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Effect of arsenate on adsorption of Zn(II) by three variable charge soils.

Introduction

In terrestrial ecosystems, sorption-desorption reactions on soil colloid surfaces control the concentration of heavy metal in soil solution, and hence may affect their bioavailability, leaching, and toxicity (Backes et al. 1995). Arsenic and zinc contamination are common co-occurrences in many contaminated environments including mining areas, soils, and sediments (Kalbitz and Wennrich 1998; Martley et al. 2004; Liu et al. 2005; An et al. 2006). Hence, studies related to the interaction of multiple metal pollutants in soils would have greater practical importance to understand the fate of these pollutants in soils.

Anion-induced cation adsorption was reported for several cations and the possible mechanisms involved for the increased uptake of cation in the presence of anions was also described (Bolan et al. 1999). They included (i) precipitation (e.g. phosphate-induced Pb adsorption as lead phosphate); (ii) co-adsorption of anion and cation as an ion pair (e.g. ion-pair adsorption of sulfate and calcium); (iii) surface complex formation (e.g. cadmium and phosphate complex formation); and (iv) surface charge-induced cation adsorption (phosphate-induced cadmium adsorption). Co-adsorption of Zn(II) with oxyanions has also been investigated on soils, clays, and oxides (Agbenin 1998; Pardo 1999; Swedlund and Webster 2001; Grafe et al. 2004; Schwab et al. 2004; Adebowale et al. 2005; Grafe and Sparks 2005; Wang and Harrell 2005). Both phosphate and sulfate enhanced the adsorption of Zn(II) by acid soils and kaolin (Agbenin 1998; Pardo 1999; Adebowale et al. 2005; Wang and Harrell 2005). Adebowale et al. (2005) found that phosphate-modified kaolin showed maximum affinity for heavy metal cations, followed by sulfate-modified, and unmodified clay. Agbenin (1998) suggested that the strong affinity of P-treated soil for Zn(II) was probably a result of the formation of Zn-phosphate complexes on the soil surface and perhaps precipitation likely to take place at sufficiently large concentrations of P and Zn. Pardo (1999) suggested that the primary reason for Zn adsorption enhanced by phosphate was variations in soil-suspension pH and the concomitant changes in surface charge of variable charge soils; the second was that the adsorbed P increased Zn-specific adsorption on organic matter and/or hydrous oxides of the soils. The enhancement of Zn adsorption induced by citrate on Fe/Al oxides was also observed and the formation of Zn-citrate-oxide ternary surface complex was suggested as a main mechanism (Schwab et al. 2004). Grafe et al. (2004) investigated the co-sorption of arsenate and Zn(II) at the goethite-water interface at pH 4 and 7 with EXAFS spectroscopy. The results showed that Zn(II) sorption was greatly enhanced in the presence of arsenate. They suggested that the presence of arsenate possibly decreased the net surface positive charge, which favoured the sorption of Zn on goethite surface. Arsenate and Zn(II) sorbed on the goethite surface above site saturation resulted in the formation of an adamite-like surface precipitate at pH 7, while at pH 4, arsenate and Zn(II) existed as co-sorbed species on the goethite surface. Collins et al. (1999) investigated Cd(II) sorption in presence of some oxyanions on the goethite with EXAFS spectroscopy, and their study showed that the mechanism of Cd(II) sorption was solely due to electrostatic interaction in the presence of phosphate and sulfate.

