KINETICS OF PHOSPHATE SORPTION AND DESORPTION BY SYNTHETIC ALUMINOUS GOETHITE BEFORE AND AFTER THERMAL TRANSFORMATION TO HEMATITE

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1 Clay Minerals (1996) 31, KINETICS OF PHOSPHATE SORPTION AND DESORPTION BY SYNTHETIC ALUMINOUS GOETHITE BEFORE AND AFTER THERMAL TRANSFORMATION TO HEMATITE H. D. RUAN AND R. J. GILKES Soil Science and Plant Nutrition, Faculty of Agriculture, University of Western Australia, Nedlands, 6907, Australia (Received 16 January 1995; revised 19 June 1995) A B S TRACT: Goethite is an important component of many soils and its transformation to hematite by fire may affect phosphate retention. This study deals with the kinetics of phosphate sorption and desorption by synthetic aluminous goethites and their dehydroxylated products. Phosphate sorption and subsequent desorption were measured for periods up to 128 h. Phosphate sorption increased with equilibrium solution concentration, A1 substitution and surface area, and decreased with increasing crystal size. The trend in amount of phosphate sorbed was: partially dehydroxylated goethite > hematite > goethite. Sorption and desorption kinetics are well described by the modified Elovich equation. Desorption of a minor proportion of the sorbed P was rapid and this was sometimes followed by minor resorption. The amount of P desorbed directly reflected the amount that was initially sorbed. Acid dissolution of the Fe oxides with sorbed phosphate suggested that most phosphate was present at the crystal surface and not in relatively poorly accessible micropores. Aluminium substitution in Fe oxides in soils is very common and such materials usually consist of small crystals with a large surface area that provide a large capacity to sorb anions. Iron oxides affect the phosphate (P) sorption characteristics of soils and other natural systems (Barrow & Shaw, 1975; Torrent, 1987; Torrent et al., 1990). Phosphate sorption by Fe oxides is considered as specific sorption or chemisorption (Schwertmann & Taylor, 1989) and is influenced by many intrinsic properties of Fe oxides as well as extrinsic factors, e.g. P concentration in solution (Laverdi6re & Karam, 1984), time of contact (Barrow, 1983), ph (Barrow, 1984), temperature (Barrow, 1983) and cation status (Stoop, 1983). The kinetics of P sorption by soils and synthetic goethites consist of a rapid reaction followed by a slow reaction (Ainsworth & Sumner, 1985). Ainsworth et al. (1985) and Torrent et al. (1990) indicated that the kinetics of P sorption may be related to, inter alia, crystal morphology, synthesis mode, A1 substitution and specific surface area. Hingston et al. (1974) believed that the apparent lack of reversibility of sorption isotherms (i.e. hysteresis) could be related to the different kinetics of sorption and desorption. Phosphate sorption is apparently only partly reversible although full reversibility may occur over a very long time (Madrid & Posner, 1979). A high proportion of the phosphate retained on goethite after desorption is isotopically exchangeable so that hysteresis between sorption and desorption may be attributed to the slow desorption of phosphate (Neoh, 1975). Colombo et al. (1994) consider that the presence of sites of different affinity for P could be a cause of hysteresis as could be the limited accessibility, by competing ions, to P sorbed on sites within nanometre size micropores and defects within crystals. Such sites are particularly abundant in hematite that is formed by the dehydroxylation of goethite at low temperatures (Watari et al., 1979, 1983). Micropores have been considered to influence P desorption from ferrihydrite (Willett et al., 1988) and hematite (Colombo et al., 1994). This study examines the influence of properties of heated synthetic aluminous goethite on the kinetics of P sorption and desorption. Such materials are likely to exist in nature; fully or partly dehydroxylated goethite occurs in goethitic soils that have been heated by forest fires or by burning of agricultural residues which is a common management practice. Similar materials are also O 1996 The Mineralogical Society

