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Novel slow-releasing micronutrient fertilizers. 1. Zinc compounds

Authors:
  • Raman Centre for Applied and Interdisciplinary Sciences
  • Raman Centre for Applied and Interdisciplinary Sciences

Abstract

A new concept for slow-releasing micronutrient compounds is proposed, based on the short-chain polyphosphate framework. The development of a zinc fertilizer, the first of such compounds, is described. Kinetic studies indicate that zinc phosphates polymerize in several linear stages. The polyphosphates are, in general, extremely water soluble and hygroscopic; moreover, those with lower solubility contain a high proportion of unavailable Zn2+. Initially, it was not possible to obtain a zinc polyphosphate having low water solubility and high nutrient availability. Solubility and hygroscopicity were attributed to free acid groups on the polyphosphate chain; neutralization of these groups indeed removed these undesirable characteristics. The fertilizers formulated have an average chain length of 2.35, contain some crystalline phases, have low water solubility, and are almost completely soluble in dilute HCl, citrate, diethylenetriaminepentaacetate, etc. Plant experiments indicate that the zinc polyphosphate is either equivalent to or better than ZnSO4.
1218
Ind.
Eng.
Chem. Res.
1993,32,
1218-1227
Novel Slow-Releasing Micronutrient Fertilizers.
1.
Zinc Compounds
Sanjay
K.
Ray, Chandrika Varadachari, and Kunal
Ghosh'
Department
of
Agricultural Chemistry
&
Soil Science, University
of
Calcutta,
35
B.C.
Road,
Calcutta
700
019,
India
A
new concept for slow-releasing micronutrient compounds is proposed,
based
on the short-chain
polyphosphate framework. The development of
a
zinc fertilizer, the first
of
such compounds, is
described. Kinetic studies indicate that zinc phosphates polymerize in several linear stages. The
polyphosphates are, in general, extremely water soluble and hygroscopic; moreover, those with lower
solubility contain a high proportion of unavailable Zn2+. Initially, it was not possible to obtain a
zinc polyphosphate having low water solubility and high nutrient availability. Solubility and
hygroscopicity were attributed to free acid groups on the polyphosphate chain; neutralization of
these groups indeed removed these undesirable characteristics. The fertilizers formulated have an
average chain length of
2.35,
contain some crystalline phases, have low water solubility, and
are
almost completely soluble in dilute HC1, citrate,
diethylenetriaminepentaacetate,
etc.
Plant
experiments indicate that the zinc polyphosphate is either equivalent to or better than ZnS04.
1.
Introduction
In view of the alarming environmental hazards and low
utilization efficiency associated with the use of soluble
salts
as fertilizers, the major emphasis in fertilizer research,
at present, is on the development of new materials which
have low water solubility and consequently are not readily
removed from the soil by leaching. Such
slow-release
or
controlled-release
formulations must, however, fulfill the
basic requirement that the nutrient ions in them be
available for plants in spite of their sparingly soluble
character. Most of the sparingly soluble inorganic salts
are, for this reason alone, not suitable for use as slow-
release fertilizers. The three main categories of slow-
release formulations include (i) organic-based compounds
such as the urea-formaldehydes and resin chelates, (ii)
membrane-coated compounds, and (iii) phosphate glasses
(frits) and metaphosphates (Roberts, 1975;Sauchelli, 1964,
1967, 1969; Silverberg et al., 1972; Volfkovich, 1972).
Although they possess suitable properties, the major
drawback of most of these formulations, is their cost which
makes them uneconomical for use, except for high-value
crops.
Phosphate glass frits are being used to some extent as
slow-releasing sources of micronutrient ions (Roberts, 1973,
1975,1977). Such glasses are usually produced by fusing
NHrH2P04 or NaH2P04 with the oxides of the micro-
nutrient ions at temperatures between
800
and 1400
"C
and then rapidly quenching the melt. However, the
drawback to large-scale utilization of such products is the
highly corrosive reaction conditions which necessitate the
use of expensive materials for furnace construction thereby
increasing the cost of product. This difficulty may be
overcome by producing long-chain metaphosphates which
require much lower operating temperatures (around
500
"C) and do not involve corrosive melts. Poly- and
metaphosphates of the macronutrients like
K+
or NH4+
are well-known and have been intensively studied (Flem-
ing, 1969; Huffman and Newman, 1970; Sauchelli, 1967).
Micronutrients may sometimes be included in such
compositions by adding the appropriate ions prior to
reaction (Ray, 1972; Volfkovich, 1972). However, most
such proposed compounds are essentially macronutrient
fertilizers which contain a small proportion of micronu-
trient ions. Apart from this, the major disadvantage of
the
micronutrient-incorporated
metaphosphates is their
08s8-5ss5/93/2632-121~~04.00/0
very slow rate of dissolution and consequent poor avail-
ability of the micronutrient ions to plants.
In spite of the existing limitations of the phosphate
glasses and the metaphosphate compounds, slow-releasing
micronutrient fertilizers based on the polyphosphates
appear to be the most promising from both the economic
and the chemical points of view. The ability of phosphoric
acid to form a linear polymeric chain (Van Wazer, 1966)
provides a convenient long-chain backbone on which the
heavy metal cations can be complexed. Moreover, one
may visualize the formation of compounds of varying
solubility by controlling the chain length of the phosphate
polymer. Therefore, in principle, it is possible to synthesize
fertilizers having any desired solubility characteristic.
Generally, solubility decreases with increasing chain length
(Thilo, 1962). Solubilization
is
also aided by hydrolytic
cleavage of
P-0-P
bonds (Ohashi, 1964). Another unique
feature of the polyphosphate-based fertilizer is that this
concept is not only applicable to the
all
the macro- and
micronutrient cations (Ca2+, Mg2+, NHd+, Zn2+, Cu2+, Fe2+,
etc.) but also the two important micronutrient anions,
viz.,
B3+
and MOB+, can
also
be included in it. This is
because during the polymerization of phosphate the borate
as well as molybdate ions can copolymerize forming
polyborophosphates and polymolybdophosphates (Van
Wazer, 1966). Thus, by utilizing a single technique,
fertilizers of almost any of the micronutrient ions can be
prepared. This concept admits a great deal of flexibility,
both in the composition of the product and in the solubility
and availability of the nutrient ion.
In spite of this apparently simple approach, today no
satisfactory slow-releasing micronutrient fertilizers are
available for widespread use. The phosphate-based com-
pounds, which have the distinct advantage of low-cost raw
materials, have yet to make any significant impact. This
is mainly because only two categories of polyphosphates
are presently available, viz., the phosphate glass frits and
the metaphosphates, both of which have their limitations
as mentioned earlier. It appears that if a third type of
polyphosphate could be synthesized which would show
greater solubility (and plant nutrient availability) than
the metaphosphates but at the same time would avoid the
very high temperatures as required for producing phos-
phate frits, a comparatively inexpensive slow-releasing
micronutrient fertilizer may be developed. Perhaps the
main reason why this has not yet been possible
is
due to
the difficult nature of the polyphosphates themselves. We
0
1993
American Chemical Society
have observed, in preliminary trials, that if the solubility
of a metal polyphosphate is attempted
to
be increased by
decreasing the chain length, the sample becomes hygro-
scopic and a large fraction of it becomes water soluble.
If
the amount of the water-soluble polyphosphates is de-
creased by increasing the degree of polymerization, then
the amount of the nonavailable nutrient fraction increases.
