Kinetics of phosphate adsorption on iron oxides formedunder the influence of citrate

C. Liu and P. M. Huang1

Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, Saskatchewan, CanadaS7N 5A8. Received 5 August 1999, accepted 11 April 2000.

Liu, C. and Huang, P. M. 2000. Kinetics of phosphate adsorption on iron oxides formed under the influence of citrate. Can.J. Soil Sci. 80: 445–454. The influence of organic acids on the formation of Fe oxyhydroxides and oxides has been intensivelystudied. However, scant attention has been paid to the subsequent effect on surface chemistry of the Fe oxides formed. The kinet-ics and mechanisms of phosphate adsorption by the Fe oxides formed in the presence of citrate ligands at initial citrate/Fe(II) molarratios (MR) of 0, 0.001, 0.01, and 0.1 were investigated using the conventional batch method. The adsorption studies were con-ducted at the initial phosphate concentration of 0.5 mM and pH 4.0 during the reaction period from 2 min to 56 h at 278, 288, 298,and 313 K. The results show that the phosphate adsorption followed multiple second-order kinetics and had two distinct rates ineach reaction system. The amount, rate coefficient, activation energy and pre-exponential factor of phosphate adsorption by the Feoxides formed greatly varied with their structural and surface properties. These properties, which included crystal structure, spe-cific surface area, surface porosity, surface geometry, and point of zero salt effect(PZSE), differed significantly with the initialcitrate/Fe(II) MR at which Fe oxides were formed. The results of this study have cast the light on the role of organic acids such ascitric acid in influencing the surface chemistry of naturally occurring Fe oxides through fundamental structural perturbation andthe impact on the dynamics of phosphate in terrestrial and aquatic environments.

Key words : Kinetics, activation energy, pre-exponential factor, phosphate, iron oxides, citric acid, structural perturbation

Liu, C. et Huang, P. M. 2000. Cinétique de l’adsorption des phosphates sur les oxydes de fer formés sous l’influence des cit-rates. Can. J. Soil Sci. 80: 445–454. Bien que l’influence des acides organiques sur la formation des oxyhydroxydes et des oxy-des de fer ait fait l’objet d’études intensives, on s’est peu intéressé à l’effet des oxydes ainsi formés sur la chimie du sol de surface.Nous étudions selon la méthode classique par fractionnement la cinétique et les mécanismes de l’adsorption des phosphates sur lesoxydes de Fe formés en présence de ligands de citrate, pour des rapports molaires (RM) citrates/Fe (II) de 0, 001, 0,01 et 0,1. Lesétudes d’adsorption étaient réalisées à la concentration initiale de phosphate de 0,5 mM au pH 4,0 durant des périodes allant de 2min à 56 h, à 278, 288, 298 et 313 K. Les résultats obtenus montrent que l’adsorption du phosphate suivait une cinétique multipledu second degré comportant deux taux distincts, dans chaque système de réaction. La quantité, le coefficient de taux, l’énergied’activation et le facteur pré-exponentiel d’adsorption des phosphates sur les oxydes de Fe formés variaient grandement selon lespropriétés structurales et les propriétés de surface des oxydes : structure cristalline, surface spécifique, porosité, géométrie ainsique le point effet zéro des sels (PZSE). Ces dernières propriétés différaient de façon significative selon le RM citrate/Fe (II) ini-tial auquel les oxydes avaient été formés. Nos observations jettent de la lumière sur l’influence des acides organiques, commel’acide citrique, sur la chimie de surface des oxydes de Fe naturels par voie de perturbations structurales fondamentales et sur l’im-pact de la cinétique des phosphates dans les milieux terrestres et les milieux aquatiques.

Mots clés : Cinétique, énergie d’activation, facteur pré-exponentiel, phosphate, oxyde de fer, acide citrique, perturbation structurale

The chemical reactions that occur in soils and aquatic envi-ronments are dynamic processes (Sparks 1998) and, there-fore, a kinetic study is essential to describe and predict theadsorption and desorption reactions occurring in naturalenvironments. Phosphorus is a major nutrient in soils(Tisdale et al. 1993) and a pollutant in aquatic environments(Bagotskii and Vavilin 1989); its fate in soils and associatedenvironments has been of interest to scientists concernedwith plant nutrition and environmental protection for a longtime. Phosphate adsorption by soils is affected by a numberof soil components, including Fe oxides (Borggaard 1983).Thus, close relationships between phosphate adsorption andthe total amount (dithionite-citrate-bicarbonate-extractable

Fe) and/or the noncrystalline fraction (oxalate-extractableFe) of Fe oxides in soils and sediments have frequently beendemonstrated (Ballard and Fiskell 1974; Schwertmann andSchieck 1980; Le Mare 1981; Borggaard 1983;Schwertmann and Taylor 1989).

The kinetics of phosphate adsorption by pure Fe oxideshas been intensively studied over the last two decades (Kuoand Lotse 1973; Parfitt and Atkinson 1976; Anderson et al.1985; Bolan et al. 1985; Hansmann and Anderson 1985;Madrid and de Arambarri 1985; Shang et al. 1993).However, pure Fe oxides seldom occur in natural environ-ments, since other ions present in the environment can influ-ence their formation (Krishnamurti and Huang 1990;Cornell and Schwertmann 1996). The presence of low-mol-ecular-weight organic acids such as citric acid during theformation of Fe oxides can greatly modify the surface prop-

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1To whom correspondence should be addressed.