2 D. Ruan and R. J. Gilkes formed during industrial process such as the high temperature Bayer processing of goethitic bauxite. MATERIALS AND METHODS Synthesis of Fe oxides The Al-goethite was synthesized from ferrous salts using a method similar to that of Goodman & Lewis (1981). Quantities of A1C13.6H20 and FeCIz.4H20 were mixed to give [A13+/FeZ++A13+] mole percentages equivalent to 0, 10, 20 and 30 mole% A1. Cation concentrations (i.e. Fe z+ + AI 3+) were set to 0.1 M in a total solution volume of 4000 ml. Sodium bicarbonate solution was used to buffer the system at approximately ph 7 during the process of oxidation with ~5 mmol/l excess of NaHCO3 being present. Air for oxidation was delivered through a sintered glass bubbling tube at ml/min. The ph remained at and then rose to ~ 8.2 at the completion of oxidation. A solution of 0.2 r~ ammonium oxalate at ph 3.0 was used in the dark to extract poorly crystalline compounds from precipitates, with five consecutive extractions being made at which stage dissolved Fe was at a constant low level (McKeague & Day, 1966). The precipitate was washed three times with deionized water and twice with acetone and then dried at 110~ in an oven. The presence of residual oxalate was investigated by infrared (IR) spectrometry but none was detected. The dry materials were gently crushed and stored in a desiccator. All subsequent measurements described below were carried out on the materials remaining after oxalate extraction. The Al-substituted hematite was formed by heating sub-samples of AIsubstituted goethite in a muffle furnace for one h at temperatures ranging from ~ Each sub-sample was heated at a selected temperature. Characteristics of Fe oxides The chemical composition of the goethites was determined by dissolving 10 mg of sample in 20 ml of concentrated HC1 at 60-80~ After the solid had completely dissolved, the solution was made up to 100 ml with deionized water. The Fe and A1 in solution were determined using atomic absorption spectrometry (AAS). An air/acetylene flame was used for Fe and a nitrous oxide/acetylene flame was used for AI. The measured amounts of AI substitution were 0, 9.7, 19.7 and 30.1 mole% for nominal A1 molar contents of 0, 10, 20 and 30 mole%, respectively. X-ray diffraction (XRD) analyses using Cu-K~ radiation were carried out on a computer-controlled Sieray modification of a Philips 1050 vertical goniometer with a graphite diffracted-beam monochromator. Sub-samples of goethite and hematite were taken from the original and heated materials and prepared for XRD by grinding 200 mg of each sample and back filling an aluminium plate holder. A quantity of 10% by weight of NaC1 was added during grinding to provide an internal standard for spacing and line broadening measurements. Scan speed was 0.3 ~ 20 per rain and step size was 0.01 ~ 20 so that counting time was 2 s/step. Patterns were run from 10 to 70 ~ 20. Mean coherence length (MCL) was calculated from the corrected widths at half height of XRD reflections using the XPAS program (Singh & Gilkes, 1992). The [hematite/(goethite + hematite)] XRD intensity ratio was calculated from the ratio of areas under the 110 reflection of goethite and the 102 reflection of hematite, the integrated areas being provided by the XPAS program. Transmission electron microscopy (TEM) was carried out by depositing a drop of suspension of Fe oxides dispersed in distilled water on a carboncoated copper grid. Micrographs were obtained using an Hitachi HUIlB transmission electron microscope. Specific surface area was determined by the BET method (six-point linear plot) using nitrogen adsorption. Phosphate sorption kinetics experiment A KH2PO4 stock solution in 0.1 M KC1 (ph 6) containing 100 gg P ml -~ was made first and was then diluted to 20 and 40 lag ml -j with 0.1 M KCI. Twenty milligram sub-samples of Fe oxide were placed in 50 ml acid-washed polythene bottles before adding 20 ml of P solution (20 and 40 gg ml l). Two drops of toluene were added to each bottle to retard microbial growth. The final volume was made up to 20 ml with 0.1 M KC1 to maintain a solid:solution ratio of 1:1000. The bottles were sealed, placed in an insulated chest on a horizontal platform shaker and shaken longitudinally with a speed of 120 cycles/rain at 25 -I- I~ At intervals of time after P addition (0.5, 1, 2, 4, 8, 16, 32, 64 and 128 h), 1 ml of suspension was withdrawn with a plastic syringe and passed through a 0.22 gm

