Maataloustieteellinen A ikakauskirja Vol. 62: I—B, 1990 Sorption capacity of phosphate in mineral soils I Estimation of sorption capacity by means of sorption isotherms RAINA NISKANEN University of Helsinki, Department of Agricultural Chemistry, SF-00710 Helsinki, Finland Abstract. The sorption capacity ofphosphate in seven soil samples (clay content I—7o %, organic carbon content 0.8—10.7 %, soil pH 4.2—5.3, oxalate-extractable Al 11—222 and Fe 11—202mmol/kg soil) was studied by means of sorption isotherms. The soils were equilibrat- ed, for two to seven days at + 5 and + 20°C, with solutions containing phosphate o—lo0—10 mmol/1 (0 —200 mmol/kg soil) at a constant ionic strength of0.01 . Prolongation of the reaction time increased the sorption of phosphate only partially. The rise in temperature, from +5 to + 20°C, increased the sorption from higher phosphate concentrations. At + 20°C, the sorption curves of three soils showed a sorption maximum of 4, 19 and 34 mmol/kg soil. The sorption data of six soils was in accordance with the Langmuir equation; the sorption maximum ranged from 15 to 119 mmol/kg soil, and were of the same magnitude as the maximums determined ex- perimentally. Index words: Langmuir equation, equilibrium constant, sorption maximum, temperature Introduction The sorption of phosphate reflects the abil- ity of soil to retain anions of weak acids. This property is related to the content of active hydrous oxides of aluminium and iron in soil. The retention of phosphate is usually ex- pressed by the amount or proportion of ad- ded phosphate sorbed under chosen condi- tions. The ‘anion exchange capacity’ of soil, determined according to the method devel- oped by Piper (1944), was previously used to measure the phosphate sorption capacity and to estimate the degree of saturation (e.g. Williams et al. 1958, Williams 1959, 1960, Williams and Knight 1963). In determina- tion of ‘anion exchange capacity’, soil is treat- ed with 0.33 M ammonium phosphate at pH 4, and the sorbed phosphate is extracted with hot sodium hydroxide. It is acknowledged, however, that this method is rather drastic (Williams 1960) and may not give a realistic picture of the sorption capacity. Nowadays phosphate sorption is often 1 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=9w5ypmXFI9EJVdT4.fJl7N3zJzAyrxY6IkV94sw.HG9nv02nnWehZWFPBVD7qzYF0VIvOMmFTqjuJa3Y7j-WXonkqJbNUzb2p5fbH-50riq-cmEUhlxO6kgEXihyzPAdawo3HKZxbQKf9V6iWkIKZA6q5qT2L9we3qmBz-VeR26lVqjTWTLF2MvB1BRD8wb7aZOiufYvRjFhIg described by sorption isotherms, interpreted by means of adsorption equations. Earlier studies have applied only the Freundlich equa- tion (e.g. Russell and Prescott 1916, Teräs- vuori 1954, Kaila 1959 a, 1959 b, 1963). The Langmuir equation is first applied to phos- phate sorption by Olsen and Watanabe (1957). The applicability of the Langmuir equation to describe phosphate sorption on Finnish soils has not been studied in detail. The Freundlich equation was originally em- pirical, and lacked a theoretical foundation (Bohn et al. 1985). It implies that the energy of adsorption decreases logarithmically as the fraction of covered surface increases. The Freundlich equation can be derived theoreti- cally by assuming that the decrease in adsorp- tion energy occurring as the surface coverage increases is due to surface heterogeneity, but in most cases, the surface heterogeneity is un- known. The frequent good fit of adsorption data to this equation is influenced by the in- sensitivity of log-log plots and the flexibility afforded curve fitting by the two empirical constants. Further, the Freundlich equation has the limitation that it does not predict a maximum adsorption capacity. Theoretically, the Langmuir equation can be used to describe