium on Arsenic Sorption

E. Smitha, R. Naidu*,a and A. M. Alstonb

a CSIRO Land and Water, Private Bag No. 2, Glen Osmond, Adelaide, SA 5064, Australia
b Department of Soil and Water, University of Adelaide, Waite Campus, Private Mail Bag No. 1, Glen Osmond, Adelaide, SA 5064, Australia

* Corresponding author (Ravi.Naidu{at}csiro.au)

Received for publication November 30, 1999.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
There are more than 10000 arsenic (As) contaminated sites in Australia. The ability of soils at these contaminated sites to sorb As is highly variable and appreciable amounts of As have been recorded in the subsurface soils. The potential risk of surface and ground water contamination by As at these sites is a major environmental concern. Factors that influence adsorption capacity of soils influence the bioavailability and subsequent mobility of As in soils. In the present study we investigated the effect of PO3-4 and Na+ and Ca2+ on the sorption of AsV and AsIII by an Oxisol, a Vertisol, and two Alfisols. The presence of P (0.16 mmol L-1) greatly decreased AsV sorption by soils containing low amounts of Fe oxides (<100 mmol kg-1), indicating competitive adsorption between P and AsV for sorption sites. In contrast, the presence of a similar amount of P had little effect on the amount of AsV adsorbed by soils with high Fe content (>800 mmol kg-1). However, AsV sorption substantially decreased from 0.63 to 0.37 mmol kg-1 as P concentration was increased from 0.16 to 3.2 mmol L-1 in selected soils. This suggests increased competition between P and AsV for soil sorption sites, through either the higher affinity or the effect of mass action of the increasing concentration of P in solution. A similar effect of P on AsIII sorption was observed in the low sorbing Alfisol and high affinity Oxisol. However, the amount of AsIII sorbed by the Oxisol was much greater than the Alfisol for all treatments. The presence of Ca2+ increased the amount of AsV sorbed compared with that of Na+ and was manifested through changes in the surface charge characteristics of the soils. A similar trend in AsIII sorption was recorded with changes in index cation, although the effect was not as marked as recorded for AsV.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
CONCENTRATIONS OF arsenic (As) in the soil solution are strongly influenced by adsorption–desorption processes involving several soil components. The key components include the crystalline layer silicate minerals (Frost and Griffin, 1977; Goldberg and Glaubig, 1988; Xu et al., 1991), and hydroxides of iron (Fe), aluminum (Al) (Anderson et al., 1976; Pierce and Moore, 1982), and manganese (Mn). Significant amounts of As are also adsorbed by short-range order secondary aluminosilicates, imogolite, and allophane and ferrihydrite. These minerals are commonly found in Andisols and in Spodic horizons (Gustafsson et al., 1995). The high positive surface charge density under low pH conditions makes these materials effective sorbents for AsV (Gustafsson and Jacks, 1995; Lindberg et al., 1997).

Both AsV and AsIII are adsorbed on oxide surfaces. However, their affinity for oxide surfaces varies depending on pH and soil and solution chemistry. The adsorption optimum for AsIII is around pH 7.0, while AsV adsorbs optimally at pH 4.0 (Pierce and Moore, 1982). More recent studies by Raven et al. (1998) and Jain et al. (2000) indicate that at solution pH above pH 8 AsIII is sorbed at a higher rate than AsV by ferrihydrite (pure system). It is, however, not clear from these studies whether similar trends could be expected in soils that constitute of a mixture of layer silicate minerals and a mixed oxide system. Indeed, there is a paucity of information on sorption dynamics of arsenic in soils. The proportion of As sorbed from solution by the soil colloidal surfaces depends on the number of sites available for sorption. The presence of specifically sorbed ligand ions such as phosphate (P) have been reported to suppress the sorption of As, while other ions such as chloride (Cl-), nitrate (NO-3), and sulfate (SO2-4) have an insignificant effect on AsV sorption (Livesey and Huang, 1981). Roy et al. (1986) reported that P and molybdate (Mo) suppressed the sorption of AsV on a Cecil clay (Typic Hapludult), with P being more effective than Mo. Manning and Goldberg (1996) have reported similar competitive effects between P and AsV, and Mo and AsV on pure mineral surfaces. These investigators found that AsV is readily desorbed from Fe(OH)3 with an increase in pH and also due to competing anions like PO4, MoO4, and SO4 for the adsorption sites. The geochemical behaviors of AsV and P are strikingly similar and both complex with Fe, Al, and, under specific circumstances, Mn (Schulze et al., 1989; Schwertmann and Taylor, 1989; Lund and Fobian, 1991).

