(EDTA) and nitrilotriacetic acid (NTA)

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Iowa State University Capstones, Theses and
Retrospective Theses and Dissertations
Dissertations
1973
Characterization of soil phosphates with chelates:
ethylenediaminetetra-acetic acid (EDTA) and
nitrilotriacetic acid (NTA)
Lambert Akparu Nnadi
Iowa State University
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Recommended Citation
Nnadi, Lambert Akparu, "Characterization of soil phosphates with chelates: ethylenediaminetetra-acetic acid (EDTA) and
nitrilotriacetic acid (NTA) " (1973).Retrospective Theses and Dissertations. 5110.
https://lib.dr.iastate.edu/rtd/5110
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NNADI, Lambert Àkparu, 1939-
ŒARACTERIZATION OF SOIL PHOSPHATES WITH
ChhiAii:S--mrinjiNEDIAMIfEIEIRÂÂCEriC ACID
(EDTA) AND MTRILOTRIACETIC ACID (NTA).
Iowa State University, Ph.D., 1973
Agroncsny
; University Microfihns, A aEwKCompany, Ann Arbor, Michigan
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Characterization of soil phosphates with chelates -
ethylenediaminetetraacetic acid (EDTA) and
nitrilotriacetic acid (NTA)
by
Lambert Akparu Nnadi
A Dissertation Submitted to the
Graduate Faculty in Partial Fulfillment of
The Requirements for the Degree of
DOCTOR OF PHILOSOPHY
Department: Agronomy
Major: Soil Fertility
Approved;
Signature was redacted for privacy.
Signature was redacted for privacy.
Signature was redacted for privacy.
Iowa State University
Ames, Iowa
1973
ii
TABLE OF CONTENTS
Page
PART 1. INTRODUCTION 1
PART 2. LITERATURE REVIEW 4
PART 3. MATERIALS AND METHODS 40
PART 4. DEVELOPMENT OF METHOD FOR DETERMINATION OF
INORGANIC P IN AQUEOUS SOLUTIONS OF EDTA OR
NTA 52
PART 5. DEVELOPMENT OF METHOD FOR DETERMINATION OF
INORGANIC P IN EDTA OR NTA EXTRACTS OF SOILS 68
PART 6. FACTORS AFFECTING EDTA- AND NTA-EXTRACTABLE
INORGANIC P AND ASSOCIATED CATIONS IN SOIL 79
PART 7. RELATIONSHIPS BETWEEN INORGANIC P AND ASSOCIATED
CATIONS EXTRACTED BY EDTA OR NTA SOLUTIONS 106
PART 8. SUMMARY AND CONCLUSIONS 151
LITERATURE CITED 157
ACKNOWLEDGMENTS 168
APPENDIX A. AMOUNTS OF P AND METAL IONS EXTRACTED
FROM SOME SURFACE SOILS BY EDTA SOLUTIONS
OF DIFFERENT pH VALUES AND CONCENTRATIONS 169
APPENDIX B. AMOUNTS OF P AND METAL IONS EXTRACTED FROM
SOME SURFACE SOILS BY NTA SOLUTIONS OF
DIFFERENT pH VALUES AND CONCENTRATIONS 178
APPENDIX C. AMOUNTS OF P AND METAL IONS EXTRACTED FROM
SURFACE AND SUBSOILS WITH 1Û0 inM EDTA
ADJUSTED TO pH VALUES OF 4, 6, F, OR 10 187
APPENDIX D. AMOUNTS OF P AND METAL IONS EXTRACTED FROM
SURFACE AND SUBSOILS WITH 100 mM NTA
ADJUSTED TO pH VALUES OF 4, 6, 8, OR 10 192
APPENDIX E. SIMPLE CORRELATION COEFFICIENTS AMONG P
AND METAL IONS IN EDTA OR NTA EXTRACTS
OF SOILS 197
1
PART 1. INTRODUCTION
The soluble salts of ethylenediaminetetraacetic acid
(EDTA) and nitrilotriacetic acid (NTA) have wide industrial
and agricultural applications. NTA is becoming increasingly
well known because of its potential use as a partial substitute
for polyphosphates in the detergent industry. Because of the
comparatively low cost of manufacture and the high chelating
power, increasing quantities of NTA have been used for deter­
gent formulations since 1966.
EDTA and NTA are now widely employed in agriculture as
carriers for micronutrients (Zn, Mn, Fe, Cu, Mo). NTA chelated
micronutrients have been mostly granular formulations for zinc
and iron; EDTA chelated micronutrients, which have been in use
for a longer period of time than NTA, include powdered, granu­
lar, and liquid formulations for iron,- zinc,- copper, and
manganese.
EDTA is used as a food preservative. Both chelates find
extensive use for removal of hardwater scales from industrial
plants. NTA has been used for 2,4-D herbicide concentrates
which are completely soluble in water.
