Effects of crystallinity of goethite. II. Rates of sorption and desorption of phosphate.pdf

7/27/2019 Effects of crystallinity of goethite. II. Rates of sorption and desorption of phosphate.pdf

1/14

European Journal of Soil Science,March 1997, 48,101-1 14

Effects of crystallinity of goethite: 11. Rates of sorptionand desorption of phosphateR . S T R A U S S a * , G . W . B R U M M E R a & N . J . B A R R O W baDepartment of Soil Science, Universiry of Bonn, NuJallee 13, 53115 Bonn: and bCSIRO Division of Land and Water,Wem bley, Western Australia 601 4, Australia

SummaryEight samples of goethite ranging in surface area from 18 to 1 32 m2 g-' were mixed withphosphate at a range of pH values fo r periods which ranged from 0.5 h t o 6 weeks. The sam ple witha surface area of 18m2 g- ' had been hydrothermally treated to improve its crystallinity. Its rate ofreaction with phosphate depended on pH but was complete within a day. Its maximum observedreaction was close to the theoretical maximum for surface adsorption of 2.5 pmol m-2. For theother samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2.5 pmolem P 2 . The duration and extent of the reaction depen ded on the crystallinity of the goethite. Theresults were closely described by a m odel in which the phosphate ions w ere initially adsorbed on tocharged external surfaces. The phosphate ions then diffused into the particles. This was closelydescribed using equations for diffusion into a cylinder.

Samples of goethite which had been loaded with phosphate dissolved more slowly in HC1, andhad a longer lag phase, than phosphate-free goethite. For the hydrothermally treated goethite, HC 1removed much of the phosphate when only a small proportion of the iron had been dissolved. For apoorly crystallized goethite, it was necessary to dissolve m uch more of the iron to obtain a similarremoval of phosphate. Brief treatment with NaOH removed most of the phosphate from thehydrothermally treated goethite but only half the phosphate from a poorly crystallized goethite.These results are consistent with the idea that phosphate ions were not only bound on externalsurface sites but had also penetrated into meso- and micro-pores between the domains of thegoethite crystals and were then adsorbed on internal surface sites. This penetration tied the domainstogether more firmly thus increasing the lag phase for dissolution. Differences between sites forphosphate adsorption are therefore caused mainly by their location on either external or internalsites. Models that ignore this are incomplete.

Introduction assumed that reaction is confined to an initial adsorptionIt has been known for some time that the effectiveness ofphosphate fertilizers decreases with time even when there isno removal of phosphate in produce. This decline is largelycaused by a slow reaction between phosphate and soil(Barrow, 1980; Parfitt e t al., 1989; Mendoza, 1992). As thereis certainly a fast initial adsorption reaction the magnitude ofwhich is affected by such factors as ionic strength and pH, itfollows that there must be at least two distinct reactionsinvolved when phosphate reacts with soil.

Iron oxides are important in the reaction of phosphate withsoil. However, in many studies with iron oxides it has been

reaction. Values for the maximum phosphate adsorption havebeen calculated on that basis. Schwertmann (1988) calculatedthat the maximum surface phosphate adsorption on goethite is2.51 pmol m-'. Many of the published values for goethite, assummarized by Goldberg & Sposito (1984), are close to thisvalue. Yet, in many cases, the maximum adsorption may havebeen underestimated because the pH was too high. Severalauthors have used pH 3.5 (Atkinson, 1969; Golden, 1978;Ainsworth & Summer, 1985), while Torrent et al. (1990)estimated maximum sorption at pH 6. In the present work weshow that adsorption is greater at pH 2 than at pH 3.5 andmuch greater than at pH 6 and suggest that previously

~published values underestimate maximum sorption. We furthermaximum observed sorption was close to a value of 2.5 p o lm-2. For less well crystallized samples it was greater.

*Present address: Fichtner. Postfach 101454, 70013 Stuttgart, Germany. show that it was Only for a crysta11ized goethite that theCorrespondence: G. W. Brilmmer. Email: [email protected] 3 July 1995; revised version accepted 13 September 1996

0 997 Blackwell Science Ltd. 101


2/14

102 R. Strauss et al.

Despite the emphasis on a presumed surface adsorption,there is appreciable evidence that reaction with iron oxides isno t always rapid. For example, Torrent et al. (1990) showedthat sorption after 75 days was greater than after one day for arange of goethites of differing crystallinity. Analogous studiesof the reaction between goethite and sev eral heavy metals havealso shown a fast initial reaction followed by a continuingreaction (Gerth & Briimmer, 1983; Briimmer et al. 1988;Gerth et al. , 1993; Bibak et al. 1995). The explanationsfor this continuing reaction listed by Torrent (1991) includediffusion through crystal defects (Barrow, 1983, 1987),diffusion through surface micro- or meso-pores (Madrid &de Arambarri, 1985), and migration to surface sites withinaggregated particles (Willett et al. , 1988). Analogously,Torrent et al. (1992) suggested that phosphate diffusedto surface sites in micro-pores. There are some similaritiesamongst these suggestions, for if a defect is large enough it canbe considered to be a pore and the surface of such po res insidethe crystals would contain surface sites. Meso- and micro-pores between goethite domains have been emphasized aspathways for the diffusion process by Briimmer et al . (1988),Gerth et al . ( 1 993), Bibak et al. (1995) and Fischer ef al.(1996).

In the present work we studied the reaction of the goethiteswith phosphate for up to six weeks. We think that ourobservation of little continuing reaction with a sample of well-crystallized goethite supports explanations based on slowdiffusion into pores. We also think that they do not favour theother explanations listed by Torrent (1991): precipitation ofiron phosphate on the surface (Martin et al . , 1988), diffusionthrough a coating of iron phosphate (van Riemsdijk et al. ,1984, Van der See & van Riemsdijk,l991), and burial ofphosphate by progressive coagulation (Anderson et al. , 1985).We provide two further lines of evidence for our conclusionthat phosphate has diffused into pores. One is based on ourability to describe closely the observations using a model inwhich diffusion into pores is assumed. The other is based onmore direct tests. Two approaches were used. In one, samplesof goethite which had reacted with phosphate were dissolvedslowly in hydrochloric acid. We measured and compared therates of release of phosphate and iron. In the other approachsodium hydroxide was used to desorb phosphate from thesurface, but the period of reaction was short so that reversediffusion of any previously penetrated phosphate was small.Both approaches were used to estimate the amount ofphosphate penetrated into inner surfaces of goethite crystals.