3 Phosphate sorption and desorption by goethite 65 Millipore filter into a 10 ml polythene bottle. Phosphate in the filtered solution (diluted with 0.1 M KC1 for expected concentration) was determined by the phospho-molybdate blue method (Watanabe & Olsen, 1965). Absorbance values were obtained at a wavelength of 710 nm. The reduction in P content in solution was assumed to be the amount of P sorbed. Phosphate desorption kinetics experiment Phosphate desorption kinetics measurements were carried out with the sub-samples of goethite and its dehydroxylated products that had sorbed P from solutions of initial concentrations of 20 and 40 lag P m1-1 in 0.1 M KCI for 128 h. After P sorption had occurred, the suspension was centrifuged and the solid was washed twice with acetone to remove residual solution. To measure desorption, 11 ml of 0.1 M KC1 together with two drops of toluene to retard microbial growth were then added to maintain a solid:solution ratio of 1:1000. This suspension was sealed in a polythene bottle, placed in an insulated chest at 25 + I~ and shaken longitudinally with a speed of 120 cycles/ rain on a horizontal platform shaker. At various times (0.5, 1, 2, 4, 8, 16, 32, 64 and 128 h), 1 ml of aliquot was withdrawn with a plastic syringe, passed through a 0.22 lain Millipore filter into a 10 ml polythene bottle. The filtered solution was diluted with 0.1 M KC1 and P was determined by the phospho-molybdate blue method (Watanabe & Olsen, 1965). Absorbance values were obtained at a wavelength of 710 nm. The amount of P in solution was assumed to be the P released from the oxide. Acid dissolution of P and Fe and resorption of P during acid dissolution A sub-group of samples of oxides after 128 h of P sorption at an initial concentration of 40 lag ml -~ was used for acid dissolution to measure the congruency of dissolution of Fe and P. The suspension remaining after 128 h P sorption was centrifuged and the solid was washed twice with acetone to remove solution. Then 1 M HC1 solution was added to the residue to maintain a solid:solution ratio of 1:2000. The suspension was sealed in a polythene bottle, placed in an insulated chest at 30 + I~ and shaken longitudinally with a speed of 400 cycles/min on a horizontal platform shaker. An aliquot (1 ml) was withdrawn from the suspension with a plastic syringe at time intervals of 1, 2.5, 5, 10, 20, 40, 80, 160, 320, 640, 1280 and 2560 min, passed through a 0.22 lam Millipore filter into a 10 ml polythene bottle. The filtered solution was diluted with 1% (v/v) HC1 for phosphate and Fe determinations using the phospho-molybdate blue method (Watanabe & Olsen, 1965) and atomic absorption spectrometry, respectively. A test for P resorption by Fe oxide during acid dissolution was carried out on 0.01 g sub-samples of , 19.7 and 30.1 mole% Al-substituted goethites in 50 ml polythene containers. Quantities of 20 ml of a solution consisting of 20 lag P m1-1 in 0.1 M HC1 was added to each container to provide a solid:solution ratio of 1:2000. Further procedures followed those for the acid dissolution of P and Fe as described in the previous paragraph. RESULTS AND DISCUSSION Properties of goethite Properties of synthetic Al-substituted goethites and their dehydroxylated products are shown in Table 1. Goethite started to transform to hematite at ~200~ and had completely altered at 260~ The [hematite/(hematite + goethite)] XRD intensity ratio increased with increasing temperature in the range ~ Weight loss over the temperature interval 110 to 620~ increased as A1 substitution increased. About 4-6% excess water remained in the samples when goethite had completely transformed to hematite at 260~ Surface areas of goethite and hematite ranged from 147 to 233 m 2 g-i and crystal size (mean coherently diffracting length) ranged from 5 to 11 nm. The AI substitution in goethite reduced growth along the crystallographic a-axis direction and increased surface area (Schulze & Schwertmann, 1984). Particles of non-substituted goethite had a lath-like shape with a domainic structure whereas particles tended to be equant and smaller with increasing A1 substitution (Fig. 1). Particles of hematite, formed from goethite at temperatures equal to or lower than 270~ inherited the crystal shape of the precursor goethite (Fig. 1). Full details of the properties of these samples have been published previously (Ruan & Gilkes, 1995a). A microporous fabric (particularly for low A1 samples) and structural defects exist in crystals that have only partly altered from goethite to hematite (Watari et al., 1983). These properties may influence the kinetics of P