Barrow (1974), however, showed that competition for sites is reversed at high AsV and low P concentrations, indicating that the mass action effect may dictate the extent of the adsorption of anions irrespective of the nature of ligand ions.

While there are numerous studies on the effects of pH and ion competition on AsV in pure mineral and soils systems, limited effort has been directed toward comparing the effects of anion competition and cations (e.g., Na+ and Ca2+) on sorption of AsV and AsIII in soils. Such a study is needed given the widely different composition of soil solutions. For example, soil solution composition in Australian soils varies from Na+ dominated to Ca2+ dominated (Naidu et al., 1995). In a previous study (Smith et al., 1999), we investigated the effect of pH and ionic strength on the chemodynamics of As adsorption and the implications for As bioavailability in soils. In this paper we further this work by investigating the sorption of AsV and AsIII in four contrasting soils to quantify the importance of selected cations and anions in the sorption processes that regulate As species in soil solution.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils and Their Properties
Four surface soil samples (0–150 mm) collected from northern New South Wales, Australia were used in this study. The soils (two Alfisols, Oxisol, and a Vertisol) studied were selected on the basis of their physical and chemical properties (Table 1). Soil samples collected from the field were air-dried and crushed to pass through a 2-mm stainless-steel sieve and stored in airtight polythene containers.


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  		Table 1. Pertinent properties of selected soils.

 
Soil pH was determined in 1:5 (w/v) soil water suspensions following 16 h equilibration; cation exchange capacity was determined as outlined by Rayment and Higginson (1992). Particle size was determined by the pipette method (USDA Soil Conservation Service, 1982). Oxalate and citrate–dithionite extractable Fe and Al were determined as outlined in Blakemore et al. (1987). Total carbon (TC) was analyzed with a Leco (St. Joseph, MI) CNS-2000. Total As concentration was estimated, following aqua regia microwave digestion of soils (Nieuwenhuize et al., 1991), by flame atomic absorption spectrometry (FAAS) with hydride generation (Voth-Beach and Shrader, 1985).

Competitive Sorption Isotherms
Effect of Phosphorus on Arsenic(V) and Arsenic(III) Sorption
Batch sorption studies were used to determine the effect of P on AsV and AsIII sorption. Soils (1 g) were shaken on an end-over-end shaker for 24 h at 20 ± 2°C in separate polyethylene tubes, with 20 mL of 0.03 mol L-1 NaNO3 solution containing varying concentrations (0–0.66 mmol L-1) of AsV, added as Na2HAsO4·7H2O, and 0.16 mmol L-1 of P added as KH2PO4. A similar method was used for the study of AsIII sorption except 20 mL of 0.03 mol L-1 NaNO3 solution containing varying amounts (0–0.26 mmol L-1) of AsIII, added as NaAsO2, and 0.16 mmol L-1 of P added as KH2PO4, was added to 1 g of soil and shaken for 24 h. Preliminary kinetic studies showed that a 24-h contact time was sufficient for soil and solution equilibrium (Smith et al., 1999). This preliminary study also revealed no transformation of AsIII to AsV during the batch sorption study. At the end of the shaking period, samples were centrifuged at 11950 x g (10000 rpm) for 10 min and the supernatant decanted and filtered through Whatman (Maidstone, UK) 541 filter paper. The As concentration in the extract was analyzed using a hydride generation flame atomic absorption spectrometer and the amount of As sorbed was calculated from the difference between the amount added and amount remaining in solution. The sorption data were fitted to sorption equations and the best fit was obtained with the Freundlich equation, which gave the highest values of coefficient of determination (R2 values).