This observation may be ascribed
to
the fact that in any
polyphosphate compound there is a very wide distribution
of chain lengths; consequently, there is also a very wide
variation in the properties. Thus, in any such polyphos-
phate, a certain proportion of the components would be
short chain and water soluble whereas another portion of
it would be long chain, water insoluble, nonexchangeable
and also unavailable
to
plants. Both these properties are
deleterious
to
the ideal fertilizer. It is not possible
to
obtain
compounds in which the soluble polyphosphate fractions
are not present and, at the same time, to limit the size of
the long-chain-length fractions
so
that the micronutrients
remain in an available form. This problem is particularly
acute with the transition metal polyphosphates for which
there is a very marked change in properties with even
small variations in chain length. Another problem of a
more practical nature is that most of the polyphosphates,
except the very long chain metaphosphates, are extremely
hygroscopic. Such hygroscopic compounds will not be
acceptable
as
fertilizers.
It
is probably for this reason
alone that almost every attempt
so
far has been to
synthesize long-chain metaphosphates rather than short-
chain polyphosphates. Such compounds, consequently,
contain unavailable nutrient forms and are not completely
satisfactory fertilizing agents.
Investigations were undertaken by us in an attempt
to
develop effective slow-releasing micronutrient fertilizers
based on the alternative short-chain polyphosphate con-
cept proposed above. The work involved, first, formulating
asuitable compound. Here, the problem was to synthesize
compounds in which the nutrient ions
will
remain available
but will have low water solubility and will be dry and
nonhygroscopic. After the synthesis routes were estab-
lished, the compounds were characterized and assessed
by studying their chemical and crystal nature, solubility
in various media, and plant-growth responses. This article
presents a report of the first of a series of such micro-
nutrient fertilizers which have been developed.
A
slow-
releasing zinc fertilizer is described here under three
subsections which include (i) kinetics
of
polymerization
of zinc phosphates and studies on some properties of the
products with a view to assessing the suitability of the
various compounds
as
potential fertilizers; (ii) formulation
of the fertilizer compound,
after
modification of the chosen
polyphosphate, to minimize its undesirable characteristics;
and (iii) characterization and assessment of its fertilizing
potential.
2.
Methodology
The materials used for kinetic studies were ZnO
(R
May
&
Baker) and Hap04 (AR BDH). In all experiments, the
acid was diluted to a concentration of abour 46.4% PzOs.
The exact strength of acid used was determined by pH-
metric titration (Van Wazer et al., 1954); standardization
was done at 3-day intervals (Varadachari, 1992).
All reactions were carried out in a platinum crucible. In
the preweighed crucible,
0.5
g of ZnO was taken and
moistened with 0.4 mL of water; water is necessary to
avoid the vigorous reaction and spattering losses which
otherwise occurs on addition of HsP04. Finally the
required quantity of H3PO4 was pipetted in. Weights were
Ind. Eng. Chem. Res., Vol. 32,
No.
6, 1993
1219
recorded at each stage. The crucible was then placed in
amufflefurnacemaintainedat
15OOC (*0.5OC)andheatsd
for 30 min. This was done in order
to
remove most
of
the
excess water which causes spattering at higher temper-
atures. The crucible was kept in a desiccator (over fused
CaClz), and the furnace temperature was increased
to
300
OC
(fl
"C), 350 OC
(fl
"C),
or
400 "C
(f1.5
OC).
After
the furnace temperature stabilized, the crucible was placed
in it and heated for the required period of time.
It
was
then cooled in a desiccator (over PzO5) and weighed. The
contents of the crucible were finally washed, filtered, and
made
to
volume. The residue on the filter paper was dried
and stored [details in Varadachari, (199211.
From the
known
weight and concentration of HaPo4
solution initially taken, the actual amount of Hap04
(excluding all the water) was calculated; this quantity is
henceforth designated
as
[HsP041. The weight loss of the
reaction system per gram of [HaPodl was then calculated
from the initial weight of ZnO
+
[HaPOJ minus the fiial
weight
after
heating. The range of error in these values
is about
f0.1%
.
The ratio
R
of the reacted product (ZnO
+
HzO)/PzOs mole ratio, which is an index of the degree
of polymerization, was also similarly determined. The
composition of the polyphosphate residue consists entirely
of ZnO, H20, and P205; since total weight
=
weights of
(ZnO
+
H20
+
P205)
and since weights of ZnO and PzO5
are
known
quantities (amounts initially added), the weight
of H20 in the residue can be easily determined. Conse-
quently, the value of
R
can be deduced.
It
may be noted
that the HzO evaluated here is only the structural
component of the acid polyphosphates such
as
in H&07
(P20~2H20)
or
Zn(H2PO& (ZnO-P205*2H20).
The solutions obtained after washing the reaction
producta with water were analyzed for Zn2+ and
P;
Zn2+
was determined (Rush and Yoe, 1954) by the zincon
method (f0.005 ppm). For the analysis of
P
(f0.02 ppm),
the samples were first depolymerized
to
the orthophos-
phate by heating in 0.1 N HC1 at
100
"C for 96
h;
the
P
in solution
WW
then determined
as
the chlorostannous
reduced molybdophosphate blue complex in HC1 medium
(Jackson, 1973). Analysis of the insoluble portion of the
reaction residue was done by dissolution in 5
N
HC1 prior
to Zn2+ determination
as
the zincon complex;
P
was
determined
as
stated above after fusing of the residue
with NaOH and dissolution in HC1 solution.
The reaction products were also subjected to the
following qualitative solubility tests, viz., solubility in 0.1
and 1.0 N HC1, 0.33 M citric acid
(GR
SM), and 0.02 M
ethylenediaminetetraacetate
(EDTA)
(AR
BDH). If the
sample dissolved in an excess of the reagent within 20 min
(without heating), it was taken
to
be
soluble;
if
a few
particles remained even after 60 min, it was termed
slowly
soluble;
if no significant dissolution was observable even
after 60 min, then it was termed
insoluble.
Number-average chain length
(a')
was determined by
dissolving the sample in 0.1 N HC1, adding &[Fe(CN)a]
(GR
SM)
(25
mg for
5
mg of Zn2+), to complex Zn2+, and
then titrating with NaOH, first without and then with the
addition
of
AgN03 (AR BDH)
(Van
Wazer
et
al.,
1954).
Chemical analysis of the fertilizer compounds
for
Zn2+
and
P
was done
as
described earlier for the insoluble
residual products of reaction. In addition, Ca2+ was
determined by atomic absorption spectrometry in the
solution obtained by dissolving the sample in
1.0
N HC1;
NH4+ was determined after distillation of the sample with
NaOH and absorption in HzS04 (Black, 1965).
IR spectra of the samples were recorded on a Perkin
Elmer Model 577 spectrometer, with the scan range
of
1220
Ind. Eng. Chem. Res., Vol. 32,
No.
6,
1993
4000-200-cm-1 resolution (f5 cm-1) using pellets contain-
ing KBr as matrix. X-ray diffraction (XRD)
was
recorded
on a Philips PW 1140 X-ray diffractometer using
Ni-
filtered Cu Ka radiation at a scanning speed of
l"
28/min
(precision in
28,
f0.1").