4 66 H. D. Ruan and R. J. Gilkes TABLE 1. Properties of goethites (Gt) synthesized from the ferrous system and hematites (Hm) formed by dehydroxylation of goethites. A1 substitution Heating IHm/ 2WL 3MCLGtllo MCLHmlI0 4SA (mole%) (~ (Gt+Hm) (%) (nm) (nm) (mzg-~) i t ~Hm/(Gt+Hm) = XRD intensity ratio calculated from the ratio of areas under the 110 reflection of goethite and the 102 reflection of hematite. ~WL = weight loss between 110 and 620~ (TGA). 3MCL = mean coherence length derived from XRD line broadening. 4SA = BET specific surface area. sorption and desorption and it is this hypothesis that is the principal subject of this investigation. Kinetics of phosphate sorption Amounts of P sorbed for two initial P concentrations (20 and 40 gg ml -l) in solution were determined at various times. Phosphate was sorbed rapidly during ~ 1 h followed by a slow sorption that had virtually ceased by 128 h. The amount of P sorbed increased as A1 substitution and equilibrium solution concentration increased. The largest amounts of P sorbed were 19 mg g-i (30.1 mole% A1 goethite heated at 220~ and 24 mg g-i (30.1 mole% A1, 240~ for initial P concentrations of 20 and 40 gg ml -l, respectively. The largest amounts of P sorbed were mostly by the partially dehydroxylated samples. Elovich-type equations have been applied to chemisorption (Low, 1960) and heterogeneous isotopic exchange reactions (Atkinson et al., 1970). For P sorption the modified Elovich equation can be expressed as, Xs = a~ + b~ lnt (1) where X~ is the amount of P sorbed at time t, a~. is the intercept which is dependent on the type of sorbent and is independent of sorbate (i.e. form of P added) and b,. is the slope which is a function of both sorbent and sorbate and may be considered to be the rate constant (Chien & Clayton, 1980; Torrent, 1987). Plots of the data for eqn. (1) are shown in Fig. 2 and most of the experimental data are quite well described by the equation (r > 0.95"**, P < 0.001). Slight systematic departures from linearity (i.e.

5 Phosphate sorption and desorption by goethite 67 Fro. 1. Transmission electron micrographs of goethites and their hydroxylated products, (a) 0 mole% AI (110~ (b) 30.1 mole% AI (110~ (c) 9.7 mole% AI heated at 200~ (d) 0 mole% A1 heated at 240~ (e) 0 mole% A1 heated at 260~ and (f) 30.1 mole% AI heated at 260~ Bar in (f) represents scale for all examples except (d).

6 68 H. D. Ruan and R. J. Gilkes E.Q Q- 30 (a) 110~ 10 o 30- (b) (c) 270~ r=0.999"** r=0.998**~ r=0.087"*" r=0.005"** 220~ r=0.005"* 200~ C i 9 i 9 i, ],, 9 i r=0.990"" r=0.98b *~ r=0.970"* 9 0 mole% AI mole% AI r=0.987"** mole% AI mole% AI r=0.984 "*~ ~ r=0,989"* 0 9, 9,,, -,, i -, Int (t in hour) r 0990 FIG. 2. Representative plots of kinetics of phosphate sorption for an initial concentration of 40 I, tm m1-1 described by the modified Elovich equation; (a) goethite, (b) dehydroxylated goethite and (c) hematite. curvatures) exist, especially for the dehydroxylated samples. The experimental data were also fitted to a modified Freundlich equation (Kuo & Lotse, 1973; Barrow & Shaw, 1975) and a modified first order kinetics equation (Kabai, 1973). Overall the closeness of fit of data to these three equations is in the sequence: modified Elovich ~ modified first-order kinetics > modified Freundlich. Other kinetics equations, such as first-order kinetics (Griffin & Jurinak, 1974), second-order kinetics (Kuo & Lotse, 1972) and diffusion equations (Cook, 1966) were also evaluated but provided poorer fits to the data. The amount of P sorbed and coefficients of the modified Elovich equation Torrent et al. (1990) reported that the amount of P sorbed after one day at an equilibrium concentration of 0.19 mmol ranged from 2.2 to 2.8 p, mol m -2 for goethites of different crystal morphology with values of surface area in the range m 2 g-1. Similarly Ainsworth & Sumner (1985) indicated that for an initial concentration of 1.2 mmol, the amount of P sorbed per unit area increased from 2.2 to 4.1 gmol m -2 as A1 substitution in goethite increased from 0 to 30 mole% and surface area ranged from 68 to 284 m 2 g-l. For the present samples, the amount of P sorbed at 128 h for an initial concentration of 1.3 mmol at ph 6 was in the range , , and p, mol m -2 for 0, 9.7, 19.7 and 30.1 mole% AI, respectively, for goethite samples that had been heated at various temperatures from 110 to 270~ and with surface area values ranging from 147 to 233 m 2 g-l. Highly