J.RADIOANAL.NUCL.CHEM. ,LETTERS 155 (5) 359-369 (1991)

INTERACTIONS BETWEEN 5,Fe(III)-POLYPHOSPHATESq- AND CLAYEY MEADOW SOIL

A. Hargitai-T6th, J. K6nya*

Department of Chemistry, G. Bessenyei Teacher's Training College, P.O. Box 166, H-4401Nyfregyh~za, Hungary

*Isotope Laboratory, L. Kossuth University, P.O. Box 8, H-4010 Debrecen, Hungary

Received 22 August 1991 Accepted 29 August 1991

Interaction between iron(III)-diphosphate and iron(III)-triphosphate and Ca-form of a clayey meadow soil was followed oVer.a period of three days using radiotracer technique and kinetic evaluation of the results performed. 59Fe served to deter- mine the quantity of iron, ~JCa to measure the calcium, and phosphorus was measured spectrophotometrically. Approximately 80% of both iron chelates disappeared from the solution during the time of {he ~ experiment as a result of two well distinguishable reactions. One of them is a rapid fnter- facial process of about 10 minutes and the other is a slow reaction leading to the decomposition of iron(III)-polyphosphate chelates. The two processep could be sepa- rated using the Christiansen equation.

INTRODUCTION

Recently polyphosphates are components of many

liquid fertilizers. A useful property of polyphosphates

is the formation of soluble complexes with some of the

359 Elsevier ~equoia S. A., Lausanne A kaddmiai K iad6, Budapest

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN s 9Fe(III).POLYPHOSPHATES AND SOIL

essential trace metal ions. Heavy metal ions form pre-

cipitation with orthophosphate-containing liquid fer-

tilizers, however, their solubility is considerably

higher in polyphosphate solutions. In order to under-

stand the behaviour of polyphosphate complexes under

field conditions their interactions with soils are to

be examined.

It is probable that in case of diphosphates two of

the coordinated water molecules of the metal are re-

placed by two phosphate oxygens of the ligand and a

six-membered chelate is formed (formula I). The complex

formation can occur through a further coordination site

within the same molecule in the open-chain polyphospha-

tes containing three or more phosphorus atoms. This ar-

rangement is illustrated by formula II (Chabarek and

Martell I ) :

O O O II O ~_ II II

-0-- P J P --O-- 0 -- P -- O__

O O 0 ~ P O /P~O\o__

O--M-- 0

I. II.

Iron chelates are often added to soils as micronut-

rient fertilizers against chlorosis. The effectiveness

of chelating agents as metal carriers in soils depends

on their ability to keep iron in soluble, mobile forms.

Wallace and Lunt 2, Norvell and Lindsay 3, Lahav and 4

Hochberg , etc. studied the interactions between numer-

ous chelates and soils. The iron is removed from the

solution as a result of two well separable reactions. 2-6

One of them is a rapid interfacial process and other

is a slow reaction leading to the decomposition of iron

chelates and precipitation of Fe(OH) 3 (Refs 2, 3, 5, 6).

360

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN s 9Fe(HI).POLYPHOSPHATE S AND SOIL

TABLE I

Some characteristic data of the soil

Soil type and place of origin

pH {KCI) Humus, CaCO 3 , Exchangeable % % calcium,

me/100 g

Clayey meadow soil (J~szlad~ny) 6.99 2.6 2.2 49.8

In this paper we report how these two processes can

be separated by following the interactions between

iron(III)- diphosphate and triphosphate and Ca-form of

a clayey meadow soil over a period of three days. The

quantity of iron was determined by labeling with 59Fe,

45Ca served to assay for calcium and phosphorus was

measured spectrophotometrically. These examinations are

in track with our previous papers 5-8 dealing with dif-

ferent iron chelates.

EXPERIMENTAL

A clayey meadow soil from J~szlad~ny (Great Hungari-

an Plain) was used. Some characteristics of the soil

are presented in Table I. The soil was converted to 9

Ca-form and the exchangeable calcium was labeled with 45Ca8.

The iron chelate solutions were prepared from inac-

tive Fe(CIO4) 3 and either Na4P207 or Na5P3OI0 solutions

and 59Fe preparation. The concentration of iron(III)-

polyphosphate solutions for iron was Ix10 -4 mol dm -3

the molar ratio of iron:ligand=1:1.05 and pH of the so-

lutions 7. The stability of these solutions was chec-

361

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN s 9 Fe(III)-POLYPHOSPHATES AND SO/I,

ked 8 over a period of 4 weeks. We could not observe any

pecipitation of Fe(OH) 3.

