Phosphate adsorption at variable charge soil/water interfaces as influenced by ionic strength.
Phosphorus (P) is an indispensable element for all living protoplasmic organisms and an essential nutrient for growth, but it also causes water eutrophication (Arai and Sparks 2007). The adsorption of phosphate by soils can affect the mobility and availability of P in soils and inhibit the loss of P from soils to surface water and ground water.
There are large areas of acidic, variable charge soils distributed in tropical and subtropical regions of southern China (Institute of Soil Science 1990), and also in other tropical and subtropical regions of the world. Because of intensive weathering and leaching, variable charge soils, enriched with Fe/Al oxides, carry both negative and positive surface charges (Yu 1997). Such soils show appreciable adsorption and fixation capacity for phosphate. Over the past few decades, the adsorption and desorption of P in soils has been studied extensively. Arai and Sparks (2007) presented a comprehensive review on the adsorption of phosphate on the surfaces of soils, clay minerals, and metal oxides and summarised the progress of investigation. The adsorption of phosphate on the surfaces of variable charge soils and minerals generally increases with decreases in solution pH (Arai and Sparks 2007). The adsorption of phosphate on acidic soils is characterised by an initial rapid reaction followed by a slower reaction (Barrow and Shaw 1975; van der Zee and van Riemsdijk 1988). It has been reported that the adsorption capacity of phosphate by these soils is closely related to the free Fe/Al oxides extracted by the dithionite-citrate-bicarbonate (DCB) method and amorphous Fe/A1 oxides extracted by acidic ammonium oxalate method. These studies clearly conclude that the Fe/Al oxides in soils play an important role in the process of phosphate adsorption (Mattson 1931; Lopez-Hernandez and Burnhan 1974; Parfitt 1979; Sei et al. 2002; Arai et al. 2005).
Both macroscopic and microscopic studies have shown that phosphate is mainly adsorbed on metal oxides through the formation of inner-sphere surface complexes (Atkinson et al. 1974; Parfitt and Atkinson 1976; Parfitt et al. 1976; Parfitt 1979; Arai and Sparks 2007). Electrophoretic mobility (EM) is a useful microscopic approach to indirectly distinguish bulk surface complexes at colloidal-water interfaces. The adsorption of phosphate on ferrihydrite, goethite, and boehmite lowered the isoelectric point (IEP) and suggested the formation of inner-sphere complexes (Hansmann and Anderson 1985; Tejedor-Tejedor and Anderson 1990; Arai and Sparks 2001). However, little information is available on the relationship between EM and phosphate adsorption in authentic soils.
The inner-sphere and outer-sphere complexes can be distinguished by studying the effect of ionic strength on anion partitioning (Hayes et al. 1988). It has reported that the formation of inner-sphere complexes is apparently unaffected by change in ionic strength, whereas the presence of outersphere complexes is likely to be influenced significantly by change in ionic strength because of competitive adsorption with counterions. However, it has been observed that a characteristic pH usually occurs above which adsorption of phosphate by goethite and soils increases with an increase in ionic strength and below which a reverse trend occurs (Barrow et al. 1980; Bolan et al. 1986). Further, an adsorption model was developed by Bowden et al. (1980) to describe the adsorption mechanism of phosphate, selenite, and citrate by goethite. This model is applied to explain the effect of ionic strength and pH on the adsorption of phosphate on goethite and soils (Barrow et al. 1980; Bolan et al. 1986). According to the model, the effect of ionic strength on adsorption operates through its effect on electrostatic potential in the plane of adsorption, rather than through its effect on surface charge (Barrow et al. 1980). But such complex model applications are generally not subject to direct experimental confirmation because they employ several fitting parameters that cannot be analytically measured (McBride 1997).
The objectives of this investigation are (1) to measure the effect of phosphate adsorption on zeta potential of colloids of variable charge soils; (2) to use the adsorption model developed by Bowden et al. (1980) to interpret the effect of ionic strength on phosphate adsorption by variable charge soils; (3) to test the hypothesis of the adsorption model with zeta potential measurements.
