competitive adsorption of oxalate and phosphate on

Clay Science 13, 181-188 (2008)

COMPETITIVE ADSORPTION OF OXALATE AND PHOSPHATE

ON ALLOPHANE AT LOW CONCENTRATION

MOHAMMED ABDALLA ELSHEIKH, ZAENAL ABIDIN, NAOTO MATSUE and TERUO HENMI

Laboratory of Applied Chemistry for Environmental Industry, Faculty of Agriculture, Ehime University,

3-5-7 Tarumi, Matsuyama 790-8566, Japan

(Received August 29, 2007. Accepted March 3, 2008)

ABSTRACT

The adsorption of an anion such as oxalate and phosphate at low concentration by soils is important be-

cause their contents in the soils are usually low. The adsorption of oxalate and phosphate on allophane at ini-

tial concentrations up to 200ƒÊM showed that the amount of adsorption of phosphate was higher than that of

oxalate, while the adsorption energy of oxalate estimated by the Freundlich parameter n was higher than that

of phosphate. The high adsorption energy for oxalate was also suggested by molecular orbital calculations ,where changes in the total energy of the adsorption reactions of oxalate were compared with that of phosphate .

The amount of adsorption of oxalate or phosphate increased with raising the concentration of the background

electrolyte (i.e. NaCl). This was ascribed to development of anion exchange site on allophane under the high

concentration of NaCl. The amount of adsorption of oxalate coexisting with 200ƒÊM of phosphate decreased

as compared to that of oxalate alone. However, the adsorption strength of oxalate estimated by the Langmuir

K and Freundlich n parameters was enhanced by the coexisting phosphate. For the adsorption of phosphate

in coexisting oxalate, oxalate also had the effect on the adsorption strength of phosphate. Especially for the

adsorption of oxalate, it was clearly observed that the amounts of Si and Al released from allophane with the

adsorption abruptly increased at the equilibrium oxalate concentration of about 100ƒÊM. This means that , atthe equilibrium concentration, the adsorption schema of oxalate changed from the ligand exchange reaction

between hydroxyl group and oxalate to the replacement reaction of silicate with oxalate. The critical equi-

librium concentration shifted to about 70ƒÊM with the addition of 200ƒÊM phosphate. This indicates that the

coexisting phosphate also consumed hydroxyls (Al-OH) leading oxalate to begin replacement with silicate

at lower equilibrium concentration. It is concluded that, at low concentration, coexisting phosphate affect not

only amount and strength but also mechanism of the adsorption of oxalate, and vice versa.

Key words: allophane, oxalate, phosphate, competitive adsorption, low concentration

INTRODUCTION

Phosphate is an essential nutrient element for plant growthand has great importance from agronomic and environmentalaspects; adsorption of phosphate by soils and soil constitu-ents has been the subject of extensive reviews (Parfitt, 1978;Guppy et al., 2005). This anion is easily adsorbed on allo-

phonic soils (Pardo and Guadalix, 1990; Beck et al., 1999)and very strongly adsorbed on the natural nano-ball allophane

(Johan et al., 1997). Phosphorus is less abundant in soils thannitrogen and potassium, and total phosphorus in surface soilsvaries from 1.6 to 48.3mmolkg-1, and total P content in a soilhas little or no relation to the availability of phosphorus to

plants (Brady and Weil, 2002).Low-molecular-weight organic acids such as formic, acetic,

oxalic, citric and malic acids are often produced in soils fromthe decomposition of organic matter, root exudates, and micro-

bial metabolites (Fox and Comerford, 1990; Xu et al., 2003).Among many biomolecules recognized in soils, oxalate playsan important role in many ecosystems, in both well-drainedand poorly drained soils (Vedy and Bruckert, 1982). For manyorganisms, oxalate is a metabolic end product (Cromack et al.,1979). Organic acids have capacity to complex metals in solu-tion, and the degree of complexation depends on: (a) numberand proximity of carboxyl and hydroxyl groups of the organicacid, (b) concentration of the organic acid, (c) type of the sur-face sites, and (d) pH and ionic strength of the soil solution.Oxalate anion is known to easily adsorb on the natural nano-ball allophane (Hanudin et al., 1999).