MethodsEight samples of goethite were investigated. Details of theirpreparation and of their crystal properties were given byStrauss et al. (1997). The samples are identified using theirsurface area. Thu s Goe-18 indicates a sample with asurface area of about 18 m2 g- . The three samples of goethite

which contained aluminium are designated as, for example,Al-G oe-4 4. The crystallinity was reflected by the surface areawith the best crystallized samples having the smallest specificsurface area. The sample Goe-18 was especially well crystal-lized as it had been hydrothermally treated.

Effects of time and amount of phosphate applicationon sorptionAdsorption measurements were made using 20 mg of goethitein the presence of 10ml of 0.01 M NaN 03 and at pH between 2and 11 . There were two series of treatments. In one, both 10and 20 mg of P 1 - were used as initial concentrations, and thesamples were shaken for 0.5 h, 1 h, 2 h, 8 h, 1 d, 3 d, 1 w, 3 w,and 6 w. In this series, the possible surface loading of Goe-18at 10 mg of P is the same as that for Al-G oe-36 at 20 mg ofP1 - . In the other series, reaction was for 1 h and a range ofinitial concentrations were used. The range depended on theadsorption by the particular goethite samples (see later). Forboth series there were 12 differing pH treatments for eachcombination of time and P concentration. The tubes were thenshaken in an end-over-end shaker at 20 cycles per minute at20C. After centrifuging, an aliquot of 1 to 5 ml was taken forphosphate determination. The pH was measured in theremaining solution.

The amount of phosphate sorbed was calculated from thedifference between the initial and the measured concentration.For the series for which time was varied, sorption at giventimes and at given values of pH was interpolated using curvesfitted visually to plots of sorption against pH. In terpolation wasnecessary both to enable presentation of the results throughtime and to decrease the computations required in modellingthem. Visual fitting of curves was preferred because of thecomplicated shapes of the sorption curves. For the series forwhich reaction time was 1 h, the original data are presentedand were used for modelling.

Phosphate desorption and goethite dissolution in 5 M HCIThe fou r samples of goethite used for the studies on dissolutionin acid were: Goe-23, Goe-46, Goe-60 and Goe-132.Preliminary experiments showed that a concentration of 5 MHC1 provided a good compromise between too rapid and tooslow dissolution.

In order to load the goethites with phosphate samples of 2 gof goethite were mixed with 250 ml of 0.01 M NaN03containing phosphate at a pH close to 4. The initial phosphateconcentrations were proportional to the surface area of thefour goethite samples. After mixing, the suspensions wereincubated at 40C with vigorous shaking once a day for 7 d.A sample of the suspension was then analysed for pH andphosphate in solution. The measured values for phosphatesorption in pmol m-2 were: Goe-23, 2.58; Goe-46, 2.62;Goe-60, 2.81; Goe-132, 2.56.

0 997 Blackwell Science Ltd, European Journal of Soil Science, 48, 101-1 14


3/14

Sorption and desorption o phosphate on goethite 103

After the adsorption stage the addition of HCI followedwithout prior separation of the equilibrium solution by centri-fuging. This was to prevent the dispersed goethite particlescoagulating. Reproducibility of the results decreased whencoagulation occurred. An aliquot of the suspension was trans-ferred to a centrifuge tube, HC1 was added, the centrifugetubes were closed and shake n end-over-end at 20C for variousperiods and were then centrifuged. The supernatant solutionswere analysed for phosphate and iron. All investigations werein duplicate. The data for goethite dissolution were comparedwith those reported for unphosphated goethite by Strauss et al.(1997).

pH and phosphate concentration were measured. A volume of10 rnl of 0.1 M NaOH was added, and the oxide wasresuspended. The centrifuge tubes were shaken for 30 minutesat 20C, centrifuged, and phosphate was determined in thesupernatant solution. The phosphate which was calculated asbeing sorbed includes that which remained in solution in thecentrifuge tubes after decanting the original solution. Themaximu m error this would cause was estima ted to be only 2%.

ReSUltSEffects of time and amount o phosphate applicationon sorption

Phosphate desorption by sodium hydroxide For the well-crystallized samples, Goe- 18 and Goe-23,For the studies on phosphate desorption at high pH Goe-18was also included. Samples of 20 mg of goethite weresuspended in 10 m1 of 0.1 M NaN03 solution at pH valuesbetween 2 and 1 1. The initial phosphate concentrations were10 and 20 mg 1-'. Reaction was for one week at 40C. Thetubes were centrifuged, the supernatant was removed, and

reaction with phosphate stopped within a day. For the othersamples reaction continued for much longer (Fig. 1 and 2). Thelarger the specific surface area, the longer the reactioncontinued but, in all cases, reaction had reached an end-pointafter three weeks (504'h ). Continuation to 6 weeks producedno further change. Mod elling of the reaction from three weeks

0

100

80

200

Goe-18

- I I I 8

1 10 100

Go e -60

0 I I I1 10 100

100

80

60

40

20

01 10 100 1 10 100

Timelh Time/hFig. 1 Time course of phosphate reaction with Goe-18 and Goe-60. The mass of goethite used was 2.01 g 1 - ' and the initial concentration ofphosphate was 20 mg 1 - I . The upper figures express sorption as a percentage of the amount of phosphate added; the lower figures express it as apercentage of the final adsorption for each given pH value. The lines in the upper figures were obtained by fitting the model to the data of this figureonly; the lines in the lower figures are polyno minals fitted to these data0 997 Blackwell Science Ltd, European Journal of Soil Science, 48, 101-114


4/14

104 R. Strauss et al.r I

Goe- 3Initial P2

1

n

2OTE-i ln

U 2 4 6 8 10

I

" 2 4 6 8 10

R t 1AIGoe-36Initial P0 2

AlGoe - 30Initial P0 2Ta 2

fi5 1 'n0 " 2 4 6 8 104 6 8 10

AIGoe-44Initial P0 2TE-

B1n..

2 4 6 8 10 2 4 6 8 10

G08-1323 Initial Pa 50 7v 15

Goe- 60Initial P0 25-:I0

2 4 6 8 10PH

2 4 6 8 10PH

Fig. 2 Effect of the indicated amounts of addition of phosphate (mg P 1- ' ) on phosphate sorption after 1 h for the eight samples of goethite. In allcases, the mass of goethite used was 2.01 g 1- ' . The lines indicate the fit of the model using the parameters shown in Table I. The model wassimultaneously fitted to this data and to th e data in Fig. 3 and 4.