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erties of the Fe oxides formed, such as surface structure, sur-face geometry, specific surface area, surface porosity, sur-face charge, and PZSE, through fundamental structuralmodifications (Liu and Huang 1999). Ainsworth andSumner (1985) found that Al substitution in goethite affect-ed the pseudo-first-order rate constant of phosphate adsorp-tion by the goethite at all pH levels and lower phosphateconcentrations. It is postulated that Al substitution may ster-ically hinder the sorption of phosphate. The isomorphoussubstitution of the smaller Al3+ ion (0.051-nm radius) forthe larger Fe3+ ion (0.064-nm radius) may cause aberrationsin the surface that would affect the adsorption process(Ainsworth and Sumner 1985). Two possible aberrationsthat could affect the adsorption process are the primary sur-face structure involving the symmetry and spacing of thesurface atoms and the secondary surface structure consistingof steps, edges, and other microtopographical features(Ainsworth and Sumner 1985). These changes in surfacegeometry appear to affect the activation energies, thestrengths of adsorption and desorption, and the rate of phos-phate adsorption onto the goethite. However, little is knownof the influence of surface properties resulting from funda-mental structural modifications of Fe oxides by the presenceof organic ligands during their formation on the kinetics ofadsorption of phosphate.

The kinetics of phosphate adsorption by soils or soil con-stituents is characteristically a rapid reaction followed by aslow reaction (Barrow et al. 1981; Ainsworth and Sumner1985). Kinetic models, that have been used to investigatephosphate adsorption by soil and soil materials are first-order and second-order kinetic equations, the two-constantrate equation, and the Elovich equation (Atkinson et al.1970; Kuo and Lotse 1972, 1973; Griffin and Jurinak 1974;Vig et al. 1979; Chien and Clayton 1980; Chien et al. 1980;Sharpley 1983; Shang et al. 1993). These equations wereused to calculate the rate coefficients of the reactions.Although the activation energies of phosphate adsorption onshort-range ordered Fe precipitates were reported (Shang etal. 1993), scant attention has been paid to the investigationof the pre-exponential factor in the Arrhenius equation inphosphate adsorption.

The objective of this study was to investigate the kineticsand mechanisms of phosphate adsorption by the Fe oxidesformed at citrate concentrations ranging from 10 µM to1000 µM. This concentration range of citrate is common insoil environments (Robert and Berthelin 1986).

MATERIALS AND METHODS

Preparation and Characteristics of Iron OxidesIron oxides were formed in a 0.01 M ferrous perchloratesolution at pH 6.00 ± 0.05 and 296.5 K at initial citrate/Fe(II) molar ratios (MRs) of 0, 0.001, 0.01, and 0.1. Thecitrate concentrations were 0, 10, 100, and 1000 µM, respec-tively. A 0.01 Mferrous iron perchlorate solution was madeby dissolving 7.257 g of Fe(ClO4)2·6H2O (Analar grade,Pflatz and Bauer, Waterbury, CT) in 2 L of deionized dis-tilled water in which N2 gas was constantly bubbled.Suitable amounts of the ligand (C6H8O7·H2O, Analar grade,

BDH Inc., Toronto, ON) were added into the solution toresult in the different initial citrate/Fe(II) MR values. ThepH of the solution was adjusted to 6.00 with 0.1 M NaOHusing a Metrohm titroprocessor (683 model, Metrohm Ltd.,Herisau, Switzerland) in a set-mode. When the pH of thesolution was stabilized at 6.00, the N2 bubbling wasreplaced by air with a flow rate of 40 mL min–1. The flowrate of air was controlled by a regulator. The air was passedthrough 1 M NaOH solution to remove the CO2. The oxida-tion of ferrous iron was terminated after 2.5 h. The suspen-sions were filtered through a membrane (pore size of 0.025µm) and washed with deionized distilled water until the con-ductivity of the filtrate was about 5 µS cm–1. Finally, theprecipitates were freeze-dried.

The Fe oxide minerals were examined by X-ray powderdiffraction (XRD) on a Rigaku diffractometer (RigakuCompany, Tokyo, Japan) with Fe-Kα radiation filtered by agraphite monochromator at 40 kV and 130 mA. The X-raydiffractograms were recorded from 4° to 90°2θ with0.01°2θ steps at 10°2θ per min and 2 s counting time.

The specific surface area of the Fe oxides was measuredby using a five-point BET N2 adsorption isotherm (Greggand Sing 1982) obtained with a Quantachrome Autosorb-1apparatus (Quantachrome Corp., Syosset, NY). Prior to N2adsorption, 100-mg samples were outgassed for 24 h at 10mTorr and room temperature (296.5 ± 0.5 K). During N2adsorption the solids were thermostated in liquid N2 (77–78K). The pore specific surface area of the Fe oxides was alsodetermined from the 93-point N2 adsorption isotherms usingthe t-plot method of de Boer and the average pore diameterwas estimated using the Kelvin equation (assuming cylin-drical pores) (Gregg and Sing 1982). The PZSE of the Feoxides was determined in 0.01, 0.1 and 1 M NaCl solutionsby the potentiometric method of Parks and de Bruyn (1962)as modified by Atkinson et al. (1967). The automatic titra-tion as described by Sakurai et al. (1989) used a Metrohmtitroprocessor (682 model, Metrohm Ltd., Herisau,Switzerland). The organic carbon content of the Fe oxideswas determined with a Leco CR12 C analyzer (Leco Corp.,St, Joseph, MI) (Wang and Anderson 1998).