Allophane is a main component of volcanic ash soils and

podzols, and a series name of naturally occurring hydrousaluminum silicate with various chemical compositions (Wadaand Harward, 1974). Previously, the aluminum silicate hadnot been believed to have definite morphology and structure.

182 M.A. Elsheikh et al.

However, Kitagawa (1971) and Henmi and Wada (1976)

found that allophane, at least separated from volcanic ash

soils and weathered pumice grains, had definite hollow spheri-

cal morphology (nano-ball with external diameter between

3.5 and 5.0nm). The chemical composition varies from 0.5

to 1.0 with respect to Si/Al atomic ratio. The wall structure

of the nano-ball allophane has been proposed as aluminum-

nesosilicate structure composed of the curved gibbsite sheet

with monomeric SiO4 tetrahedra attached to it.

To investigate the competitive adsorption of oxalate and

phosphate on soil components is important to understand the

behavior of the biomolecules in soil environments and their

availability for plants. The study on the competitive adsorp-

tion between phosphate and organic ligands has received

attention from many researchers. For example, the competi-

tive adsorption was investigated onto a tropical soil (Lopez-

Hernandez et al., 1986), synthetic clay minerals and oxides

(Violante and Gianfreda, 1993, 1995), spodosols (Bhatti et al.,

1998), and acid soil (Hu et al., 2001). Jara et al. (2006) stud-

ied the competitive adsorption of phosphate with oxalate on

the synthetic and natural allophanic soil (allophanic material

content of 42%). A review (Guppy et al., 2005) on the com-

petitive adsorption between phosphate and the low-molecular-

weight organic acids in soils showed that the previous studies

were usually carried out at high initial concentrations (1-100

mM), and the resultant amounts of adsorption were much

higher than those found in soils. Therefore, objective of this

study was to investigate adsorption and competitive adsorp-

tion of oxalate and phosphate on the nano-ball allophane at

lower initial concentrations than 1mM. Because the adsorp-

tion experiments are conducted under such low oxalate and

phosphate concentrations, the effect of NaCl concentration

as a background electrolyte on the adsorption was also exam-

ined. Analysis of the adsorption mechanism was carried out

by using the recent information about detailed morphology

and chemical structure of the wall of the aluminum silicate

(Abidin et al., 2007). Figure 1a shows whole structure of the

allophane unit particle, and Fig. 1b represents the schematic

arrangement of atoms at the pore region of the particle, where

oxalate and phosphate are mainly adsorbed.

MATERIALS AND METHODS

Pumice grains containing the nano-ball allophane were col-

lected from a volcanic ash soil near Mt. Daisen in Tottori pre-

fecture, Japan. In order to obtain the pure nano-ball allophane

that is free from contaminants such as volcanic glass, opaline

silica and imogolite, only the inner portion of the pumice

grains was used (Henmi and Wada, 1976). The fraction with

diameter of less than 0.2ƒÊm was collected by centrifugation

after ultrasonification at 28kHz and dispersion at pH 4. The

collected sample was flocculated by saturated NaCl solution

and washed with water, then stored as suspension of pH 6 in

10mM NaCl. The prepared allophane was subjected to ƒ´-ray

diffractometry, infrared spectroscopy and thermal analysis,

and was proven to be free from the above-mentioned contami-

nants. The Si/Al ratio of allophane used was determined as

0.67 by the acid oxalate method (Higashi and Ikeda, 1974),

and content of the other metals except for Na was negligible.

a

b

FIG. 1. (a) Whole structure of allophane unit particle and (b) schematic

presentation of atomic arrangements at pore region of the particle.

Pore opening is 0.3 to 0.5nm.

The allophane used belongs to the low Si/Al type (Henmi and

Wada, 1976).