Large areas of tropical and subtropical regions are widely distributed with variable charge soils. These soils usually carry both positive and negative charges on their surfaces, and therefore can adsorb both anions and cations (Yu 1997). Thus, co-sorption of heavy metals and oxyanions may also exist simultaneously in these variable charge soils. For example, the presence of Cr[O.sub.4.sup.2-] or arsenate increased the adsorption of Cu(II) and/or Cd(II) by the soils (Xu et al. 2004, 2005; Liang et al. 2007). Although the co-sorption of arsenate and Zn(II) was studied in detail at the goethite-water interface (Grafe et al. 2004; Grafe and Sparks 2005). In this context, a literature survey revealed that there is no information available related to the co-sorption of arsenate and Zn(II) in authentic, variable charge soils. The occurrence/contamination of these 2 ions, i.e. arsenic and zinc are very common in soils around mining areas in south of China (Liu et al. 2005; An et al. 2006). Therefore, the objective of this study is to evaluate the effect of arsenate on Zn(II) adsorption by 3 variable charge soils from the south of China and to discuss the mechanisms involved at soil solid/solution interface.

Materials and methods

Soils

Three variable charge subsoils, a Hyper-Rhodic Ferralsol (102[degrees]43'E, 25[degrees]3'N), a Rhodic Ferralsol (110[degrees]10'E, 20[degrees]20'N), and a Haplic Acrisol (116[degrees]17'E, 28[degrees]23'N) were collected, respectively, from Kunming, Yunnan Province, Xuwen, Guangdong Province, and Jinxian, Jiangxi Province, China. These soils were distributed widely in southern China, and were representative of major types of soils in these regions. The soil samples were air-dried and ground to pass a 60-mesh sieve. Selected properties of these soils were given in Table 1.

Adsorption/desorption experiments

A stock solution of 0.01 mol K[H.sub.2]As[O.sub.4]/L was made using reagent-grade K[H.sub.2]As[O.sub.4] followed by a series of K[H.sub.2]As[O.sub.4] solutions with various concentrations (0.5, 0.8, 1.0, 1.5, 2.0 mmol/L) prepared by the dilution method for the concentration dependence experiments and 1.0 mmol/L for other experiments. In addition the KN[O.sub.3] was used to make the constant background [K.sup.+] concentration. KN[O.sub.3] at 2.0 and 1.0 mmol/L was prepared for the control of the concentration dependence experiments and other experiments, respectively. Zn[(N[O.sub.3]).sub.2] at 0.01 or 0.1 mol/L was prepared using reagent-grade Zn[(N[O.sub.3]).sub.2] x 3[H.sub.2]O salt. The solution pH was adjusted by drop-wise addition of NaOH (5.0 mol/L) or HN[O.sub.3] (6.0 mol/L).

Samples of 1.000g soil in duplicate were weighed into centrifuge bottles, and the bottle and soil sample were weighed together as [W.sub.1] (g), then 25 mL of K[H.sub.2]As[O.sub.4] solution or KN[O.sub.3] solution (control) was added into each of the bottles. Suspensions were shaken in a constant-temperature water bath (25 [+ or -] 1[degrees]C) for 2 h. After standing for another 2 h, a small volume of Zn(II) solution with a concentration of 0.01 or 0.1 mol/L was added by pipette to make the initial concentrations of Zn(II) 0.1, 0.25, 0.5, 0.8, 1.0, 2.0 mmol/L for carrying out the concentration dependence study, and 1.0 mmol/L for remaining experiments. The suspensions were shaken for another 2 h. After standing overnight, the solution was separated from the solid phase by centrifugation at 3000g for 10 min, followed by further filtration through a 0.45-[micro]m pore membrane filter. The Zn(II) and As(V) in solution were determined by atomic absorption spectrophotometric method and ICP-AES method, respectively. The amount of Zn(II) or arsenate adsorbed was calculated from the difference between the total amount added and the amount remaining in solution. After the adsorption experiment, the pH values of equilibrium solutions were determined and the same values are reported in the figures and tables.