Roughness of a surface is characterized by the fluctua-tions of the height of the surface relative to an appropriatereference plane (Gouyet et al. 1991). Several parameters areapplied in surface science to describe surface roughness ofan object. Mean surface roughness, which is the averagedeviation of nonplanar surface from its center plane, is oneof these parameters. When the material surfaces are irregu-lar and rough, the mathematical description of the topogra-phy can also be achieved by the fractal dimension. A fractalis an object that appears the same, regardless of the scale ofobservation (Anderson et al. 1998). The fractal dimension ofa material surface lies between 2 and 3. A surface fractaldimension 2 is for a planar surface and a surface fractaldimension 3 is for an extremely rough surface that is soirregular that it takes up the whole of three-dimensionalspace. The mean surface roughness and surface fractaldimension of the Fe oxides formed at varying citrate/Fe(II)MR values were determined based on a scanning area 100nm × 100 nm using atomic force microscopy (NanoScopeTM

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III, Digital Instruments, Santa Barbara, CA) as described byLiu and Huang (1999). After a height image was captured,the computer automatically recorded all the three-dimen-sional data of the surface. The mean surface roughness andsurface fractal dimension of the Fe oxides were respective-ly obtained by executing roughness and fractal command inoff-line of the NanoScopeTM III Software version 3.0(Digital Instruments, Inc. 1993) at the same fractal Z ratio of1. Since the scanning area (100 nm × 100 nm) is very small,the determination of the mean surface roughness and surfacefractal dimension were conducted in 15 replicates.

The basic properties of the Fe oxides are listed in Table 1.

Kinetics of Phosphate AdsorptionThe phosphate adsorption by the Fe oxides formed at theinitial citrate/Fe(II) MRs of 0, 0.001, 0.01 and 0.1 wasinvestigated by the conventional batch method. The Feoxides were dispersed as suspensions by ultrasonification(Sonifier, Model 350, Danbury, CT) at 150 W for 2 min. Analiquot of the suspension containing 50 mg of the Fe oxideswas transferred to a series of 125-mL flasks which con-tained 30 mL of deionized distilled water and then shakenovernight at 278, 288, 298, and 313 K. The pH of the sus-pensions was adjusted to 4.0 with 0.1 M HClO4 or NaOH.The total volume of the Fe oxide suspension after the pHadjustment was 35 mL. An aliquot of 5 mL of phosphatestock solution (pH 4.0) as NaH2PO4 (4 mM) containingNaClO4 (0.08 M) was added to each flask to adjust the con-centrations of phosphate and NaClO4 to 0.5 mM and 10mM, respectively, bringing the final volume of the mixedsuspension to 40 mL. The mixed suspension was shaken for0.033, 0.083, 0.25, 0.5, 1, 2, 4, 8, 12, 15, 24 and 56 h at 278,288, 298 and 313 K. The suspensions at the end of eachadsorption period were filtered through a 0.1 µm Milliporemembrane within 15 s. The concentration of phosphateremaining in the filtrates was determined by the molyb-dophosphoric blue colorimetric method (Jackson 1958). Theamount of phosphate adsorbed was determined by taking the

difference between the initial and final concentrations ofphosphate in solutions.

Statistical AnalysisThe whole experiment was carried out in duplicate. Two Feoxide samples formed at each citrate/Fe(II) molar ratio wereused for phosphate adsorption experiment and for the deter-mination of the surface properties. The statistical analysis inthe present study was carried out by calculating least signif-icant difference (LSD) values based on the standard error ofthe experimental data and the t values at 95% and 99% con-fident levels. The LSD value of the rate coefficient and thatof the activation energy were calculated based on the stan-dard error of the slope of the rate equation and that of theArrhenius equation, respectively. The LSD value of the pre-exponential factor was computed on the basis of the stan-dard error of the intercept of the Arrhenius equation. TheLSD values of the mean surface roughness and the surfacefractal dimension were calculated based on 15 replicates.

RESULTS AND DISCUSSION

Amount of P Adsorbed by the Fe OxidesThe amounts of phosphate adsorbed by the Fe oxides, whichhad been formed at the various initial citrate/Fe(II) MRs, atthe end of the 56-h reaction period at different temperaturesare presented in Table 2. The amounts of phosphateadsorbed by the different Fe oxides increased with increas-ing temperature, since there is higher energy for the bondbreaking and bond formation at a higher temperature.Except for the data obtained at 278 K, the amount of phos-phate adsorbed per unit weight of the Fe oxides initiallydecreased with an increase in the initial citrate/Fe(II) MRfrom 0 to 0.001 and then steadily increased with the increasein the MR to 0.01 and 0.1. The Fe oxides formed at the ini-tial citrate/Fe(II) MRs of 0.1 and 0.001 adsorbed the largestand smallest amounts of phosphate, respectively, at 288, 298and 313 K; the former had the largest specific surface andthe latter had the smallest specific surface (Table 1). The Fe

Table 1. Basic surface properties of the iron oxides formed at variousinitial citrate/Fe(II) MR

Initial citrate/Fe(II) MR

Property 0 0.001 0.01 0.1 LSD0.05z

Dominant mineraly G, M L L, nc. nc. —Specific surface area 135.2 102.8 189.3 209.6 1.6

(m2 g–1)Micropore area NDx ND 22.2 136.1 1.8

(m2 g–1)Mean surface roughness 1.36 1.01 1.94 3.68 0.36

(nm)Surface fractal dimension 2.17 2.15 2.19 2.24 0.02Organic C content 0.2 1.1 6.2 55.7 1.9

(g kg–1)PZSE 7.0 6.0 5.6 3.9 0.6zLeast significant difference at a 95% confident level.yG, goethite; M, maghemite; L, lepidocrocite; and nc, X-ray noncrystalline.xNot detectable.