Solubility of the fertilizer compounds in the following
reagents was noted, viz., in
0.1
N
HC1, 0.33 M citric acid
(GR E. Merck), 1.0
N
ammonium citrate (pH
8.5),
0.005
M
diethylenetriaminepentaacetate
(DTPA) (AR Ferak-
Berlin),
0.5
N
ammonium oxalate (AR BDH) (pH
8.51,
and a mixture of
0.5
N
ammonium acetate (AR IDPL) and
0.02 M EDTA
(AR
BDH) (pH 4.65). These reagents are
popularly used for extracting and evaluating available
forms of micronutrients from soils (Black, 1965; Cox and
Kamprath, 1972). To 0.1 g of the fertilizers 20 mL of the
reagent
was
added and agitated for 2 h. The solution was
then filtered, washed, and made
to
volume. Soluble Zn2+
was
determined by atomic absorption spectrometry
(AAS)
with a precision of
fO.O1
ppm Zn. Here the zincon method
described earlier
was
not used because citrate and oxalate
were observed to cause interference in color development.
The rate of solubilization of Zn2+, from the fertilizers,
in water
was
also studied 0.1 g of the compounds was
taken and 10 mL of water was pipetted into each. The
solutions were agitated for
2
h each day and then allowed
to stand. After 24,48,72,96, and 120 h of contact time,
the solutions were filtered, washed, and made to volume;
Zn2+ in these solutions was determined
as
before, by the
zincon method.
Plant-growth experiments were carried out in porcelain
pots. Soils (0-15 cm) were collected from (i) Pusa, Bihar,
India, and (ii) Mal, West Bengal, India. Both these soils
are reported to the responsive to Zn2+ fertilization (Kanwar
and Randhawa, 1978). Characteristics of these soils are
as
follows. (a) Pusa: Old alluvium; Haplaquept; pH
8.75;
organic carbon, 0.60
%
;
available Zn2+ (0.005 M DTPA),
0.79 ppm. (b) Mal: Terai (near Himalayan foothills)
alluvium; Haplaquept; pH 4.85; organic carbon, 1.46%;
available Zn2+ (0.005 M DPTA),
0.88
ppm. In each pot,
1
kg of soil
was
weighed. The soils were then treated with
superphosphate (100 mg of PzOs/kg). Here, an excess of
superphosphate fertilizer
was
added
so
that the plant's
requirement of nutritional
P
would be completely met;
response to additional
P
in the insoluble polyphosphate
fertilizer would be of relatively little consequence. Zinc
was added
as
ZnS04.7H20
as
well as zinc calcium poly-
phosphate at the rates of
0,
2.025, 4.05, 8.10, 12.15 ppm
Zn2+ which are equivalent to 0,5,10,20, and 30 kg/ha of
ZnSO4, respectively.
All
fertilizers were mixed with the
soil
4
days prior to transplanting. Paddy (IET 4094) was
grown in a nursery bed and transplanted when the seedlings
were 3 weeks old. Each pot contained one plant for the
Pusa soil and two plants for the Mal soil; the former
was
grown
as
a summer crop and the latter
as
a winter crop.
At
each fertilizer level, four replicates were performed.
After harvesting, grains were separated from the straw
and then grain weight and straw weight were recorded
after drying at
60
"C. The straw was cut into small chips
(with stainless steel scissors) and analyzed for Zn2+ after
digestion with triacid mixture (Jackson, 1973). In this
case again the zincon method
was
unsuitable because of
interference; hence analysis
was
done by
AAS.
Results of
these experiments were thereafter statistically analyzed.
3.
Results and Discussion
3.1.
Kinetics
of
Zinc Phosphate Polymerization and
Nature
of
the Reaction Products.
Rate curves for
dehydration of the system ZnO
+
in which Zn:P
280
-
m0c
M
350%
P
-
300°C
0'
n
x
240.
4
,qoo
120
I
0
LO
bo
120
TIME
OF
HEATING
(min)
Figure
1.
Kinetics
of
dehydration in the system ZnO-HsOd (Zn:P
=
1:2).
,001
1
3
"
220
I
3
uu
180
loo
1u)
180
010
60
TIME
OF
HEATING
(min)
Figure
2.
Kinetics
of
dehydration in the system ZnO-HaPOr (Zn:P
=
1:2.16).
n
X
1
3sO'c
I
5
-
w
3
0
40
80
120
TIME
OF
HEATING
(min)
Figure
3.
Kinetics
of
dehydration in the system ZnO-HsO, (Zn:P
=
1:1.58).
=
1:2,
1:2.16, and 1:1.58, are shown in Figures 1-3 at
reaction temperatures of 300, 350, or 400 "C. In Figure
1,
the initial reacting species is Zn(H2P04)2 (since 2n:P
=
1:2).
Therefore, the reaction may be represented as Zn-
Ind. Eng. Chem. Res., Vol. 32,
No.
6,1993
1221
200-
CI
0
x
24Qb
c
c
9'
f
c200.
%
9
2
3
m
-
160-
I
CI1
120
(H2PO4)2
-
Zn(PO3)z
+
2H20. The formation of the
metaphosphate Zn(POs)z, thus, involves the elimination
of
2
mol of
HzO
for every mole of Zn(H2P04)~.
Compounds which are less polymerized (the polyphos-
phates) are not completely condensed and hence lose less
water. For the compounds shown in Figure
1,
the
theoretically maximum possible water loss corresponding
to complete polymerization is 0.2756 g/g of [H3P041. Here,
none of the reaction products are completely dehydrated
indicating the formation of polyphosphate rather than
the metaphosphate. This is also evident from the fact
that none of the curves show plateau formation which
reveals the attainment of an equilibrium state.
An
unusual aspect of these curves (Figures 1-3) is the
apparent lack of similarity between nature of dehydration
kinetics (as seen from the curve shapes) at 300,350, and
400 "C. Any reaction of a particular order
n
is normally
expected to have similar curve shapes at different tem-
peratures since the value of
n
which determines the nature
of the curves (defined by the kinetic equation dc/dT
=
kc")
does not change with temperature. However, closer
inspection reveals interesting features which could explain
this apparent abnormality. Thus, at 300
"C,
Figure
1
shows
two linear regions: one at the (127-155)
X
g/g of H3-
PO4 and the other at the (160-169)
X
10-3 g region.
At
350
"C (Figure l), only a single straight line is obtained covering
the region (158-182)
X
10-3 g. By overlapping the data for
300 and 350
"C,
it can be seen that whereas the region at
(160-169)
X
10-3 g (for the sample reacted at 300 "C) is
a straight line, this region is also a straight line for the
sample reacted at 350
"C.
Other regions, however, are not
common for both samples, and therefore, their natures
cannot be compared.
Studies with ZnO
+
H3P04 mixtures containing different
proportions of Zn:P also reveal such behavior. Thus, in
Figure
2
(Zn:P
=
1:2,16), the linear region at (225-232)
X
10-3
g for the sample reacted at 350 "C is also shown by
the sample reacted at 400 "C; the other linear region at
(242-262)
X
10-3
g
is similarly present in the sample reacted
at 400 "C. Figure 3 (Zn:P
=
1:1.58) shows asingle straight
line from 232
X
10-3 to 255
X
10-3 g. The interesting feature
here is that common linear regions can be observed not
only within samples containing the same Zn:P ratios but
also between samples having different Zn:P ratios. For
example, the region at (225-232)
X
10-3 g which is linear
for the Zn:P
=
1:2.16 sample reacted at 350 "C (Figure
2)
is also linear for the Zn:P
=
1:2
sample reacted at 400 "C
(Figure
1)
and the Zn:P
=
1:2.16 sample reacted at 400 "C
(Figure
2).