After the adsorption experiments, the bottle with soil and residual solution in it was then weighed as [W.sub.2] (g). In order to perform the desorption experiments, 25 mL of 1.0 mol KN[O.sub.3]/L solution was added into each of the bottles to replace the pre-adsorbed Zn(II). The suspension was then shaken for 1 h, and the solution was separated by centrifugation and further filtration as was performed for the adsorption experiment. The Zn(II) content in bulk solution was determined by atomic absorption spectrophotometric method as mentioned above. Given that the specific gravity of the residual solution after the adsorption experiments is equal to that for water at 25[degrees]C (1.0 g/mL), then [W.sub.2] - [W.sub.1] is equal to the volume of the residual solution (entrained solution) (mL). The amount of Zn(II) desorbed by KN[O.sub.3] was calculated with the following equation:

Zn[(II).sub.des] (mmol/kg) = {[[Zn(II)].sub.K] x (25 + [W.sub.2] - [W.sub.1]) -[[Zn(II)].sub.ad] x ([W.sub.2] - [W.sub.1])}/W (1)

where [[Zn(II)].sub.K] is the concentration of Zn(II) in the equilibrium solution after desorption (mmol/L), [[Zn(II)].sub.ad] is the concentration of Zn(II) in the equilibrium solution after adsorption (mmol/L), [W.sub.1] is the total weight of soil sample and bottle (g), [W.sub.2] is the total weight of soil sample and bottle together with the residual solution (g), and W is the mass of soil sample in adsorption experiments (1.0 g).

All data were reported as mean [+ or -] standard error of the replicates.

Zeta potential determination

Colloid samples (0.025 g) of 3 variable charge soils (<2 [micro]m in diameter) were placed in 250-mL plastic bottles, and 200 mL of 1.0 mmol K[H.sub.2]As[O.sub.4]/L solution or 1.0 mol KN[O.sub.3]/L solution (as control) was added to each of the bottles. The pH of suspensions was adjusted to a range from 3.0 to 6.5 with NaOH (5.0 mol/L) or HN[O.sub.3] (6.0 mol/L). Then the suspensions were dispersed ultrasonically at a frequency of 40 kHz and a power of 300 W for 1 h at 25 ([+ or -] 1) [degrees]C. After standing for 3 days, the zeta potential was determined with a JS94G + microelectrophoresis apparatus made in China and the values of zeta potential were calculated using the computer with the specific software (Hou et al. 2007). The suspension pH was also checked.

Results

Adsorption isotherms and desorption curves for Zn(II)

The pH values of these 3 variable charge soils used were 4.80, 5.10, and 5.48 (Table 1); hence, in order to compare the adsorption of Zn(II) at the same pH, the experiments for the adsorption isotherms were conducted at constant pH 5.2 for the 3 soils. The adsorption isotherms of Zn(II) and the desorption curves of pre-adsorbed Zn(II) are presented in Figs 1-3. The adsorption of Zn(II) was increased with the increase in its equilibrium concentration in soil solution. Moreover, the presence of arsenate induced the increase in Zn(II) adsorption. When these 3 variable charge soils were compared, the increment of Zn(II) adsorption induced by arsenate followed the order Hyper-Rhodic Ferralsol > Rhodic Ferralsol > Haplic Acrisol. For example, when the adsorption equilibrium concentration of Zn(II) was 0.2 mmol/L, the increment was 6.3, 5.6, and 5.2 mmol/kg for the Hyper-Rhodic Ferralsol, the Rhodic Ferralsol, and the Haplic Acrisol, respectively.

The changing patterns of the desorption of pre-adsorbed Zn(II) in the presence of arsenate were similar to those of the adsorption of Zn(II) (Figs 1-3). The presence of arsenate during Zn(II) adsorption led to an increase in the amount of Zn(II) desorbed.