Adsorption experiments of oxalate and phosphate were car-

ried out with 25mL of the allophane suspension (containing

100mg of allophane on the basis of drying up allophane at

105•Ž in an oven) in a 250mL polypropylene centrifuge bot-

tle. Oxalate (Na2C2O4) or phosphate (NaH2PO4) solution was

Competitive adsorption of oxalate and phosphate on allophane at low concentration 183

mixed with NaCl solution and water, and pH was adjusted to

6.0 by HCl or NaOH solution: oxalate or phosphate concen-

tration of the prepared solution was 0 to 2.67mM, and the

concentration of NaCl was 13.3mM. The pH value of 6.0 was

selected because the zero point of salt effect on this allophane

was 6.0 (Johan et al., 1999). To the centrifuge bottle contain-

ing 25mL of the allophane suspension, 75mL of the prepared

solution was added to give initial oxalate or phosphate con-

centration of 0 to 2mM and NaCl concentration of 10mM.

The adsorption experiment of oxalate (or phosphate) at initial

concentrations of 0 to 200ƒÊM were conducted in the absence

or presence of 200ƒÊM of phosphate (or oxalate). The oxalate

or phosphate adsorption experiment at the low concentration (0

to 200ƒÊM) was also conducted with 0 and 100mM NaCl, in

addition to with 10mM NaCl.

The mixture was shaken for 24h, and then centrifuged

at 59000ms-2 for 15min. Supernatant was analyzed colori-

metrically for phosphate by the ascorbic molybdate method

(Murphy and Riley, 1962), by the KMnO4 titration method

for oxalate, and by atomic absorption spectrophotometer for

Si and Al. The oxalate did not interfere with phosphate de-

termination at the concentrations. The amounts of adsorption

of oxalate and phosphate were calculated from the difference

between initial and final concentrations.

An anion exchange capacity (AEC) of allophane used at

the equilibrium pH 6.0 was determined by adsorption of Na+

and Cl- to allophane to use 10 or 100mM of NaCl solution.

The allophane suspension was washed 3 times with 100mL

of 10 or 100mM NaCl solution at pH 6, and equilibrated with

the solution. After the final centrifugation, volume of the en-

trained NaCl solution was determined by mass measurement.

Then the residue was washed 3 times with 100mL of 1M

NH4NO3 solution to extract Cl in the system. AEC was calcu-

lated as the difference between Cl extracted and entrained.

Molecular orbital calculation was performed by Gauss-

ian03 Revision D.01 program with ONIOM method (Frisch

et al., 2004), where B3LYP/6-31G(d, p) and HF/3-21G basis

functions were applied for adsorption region and other region

of the allophane model cluster, respectively.

RESULTS AND DISCUSSION

Adsorption isotherms of oxalate and phosphate

Adsorption isotherms of oxalate and phosphate on allo-

phane at pH 6.0 and at initial concentrations from 200 to 2000

ƒÊM are shown in Fig. 2. The amount of adsorption of phos-

phate was always higher than that of oxalate; this is consis-

tent with the previous studies (Violante and Gianfreda, 1993;

Violante et al., 1996; Jara et al., 2006). The amount of adsorp-

tion of phosphate was more than 150ƒÊmolg-1 at about 20ƒÊM:

the concentration of phosphate on allophane is much higher

than that in soils (1.6 to 48.3ƒÊmolg-1: Brady and Weil, 2002).

Although the adsorption isotherms of phosphate and oxalate

have a gentle slope as shown in Fig.2, steep increases of the

amount of adsorption of phosphate and oxalate are expected

at the low equilibrium concentration. The strong interaction of

oxalate or phosphate with allophane is predicted by the sud-

den increase of the amount of adsorption of phosphate or oxa-

late at the low concentration. Oxalate (Hanudin et al., 1999)

FIG. 2. Oxalate and phosphate adsorption isotherms on allophane at

high initial concentration (200 to 2000ƒÊM).

and phosphate (Johan et al., 1997) adsorption behaviors were

examined by using allophane in the same region, but the low-

est initial concentrations of the two anions were 500ƒÊM for

oxalate and 200ƒÊM for phosphate.