Moreover, the linear region at (232-262)
X
10-3 g for the Zn:P
=
1:2.16 sample reacted at 350
"C
(Figure
2)
is
also
shown by samples at Zn:P
=
1:2
reacted at 400
"C (Figure
1)
as well as Zn:P
=
1:1.58 at 350 "C (Figure
3) and Zn:P
=
1:2.16 at 400 "C (Figure
2).
In fact, the
break in the curve occurs in the same position, viz., at
232.5
X
10-3
g, in the first three samples mentioned above.
The implications of these observations are as follows:
The reaction of ZnO and H3P04, in which polymerization
occurs by the elimination of water, is kinetically a zero-
order process
as
evidenced by the straight-line shapes of
the curves. However, breaks in these lines suggest that
polymerization occurs in linear stages, each stage being
characterized by the degree of weight loss (Le., degree of
polymerization). Once a particular degree of polymeri-
zation is reached (i.e., polyphosphates of a particular
average chain length are formed), then a quasi-equilibrium
stage results. After a period of time, polymerization begins
once again at a rate different from the former one. This
process continues as before until another breakpoint
2
c
6
Figure
4.
Empirical dehydration
curve
for the system ZnO-HsPOI.
(characterized by the degree of polymerization) is reached,
whereupon a further change in the rate of polymerization
occurs. The nature of polymerization of the zinc phos-
phates may, thus, be represented by the empirical curve
in Figure 4. An initial dehydration up to 155
X
10-3 g is
followed by another of slower rate up to 180
X
lo9
g; then
a sharp rise occurs which continues up to 232
X
103
g and
followed by a slower rate until polymerization is complete.
On increase of the reaction temperatures from 300 to 400
"C, only the slopes of the lines will increase; apart from
this, other features of the curves, particularly the bound-
aries of the breakpoints, will remain unaltered even with
changes in reaction conditions.
Table
I
gives values of
R
for the reaction products.
R,
which is an index of the degree of polymerization, equals
3.0 for the unpolymerized orthophosphate and reaches a
limiting value of 1.0 for the infinite linear chain meta-
phosphates; values lower than 1.0 indicate cross-linked
ultraphosphates (Van Wazer, 1966). Here, it is seen that
R
values are greatest at Zn:P
=
1:2 ratios (i.e., polymer-
ization is lowest) as compared to higher
or
lower Zn:P
ratios under identical heating conditions. At Zn:P
=
1:2.16,
the presence of free H3P04 is probably responsible for
increasing condensation. In samples deficient in H3P04
(Zn:P
=
1:1.58), the initial amount of structural H2O is
itself less, and hence even if the same amount of water is
removed by reaction, the
R
value (ZnO
+
HzO/P205) will
be lower than that of the Zn:P
=
1:2 residue.
Amounts of water-soluble Zn2+ and
P
in the polyphos-
phate products obtained from the aforementioned kinetic
studies are shown in Table
11.
Products from Zn:P
=
1:2
reaction mixtures at 300 and 350 "C contain a very high
proportion of water-soluble components; only at 400 "C
after 50-min reaction do the samples become sparingly
soluble. However, at
all
temperatures, solubility trends
are irregular. This irregularity is probably due to the fact
that the polyphosphates hydrolyze rapidly, and therefore,
during the period of filtration and washing, some insoluble
compounds may be hydrolyzed to the soluble forms.
Therefore, the solubility data reflect only an average
picture. They are, however, useful for assessing the overall
trend in the solubility characteristics.
It
is also interesting
to note that even the highly polymerized compounds, with
R
values around 1.0, contain significant amounts of water-
soluble components (Tables I and
11).
This indicates a
very wide distribution of the chain length such that
1222
Ind. Eng. Chem. Res., Vol. 32,
No.
6, 1993
Table
I.
Kinetics
of
Water
Loss
in the Reaction
ZnO
+
Ha04
and Corresponding
R
Values Table
11.
Water Solubility
of
Reaction Products
of
the
Reaction
ZnO
+
HQO4
2n:P
(molar ratio
of reaction reaction time of
wt
loss
(g/g
R
mixture) temp
(OC)
heating
(min)
of (M2O/P2Os)
Zn:P
(molar ratio timeof
Zn2+(%
P(%
P/Zn
of reaction reaction heating soluble) soluble) (molar ratio
mixture)
temp
("C)
(min)
(w/w)
(w/w)
of
solution)
1:2
1:2.16
1:1.58
300 10
20
30
40
50
70
90
350 15
30
45
60
75
90
400
10
20
30
40
50
60
80
100
350 45
60
75
90
105
150
180
400 10
20
30
40
45
50
60
350 15
30
45
60
75
90
105
0.1267
0.1388
0.1556
0.1594
0.1620
0.1659
0.1690
0.1581
0.1651
0.1672
0.1728
0.1786
0.1802
0.1807
0.1864
0.1884
0.1894
0.2333
0.2389
0.2446
0.2516
0.2244
0.2313
0.2296
0.2327
0.2328
0.2514
0.2627
0.2189
0.2340
0.2421
0.2418
0.2844
0.2863
0.2875
0.2284
0.2359
0.2401
0.2439
0.2477
0.2515
0.2559
2.62
2.49
2.31
2.27
2.23
2.19
2.16
2.28
2.20
2.18
2.12
2.07
2.04
2.04
1.98
1.95
1.94
1.46
1.40
1.34
1.27
1.16
1.09
1.10
1.07
1.07
0.86
0.74
1.22
1.06
0.97
0.97
0.61
0.49
0.47
1.78
1.70
1.63
1.62
1.58
1.53
1.49
although the average value is very high (low
R),
a small
proportion of short-chain compounds are also present
which remain soluble.
Compositions of the insoluble residues (Table
111)
also
show haphazard trends. This may be attributed
to
hydrolysis
as
well
as
simultaneous precipitation (of in-
soluble ortho- or pyrophosphate) reactions that are
unavoidable during residue recovery.
3.2.
Formulation
of
the Fertilizer.
Solubility char-
acteristics of the various polyphosphates are shown in
Table
111.
It
may be observed that the products obtained
at 300 "C with Zn:P
=
1:2
are
all
soluble in 0.1
N
and
1
N
HC1 and in0.33 M citric acid. However,
all
these samples
are extremely hygroscopic and become sticky when exposed
to the atmosphere for a few minutes.
At
350 "C, the
samples remain soluble in these reagents till a
R
value of
2.12
(Table
I)
is reached; beyond this stage the products
become much less soluble. In fact, products with
R
values
<1.4
are insoluble even in
1
N
HC1. The only apparent
advantage with such compounds, obtained at 400 "C, is
that they are less hygroscopic, and more powdery than
the more soluble products obtained at 300 or 350 "C.
With samples prepared at Zn:P
=
1:2.16 ratios, solubility
decreases further. This is to be expected, in view of their
higher chain lengths (vide
R
values, Table
I).
