Maataloustieteellinen A ikakauskirja Vol. 62: 17—20, 1990 RESEARCH NOTE Temperature dependence of phosphate sorption in mineral soils RAINA NISKANEN University of Helsinki, Department of Agricultural Chemistry, SF-00710 Helsinki, Finland Abstract. The sorption of phosphate was studied in 33 mineral soil samples at the tem- peratures of +5°C and +2O°C. The soils were equilibrated for 2 days with a solution con- taining phosphate 0.1 mmol/1 at an ionic strength of 0.01. At +5°C, the sorption remained unchanged or increased slightly as compared to the sorption at +2O°C. The differences be- tween sorbed amounts ranged from —0.03 to 0.29 mmol/kg soil and were statistically signifi- cant in 11 samples. Index words: reaction heat, oxalate-extractable Al and Fe, sorption equilibrium, temperature Introduction In a previous study on seven mineral soils (Niskanen 1990), the rise of temperature from +s°Cto + 20°C increased the sorption of phosphate from high P concentrations but the sorption from low phosphate concentra- tions was not significantly dependent on tem- perature. There the effect of rise in tempera- ture on the sorption from high phosphate con- centration may have been due to accelerated diffusion which increased the migration of phosphate onto the sorption surfaces of soil. Hartikainen (1979) found that the effect of the rise in temperature on sorption from low phosphate concentrations was slightly posi- tive, negative or nonexistent. Because the soil material in the above studies was limited and the results were contradictory, the aim of this paper was to study in a larger soil material the effect of temperature on phosphate sorption from a low P concentration. Material and methods The material was collected at 23 sampling sites, and it consisted of 16 surface soil sam- ples and 17 samples from deeper soil layers (Table 1). At 10 sampling sites, both surface and deeper layer samples were taken. The samples were air-dried and ground to pass a 2-mm sieve. Soil pH was measured in soil- -0.01 M CaCl 2 suspension (1 :2.5 v/v). The organic carbon content was determined by 17 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND Table 1. Soil samples. Sample Locality Sampling pH Organic C, Particle-size distribution Oxalate-extractable No. depth, cm (CaCl2) % (itm), % Fe AI Fe/Al <2 2—20 >2O ~ "mmol/kg soil 1 Vaala 20—40 4.2 2.8 2 Hyvinkää o—2o 4.5 3.3 3 Turenki 30—60 6.9 0.9 4a » o—3o 5.6 3.5 4b » 30—60 7.2 1.0 5a » o—3o 6.2 2.7 5b » 30—60 6.5 0.7 6a Naantali o—3o 7.3 1.6 6b » 30—60 7.1 0.7 7 Imatra 20—40 4.8 3.7 8 Hyvinkää o—2o 5.0 3.7 9a Naantali o—3o 7.2 2.3 9b » 30—60 6.6 1.7 10a » o—3o 6.2 2.8 10b » 30—60 5.9 1.6 11a Salo o—3o 7.1 3.1 11b » 30—60 6.9 2.3 12 Rajamäki o—2o 5.2 3.0 13 Säkylä o—3o 7.2 14.6 14a Salo o—3o 6.6 2.4 14b » 30—60 5.9 1.3 15a Säkylä o—3o 7.0 3.1 15b » 30—60 6.1 1.5 16a Salo o—3o 6.7 2.7 16b » 30—60 6.3 1.6 17 Säkylä 30—60 5.5 1.4 18 Imatra 20—40 5.4 3.2 19 Hyvinkää o—2o 5.3 1.9 20 Viikki 20—40 5.6 1.7 21 Nurmijärvi 20—40 4.1 2.9 22 Salo o—3o 6.5 4.4 23a Anjala o—3o 6.1 2.5 23b » 30—60 6.0 2.0 x 6.1 2.7 s 0.9 2.3 range 4.1—7.3 0.7—14.6 3 16 81 60 33 1.8 4 12 84 93 186 0.5 5 15 80 40 101 0.4 7 18 75 59 111 0.5 4 18 78 35 101 0.3 8 23 69 53 103 0.5 6 24 70 31 78 0.4 10 7 83 65 32 2.0 7 5 88 53 19 2.7 14 19 67 57 137 0.4 15 24 61 66 106 0.6 19 14 67 70 19 3.8 28 20 52 75 40 1.9 20 