significant relationships exist between the amount of P sorbed and surface area (r = 0.78**** and 0.89**** for 20 and 40 gg m1-1, respectively, P = ). Coefficients, as and b~., from the modified Elovich equation, are also closely related to surface area (Table 2). Thus P retention by these goethites and their dehydroxylated products can be considered as predominantly a simple surface sorption reaction with no major influence of mineral species. The linear relationships between a~ and b~ and mole% A1 substitution are also highly significant (P < 0.01, Table 2). The amount of P sorbed is indicated by the a~. coefficient and varied with the extent of transformation of goethite to hematite, increasing for temperatures where goethite had partly changed to hematite, and then decreasing for higher temperatures as surface area decreased and hematite crystal size increased. Thus the values of coefficients a,. and b, from the modified Elovich equation show no simple trends with heating temperature. The rate constant (b,.) for Al-hematite is smaller than for A1- goethite whereas there was little difference for the zero Al-substituted Fe oxides (Table 3). The partially dehydroxylated goethites consisted of microporous mixtures of goethite and hematite of small crystal size (MCL) and with the highest surface area (Table 1) so that P sorption increased. Heating at above 220~ induced the development of structural defects including voids formed by the

7 Phosphate sorption and desorption by goethite 69 TABLE 2. Correlation matrix (r values) for P sorption, desorption and retention and various properties of synthetic variously dehydroxylated Al-goethite t. Variable P, Pa Pr as bs ad ba-i bd-ii 20P Mole%Al 0.91"*** -0.92**** 0.92**** 0.83**** 0.62** -0.87**** -0.75*** T~ * * 0.47* SA 0.78**** -0.80**** 0.79**** 0.84**** 0.75*** -0.54* -0.73*** MCL -0.53* * -0.59** P Mole%Al 0.93**** 0.52* 0.93**** 0.86**** 0.79**** 0.59** * T~ * SA 0.89**** 0.49* 0.87**** 0.84**** 0.75*** 0.61"* MCL -0.52* * -0.51" * 0.36 * **, *** **** P = 0.05, 0.01, and , respectively. ~" Ps, Ptl, Pr = the amounts of P initially sorbed, P desorbed after 128 h and P retained (mg g-l); SA = surface area (m 2 g-l); Toc = heating temperature (~ and MCL = mean coherence length (nm) derived from XRD line broadening of the 110 reflections for goethite and hematite, respectively; a (mg g-l) is the intercept (time = 1 h), as for sorption and aa for desorption; b (rag g-1 h-l) is the slope of the modified Elovich equation, bs for sorption and b d for desorption, ba-i and ba-ii are the slopes of part I and part II of the two stage fits for desorption; 20P and 40P indicate initial P concentrations of 20 and 40 I-tg m1-1 for P sorption. TABLE 3. Coefficients t derived from linear fits of P sorption data to the modified Elovich equation as affected by Al substitution and heating for various initial P concentrations in solution. A1 Heating P concen~ation as b~ r (mole%) (~ (~g m1-1) *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** t as (mg g-l) is intercept depending on properties of sorbent and b~ (mg g-lh-l) is the slope depending on properties of both sorbent and sorbate; ***, P =

8 70 H. D. Ruan and R. J. Gilkes Surface P sorbed onto surface sites (a) original goethite _ domains (b) dehydroxylated goethite --~ hematite, showing voids (c) High temperature hematite, voids healed FIG. 3. Schematic of phosphate sorption by (a) goethite (ll0~ (b) dehydroxylated goethite (~220~ and (c) hematite (t> 260~ escape of evolved water and formation of nm-size micropores associated with the volume change necessary for this topotactic transformation (Watari et al., 1983). The nature of these morphological changes and their possible effects on P sorption are illustrated by the sketch shown in Fig. 3. The reduction in the amount of P sorbed resulting from heating at higher temperatures between 220 ~ and 260~ for 0 to 19.7 mole% AI and between 240 ~ and 260~ for 30.1 mole% AI was due to the decrease in surface area that resulted from the growth of hematite crystals and healing of structural defects due to surface and volume diffusion (Ruan & Gilkes, 1995a). Note that despite these considerable changes in crystal structure and morphology due to heating, it appears that it is the total exposed area that largely determines the extent of P sorption. Stepwise linear regression analysis was used to develop a highly predictive relationship between the maximum amount of P sorbed (Ps) for an initial P concentration of 40 gg ml -~ and the above mentioned properties of heated goethites. P~ = X Xz X3 r = 0.95, where Xl, X2 and X3 are mole% A1 substitution, surface area and mean coherence length, respectively,