In a separate study, varying amounts (0–6.45 mmol L-1) of P and AsV (0.66 mmol L-1), prepared in a background solution (0.03 mmol L-1 NaNO3), were added to 1 g of the Oxisol soil. The range in concentrations of P added to solution were the average concentrations of solution P present in soils from contaminated cattle dip sites (Smith, unpublished data, 1998). Samples were shaken on an end-over-end shaker for 24 h at 20 ± 2°C, centrifuged at 11950 x g (10000 rpm) for 10 min and the supernatant decanted and filtered through Whatman 541 filter paper. Arsenate in solution was determined as described above and P in solution was determined by inductively coupled plasma optical emission spectroscopy (ICP–OES).

Effect of Index Cations
The effect of index cations on AsV and AsIII sorption was investigated as described above for competitive sorption isotherms, using either sodium (0.03 mol L-1 NaNO3) or calcium [0.01 mol L-1 Ca(NO3)2] nitrate as the background electrolyte. Comparative anion competition studies using P in either 0.03 mol L-1 NaNO3 or 0.01 mol L-1 Ca(NO3)2 background solutions were also conducted. Following equilibration, soil suspensions were collected and elemental analyses of suspension were conducted by inductively coupled plasma optical emission spectroscopy. Arsenic in the supernatant was analyzed using hydride generation with a hydride generation flame atomic absorption spectrometer.

Surface Charge Characteristics
During the sorption study it was found that whenever Ca2+ was used as the index cation, the amount of AsV sorbed was markedly more than that recorded in the presence of Na+. To further investigate the effect of Ca, the soil surface charge was examined in Ca-saturated and Na-saturated soils using an ion adsorption method similar to that described by Naidu et al. (1990). Soil samples (2 g) were equilibrated with 20 mL of 1 mol L-1 NaCl on an end-over-end shaker for 1 h. Following the initial equilibration, the samples were centrifuged for 10 min at 2987 x g (5000 rpm) and the supernatant decanted. The sample residues were equilibrated with 30 mL of deionized H2O for 10 min on an end-over-end shaker to decrease the concentration of entrained solution and then centrifuged at 11950 x g (10000 rpm) for 5 min and the supernatant discarded. The soils were then saturated with 20 mL of 0.03 mol L-1 NaCl and equilibrated on an end-over-end shaker for 30 min. The samples were centrifuged for 20 min and supernatant discarded. This saturation step was repeated twice more; however, after the third shaking, the supernatant was filtered and saved for later analysis. The tubes were then weighed and the sorbed Na+ and Cl- were extracted by shaking for 30 min with five 20-mL lots of 0.5 mol L-1 NH4NO3 solution. Sodium in the solution was measured using ICP–OES, and Cl- was measured using the mercuric thiocyanate indicator method as described by Florence and Farrer (1971). The intensity of the Cl complex was measured by ultraviolet-visible spectrophotometry at 460 nm.

