Maataloustieteellinen A ikakauskirja Vol. 61: 73—78, 1989 Extractable aluminium, iron and manganese in mineral soils 1 Dependence of extractability on the pH of oxalate, pyrophosphate and EDTA extractants RAINA NISKANEN University of Helsinki, Department of Agricultural Chemistry, SF-00710 Helsinki, Finland Abstract. Al, Fe and Mn in two mineral soils were extracted by 0.05 M and 0.02 M oxalate and pyrophosphate and 0.02 M EDTA solutions the pH of which was adjusted to values ranging from 1.7 to II .0. The extractability of metals tended to decrease as the pH rose and as the deprotonation of extractant acid, expressed as pKa values, progressed. The reduction in ex- tractability of metals by oxalate was rather steep at pH > 4, whereas the extractability by pyrophosphate remained moderate at a wider pH range. The extractability of metals by EDTA (pH 3.6—7.3) was lower than that by oxalate and pyrophosphate. Extractability was lower in the absence of the studied oxyacid anions and with 0.01 M KCI as the supporting electrolyte at apH between 2 and 11 than in their presence. Index words: amorphous Al, Fe and Mn, acetate-extractable Al, complexing agents, protolysis constants, soil extraction Introduction The removal of aluminium, iron and man- ganese from soil by solutions of weak acid anions and the adsorption of these anions on soil oxides are based on complexation reac- tions. At low concentration anions are sorbed on oxides; at high concentration they act as extractants of oxides. The ability of weak acid anions to form metal complexes depends on the form of anions, which in turn depends upon pH. Inflexions in the adsorption ’enve- lopes’ of w ak acid anions occur in the vicinity of pHs corresponding to pK a values of acid at issue (Kingston et al. 1967, 1968). The ex- tractability of amorphous aluminium, iron and manganese oxides from soil by solutions of weak acid anions is seldom studied as a function of the extractant pH. In a previous study (Niskanen 1989), the release of soil aluminium and iron by fluoride strongly in- creased as the pH of the extractant solution decreased. The aim of this paper was to study the effect of the extractant pH on the disso- 73 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND Table I. Soil samples. Fine sand Sand Sampling depth, cm o—2o 20—40 P H(CaCI2 ) 5.1 4.6 Organic C, % 3.6 0.8 Particle-size distribution: <2 urn, % 13 2 2—20 urn, % 20 I 20—60 urn, Wo 27 7 60—200 urn, % 31 35 >2OO urn, «It 9 56 Tamm's acid ammonium oxalate-soluble Al mmol/kg soil 186 104 Fe mmol/kg soil 53 32 Mn umol/kg soil 2 200 530 0.1 M Na4 P 207 -soluble Al mmol/kg soil («It of Tamm's oxalate-soluble) 92 (49) 54 (52) Fe mmol/kg soil (It of Tamm's oxalate-soluble) 23 (44) 10 (30) Mn umol/kg soil («It of Tamm's oxalate-soluble) 2 280(103) 110(21) 1 M ammonium acetate-soluble Al mmol/kg soil («It of Tamm's oxalate-soluble) 6 (3) 10 (9) lution of aluminium, iron and manganese from soil by solutions of anions of three oxy- acids commonly used in soil research. Material and methods The material consisted of a fine sand sample from the surface layer of a cultivated soil (South Karelia, Imatra) and a sand soil sample from a deeper layer of virgin soil (Viikki Ex- perimental Farm, University of Helsinki) (Table 1). The samples were air-dried and ground to pass through a 2-mm sieve. The pH of the soil was measured in a soil-0.01 M CaCl 2 suspension (1:2.5 v/v) (Ryti 1965). The organic carbon content of the soil was determined by a modified (Graham 1948) Alten wet combustion method, the particle- size distribution by the pipette method (Elonen 1971). The extraction methods for soil aluminium, iron and manganese are given in Table 2. Ex- tractions I—31 —3 were used as reference methods. The effect of pH on extractability was studied by using dilute oxalate, pyrophosphate and EDTA solutions. The pK a values of the acids are given in Table 3. Acid oxalate solutions were prepared from oxalic acid and ammoni- um oxalate, basic oxalate solutions from am- monium oxalate by adjusting the pH with 5 % NH 4OH. The pH of K 4P207 solutions was adjusted with 5 M HCI and NaOH, the pH of EDTA solutions with CH 3 COOH and NaOH. It was not possible to prepare EDTA solutions with pH <3.6 because at a lower pH the H 2-EDTA of low solubility precipi- tated. Extractability without anions of oxy- acids was studied by using KCI solutions, the pH of which was adjusted with 5 M HCI and NaOH. The pH of soil-solution suspensions was measured before and after shaking. Aluminium, iron and manganese in filtrates were determined by atomic absorption spec- trophotometry, Fe and Mn with air-acetylene flame and A 1 with N,O-acetylene flame. The experiment was carried out in duplicate. Table 2. Extraction methods. Extractant pH Extraction Shaking Reference ratio, w/v time, h 1. 