The adsorption isotherms of oxalate and phosphate at initial

concentrations up to 200ƒÊM are shown in Fig. 3. Equilibrium

pH increased from 6.00 with raising the initial oxalate and

phosphate concentrations, and reached to 6.68 (oxalate) and

6.48 (phosphate) at the initial concentration of 200ƒÊM. The

adsorption isotherm of oxalate was a typical Langmuir type:

the amount of adsorption increased rapidly at low equilibrium

concentrations and became gradual at high concentrations (Fig.

3a). On the other hand, the amount of adsorption of phosphate

increased almost linearly in this experimental condition (Fig.

3b). Furthermore, the adsorption of phosphate started at equi-

librium concentration of about 8ƒÊM: this type of isotherm

was observed for a sample containing phosphate originally

(White and Taylor, 1977). However, the allophane used in this

study contained very small amount of P (5mgkg-1), and no

phosphate was detected in the supernatant of the blank run.

The adsorption isotherm of phosphate in Fig. 3b indicates that

at equilibrium concentrations of less than 8ƒÊM, no phosphate

is adsorbed on this allophane.

Effect of background NaCl concentration

It is considered that 10mM of NaCl as the background

electrolyte may affect the adsorption of phosphate or oxalate.

Especially, the background electrolyte might have some ef-

fects on adsorption of phosphate until 8ƒÊM of the equilibrium

concentration. To confirm this, adsorption experiments of

phosphate with 0 and 100mM NaCl solutions were con-

ducted, and the results are shown in Fig. 4b. The starting point

of the adsorption of phosphate shifted little from 8ƒÊM (Fig.

4b). These results indicate that the coexisting chloride anion

do not compete with the phosphate anion toward allophane.

Therefore the reason why phosphate did not adsorbed at equi-

librium phosphate concentration of less than 8ƒÊM remains

in question, and further study is necessary to clarify this phe-

nomenon.

Adsorption of phosphate was influenced unexpectedly by


(a)

(b)

FIG. 3. (a) Oxalate and (b) phosphate adsorption isotherms on allophane

at low concentration (up to 200ƒÊM) in the absence and presence of

200ƒÊM of the counterpart.

the background electrolyte. The adsorption of phosphate in-

creased with elevating the concentration of the background

electrolyte. That cause may be due to increase in the anion

exchange sites on allophane (A1-OH2+). It is known that cat-

ion and anion exchange capacities of allophane samples at a

constant pH increase with increasing ionic strength of the bulk

solution (Okamura and Wada, 1983). The AEC value of the

allophane at equilibrium pH of 6 was 100 and 170ƒÊmolg-1 in

10 and 100mM, respectively. Therefore, high concentration

of NaCl developed much anion exchange site, Al-OH2+Cl-

, and then phosphate anion was attracted toward the positivecharge of the Al-OH2+ to replace Cl-. The similar trend wasobserved for the adsorption of oxalate. However, the adsorp-tion of oxalate with 100mM NaCl was lower than that with10mM NaCl (Fig. 4a). This may be due to decrease in theactivity coefficient of divalent oxalate ion: the activity coef-

(a)

(b)

FIG. 4. (a) Oxalate and (b) phosphate adsorption isotherms on allophaneat different background NaCl concentrations.

ficient of oxalate ion was calculated as 0.67 in 10mM NaCl,

and 0.42 in 10mM NaCl, by using the extended Debye-

Huckel equation. These results are very important in soil

solution chemistry, indicating that high concentration of Cl-

and also probably NO3- in soils never prevents adsorption of

phosphate and low-molecular-weight organic anions. The high

concentrations of the monovalent anions, in fact, accelerate

adsorption of oxalate and phosphate on the variable charged

soil components.