Table 2. The amounts of phosphate adsorbed in the Fe oxide systemsat different temperatures at the end of a 56-h reaction period

Initial citrate/Fe(II) Temperature (K)

MR 278 288 298 313

(cmol kg–1)z

0 15.3 22.6 28.6 32.30.001 16.3 17.9 20.5 22.40.01 17.0 19.1 22.8 25.60.1 20.4 33.2 36.8 40.0

(µmol m–2)yx

0 1.13 1.67 2.12 2.390.001 1.58 1.74 1.99 2.180.01 0.90 1.01 1.20 1.350.1 0.97 1.58 1.76 1.91zLSD0.05 = 0.6 and LSD0.01 = 0.9 for phosphate adsorbed per unit weightof Fe oxides.yLSD0.05 = 0.07 and LSD0.01 = 0.09 for phosphate adsorbed per unit area.xbased on the specific surface area determined by N2-BET method.

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oxides formed at the MR of 0 adsorbed the lowest amountof phosphate at 278 K. The Fe oxides formed at the initialcitrate/Fe(II) MR of 0 were predominantly goethite andmaghemite, whereas the Fe oxides formed at the MRs of0.001, 0.01 and 0.1 were lepidocrocite and/or X-ray non-crystalline oxides. Goethite is a more stable phase of Feoxide than lepidocrocite (Krishnamurti and Huang 1993;Cornell and Schwertmann 1996). The surface –OH and/or–OH2 of goethite may be bonded to Fe more strongly thanthose of lepidocrocite (Benjamin and Leckie 1981; Hiemstraet al. 1989a,b). Goethite may need higher energy comparedwith lepidocrocite to adsorb phosphate by replacing the–OH and –OH2 groups. At the low temperature, there is notenough energy to break the same number of Fe–OH bondson the goethite surfaces as the number of Fe–OH bonds onthe lepidocrocite.

Although the Fe oxides formed at the initial citrate/Fe(II)MR of 0.01 had a larger specific surface area compared withthose formed in the absence of citrate, more phosphate wasadsorbed per unit weight of the latter oxides at 288, 298 and313 K, indicating that the citrate anion coprecipitated withthe Fe oxides may block some of the phosphate adsorptionsites. In the present study, citrate ligand was introduced tothe system when the Fe oxides were synthesized. After theFe oxides were formed, the phosphate adsorption experi-ment was carried out. Therefore, citrate ligands had alreadyoccupied some reactive sites on the Fe oxide surface beforephosphate was added into the Fe oxide systems. This phe-nomena is referred to as blocking effect. Further, citrate hasa relatively large molecular structure. The presence of cit-rate on the Fe oxide surface would block some reactive siteson the Fe oxide surface through steric effect and thusreduced the adsorption of phosphate. In addition, the copre-cipitation of citrate with Fe oxides during the formation ofFe oxides also changed the porosity of the Fe oxides.Especially when the micropore was formed at citrate/Fe(II)MRs of 0.01 and 0.1 (Table 1), the pore with the diameterless than that of phosphate ions would block the entry ofphosphate ions into these micropore surface. This interpre-tation is substantiated by the amount of citrate coprecipitat-ed with Fe (Table 1) and the amount of phosphate adsorbedper m2 of Fe oxides (Table 2). The amount of citrate copre-cipitated with the Fe oxides increased with increasing initialcitrate/Fe(II) MR. The phosphate adsorbed per unit surfacearea of the Fe oxides decreased when the initialcitrate/Fe(II) MR increased, except for the initialcitrate/Fe(II) MR of 0.1. At this MR, the Fe oxides formedadsorbed more phosphate than that formed at the initial cit-rate/Fe(II) MR of 0.01. Citrate at the initial citrate/Fe(II)MR of 0.1 inhibited the crystallization of Fe oxides throughits incorporation into the structural network of the Fe oxide,resulting in the formation of X-ray noncrystalline Fe oxides.The higher values of mean surface roughness and surfacefractal dimension of the noncrystalline oxides formed at theinitial citrate/Fe(II) MR of 0.1 (Table 1) indicate that theirsurface was much rougher compared with the Fe oxidesformed at the MRs of 0, 0.001, and 0.01. More edges andcorners exist on the rougher surface. Therefore, compared

with the faces, more Fe-OH and Fe-OH2 groups should beexposed per unit area of edges and corners. Consequently,the noncrystalline Fe oxides formed at the initialcitrate/Fe(II) MR of 0.1 would have more exposed Fe-OHand Fe-OH2 functional groups per unit area, which areaccessible to phosphate. Another exception was that thephosphate adsorbed per unit surface area of the Fe oxidesformed at the initial citrate/Fe(II) MR of 0.001 at 278 and288 K was higher than that at the initial citrate/Fe(II) MR of0 at 278 and 288 K. This is also attributed to the fact thatmore energy is required for the replacement of the –OH and–OH2 groups on goethite by phosphate compared with thoseon lepidocrocite.