Effect of arsenate on Zn(II) adsorption and desorption of pre-adsorbed Zn(II) at different pH

The Zn(II) adsorption by the 3 variable charge soils was highly pH-dependent (Figs 4-6). Almost all Zn(II) added was sorbed at higher pH when the initial concentration of Zn(II) was 1.0 mmol/L. The adsorption edge for a given pH range was shifted markedly to lower pH due to the presence of arsenate, indicating the substantial enhancement of Zn(II) adsorption by arsenate even at lower pH. This is evidence that arsenate adsorption increased the surface negative charge and consequently the electrostatic attraction of cation at the solid surface (Xu et al. 2004).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The desorption of pre-adsorbed Zn(II) in the presence of arsenate was greater than that in the control, and the increase in desorption due to the presence of arsenate during Zn(II) adsorption increased with the rise in pH (Figs 4-6). However, in both control and arsenate systems, the changing pattern of the desorption of pre-adsorbed Zn(II) with pH was found to be different from that for Zn(II) adsorption, which was increased with increasing pH. The desorption of pre-adsorbed Zn(II) in both the control and the presence of arsenate initially increased with the increase in pH and reached a maximum value followed by a gradual decrease with further increase in pH (Figs 4-6).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Effect of arsenate concentration on Zn(II) adsorption and desorption of pre-adsorbed Zn(II)

In order to study the adsorption mechanism of Zn(II), the effect of arsenate concentration on Zn(II) adsorption and desorption of pre-adsorbed Zn(II) in 3 variable charge soils was studied, and the results are presented in Table 2. It noted that both arsenate adsorption and the enhanced adsorption of Zn(II) and desorption of pre-adsorbed Zn(II) in the presence of arsenate in these 3 soils increased with the increase in arsenate dose. From the data on adsorption and desorption of Zn(II) in control and arsenate systems, the increment of adsorption ([DELTA] adsorption) and desorption ([DELTA] desorption) due to the presence of arsenate can be calculated (Table 2). The results indicate that Zn(II) adsorption in the Hyper-Rhodic Ferralsol was more affected by arsenate than that in the Rhodic Ferralsol, while adsorption was less affected by arsenate in the Haplic Acrisol than the Rhodic Ferralsol. This is consistent with the content of free iron oxides in these soils (Table 1). The presence of more iron oxides results in greater adsorption of arsenate in the soil and thus induces more Zn(II) adsorption onto the soil surface. In the case of the desorption of pre-adsorbed Zn(II), it changed in the same way as Zn(II) adsorption.

Discussion

Three possible mechanisms may be involved in the co-sorption of oxyanions and cations: enhancement by electrostatic interaction (e.g. Diaz-Barrientos et al. 1990); formation of a ternary cation-anion-surface complex (e.g. Lamy et al. 1991); formation of a surface precipitate (e.g. Hawke et al. 1989). However, the relative contribution of different mechanisms to a certain reaction system depends on the properties of adsorbent and adsorbate and reaction conditions. The results obtained in the present investigation suggested that the enhancement of electrostatic interaction between Zn(II) and variable charge soil surface was one of the main reasons for the increase in Zn(II) adsorption in the presence of arsenate, since the presence of arsenate increases not only Zn(II) adsorption on variable charge soils but also the desorption of the pre-adsorbed Zn(II) from the soils (Figs 1-6). If the adsorbed ions can be desorbed by the unbuffered salts such as KN[O.sub.3], they are termed electrostatically adsorbed ions (Xu et al. 2005). Therefore, the Zn(II) desorbed by KN[O.sub.3] was adsorbed through electrostatic attraction by the soils, and the proportion of [DELTA] desorption in [DELTA] adsorption in Table 2 roughly represented the relative contribution of electrostatic adsorption to the total increment of Zn(II) adsorption due to the presence of arsenate. It ranged from 67.0% to 93.5% in the Hyper-Rhodic Ferralsol, from 76.0% to 83.6% in the Rhodic Ferralsol, and from 53.7% to 84.3% in the Haplic Acrisol, which was similar with that in Cd(II)-arsenate-variable charge soil system (Liang et al. 2007). Therefore, the electrostatic interaction is perhaps the main mechanism for the enhancement of Zn(II) adsorption by arsenate in variable charge soils, which was also in agreement with the conclusions obtained from Zn(II)-arsenate-goethite system at low pH and Cd(II)-phosphate (or sulfate)-goethite system (Collins et al. 1999; Grafe et al. 2004).