0 997 Blackwell Science Ltd, European Jounral of Soil Science, 48, 101-114


5/14

to six weeks would have required many more time steps. It wastherefore not done, and the data for 6 weeks are not presented.

Reaction was initially slower at high pH. This is shown inFig. Ic and Id by expressing sorption as a percentage of thefinal value. At low pH reaction with Goe-18 was almostcomplete after 2 h; at high pH it was still incomplete at the 8 hsampling. However, reaction was apparently complete by thenext sampling at one day. We therefore conclude that only thefast, initial, adsorption reaction was present for this sample.The effect of pH on kinetics show s the difficulty of separatingthe postulated fast and slow reactions (when both are present)by attempting to choose particular sampling times. If one wereto choose, say, 1 h, the fast reaction would be incomplete athigh pH. Yet, if one chose longer periods, the slow reactionmay have become increasingly important-depending on thecrystallinity of the goethite.Figure 2 shows that, after 1 h reaction, there was a steadydecrease in sorption with increasing pH up to about pH 6. Th edecrease was much steeper at pH values higher than about 7.Part of the decrease at high pH occurs because, after 1 h, theinitial reaction was still incomplete at high pH. Maximumsorption was not achieved at pH 6 and a conce ntration of 6 mgP 1-' (194 p ~ )Figs 2 and 3). These were the conditionschosen by Torrent et al. (1990) who, however, measuredsorption after one day. Sorption increased with both decreasingpH and increasing phosphate concentration. Table 1 presentsvalues for sorption interpolated from curves fitted to sorptiondata at given pH values. On the average, values for pH 6 and6 mg P 1- ' are about two thirds of those at pH 2 and 30 mg P1-'. Figures 2 and 3 both show that sorption at pH 2 wasgreater than at pH 3. This supports our contention thatmeasurements made at pH 6 or even at pH 3.5 underestimatethe maximum sorption.

Short-term sorption curves such as those in Fig. 3 approacha smaller maximum sorption as the pH increases. This iscaused by a decrease in the electrical potential of the reactingsites. The maximum amount of long-term sorption wassimilarly affected by pH. This is shown by the trend of thecurves in Fig. 4 to approach an end-point which decreases w ithincreasing pH. That these curves have a similar shape isshown by plotting sorption as a percentage of the final valuefor Goe-60 (Fig. Id). After the 8 h sampling, points fell closeto a comm on line. This result shows that the region into whichthe phosphate diffused, and the surface on which it wasinitially adsorbed, were affected by pH in a sim ilar way. If thiswere not the case, and diffusion was into an uncharged region,it would have continued for much longer at high pH.

Phosphate desorption and goethite dissolution in 5 M HCIFigure 5 shows that the release of phosphate was much fasterthan the release of iron from the goethites. Furthermore, theshapes of the release curves w ere different: the curves for ironwere sigmoid; those for phosphate were not. This is shown by

Sorption and desorption of phosphate on goethite 1052.5 I I I DH' 1

-V V 8

0 200 400 600 800P concentration/pM

Fig. 3 The sorption curves (lines) at constant pH derived from thefitted mod el for Goe-18 together wi t h interpolated values (points) forsorption at the specifie d pH values.

the curves fitted to describe the rate of reaction. The functionused was:

Ct = C , - Crexp(-(kt)a), (1)where C, is the amount of phosphate or iron dissolved at timet, C, indicates the maximum amount dissolved, C,-Crindicates the amount dissolved at zero time, and k is a measureof rate. The greater the value of k, the faster the dissolution. Thevalue of a reflects the degree to w hich the curve is sigm oid. Avalue greater than unity indicates a sigmoid curve; a valueof unity indicates an exponential curve; and a value less thanunity indicates a curve that rises mpre steeply than an expo-nential curve (Barrow & Mendoza, 1990). Table 2 shows that,for iron dissolution in the presence of phosphate, the values of awere greater than unity, whereas for phosphate d issolution theywere sm aller than unity. The larger values for the k coefficientsfor phosphate than for iron, and also the curves in Fig. 5 , showthat the release of phosphate w as faster than the release of iron.Similarly, the rate of release of both iron and phosphate tendedto increase as the surface area of the goethites increased.In all cases the release of iron from the phos phated goethiteswas appreciably slower than from unphosphated goethites(Fig. 5) . Not only were the k terms smaller for the phosphatedgoethites but the a terms were also larger (Table 2). Th edifferences in the a terms show that the curves for dissolutionof the phosphated goethites were more sigmoid-that is, therewas a longer lag phase.

In Fig. 6 the release of phosph ate after increasing periods isplotted against the release of iron from the four differentgoethites. For Goe-23 most of the phosphate was releasedwhen only small amounts of iron had been dissolved; when

0 997 Blackwell Science Ltd, European Journal of Soil Science, 48, 101-1 14


6/14

106 R. Struuss et al.

40

30

20

10

0 -

15

---.

60

40

20

-*

.

I20 -15 -

c icC8k i 5n

0 n l I

AlGoe-36

1 10 100

AlGOe -30308

20cCQ I -d 10

+I+ + +0 1 10 100I

40

20

0 1 10 10010 100

80 1 10 080

60

40

20

Go e-608v)

0' I1 10 100Tirneh

0 ' 11 10 100Tirneh

Fig. 4 Time course of phosphate sorption for all eight sam ples of goethite. In all cases the mass of goethite used was 2.01 g 1- ' , and the initialconcentration of phosphate was 20 mg 1 - . Th e open circles indicate pH 2 and the other symbols indicate pH 3 to 11 in sequence down he graph.The ines indicate the fit of the model using the parameters shown in Table 1. The model was simultaneously fitted to these data and to the datain Figs 2 and 3.