9 Phosphate sorption and desorption by goethite 71 4~ (a) 110~ 3~ r ~.~, 2. 6, 5- (d) 110~ ~ 0 rnole% AI,t g.7 rnole% AI mole% AI 30.1 mole% AI " I I P'"I ' I ' I "' 3 i I ~ i '% ' '1 "1",! i 4- E 2. 0 "0 1 (b) 220~ 230~ 230~ - _ r 240~ (e) 220~ - 220~ r 210~ 200~ i i! i 4-3. (e) 270~ ~ ' 6. (f) 270~ I A i "1 i! ' I ~ i i i i! i Time (h) Ftc. 4. Representative plots of the amount of P desorbed from Fe oxides as a function of time where P sorption had occurred at initial P concentrations of 20 Ixg ml -~ (a, b, c) and 40 tag ml -~ (d, e, f). Kinetics of P desorption Figure 4 shows that P desorption in 0.1 M KC1 at ph 6 was initially rapid and was then slow or reversed with prolonged time. Generally there was substantial desorption within the first minute of contact followed by either further minor desorption or resorption during 128 h. The samples which had been equilibrated in the 20 ~tg P m1-1 solution (Fig. 4a,b,c) largely showed minor continuing desorption from 1 rain up to ~20 h followed by minor resorption up to 128 b. Resorption was greater and commenced earlier for the high AI and

10 D. Ruan and R. J. Gilkes heated samples. Resorption occurred earlier for hematite (Fig. 4c) than for goethite (Fig. 4a). For samples where P sorption had occurred at 40 gg P ml -l (Fig. 4d,e,f), most isotherms showed very rapid desorption followed by slow desorption up to 20 h which then remained at constant values until 128 h. Resorption only occurred for goethite with large amounts of A1 substitution. The P desorption data were fitted to the modified Elovich equation, Xj = ad + bet lnt (2) where Xd is the amount of P desorbed at time t, ad is the intercept and b d is the slope which provides an indication of the rate of desorption (Torrent, 1987). An adequate description of most data is obtained by two straight lines (slopes b,vi and b,t-ii) when fitted to eqn. (2) (plots not shown). Negative values of b,t-ii (i.e. resorption) were obtained for samples where P sorption had occurred at 20 gg P ml -~, except for 0 mole% Al-hematite formed from goethite heated at 220~ Positive values of b,t-ii were obtained for samples where P sorption had occurred from a solution with an initial concentration of 40 gg P ml i except for 30.1 mole% Al-goethite heated at l l0~ These data could also be adequately fitted to modified Freundlich and first-order kinetics equations. The degree of fit was usually best for the modified Elovich equation and the same systematic departures from single straight lines were apparent for each equation, especially for partially dehydroxylated samples. Phosphate desorption/retention in relation to the properties of goethite and its dehydroxylated products The linear correlation coefficients between P sorption, desorption and retention measurements and crystal properties of the oxides are shown in Table 2. For both the low (20P) and high (40P) initial concentrations for P sorption (i.e. 20 and 40 lag P ml -~) the amount of P retained after 128 h desorption (Pr) was positively and significantly related to mole% AI and surface area (SA) as was also the case for P sorbed (Ps). For the 20P samples the amounts of P desorbed at 128 h and 1 h (a~l) were also closely related to these crystal properties (i.e. mole% A1 and SA) but in this case the relationships are negative. For the 40P samples these relationships are positive and less significant than for the 20P samples. Thus it appears that the amounts of P initially sorbed and the amounts retained after 1 and 128 h desorption directly reflect the surface area of the Fe oxides. The close relationship with mole% A1 is mostly a consequence of surface area being correlated with mole% A1 (r = 0.93, P = ). The slopes of the Elovich equation for desorption (bj-i and bd-ii) are usually not closely related to crystal properties; with the exception of the initial slope (ba-i) for 20P where the slope decreased systematically with mole% A1 and surface area. The existence of simple relationships between P sorption and desorption parameters and surface area indicates that the microporosity induced by heating simply affects changes in P sorption/desorption