Analysis with the Langmuir and Freundlich equations

The adsorption data of oxalate and phosphate in Fig. 3 are

plotted according to linear form of the Langmuir equation

below,

C/X=1/XmK+C/Xm (1)

where X=adsorption amount of oxalate or phosphate (ƒÊmol

g-1), K=a constant related to the binding energy (L ƒÊmol-1),

Xm=maximum adsorption of oxalate or phosphate (ƒÊmol


(a)(a)

(b)

(b)

FIG. 5. Langmuir plots for adsorption of (a) oxalate and (b) phosphate

in the absence and presence of 200ƒÊM of the counterpart.

FIG. 6. Freundlich plots for adsorption of (a) oxalate and (b) phosphate

in the absence and presence of 200ƒÊM of the counterpart.

g-1), and C=equilibrium concentration of oxalate or phos-

phate (ƒÊmol L-1). Adsorption of oxalate or phosphate was

plotted according to the equation (1) (Fig. 5). The adsorption

of oxalate was satisfied well with the equation (1) in Fig. 5a.

On the other hand, adsorption of phosphate did not correspond

to the equation (1) as shown in Fig. 5b. These suggested that

oxalate was adsorbed on surface aluminol groups of the al-

lophane sample very strongly, and the adsorption energy of

oxalate was assumed to be greater than that of phosphate.

This assumption was supported by fitting the adsorption

data in Fig. 3 to logarithm form of the Freundlich equation

below,

(2)

where X=adsorption amount of oxalate or phosphate (ƒÊmol g

-1), and C=equilibrium concentration of oxalate or phosphate

(ƒÊmol L-1) whereas k and n are empirical constants. The n is

related to the adsorption strength, and k to both the strength

and amount of adsorption. The Freundlich plots for adsorp-

tions of oxalate and phosphate are shown in Fig. 6, and in

both cases, data points fitted the linear form of the equation

(2). Table 1 summarizes the Langmuir parameters obtained

for the adsorption of oxalate, and the Freundlich parameters

for adsorptions of oxalate and phosphate. When we compared

the Freundlich parameters between adsorptions of oxalate and

phosphate, the parameter n related to the adsorption energy

of oxalate is higher than that of phosphate. The parameter k

related to both strength and amount of adsorption of oxalate is

also higher than that of phosphate.


Analysis by molecular orbital method

The adsorption strength of the two anions on aluminol

groups of the nano-ball allophane were also estimated by themolecular orbital method, where changes in the total energy of

the following adsorption reaction equations were calculated.

(3)(4)

In these adsorptions, we assumed a simple monodentate li-

gand exchange reaction between Al-OH of allophane and theanions. Change in the total energy is given by subtracting sum

of the total energy of the reactants from that of the products.

The calculated values were -360kJ mol-1 for adsorption of

oxalate and -330kJ mol-1 for adsorption of phosphate. That

means that the adsorption energy of oxalate was stronger than

that of phosphate.

Competitive adsorption of oxalate and phosphate

Results of the competitive adsorption experiments between

oxalate and phosphate on allophane used are plotted in Fig.

3. The adsorption of oxalate or phosphate was evidently de-

pressed by the competitor (i.e. phosphate or oxalate). When

the amount of adsorption of oxalate with initial concentration

of 200ƒÊM was plotted to coexisting phosphate concentra-

tions, the adsorption of oxalate decreased almost linearly with

increasing coexisting phosphate concentration. Similarly, in-

creasing coexisting oxalate linearly depressed the adsorption

of 200ƒÊM phosphate.

We adopted the competitive adsorption data of oxalate and

phosphate with coexisting with 200ƒÊM of the counterparts

to the Langmuir and Freundlich equation (Figs. 5 and 6). The

adsorption of phosphate with 200ƒÊM of oxalate did not fit to

linear form of the Langmuir equation as well as the adsorption

of phosphate without oxalate as the competitor (Fig. 5b). The

Langmuir parameters obtained for the adsorption of oxalate,

and the Freundlich parameters for adsorptions of oxalate and

phosphate are given in Table 1.