The
compounds prepared from Zn:P
=
1:1.58 are likewise less
1:2
1:2.16
1:1.58
300
350
400
350
400
350
10
20
30
40
50
70
90
15
30
45
60
75
90
10
20
30
40
50
60
80
100
45
60
75
90
105
150
180
10
20
30
40
45
50
60
30
45
60
75
90
105
52.29
67.23
65.36
72.21
68.48
73.46
06.74
69.41
70.67
70.03
66.61
61.94
57.58
77.19
79.06
68.48
59.76
7.47
6.23
3.32
1.62
63.13
66.88
61.26
55.00
56.25
30.00
20.75
57.50
63.75
49.37
55.00
4.73
5.00
5.55
44.81
28.32
47.30
37.34
27.69
28.32
79.18
82.76
85.55
81.78
82.60
84.85
82.96
86.94
83.83
83.58
78.01
74.96
72.06
91.15
91.00
81.66
70.14
14.11
12.67
5.50
2.92
81.48
86.44
84.82
59.33
30.70
89.23
76.93
86.21
11.96
12.29
16.20
59.34
47.54
55.25
45.35
31.39
30.18
3.20
2.60
2.76
2.39
2.55
2.44
2.66
2.64
2.50
2.52
2.47
2.55
2.64
2.49
2.43
2.52
2.48
3.99
4.29
3.50
3.80
2.81
3.32
3.18
4.17
3.12
3.28
3.29
3.31
5.34
5.19
6.16
2.80
3.54
2.47
2.56
2.39
2.25
soluble than the Zn:P
=
1:2
compounds. Both the Zn:P
=
1:2.16 and 1:1.58 compounds are initially fairly dry, but
after exposure
to
the atmosphere for a few hours, stickiness
appears and the powders are no longer free flowing.
From this brief survey of the characteristics of the
polyphosphates, it would seem that none of them is
completely suitable for use
as
a fertilizer material. Most
of the compounds are hygroscopic; those which are more
dry are far
too
insoluble. However, even compounds which
are less sticky and fairly well polymerized contain a
significant fraction of water-soluble Zn2+. In compounds
in which the water soluble fraction is reduced to
<lo%
the solubility
in
0.1
N
HC1 is
also
very low; such compounds
are, therefore, not acceptable. On the other hand, the
products obtained at
300
"C with Zn:P
=
1:2
contain
>80
5%
Zn2+ in water-soluble forms; moreover, they are not only
very hygroscopic but
also
acidic, with a pH of around 2.
At
this stage, the problem appeared
to
be insurmount-
able. Although investigations on the effect of decreasing
as
well
as
increasing the amount of
P
were made specifically
to observe if the problem of hygroscopicity and water
solubility could be avoided, this approach
was
not suc-
cessful. Apparently the chain length distribution pattern
of the zinc polyphosphates is
too
wide
to
permit synthesis
of a compound low in water-soluble forms
as
well
as
unavailable forms.
As
this is an inherent characteristic of
the polymerization process itself, the only solution would
Ind. Eng. Chem.
Res.,
Vol. 32,
No.
6,1993
1223
Table 111. Water-Insoluble Residue
of
the Products
of
the Reaction
ZnO
+
&POI:
Contents
of ZnN
and P and Solubility*
Zn:P PJZn solubility in
(molar ratio
of
reaction time
of
Zn2+
(%)
P
(%)
(molarratio
0.1
N
LON 0.33M 0.02M
reaction mixture)
temp
(OC)
heating (min) (w/w) (w/w)
of
residue) HC1 HC1 citric acid EDTA
1:2.16 350
400
1:2 300
10
39.06
20 38.04
30 38.65
40 39.47
50
70 39.88
90
37.69
350 15 35.77
30 34.75
45 36.39
60 36.39
75 32.90
90 34.11
400 10 40.30
20 35.77
30 34.95
40 36.96
50
60
80 29.02
100
27.79
45 37.31
60 46.64
75 42.76
90
35.34
105 39.40
150 33.92
180
34.05
10 41.59
20 45.10
30 33.43
40 37.31
45 29.93
50 36.54
60 30.32
1:1.58 350 30 45.23
45 35.99
60 47.30
75 42.37
90
42.76
105 39.08
a
Abbreviations:
S,
soluble;
SS,
slowly soluble; I, insoluble.
25.70
20.22
25.36
26.73
24.33
18.84
25.70
24.67
22.62
20.56
25.05
22.69
14.39
21.07
34.44
27.45
28.37
28.19
19.35
22.26
24.29
25.83
23.46
27.55
28.48
19.75
21.34
27.96
31.25
39.89
39.98
21.10
17.83
6.84
7.53
6.18
20.56
21.90
1.39
1.12
1.38
1.43
1.29
1.06
1.52
1.50
1.31
1.19
1.61
1.40
0.75
1.24
2.08
1.57
2.06
2.14
1.09
1.01
1.20
1.54
1.26
1.71
1.77
1.00
1.00
1.77
1.77
2.81
2.31
2.17
0.83
0.40
0.34
0.31
1.01
1.18
S
S
S
S
S
S
S
S
S
S
S
I
I
S
S
ss
I
I
I
I
I
S
S
S
ss
I
I
I
S
S
ss
I
I
I
I
S
S
ss
ss
I
I
S
S
S
S
S
S
S
S
S
S
S
S
ss
S
S
S
S
ss
ss
I
I
S
S
S
S
S
I
I
S
S
S
S
I
I
I
S
S
S
S
ss
ss
S
S
S
S
S
S
S
S
S
S
ss
I
I
ss
ss
I
I
I
I
I
I
ss
ss
ss
I
I
I
I
ss
ss
I
I
I
I
I
ss
ss
ss
I
I
I
be to find out some other means of overcoming the
solubility problem.
Since most of the zinc polyphosphates are not completely
dehydrated, they are expected
to
contain free acid groups.
This is supported by the fact that the pH of the
polyphosphates is usually C2. It was conjectured that the
high water solubility of these polyphosphates could be
due to the presence of such free acid groups on the chain
and that reducing the free acid group content would also
reduce solubility. To test this hypothesis, a sample was
prepared at 300 OC by heating for 60 min with a Zn:P
=
1:2; a small amount of water was added
to
form a suspension
and then it was neutralized with CaC03
(R
May
&
Baker)
until the pH increased
to
3.7. The suspension was then
dried in a vacuum desiccator over fused CaC12. When
tested, the water solubility of this compound was observed
to decrease
to
about
1%
Zn2+ from the original value of
70
%
Zn2+ for the unneutralized sample. The compound
was also observed to retain ita solubility in 0.1
N
HC1,0.33
M citric acid, and 0.02 M EDTA. Moreover, it was also
nonhygroscopic and powdery. In short,
it
possessed all
the properties of an ideal slow-releasing fertilizer. This
experiment confirmed the fact that free acid groups are
responsible for both high water solubility and hygroscop-
icity of the polyphosphates.
Of all the polyphosphate residues studied, only the
compounds formed at 300 "C at Zn:P
=
1:2
show good
S
S
S
S
S
ss
S
S
ss
ss
I
I
I
I
I
I
I
I
I
I
ss
ss
ss
I
I
I
I
ss
ss
I
I
I
I
I
I
I
I
I
I
I
solubility in
0.1
J
HC1, 0.33
M
citric acid, and
0.02
M
EDTA (and thereby contain Zn2+ in completely available
forms). Taking other ratios of Zn:P does not appear
to
be
of any particular advantage. On the contrary, with an
excess of H3PO4 (Zn:P
>
21, more free acid groups
will
have
to
be neutralized, whereas with a mixture deficient
in H3PO4, there is a chance of unreacted ZnO remaining.
Therefore, a reaction mixture containing Zn:P
=
1:2,
appears to be most suitable.
Tables
I1
and
I11
show that the products obtained at
300 OC (Zn:P
=
1:2) are
all
extremely soluble
in
water
as
well
as
in 0.1
N
HC1 and 0.33 M citric acid. However,
solubility in 0.02 M EDTA decreases below a
R
value of
2.16 (Table
I).