Although the phosphate adsorbed per unit area of the Feoxides formed at the initial citrate/Fe(II) MR of 0.1 washigher than that at the initial citrate/Fe(II) MR of 0.01, it wasstill lower than those adsorbed at the initial citrate/Fe(II)MRs of 0 and 0.001 (Table 2), indicating that the citratecoprecipitated with Fe had a blocking effect. However, theamount of phosphate adsorbed per unit weight of the Feoxides formed at the initial citrate/Fe(II) MR of 0.1 was thehighest (Table 2), since these noncrystalline Fe oxides hadthe highest specific surface area (Table 1). The mechanismresponsible for increasing phosphate adsorption is the cit-rate-induced formation of surface area and reaction sites onthe Fe oxides. This mechanism is different from the simplecompetition of citrate with phosphate proposed by Earl et al.(1979) and Bowden et al. (1980) for the reaction sites. Theincrease in specific surface and reaction sites for phosphateadsorption evidently more than compensated for the block-ing of the phosphate adsorption sites by citric acid. In thestudies of Earl et al. (1979) and Bowden et al. (1980), bothcitrate and phosphate were added into the Fe oxide suspen-sion at the begining of the adsorption reaction. Citrate sim-ply competed with phosphate for the adsorption sites and,thus, consistently decreased the phosphate adsorption. In thepresent systems, citrate, which was present during the for-mation of Fe oxides, significantly modified the surfaceproperties of the Fe oxides through fundamental structuralchanges and the blocking of the reaction sites, and, thus,influenced phosphate adsorption. Kwong and Huang (1978)reported that citric acid also increases the ability of alu-minum hydroxides to adsorb phosphate through fundamen-tal structural modification.

Rates of Phosphate AdsorptionAfter 56-h reaction period, 51.2–92% of the phosphate ini-tially added at the beginning of the reaction was adsorbed at298 K (Fig. 1). The percent fraction of phosphate adsorbedat the end of a 24-h reaction period was very close to that ofphosphate adsorbed at the end of a 56-h reaction period.Furthermore, the processes of phosphate adsorption by theFe oxides were very fast. The 41.8–74.5% and 68.2–87.4%of phosphate adsorbed by the Fe oxides after 56-h reactionperiod had already been adsorbed in the first 2-min and 1-hreaction periods, respectively (Fig. 1). Generally, the con-centration of phosphate in solution decreased rapidly in the0.033–1 h reaction period and more slowly in the 1–24 hreaction period, indicating that phosphate adsorption con-

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sisted of a multiple rate process. The multiple-rate charac-teristic of phosphate adsorption is possibly related to theheterogeneity of adsorption sites. The different accessibilityof surface pores and multiple sorption sites with differentbinding strengths result in an apparent heterogeneity of thesurface (Benjamin and Leckie 1981; Madrid and deArambarri 1985; Hiemstra et al. 1989a,b) and would causedifferences in the adsorption rate. The phosphate adsorptionmay replace the positively charged OH2 groups through lig-and exchange and, thus, tends to lower the positive surfacepotential and increase negative charge (Parfitt and Atkinson1976). Therefore, the electrostatic repulsion becomesstronger as the adsorption proceeds (Hansmann andAnderson 1985; Shang et al. 1993).

The kinetic and empirical equations, including the zero-order, first-order, and second-order rate equations, theElovich equation, the modified Freundlich equation, and theoverall diffusion equation, were used to fit the phosphateadsorption data. The degree of fit of the rate equations to the

data was examined by using the determination coefficient (r2)and probability (P) of the linear regression analysis as well asthe standard error (SE). As an example, the statistical resultsfor the degree of fit of the rate equations to the phosphateadsorption by the Fe oxides formed at the initial citrate/Fe(II)MR of 0.1 at 298 K are presented in Table 3. The multiplesecond-order kinetic equation better describes the phosphateadsorption by the Fe oxides than the zero-order and 1st-ordermodels. The first reaction occurred between 0.033 and 1 hand the second reaction between 1 and 24 h. Although thephosphate adsorption also can be satisfactorily described bythe Elovich, modified Freundlich and overall diffusion equa-tions, the parameters derived by the Elovich and modifiedFreundlich equations are not well defined physicochemicallyand the equations may not provide the rate coefficients (Kuoand Lotse 1973; Bolan et al. 1985). The overall diffusionequation is often used to indicate that diffusion-controlledphenomena are rate-limiting and only “apparent” diffusioncoefficients can be obtained (Sparks 1999).

Fig. 1. The concentration of phosphateremaining in solution vs. time in the systemof Fe oxides formed at the initialcitrate/Fe(II) molar ratios (MRs) of (a) 0,(b) 0.001, (c) 0.01, and (d) 0.1. The stan-dard errors of all the experimental datawere less than 5%.