18 cm 3 double-distilled water was added to 50 mg of

the air-dried soil labeled with 45Ca and the mixture

stirred at constant speed for 30 min. This time period

proved to be enough for the equilibrium between the two

phases to set in. 2 cm 3 Ixi0 -3 mol dm -3 iron(III)-poly-

phosphate solutions labelled with 59Fe was added to the

suspensions. Reaction periods were varied from 5 min to

3 days.

After the experimental periods the phases were sepa-

rated by Sartorius membrane filter. The y-radiation of

59Fe both in the solid and liquid phases was measured

on a NaI(TI) scintillation crystal. The quantity of

45Ca was determined by liquid scintillation method 5.

The concentration of phosphorus was measured in form of

molybdo-vanadate complex 10 spectrophotomerically after

the iron-polyphosphate chelates were converted to 8

phosphate by hydrolysis . The pH of the solutions was

between 7.12 and 7.36.

The average standard deviation of the measurements

for iron is +3% and for calcium +5%.

RESULTS AND DISCUSSION

The experimental data of the interaction between

Fe(III)-polyphosphates and clayey meadow soil are shown

in Fig. I and Table 2.

The changes in concentration of iron(III)-diphosphate

(la), iron(III)-triphosphate (2a) and the two ligands

(diphosphate: (Ib) and triphosphate: (2b)) as a function

of time are illustrated in Fig. I. There is a signifi-

cant difference between temporal changes in the concen-

362

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN s 9 Fe(III)-POLYPHOSPHATES AND SOIL

J I 0 ~ 2

"o a . b 1 ~ 0.8

~:~ o : diphosphate ~'~ 0.6I ~ �9 : triphosphate

I 0.0l:.l- f i J J I

T o

0 0.5 1.0 1.5 2.0 2.5 3.0 t,d

Fig. 1. Changes in the concentration of iron(III)-poly- phosphate chelates (a) and polyphosphate ligands (b)

trations of iron chelates and ligands. The distribution

of polyphosphate ligands between the solid and liquid

phases attained the equilibrium state in the first ten

minutes, the concentration of iron chelates however,

showed a continuous temporal decrease. During the ex-

perimental period the system did not reach the equilib-

rium state. There is no significant difference between

the chelating ability of diphosphate and triphosphate

ligands. The igK s values of the formation constants of

the iron(III)-diphosphate and iron(III)-tripnosphate

are nearly the same (the values of igK s are about 6-7

depending on the circumstances of d~termination11).

The data in Table 2 clearly show that the polyphos-

phate concentration measured after the first ten minu-

tes was approximately the same than the average con-

centration measured over a 3-day period. However, the

quantity of iron disappeared from the solution in case

of iron(III)-diphosphate increased from 10.2% (tenth

363

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN s 9 Fe(III)-POLYPHOSPHATES AND SOIL

t'N

- , - I , - . I

4-1

..a ID.,

O

,- - I O

I"O I=

O 1~ - ," t

0

-,-I

~-t g..l 0

m 0

,--t

I]1

0

oa

0

~D

I O

tt3 �9 o Q O

w- ('--

!

O

O LOO

�9 ~

LO I O r

X O~c~ Lq

�9 O~ v--t~.

I O

.~r

O ~ .O

CXl ~--

I O

O3.-~ .O",

O �9 +1 ~.-- O +1 LOO

v-o0

I O

LO ~DOq

v--OO

I O

X A

Cq

+I Ov- O +I ~

I O

X

O , .O

r

4J t~ r O~

r-4Ln ~ Ln ~O ~ O

�9 O �9 OO ~'~O

O 0 ,-4 t~ O I~ O - H ~

I-.-I ~ ID.4 r-..I ~

364

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN 5 9 Fe(III)-POLYPHOSPHATES AND SOIL

minute) to 79.3% (third day) and in case of iron(III)-

triphosphate from 8% to 78.7%.

The processes can be divided into two groups: the

rapid process taking place in 10 min and the slow reac-

tion requiring several days.

The lesser part of iron-polyphosphate chelates due

to a rapid sorption process was fixed on the boundary

surface, S. The following reaction can be considered:

k FeL + S < sorp, FeL-S (1)

where L stands for the polyphosphate ligands, and

ksorp is the rate constant of the sorption process.

The fact, that equivalent quantities of iron and

polyphosphate disappeared from the solution in first

ten minutes (Table 2) proved reaction I had taken place.