Surface Area
The surface area of the soils was determined by single-point Brunauer–Emmett–Teller (BET) N2 sorption with a Quantachrome (Boynton Beach, FL) Quantasorb surface area analyzer (Brunauer et al., 1938). Two standard clays, Fullers earth (montmorillonite) and Kent clay (kaolinite), were used as reference materials. The surface areas of the reference materials were determined as 97 and 41 m2 g-1, compared with the actual surface areas of 100 and 40 m2 g-1, respectively. Surface areas of the soils studied are in Table 1.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Properties
The physical and chemical properties of the study soils are shown in Table 1. The properties of the soils varied widely for the four soils investigated: pH 5.0 to 6.9, clay 70 to 420 g kg-1, and citrate–dithionite extractable Fe and Al ranging from 23 to 2800 mmol kg-1 and 7 to 605 mmol kg-1, respectively. The total background concentration of As in all soils was low, ranging from 0.3 to 1.53 mg As kg-1 in the Alfisol and Oxisol soils, respectively. X-ray diffraction examination of the clay fraction (<1.4 µm) reveals the presence of dominant amounts of kaolin and goethite in the Oxisol and expansive layer silicate and trace amounts of interstratified minerals in the Alfisols and Vertisols. The presence of goethite is further confirmed by the citrate–dithionite extractable data that reflects the presence of significant amounts of free Fe oxide minerals in these soils.

Arsenic(V) Sorption in the Presence of Phosphorus
The effect of P on the sorption AsV is shown in Fig. 1 . The largest effect of P on AsV sorption was observed in the Alfisol soils (C and H) with the amount of AsV sorbed by these soils decreasing between 44 and 65%, respectively. In contrast, there was little effect of adding P in the background solution to the sorption of AsV in the Vertisol and Oxisol soils.



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  		Fig. 1. Effect of different ions on the sorption of AsV by selected soils. Bars represent the standard errors of the means. Where no bar is seen, error is smaller than the symbol.

 
The suppressed sorption of AsV in the presence of P by soil and pure mineral surfaces has been reported by numerous investigators (Livesey and Huang, 1981; Roy et al., 1986; Goldberg, 1986; Melamed et al., 1995; Manning and Goldberg, 1996). For example, Livesey and Huang (1981) reported that P (10-4 to 10-2 mol L-1 PO3-4) significantly suppressed the sorption of As, while the addition of Cl-, NO-3, and SO2-4 (10-4 to 10-2 mol L-1) had little significant effect on As sorption by four surface soils. In contrast, Manning and Goldberg (1996) reported that, when AsV and P were present in equimolar concentrations (6.7 x 10-7 mol L-1), the presence of P in solution only slightly decreased AsV sorption on kaolinite and illite. A further 10-fold increase in the solution P concentration to 6.7 x 10-6 mol L-1, decreased AsV sorption on kaolinite and illite by 67 and 82%, respectively. The increased competition for available sorption sites between AsV and P with increasing solution P concentration was probably responsible for the decline in the amount of AsV sorbed. As with the observations reported in the literature, the present study found that in some soils the effect of P might be more prominent than that observed in other soils. The variable effect of P on As sorption may be attributed to the varying sorption capacity of soils. In soils with low sorption capacity, the competitive effect of P was more evident than in soils with high As sorption capacity. This suggests that the competitive effect of P is possibly apparent in low As sorbing soils when the number of sorption sites are low. Conversely in high As sorbing soils, and at low solution As concentration, such an effect is minimized due to the availability of sorption sites for both competing ligand ions. It might be argued that if binding energy is the major factor controlling the effect of ligand ions, such as P and AsV, then irrespective of the number of available sorption sites, P sorbed will always be greater than AsV due to its higher binding coefficient (Hingston et al., 1972) (if the mass action effect does not prevail). This is not observed with all soils as at low solution concentrations, when the sites are undersaturated with AsV and P, both species are almost equally sorbed. However, at increasing P solution concentrations the strongly binding P competes effectively for the limited sites available for sorption. This hypothesis is supported by the sorption data (Fig. 1) for soils with low sorbing capacity for AsV (Alfisols). In these soils (Alfisols), the effect of P appears to be pronounced even at low AsV concentrations. In contrast, at low solution concentrations of AsV and P, there is little effect of P on AsV sorbed by the Vertisol and Oxisol soils, suggesting that both ions are strongly retained by soil colloids. Increasing P in solution results in increased competition between P and AsV for sorption sites and a subsequent decrease in the amount of AsV sorbed.