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The 2nd-order kinetic equation was chosen in this studyto estimate the rate coefficients of phosphate adsorption andthe temperature effect on the phosphate adsorption based onthe Arrhenius equation. The 2nd-order kinetic plots for thephosphate adsorption on the Fe oxides, which were formedat the initial citrate/Fe(II) MR of 0.1, at different tempera-tures are used as an example and illustrated in Fig. 2. Therate coefficients of phosphate adsorption by the Fe oxidesgenerally increased with increasing temperature (Table 4).The rate coefficients of the first reaction were about 4–19times those of the second reaction, except that the rate coef-ficient for the second reaction of phosphate adsorption bythe Fe oxides, which were formed at the initial citrate/Fe(II)MR of 0.1, at 313 K was 1.5 times that of the first reaction.The second reaction of phosphate adsorption by the Feoxides formed at the citrate/Fe(II) MR of 0.1 probablyoccurred predominantly on the micropore surface area and,thus, was more temperature dependent and required a longerinduction period. This phenomenon occurring in the systemof the Fe oxides formed at the citrate/Fe(II) MR of 0.1 dif-fers from that in the systems of Fe oxides formed at othercitrate/Fe(II) MRs. This is attributed to the differences in thesurface properties between the Fe oxide formed at the cit-rate/Fe(II) MR of 0.1 and those formed at the citrate/Fe(II)MRs of 0, 0.001, and 0.01. Especially, 65% of the surfacearea of the Fe oxides formed at the citrate/Fe(II) MR of 0.1was composed of micropore area (Table 1). The greater tem-perature-dependent and longer induction period were appar-ently the nature of adsorption reaction occurring on themicropore surface. The movement of phosphate ions intothe micropore would be facilitated at higher temperatures.Transport of phosphate ions to the micropore surface of theFe oxide would require more time compared with transportto the mesopore surface. This would account for the longerinduction for the phosphate adsorption on the Fe oxideformed at the MR of 0.1 compared with the adsorption inother Fe oxide systems. Therefore, compared with other Feoxide systems, the higher micropore surface area of the Feoxides formed at the citrate/Fe(II) MR of 0.1 would accountfor the greater rate coefficient. Generally, the rate coeffi-cient of phosphate adsorption by the Fe oxides formed at

various initial citrate/Fe(II) MRs followed the order of theinitial citrate/Fe(II) MR: 0.1 > 0 > 0.01 > 0.001.

The phosphate adsorption by the Fe oxides formed at theinitial citrate/Fe(II) MR of 0.1 had the highest rate coeffi-cients in the two reactions, which may be attributed to itshighest specific surface area among the oxides studied, theirX-ray noncrystalline nature (Table 1), and the greater num-ber of reaction sites per unit surface area as indicated bytheir highest mean surface roughness and surface fractaldimension. The rate coefficients of the first and second reac-tions of phosphate adsorption at 298 and 313 K on the Feoxides formed at the initial citrate/Fe(II) MR of 0 were high-er than those by the Fe oxides formed at the MR of 0.01.First, this is attributed to the blocking of some of the reac-tion sites by citrate and steric effects of citrate coprecipitat-ed with Fe in the Fe oxides formed at the MR of 0.01.Second, it is attributed to the higher PZSE of the Fe oxidesformed at the citrate/Fe(II) MR of 0 than at the MR of 0.01(Table 1). Therefore, the Fe oxides formed at the initial cit-rate/Fe(II) MR of 0 would have more net positive chargesthan those formed at the initial citrate/Fe(II) MR of 0.01 atthe same pH value.

Compared with the Fe oxides formed at the MR of 0.01,the Fe oxides formed at the MR of 0 should more stronglyattract the phosphate anions and account for the increasedrate coefficients at higher temperatures. However, theincreased rate coefficient of phosphate adsorption by the Feoxides formed at the citrate/Fe(II) MR of 0 caused by theelectrostatic attraction was not significantly pronounced atlower temperatures. As discussed above, goethite andmaghemite were predominantly formed at the initial cit-rate/Fe(II) MR of 0, whereas poorly crystalline lepi-docrocite with some X-ray noncrystalline Fe oxides wereformed at the initial citrate/Fe(II) MR of 0.01. Sincegoethite is more stable than lepidocrocite (Krishnamurti andHuang 1993; Cornell and Schwertmann 1996), the rate coef-ficient of phosphate adsorption by the Fe oxides formed atthe initial citrate/Fe(II) MR of 0 (mainly goethite andmaghemite) would be more temperature dependent than thatof phosphate adsorption by the Fe oxides formed at the cit-rate/Fe(II) MR of 0.01. Thus, at low temperatures, the elec-

Table 3.The comparison of the degree of fit of the kinetic models to the data of phosphate adsorption by the Fe oxidesz

r2 P SE (cmol kg–1)

Model fy sx f s f s

Zero-order 0.76 0.74 5.39 × 10–2 2.70 × 10–2 2.78 2.11First-order 0.80 0.88 4.22 × 10–2 1.71 × 10–3 2.27 3.90Second-order 0.83 0.97 3.15 × 10–2 6.71 × 10–5 1.08 1.40Overall 0.90 0.87 1.45 × 10–2 2.07 × 10–3 1.66 1.48diffusionElovich 0.96 0.97 4.15 × 10–3 6.41 × 10–5 1.10 0.74Freundlich 0.95 0.92 4.59 × 10–3 2.04 × 10–4 1.15 0.91(Modified)zThe Fe oxide was formed at the initial citrate/Fe(II) MR of 0.1; the kinetic experiment was conducted at 298 K; r2 is the determination coefficient; P is theprobability; SE is the standard error, which was calculated by the difference between measured amount of P adsorbed A and calculated amount of P adsorbedA* based on the equation SE = [∑(A – A*)2 / (n – 2)]1/2.yThe first reaction.xThe second reaction.