15 65 103 38 2.7 22 14 64 96 34 2.8 20 31 49 92 42 2.2 32 17 51 100 40 2.5 23 25 52 66 44 1.5 25 43 32 67 130 0.5 26 18 56 76 33 2.3 45 27 28 91 42 2.2 27 42 31 168 32 5.2 30 45 25 180 30 6.1 28 10 62 79 42 1.9 48 10 42 89 49 1.8 29 45 26 182 36 5.1 33 39 28 77 62 1.3 36 36 28 53 53 1.0 45 19 36 162 68 2.4 45 45 10 134 99 1.4 46 27 27 64 56 1.2 48 29 23 72 56 1.3 54 25 21 70 56 1.3 25 24 52 84 64 1.9 15 12 23 40 39 1.5 3—54 5—45 10—88 31 182 19—186 0.3—6.1 a modified Alien wet combustion method (Graham 1948). The particle-size distribution of the inorganic matter was determined by the pipette method (Elonen 1971). Aluminium and iron were extracted by 0.05 M ammonium oxalate (pH 2.9, ratio 1 : 20 w/y, shaking time 2 h) (Niskanen 1989), and determined by atomic absorption spectrophotometry. To determine the phosphate sorption, 5 g of soil was treated at +5°C and + 20°C for 2 days with 100 ml of solution containing KH 2P0 4 0.1 mmol/1. The ionic strength of the solution was adjusted with KCI to 0.01. To inhibit microbial activity, the solution con- tained 0.01 % NaN 3 . The suspensions were shaken daily for 8 hours. The pH of suspen- sions was measured at the beginning and the end of the experiment. The phosphorus con- centration of the filtrates was determined by a modified molybdenum blue method (Kaila 1955). The amount of retained phosphate was calculated as the difference between the phos- 18 19 phate present initially and that remained in the supernatant. The experiment was carried out in duplicate. Results and discussion The initial pH of soil suspensions was 5.0 — 7.8, and after the sorption of 2 days, pH rose or decreased slightly in most cases, generally by no more than 0.5 pH units. On the aver- Table 2. Sorption of P in experimental soils. Sample Sorption of P Change in sorption No. at +2O°C of P with a drop ~~ ~ ~ in temperaturemmol/kg %of , 0p soil addition mmol/kg 1 1.30 65 0.13* 2 2.00 100 0.00"* 3 1.94 97 0.01» 4a 1.76 88 0.06* 4b 1.91 95 0.02» 5a 1.89 94 0.04* 5b 1.93 97 0.04* 6a 0.80 40 —0.03» 6b 1.18 59 0.21» 7 1.97 98 0.01» 8 1.99 99 0.01» 9a 0.69 34 0.10» 9b 1.41 71 0.14» 10a 0.71 35 0.02» 10b 1.38 69 0.13» 11a 0.69 35 0.17» lib 1.64 82 0.05» 12 1.53 77 0.22* 13 1.87 94 0.00» 14a 1.12 56 0.15» 14b 1.80 90 0.06» 15a 0.91 46 0.13* 15b 1.68 84 0.12» 16a 0.50 25 0.29* 16b 1.83 92 0.05* 17 1.81 91 o.lo** 18 1.22 61 0.21» 19 1.61 80 o.2s*** 20 1.92 96 0.05» 21 2.00 100 0.00» 22 1.67 83 0.07» 23a 1.63 81 0.07» 23b 1.79 90 0.07* x 1.52 76 0.09 s 0.46 0.08 range 0.50—2.00 —0.03—0.29 age, experimental soils sorbed three quarters of the added phosphorus (Table 2). There were 14 samples which sorbed 90 °/o or more of the added phosphorus. The sorption of phosphate tended to be higher in most soils at the temperature of + 5°C than at +2O°C (Table 2). The differ- ence was statistically significant in 11 samples. In the study of Hartikainen (1979) on a ma- terial of six soil samples, there was also a ten- dency for the phosphate sorption to be higher at +5°C than at +2O°C, when sorption oc- curred from a P concentration of 0.02 mmol/1 during 24 hours. From the P concentration of 0.06 mmol/1, two soils sorbed P more at + 5°C than at + 20°C. However, the response to changes in temperature was slight. The minor effect of temperature is due to the small change in the standard enthalpy in the adsorption of