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Development of a new slow-release micronutrient fertilizer of iron−manganese is described. Studies include formulation, synthesis, kinetics, characterization, and testing. The compound is a partially polymerized, iron−manganese−magnesium polyphosphate, which is produced at 200 °C by reaction of iron, manganese, and magnesium oxides with phosphoric acid, followed by neutralization to pH 5.6. Polymerization is optimal at 21.2% with an Fe/Mn/Mg/P molar ratio of 1:0.51:1.15:7.33. Condensation kinetics show multistage processes with plateau formation at the end of each stage. The compound is crystalline with new XRD patterns indicative of a regular arrangement of polyphosphate chains. ESR spectra reveal Mn predominantly in the IV state. The product has ideal slow-release characteristics of low water solubility but high citrate and DTPA solubility, indicating high bio-availability of the micronutrients. Plant trials with chilli show a 45.6% increase in yield, at Fe 2 kg/ha−Mn 1 kg/ha as the slow-release fertilizer. The compound appears to be a promising, environmentally friendly alternative for Fe−Mn fertilization.
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GENERAL RESEARCH
Development of a Novel Slow-Releasing Iron-Manganese Fertilizer Compound
Ishita Bhattacharya,Siladitya Bandyopadhyay,Chandrika Varadachari,†,* and Kunal Ghosh
Raman Centre for Applied and Interdisciplinary Sciences, 16A Jheel Road, Calcutta 700 075, India, and
Department of Agricultural Chemistry & Soil Science, UniVersity of Calcutta, 35 BC Road,
Calcutta 700 019, India
Development of a new slow-release micronutrient fertilizer of iron-manganese is described. Studies include
formulation, synthesis, kinetics, characterization, and testing. The compound is a partially polymerized, iron-
manganese-magnesium polyphosphate, which is produced at 200 °C by reaction of iron, manganese, and
magnesium oxides with phosphoric acid, followed by neutralization to pH 5.6. Polymerization is optimal at
21.2% with an Fe/Mn/Mg/P molar ratio of 1:0.51:1.15:7.33. Condensation kinetics show multistage processes
with plateau formation at the end of each stage. The compound is crystalline with new XRD patterns indicative
of a regular arrangement of polyphosphate chains. ESR spectra reveal Mn predominantly in the IV state. The
product has ideal slow-release characteristics of low water solubility but high citrate and DTPA solubility,
indicating high bio-availability of the micronutrients. Plant trials with chilli show a 45.6% increase in yield,
at Fe 2 kg/ha-Mn 1 kg/ha as the slow-release fertilizer. The compound appears to be a promising,
environmentally friendly alternative for Fe-Mn fertilization.
Introduction
The use of soluble salts as micronutrient fertilizers is a cause
for serious environmental concern. Dosages far exceed plant
uptake and could lead to a buildup of these metal ions in soils.
Leaching losses of anionic micronutrients and chelated forms
may cause groundwater contaminantion. High wastage and very
low use-efficiency are major economic drawbacks. Conse-
quently, micronutrient fertilizer application has not been popu-
larized to the extent necessary and essential in many regions of
the world. The solution, to both the environmental and economic
problems associated with the widespread use of water-soluble
fertilizers, is in the development of cost-effective slow-release
compounds. For a water-insoluble compound to function as a
fertilizer, nutrients may either be released slowly into the
solution by hydrolysis or diffusion or they may be present in
forms that are extractable by plant roots. The majority of slow-
releasing fertilizers developed to date belong to the first
category, that is, either hydrolysis or diffusion controls the
release of nutrients to the plants, as in urea formaldehyde, via
membrane-coated fertilizers or glass frits, and so forth.1-3The
other category of slow-release fertilizer is that in which nutrients
are present in exchangeable or chelate-extractable positions.
These mimic the natural forms of available nutrients in soils.