If the number of sites is defined by x, and assuming x equals sorption maxima from the As sorption isotherm for each soil, then the sorption density (Ps) is given by Eq. [1]:

[1]

It is apparent from the sorption density (Table 2) that the amount of AsV sorbed per unit area is between 5 and 10 times higher in low sorbing soils (Alfisols) compared with the high sorbing soils (Vertisol and Oxisol), indicating the presence of a small number of high affinity sorption sites per unit area of the low sorbing soils. This is consistent with the surface area of the four soils that ranges from <1 to 2 m2 g-1 in the low sorbing soils to 27 and 63 m2 g-1 in the high sorbing soils. Comparison of the Kd values further illustrates the marked competition effect of P in the low sorbing soils. As discussed above such a trend in As sorption in the presence of P may be related to the number of sites available for sorption and the strength of the two ligand ions under study. Greater competition for As in the low sorbing soils illustrates competition for sorption sites by both As and P, and the latter, being a stronger ligand, is preferentially sorbed by the soil. Competition is higher in the low sorbing soils due to the much smaller pool of sorption sites. In contrast, the presence of large number of sites in the high sorbing soils reduces competition between the two ligand ions. It is likely that the competitive effect will be much more apparent at higher ligand ion concentrations when the number of sorption sites is less than the number of ions available for binding.


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  		Table 2. Effect of P on AsV sorbed by selected soils.

 
Table 2 shows that Kd in the presence of P (Kd+P) decreases between 50 and 70% in the low sorbing soils compared with 7 to 17% in the high sorbing soils. The competitive effect of P is also reflected in the differences observed in the ratio between Kd+P and Kd-P. In high AsV sorbing soils there is no change in the ratio between Kd+P and Kd-P, indicating that P has little effect on the amount of AsV sorbed. However, in low sorbing soils, the ratio between Kd+P and Kd-P increases, indicating that much more AsV is in soil solution in the presence of P. This confirms the hypothesis that the competitive effect of P on AsV sorption is dependent on the number of available sorption sites and the extent of saturation of those sites by the two anions.

To investigate further the effect of P, As sorption in solutions containing varying concentrations of P were studied using the Oxisol soil and the results are shown in Fig. 2 . The results indicate that at very low P concentrations (<1 mmol L-1) there is little marked difference in the amount of As sorbed. However, increasing the P concentration from 0 to 6.45 mmol P L-1 in the binary AsV/P sorbate solutions, decreased AsV sorbed on the Oxisol from 12.6 to 5.6 mmol kg-1 when 0.66 mmol L-1 of AsV was initially added to the solution. A fourfold difference between P and AsV concentrations in added solution resulted in only a 57% decrease in the amount of AsV retained by the soil. Darland and Inskeep (1997) and Manning and Goldberg (1996) have reported similar findings. Darland and Inskeep (1997) reported that even with a solution P concentration that exceeded the sorption capacity of an aquifer sand column by twofold, some previously sorbed AsV still remained sorbed to the sand. These results, and those of other researchers, raise questions that the only mechanism(s) of AsV sorption by soils in the presence of P is due to competition.



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  		Fig. 2. Effect of increasing soil solution P concentration on AsV sorption by an Oxisol (AsV concentration added equaled 0.66 mmol L-1).

 
There is evidence that sorption of AsV and P by Fe and Al oxides is through inner-sphere surface complexes (Hingston et al., 1971; Harrison and Berkheiser, 1982; Lumsdon et al., 1984; Waychunas et al., 1993, 1996; Fendorf et al., 1997) and the presence of P has generally been reported to decrease AsV sorption when both anions are present in solution (Hingston et al., 1971; Roy et al., 1986; Manning and Goldberg, 1996; Darland and Inskeep, 1997). Similarly, in this study even at the highest P concentration added, some AsV was still retained. Assuming P and AsV are competing for the same sorption sites, the addition of a greater concentration of P than AsV should result in a substantial decline in AsV sorbed. Sorption of AsV even in the presence of high concentrations of P (which exceed the AsV saturation concentration for the soil) confirms the presence of sorption sites specific for AsV and P and other sites that are common to both. Similar hypotheses have been postulated by Hingston et al. (1971) and Manning and Goldberg (1996) after studying the sorption of AsV and P by pure mineral surfaces, but much work still needs to be done to clarify the mechanisms that allow different ions to be preferentially sorbed by some sites in the soil and not others.