In order
to
obtain a compound with high
solubility
in
all
these reagents, a residue with a
R
value
slightly higher than 2.16 appeared
to
be optimum. Thus,
a zinc polyphosphate with a 2n:P ratio of 1:2 and a
R
value
of 2.19 was chosen
as
the polyphosphate base which would
be subsequently modified by neutralization
to
produce
the fertilizer. The number-average chain length
(tt')
of
this polyphosphate
(as
determined by titrimetric analysis)
is 2.35.
Small amounts of the proposed fertilizer were then
prepared. Mixtures of ZnO and Haor (Zn:P
=
1:2)
were
reacted
as
described
in
section
2,
at 300 "C for
60
min; the
final weight loss recorded was 0.166 g/g of [HaPOd]. This
corresponds
to
a
R
value of 2.19. The product was then
1224
Ind. Eng. Chem. Res., Vol.
32,
No.
6, 1993
2110
H,PO,
c
J.
I
5.
ZnZ*
-
polypho6phate
I
I
Heat at
1sOoC
Zn(
Hp,
1
Heat at
300.C
Add rater and
mix
to
form
6
slurry
Add
CaC03/NH,0H.
mix
and
allow to 6tand
for
6
hours
&
Dry
and grind
Figure
5.
Flow
chart for the production
of
zinc polyphosphate
fertilizer.
cooled
to
room temperature and made into a slurry with
water. To this,
1.02
g of CaCO3
or
16.2
mL of
0.5
N
"4-
OH (AR BDH) (per
1
g of ZnO) was added, and the mixture
was stirred and allowed to stand for
6
h. It may be
mentioned that, in addition to CaC03, NH40H solution
may also be successfully used
as
the neutralizing agent.
Since neutralization reaction is slow, sufficient time has
to be allowed for the reaction to complete. This entire
process has been depicted in the flow chart shown in Figure
5.
Several such batches were prepared to obtain a sizable
quantity for further studies.
3.3.
Characterization
of
the Fertilizer Compounds.
Chemical composition (in percent) of the zinc calcium
polyphosphate is ZnO,
26.99;
PzOS,
44.93;
CaO,
14.15;
HzO+
(structural),
6.93;
HzO- (adsorbed),
6.60.
This corresponds
to the formula
Zno.33Ho.7,Cao.~6Po.as0z.s4.
For
the zinc
ammonium polyphosphate, the chemical composition is
as
follows: ZnO,
29.57;
PzOS,
52.84;
NH4+,
6.62;
HzO+,
5.64;
HzO-,
4.70;
its formula is
Zno.3sHo.s3(NH4)o.~Po.740~.~1.
IR spectra of the compounds are shown in Figure
6.
The two compounds reveal almost the same absorption
behavior except in the region around
1400
cm-l. The NH4+
ion absorbs strongly around
1400
cm-l; NH4HzP04 has
twin absorptions at
1450
and
1400
cm-l which coalesce to
a single peak at
1450
cm-l for (NH&HP04. However, the
latter shows an additional absorption at
1715
cm-'
(Corbridge and Lowe,
1954;
Nyquist and Kagel,
1971).
The spectra recorded here (Figure
6)
reveal broadening of
the absorptions in these regions; this could arise due to
the presence of ammonium polyphosphates in addition to
the orthophosphates. Stronger absorptions at
3000
cm-l
for the ammonium fertilizer may be attributed to NH
stretching in addition to the OH stretching of H-bonded
water molecules. Absorptions due to
P-0
ionic stretching
and
deformation at
1180-1050
and
560
cm-I, respectively
(Corbridge and Lowe,
1954),
are also evident in the spectra
of both compounds (Figure
6).
The position of the
P-0-P
absorption which is centered at
1100
cm-1 indicates short-
chain units of the tripoly type rather than the longer chain
polymers; in the latter type of compounds, such absorptions
15W
2000
ICm
I100
IW
La
100
LOO0
1500
WAVENUMBER
(CM"1
Figure
6.
Infrared absorption spectra
of
(a) zinc calcium poly-
phosphate
and
(b)
zinc ammonium polyphosphate.
are usually centered at longer wavelengths of around
1250
cm-' (Corbridge and Lowe,
1954).
This conclusion is
also
supported by the chain-length analysis data wherein a
n'
of
2.35
was obtained.
X-ray diffraction analysis (Table IV) of the samples
reveals some interesting features. Both compounds show
a broad diffraction band around
6
X
10-'
nm indicating
an amorphous phase and strong sharp lines suggesting
crystalline phases, too. The strongest diffraction is at
(2.03-2.04)
X
10-l
nm. Numerous other bands are
also
common to both compounds. These may, therefore, be
attributed
to
the basic zinc polyphosphate structure which
is the same for both.
It
may be mentioned that a cupric
polyphosphate fertilizer which was studied also showed
an identical strong peak at
(2.03-2.04)
X
10-1
nm together
with many other diffractions (unpublished; Ray,
1991)
which are common with those of the zinc salts. Here, such
lines which are common to the polyphosphates of both
zinc and copper have been tentatively assigned to the basic
polyphosphate skeletal structure (Table IV).
Apart from the crystalline polyphosphates, other crys-
talline compounds are also evident. In the zinc calcium
polyphosphate, calcium pyrophosphate may be present
whereas the corresponding ammonium form probably also
contains the tripolyphosphate (NH4)4HzP4013,
("4)Z-
Zn(PzOd, and (NH4)ZnH3(P04)2"20 (JCPDS,
1978).
In
addition to these, there are other reflections which may
be due to ammonium polyphosphates and which are also
shown by a cupric ammonium
salt
(unpublished; Ray,
1991).
Solubilization of the zinc fertilizers in water over a period
of several days is shown in Table V. After the initial
solubilization of
1.25%
and
7.50%
Zn2+ €rom the calcium
and ammonium forms, respectively, further dissolution is
practically negligible even after
120
h. It appears that
low-molecular-weight fractions contribute to the initial
solubility and subsequent hydrolytic dissolution is ex-
tremely slow. Hydrolysis
is
probably inhibited
to
a large
extent due
to
the cross-linking of chains by the divalent
cations, Zn2+ and Ca2+, which limits the accessibility of
H2O molecules to the
P-0-P
groups.
In dilute acids, viz.,
0.1
N
HC1 and
0.33
M
citric acid,
both fertilizers are completely soluble. Solubilization in
0.1
N HC1 may occur by exchange of Zn2+ ions on the
polymer chains with H+ ions from solution. In
0.33
M
citric acid, however, complexation would be an additional
factor of Zn2+ solubilization. In near neutral and alkaline
media,
too,
the fertilizers are highly soluble. Such excellent
solubility in organic complexanta indicates that the Zn2+
Ind. Eng. Chem.
Res.,
Vol.
32,
No.
6,1993
1226
10.05
9.21
6.71
(b)
4.67
4.00
3.561
3.255
3.100
3.058
2.867
2.637
2.607
2.550
2.481
2.411
2.356
2.232
2.122
2.040
1.914
13.4
9.21
7.76
7.25
6.71
6.51
5.98
5.61
5.40
4.79
4.44
3.480
3.327
3.198
3.079
2.998
2.867
2.797
2.607
2.481
2.344
2.036
Table
IV.
X-ray Diffraction Characteristics of the Zinc
Calcium
and
Zinc Ammonium Polyphorphatesa
grain output with the addition of zinc calcium polyphos-
phate fertilizer. By contrast, soils containing ZnSO4 do
d
(A)
Z
assignment
not show any such trend probably because of the precip-
itation of insoluble zinc hydroxides, phosphates, etc. under
the highly alkaline conditions which reduce the quantity
of Zn2+ available. Statistical analysis of the data reveal
Zinc Calcium Polyphoephate
6
capy
8
PP
19
PP
6
capy,
PP(?)