Synthetic ion-exchange-resin-based fertilizers4belong to this
category as well as the more recently developed phosphate-
polymer-based compounds.5,6 Those in the latter group are
superior because release rates in hydrolysis and diffusion-
controlled mechanisms may not match rates of plant uptake,
and thus such fertilizers are not always effective. Furthermore,
fertilizers with incorporated heavy-metal micronutrients are
highly insoluble and have not proved to be successful. Com-
pounds developed for Zn and Cu7,8 have overcome some of the
major drawbacks associated with earlier slow-release formula-
tions. Uniquely, these are short-chain polyphosphates in which
problems of hygroscopicity, stickiness, and water solubility
inherent to such partially polymerized linear polyphosphates can
be successfully overcome to produce very effective fertilizers.5,6
The nutrient ions in these compounds have low water solubility
but are potentially plant-available by virtue of their high
solubility in organic acids such as citrate, and so forth, which
are excreted by plant roots (hence solubility in organic acids
can be used as an index of nutrient availability9). Thus, nutrient
availability from such compounds is not controlled by hydrolysis
rates but by active extraction by the roots.
In view of the promising characteristics of this type of slow-
release formulation, the next step must be to extend this concept
to the development of fertilizers of other micronutrient cations.
Iron and manganese are two other important micronutrients
widely used as fertilizers. They are used mostly in the form of
sulfates or EDTA salts, both of which are water-soluble.
Dosages for ferrous or manganous sulfate fertilization are
necessarily high (10-50 kg/ha) because of rapid transformation
in the soil.9Fe-EDTA salts are more efficient but are expensive
and, therefore, not widely used for cereals and low-value crops.
Development of effective and relatively inexpensive slow-release
iron and manganese fertilizers could improve usage of iron and
manganese, particularly in cereals and vegetables grown in
developing countries. This would improve crop yields in these
densely populated regions and also provide more iron nutrition
in the diet.
Here, we report the development of a novel slow-releasing
iron-manganese compound based on a short-chain polyphos-
phate structure. The study involved three stages, namely, (i)
formulation and synthesis of a slow-release compound with
desired properties, (ii) fundamental studies (including reaction
* To whom correspondence should be addressed. Phone: 91-33-
2483 0029. Fax : 91-33-2418 0610. E-mail: rcais@cal3.vsnl.net.in.
Raman Centre for Applied and Interdisciplinary Sciences.
University of Calcutta.
2870 Ind. Eng. Chem. Res. 2007, 46, 2870-2876
10.1021/ie060787n CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/10/2007
kinetics) in the iron-manganese-(magnesium)-phosphate
system, and (iii) characterization and testing of the compound
developed.
Methodology
1. Formulation and Synthesis. Preliminary experiments were
conducted working on the premise that the end product should
possess the desired properties of low water solubility and high
citrate/DTPA solubility.5,6 The following materials were used:
(i) synthetic goethite [R-FeO(OH)], (ii) manganese dioxide (GR,
E Merck), and (iii) orthophosphoric acid (GR, E Merck).
Goethite was analyzed for Fe content after digestion in
concentrated HCl followed by spectrophotometric determination
as the o-phenanthroline complex.10 The Fe content of goethite
was 62%.
The Fe/Mn weight ratio used was fixed at 2:1 (molar ratio
of 1:0.51) based on an average ratio of iron and manganese
contents observed in normal leaf tissues of a large number of
plant species.9Sufficient H3PO4was added to convert all cations
to the dihydrogen phosphates; therefore, molar ratios were
Fe/Mn/P )1:0.51:5.04. Reactants were taken in a borosilicate
glass beaker and placed in a muffle furnace at 200 °C. Samples
were taken at regular intervals, neutralized with ammonia to
pH 5.6-6. This pH range was experimentally obtained for
optimum product properties. (Details are given in the Results
section). Neutralized products were tested for solubility in 0.33
M citric acid and 0.005 M DTPA. The objective was to obtain
a compound which was almost completely soluble in citrate
and DTPA since solubility in these reagents may infer plant
availability.5,9 Weight loss in the system was recorded at periodic
intervals.