the chemical equilibrium for the ligand exchange reactions of anions (Aura 1980). It is assumed that the adsorp- tionenergy does not vary with surface cover- age. The Langmuir equation has the advan- tage that it defines a limit for adsorption on a given array of sites meeting the Langmuir model criteria. This apparent limit has been used to estimate the adsorption capacity of soils for, e.g. phosphate. The applicability of the Langmuir equation can be improved by using the equation for the two adsorption sur- faces (Syers et al. 1973). This study was performed in order to inves- tigate preliminarily the availability of the Langmuir isotherms in estimating the phos- phate sorption capacity in Finnish soils. Material and methods The material consisted of seven mineral soil samples; three surface soils and four deeper layer soils (Table 1), which were air- dried and ground to pass a 2-mm sieve. The particle-size distribution of the inorganic mater in the soil samples was determined by the pipette method (Elonen 1971). The pH of the soil was measured in a soil-0.01 M CaCL suspension (1:2.5) (Ryti 1965). The organic carbon content of the soil samples was determined using a modified (Graham 1948) Alten wet combustion method. Aluminium and iron were extracted (ratio 1:20 w/v, shak- ing time 2 h) with 0.05 M ammonium oxalate (pH 2.9) (Niskanen 1989) and determined by atomic absorption spectrophotometry. Isotherms for phosphate sorption were de- termined at +5 and +2O°C. The soils were treated with solutions of different phosphate concentrations at a constant ionic strength of Table !. Soil characteristics. Sample Locality Depth, pH Organic Particle-size distribution (nm) % Oxalate- N°- Cm (CaCy C ' % <2 2-20 20-60 60-200 >2OO eXUaCtable Al Fe Fe/Al mmol/kg 1 Vaala 20—40 4.2 1.3 1 3 3 93 0 84 3 0.04 2 Viikki 20—40 4.6 0.8 2 17 35 56 91 39 0.42 3 Viikki o—2o 5.3 4.4 10 7 15 61 7 24 144 6.05 4 Imatra o—2o 5.1 3.6 13 20 27 31 9 160 52 0.33 5 Viikki 20—40 5.0 1.0 26 2 23 42 6 11 11 1.00 6 Viikki 20—40 4.8 2.6 47 30 18 5 0 68 202 2.99 7 Imatra o—2o 4.7 10.7 70 18 6 3 3 222 65 0.29 2 0.01 adjusted with KCI as follows (mmol/1): KH 2P0 4 0 0.5 1.0 1.5 2.5 3.5 5.0 7.0 8.5 10.0 KCI 10.0 9.5 9.0 8.5 7.5 6.5 5.0 3.0 1.5 0 To inhibit microbial activity, the solutions contained 0.01 °7o NaN3 . Soil, 5 g, was treated with 100 ml solution (P-addition o—2oo mmol/kg soil) for 2 —7 days. The pH of suspensions was measured in the beginning of the experiment. The sus- pensions were then shaken for four hours, al- lowed to stand overnight; they were shaken during the reaction period daily for eight hours and left to stand overnight. At the end of the experiment, the suspensions were shaken for half an hour, the pH of the sus- pensions was measured and the supernatants were filtered. The phosphorus concentration of filtrates was determined by a modified molybdenum blue method (Kaila 1955) and by an ammonium vanadate method (Jackson 1958). The amount of retained phosphate was calculated as the difference between thephos- phate quantity present initially and that re- maining in the supernatant. The experiment was carried out in duplicate. Results and discussion The sorbed amounts of phosphate and the corresponding phosphate concentrations in the equilibrium solution are presented in Fig. 1. As compared with doses used in the fertilization practice, the added amounts of phosphorus were very high; 1 mmol of P per kg soil corresponds to 62 kg per hectare (bulk density of soil 1 kg/dm 3 , the depth of plough Table 2. Final pH of suspensions. Soil pH 4.9—5.21 2 5.3—5.7 3 5.4—5.9 4 5.5—5.7 5 5.5—5.6 4.9—5.26 7 4.7—5.1 layer 20 cm). Such high amounts were used because thepurpose was to saturate the phos- phate sorption capacity of