20
ZnCapp(?)
12
ZnCapp(?)
8
PP(?)
19
PP, CapY
4
PP
9
ZnO
8
Capy
3
ZnO
10
Cap
6
ZnO
3.8
PP(?)
15
PP
5
10
100
PP, CapY
4
PP(?)
Zinc Ammonium Polyphosphata
5
24
6
6
8
3
10
MA.
A&d?
10
APp,
AznP
5
APP
14
6
13
11
8
6
PP
11
APD
3
Zno
8
UP
8
bo
5
ZnO
22
PP
100
PP
Abbreviations: AZnP, (NH4)ZnH~(PO4)~.H20; AZnPy,
(NH4hZn(P107)a; APp, ammonium polyphosphate; AP4,
~“4)rHS4Ou;
CaPy, C~L~HPZOT~HIO; PP, polyphoephate frame-
work;
ZnCaPp, zinc calcium polyphoephate; ZnO, zinc
oxide.
in the micronutrient fertilizers should be readily assim-
ilable by plants.
It
may be mentioned that the ability of
a soil
to
provide Zn2+ for plant growth is usually assessed
by the amount that is dissolved by HC1, EDTA, DTPA,
or citric acid (Cox and Kamprath, 1972; Jackson, 1973)
depending on the soil conditions. Moreover, it
haa
also
been observed that water-insoluble zinc fertilizers which
readily dissolve in dilute HC1 (Jackson, 1973) are all
suitable sources of zinc for plants. The fertilizer com-
pounds may, thus, be judged
to
contain Zn2+ in ionic
binding sites which are closely comparable
to
those present
in soils; more strongly bound (and less soluble) Zn2+ ions
would be difficult for plants
to
assimilate. Consequently,
the zinc fertilizers closely simulate natural sources of zinc
in the soil which have low water solubility but are available
for plant uptake over the entire period of growth.
For a further assessment of the fertilizing action of the
polyphosphates, plant-growth experiments were done.
Results of the trials carried out with two different types
of soils using the zinc calcium polyphosphate
as
well
as
zinc sulfate (for a comparative assessment) are shown in
Table
VI.
Grain yields of plants grown on an alkaline soil
from Pusa, Bihar, show a definite trend of increase in
that increase in yield over the control due
to
the slow-
releasing zinc fertilizer is significant at the
5%
level
(LSDo.05) when the Zn2+ dosage is
4.05
and 12.15 ppm.
It
is also noteworthy that, at three different levels (Table
VI), there is a statistically significant increase in yield
with the zinc calcium polyphosphate-treated soils over
ZnSO4-treated soils.
Experiments with other acidic soil from Mal, West
Bengal,
also
show good response
to
the slow-releasing zinc
fertilizer. Thus, at Zn2+ dosages of 8.10 and 12.15 ppm,
there is a significant increase (LSD0.m) in yield over the
control; ZnSO4-treated soils also produce higher grain
yields than the control but the increase is significant at
only one level (8.10ppm Zn2+). Yields from ZnSO4-treated
and zinc calcium polyphosphate-treated soils are statis-
tically similar at similar fertilizer levels.
On the whole, the results of these trials indicate that
the zinc calcium polyphosphate is either
as
good
as
or
better than ZnSO4
as
a fertilizing compound for zinc. The
possibility of the phosphorus content of the new fertilizer
itself causing an increase in yield was eliminated by adding
excess superphosphate
to
all the pots
so
that phosphorus
hunger would be subsided and response
to
more phos-
phorus itself would be poor. To further confiim the
fertilizing capabilities of zinc calcium polyphosphate, the
straw was analyzedfor Zn2+ uptake (Table VII). The data
reveal a definite increase in Zn2+ content of the straws
with both types of fertilizers. Increase in the Zn2+
concentrations is statistically significant at LSD0.m at three
levels with both ZnSO4 and zinc calcium polyphosphate
treatments. The overall trend in the data (Table VII)
also suggests that Zn2+ contents are higher with the plants
grown in polyphosphate-treated soils than with those
grown in ZnSO4-treated
soils.
This difference is, however,
significant (LSD0.m) at only one fertilizer level
(4.05
ppm
Zn2+ dosage).
All
the aforementioned results, thus, suggest that the
slow-releasing zinc calcium polyphosphate is an effective
material for zinc fertilization. The Zn2+ in this compound
can be taken up by plants
as
readily
as
from ZnSO4.
Previous chemical data also support this property.
4.
Summary and Conclusion
The basic concept of this investigation was to develop
novel slow-releasing micronutrient fertilizer compounds
based on the polyphosphate framework. Polyphosphates
appeared
to
offer a distinct advantage over other slow-
release formulations in view of their low raw material costs.
From the chemical angle,
too,
the polyphaphates appeared
to
be highly suitable in view of their versatile amenable
nature.
The two types of phosphate-based slow-release fertilizers
which have been recommended
so
far are the
glass
frits
and the long-chain metaphosphates. In this investigation,
it has been attempted
to
synthesise a third type of
compound which would overcome the major drawbacks of
the two earlier types of compounds. In short, these
compounds would contain nutrients in available forms
and would
also
be fairly easy
to
produce.
The first micronutrient fertilizer thus developed was
the zinc compound which has been described here.
It
was
hoped that by solving the problem with zinc it would be
1226
Ind. Eng. Chem.
Res.,
Vol.
32,
No.
6,
1993
Table V. Solubilization of Zinc Polyphosphate Fertilizers
in
Water and Various Reagents
(i) Kinetics of Solubility of Zinc Polyphosphate Fertilizers in Water
%
Zn2+ soluble after
fertilizer Oh 24 h 48 h 72 h 96 h 120 h
zinc calcium polyphosphate 1.25 1.25 1.43 1.48 1.48 1.52
zinc ammonium polyphosphate 7.50
7.50
7.37 7.58 7.32
7.50
(ii) Solubility of Zinc Polyphosphate Fertilizers in Various Reagents
%
Zn2+ soluble in
1.0
N
0.5
N
ammonium
0.5
N
0.1
N
0.33 M citric ammonium acetate
+
0.02 M EDTA 0.005 M ammonium
fertilizer HCl acid citrate (pH
8.5)
(pH 4.65) DTPA oxalate (pH 8.5)
zinc calcium 100.00 100.00 94.20 94.83 100.00 76.85
polyphosphate
zinc ammonium polyphosphate 100.00 100.00 97.81 97.49 99.80 97.81
Table VI. Averme Grain and Straw Yields of Paddy
on
ADDlication
of
Zinc Sulfate and Zinc Calcium PoluDhosDhate
~
av grain yield (9)
at
Zn2+ dose (ppm)
soil treatment
0
2.025 4.05 8.10 12.15
Pusa ZnSOd 3.86 3.77 2.93 2.41 3.59
Mal ZnSO4 10.08 10.38 11.26 12.33"
11.88
zinc calcium polyphosphate 3.86 4.96
5.590gb
4.80b 6.08"~~
zinc calcium polyphosphate 10.08 11.49 11.93 12.9W 12.42"
av straw yield (9) at Zn2+ dose (ppm)
0
2.025 4.05 8.10
12.15
4.83 4.38 4.84 4.67 4.86
4.83 4.60 5.19 5.68 5.39
7.88 8.09 8.58 8.45 8.98
7.88 9.11 9.91" 8.60 9.67"
Significant increase in yield over the control (LSD0.w). Significant increase in yield over ZnSO,
(LSD0.w).