Characterization of the Fe-Mn system showed that products
with desirable properties could not be obtained. Notably, all of
the products had low solubility in citrate and DTPA. In was
envisaged that addition of a bivalent cation could result in the
formation of cross-linked chains that would be more susceptible
to cleavage. Thus, some P-O-M2+-O-P cross-linkages could
be added which would induce some weak points in the crystal
and make the structure more soluble. Following preliminary
trials with Ca2+,Mg
2+,Na
+, and K+, we observed the best
results with Mg2+. Another set of experiments was, therefore,
conducted with MgO (GR, E Merck) as an additive. Here, Mg
content was varied from Fe/Mg )1:2.3 to 1:0.29 with sufficient
H3PO4added to convert the cations to the dihydrogen phos-
phates.
Bench-scale studies were then repeated with potentially
promising raw materials: (i) iron ore or red oxide of iron
(hematite), (ii) manganese ore (pyrolusite), (iii) roasted mag-
nesite, and (iv) commercial phosphoric acid. Hematite (0.1 g)
was digested10 in a mixture of 10 mL of concentrated HF and
3 mL of concentrated HClO4(GR, E Merck) on a hot plate,
and the solution was analyzed for Fe2+/3+and Al3+, with
Fe2+/3+determined as the o-phenanthroline complex as described
above and Al3+determined as the calcium aluminum alizarin
Red S complex.10 Si4+in all the ores was determined as the
blue silicomolybdate after fusing the sample with NaOH beads
and extracting into dilute HCl.10 Pyrolusite was analyzed after
dissolving the sample in 10 mL of concentrated H2SO4with
dropwise addition of 3 mL of 30% H2O2followed by heating.10
Mg2+and Ca2+contents of magnesite were determined by AAS
after dissolution of the sample in 6 M HCl. P content
of H3PO4was determined as the blue phosphomolybdate.10
Results of chemical analysis data were as follows: (a) hema-
tite: 46.28% Fe, 4.33% Al, 8.99% Si; (b) pyrolusite: 49.31%
Mn, 1.08% Al, 2.21% Fe, 1.94% Si; (c) magnesite: 41.72%
Mg, 0.71% Fe, 11.56% Si; and (d) commercial phosphoric
acid: 25.54% P.
Bench-scale studies with ores were done in borosilicate
beakers. The optimum molar ratio of Fe/Mn/Mg/P determined
by trials was 1:0.51:1.15:9.9. Here, excess H3PO4was added
to compensate for impurities (such as Al3+,Ca
2+,Si
4+), which
may consume the acid. Each batch consisted of8gofhematite,
3.76 g of pyrolusite, 4.44 g of roasted magnesite, and 79.57 g
of phosphoric acid. All components were taken in a preweighed
beaker and placed in a muffle furnace at three test tempera-
tures: 175, 200, and 250 °C. Condensation kinetics was studied
by recording the weight at regular intervals. Product charac-
teristics were evaluated by taking samples at periodic intervals,
neutralizing them with ammonia to pH 5.6, and then testing
them for solubility in 0.33 M citric acid The sample produced
a nearly colorless solution within 30 min when at the optimum
polymerization stage. The polyphosphate liquid was then
neutralized with ammonia solution (10% NH3), and its weight
was recorded. It was dried in an oven at 80 °C, and the weight
of the sample was recorded once more. The sample was ground
and sieved through 150 mesh. Several batches of fertilizer were
produced for analysis and testing.
2. Reaction Kinetics. Kinetics (laboratory-scale) was studied
using synthetic goethite, manganese dioxide, magnesium oxide,
and orthophosphoric acid. Here, H3PO4was diluted to about
40% P2O5to make it easier to pipet. The exact strength of the
acid was determined by spectrophotometric analysis as the blue
molybdophosphate complex.10 Standardization of the acid was
done at weekly intervals.
Details of the reaction studies were described earlier.5,11
Briefly, reactions were carried out in preweighed platinum
crucibles containing weighed amounts of FeO(OH), MnO2,
MgO, and H3PO4in the molar ratio Fe/Mn/Mg/P )1:0.51:
1.15:7.33. This ratio was observed to be best-suited for fertilizer
formulation. After addition of the reagents, the weight of the
crucible and its contents were recorded. The crucible was then
preheated in a muffle furnace at 150 °C((0.5 °C) for 30 min
to remove excess water and to avoid spattering of the contents
at higher temperature. It was stored in a desiccator, and the
furnace temperature was raised to 165, 175, 200, and 225 °C
((0.5 °C). After the required period of heating, the crucible
was cooled in a desiccator and weighed.