through surface area and has no other effect. Acid dissolution of Fe oxides and P resorption Sigmoidal dissolution curves were obtained for goethite in 1 M HC1 whereas deceleratory dissolution curves were obtained for partially dehydroxylated goethite and hematite (graph not shown). This finding is consistent with the results reported by Schwertmann (1984) and Ruan & Gilkes (1995b). If it is assumed that the P sorbed by Fe oxides is all located on surface sites that are fully exposed to solution and that dissolution occurs at an equal rate from all exposed surfaces (Cornell et al., 1974), then most sorbed P would be released during the early stages of acid dissolution of the oxides. If some P is located in micropores that are not readily accessible to the acid, then diffusion into and out of these micropores will retard dissolution and this P may not be released until considerable amounts of Fe oxide have dissolved. Figure 5 shows that most P dissolved rapidly, especially for goethite (110~ and goethite/hematite (220~ where more than 90% of the P had dissolved before 20% of the Fe oxide had dissolved. In contrast, 85% of the hematite (260~ had dissolved before 90% of the P had dissolved, indicating that some P may have been located in poorly accessible micropores within hematite. These data need to be interpreted with caution as resorption of P by Fe oxides probably occurred during acid dissolution. This possibility was investigated by dissolution of goethites in acid containing 20 lag P m1-1. Up to 20% of the P present in the acid was sorbed by the goethite

11 - Phosphate sorption and desorption by goethite A ~1~ ~ 0 O0 100" (a) 0 mole% AI v 6O _> o 40,._~ ~ /k Goethite (110~ 9 Goethite/hematite (220~ O Hematite (260~ i i i i i Fe dissolved (%) FIG. 5. Plot of % P dissolved vs. % Fe dissolved in 1 M HC1 for Fe oxides with sorbed P. (Fig. 6). This proportion is similar to the amount of P that appears to be retained by Fe oxides during the early stage of dissolution (Fig. 5). Thus it is probable that most sorbed P was released from goethite and 220~ heated goethite during the initial stage of dissolution and that little P was present in inaccessible or protected sites in micropores. Resorption of P is unlikely to explain fully the retention of some P during dissolution of hematite (i.e. 260~ heated goethite, Fig. 5). In this instance, the more rapid dissolution of Fe from the fully exposed exterior surface of crystals of hematite (up to eight times faster than for goethite on a unit surface area basis, Ruan & Gilkes, 1995b) would contrast more strongly with the relatively slower dissolution of that part of the P within relatively poorly accessible micropores within the hematite where diffusion into and out of the micropores provide rate limiting constraints. CONCLUSIONS Phosphate sorption by synthetic Al-substituted goethites and their dehydroxylated products is a surface reaction consisting of an initially rapid reaction followed by a slow reaction. The coefficients derived from the modified Elovich kinetics equation changed in a complex manner with heating temperature. These changes reflect the increase in surface area due to formation of microporosity in the first-formed hematite followed by healing of structural defects and a reduction of surface area at higher temperatures. The amount of P desorbed from these Fe oxides also predominantly reflects the surface area of the o 0 III 0._~ "O 9 10 loo. c t~ Q (b) 30.1 mole% AI! i i i i Time (rain) FIG. 6. Phosphate sorption during acid dissolution of Fe oxides, dashed line indicating 100% of added P remaining in solution. oxides and is well described in most instances by a two-line Elovich plot. Acid dissolution data indicate that for goethite and goethite/hematite, little or no P was located in relatively poorly accessible micropores. Some P may be present in micropores in hematite but this partitioning of P between exterior surface and micropore sites was hardly reflected in the kinetics of P desorption from hematite. REFERENCES AINSWORTH C.C. & SUMNER M.E, (1985) Effect of aluminum substitution in goethite on phosphorus adsorption. II. Rate of adsorption. Soil Sci. Soc. Am. J. 49, AINSWORTH C.C., SUMNER M.E. & HURST V.J. (1985) Effect of aluminum substitution in goethite on phosphorus adsorption: I. Adsorption and isotopic exchange. Soil Sci. Soc. Am. J. 49,

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