It was reported that arsenate can be specifically adsorbed by variable charge soils and minerals (Fe/Al oxides) (Fendorf et al. 1997; Arai et al. 2001) and the specific adsorption of arsenate resulted in the increase in negative surface charge of variable charge soils (Xu et al. 2004), which further induced an increase in the electrostatic adsorption of cations. This phenomenon was also supported by the results of the effect of arsenate on the zeta potentials of soil suspensions, as shown in Figs 7-9. The presence of arsenate led to a decrease in the zeta potential values of soil suspensions, which suggested that arsenate adsorption reduced the surface potential of soil colloid so that the surface became more attractive for Zn(II). The decrement of zeta potential due to the presence of arsenate (the difference of zeta potential between the arsenate system and control) was estimated from Figs 7 to 9, and it was found to be 31, 33, and 35 mV, respectively at pH 4.5, 5.0, and 5.5 for the Hyper-Rhodic Ferralsol, whereas, it was found to be 29, 33, and 30 mV for the Rhodic Ferralsol, and 22.0, 28.0, and 25.5 mV for the Haplic Acrisol, respectively, for the same pH values. Thus, arsenate affects the zeta potentials in suspensions of 3 variable charge soils with the order Hyper-Rhodic Ferralsol > Rhodic Ferralsol > Haplic Acrisol, which is consistent with the amount of arsenate adsorbed by these soils (Table 2) and the content of free iron oxides in the soil samples (Table 1). Iron oxides are supposed to be the main constituent in the soils for the adsorption of anions (Yu 1997); therefore, the greater the content of free iron oxides in soil, the greater the amount of arsenate adsorbed by the soil, and hence, the greater the effect of arsenate on zeta potential and Zn(II) adsorption. Since the electrostatic adsorption of ion by soils is determined by electrostatic attraction force between ion and surface of soil colloid, it is to be believed that the change in surface charge (or surface potential) is the principal reason for the enhancement of electrostatic sorption of Zn(II) in the presence of arsenate. With the increase in arsenate added, the adsorption of arsenate by the soils increased; the greater the adsorption of arsenate, the more the enhancement of arsenate on Zn adsorption (Table 2). From the amount of arsenate adsorbed and [DELTA] adsorption of Zn(II) in Table 2, the moles of Zn(II) adsorption per unit mole of arsenate adsorption were calculated and the results were also listed in Table 2. The ratio followed the same order as the effect of arsenate on zeta potential: Hyper-Rhodic Ferralsol > Rhodic Ferralsol > Haplic Acrisol, except for Haplic Acrisol at higher initial concentration of arsenate. This further suggested the increase in surface charge due to arsenate adsorption resulted in the enhancement of Zn(II) adsorption by the soils. The higher ratio for Haplic Acrisol at higher initial concentration of arsenate was possibly due to the formation of precipitate of Zn(II) with arsenate.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The pH is an important factor influencing adsorption and desorption of heavy metals. With increasing pH, the negative surface charge on variable charge soils increases, which consequently induces more Zn(II) to be adsorbed by the soils through electrostatic attraction. On the other hand, [Zn.sup.2+] hydrolyses at higher pH to form ZnO[H.sup.+] and the affinity of ZnO[H.sup.+] for clay surface is much stronger than that of [Zn.sup.2+]. Thus, ZnO[H.sup.+] can be adsorbed more easily than [Zn.sup.2+]. It is possible to form surface precipitate of Zn(II) at higher pH, which also increased the apparent adsorption of Zn(II). These 3 factors are likely to be involved in the increase in Zn(II) adsorption with the increase in pH in control and arsenate systems (Figs 4-6). The adsorbed hydroxyl-Zn ions and the surface precipitate of Zn(II) are difficult to desorb by unbuffered salts such as KN[O.sub.3], and the proportion of adsorbed Zn-OH in total adsorbed Zn(II) increased with increasing pH. Therefore, the desorption of pre-adsorbed Zn(II) decreased at higher pH range (Figs 4-6).