The actual amount of H3PO4(excluding water), designated
as [H3PO4], was calculated for each system from the known
weight and concentration of H3PO4solution initially taken.
Weight loss in the reaction system was obtained from the initial
weight of FeO(OH) +MnO2+MgO +[H3PO4] minus the
final weight after heating. Range of error in these values is about
(0.2%. Theoretical H2O loss for complete (100%) polymeri-
zation to metaphosphate was evaluated according to the fol-
lowing three pairs of reactions :
FeO(OH) +3H3PO4fFe(H2PO4)3+2H2O (1a)
Fe(H2PO4)3fFe(PO3)3+3H2O (1b)
MnO2+4H3PO4fMn(H2PO4)4+2H2O (2a)
Mn(H2PO4)4fMn(PO3)4+4H2O (2b)
Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007 2871
Since the molar ratio of Fe/Mn/Mg in our system is 1:0.51:
1.15, the total water loss for complete polymerization is (5 ×
1) +(6 ×0.51) +(3 ×1.15) mol )11.51 mol )207.18 g.
This system contains 7.33 mol of H3PO4. Thus, a weight loss
of 0.104 g/g [H3PO4] in the system corresponds to the formation
of dihydrogen phosphate, and a weight loss of 0.288 g/g
[H3PO4] corresponds to 100% polymerization. The degree of
polymerization (expressed as a percent) of a system with a
recorded weight loss of θg/g H3PO4)[(θ-0.104)/(0.288 -
0.104)] ×100.
Amount of Fe3+/2+solublized by 0.33 M citric acid, from
the polyphosphate, was determined by spectrophotometric
analysis as the ferrous o-phenanthroline complex.10 It was
experimentally determined that the solution was required to
stand for 24 h after addition of reagents in order to overcome
the inhibiting effect of citrate ion on color development.
Number-average chain length (n) of the polyphosphate was
determined by dissolving the sample in 0.5 M H2SO4and
removing Fe3+,Mn
4+, and Mg2+interference by solvent
extraction as the 8-hydroxyquinoline complex followed by
titrimetric analysis.12
3. Characterization and Testing. Sufficient fertilizer (iron-
manganese-magnesium-ammonium polyphosphate) was pro-
duced at the bench scale using commercial hematite, pyrolusite,
roasted magnesite, and phosphoric acid (as described in section
1). This was done by mixing raw materials in the molar ratio
Fe/Mn/Mg/P )1:0.51:1.15:9.9 and heating at 200 °C for 70
min, corresponding to a weight loss of 0.144 g/g [H3PO4]in
the system. The polyphosphate was neutralized with ammonia
solution (10% NH3) to pH 5.6, dried at 80 °C, and ground
and sieved through 150 mesh. Samples were dissolved in
concentrated HCl and analyzed for Fe3+/2+and Mg2+contents,
while Mn4+was analyzed after dissolving the sample in
H2SO4-H2O2as described earlier.
Solubility of the compound in water, 0.33 and 0.104 M (2%)
citric acid (GR, E Merck), and 0.005 M DTPA (AR, Ferak-
Berlin) was determined. To 0.05 g of the fertilizer, 50 mL of
0.33 M citrate, 150 mL of 2% citrate or 150 mL of 0.005 M
DTPA was added, and the suspension was agitated in a
horizontal shaker for 30 min, after which samples were filtered
(Whatman 42), washed (with about 75 mL of deionized water),
made to volume, and analyzed for Fe3+/2+,Mn
4+,andPas
described above, after digestion with triacid (HClO4-HNO3-
H2SO4)13 to oxidize chelates. To study rates of solubilization
of the fertilizer in water, 0.05 g of the compound was taken,
and 10 mL of water was added to each. The suspensions were
allowed to stand. After 3, 6, 9, 12, 15, 18, and 21 days of contact
time, the solutions were filtered (Whatman 42), washed (with
30 mL of deionized water), made to volume, and analyzed for
Fe3+.