Materials and methods
Three kinds of soil samples, i.e. 20xisols collected from Kunming, Yunnan Province (102[degrees]43'E, 25[degrees]3'N), and Xuwen, Guangdong Province (110[degrees]10'E, 20[degrees]20'N), and 1 Ultisol collected from Jinxian, Jiangxi Province (116[degrees]17'E, 28[degrees]23'N), were used in the investigation. These soils were classified based on the US Soil Taxonomy. Both Oxisols were derived from basalt and the Ultisol was derived from Quaternary red earth. These soil samples, occupying large areas, are representative of major types of soils in subtropical regions of southern China. They had never been cultivated before and were covered by a mixed forest of Cinnamomum camphora, Pinus massoniana, and Glyptostrobus pensilis at the time of sampling. In these regions, the soils are very deep and the whole profile is homogeneous except for the surface layer, which has comparatively high organic matter content. The samples were thus taken below 0.4 m depth. Soil samples are air-dried and ground to pass a 0.25-mm sieve. Properties of these soil samples are given in Table 1.
The clay fraction <2 [micro]m in diameter was separated from the soil samples by pipette method, in which particles in the upper portion of the suspension were collected at a specific time after stirring (Pansu and Gautheyrou, 2006). The clay fraction was dried at room temperature and ground for use in zeta potential measurements after purification by electrodialysis under a voltage gradient of 10 V/cm between the 2 electrodes until the electrical conductance of the suspension did not decrease appreciably.
Zeta potential determination
The effect of phosphate adsorption on zeta potential of soil colloids was implemented by placing 0.050-g colloidal samples of the soils (<2 [micro]m in diameter) into 250-mL plastic bottles and adding 200mL of 0.1 mM Na[H.sub.2]P[O.sub.4] or 0.1 mM NaN[O.sub.3] (as control) solutions. The pH of these suspensions was adjusted to range from 3.0 to 7.0 by drop-wise addition of NaOH (5.0 M) or HN[O.sub.3] (6.0 M) solutions. These suspensions were dispersed ultrasonically at a frequency of 40 kHz and a power of 300 W for l h at 25 ([+ or -] 1)[degrees]C. After standing for 3 days, the zeta potential was measured using JS94G+ microelectrophoresis apparatus (Shanghai Zhongchen Digital Technique Equipment Ltd Co., Shanghai, China) and the values of zeta potential were calculated using a computer with specific software (Hou et al. 2007). The suspension pH was also checked.
Similarly, the effect of ionic strength on zeta potential of soil colloids was measured for soil colloidal suspensions containing 0.1 and 10mM NaN[O.sub.3] (or 10mM KN[O.sub.3]). The remaining experimental procedure was as mentioned above.
Effect of ionic strength on phosphate adsorption
The 1.0 mM Na[H.sub.2]P[O.sub.4] solution containing 0.0l M or 0.6M NaN[O.sub.3] as supporting electrolyte was prepared. The required solution pH was maintained by drop-wise addition of NaOH (5.0 M) or HN[O.sub.3] (6.0 M) solutions.
Samples of 0.50g soil in duplicate were weighed into centrifuge bottles, and 25 mL of Na[H.sub.2]P[O.sub.4] solution was added to each bottle. Suspensions were shaken in a constant-temperature water bath (25 [+ or -] 1[degrees]C) for ~2 h. After standing for another 22 h, the solution was separated from the solid phase by centrifugation at 3000G for 10min, followed by filtration using a 0.45-[micro]m micropore membrane filter. Phosphorus in solution was determined by ascorbic acid-N[H.sub.4]-molybdate blue colourimetric method. The amount of P adsorbed was calculated from the difference between the total amount added to, and the amount remaining in, the bulk solution. In order to compare the effect of NaN[O.sub.3] and KN[O.sub.3], 0.6 M KN[O.sub.3] was used as supporting electrolyte and the same experiment was performed to determine the phosphate adsorption in presence of KN[O.sub.3]. At the end of adsorption experiments, the pH values of equilibrium solutions were determined. All data are reported as a mean [+ or -] standard error of the replicates.
Point of zero salt effect of soils
Potentiometric titrations were carried out to obtain the soil point of zero salt effect (PZSE) (Pansu and Gautheyrou 2006). Exactly 2.0g of soil sample saturated with [Na.sup.+] was taken in a plastic beaker; 40mL of 0.001 M NaN[O.sub.3] was then added and the suspension was agitated continuously for ~5 min using a bar magnet followed by measurement of the suspension pH. An automatic titrimeter along with a combined electrode assembly was used to titrate the suspension with 0.1 M HN[O.sub.3] or NaOH at a regulated dose of HN[O.sub.3] or NaOH addition, i.e. 0.05 mL/2 min. The procedure was repeated with 0.005 M and 0.01M NaN[O.sub.3] solutions. The adsorption of [H.sup.+] or OH by soils was calculated and the intersection of [H.sup.+] and OH adsorption-pH curves at different ionic strengths was reported as the soil PZSE.