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trostatic attraction to increase the rate of phosphate adsorp-tion by the Fe oxides formed at the initial citrate/Fe(II) MRof 0 would be masked by the greater temperature depen-dence of the rate coefficient of phosphate adsorption. Thephosphate adsorption by the Fe oxides formed at the MR of0.001 had the lowest rate coefficients (Table 4), which isattributed to their lowest specific surface among the Feoxides studied (Table 1). Therefore, the specific surfacearea, surface porosity, surface geometry described by themean surface roughness and surface fractal dimension, sur-face charge and the coprecipitated citrate ligands togetherinfluenced the rate coefficient of phosphate adsorption. Thecoprecipitation of citrate ligands affected the rate coefficientof phosphate adsorption by distorting the structure of Feoxides formed and modifying their surface geometry andsurface charge.

Activation Energy and Pre-exponential FactorTo further understand the effect of temperature on phos-phate adsorption on the Fe oxides, the Arrhenius equationwas used to calculate the activation energy (Ea) as well asthe pre-exponential factor:

k = A e–Ea/RT

where k is the rate coefficient, A is the pre-exponential fac-tor (frequency factor), Ea is the Arrhenius activation energy,R is the universal molal gas constant (8.314 J K–1 mol–1), andT is the absolute temperature. When ln k is plotted versus

Fig. 2. The second order kineticplottings of phosphate adsorptionon the Fe oxide formed at the ini-tial citrate/Fe(II) molar ratio(MR) of 0.1 at (a) 278, (b) 288,(c) 298, and (d) 313K.

Table 4. The rate coefficient of the 2nd-order kinetic model ofphosphate adsorption by the Fe oxides at different temperatures

Initial citrate/Fe(II) Temperature (K)

MR 278 288 298 313(mol–1 h–1)

First reactionz

0 245ab 484cd 1052e 1613f0.001 142a 242ab 290bc 495d0.01 190a 323bc 515d 599d0.1 634d 1185ef 2216g 3215h

Second reactiony

0 13a 25bc 62e 131f0.001 11a 18ab 19abc 31bcd0.01 15ab 20abc 38cde 59e0.1 42de 176f 600g 4706hzThe values followed by the same letter are not significantly different at95% confidence level for the first reaction for both columns and rows.yThe values followed by the same letter are not significantly different at95% confidence level for the second reaction for both columns and rows.

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1/T, the activation energy and the pre-exponential factor canbe obtained based on the slope and intercept, respectively.

The activation energy for phosphate adsorption by the Feoxides ranged from 19 to 97 kJ mol–1 phosphate adsorbed(Table 5). The activation energy of a diffusion-controlledprocess in solution is about 25 kJ mol–1 (Sparks 1986).However, in heterogeneous systems such as mineral-waterinterfaces, diffusion occurs not only in the bulk solution butalso in micropores and macropores, in the films around solidparticles, on the solid surface, and inside solid particles(Sparks 1989). Therefore, the activation energy for the dif-fusion processes in heterogeneous systems is higher thanthat in solutions. Film diffusion typically has an energy ofactivation of 17–21 kJ mol–1 and intraparticle diffusion hasEa values of 21–42 kJ mol–1 (Sparks 1986). Thus, low Eavalues (< 42 kJ mol–1) indicate diffusion-controlled process-es whereas higher Ea values (> 42 kJ mol–1) indicate chem-ically controlled processes (Sparks 1989).

The data show that the rate-limiting step of most of thephosphate adsorption reactions by the Fe oxides was a dif-fusion process, except for the second reaction of phosphateadsorption by the Fe oxides formed at the initial MR of 0.1as indicated by the activation energy of 97 kJ mol–1, wherethe rate-limiting process was evidently a chemical process,which may involve ligand exchange of citrate by phosphate.Further, the Fe oxides formed at the initial citrate/Fe(II) MRof 0.1 had the lowest PZSE values and, thus, the least netpositive charges at pH of 4.0 (Liu and Huang 1999), and thelargest micropore surface areas (Table 1). The negativecharges on the Fe oxides would result in an electrostaticrepulsion of phosphate anions and make it difficult for phos-phate to approach the Fe oxide surface. Due to steric effects,micropores would slow down the diffusion of phosphateions to the Fe oxide surface compared with mesopores.However, the activation energy for the first reaction of phos-phate adsorption on the Fe oxides formed at the MR of 0.1was not significantly different from those for the first reac-tion of phosphate adsorption on the Fe oxides formed at theMRs of 0, 0.001 and 0.01. The first reaction of phosphateadsorption by the Fe oxides formed at the citrate/Fe(II) MRof 0.1 could predominantly occur through ligand exchangereactions with –OH2/–OH groups rather than with citrateligands. The second reaction of phosphate adsorption by theFe oxides formed at the initial MR of 0 had a higher activa-tion energy than those formed at the initial MRs of 0.001and 0.01 (Table 5). The Fe oxides formed at the MR of 0were composed of goethite and maghemite whereas the Fe

oxides formed at the MRs of 0.001 and 0.01 were predomi-nantly composed of lepidocrocite (Table 1). As discussedbefore, goethite may require more energy to adsorb phos-phate by replacing the –OH and –OH2 groups comparedwith lepidocrocite.