IR spectra of the sample were recorded on a Perkin-Elmer
FTIR RX1 instrument with the scan range of 4500-450 cm-1
(resolution (5cm
-1) using KBr pellets. XRD was recorded
on a JDX-8030 X-ray diffractometer using Ni-filtered Cu KR
radiation at a scanning speed of 2°2θ/min. The ESR spectrum
was recorded at room temperature with powdered samples using
a JEOL (model JES-RE1X) spectrometer.
Plant growth experiments were carried out in pots using the
iron-manganese-magnesium-ammonium polyphosphate. Sur-
face soil from a black soil region (Vertisol) was collected from
Gundkheri, Nagpur, Maharashtra, India. Characteristics of this
soil are as follows: Udic Chromustert, pH 7.95, ECe0.155
dS/m, organic carbon 0.74%, available Fe and Mn 1.80 mg/kg
and 3.3 mg/kg, respectively (determined by 0.005M DTPA
extraction),19 and exchangable Mg2+11.84 cmol(p+)/kg (de-
termined by AAS). Each pot contained 2 kg of soil. The
treatments consisted of the following: (i) a control (where only
NPK fertilizers but no Fe-Mn fertilizers were added), (ii) four
different levels of Fe-Mn micronutrients as the slow-release
fertilizer, and (iii) the same four levels of micronutrients as
ferrous sulfate and manganous sulfate. Each treatment had 6
replicates. All pots were equalized for additions of N, P, K,
Mg, and SO42-by calculated additions of urea, DAP, KCl,
MgSO4, and K2SO4. The fertilizers were thoroughly mixed into
the soil before planting. Chilli (Capsicum frutescens) was grown
as the test crop. Chillies were harvested and their weights
recorded. Due to pest attack some plants were damaged, and
this also introduced more error in statistical analysis of the data.
Vitamin C was analyzed by extracting 0.5 g in 4% oxalic acid
followed by titrimetric analysis using 2,6-dichlorophenol in-
dophenol.15 Samples were also oven-dried, ground, and digested
in the triacid mixture. Fe3+/2+in the extract was determined as
described above. Results of the experiment were statistically
analyzed for the significance of the differences in the mean
values at 1% and 5% levels.
Results and Discussion
1. Formulation and Synthesis. Trials with the Fe-Mn-P
system showed that citrate (0.33 M) solubility of all products
MgO +2H3PO4fMg(H2PO4)2+H2O (3a)
Mg(H2PO4)2fMg(PO3)2+2H2O (3b)
Figure 1. Rates of reaction at different temperatures (bench-scale studies
with commercial materials).
Figure 2. Schematic diagram of the process of production of slow-release
iron-manganese fertilizer.
2872 Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007
was low; no product could be synthesized which had good
solubility in citrate. Low solubility could be attributed to the
fact that Fe3+and Mn4+form very strong cross-linkages between
adjacent P-O-P chains, and thereby a very stable structure is
formed which cannot be made soluble by breaking of P-O-
P-Fe/Mn-O-P-bonds. Therefore, Mg2+was introduced into
the structure to provide weak points susceptible to solubilization.
The products formed with Mg as an additive were more
promising. At the lowest Fe/Mg molar ratio of 1:0.29, there
was a significant improvement in solubility; even so, some
citrate-insoluble residue was observed in all products. With
further increase in the Fe/Mg ratio to 1:1.15, an improvement
in solubility occurred. The product obtained at 200 °C with 45
min heating produced a clear solution with citrate (0.33 M).
The mechanism of dissolution is essentially due to chelation of
the cations, which are removed from the structure by citrate or
DTPA leaving behind a polyphosphate chain. By addition of
Mg2+, weaker linkages are introduced. The initial dissolution
of Mg2+by chelation exposes the structure to further breakdown
and facilitates solubilization. Lower levels of heating yielded
products that left a whitish insoluble residue in citrate. Over-
polymerized products, on the other hand, produced a black
residue on citrate treatment. Weight-loss data showed that the
optimum level of polymerization corresponds to a weight loss
of 0.143 g/g H3PO4or 21.2% polymerization.