0.18 M ammonium oxalate, 0.10 M oxalic acid 3.3 1:20 2 Tamm 1922 2. 0.1 M Na 4 P 20 7 10.0 1:20 4 McKeacue 1967 3. 1 M ammonium acetate 4.8 1:10 2 McLean et al. 1958 4. 0.05 M oxalate 1.7—9.7 1:200 3 5. 0.02 M oxalate 2.0—8.4 1:100 3 6. 0.05 M K 4P 207 2.4—11.0 1:100 3 7. 0.02 M K 4P 207 2.5—10.1 1:100 3 8. 0.02 M Na 2-EDTA 3.6—7.3 1:100 3 9. 0.01 M KCI 2.3—11.1 1:100 3 74 Table 3. pK, values of oxalic, pyrophosphoric and ethy- lenediamine tetraacetic acids (Martbll and Calvin 1956, Anon. 1984). Acid pK, pK 2 pK, pK4 Oxalic 1.24.2 0.91.5 5.88.2Pyrophosphoric Ethylenediamine tetraacetic 2.02.7 6.210.3 Results and discussion The removal of metals by 0.05 M oxalate decreased as the pH increased, the reduction in extractability being rather steep at pH > 4 (Figs. I—3).1 —3). The minimum extractability of aluminium was measured when pH was about 9 (Fig. 1). At low pH, 0.05 M oxalate ex- tracted more metals than did Tamm’s oxalate. The pH at which the removal of metals was equal to the extractability by Tamm’s oxalate differed to some extent in both soils, being about 4 for aluminium and iron in fine sand soil and about 4.5 for those in sand soil (Figs. I—2). The removal of manganese by 0.05 M and Tamm’s oxalate was equal at pH 4.3 (fine sand) and 2.0 (sand) (Fig. 3). The 0.02 M oxalate was a less efficient ex- tractant than 0.05 M oxalate (Figs. I—3). The extractability of iron in fine sand soil by 0.02 M oxalate was at the highest only 80 % of Tamm’s oxalate-extractable iron (Fig. 2). As pH rose, the decrease in the extracta- bility of metals by K 4P207 sloped more gently than with oxalate extraction (Figs. I—3). As compared to oxalate, the extracting ability of pyrophosphate remained moderate at a wider pH range. The iron extracted by 0.05 M K 4P 207 at pH 10 did not deviate much from that extracted by 0.1 M Na4P2 07 , whereas more aluminium was extracted from sand soil by 0.05 M K 4P 2 O v than by 0.1 M Na4P 207 . The efficiency of 0.02 M K 4P 207 as the ex- tractant was less than that of 0.05 M K 4P 2 07 . As compared to oxalate and pyrophosphate extraction, the extractability of aluminium and iron by 0.02 M EDTA was lower (Figs. I—2). The extractability was nearly un- changed in the pH range of 5 —7; outside this range it slightly increased (Figs. I—2).1 —2). In the studied pH range, the extractability of aluminium by EDTA was not much higher than that by ammonium acetate at pH 4.8 (Table 1). The solubility of iron by EDTA was Fig. I. Extractability of aluminium (% of Tamm’s oxalate-extractable Al) versus extractant pH 75 less than 10 % of that by Tamm’s oxalate (Fig. 2). The extractability of manganese in fine sand soil by EDTA decreased as the pH increased, whereas no manganese was ex- tracted from sand soil (Fig. 3). In the absence of oxyacid anions and with 0.01 M KCI as supporting electrolyte, the solu- bility of metals was lower than in the presence of those anions (Figs. I—3).1 —3). An increasing H + concentration in the KCI solution en- hanced the dissolution of metals as a result of the formation of aquocations in a reverse hydrolysis reaction. Aluminium and man- ganese in fine sand soil were solubilized more readily than iron (Figs. I—3).1 —3). The method for the selective extraction of ‘active’ aluminium oxides by 0.5 M CaCl 2 at pH 1.5 used by Tweneboah et al. (1967) is actually based on this different solubility of aluminium and iron at low pH. An increasing OH concentration enhanced the release of metals. The extractability of aluminium and iron in fine sand soil by 0.01 M KCI increased when the pH exceeded 8, that of manganese when the pH exceeded 10 (Figs. I—3).1 —3). The release of aluminium by 0.05 M oxalate increased when the pH exceeded about 9 (Fig. 1). Oxalic, pyrophosphoric and ethylenedia- mine tetraacetic acids are di- and tetraprotic acids the pK a values of which are given in Table 3. According to the theory of Hinoston et al. (1967, 1968), the adsorption ‘envelopes’ of anions of these acids should have a maxi- mum at a pH that corresponds to pK,. Thereafter the ‘envelopes’ should have a decreasing slope that is most marked at a pH corresponding to the highest pKa . The graphs of the solubility of metals versus pH (Figs. I—3)1 —3) showed