Table VII. Uptake of Zna+ by Paddy Straw
on
Application
of
Zinc Sulfate and Zinc Calcium Polyphosphate to Pusa
Soil
Zn2+ content (mg/kg
of
paddy straw)
at Zn2+
dose
(ppm)
treatment
0
2.025 4.05 8.10 12.15
ZnSO4 64.38 85.63" 73.50 83.75O
86.00"
zinc calcium 64.38 89.38" 94.63"~~ 84.38" 78.88
polyphosphate
6
Significant increase over the control (LSD0.w). Significant
increase over ZnSOd (LSD0.d.
possible to gain some understanding of the chemistry of
the process which in turn would facilitate development of
other micronutrient fertilizer compounds.
Initially, the kinetics of polymerization of ZnO
+
H3-
PO4 mixtures, at various Zn:P molar ratios and at various
temperatures, were studied; the structural water loss and
R
value (M20/P206 molar ratio) of the products were
evaluated. Results of this investigation gave interesting
information on the nature of the polymerization process.
It was concluded that polymerization of zinc phosphates
is a zero-order process which shows linear rata that change
with the degree of polymerization. Such a changeover
from one rate constant to another is fairly sharp, and the
breakpoint is constant regardless of temperature of
reaction and, to some extent, the ratio of reactants.
Analysis of the water-soluble Zn and
P
in these products
showed that
all
of
them contained a very high proportion
of water-soluble components. Only in the very highly
polymerized compounds did the water solubility decrease
considerably. On the other hand, the solubility
of
these
polyphosphates in dilute HCl(O.1 and
1.0
N) and in various
complexants
(0.33
M citric acid and0.02 M EDTA) showed
that whereas the small chain compounds initially formed
were soluble, with an increase in chain length the com-
pounds became rapidly insoluble particularly in the
complexants. Since solubility in dilute HC1 and in
complexants may be taken
as
an index of nutrient
availability, the higher chain compounds obviously contain
a portion of Zn2+ in unavailable forms. Thus, owing to
the very high dispersion in chain lengths in the zinc
polyphosphates, it was not possible to synthesise com-
pounds having low water solubility but high solubility in
complexants. Another problem with the polyphosphates
was their extreme hygroscopicity.
Subsequently, it was concluded that the presence of
free acid groups was responsible for high water solubility
and that this could be reduced simply by neutralizing the
acid groups. The desired formulation for the zinc fertilizer
was, therefore, developed by choosing an optimum poly-
phosphate base and then reacting it with a base like CaC03
or
NH40H. It was observed that such a treatment also
removed the hygroscopicity and resulted in a dry and
powdery product. The synthesis routes for these com-
pounds are also very simple.
The fertilizers (viz., the calcium and ammonium salts
of the zinc polyphosphate) were then characterized by
chain-length analysis, chemical composition, solubility
properties, IR characteristics, XRD measurements, and
plant-growth experiments. The compounds are short-
chain polyphosphates which are sparingly soluble in water
but slightly soluble in
0.1
N HC1,
0.33
M citric acid,
1.0
N ammonium citrate (pH
8.5),
0.02 M EDTA, and
0.005
M DTPA. IR spectra also suggest the presence of short-
chain polyphosphates. XRD analysis shows an amorphous
phase witha broad hump
at
6 X
10-l
nm; crystalline phases
are
also
present. The strongest reflection is at
(2.03-2.04)
X
10-1
nm which could be due to
a
polyphosphate skeletal
structure. The other crystalline phases present include
Cal.~HP20~2H20, (NH4)ZnHs(PO&H20, (NH&Zn-
(P20712, and (NH4)32P4013 in the calcium form and
ammonium forms. Plant-growth experiments, carried out
with two types of soils, showed statistically significant
increases in yield due to the addition of the zinc calcium
polyphosphate fertilizer. Zn2+ contents of the straw also
showed significant uptake of this nutrient. Results
indicate that the zinc calcium polyphosphate is either
as
good
as
or
even better than ZnSO4
as
a fertilizing
compound.
In conclusion, it may be stated that the concept
of
a
polyphosphate-based compound appears
to
be
well
suited
for slow-releasing fertilizer formulations. Firstly, the
compounds are not only insoluble in water but also contain
nutrients in readily available forms. Such a juxtaposition
of desirable properties is very rarely observed in slow-
releasing materials.
This
dual characteristic not only
reduces drainage losses but
also
ensures an ever-ready
supply of nutrients at any stage of the growth of the plant.
Moreover, this supply is not dependent on hydrolysis rates
and, therefore,
will
be much less influenced by soil factors.
Secondly, from the commercial aspect, too, the polyphos-
phate formulation possess certain advantages, viz., cheap
raw materials, technically simple synthesis routes, and
relatively low operating temperatures.
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... The results of field trials with one such biorelease fertilizer, viz., a bio-release Zn fertilizer are herein reported. This compound has a water solubility of 2.4% but the entire amount of Zn is solubilized by common extractants like diethylenetriamine pentaacetic acid (DTPA), citric acid, ammonium citrate, ammonium oxalate, and 0.1 n HCl (Ray et al., 1993). The phosphate chain has a widely distributed chain length (Ray et al., 1998) in which Zn 2+ ions substitute for H + atoms of P-OH groups. ...
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It is impossible to consider in a short article each of the publications relevant to this field, which has appeared in the scientific and patent literature, they already far exceed a thousand in number. In the following pages, only the basic outline of the development of this branch of research and the present state of our knowledge will be presented, primarily from the standpoint of preparative and structural chemistry. Since the condensed arsenates acid mixed condensed species, the so called arsenatophosphates have played an essential role in elucidating the structure of some condensed phosphates, these will be considered in a special section. Derivatives of considered phosphates, in which oxygen is partly replaced by another element, such as nitrogen will be mentioned only where necessary. In the more recent literature, there are also a number of publications that are physico-chemical and theoretical in character. These will be considered only in, so far, as they contribute essentially to special problems of the structure of these substances or to the understanding of their most important properties.
Article
The infra-red spectra of 92 salts of phosphorus oxy-acids have been examined in the rock-salt region. Characteristic frequencies are suggested for P-N and P-O-P linkages. Confirmation of a number of frequency assignments made by other workers has been obtained. The absorption frequencies are found to be largely independent of the positive ion, but show considerable variation with the nature of the anion. General absorption bands characteristic of each class of negative ion are listed and their possible use in analysis is indicated. Effects of crystal and molecular structure on the absorption spectra are discussed, and some useful structural information is derived.
Article
The analysis of the NMR spectrum of ethylphenylphosphine (ABC3MX system) gives the following results: This secondary phosphine has a stable tetrahedric bond structure with respect to the NMR time scale, the geminal coupling constant for the methylene group attached to the phosphorus atom is negative and the two 2JP-C-H differ by about 5 cps.
Article
The purpose of this investigation was to develop a new colorimetric method for zinc and copper utilizing the compound 2-carboxy-2′-hydroxy-5′-suIfoformazylbenzene (Zincon). Both elements form a blue complex with this reagent. The zinc complex is stable over the pH range 8.5 to 9.5 while the copper complex is stable in the pH range 5.0 to 9.5. This difference in effect of pH permits the determination of zinc and copper in the presence of each other. Both complexes follow Beer's law over the concentration range 0.1 to 2.4 p.p.m. of the element. The sensitivity is 0.003 γper sq. cm. for both zinc and copper. An ion exchange procedure is described for the separation of zinc from interfering ions.