The calculated Langmuir Xm value for the adsorption of

oxalate decreased with the coexistence of 200ƒÊM phosphate,

from 115 to 83ƒÊmol g-1 (Table 1), and this corresponds to

about 32% reduction. On the other hand, with the addition of

phosphate, Langmuir K value for the adsorption of oxalate in-

creased from 0.09 to 0.13L ƒÊmol-1, indicating increasing ad-

sorption energy of oxalate onto allophane. Similar results were

obtained from the Freundlich parameters for the adsorption of

Table 1. Langmuir and Freundlich parameters for oxalate and phosphate

adsorption on allophane in the absence and presence of 200ƒÊM of the

counterpart

oxalate and phosphate in Table 1. The above described chang-

es in the Langmuir and Freundlich parameters indicate that

the addition of phosphate (or oxalate) decreases the amount

of adsorption of oxalate (or phosphate), though adsorption

strength of oxalate (or phosphate) increases by the coexisting

phosphate (or oxalate). The increase in the adsorption strength

may be due to simultaneous adsorption of the both anions

onto the surface of allophane. For example, when oxalate is

added to allophane, the oxalate begins to adsorb on accessible

sites of allophane, and next, on adsorption sites with less ac-

cessibility. The coexisting phosphate might impose a part of

the oxalate to adsorb on the less accessible site, and the result-

ing adsorption strength increased. Detailed discussion on the

above mentioned two kinds of adsorption site is made below.

Figure 7 shows the amounts of Si and Al released from

allophane after the adsorption of oxalate (or phosphate) with

and without coexisting 200ƒÊM phosphate (or oxalate). The

release of Si and Al increased with elevating adsorption of

oxalate and phosphate. When we compared the adsorption of

oxalate to that of phosphate, much Si and Al were released

for oxalate: oxalate has an ability to dissolve low crystallin-

ity silicate minerals such as allophane (Higashi and Ikeda,

1974). In the case of adsorption of oxalate without phosphate,

the release of Si or Al abruptly increased at around adsorp-

tion of 100ƒÊmol g-1 (Fig. 7). This indicates that, at the low

oxalate concentration, oxalate adsorbed on allophane mainly

by simple adsorption onto the aluminol group; oxalate might

be adsorbed by replacement of Si with increasing amount of

adsorption of oxalate, as suggested by Hanudin et al. (1999).

The release of Al may be due to dissolution as aluminum

oxalate complexes, and also due to induced effect of the Si

dissolution which partially destroyed the nano-ball allophane

structure (Abidin et al., 2005).

With the addition of 200ƒÊM of phosphate to the adsorption

of oxalate, the abrupt increase of the release of Si and Al oc-

curred at low adsorption as compared with no phosphate ad-

dition (70ƒÊM; Fig. 7). The adsorption mechanism of oxalate,

that is to say the replacement of Si with oxalate, started from

the low amount of adsorption of oxalate, because the alumi-

nol group as the adsorption site of oxalate was occupied by

phosphate. For the adsorption of phosphate with 200mM of

oxalate, the sudden release of Si was observed from about 120

mmol g-1 of adsorption (Fig. 7). In adsorption of oxalate (or

phosphate) including phosphate (or oxalate) as the competitor,

each of the competing anions will affect not only amount and

energy of adsorption of oxalate (or phosphate), but also the

mechanism of adsorption of oxalate (or phosphate).

CONCLUSIONS

At low initial concentrations up to 200ƒÊM, the amount

of adsorption of phosphate on the nano-ball allophane was

higher than that of oxalate, but the adsorption strength of

oxalate was higher than that of phosphate. High concentra-

tion of the monovalent anion such as chloride has shown to

enhance the adsorption of oxalate and phosphate, may be due

to forming of anion exchange site, Al-OH2+Cl-, on allophane.

The adsorption mechanism of the anions was firstly the li-

gand exchange with hydroxyl, and then would change to the


FIG. 7. Amount of (a) Si and (b) Al released from allophane with ad-

sorption of oxalate and phosphate in the absence and presence of

200ƒÊM of the counterpart

replacement reaction with Si. In binary system of the two an-

ions, coexistence of the counterpart depressed the amount of

adsorption, but enhanced the adsorption strength. It is notable

that the coexistence also affected the adsorption mechanism of

the counterpart.