The presence of arsenate increased the soil negative surface charge and decreased the surface potentials and IEP (isoelectric point); thus, the adsorption edge of Zn(II) was shifted towards the lower pH compared with the control. The effect of arsenate on Zn(II) adsorption also varied with the change in pH. The increments of the desorption of pre-adsorbed Zn(II) increased with the rise in pH in the 3 variable charge soils, which suggested that the rise in pH favoured the electrostatic interaction between Zn(II) and soil surface induced by arsenate. This may be explicable with the difference of zeta potential between control and arsenate systems, which increased with the rise of pH (Figs 7-9) and thus the dissociation of adsorbed arsenate may lead to more negatively charged surface on these soils at higher pH region.

Conclusion

The batch experiments were carried out to explain the mechanisms involved in the effect of arsenate on Zn(II) adsorption by 3 variable charge soils. It was noted: (1) variable charge soils have a great adsorption capacity for arsenate because the soils have large contents of iron and aluminium oxides; (2) the presence of arsenate induced the increase in the adsorption of Zn(II) in variable charge soils, and the extent of the effect was greatly dependent on the initial concentrations of arsenate and Zn(II), the system pH, and the nature of soils, particularly the content of iron oxides present in the soils; (3) the enhanced adsorption of Zn(II) in the presence of arsenate was mainly due to the increase in soil negative surface charge that occurred through the specific adsorption of arsenate on the soil and thus the increase in electrostatic attraction of soil surface for Zn(II).

Acknowledgments

The financial support from the National Natural Science Foundation of China (No. 20577054) and National Basic Research and Development Program of China (2002CB410808) is gratefully acknowledged.

Manuscript received 29 December 2006, accepted 7 August 2007

References

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Jing Liang (A,B), Ren-kou Xu (A,D), Diwakar Tiwari (C), and An-zhen Zhao (A)

(A) State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, PO Box 821, Nanjing, China.

(B) College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China.

(C) Department of Chemistry, Mizoram University, Tanhril Campus, Aizawl 796009, India.

(D) Corresponding author. Email: rkxu@issas.ac.cn
Table 1. Properties of the tested soils

Soil Depth OM (A)
 (m) (g/kg)

Hyper-Rhodic Ferralsol 0.50-1.10 12.9 [+ or -] 0.26
Rhodic Ferralsol 0.40-0.70 13.8 [+ or -] 0.19
Haplic Acrisol 0.80-1.30 4.4 [+ or -] 0.06

Soil pH (B) CEC (C)
 (cmol/kg)

Hyper-Rhodic Ferralsol 5.48 11.88 [+ or -] 0.26
Rhodic Ferralsol 5.10 8.02 [+ or -] 0.11
Haplic Acrisol 4.80 8.91 [+ or -] 0.13

Soil [Fe.sub.2][O.sub.3] Dominant
 (D) (g/kg) clay mineral (E)

Hyper-Rhodic Ferralsol 148.4 [+ or -] 0.72 K, G, H (V, Go)
Rhodic Ferralsol 108.3 [+ or -] 0.58 K, G, H (Go)
Haplic Acrisol 51.1 [+ or -] 0.07 K, I (V)

(A) Dichromate method.

(B) Soil : water 1:2.5.

(C) Ammonium acetate method.

(D) Dithionite-citrate-bicarbonate (DCB) method.

(E) K, Kaolinite; G, gibbsite; H, hematite; I, hydrous mica;
V, vermiculite; Go, goethite (determined by X-ray diffractometry).