0 997 Blackwell Science Ltd, European Journal of Soil Seience,48, 101-1 14


7/14

Sorption and desorption ofphosphate on goethite 107Table 1 Measured phosphate sorption and parameters for the model fitted the describe phosphate sorption by eight samples of goethite

Goe-18 Goe-23 Goe-46 Goe-60 Goe-132 Al-Goe-30 AI-Goe-36 A1-Goe-44Measured propertiesP sorbed 2a Ipmol m- 2.30 2.86 2.89 3.12 3.23 2.96 2.86 2.99P sorbed 6b Ipmol rn- 1.42 2.00 1.66 2.09 2.30 1.70 1.80 2.07P sorbed 3w c Ipmol m- 2.40 2.94 3.31 3.96 n.d. 3.28 3.59 3.32Excess over 2.5d Ipmol m- -0.1 0.44 0.81 1.46 n.d. 0.78 1.09 0.82Fitted parametersMax. internal sorption /pmol m-2 0.16 0.62 2.73 2.95 5.70 1.72 2.5 1 1.48Capacitance GSA F m-2 2.89 2.99 2.43 3.33 3.19 2.80 2.83 2.84Binding constant K, / 1 pmol m-I 5.01 52.1 12.6 7.18 19.05 2.58 11.54 42.03Diffusion term (D/ ,a2) h- 0.0 138 0.0856 0.0103 0.0101 0.0017 0.047 0.0122 0.0293R2 0.993 0.997 0.995 0.994 0.995 0.992 0.995 0.997aPhosphate sorbed at pH 2 at a concentration of 30 mg P I - after 1 h.bPhosphate sorbed at pH 6 at a concentration of 6 mg P I- after 1 hCPhosphate sorbed at pH 2 with an initial concentration of 20 mg P I - an d 2.01 g of goethite I- after 3 weeksdVa lue in previous line minus 2.5 pmol m-.

Rate constant k l / h 1 pmol- 3.38 25.9 4.03 2.91 7.59 4.60 4.68 20.10

k a R2able 2 Values of the C and a parametersan d for R2 or the equationa used to describePb Fe(+P) Fe(-P) P Fe(+P) Fe(-P) P Fe(+P) Fe(-P)he rate of dissolution of four samples ofgoethite with and without sorbed P in 5 MHC 1 Goe-23 0.280 0.0202 0.0223 0.470 1.81 1.15 0.999 0.904 0.996

Goe-46 0.354 0.0322 0.0573 0.359 1.42 1.31 0.997 0.998 0.998Goe-60 0.293 0.0666 0.1072 0.492 1.59 1.82 0.998 0.998 0.998Goe-132 3.073 1.360 2.065 0.703 1.14 0.98 0.997 0.999 0.997aThe equation used was: C, = C , - C,exp[ - (k r y ) , where C, indicates the amount of P or Fedissolved and f is the time in hours.bP indicates phosphate released from phosphated goethites, Fe (+P) indicates iron released fromphosphated goethites, Fe( -P) indicates iron released from unphosphated goethites.

only 10% of the iron had been dissolved, about 90% of thephosphate had already been released. At a given value for irondissolved, the phosphate released decreased in the sequenceGoe-23 > Goe-46 z Goe-60 z Goe-132. For Goe-132, when10%of the iron had been dissolved, only 45% of the phosphate

little affected by either the pH of the initial sorption reaction orby the level of addition of phosphate (Fig. 8). For somesamples, there was a slight downward trend with pH, for othersa slight upward trend.

had been released. Thus, the proportion of the phosphatebound within the crystal increased with increasing specific Development of a model to describe the observationssurface area. This in turn indicates decreasing crystallinity andincreasing porosity (Strauss et al., 1997).Phosphate desorption by sodium hydroxideFigure 7 shows that for Goe- 18most of the phosphate initiallysorbed was desorbed after exposure for 3 0 min to 0.1 M NaOH.With increasing surface area of the goethites the proportionthat was desorbed decreased. For Goe-132 only about half ofthe previously sorbed phosphate was desorbed.

When the pH at which the sorption occurred was varied boththe phosphate initially sorbed, and the phosphate subsequentlydesorbed, decreased with increasing pH (Fig. 7). Theproportion of the sorbed phosphate which was desorbed was

In order to model sorption of phosphate it is necessary toaugment the model used by Strauss et al. (1997) to describecharge. In that model the m ean charge on the adsorbed protonsis allocated to a plane ( S plane) close to the surface arid thecharge on the counter ions to a plane slightly further from thesurface. When phosphate is present its adsorption is assumedto be into a plane (A plane) between the protons and thecounter ions (Barrow, 1987). The position of this plane iseffectively determined by the capacitance between the twoplanes, GSA.As the concept is that this is the mean position ofthe charge on the phosphate ion, the effect is similar to thatrecently obtained by Hiemstra & Van Riemsdijk (1996) whodistributed the charge on the adsorbed phosphate between theother two planes.

0 997 Blackwell Science Ltd, European Jountal of Soil Science, 48, 101-1 14


8/14

108 R. Struuss et al.

0fPe9,LLPb

0 20 40 60 80 100 120100

80

60

40

20Go e- 0

0 0 20 40 60

10 0

80

60

40

20Go e- 6

00 20 40 60 80 100 120

" 0 1 2 3 4Time/hFig. 5 Effect of time on the dissolution of iron and of phosphate by 5 M HC I from four samples of goethite pre-treated with phosphate(0 hosphate, 0 iron from samples pre-treated with phosphate, c] iron from samples not pre-treated with phosphate). The lines are drawn from

fitted curves and som e of the parameters are given in Table 2.

7i 0 G o e - 2 30 Go e - 4 6v G o e - 6 0v Go e - 1 3 220 40 60 80 10 0iron disso lved/%

It is proposed that the initial adsorption reaction is relativelyrapid and that this is followed by a diffusive penetrationtowards the center of the particle. Although the initial reactionis relatively rapid, it is nevertheless not instantaneous and it isnecessary to include equations to describe its rate.Rate ofrhe initial reaction. The results show that the rate ofthe initial reaction was slower at high pH. This indicates thatits rate was controlled by the rate of reaction at the chargedsurface rather than by the rate of transfer through a liquidfilm. It has been suggested by Barrow et ul . (1981) thatEquation (9.24)of Bockris & Reddy (1970) is appropriate:

_ - klapyc,m, exp( % F Y a / R T )dt (2)d O- 2 0 , exp(-dFY',/RT),

Fig. 6 The release of phosphate relative to the release of iron (bothas a percentage of the total present) w hen four samples of phosphatedgoethite were reacted with 5 M HC I for a range of times.

where 0, s the concentration of Occupied Sites at time t andis therefore a measure of the amount of sorption (pmol 1 -'),k, and k2 are the rate constants (h l2 pmol-? and h 1 pmol-'),