Arsenic(III) Sorption in the Presence of Phosphorus
The effect of P on AsIII sorption is shown in Fig. 3 and was similar to that of AsV. In low AsIII sorbing soils, such as the Alfisols, the presence of P in solution decreased the amount of AsIII sorbed from 0.38 to 0.1 mmol kg-1 at an equilibrium AsIII solution concentration of 0.2 mmol L-1 (Fig. 3). A similar decline in the amount of AsIII sorbed in the presence of P is also observed in the Oxisol (Fig. 3). However, the amount of AsIII sorbed by the Oxisol is much greater than the Alfisols for all the treatments. Such a large difference in sorption in the presence of P may be attributed to the widely different mineralogy of the soils and the mechanism of AsIII sorption. Arsenite has been shown to sorb predominantly on oxide surfaces (Anderson et al., 1976), which are also the active sorption sites for P (Naidu et al., 1990). Enhanced sorption of P appears to saturate sorption sites and decrease AsIII sorption and this effect may be more pronounced in low sorbing soils as recorded for the Alfisols. Sun and Doner (1996) studied the sorption of AsIII and AsV at pH 5.5 on goethite with transmission–Fourier transfer infrared (FTIR) and attenuated total reflectance FTIR spectroscopy. They postulated that AsIII formed an inner-sphere complex, which is similar to that reported for other anions such as P.



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  		Fig. 3. Effect of different ions on the sorption of AsIII by selected soils. Bars represent the standard errors of the means. Where no bar is seen, error is smaller than the symbol.

 
They suggested that the H2AsO-3 ion was the main form of AsIII sorbed by goethite at pH 5.5. This is surprising given that the major species present at pH 5.5 is the neutral H3AsO3 molecule. MINTEQA2 (Allison et al., 1990) speciation studies reveal that >99% of AsIII is present as the H3AsO3 species at pH 5.5, with <1% as H2AsO-3. An increase in solution pH has little effect on the distribution of AsIII species except above pH 7.5, where the distribution of H2AsO-3 ions in solution increases abruptly as pK1 is approached (pK1 = 9.2). If the H2AsO-3 ion is the major species sorbed by soil components, then the sorption process must be a multistep mechanism that is dictated by the kinetics of both the dissociation of H3AsO3 and the sorption of AsIII. The first component of this process may be the pH-dependent dissociation of the H3AsO3 ion (Eq. [2]), where the concentration of As is controlled by the pH and pKa of the dissociation process:


Rapid removal of the H2AsO-3 by the sorption process will shift the equilibrium toward the ionization of more H3AsO3 as shown above in Eq. [2]. This suggests that the partition coefficient (Kd) may be the key factor controlling H2AsO-3 release in soil solution.

If H2AsO-3 is the major species sorbing on soil particles, then the large effect of P on AsIII sorption may be attributed to competition between P and AsIII for similar sorption sites. Manning et al. (1998) confirmed the formation of an inner-sphere between AsIII and Fe on goethite using extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure spectroscopy. Using EXAFS analysis they reported that AsIII most likely formed a bidentate, binuclear bridging complex with Fe.

MINTEQA2 (Allison et al., 1990) speciation shows that greater than 90% of P was present as H2PO-4 at solution pH of this study. The competitive effect of P is apparent from the As Kd values, which decrease in the presence of P (Table 2). The Oxisol has a greater Fe oxide content than the Alfisol and AsIII sorption was less affected by the addition of P. This further supports our hypotheses that the number of available sorption sites dictates the competitive effect of P to a large extent.