soils. The pH of the suspensions did not change much during the sorption or with an increas- ing phosphate concentration. The final pHs of the suspensions are given in Table 2. Prolongation of the reaction time did not significantly increase phosphate sorption on experimental soils at +2O°C. Prolongation of the reaction time from three to seven days in- creased the phosphate sorption on soil 3 at + 5°C from lower phosphate concentrations. The reaction time used in the experiment was rather long. It is often shorter in sorp- tion studies, e.g. 18 (Bache and Williams 1971) or 24 hours (Olsen and Watanabe 1957, Singholka et al. 1975) for phosphate. According to Haseman et al. (1950), the most rapid sorption of phosphate takes place in as little as half an hour. Rajan and Watkinson (1976) considered a reaction time of three hours to be sufficient, because allophane sam- ples studied retained during this time 80 % of the phosphate sorbed during four days. Ac- cording to Olsen and Watanabe (1957), phosphate sorbed during one day is 84— 100 % of that sorbed during three days, and is exchangeable by 32P. After the rapid initial reaction, the sorption of phosphate can pro- ceed slowly for weeks, but then not only sur- face adsorption is involved (Haseman et al. 1950, Olsen and Watanabe 1957, Juo and Maduakor 1974). The rise in temperature from +5°C to + 20°C increased the sorption of phosphate at higher concentrations, a finding in accor- dance with those of some earlier studies (Low and Black 1950, Muljadi et al. 1966, Kuo and Lotse 1974). This effect may be caused by the increased rate of phosphate sorption at higher temperature (Haseman et al. 1950, Gardner and Jones 1973). When the sorp- tion time is long and the phosphate concen- tration is high, phosphate is adsorbed not only on surfaces but is also migrated into fine pores of hydrous oxides. The rate of migration, in 3 4 Fig. 1. Sorption of phosphate on experimental soils as a function of the phosphate concentration in the equilibrium solution. particular, increases with therise in tempera- ture. At low phosphate concentrations, the soils adsorbed nearly all phosphate, nor did the sorption depend significantly on temperature. In the study of Hartikainen (1979), the ef- fect of the rise in temperature on the phos- phate concentration in the equilibrium solu- tion in contact with acid soils was slightly posi- tive, negative or nonexistent. The slight effect is due to the fact that the change of the stan- dard enthalpy in the adsorption of phosphate is slight, and changes in entropy largely de- termine the equilibrium of phosphate adsorp- tion in acid soils (Aura 1980). Deeper layer soils 1 and 2 represented soils with a low clay content. The sorption of phos- phorus was greater on soil 1 than on soil 2 (Fig. 1). The sorption curve of soil 1 at 20°C (3 d) flattened, when the sorption of P was about 30 mmol/kg soil. Soil 2 had a sorption maximum (3 d, 20°C) of about 19 mmol/kg soil. The concentration of extractable alumini- um was nearly equal in both soils, whereas the concentration of iron was greater in soil 2 (Table 1). On the basis of the sum of Al and Fe, the sorption of P may have been greater in soil 2. The oppositeresult might have been caused by the lower pH of soil 1 (Table 1). The sorption of phosphate increases with a drop in pH (Hingston et al. 1972). In addi- tion, soil 2 was coarser in texture than soil 1. The coarser texture means a smaller surface area and a smaller reacting surface. Surface soils 3 and 4 had nearly equal pHs and clay and organic carbon contents, but soil 3 contained mainly extractable iron and soil 4 aluminium (Table 1). The sorption curve of soil 3 (3 d, 20°C) showed no sorption maxi- mum, but the curve still rose with the