0 997 Blackwell Science Ltd, European Journal of Soil Science, 48, 101-114


9/14

Sorption and desorption of phosphate on goethite 109

b

ba

I I I I 125

20

15

10

5

n" 2 4 6 8 10PH

100 I I I I I 1

80

60

40

20

0

-20 2 4 6 8 10DH

50 I I I IGoe-23

40

30

20

10

I I 1 I I2 4 6 8 10

PH100

80

60

40

20

0 2 4 6 8 10PH

100

0 Observed sorption0 Observed desorption80

Upper line is m odelled total sorptionLower lin e is m odelled external sorption0 ..a@ a a a40

20

4 6 8 100 2PH

Fig. 7 Effects of the pH of initial sorption of phosphate from a solution containing 10mg P 1 and on the subsequent desorption of phosphate ina solution of NaOH after 1 h. The lines were fitted to the data in Figs 2, 3 and 4. The upper lines show the modelled values for total phosphatesorption; the lower lines show the modelled values for the amount of phosphate adsorbed on the external surface and therefore more likely to berapidly adsorbed.0 997 Blackwell Science Ltd, Eumpean Journal of Soil Science, 48, 101- 14


10/14

110 R. Strauss et al.60 1 I I I I5 0 1 1

a's$? 30a.-

s! 20m-a

10

I I I I2 4 6 8 10

PHFig. 8 The measured values for the proportion of the sorbedphosphate which was retained after brief treatment with NaOH.Hollow symbols indicate an initial concentration for phosphatesorption of 10 mg P I - ' , filled symbols 20 mg P 1 - ' .

up is the fraction of the total phosphate dissociated to HPOi-,c , is the total concentration of phosphate in solution(pmol 1-I), m, is the concentration of vacant sites at time t(pmol I - ' ) , Ya is the electrical potential in the plane ofadsorption at time f (mV), and F is the Faraday, R the gasconstant and T (K) the temperature. The terms % and d recalled transfer coefficients and are described by Bockris &Reddy (1970) in their Equation (9.25). The magnitude of thesecoefficients is determined by the position of the rate-determining step in the sequence of steps involved in theoverall reaction (Barrow et al., 1981). The observed behaviourof decreasing rate of reaction with increasing pH is reproducedwhen % is 2 and d s zero. These values were used here.They indicate that the rate-determining step precedes theelectron transfer steps and does not itself involve an electrontransfer. The adsorption at equilibrium is given by:

(3 )N KiupyC, exp(2YaF/RT)1 + N KiapyC, exp(2YaF/RT) '=where N is the maximum adsorption, C, is the totalconcentration at equilibrium (in pmol I - ' ) and the bindingconstant Ki is equal to the ratio of kl lk2. This is equivalent tothe adsorption equation of Bowden et al. (1977).Rate of the continuing reaction. The observation that theamount of phosphate reacting via the continuing reaction

decreased with increasing pH suggests that the reacting surfaceadopted an electric potential determined by the external pH.The observation that the rate of the second reaction was muchslower than the initial reaction suggests that the rate waslimited by a different mechan ism-probab ly by the rate ofdiffusion to the reacting sites rather than the rate of reactionwith the sites. The observation that reaction was completedwithin the experimental period suggests that phosphate hadpenetrated to the end of the diffusion pathways of the goethiteparticles. Models for which penetration was small relative tothe radius of the particles were therefore inappropriate. Weargued from the results of Strauss et al . (1997) and Fischere f ~ l .1996) that the goethite particles could be approximatedto cylinders. The problem is then one of diffusion intocylinders from a stirred solution of limited volume, andEquation 5.33 of Crank (1964) is appropriate:

where M,,he amount of the solute in the cylinder after timet , is expressed as a fraction of the amount at equilibrium,M,, j? is the final fractional uptake by the cylinder; a isthe radius of the cylinder; the qns are the positive, non-zero roots of Equation (5.34), the values of which aretabulated in Table 5.1 of Crank (1964); and D is the diffusioncoefficient. It is convenient to regard the ratio D/a2 as asingle parameter.

Formulation of the model. The results had suggested thatdiffusion of phosphate occurred into a region carrying anelectric potential which was influenced by the pH of thesolution. It was therefore necessary to allocate an electricpotential to this surface. The resuits of Fig. 8 were important inconstraining the model in this regard. They could only bereproduced if it was assumed that the internal phosphateexperienced the same changes in potential with changes inpH as the external phosphate.

For the external surface the parameters estimated were: thebinding constant Ki he rate constant for the forward reactionk , , and the capacitance GSA.The maximum adsorption wastaken as 2.5 pmol m - 2 . For the internal surface the parametersestimated were: the maximum adsorption and the diffusionterm D/d. he binding constant far the internal surface wasset equal to that for the external surface.

The program was written in BASIC. Details of the programand copies of it are available from N.J. Barrow at [email protected]. T he data sets are shown in Figs 2 and 4.For each sample of goethite there were typically 160 obser-vations through time (10 pH values by 8 times by 2 levels of P)plus 84 observations at 1 h (12 pH values by seven levels ofadded P).There were 200 iterations allowed in the simplexprocedure used to optimize the choice of parameters, and the

0 997Blackwell Science Ltd, European Journal of Soil Science, 48, 101-114


11/14

Sorption and desorption of phosphate on goethite 111procedure was restarted twice at the previous best values,giving 600 iterations in total.

loo I I I

Output fro m the modelThe model described the observations very closely (Figs 2, 3and 4) nd the average value for R 2 was 0.995 (Table 1). Whenthe internal surface was excluded from the model, the changesin the value of R 2 for Goe-18 were on fourth decimal placeonly. This shows that internal penetration w as unimportant forthis sample. As the maximum external adsorption N was set at2.5 pmol m-' for all goethites, this meant that, for Goe-18, theeffects of pH, initial concentration of P, and time were alldescribed using only three adjustable parameters. The import-ance of internal penetration increased with increasing surfacearea. Thus for Goe-23, excluding internal penetration from themodel significantly (P i .05)decreased the value of RZ. hatis, there was a significant though small effect even for thiswell-formed sample (Fig. 4).This figure shows that the slowreaction continued for much longer for the goethites withlarger surface area. For these goethites, the model was alsovery efficient in describing the observations as only fiveadjustable parameters were needed to closely describe theeffects of different pH, initial concentration of P, and time.

