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View of Extractable aluminium, iron and manganese in mineral soils: I Dependence of extractability on the pH of oxalate, pyrophosphate and EDTA extractants

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MaataloustieteellinenAikakauskirja 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-00710Helsinki, Finland

Abstract.Al,Feand Mnintwo mineral soilswereextracted by0.05 Mand0.02 Moxalate and pyrophosphate and0.02 M EDTAsolutions the pH of whichwasadjusted tovalues ranging from 1.7toII .0. The extractability of metals tendedto decreaseas thepH roseand asthe deprotonationof extractantacid, expressedaspKavalues, progressed.The reduction in ex- tractabilityof metals by oxalate was rather steepat pH > 4, whereas the extractability by pyrophosphateremained moderate atawider pH range. The extractability of metals byEDTA (pH 3.6—7.3) was lower than that by oxalate and pyrophosphate. Extractabilitywas lower inthe absence of the studied oxyacid anions and with0.01 MKCIasthe supporting electrolyte at apH between 2and 11than intheir presence.

Index words: amorphousAl,Fe andMn,acetate-extractableAl,complexingagents,protolysis constants,soilextraction

Introduction

The removal ofaluminium, 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 anionsare sorbed on oxides; at high concentration they act as extractantsof oxides. The ability of weak acid anions toform metal complexes depends on the form of anions, which in turn depends uponpH. Inflexions in the adsorption ’enve- lopes’ ofw ak acid anionsoccurin the vicinity

of pHs corresponding topKa values of acid at issue (Kingston etal. 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 theextractant pH. Inaprevious 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 paperwastostudy the effect of theextractant pH on the disso-

JOURNAL OF AGRICULTURALSCIENCE IN FINLAND

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Table I. Soil samples.

Fine sand Sand

Samplingdepth, cm o—2o 20—40

PH(CaCI2 ) 5.1 4.6

Organic C, % 3.6 0.8

Particle-size distribution:

<2 urn, % 13 2

2—20urn, % 20 I

20—60urn, Wo 27 7

60—200urn, % 31 35

>2OO urn, «It 9 56

Tamm's acid ammonium oxalate-soluble

Almmol/kg soil 186 104

Fe mmol/kg soil 53 32

Mnumol/kg soil 2 200 530

0.1 MNa4P207-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)

Mnumol/kg soil («It of Tamm's

oxalate-soluble) 2 280(103) 110(21) 1 Mammonium 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 ofafine sand sample from the surface layer of a cultivated soil (SouthKarelia, Imatra)andasand soilsample froma deeper layer of virgin soil (Viikki Ex- perimental Farm, University of Helsinki)

(Table 1). The samples were air-dried and ground topass througha2-mm sieve. The pH of the soil was measured in a soil-0.01 M CaCl2 suspension (1:2.5 v/v) (Ryti 1965).

The organic carbon content of the soilwas determined by a modified (Graham 1948) Alten wet combustionmethod, the particle- size distribution by the pipette method (Elonen 1971).

The extraction methods for soilaluminium, iron and manganesearegiven in Table2. Ex- tractions I—31 —3wereusedasreference methods.

Theeffectof pHonextractabilitywasstudied by using diluteoxalate, pyrophosphate and EDTA solutions. The pKavalues of the acids are given in Table 3. Acid oxalate solutions wereprepared from oxalic acid and ammoni- um oxalate, basic oxalate solutions from am- monium oxalate by adjusting the pH with 5 % NH4OH. The pH of K 4P207 solutions was adjusted with 5 M HCI and NaOH, the pH of EDTA solutions with CH3COOH and NaOH. Itwasnot possibletoprepareEDTA solutions with pH <3.6 because at alower pH the H2-EDTA of low solubility precipi- tated. Extractability without anions of oxy- acidswasstudied by using KCI solutions, the

pHof whichwasadjusted with 5 M HCI and NaOH. The pH of soil-solution suspensions was measured before and after shaking.

Aluminium, iron and manganese in filtrates weredetermined 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 ammoniumoxalate,

0.10 M oxalic acid 3.3 1:20 2 Tamm1922

2. 0.1 M Na4P207 10.0 1:20 4 McKeacue 1967

3. 1 M ammonium acetate 4.8 1:10 2 McLeanetal. 1958

4. 0.05 M oxalate 1.7—9.7 1:200 3

5. 0.02Moxalate 2.0—8.4 1:100 3

6. 0.05 M K 4P207 2.4—11.0 1:100 3

7. 0.02 M K 4P207 2.5—10.1 1:100 3

8. 0.02 MNa2-EDTA 3.6—7.3 1:100 3

9. 0.01 M KCI 2.3—11.1 1:100 3

74

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Table3.pK,values ofoxalic,pyrophosphoricand ethy- lenediamine tetraacetic acids (Martblland Calvin1956, Anon. 1984).

Acid pK, pK2 pK, pK4

Oxalic 1.24.2

0.91.5 5.88.2

Pyrophosphoric Ethylenediamine

tetraacetic 2.02.7 6.210.3

Results and discussion

The removal of metals by 0.05 M oxalate decreased asthepH increased, the reduction in extractability being rathersteepatpH > 4 (Figs. I—3).1—3). The minimum extractability of aluminiumwasmeasured when pHwasabout 9 (Fig. 1). At low pH, 0.05 M oxalate ex- tractedmoremetals than did Tamm’s oxalate.