greatest addition of P, the sorption being about 30 mmol/kg soil (Fig. 1). Soil 4 had a sorption maximum of 34 mmol/kg soil; thereafter the sorption decreased rather steeply. Soil 5, which had low contents of aluminium and iron, showed a sorption curve course in equal to that of soil 4; the maximum sorption was 4 mmol/kg soil (Fig. 1). Sorption curves which have a sorption max- imum and thereaftera decreasing course have seldom been presented in the literature. In the study of Rajan (1978), which concerned the sorption of sulphate on aluminium hydroxide, however, the sorption curve had a maximum and thereafter the sorption decreased as the concentration of sulphate in the reacting so- lution increased. The cause of the decreasing sorption is not clear, but it seems to be con- nected with a saturated sorption capacity. Decreasing sorption may perhaps result from the breaking of sorption surfaces. Rajan (1978), however, found no noteworthy release of aluminium during sorption. Also in the present study, therelease of Al and Fe did not seem to be very great. When experimental soils were extracted for four hours by 0.05 M KH 2P04, the concentration of which was fivefold that of the strongest sorption solu- tion, the amounts of iron and aluminium released were 40—70 and 370—740 pmol/kg soil, respectively. Soils 6 and 7 represented clay soils with nearly equal pH, and the sum of extractable aluminium and iron was also about the same; however, soil 6 contained mainly iron and soil 7 aluminium (Table 1). The sorption curves of both soils still rose with the greatest additions of P (Fig. 1). With the addition of 200 mmol/kg soil, the sorption on soil 6 was 66 mmol/kg soil (3 d, 20°C); on soil 7 it was 121 mmol/kg soil (7 d, 20°C). The great difference in sorption between soils 6 and 7 may have been caused partially by the much higher content of organic carbon in soil 7 (Table 1). Soil organic matter inhibits crystal- lization of hydrous oxides and enhances their reactivity (Williams et al. 1958, Schwert- mann et al. 1968). Langmuir equations and sorption maxi- mums for experimental soils are given in Tab- le 3. The Langmuir equation is based on the assumption that the energy of adsorption does not vary with the surface coverage, and may be written in linear form c_ _ _c_ 1 X x m kxm 5 where x is the quantity of sorbed phosphate, c the equilibrium phosphate concentration, xra the adsorption maximum and k a con- stant. A plot of c/x against c should give a straight line of slope l/xm, from which an adsorption maximum can be calculated, and the equilibrium constant k relating to bond- ing energy can be calculated from the inter- cept. The plots are curved over wide concen- tration range (Gunary 1970), which indicates that the bonding energy is not in fact constant and that there is no well-defined maximum. A possible reason for this is that sorbed phos- phate migrates to sub-surface layers. The Langmuir equation is limited to the range for which experimental data are available. Even in systems where the energy of adsorption is not strictly constant, the Langmuir equation may still describe adsorption over a portion of the adsorption range, since the variation in the energy of adsorption should be slight if only one type of bonding mechanism predominates. The sorption data of ex- perimental soils followed the Langmuir equa- tion rather well when data obtained for a P addition of 0 mmol/kg soil were excluded (Table 3). It was not necessary to use the Langmuir equation for the two adsorption surfaces. Soil 5 is not included in Table 3 be- cause the sorption capacity of this soil was low, and the sorption data were too limited for the Langmuir equation to be applied. The Langmuir sorption maximum was the lowest in