REFERENCES

ABIDIN, Z., MATSUE, N. and HENMI, T. (2005) Molecular orbital analysison dissolution of nano-ball allophane under alkaline condition. ClaySci., 13, 1-6.

ABIDIN, Z., MATSUE, N. and HENMI, T. (2007) Differential formation ofallophane and imogolite: Experimental and molecular orbital study. J.Computer-Aided Materials Design, 14, 5-18.

BECK, M.A., ROBARGE, W.P. and BUOL, W. (1999) Phosphorus retentionand release of anions and organic carbon by two Andisols. Eur. ISoil. Sci., 50, 157-164.

BHATTI, J.S., COMERFORD, N.B. and JOHNSTON, C.T. (1998) Influence ofoxalate and soil organic matter on sorption and desorption of phos-phate onto a spodic horizon. Soil Sci. Soc. Am. J., 62, 1089-1095.

BRADY, N.C. and WEIL, R.R. (2002) The Nature and properties of soils.13th Ed. Prentice Hall. Upper Saddle River, New Jersey.

CROMACK, K., SOLLINS, P., GRAUSTEIN, W., SPEIDEL, K., TODD, A., SPYCHER,H., Li, C. and TODD, R. (1979) Calcium oxalate accumulation and soil

weathering in mats of the hypogeous fungus Hysterangium crassum.Soil Biol Biochem., 11, 463-469.

FOX, T.R. and COMERFORD, N.B. (1990) Low-molecular-weight organicacids in selected forest soils of the south eastern USA. Soil Sci. Soc.Am. J., 54, 1139-1144.

FRISCH, M.J., TRUCKS, G.W., SCHLEGEL, H.B., SCUSERIA, G.E., ROBB,M.A., CHEESEMAN, J.R., MONTGOMERY, JR. J.A., VREVEN, T., KUDIN,K.N., BURANT, J.C., MILLAM, J.M., IYENGAR, S.S., TOMASI, J., BARONE,V., MENNUCCI, B., COSSI, M., SCALMANI, G., REGA, N., PETERSSON,G.A., NAKATSUJI, H., HADA, M., EHARA, M., TOYOTA, K., FUKUDA, R.,HASEGAWA, J., ISHIDA, M., NAKAJIMA, T., HONDA, Y., KITAO, O., NAKAI,H., KLENE, M., LI, X., KNOX, J.E., HRATCHIAN, H.P., CROSS, J.B., BAK-KEN, V., ADAMO, C., JARAMILLO, J., GOMPERTS, R., STRATMANN, R.E.,YAZYEV, O., AUSTIN, A.J., CAMMI, R., POMELLI, C., OCHTERSKI, J.W.,AYALA, P.Y., MOROKUMA, K., VOTH, G.A., SALVADOR, P., DANNENBERG,J.J., ZAKRZEWSKI, V.G., DAPPRICH, S., DANIELS, A.D., STRAIN, M.C.,FARKAS, O., MALICK, D.K., RABUCK, A.D., RAGHAVACHARI, K., FORES-MAN, J.B., ORTIZ, J.V., CUI, Q., BABOUL, A.G., CLIFFORD, S., CIOSLOWS-KI, J., STEFANOV, B.B., LIU, G., LIASHENKO, A., PISKORZ, P., KOMAROMI,I., MARTIN, R. L., FOX, D. J., KEITH, T., AL-LAHAM, M.A., PENG, C.Y.,NANAYAKKARA, A., CHALLACOMBE, M., GILL, P.M.W., JOHNSON, B.,CHEN, W., WONG, M.W., GONZALEZ, C., POPLE, J.A., GAUSSIAN 03, re-vision C.01 Inc. Wallingford CT 2004.

GUPPY, C.N., MENZIES, N.W., MOODY, P.W. and BLAMEY, F.P.C. (2005)Competitive sorption reactions between phosphorus organic matter insoil: a review. Aust. J. Soil. Res., 43, 189-202.