The maximum internal sorption was estimated in the modelin units of pmol of phosphate. In order to compare the valueswith those for external sorption it is expressed relative to theBET surface area. The regression estimates of the internalsorption per unit of surface area increased as the surface areaincreased (Table 1). These values should be interpreted withcaution because they depend on the values allocated to theother parameters and especially the value of the bindingconstant. The values allocated by the model (V,) exceed theobserved excess sorption over the calculated surface maximumof 2.5 pmol rn-' (V,) (Table l) , but were correlated withthem (V, = 0.25 + 1.97 V , , r = 0.90). A small part of thelarger value for V,,, arises because the observed maxima (V,)were calculated from a particular amount added, rather thanfrom a specified final concentration. Especially for the sampleswith a large surface area, slightly larger values might beobtained with a greater amount added. However, the mainreason is the assumption in the model that penetration ofphosphate increases the negative surface charge and thereforedecreases the sorption on the external surface at a given pH.Hence, the observed excess sorption over 2.5 pmol m-'underestimates the internal surface.Tw o factors influence the other model parameters in Table 1.One is that values for the diffusion term for Goe-18 andGoe-23 are imprecise because the continuing reaction wassmall. The second is that some of the parameters arecorrelated. This means that changing values for one can bepartly compensated by change in another. Thus, because of theway the terms are defined, large values for the binding constantKi are associated with large values of the rate constant k l :

I /-1 10 100

TimelhFig. 9 Effects of time on the modelled split of phosphate betweenthe extemal and internal surfaces. The symbols show the observedvalues of total sorbed phosphate for an initial concentration of 20 mgP I - ' at pH 8; the solid lines show the fit of the model using theparameters of Table 2; the broken lines show the modelled extemalphosphate. The gap between the solid and the broken lines thereforerepresents the internal phosphate.

kl = 0.15 + 0.473 Ki, = 0.905. Similarly, increases in therate term k l can be partly compensated by changes in thediffusion term. Because of these effects, individual values ofthe parameters should be treated with caution.Figure 7 shows that the phosphate modelled as being sorbedon external surfaces was smaller than that measured asdesorbed after 30 min. This is to be expected because some ofthe phosphate desorbed within this period would come frominternal surfaces. Nevertheless, the model reproduced theeffects of the pH on the amounts of phosphate desorbed bysodium hydroxide.

Figure 9 shows the modelled partition of phosphate betweenthe internal and external surfaces at a common level ofaddition of phosphate for three samples of goethite. ForGoe-18 the modelled total sorption and the modelled externalsorption were virtually the same. For Goe-132 modelledtotal sorption increased with time, but modelled externalsorption decreased partly because increasing internal sorptiondecreased the solution concentration of phosphate and partlybecause it increased the negative charge as described above. Atthe end of the experimental period modelled external phos-phate was less than half of modelled total sorption. Thesetrends were similar but less marked for Goe-60.

DiscussionIt was only for the well-crystallized Goe- 18 that no continuingreaction could be detected after 24 hours shaking and it wasonly for this sample that the maximum observed sorption was

0 997 Blackwell Science Ltd, European Journal of Soil Science, 48,101-1 14


12/14

1 12 R. Struuss et al.slightly lower than the theoretical maximum surface adsorp-tion of 2.5 p o l m-. Furthermore, most of its sorbedphosphate was removed by 0.5 h treatment with sodiumhydroxide or by brief treatment with hydrochloric acid. Wethink this shows that it was only for this sample that thephosphate reacted solely with the surface. All of the less well-crystallized samples differed in all of these respects: reactioncontinued for up to 3 weeks; the maximum exceeded 2.5 pmolm-*; and recovery by brief treatment with acid or alkali wasincomplete. All of these observations are consistent with theconclusion that there was a continuing reaction with imper-fections or meso- and micro-pores between crystal domains(Briimmer et al., 1988; Torrent et al., 1992; Gerth et al . 1993;Fischer et al., 1996). In comparing this conclusion with thoseof previous reports questions of semantics arise. If animperfection or a defect can be invaded by a foreign moleculeit is reasonable to refer to it as a pore. Once it is so invadedreaction can be considered to be with sites on an internalsurface (Briimmer et al., 1988; Gerth et al., 1993). Wetherefore think that our conclusion is compatible w ith the ideasput forward by Barrow (1983, 1987) and Madrid & deArambarri (1985) as indicated earlier. Further, it is compatiblewith the idea of Anderson et al . (1985) in so far as theysuggested that phosphate was buried. However, they associ-ated this burial with observed changes in the coagulationbehaviour. We think that changes in the coagulation behaviourwere caused by changes in the surface charge as a result of. eaction with phosphate and were not the cause of thecontinuing reaction. On the other hand, we think that ourresults are incompatible with ideas of precipitation. Againsemantics arise, but precipitation suggests that s ome thresholdexists below which no continuing reaction occurs and abovewhich continuing reaction is marked. This is not consistentwith our results. Nor are our results compatible with formationof a sequence of layers of iron phosphate which progressinwards (van Riemsdijk et al., 1984, van der Zee & vanRiemsdijk, 1991) for we would expect this process to alsooccur w ith G oe- 18.

These conclusions are supported by the fact that the resultswere closely described by a model that involved an initialadsorption step on to the surface of the oxide followed bydiffusive penetration into the particle. We are not aware of anyother successful description of the long-term rate of reactionof anions with oxides. The reaction of the cations nickel,zinc and cadmium with goethite were closely modelled byassuming that the initial adsorption reaction was followed bydiffisive penetration of the surface (Briimmer et al . 1988;Barrow et al., 1989). However, Yiacoumi & Tien (1995a, b)could obtain only a poor description of the rate of reaction ofcadmium with aluminium oxide despite the presentation ofsome 90 equations.