that extractability by these oxy- acid anions also depended on the extractant pH and had a tendency to decrease with ris- ing pH and progressing deprotonation of the extractant acid. The extraction graphs of metals when oxalate was used showed that when the pH was higher than that correspond- ing to the highest pK a of acid, the extracta- bility graphs descended steeply. The fact that the extracting ability of or- ganic acid anions depends on pH is not sig- nificant only in choosing the pH of the extrac- tant solution; it is also important when soil Fig. 2. Extractability of iron (% of Tamm’s oxalate-extractable Fe) versus extractant pH 76 conditions are considered. Organic acids are present in soil as a consequence of root exudation and microbial activity in the rhizosphere (Curl and Truelove 1986), they act as chelators in soil formation processes (Schnitzer 1959, Hingston 1962), and they affect the availability of plant nutrients, e.g. phosphorus. In acid soil, the removal of phos- phate by organic acid anions is pH-dependent and occurs largely through dissolution and References Anon. 1984. Handbook of chemistry and physics. 65th Ed. Boca Raton, Florida. Curl, E.A. & Trueiove, B. 1986. The rhizosphere. 288 p. Berlin. 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. Hinosion, F.J. 1962. Activity of polyphenols consti- tuents of leaves of Eucalyptus and other species in complexing and dissolving iron oxide. Aust. J. Soil Res. 1: 63—73. —, Atkinson, R.J., Posni k, A.M. & Quirk, J.P. 1967. Specific adsorption of anions. Nature 215: 1459 1461. chelation of iron and aluminium (Lopez- Hernandez et al. 1979). Organic anions enhance the solubility of metals; thus, for example, the concentration of aluminium in- creases more than would be expected only on the basis of the pH (Reuss and Johnson 1986). The effect of soil acidification is to enhance the ability of organic acid anions to solubilize metals through complexation. —, Atkinson, R.J., Posni-r, A.M. & Quirk, J.P. 1968. Specific adsorption of anions on goethite. Trans. 9th Int. Congr. Soil Sci. 1: 669—677. LoPEZ-HeRNANDEZ, D., FIjORES, D., SIEGERT, G. & RoURIGUEZ, J.V. 1979. The effect of some organic anions on phos- phate removal from acid and calcareous soils. Soil Sci. 128: 321—326. Martell, A.E. & Calvin, M. 1956. Chemistry of the metal chelate compounds. 613 p. Englewood Cliffs, N.J. McKeacue, J.A. 1967. An evaluation of 0.1 M pyro- phosphate and pyrophosphate-dithionite in compar- ison with oxalate as extractants of the accumulation products in Podzols and some other soils. Can. J. Soil Sci. 47: 95—99. McLean, E. 0., Heddleson, M.R., Bariiett, R.J. & 77 Fig. 3. Extractability of manganese (% of Tamm’s oxalate-extractable Mn) versus extractant pH Holowaychuk, N. 1958. Aluminum in soils: I. Extrac- tion methods and magnitudes in clays and Ohio soils. Soil Sci. Soc. Proc. 22: 382—387. Niskanen, R. 1989. Effect of extractant pH on release of soil phosphorus, aluminium and iron by ammonium fluoride. J. Agric. Sei. Finl. 61: 67 —72. Reuss, J.O. & Johnson, D.W. 1986. Acid deposition and the acidification of soils and waters. 119 p. New York. Ryti, R. 1965. On the determination of soil pH. J. Scient. Agric. Soc. Eini. 37; 51—60. Schnitzer, M. 1959. Interaction of iron with rainfall SELOSTUS Kivennäismaiden uuttuva alumiini, rauta ja mangaani I Uuttuvuuden riippuvuus oksalaatti-, pyro- fosfaatti- ja EDTA-uuttoliuoslen pH:sta Raina Niskanen Helsingin yliopisto, Maanviljelyskemian laitos, 00710 Helsinki Kahden kivennäismaan alumiinia, rautaa ja mangaa- nia uutettiin 0,05 M ja 0,02 M Oksalaani- ja pyrofosfaatti- liuoksilla sekä 0,02 M EDTA-liuoksilla, joiden pH oli 1,7—11,0. Suuntauksena oli, että metallien uuttuminen väheni uuttoliuoksen pH:n ja uuttavan hapon dissosiaa- tioasteen kohotessa. Uuttuvuus aleni jyrkästi oksalaatti- leachates. J. Soil Sci. 10: 300 —308. Tamm, O. 1922. Eine Methode zur Bestimmung der anor- ganischen Komponente des Gelkomplexes im Boden. Statens Skogsförsöksanstalt. Medd. 19: 387—404. Stockholm. Tweneboah, C.K., Greenland, D.J. & Oades, J.M. 1967. Changes in charge characteristics of soils after treat- ment with 0.5 M calcium chloride at pH 1.5. Aust. J. Soil Res. 5; 247—261. Ms received January 12, 1988 liuoksen pH:n ollessa yli 4, kun uuttuvuus pyrofosfaa- tilla pysyi kohtalaisena laajalla pH-alueella. EDTA (pH 3,6—7,3) uutti vähemmän metalleja kuin Oksalaani ja py- rofosfaatti. Pelkkä 0,01 M KCI uutti pH-alueella 2—ll vähemmän metalleja kuin tutkittavat uuttoliuokset. 78