Point of zero net charge of soils
In order to obtain the point of zero net charge (PZNC), the negative and positive surface charges of the soils were determined by an ion-adsorption method (Qafoku et al. 2000) whereby 2.5-g air-dried soil samples were placed into each often 50-mL pre-labelled and pre-weighed centrifuge tubes. Twenty-five mL of 1 M KCl was added and tubes were shaken for 1 h followed by 5 washings with 25 mL of 0.01 M K[Cl.sup.-]. At the final washing, pH was adjusted by adding 1 M KOH or HCl and the centrifuge tubes were shaken for 4 h. The equilibrium solution was separated by centrifugation at 3000G for 10 min. The pH of the supernatant was recorded and [K.sup.+] and [Cl.sup.-] were determined using flame photometry and potentiometric titration, respectively. Then the tube with its content was weighed again. To replace the adsorbed [K.sup.+] and [Cl.sup.-] , 25 mL of 0.5 M N[H.sub.4]N[O.sub.3] was added and this step was repeated 5 times. The [K.sup.+] and [Cl.sup.-] in replaced solutions were determined by the same method for these ions in the equilibrium solutions. Then the amount of [K.sup.+] and [Cl.sup.-] adsorbed by soils was calculated based on the concentrations of [K.sup.+] and [Cl.sup.-] in the equilibrium solution and the replaced solution, and the volume of residue solution in the centrifuge tube before the replacing step. The adsorbed [K.sup.+] and [Cl.sup.-] represent soil negative and positive surface charge, respectively. The pH at which the quantities of the 2 adsorbed ion species are equal is the PZNC of the soil.
Results and discussion
Effect of phosphate adsorption on zeta potential
The zeta potential is an electrical potential at the shear plane of the electric double layer on colloid particles. Zeta potential-pH curves for the colloids of 3 variable charge soils in the presence and absence of phosphate are presented in Fig. 1. Results indicate that in the presence of phosphate, the zeta potential-pH curves shifted towards more negative values for all 3 soils. Figure 1 also implies that phosphate decreased the isoelectric point (1EP) of the soils. Quantitatively, the IEP value decreased from 4.3 to 3.3 for the Oxisol from Yunnan, from 3.8 to 3.1 for the Oxisol from Guangdong, and from 3.6 to <3.0 for the Ultisol from Jiangxi. These results are in agreement with those reported for Fe/Al oxides systems (Hansmann and Anderson 1985; Tejedor-Tejedor and Anderson 1990; Arai and Sparks 2001), where it was suggested the phosphate was adsorbed specifically by these variable charge minerals. Therefore, the phosphate was also adsorbed specifically by these variable charge soils and transferred some negative surface charge to soil surface, which ultimately decreased the zeta potential of soil colloids.
Phosphate caused greatest effect on zeta potential for the Oxisol from Yunnan, followed by the Oxisol from Guangdong, and then the Ultisol from Jiangxi. At pH 4.0, the decrease in the value of zeta potential was 23.2 mV for the Oxisol from Yunnan, 17.5 mV for the Oxisol from Guangdong and 13.8 mV for the Ultisol from Jiangxi. At pH 6.0, the corresponding values were 21.0, 12.5, and 5.5mV, respectively, for these soils. These trends are consistent with the content of free Fe/A1 oxides in these soils (Table 1). The specific adsorption of phosphate mainly occurred on the Fe/A1 oxides; hence, the highest content of free Fe/Al oxides, in the Oxisol from Yunnan, resulted in the greatest specific adsorption of phosphate on the soil and caused the maximum decrease in zeta potential of the soil colloid. On the other hand, the Ultisol, with the lowest free Fe/Al oxides content, gave the smallest decrease in zeta potential due to specific adsorption of phosphate on the soil.