The pH at which the removal of metals was equaltothe extractability by Tamm’s oxalate differed to some extent in both soils, being about 4 for aluminium and iron in fine sand soil and about4.5 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 aless efficientex- tractant than0.05 M oxalate (Figs. I—3). The extractability of iron in fine sand soil by 0.02 M oxalatewas 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 slopedmoregently than with oxalate extraction (Figs. I—3). As compared tooxalate,the extracting ability of pyrophosphate remained moderateatawider pH range. The iron extracted by 0.05 M K 4P207atpH 10 didnot deviate much from that extracted by 0.1 M Na4P207, whereas morealuminiumwasextracted from sand soil by 0.05 M K 4P2Ov than by 0.1 M Na4P207. The efficiency of 0.02 M K 4P207 as theex- tractant wasless than that of0.05 M K 4P207.

As comparedtooxalate 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

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less than 10% of that by Tamm’s oxalate (Fig. 2). The extractability of manganese in fine sand soil by EDTA decreasedas 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 assupporting electrolyte, the solu- bility of metalswaslower 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 metalsas aresult of the formation of aquocations in a reverse hydrolysis reaction. Aluminium and man- ganesein fine sand soilweresolubilizedmore readily than iron (Figs. I—3).1—3). The method for the selective extraction of ‘active’ aluminium oxides by 0.5 M CaCl2 at pH 1.5 used by Tweneboahetal. (1967) is actually basedon 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 aluminiumby

0.05 M oxalate increased when the pH exceeded about 9 (Fig. 1).

Oxalic, pyrophosphoric and ethylenedia- mine tetraacetic acidsare di- and tetraprotic acids the pKa values of which are given in Table 3. Accordingto the theory of Hinoston etal. (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 mostmarkedata 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 tendencyto 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 washigher than that correspond- ing to the highest pKaof acid, theextracta- 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 theextrac- tant solution; it is also important when soil

Fig. 2. Extractabilityof iron (% of Tamm’s oxalate-extractable Fe) versus extractant pH

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conditionsare considered. Organic acidsare 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 acidsoil,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. BocaRaton, Florida.

Curl,E.A. & Trueiove,B. 1986.The rhizosphere.288 p. Berlin.

Elonen,P. 1971.Particle-size analysis of soil.ActaAgr.

Fenn. 122: 1 122.

Graham,E.R. 1948.Determination of soil organic mat- ter bymeans of aphotoelectriccolorimeter.SoilSci.

65: 181 183.

Hinosion, F.J. 1962. Activity of polyphenols consti- tuentsof leaves of Eucalyptusand otherspeciesin complexingand dissolving iron oxide. Aust. J. Soil Res. 1:63—73.

—, Atkinson, R.J., Posnik, 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 morethan would be expected onlyon 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 adsorptionof anionsongoethite.Trans. 9th Int. Congr. Soil Sci. 1: 669—677.

LoPEZ-HeRNANDEZ, D.,FIjORES,D.,SIEGERT, G.&RoURIGUEZ, J.V. 1979. Theeffect ofsomeorganicanionsonphos- phateremoval from acid and calcareous soils. Soil Sci.

128: 321—326.

Martell, A.E.& Calvin, M. 1956. Chemistry of the metal chelatecompounds.613p.Englewood Cliffs, N.J.

McKeacue, J.A. 1967. An evaluation of0.1 M pyro- phosphateand pyrophosphate-dithioniteincompar- ison with oxalateasextractantsof the accumulation productsin Podzols and some other soils.Can.J. Soil Sci. 47: 95—99.

McLean, E.0., Heddleson, M.R., Bariiett, R.J. &

Fig. 3. Extractability of manganese (% of Tamm’s oxalate-extractable Mn) versus extractant pH

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Holowaychuk,N. 1958.Aluminuminsoils:I.Extrac- tion methods and magnitudesinclaysand Ohio soils.

Soil Sci. Soc.Proc. 22: 382—387.

Niskanen,R. 1989.Effect of extractant pH onrelease 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.119p. NewYork.

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, Maanviljelyskemianlaitos, 00710 Helsinki

Kahden kivennäismaanalumiinia,rautaa jamangaa- nia uutettiin0,05Mja0,02MOksalaani- ja pyrofosfaatti- liuoksilla sekä0,02 MEDTA-liuoksilla,joiden pH oli

1,7—11,0.Suuntauksena oli, ettämetallien uuttuminen väheni uuttoliuoksen pH:n ja uuttavan hapon dissosiaa- tioasteen kohotessa. Uuttuvuusaleni jyrkästi oksalaatti-

leachates. J. Soil Sci. 10: 300 —308.

Tamm,O. 1922.Eine Methode zurBestimmung deranor- ganischen Komponentedes GelkomplexesimBoden.

Statens Skogsförsöksanstalt.Medd. 19: 387—404.

Stockholm.

Tweneboah,C.K., Greenland, D.J. &Oades,J.M. 1967.

Changesinchargecharacteristics of soils after treat- ment with 0.5Mcalcium chloride at pH 1.5.Aust.

J. Soil Res. 5; 247—261.

Msreceived January 12, 1988

liuoksen pH:n ollessa yli4,kun uuttuvuus pyrofosfaa- tilla pysyi kohtalaisena laajalla pH-alueella.EDTA(pH

3,6—7,3) uutti vähemmän metalleja kuin Oksalaani ja py- rofosfaatti. Pelkkä0,01 MKCIuutti pH-alueella2—ll vähemmän metalleja kuin tutkittavat uuttoliuokset.

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