Effect of Cations on the Adsorption of Arsenic(V) and Arsenic(III) in the Presence of Phosphorus
The effect of different index cations on the sorption of AsV is shown in Fig. 1. Irrespective of the nature of the solution composition, the sorption of AsV by soils was enhanced by the presence of Ca2+ ions in the solution compared with Na+. This effect of index cation may be attributed in part to the effect of the divalent cation on the nature of the As species present, and the pH of the soil suspension ({Delta}pH = 0.1 to 0.3 units lower in the presence of Ca2+ than Na+). The presence of Ca2+ ions may also decrease the activity of As species through ion-pair formation, thus decreasing sorption. However, this is not evident from MINTEQA2 (Allison et al., 1990) speciation of the sorbing solution, which shows that the presence of Ca2+ in solution makes no difference to the formation of ion pairs. The effect of index cation may also operate through the specific sorption of Ca2+, leading to increased positive charge. Bowden et al. (1973)( 1977) have explained this through the effects of different cations on surface charge density. Increasing the valency of the cation makes the potential in the plane of sorption less negative, thereby increasing anion sorption. The greater effect of cationic charge on surface potential was recently demonstrated by Bolan et al. (1993) for variable charged soils. These investigators found that specific sorption of Ca2+ increased the surface positive charge and led to enhanced retention of SO2-4.

In the present study, changing the index cation from Na+ to Ca2+ at a constant ionic strength increased the surface positive charge in the Alfisol (H) and Oxisol from 5 to 10 and from 32 to 43 mmol kg-1 Cl- sorbed, respectively. If coulombic interaction was one of the mechanisms controlling AsIII/AsV sorption, then increased surface positive charge will enhance sorption as recorded above. However, the effect of Ca2+ on the amount of AsV sorbed varied with soil type. In high AsV sorbing soils, the presence of Ca2+ had little effect on the amount of AsV and P sorbed. These results further confirm the hypothesis that the effects of competing ions are dependent on the number of available sorption sites. Clearly, competition is evident in low sorbing soils or when the sorption sites are limited by the high concentration of sorbing ions.

There was a small but consistent effect of Ca2+ compared with Na+ amount of AsIII sorbed in the presence of P with both soils studied (Fig. 3). This suggests that a different sorption mechanism is important for AsIII. As previously discussed, AsIII has been shown to be specifically sorbed by Fe oxide (goethite) surfaces (Sun and Doner, 1996; Manning et al., 1998). Therefore, the presence of Ca2+ had less of an effect on AsIII than AsV, as the charge present on the soil surface plays less importance on the sorption of AsIII compared with AsV.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
These investigations found that the presence of P in solution decreased the amount of AsV and AsIII sorbed by selected soils, although the effect of P varied depending on the sorption capacity of the soil. Phosphate competes with both As species for sorption sites on the soil surface and this is pronounced for soils with limited sorption sites, but where sorption sites are not limited, both As and P are strongly retained by the soils. However, increasing P in solution did not result in a corresponding decline in AsV sorbed and indicates that some oxides surfaces contain preferential sorption sites for AsV and P and others that are common to both. Similarly, Ca2+ increased AsV sorption by all soils studied, which was manifested through changes in the surface charge characteristics of the soils. The investigations also showed that P greatly decreased the sorption of AsIII on the soils studied, although the effect was less distinct in soils with high sorption capacity. The presence of Ca2+ had little effect on the amount of AsIII sorbed, indicating that specific sorption probably plays a major role for the sorption of AsIII.


   ACKNOWLEDGMENTS
 
The authors would like to acknowledge the Cooperative Research Center for Soil and Land Management and CSIRO Land and Water, who provided financial and laboratory support for this research.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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Antimony Impurity in Lead Arsenate Insecticide Enhances the Antimony Content of Old Orchard Soils
J. Environ. Qual., March 1, 2003; 32(2): 736 - 738.
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