Effect of ionic strength on phosphate adsorption by variable charge soils
Figure 2 shows the adsorption of phosphate by the 2 Oxisols at different ionic strength and pH. The adsorption of phosphate decreased sharply with increasing pH in the 0.01 M NaN[O.sub.3] system, whereas in the 0.6 M NaN[O.sub.3] system, it changed insignificantly for a similar change in pH (Fig. 2). The Oxisol from Yunnan adsorbed relatively more phosphate than the Oxisol from Guangdong. This may be due to the higher content of free iron oxides in the former, as mentioned above. These pH-dependent data obtained at 2 different ionic strengths intersected at pH 4.60 for the Oxisol from Guangdong and at pH 4.55 for the Oxisol from Yunnan. The adsorption of phosphate was independent of the ionic strength at this intersect pH. The intersect pH for both Oxisols was lower than the soil PZSE, but near the PZNC of the soils. The PZSE was 5.40 for the Oxisol from Guangdong and 5.20 for the Oxisol from Yunnan (Fig. 3). The PZNC for the Oxisol from Yunnan was 4.9. Above the characteristic pH (i.e. intersect pH), the adsorption of phosphate by the soils increased with increasing ionic strength, and below it the reverse trend occurred. The changing trends of phosphate adsorption by variable charge soils with ionic strength are similar to those obtained for phosphate by goethite and soils (Barrow et al. 1980; Bolan et al. 1986). A model was developed (Bowden et al. 1977; Bowden et al. 1980) to explain the adsorption mechanisms in terms of pH effect, and it had been applied to explain the adsorption behaviour on goethite for the phosphate, selenite, and citrate ions. The theoretical model was also used to interpret the mechanism involved in the adsorption of phosphate on soils and goethite as a function of ionic strength (Barrow et al. 1980; Bolan et al. 1986).
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The model developed by Bowden et al. (1980) is fully used to discuss the mechanism involved in the adsorption of phosphate by variable charge soils as a function of ionic strength in this paper. The outline of the model is presented as Fig. 4. According to the model, the specific adsorption of anions on the surfaces of variable charge soils takes place in a separate plane, 'a' in Fig. 4. Because these anions have a high binding constant, they coordinate to the surface and nearer the surface than electrolyte ions. The electrolyte ions can reach the boundary of surface, and plane 'a' is between plane's' and plane '[beta]' as shown in Fig. 4. The variation in the number of electrolyte ions in plane '[beta]' and in the diffuse layer (d) will affect the potential ([[psi].sub.a]) developed in the adsorption plane (plane 'a') and thus affect the adsorption of anions (Barrow et al. 1980). For example, when pH was higher than the PZNC of the Oxisols, the soils possessed net negative charge, and the surface potential and the potential in the adsorption plane were negative. The surface charge became more negative with an increase in ionic strength as shown in Fig. 3. Under this condition, the counter-ion in plane '[beta]' and the diffuse layer was a cation ([Na.sup.+]) and the number of cations per unit area increased with the decrease in distance to the soil particle surface and an increase in electrolyte concentration. Therefore, the increase in the number of cations in plane '[beta]' and the diffuse layer made the potential at the adsorption plane (plane 'a') less negative, thus allowing increases in the adsorption of phosphate by the soils. The increase in phosphate adsorption by the soil with increasing concentration of NaN[O.sub.3] (Fig. 2) was attributed to the change of the potential in the adsorption plane induced by the change of ionic strength.
[FIGURE 4 OMITTED]
When the pH was lower than the PZNC of the soils, they possessed a net positive charge, and the surface potential and the potential at the adsorption plane were also positive. The counter-ion in plane '[beta]' and the diffuse layer was an anion (N[O.sub.3.sup.-]). Under this condition, soil surface charge (positive value) increased with increasing ionic strength (Fig. 3), whereas the increase in the number of N[O.sub.3] ions in plane '[beta]' and the diffuse layer with increasing ionic strength caused the reduction of the potential at the adsorption plane (plane 'a'), thus decreasing the adsorption of phosphate by these soils.