Our purpose in developing the model was to test the idea ofdiffusive penetration in a quantitive manner and for thispurpose, the ability to describe the data is more im portant than

the values of the parameters. This purpose differs from that ofHiemstra & Van Riemsdijk (1996) who w anted to test whetherthey could describe phosphate adsorption from detailedknowledge of the bonds formed and of the distribution ofcharge. For this purpose, the values of the parameters aremore important. They distribute the charge on the adsorbedphosphate ion between the plane in which protons are thoughtto adsorb (S plane) and a plane in which counter ions adsorb(C plane). The effect is very similar to that of our approach inwhich the charge on the phosphate ions is allocated to a meanposition between these two planes by the value of thecapacitance term (Table 1). To illustrate this considerGoe-18. For this sample, the fitted value for the capacitancebetween the S plane and the plane of phosphate adsorption(A plane) was 2.89 F m- (GsA, Table 1). The value for thecapacitance between the S plane and the counter ions was1.75 F m- (Gsc from Table 1 of Strauss et al., 1997). If weassume that the dielectric properties are uniform in this region,the relative position of the planes is proportional to thereciprocal of the capacitances. The A plane is therefore about0.6 of the distance from the S plane to the C plane. Hiemstra &Van Riemsdijk (1996) obtained a similar description of theeffects of pH, phosphate concentration and of backgroundelectrolyte as had been obtained using our approach. How-ever, they assumed that the only reaction is with the surface.They note that large values for sorption can occur with somesamples of goethite but they associate this with large positivesurface charge. We suggest that our results show that phos-phate can react with more than the surface layers and that thisneeds to be incorporated in more fundamental models of thereaction and that large values for charge are caused byanalogous diffusion of protons.The results also show that the initial reaction was slower athigh pH. The remarkably slow reaction at pH 10 was indeedcompatible with the faster reaction-at lower pH provided thatthe initial rate of reaction was limited by the rate of reaction ofthe phosphate ions with differently charged surfaces. Indeed,for Goe-18, this was sufficient to explain the entire timecourse of the reaction. It is possible that this result wasobtained because the reactants were m ixed fairly vigorously. Ifmixing were not as vigorous it is possible that diffusionthrough a surface layer would then become limiting and thatthe rate would not depend on the charge on the surface.The sigm oid dissolution curves arise because dissolution byHC 1 is a two-stage process (Strauss et al., 1997 and referencestherein). Dissolution is initially slow until attack by the acidcauses the goethite domains to separate, thus exposing a biggersurface to further reaction (Cornell et al., 1976; Schwertmann,1984; Ruan & Gilkes, 1995). We think that the more-markedlysigmoid curves for the phosphated goethites indicate thatseparation of the domains was slower and that this is directevidence that phosphate had indeed penetrated between thedomains. Th is is consistent with the observations of B iber et al .(1994) that addition of anions such 88 phosphate to the solution

0 997 Blackwell Science Ltd, European Journal of SON Science,48,101- 14


13/14

S o rp t io n a n d d e so rp t io n of p h o s p h a t e on goe th i te 11 3

phase slowed the dissolution of iron oxides. Similarly, Willett& Cunningham (1983) found that phosphate stabilized thesurface of ferrihydrite at a range of pH and Eh values.

Previous investigations of desorption of phosphate withsodium hydroxide have used a longer period of desorption.McLaughlin e t a l . , (1977) extracted phosphate from ferri-hydrite using two extractions with 0.1 M NaOH over a totalperiod of 18 h. They found that when the phosphate reactedwith the ferrihydrite for 30 d prior to desorption, only 88% ofthe phosphate was desorbed. In contrast, Willett et a l . (1988)used the sam e procedure and desorbed all of the phosphate thathad reacted with ferrihydrite for 90 d. However, femhydriteforms very small crystals and could have only short pores.Consequently results with ferrihydrite may not be transferableto goethite. The influence of crystallinity on the proportion ofthe phosphate which could not be desorbed from goethite andlepidocrocite (y-FeOOH) was investigated by Cabrera e t al .(1981). They desorbed phosphate that had reacted with theoxides for six days using sodium hydroxide with six treatmentsover a total period of 17 h. They found a significan t proportionof the phosphate remained within the crystal. They attributedthis to diffusion of the phosphate into micropores. Because ofthe slow diffusion, the desorbability of the phosphate wasvery slight. Torrent et a l . (1990) extracted phosphate that hadreacted with samples of goethites for 75 d by shaking for 16 hwith sodium hydroxide. Up to 0.66 pmol m P 2 of phosphateremained unextracted. We have shown that amounts of phos-phate extracted by 0.5 h treatment with sodium hydroxide werelarger than ou r modelled values for surface phosphate. Wesuggest that longer extraction with sodium hydroxide wouldextract an even greater proportion of the phosphate that haspenetrated into the particles.

In summary, this work has shown that differences betweensites for phosphate adsorption are mainly caused by theirlocation on either external or internal sites. Models that ignorethis are incomplete.

ReferencesAinsworth, C.C. & Sumner, M.E. 1985. Effect of aluminum substitu-

tion in goethite on phosphorus adsorption: 11. Rate of adsorption.Soil Science Society of America Journal, 49, 1149-1 153.

Anderson, M.A., Tejedor-Tejedor, M.I. & Stanforth, R.R. 1985.Influence of aggregation on the uptake kinetics of phosphate bygoethite. Environmental Science and Technology,19, 632-637.

Atkinson, R.J. 1969. Crystal Morphology and Suflace Reactivity oGoethite. Ph.D. Thesis, University of Western Australia.

Barrow, N.J. 1980. Differences amongst a wide-ranging collection ofsoils in the rate of reaction with phosphate. Australian Journal oSoil Research, 18, 215-224.

Barrow, N.J. 1983. A mechanistic model for describing the sorptionand desorption of phosphate by soil. Journal of Soil Science, 34,

Barrow, N.J. 1987. Reactions with Variable Charge Soils. Martinus733-750.Nijhoff, Dordrecht.

Barrow, N.J. & Mendoza, R.E. 1990. Equations for describingsigmoid yield responses and their application to some phosphateresponses by lupins and by subterranean clover. FertilizerResearch, 22, 18 1- 88 .

Barrow, N.J., Gerth, J . & Briimmer, G.W. 1989. Reaction kinetics ofthe adsorption and desorption of nickel, zinc and cadmium bygoethite. 11. Modelling the extent and rate of reaction. Journal oSoil Science, 40, 437-450.

Barrow, N.J., Madrid, L & Posner, A.M. 1981. A partial model forthe rate of adsorption and desorption of phosphate by goethite.Journal of Soil Science, 32, 399-407.

Bibak, A., Gerth, J. & Borggaard, O.K. 1995. Retention of cobalt bypure and foreign-element associated goethites. Clays and ClayMinerals, 43, 141-149.

Biber, M.V., Dos Santos Afonso, M. & Stumm, W. 1994. Th ecoordination chemistry of weathering: IV. Inhibition of thedissolution of oxide minerals. Goechimica et Cosmochimica Acta,

Bockris, J.OM. & Reddy, A.K.N. 1970. Modem Electrochemistry:An Introduction to an Interdisciplinary Area. Plenum Press,New York.