Table 2. Effect of initial arsenate concentration on adsorption
and desorption of Zn(II)

The pH of equilibrium solution after adsorption was 5.2
and the initial concentration of Zn(II) was 1.0 mmol/L.
[DELTA] Adsorption is the difference in Zn(II) adsorption
between the arsenate system and control, and A Desorption
is the difference in the desorption of Zn(II) pre-adsorbed
between the arsenate system and control

Arsenate Arsenate
added adsorbed Adsorption
(mmol/kg) (mmol/kg) (mmol/kg)

 Hyper-Rhodic Ferralsol

0 -- 14.83([+ or -] 0.13)
12.5 12.49([+ or -] 0.01) 17.07([+ or -] 0.15)
20.0 19.94([+ or -] 0.00) 18.35([+ or -] 0.23)
37.5 36.34([+ or -] 0.10) 21.84([+ or -] 0.00)
50.0 44.53([+ or -] 0.12) 23.46([+ or -] 0.00)

 Rhodic Ferralsol

0 -- 12.59([+ or -] 0.41)
12.5 12.47([+ or -] 0.00) 14.64([+ or -] 0.22)
20.0 19.83([+ or -] 0.02) 15.70([+ or -] 0.29)
37.5 34.35([+ or -] 0.05) 18.71([+ or -] 0.12)
50.0 40.92([+ or -] 0.13) 19.82([+ or -] 0.08)

 Haplic Acrisol

0 -- 17.75([+ or -] 0.11)
12.5 12.29([+ or -] 0.01) 19.76([+ or -] 0.22)
20.0 18.62([+ or -] 0.00) 20.61([+ or -] 0.19)
37.5 26.71([+ or -] 0.09) 23.15([+ or -] 0.18)
50.0 28.96([+ or -] 0.01) 24.61([+ or -] 0.05)

 Zn(II)

Arsenate [DELTA] [DELTA]
added adsorption Adsorption/
(mmol/kg) (mmol/kg) arsenate ads.

 Hyper-Rhodic Ferralsol

0 -- --
12.5 2.24 0.18
20.0 3.52 0.18
37.5 7.01 0.19
50.0 8.63 0.19

 Rhodic Ferralsol

0 -- --
12.5 2.05 0.16
20.0 3.11 0.16
37.5 6.12 0.18
50.0 7.23 0.18

 Haplic Acrisol

0 -- --
12.5 2.01 0.16
20.0 2.86 0.15
37.5 5.40 0.20
50.0 6.86 0.24

 Zn(II)
 [DELTA]
 Desorption/
Arsenate [DELTA] [DELTA]
added Desorption Desorption Aadsorption
(mmol/kg) (mmol/kg) (mmol/kg) (%)

 Hyper-Rhodic Ferralsol

0 9.34([+ or -] 0.20) -- --
12.5 10.84([+ or -] 0.41) 1.50 67.0
20.0 12.50([+ or -] 0.15) 3.16 89.8
37.5 15.67([+ or -] 0.26) 6.33 90.3
50.0 17.40([+ or -] 0.16) 8.06 93.4

 Rhodic Ferralsol

0 7.73([+ or -] 0.35) -- --
12.5 9.45([+ or -] 0.06) 1.72 83.9
20.0 10.09([+ or -] 0.21) 2.36 75.9
37.5 12.39([+ or -] 0.12) 4.66 76.1
50.0 13.58([+ or -] 0.03) 5.85 80.9

 Haplic Acrisol

0 15.26([+ or -] 0.12) -- --
12.5 16.34([+ or -] 0.51) 1.08 53.7
20.0 17.67([+ or -] 0.17) 2.41 84.3
37.5 19.38([+ or -] 0.15) 4.12 76.3
50.0 20.18([+ or -] 0.07) 4.92 71.7
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Article Details
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Author:Liang, Jing; Xu, Ren-kou; Tiwari, Diwakar; Zhao, An-zehn
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Sep 1, 2007
Words:5277
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