The adsorption model developed by Bowden et al. (1980) can interpret the effect of ionic strength on phosphate adsorption by variable charge soils. But McBride (1997) pointed out that such complex model applications are generally not subject to direct experimental confirmation because they employ several fitting parameters that cannot be analytically measured. Here, we tried to test the hypothesis of the model with the help of zeta potential measurements. The zeta potential is an electrical potential at the shear plane of the electric double layer on colloid particles. Although the exact location of the shear plane in the electric double layer cannot be ascertained, it is generally considered to be located near the interface between the Stern layer and the diffuse layer (Yu 1997). In Fig. 4, the location of the shear plane is the boundary of surface as shown by dash line. Therefore, the shear plane is near the adsorption plane (plane 'a') in the model described previously. Hence, the changing trends of zeta potential with pH at different ionic strengths should be similar to that on the adsorption plane for these 2 soil samples. The results for the Oxisol from Yunnan were presented in Fig. 5. Theoretically, the zeta potential v. pH curves at different ionic strength should intersect at the zeta potential of 0mV, and the intersection pH is the isoelectric point (IEP) (Hunter 1981). However, the intersection obtained from experiments normally has a small shift from the potential of 0mV, even in pure Fe/AI oxides systems (He et al. 2008). In this paper, we set the intersection pH of zeta potential v. pH curves at 2 different ionic strengths as the IEP of the soil (Fig. 5); its value was 4.25. Figure 5 indicates that the zeta potential became less negative with an increase in ionic strength, when pH was above the IEP of the soil colloid, whereas it behaved oppositely when the pH was below the IEP of the soil colloid. The changing trend of soil zeta potential with pH and ionic strength was opposite to that of the soil surface charge with pH and ionic strength (Fig. 3), although the IEP was lower than PZNC and PZSE of the soil due to the use of soil clay fraction in zeta potential determination. These results are consistent with the prediction of effect of ionic strength on the potential in the adsorption plane by the model. The results of zeta potential values provide direct support for the hypothesis that the potential in the adsorption plane increased (became less negative) with increasing ionic strength when the pH was above the PZNC of a variable charge surface and followed the reverse trend with ionic strength when pH was below the PZNC of the variable charge surface. This is perhaps the first experimental proof to support the hypothesis that the potential in the adsorption plane changed with ionic strength with an opposite trend to the surface charge as earlier suggested by Barrow et al. (1980). Figure 5 also shows that the difference of zeta potential between 2 ionic strengths increased with increasing pH when pH was above the IEP of the soil. However, it decreased with increasing pH when pH was below the IEP of the soil. These changing trends of zeta potential values are also consistent with the effect of ionic strength on phosphate adsorption at different pH. Figure 2 clearly shows that the difference in phosphate adsorption between the 2 ionic strengths increased with increasing pH when pH was above the intersection pH of the soil.
[FIGURE 5 OMITTED]
Comparison of different cations on phosphate adsorption
Based on the hypothesis of the model, the electrolytes affect phosphate adsorption through its effect on electrostatic potential in the plane of adsorption. Hence, the adsorption of phosphate was not only related to ionic strength but also to the types of electrolytes present in solutions. Figure 6 shows that the phosphate adsorption by 2 Oxisols was significantly greater with KN[O.sub.3] than NaN[O.sub.3] at higher pH values and the same concentration of electrolyte, and the difference in phosphate adsorption between KN[O.sub.3] and NaN[O.sub.3] increased with increasing pH. It is well known that both [K.sup.+] and [Na.sup.+] are adsorbed by soils mainly through electrostatic attraction, and adsorption affinity of soils for [K.sup.+] is greater than for [Na.sup.+] (Yu 1997). Sodium, being the smaller cation, has a greater hydrated radius than [K.sup.+] and as such cannot approach a soil surface as closely as [K.sup.+], and so forms a more 'diffuse' layer than [K.sup.+]. Therefore, at the same concentration, more [K.sup.+] was distributed in the shear plane of the diffuse layer as a counter-cation on the soil colloid and this made the electric potential on the shear plane and in the adsorption plane less negative than with [Na.sup.