Bowden, J.W., Posner, A.M. & Quirk, J.P. 1977. Ionic adsorptionon variable charge mineral surfaces. Theoretical-charge develop-ment and titration curves. Ausfralian Journal of Soil Research, 15,

Briimmer, G.W., Gerth, J. & Tiller, K.G. 1988. Reaction kinetics ofthe adsorption and desorption of nickel, zinc and cadmium bygoethite. I. Adsorption and diffusion of metals. Journal of SoilScience, 39, 37-52.

Cabrera, F., de Arambarri, P., Madrid, L. & Toca, C.G. 1981.Desorption of phosphate from iron oxides in relation to equilibriumpH and porosity. Geoderma, 26, 203-216.

Cornell, R.M., Posner, A.M. & Quirk, J.P. 1976. The kinetics andmechanisms of the acid dissolution of goethite cc-FeOOH. Journalof Inorganic Nuclear Chemistry, 38, 563-567.

Crank, J . 1964. The Mathematics o Difusion. Oxford UniversityPress, London.

Fischer, L., Zur Miihlen, E., Briimmer, G.W. & Niehus, H. 1996Atomic force microscopy (AFM) investigations of the surfacetopography of a m ultidomain porous goethite. European Journal ofSoil Science, 47, 329-334.

Gerth, J . & Briimmer, G.W. 1983. Adsorption und Festlegung vonNickel, Zink und Cadmium durch Goethit (cc-FeOOH). FreseniusZeitschrifi fu r Analytische Chemie, 316, 616-620.

Gerth, J ., Briimmer, G.W. & Tiller, K.G. 1993. Retention of Ni, Znand Cd by Si-associated goethite. Zeitschrqt fd r Pflanzenerndhrungund Bodenkunde, 156, 123- 129.Goldberg, S. & Sposito, G. 1984. A chemical model of phosphateadsorption by soils: 11. Noncalcareous soils. Soil Science Society oAmerica Journal, 48, 779-783.

Golden, D.C. 1978. Physical and Chemical Properties of Aluminium-Substituted Goethite. Ph.D. Thesis, North Carolina State University,Raleigh. N.C.

Hiemstra, T. & Van Riemsdijk, W.H. 1996. A surface structuralapproach to ion adsorption: the charge distribution (CD) model.Journal o Colloid and Inteflace Science, 179, 488-508.

Madrid, L. & de Arambarri, P. 1985. Adsorption of phosphate bytwo iron oxides in relation to their porosity. Journal o Soil Science,

58, 1999-2010.

121- 36 .

36, 523-530.0 997 Blackwell Science Ltd, European Journal ofsoil Science,48, 101-1 14


14/14

114 R. Strauss et al.Martin, R.R., Smart, R.St.C., & Tazaki, K. 1988. Direct observation

of phosphate precipitation in the goethite/phosphate system. SoilScience Society of America Journal, 52, 1492- 1500.

McLaughlin, J.R., Ryden, J.C. & Syers, J.K. 1977. Development andevaluation of a kinetic model to describe phosphate sorption byhydrous ferric oxide gel. Geodermu, 18, 295-307.

Mendoza, R.E. 1992. Phosphorus effectiveness in fertilized soilsevaluated by chemical solutions and residual value for wheatgrowth. Fertilizer Research, 32, 185- 194.

Parfitt, R.L., Hume, L.J. & Sparling. G.P. 1989. Loss of availabilityof phosphate in New Zealand soils. Journal o Soil Science, 40,

Ruan, H.D. & Gilkes, R.J. 1995. Acid dissolution of syntheticaluminous goethite before and after transformation to hematite byheating. Clay Minerals, 30, 55-56.

Schwertmann, U. 1984. The influence of aluminium on iron oxides:IX. Dissolution of Al-goethites in 6M HCI. Clay M inerals, 19,

Schwertmann, U. 1988. Some properties of soil and synthetic ironoxides. In: Iron in Soils and Clay Minerals. (eds J.W. Stucki, B.A.Goodman and U. Schwertmann), pp. 203-250. North AtlanticTreaty Organisation, Advanced Studies Institute S eries C, Vol. 217,D. Reidel, Dordrecht.

Strauss, R. Brilmmer, G.W. & Barrow, N.J. 1997. Effects ofcrystallinity of goethite: I. Preparation and properties of goethitesof differing crystallinity. European Journal of Soil Science, 48,

Torrent, J., Barr6n, V. & Schwertmann, U. 1990. Phosphateadsorption and desorption by goethites differing in crystal morph-ology. Soil Science Society of America Journal. 54, 1007- 1012.

371-382.

9- 19.

87-99.

Torrent, J. 1991. Activation energy of the slow reaction betweenphosphate and goethites of different morphology. AustralianJournal of Soil Research, 29, 69-74.

Torrent, J., Schwertmann, U. & Barr6n, V. 1992. Fast and slowphosphate sorption by goethite-rich natural materials. Clays andClay M inerals, 40, 14-21.

Van der Zee, S.E.A.T.M. & Van Riemsdijk, W.H. 1990. Modelfor the reaction kinetics of phosphate with oxides and soil.In : Interactions at the Soil Colloid-Soil Solution Interface,(eds G.H. Bolt et al .) ,pp. 205-239. Kluwer Academic Publishers,Dordrecht.

Van Riemsdijk, W.H., Boumans, L.J.M. & De Haan, F.A.M. 1984.Phosphate sorption by soils: I. A model for phosphate reactionwith metal-oxides in soil. Soil Science Society of AmericaJournal, 48,537-541.

Willett, LR. & Cunningham, R.B. 1983. Influence of sorbedphosphate on the stability of ferric hydrous oxide under controlledeH and pH conditions. Australian Journal of Soil Research, 21,

Willett, I.R., Chartres, C.J. & Nguyen, T.T. 1988. Migration ofphosphate into aggregated particles of ferrihydrate. Journal of SoilScience, 39, 275-282.

Yiacoumi, S. & Tien, C. 1995a. Modelling adsorption of metal ionsfrom aqueous solution. I. Reaction-controlled cases. Journal ofColloid and Interface Science, 175, 333-346.

Yiacoumi, S. & Tien, C. 1995b. Modeling adsorption of metal ionsfrom aqueous solution. 11. Transport-controlled cases. Journal ofColloid and Interface Science, 175, 347-357.

301 -308.

0 997Blackwell Science Ltd, European Journal of Soil Science, 48, 101-114

Documents

Effects of crystallinity of goethite. II. Rates of sorption and desorption of phosphate.pdf