soil 2, which had the lowest organic carbon content (Table 3). The fact that the sorption maximum of soil 1 was of the same magnitude as that of soils 3 and 4, which had more than twice the metal content, may be ex- plained by about the one unit lower pH of soil 1 as compared to the other soils. In deeper layer soil 1, the saturation degree of sorption capacity may also be lower than in surface soils 3 and 4. The sorption maximum of soil 7 was about double that of soil 6. The calcu- lated and experimentally detected sorption maximums were of the same order of magni- tude. In most cases, the Langmuir sorption maximums as well as equilibrium constants in- creased with rising temperature (Table 3). The Langmuir sorption maximum is an ap- Table 3. Langmuir equations for P sorption on experimental soils, sorption maximums and equilibrium constants. Soil Temperature Sorption Equation n r Maximum Equilibrium No. °C time, d sorption constant mmol/kg soil kx 10! I 5 3 y=20.04 +0.0355x 10 0.991*** 28.2 3 y= 14.85+0.0323x 10 o.9BB*** 31.0 3 y=48.44 +0.0546x 10 o.9BB*** 18.3 2 y= 8.65 +0.0613x 5 0.996*** 16.3 3 y= 4.51 +0.0654x 9 o.9BB*** 15.3 3 y=86.93 +0.0418x 9 0.975»** 23.9 7 y=45.66+ 0.0472X 9 0.992*** 21.2 2 y= 6.55 +0.0539x 4 0.999*** 18.6 3 y=44,23 +0.0316x 9 0.979*** 31.7 7 y=37.75+ 0.0368x 9 0.992»** 27.2 3 y= 10.25+0.0356x 6 0.999*** 28.1 2 y= 6,77 + 0.0375x 4 0.999*** 26.7 3 y= 9.85 + 0.0278x 6 0.994*** 36.0 3 y = 5.05 +0.0177x 10 0.994*** 56.5 2 y= 1.18+ 0.0171x 4 0.9997*** 58.5 3 y= 4.00 + 0.0155x 10 0.994*** 64.5 7 y= 1.15 + 0.0094x 10 0.996*** 106.5 7 y= 0.97 +0.0084x 9 0.997*** 119.3 1.77 20 2.18 2 5 18.3 1.13 20 7.09 20 14.50 3 5 5 0.48 1.03 20 8.24 20 0.71 20 0.98 4 5 28.1 3.47 20 5.54 20 2.82 6 5 56.5 58.5 64.5 3.50 20 14.47 20 3.88 7 5 8.20 20 8.60 6 parent measure which cannot be realized in sorption surface. In spite of this, the Lang- practice because surface properties are muir sorption maximums can be used to corn- changed during sorption; e.g. the sorption of pare the phosphate sorption properties of phosphate increases the negative charge of the different soils. References Aura, E. 1980. Oxygen as an exchangeable ligand in soil. J. Scient. Agric. Soc. Finl. 52; 34—44. Bache, B.W. & Williams, E.G. 1971. A phosphate sorp- tion index for soils. J. Soil Sci. 22: 289—301. Bohn, H., McNeal, B. & O’Connor, G. 1985. Soil chemistry. 2nd ed. 341 p. New York. Elonen, P. 1971. Particle-size analysis of soil. Acta Agr. Fenn. 122: 1—122. Gardner, B.R. & Jones, P.J. 1973. Effects of tempera- ture on phosphate sorption isotherms and phosphate desorption. Commun. Soil Sci. PI. Anal. 4: 83—93. Graham, E.R. 1948. Determination of soil organic mat- ter by means of a photoelectric colorimeter. Soil Sci. 65; 181—183. Gunary, D. 1970. A new adsorption isotherm for phos- phate in soil. J. Soil Sci. 21: 72—77. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. J. Scient. Agric. Soc. Finl. 51: 537—624. Haseman, J.F., Brown, E.H. & Whitt, C.D. 1950. Some reactions of phosphate with clays and hydrous oxides of iron and aluminium. Soil Sci. 70: 257—271. Kingston, F.J., Posner, A.M. & Quirk, J.P. 1972. Anion adsorption by goethite and gibbsite I. The role of the proton in determining adsorption envelopes. J. Soil Sci. 23: 177—192. Jackson, M.L. 1958. Soil chemical analysis. 498 p. Lon- don. Juo, A.S.R. & Maduakor, H.O. 1974. Phosphate sorp- tion of some Nigerian soils and its effect on cation exchange capacity. Commun. Soil Sci. PI. Anal. 5: 479—497. Kaila, A. 1955. Studies on the colorimetric determina- tion of phosphorus in soil extracts. Acta Agr. Fenn. 83: 25—47. 1959 a. Retention of phosphate by peat samples. J. Scient. Agric. Soc. Finl. 31: 215—225. 