+]. Hence, the soils adsorbed more phosphate in the KN[O.sub.3] system than in the NaN[O.sub.3] system. The difference in the effect of the 2 cations was attributed to the difference in their effect on electric potential on the adsorption plane of soil colloids. Figure 7 provided evidence for this interpretation; it is clear from Fig. 7 that the zeta potential of the colloid of the Oxisol from Yunnan in the 0.01 M KN[O.sub.3] system was less negative than that in 0.01 M NaN[O.sub.3] system. This suggests that more [K.sup.+] entered the shear plane of the electric double layer on colloid particles of the soil than did [Na.sup.+] and made the zeta potential less negative. At low pH, there was no appreciable difference in phosphate adsorption between the KN[O.sub.3] and NaN[O.sub.3] systems (Fig. 6). Under these conditions, the soils had positive charge on their surfaces and the counter ion N[O.sub.3.sup.-] was distributed in the diffuse double layer of soil colloids. Hence, electrolyte cations showed little effect on the adsorption of phosphate. These results provide further experimental support for the hypothesis of the adsorption model.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Batch experiments were carried out to assess the effect of phosphate adsorption on zeta potential of colloids of 3 variable charge soils and the effect of ionic strength on phosphate adsorption on the variable charge soils. The results obtained can be summarised as: (1) The adsorption of phosphate decreased the zeta potential of the colloids of variable charge soils and suggested that phosphate was adsorbed by these variable charge soils specifically. (2) A characteristic pH was obtained from the intersection of the phosphate adsorption-pH curves at different ionic strengths. Above this pH (intersect point), adsorption increased with increasing ionic strength and below this pH the reverse trend occurred. The characteristic pH was near the PZNC of the soils, at which there was apparently no effect of electrolyte concentration on the adsorption of phosphate by these soils. (3) The effects of ionic strength and pH on phosphate adsorption by these soils were interpreted with the help of the adsorption model developed by Bowden et al. (1980). The effect of ionic strength on the adsorption was closely related to the electrostatic potential induced at the adsorption plane. (4) The results of zeta potential values suggested that the potential in the adsorption plane changed with ionic strength as an opposite trend to surface charge of the soils. The phosphate adsorption by variable charge soils was related not only to the ionic strength but also to the types of electrolyte.
This study was supported by the Knowledge Innovation Program Foundation of the Chinese Academy of Sciences (KZCX2-YW-409) and National Natural Science Foundation of China (40701079). We thank 2 anonymous reviewers for their suggestions to improve the manuscript.
Manuscript received 7 August 2008, accepted 9 June 2009
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Yong Wang (A,B), Jun Jiang (A), Ren-kou Xu (A,D), and Diwakar Tiwari (C)
(A) State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing, China.
(B) School of Life Science and Technology, Henan Institute of Science and Technology, Xinxiang 453003, China.
(C) Department of Chemistry, Mizoram University, Tanhril Campus, Aizawl 796 009, India.
(D) Corresponding author. Email: firstname.lastname@example.org
Table 1. Properties of the tested soils Soil Depth OMA pH (B) (m) (g/kg) Oxisol from Yunnan 0.50-1.10 12.9 [+ or -] 0.3 5.48 Oxisol from Guangdong 0.40-0.70 13.8 [+ or -] 0.2 5.1 Ultisol from Jiangxi 0.80-1.30 4.4 [+ or -] 0.1 4.8 Soil CEC (c) [Fe.sub.2] ([cmol.sub.c]/kg) [0.sub.3.sup.D] (g/kg) Oxisol from Yunnan 1.9 [+ or -] 0.3 148.4 [+ or -] 0.7 Oxisol from Guangdong 8.0 [+ or -] 0.1 108.3 [+ or -] 0.6 Ultisol from Jiangxi 8.9 [+ or -] 0.1 51.1 [+ or -] 0.1 Soil Al.sub.2] [O.sub.3.sup.D] Dominant clay mineral (E) (g/kg) Oxisol from Yunnan 30.7 [+ or -] 0.6 K, G, H (V, Go) Oxisol from Guangdong 22.6 [+ or -] 0.5 K, G, H (Go), Ultisol from Jiangxi 11.5 [+ or -] 0.3 K, I (V) (A) Dichromate method. (B) Soil:water was 1:2.5. (C) Ammonium acetate method. (D) Dithionite-citrate-bicarbonate (DCB) method, (E) K, kaolinite; G, gibbsite; H, hematite; 1, hydrous mica; V, vermiculite; Go, goethitc (determined by X-ray diffractometry). (V, Go), (Go) and (V), represent only small amount of these minerals found in the soils.
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|Author:||Yong, Wang; Jun, Jiang; Ren-kou, Xu; Diwakar, Tiwari|
|Publication:||Australian Journal of Soil Research|
|Date:||Aug 1, 2009|
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