The greater part of iron-polyphosphate chelates ta-

kes part in a slow parallel reaction-characterized by

kdo, the overall rate constant of the decomposition-

described by the following equation:

FeL 3-n + 3OH- + Ca 2+ ~ kd~ Fe(OH) 3 + CaL 2-n (2)

Consequently the iron chelates react with the soluble

ions ( Ca2+, Zn 2+, Cu 2+, Mn 2+, etc.) and decompose gra-

dually. The equilibrium is shifted to the right due

to precipitation of Fe(OH) 3 and the free ligands form

calcium (or other metal) chelates.

The quantity of other metal ions (Cu 2+, Zn 2+, Mn 2+,

etc.) besides calcium can be neglected in this exami-

nation because of the 1:400=solid:liquid phase ratio.

So the dominant cation in the solution is calcium. 8-10%

of the exchangeable calcium entered the solution and

365

HARGITAI-TOTH. KONYA: INTERACTIONS BETWEEN s 9 Fe(III)-POLYPHOSPHATES AND SOIL

the distribution of 45Ca between the phases attained

the equilibrium state in the first ten minutes.

For the kinetic evaluation of the two parallel pro-

applied 5 cesses the christiansen equation can be because

there is a great difference in the values of rate con-

stants:

-ksorpt -kdot [FeLl = [iFeL(sorp)] e + [FeL~ e , (3)

o o

where [FeL(sorp)] O is the concentration of iron(III)-

polyphosphate chelates fixed on the boundary surface and

[FeLl o is the initial concentration of decomposable o

iron chelates. The sum of [FeL(sorp)] and LFeL~ is o o

equal to [FeLl i, the real initial concentration:

[FeL(sorp)] O + [FeL]o = [FeL]i (4)

The experimental data for the reaction times between

0 and 8 h were subjected to a kinetic analysis. During

this time period the reaction between iron(III)-polyphos-

phate chelates and the soil followed a firstorder kinet-

ics. Equation (3) can be solved graphically 5 (Fig. 2).

The ksorp and kdo rate constantst the reaction half

times and [FeL(sorp)] ~ and [FeLl o values are summarized

in Table 3.

The results prove that really two parallel reactions

take place. The loss of iron chelates from the solution

was partly due to the rapid sorption process of about

10 min and as a consequence 10% for the iron(III)-

diphosphate chelate (2xi0 -7 mol/0.05g) and 8% for the

iron(III)-triphosphate chelate (1.6xi0 -7 mol/0.05g) of

the total irDn(III)-polyphosphate were fixed on the

surface of the soil particles. Consequently the values

366

HARGITAI-TOTH, KONYA: INTERACTIONS BETWEEN s 9 Fe(IID.POLYPHOSPHATE S AND SOIL

o t,h

0 2 4 6 8 10 - 9 . 2 / i 1 i ~ "

11~Ctayey meadow soil , _9.4 ~ - - - - _ _ ~ l ~ O: Fe(ll l)- d iphosphote

I ~ ' b �9 �9 : Fe(lll)- t r iphosPhete

I -131-~. _ ~ -,o or_ J

I - 0 0 . 2 0.4 - I 0 . 2 ~

Fig. 2. Reaction kinetic curves of the examined inter- action. Values of the ordinate: I - in [!eL], 2 - in {[FeL]-[FeL]oe-kdot}

of kinetic analysis (Table 3, last two columns) agree

with both the measured quantities (Table 2) of fixed

iron and sorbed di- and triphosphate ligands. The dis-

&ppearance of iron chelates from the solution was part-

ly due to the slow decomposition of iron(III)-polyphos-

phates which resulted Fe(OH)3. It is most likely that

this process is a multi-stepped consecutive reaction

influenced by changes in pH. The exact evaluation of

this process would need the knowledge of the concentra- 12 tions of different protonized iron chelate species

and the values of formation constants.

Gratitude is expressed to Mrs. J. B4s~n (Research

Institute for Heavy Chemical Industry, Ves:zpr~m, Hungary)

for the valuable discussions.

367

HARGITAI-TOTH', KONYA: INTERACTIONS BETWEEN ~ SFe(III)-POLYPHOSPHATES AND SOIL

03

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368

HARGITAI-TOTH, KONYA:~TERAC~ONSBETWEENS9Fe(HI)~OLYPHOSPHATESANDSOIL

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10. B. Michelsen, Anal. Chem., 29 (1957) 60.

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7 369