1959 b. Effect of superphosphateon the retention of phosphorus by peat soil. J. Scient. Agric. Soc. Finl. 31: 259—267. 1963. Dependence of the phosphate sorption capaci- ty on the aluminium and iron in Finnish soils. J. Scient. Agric. Soc. Finl. 35; 165—177. Kuo, S. & Lotse, E.G. 1974. Kinetics of phosphate ad- sorption and desorption by lake sediments. Soil Sci. Soc. Amer. Proc. 38; 50—54. Low, P.F. & Black, C.A. 1950. Reactions of phosphate with kaolinite. Soil Sci. 70: 273—290. Muljadi, D., Posner, A.M. & Quirk, J.P. 1966. The mechanism of phosphate adsorption by kaolinite, gibbsite and pseudoboehmite. Part 111. The effect of temperature on adsorption. J. Soil Sci. 17:238—247. Niskanen, R. 1989. Extractable aluminium, iron and manganese inmineral soils. 11l Comparison of extrac- tion methods. J. Agric. Sci. Finl. 61: 89—97. Olsen, S.R. & Watanabe, F.S. 1957. A method to de- termine a phosphorus adsorption maximum of soils as measured by the Langmuir isotherm. Soil Sci. Soc. Amer. Proc. 21: 144—149. Piper, C.S. 1944. Soil and plant analysis. 368 p. New York. Rajan, S.S.S. 1978. Sulfate adsorbed on hydrous alumi- na, ligands displaced, and changes in surface charge. Soil Sei. Soc. Amer. J. 42; 39—44. & Watkinson, J.H. 1976. Adsorption of selenite and phosphate on an allophane clay. Soil Sci. See. Amer. J. 40: 51—54. Russell, E.J. & Prescott, J.A. 1916. The reaction be- tween dilute acids and the phophorus compounds of the soil. J. Agr. Sci. 8: 65—110. Ryti, R. 1965. On the determination of soil pH. J. Scient. Agric. Soc. Finl. 37; 51—60. SCHWERTMANN, U., FISCHER, W.R. & PapENDORF, H. 1968. The influence of organic compounds on the for- mation of iron oxides. Trans. 9th Int. Cong. Soil Sci. 1: 645—655. Stnoholka, S., Ellis, J.H. & Blevins, R.L. 1975. Ad- sorption and bonding energy indexes for phosphorus in four soils of northeast Thailand. Commun. Soil Sci. PI. Anal. 6: 619—628. Syers, J.K., Browman, M.G., Smillie, G.W. & Corey, R.B. 1973. Phosphate sorption by soils evaluated by the Langmuir adsorption equation. Soil Sci. Soc. Amer. Proc. 37: 358—363. Teräsvuori, A. 1954. Über die Anwendung saurer Ex- traktionslösungen zur Bestimmung des Phosphordiin- gerbedarfs des Bodens, nebst theoretischen Erör- terungen iiber den Phosphorzustand des Bodens. Pubi. Staatl. Landw. Versuchsw. Finnland Nr 141, 64 p. Helsinki. Williams, E.G. 1959. Influences of parent material and drainage conditions on soil phosphorus relationships. Agrochimica 3: 279—309. 1960. Some aspects of phosphateretention and avail- ability in soils. Trans. 7th Int. Cong. Soil Sci. 3: 604—611. 7 & Knight, A.H. 1963. Evaluations of soil phosphate status by pot experiments, conventional extraction methods and labile phosphate values estimated with the aid of P-32. J. Sci. Food Agric. 8; 555—563. , Scott, N.M. & McDonald, M.J. 1958. Soil proper- ties and phosphate sorption. J. Sci. Food Agric. 9: 551—559. Ms received May 27, 1989 SELOSTUS Kivennäismaiden fosfaatin pidätyskapasiteetti I Pidätyskapasiteetin määrittäminen sorptioisotermien avulla Raina Niskanen Maanviljelyskemian laitos, Helsingin yliopisto, 00710 Helsinki Seitsemän maanäytteen (savespitoisuus I—7o %, or- gaanisen hiilen pitoisuus 0,8—10,7 %, pH 4,2—5,3, oksalaattiuuttoinen AI 11—222 ja Fe 11—202 mmol/kg maata) fosfaatin pidätyskapasiteettia tutkittiin sorptioiso- termien avulla. Reaktioaika oli 2—7 vuorokautta ja lämpötila +5 ja +20°C. Reaktioliuokset sisälsivät fos- faattia o—lo mmol/l (0—200 mmol/kg maata) ionivah- vuuden ollessa 0,01. Kolmen maan sorptioisotermeissä ( +20°C) löydettiinkokeellisesti sorptiomaksimit, joiden suuruudet olivat 4, 19 ja 34 mmol/kg maata. Kuuden maan sorptiotulokset noudattivat Langmuirin yhtälöä; lasketut sorptiomaksimit (15 —119 mmol/kg maata) oli- vat samaa suuruusluokkaa kuin kokeellisesti löydetyt. 8