In aqueous solutions, chelating agents such as EDTA and
NTA form strong, soluble complexes with polyvalent cations and
thereby eliminate the detrimental effects often caused in
aqueous systems by metallic impurities. The dyeing, textile
2
processing, tanning, and photographic industries use those
chelates for the removal of metal-ion contaminations in their
sources of water. But the addition of such compounds to soils
or natural waters by detergent products released from sewage
treatments could cause adverse effects by binding cations
associated with soil or sediment phosphate and releasing the
phosphate to the aqueous phase of the soil-water or sediment-
water systems. This release of phosphate may lead to eutro-
phication of water resources by phosphate derived from soils
and sediments.
Since NTA complexes iron, aluminum, calcium, magnesium,
and other metals, information concerning its effects on the
release of phosphate associated with these metal ions in
soils or river, and lake sediments, is of vital importance
in relation to possible use of this compound as a substitute
for phosphates in detergents. On the other hand, the ability
of EDTA and NTA to bind Fe, Al, Ca, Mg, and other metal ions
makes them potentially useful extractants for the study of
phosphates in soils and, possibly, in soil testing for
fertilizer recommendations. Also, these compounds could serve
as a single extractant for the determination of not only phos­
phates but also other ions of interest in soils, particularly
the micronutrient elements.
A few attempts have been made to use EDTA to bind the
various metal ions in soils and thereby release the phosphate
3
associated with these metal ions. But difficulties have been
encountered in the determination of the released phosphate
because EDTA interferes with phosphomolybdenum color developed
in the colorimetric methods employed for determination of
phosphate. Probably because of this difficulty, very few
investigations have been conducted to determine the potential
use of these chelates in the characterization of soil phos­
phates, although EDTA has been widely employed for the ex­
traction of some micronutrients from soils. To my knowledge
NTA has not been used in studies related to phosphates in soils.
The objectives of this investigation were:
(a) to develop a satisfactory colorimetric method for the
determination of phosphates in aqueous solutions and soil ex­
tracts of EDTA and NTA;
(b) to study the factors that affect the release of soil
phosphates by EDTA and NTA solutions;
(c) to investigate any possible relationships between
EDTA- and NTA-extractable phosphates and calcium, magnesium,
iron, and aluminum ions in the surface and subsoils of the
major soil types in Iowa.
4
PART 2. LITERATURE REVIEW
Chemistry of Soil Phosphates
Since the investigation is related to inorganic phos­
phates in soils, it is essential that some aspects of the
chemistry of inorganic phosphates in soils be reviewed. In
addition, the literature related to the use or potential use
of chelates in studies of soil phosphates will be surveyed.
Forms of inorganic phosphates in soil
Phosphorus compounds in soils can be classified into two
broad categories: "organic and inorganic. The relative amounts
of these two categories in a given soil depend on the organic
matter content and the age of the soil. The insoluble or
slightly soluble forms of inorganic P in soils include the
oxy- and hydroxy-phosphates of Fe"*"^, Fe^^, Al^^' Mg*^' Ca^^,
+2 +4
Mn , and Ti . Of the above, iron, aluminum, and calcium
phosphates will predominate. However, magnesium phosphates
may be important in soils derived from dolomite.
In acid soils, aluminum and iron phosphates are the domi­
nant fractions. Of the various iron oxides in highly weathered
soils, goethite FeCOH)^ is the most stable. Gibbsite, AKOH)^/
is the most stable form of the aluminum compounds. Therefore,
these two compounds would be expected to control the solu­
bility of iron and aluminum phosphates in acid soils. Also
postulated to be present in acid soils are variscite.
«
5
AlPO^-2 HgO, and strengite, FePO^'ZH^O. This postulate is
supported by the solubility product calculations of Lindsay
and Moreno (1960). But Bache (1963) found that in pure
systems only at pH less than 3.1 does the solubility of
variscite control the P concentration. At higher pH values
he found that variscite dissolved with the formation of
aluminum hydroxyphosphate. He stated that strengite was
never likely to be in equilibrium with any soil solution,
but could be associated with colloids as surface complexes.
In alkaline soils the native minerals are largely hydroxy-
apatite Ca (PO^) g (OH) 2 » fluor apatite Ca (PO^) gF2, or mixtures
^Q ^^Q
of both (Bassett, 1917). Smith and Lear (1966) identified some
carbonate apatites and some substituted fluorapatite in which
carbonate plus fluorine replaced some phosphates and sodium
and magnesium replaced some calcium. Larsen (1967) in a review
of soil phosphorus suggested that there was no reason to assume
any calcium phosphate other than hydroxy-phosphate is permanent­
ly present in slightly acid, neutral, and alkaline soils. The
effect of carbonate on hydroxyapatite was to make it more
chemically reactive.
The different forms of inorganic P are associated with
different mineral fractions of the soil. Most of the in­
organic P occurs in clay fractions. Hanley and Murphy (1970)
analyzed the phosphate forms in particle-size separates in
twenty-four Irish soils and found all forms of P were highest