Although the second reaction of phosphate adsorption bythe Fe oxides formed at an initial MR of 0.1 had the highestactivation energy (Table 5), it had the highest rate coeffi-cient for phosphate adsorption (Table 4). This is evidentlydue to its highest pre-exponential factor for phosphateadsorption among the Fe oxides (Table 5). The pre-expo-nential factor values for phosphate adsorption differed by 14orders of magnitude for the different Fe oxide systems stud-ied. Compared with the Fe oxides formed at the initialcitrate/Fe(II) MRs of 0.001 and 0.01, the higher pre-expo-nential factor values for the second reaction of phosphateadsorption by the Fe oxides formed at the initialcitrate/Fe(II) MRs of 0 and 0.1 accounted for their higherrate coefficients despite their higher activation energy val-ues. Therefore, both the activation energy and pre-exponen-tial factor must be considered in interpreting the rates ofphosphate adsorption on the Fe oxides.

The pre-exponential factor is a measurement of the fre-quency at which reactant molecules collide with each other.In the phosphate adsorption by the Fe oxides formed at var-ious citrate/Fe(II) MRs, the pre-exponential factor is, thus, ameasurement of the accessibility of phosphate ions to thesurface of the Fe oxides. The specific surface area and thesurface site density govern the total number of reaction sites.Micropores with a diameter less than the hydrated diameterof phosphate ions (0.8 nm) and the blocking of surface sitesby citrate ligands would reduce the number of effectivereaction sites. The surface geometry, surface porosity andcitrate coprecipitated with Fe may have a steric effect on thecollision between reactant molecules. Therefore, the pre-exponential factor should be related to the surface area, sur-face site density, surface geometry, surface porosity, and theblocking of surface sites caused by the coprecipitated citrateions. In the present study, only four Fe oxide samples wereused to investigate phosphate adsorption and it is difficult toestablish the mathematical relationships between the pre-exponential factor and the above-mentioned surface proper-ties. This relationship is worth studying in the future.Although the Fe oxides formed at the citrate/Fe(II) MR of0.1 had a high micropore volume and a significant amountof coprecipitated citrate (Table 1), they had the highest pre-exponential factor (Table 5). This is attributed to their high-

Table 5. The activation energy and pre-exponential factor of phosphate adsorption by the Fe oxides formed at various initial citrate/Fe(II) MR

Initial Activation energyz Pre-exponential factory

citrate/Fe(II) (kJ mol–1) (mol–1 h–1)

molar ratio First reaction Second reaction First reaction Second reaction

0 40 49 8.2 × 109 2.4 × 1010

0.001 25 19 6.5 × 106 5.5 × 105

0.01 24 30 6.9 × 106 6.4 × 106

0.1 34 97 1.7 × 109 6.1 × 1019

zLSD0.05 = 18 and LSD0.01 = 26 for activation energy.yLSD0.05 = 5.2 × 104 and LSD0.01 = 7.5 × 104 for pre-exponential factor.

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est specific surface area (Table 1) that was accessible tophosphate. Further, these Fe oxides were X-ray noncrys-talline (Table 1) and thus should have a higher surface sitedensity compared with crystalline Fe oxides. The Fe oxidesformed at the citrate/Fe(II) MR of 0.001 had the lowest spe-cific surface (Table 1), which accounted for the lowest pre-exponential factor (Table 5). Although the specific surfacearea of Fe oxides formed at the citrate/Fe(II) MR of 0.01was higher than that of Fe oxides formed at the MR of 0(Table 1), the latter had the higher pre-exponential factor(Table 5). This is attributed to the development of microp-ores and the coprecipitation of a significant amount of cit-rate ligands in the former (Table 1). Therefore, somereaction sites on the Fe oxide surface were blocked by thecitrate ligands and the effective reaction sites, thus,decreased.

CONCLUSIONSThe data for phosphate adsorption by the Fe oxides, whichwere formed at the initial citrate/Fe(II) MRs of 0, 0.001,0.01, and 0.1, show that the phosphate adsorption followedmultiple second-order kinetics. The amount, rate coeffi-cient, activation energy and pre-exponential factor for phos-phate adsorption by the Fe oxides were influenced by thesurface properties of the Fe oxides such as specific surfacearea, surface charge, surface porosity and surface geometry,which varied significantly with the initial citrate/Fe(II) MRvalues during the formation of the Fe oxides. The presenceof citrate at a low concentration in the solution in which theFe oxides formed (initial citrate/Fe(II) MR of 0.001)decreased the amount and rate of phosphate adsorption dueto the improvement of lepidocrocite crystallization and theresultant low specific surface area. The presence of citrate ata high concentration (initial citrate/Fe(II) MR of 0.1)increased the amount and rate of phosphate adsorption,although the coprecipitated citrate blocked some of the reac-tion sites and increased the activation energy for phosphateadsorption. This is apparently due to the citrate-induced for-mation of reaction sites on the Fe oxides, i.e., the high spe-cific surface area and reaction site density of noncrystallineFe oxides, and the resultant high pre-exponential factor. Thedata of this study indicate that the surface properties of Feoxides formed under the influence of organic acids throughfundamental structural perturbation should greatly modifythe dynamics of phosphate in terrestrial and aquatic envi-ronments.

ACKNOWLEDGMENTSThis study was supported by the Natural Sciences andEngineering Research Council of Canada Research GrantGP2383-Huang and the University of SaskatchewanGraduate Scholarship.

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