HANUDIN, E., MATSUE, N. and HENMI, T. (1999) Adsorption of low mo-lecular weight organic acids on nano-ball allophane. Clay Sci., 11, 1,57-72.

HENMI, T. and WADA, K. (1976) Morphology and composition of allo-phane. Am. Miner., 61, 379-390.

HIGASHI, T. and IKEDA, H. (1974) Dissolution of allophane by acid oxa-late solution. Clay Sci., 4, 205-211.

HU, H.Q., HE, J.Z., LI, X.Y. and Liu, F. (2001) Effect of several organicacids on phosphate adsorption by variable charge soils of centralChina. Environ. International., 26, 353-358.

JARA, A.A., VIOLANTE, A., PIGNA, M. and MORA, M.L. (2006) Mutual in-teractions of sulfate, oxalate, citrate, and phosphate on synthetic andnatural allophones. Soil. Sci. Soc. Am. J., 70, 337-346.

JOHAN, E., MATSUE, N. and HENMI, T. (1997) Phosphate adsorption onnano-ball allophane and its molecular orbital analysis. Clay Sci., 10,259-270.

JOHAN, E., MATSUE, N. and HENMI, T. (1999) New concepts for change incharge characteristics of allophane with phosphate adsorption. ClaySci., 10, 457-468.

KITAGAWA, Y. (1971) The "unit particle" of allophane. Am. Miner., 56,465-475.

LOPEZ-HERNANDEZ, D., SIEGERT, G. and RODRIGUEZ, J.V. (1986) Competi-tive adsorption of phosphate with malate and oxalate by tropicalsoils. Soil Sci. Soc. Am. J., 50, 1460-1462.

MURPHY, J. and RILEY, J.P. (1962) A modified single solution method fordetermination of phosphate in natural waters. Anal. Chim. Acta., 27,31-36.

OKAMURA, Y. and WADA, K. (1983) Electric charge characteristics ofhorizons of Ando (B) and Red-Yellow B soils and weathered pumices.J. Soil Sci., 34, 287-295.

PARDO, M.T. and GUADALIX, M.E. (1990) Phosphate sorption in allophan-ic soils and release of sulphate, silicate and hydroxyl. J. Soil Sci., 41,607-612.

PARFITT, R.L. (1978) Anion adsorption by soil and soil materials. Adv.Agron., 30, 1-50.

VIOLANTE, A. and GIANFREDA, L. (1993) Competition in adsorption be-tween phosphate and oxalate on an aluminum hydroxide montmoril-lonite complex. Soil Sci. Soc. Am. J., 57, 1235-1241.

VIOLANTE, A. and GIANFREDA, L. (1995) Adsorption of phosphate on vari-able charge mineral: competitive effect of organic ligands. In: HuangP.M., Berthelin J. et al., editors. Environmental impact of soil compo-nent interactions vol.2. Boca Roton, Florida: CRC Press., 29-38.

VIOLANTE, A., RAO, M.A., DE CHIARA, A. and GIANFREDA, L. (1996) Sorp-tion of phosphate and oxalate by a synthetic aluminium hydroxysul-


phate complex. Eur. J. Soil. Sci., 47, 241-247.VEDY, J.C. and BRUCKERT, S. (1982) Soil solution: Composition and

pedogenic significance. 184-213. M. Bonneau and B. Souchier (ed.)Constituents and properties of soils, Academic Press, New York.

WADA, K. and HARWARD, M.E. (1974) Amorphous clay constituents insoils. Adv. Agron., 26, 211-260.

WHITE, R.E. and TAYLOR, A.W. (1977) Reaction of soluble phosphatewith acid soils: the interpretation of adsorption-desorption isotherms.J. Soil. Sci., 18, 314-328.

Xu, R., ZHAO, A. and JI, G. (2003) Effect of low-molecular weight or-

ganic anions on surface charge of variable charge soils. J. ColloidInterface Sci., 264, 322-326.

competitive adsorption of oxalate and phosphate on

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