phosphate and the principal de- pendence of adsorption equilibrium on the changes in entropy (Aura 1980). In many studies concerning the effect of temperature on phosphate sorption on soils and oxides, the sorption has been shown to increase with a rise in temperature (e.g. Muljadi et al. 1966, Kuo and Lotse 1974, Hartikainen 1979). There are, however, some investigations in which phosphate sorp- tion is reported to be exothermic (e.g. Bar- row 1979). These opposite results may be ex- plained by the different responses to changes in temperature in sorption from low and high anion concentration. There are two phases in the sorption of phosphate and other weak acid anions retained through ligand exchange. The first one, the sorption from low anion con- centration is relatively insensitive to changes in temperature. The minor effect may be posi- tive, negative or nonexistent. In the second phase, the sorption from high anion concen- tration increases with rising temperature. In the study of Reyes and Jurinak (1967) on adsorption of molybdate on iron oxide (hema- tite), the first phase of sorption continued un- til the concentration of molybdate in the equi- librium solution was about 0.1 mmol/1. There- after the sorption increased with rising tem- perature. The probable explanation for the ef- in sorption is due to an increased rate of diffu- fect of temperature on the sorption from high sion and migration of phosphate into fine anion concentration may be that the increase pores of hydrous oxides. References Aura, E. 1980. Oxygen as exchangeable ligand in soil. J. Scient. Agric. Soc. Finl. 52: 34 —44. Barrow, N.J. 1979. Three effects of temperature on the reactions between inorganic phosphate and soil. J. Soil Sci. 30: 271—279. Elonen, P. 1971. Particle-size analysis of soil. Acta Agr. Fenn. 122: 1 —122. Graham, E.R. 1948. Determination of soil organic mat- ter by means of a photoelectric colorimeter. Soil Sci. 65: 181 183. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. J. Scient. Agric. Soc. Finl. 51: 537—624. Kaila, A. 1955. Studies on the colorimetric determina- tion of phosphorus in soil extracts. Acta Agr. Fenn. 83: 25—47. Kuo, S. & Lotse, E.G. 1974. Kinetics of phosphate ad- sorption and desorption by lake sediments. Soil Sci. Soc. Amer. Proc. 38: 50—54. 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 in mineral soils. 11l Comparison of extrac- tion methods. J. Agric. Sci. Finl. 61: 89—97. 1990. Sorption capacity of phosphate in mineral soils. I Estimation of sorption capacity by means of sorp- tion isotherms. J. Agric. Sci. Finl. 62: I—B.1 —8. Reyes, E.D. & Jurinak, J.J. 1967. A mechanism of molybdate adsorption on a-l e,0,. Soil Sci. Soc. Amer. Proc. 31: 637 —641. Ms received August 25, 1989 SELOSTUS Lämpötilan vaikutus fosfaatin pidättymiseen kivennäismaihin Raina Niskanen Maanviljelyskemian laitos, Helsingin yliopisto, 00710 Helsinki Lämpötilan vaikutusta fosfaatin pidättymiseen tutkit- tiin 33:11 a kivennäismaalla. Reaktio tapahtui 2 vuorokau- den aikana lämpötiloissa + s°Cja + 20°C. Reaktioliuok- sessa oli fosfaattia 0,1 mmol/l (ionivahvuus 0,01). Läm- pötilassa + 20°C pidättyi fosfaattia 0,5—2,0 mraol/kg maata. Lämpötilassa + 5°C fosfaattia pidättyi saman ver- ran tai vähän enemmän kuin +2o°C:ssa. Ero ( —O,O3— 0,29 mmol/kg maata) oli tilastollisesti merkitsevä 11 maa- näytteessä. 20