In the next stage, the process was upgraded and studied on a
bench scale with ore-grade materials. Rates of condensation in
this system can be seen in Figure 1. Rates are essentially linear
in nature with breaks (slope changes) that indicate a multistage
process. Overall, the features are suggestive of a zero-order
reaction with phase changes.
Reactions at all three temperatures (Figure 1) have been
terminated at the optimum polymerization levels. It is, therefore,
interesting to observe that at 175 °C, weight loss for optimum
polymerization is lower than that at 200 °C, and weight loss at
250 °C is the highest. Thus, at lower temperatures, although
rates of reaction are slower, less polymerization is required to
produce the desired materials. Structural differences between
polymers obtained at different temperatures may account for
this behavior.
For the synthesis of fertilizer, a temperature of 200 °C appears
to be optimum; reaction at 175 °C is comparatively slow,
whereas at 250 °C reaction control is difficult and over-
polymerization is likely. Optimum pH for neutralization of the
polyphosphate was studied in the range pH 4-7. Product
obtained at pH <5, on drying, was hygroscopic and sticky. At
a neutralization pH >6, there was a reduction in citrate
solubility. High citrate solubility and low hygroscopicity were
Figure 3. Reaction kinetics and degree of polymerization at different
temperatures (laboratory-scale studies with pure compounds).
Figure 4. Polymerization steps showing metal tetraphosphate dimer of less-stable (b) and more-stable (a) forms, plus the stable form of a multidimensional
polymer (brickwall-like structure). Magnesium is shown as an example of a metal.
Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007 2873
obtained at pH 5.5-6.0. A schematic diagram of the process is
shown in Figure 2.
2. Polymerization Kinetics and Mechanism. Reaction
kinetics, in general, showed a multistage process (Figure 3).
At the lowest temperature studied, that is, 165 °C, there was an
initial nonlinear rise followed by a plateau. All temperatures
above 175 °C showed one or more plateaus. This clearly
indicated a multiple-stage reaction. Thus, at 225 °C, a plateau
at 32.4% polymerization was followed by a steep rise and then
another plateau at 59.5% polymerization. Reaction at 200 °C
showed three plateaus at 14.0, 39.9, and 51.2% polymerization.
It is interesting that plateau formation did not occur at the same
level of polymerization but varied with temperature; generally,
plateau regions were higher at higher temperatures.
The features may be explained by first envisaging the
fundamental reactions (eqs 4-6):
Dihydrogen phosphate polymerizes by elimination of one
mole of water between two P-OH groups of adjacent phos-
phates, forming linear polyphosphates with -P-O-P-link-
ages.11 Condensation of small chains would produce succes-
sively larger chains in a random manner. Thus, a Fe-diphosphate
(n)2) could condense with another Fe-diphosphate or
orthophosphate to produce a tetraphosphate (n)4) or a
triphosphate (n)3), respectively. It could also combine with
a Mn/Mg-phosphate to produce a mixed polymer. The reacting
species would be very heterogeneous, and this would account
for the complex reaction kinetics in the polyphosphate system.
On the basis of the information obtained from kinetic data,
further insight can be gained into the nature of polymerization.
The following facts may be considered: (i) All systems showed
similar dehydration patterns, (ii) these did not conform to regular
first-, second-, or third-order kinetics, (iii) stage-wise reaction
was in evidence, and (iv) each stage was essentially linear.
According to this polymerization mechanism initially pro-
posed elsewhere,11 the heating of dihydrogen phosphates
produces OH-ions. These ions combine with the H+of an
adjacent P-OH group with release of H2O. After polymeriza-
tion, a dimer can have two structural arrangements, with one
formation being more stable than the other due to steric factors
(Figure 4). As polymerization proceeds further, variations in
structural forms would be more numerous. The stable form in
a multidimensional polymer containing divalent metal ions can
be represented in a brick-wall-like structure (Figure 4). During
polymerization, as longer chains are produced, one or more
unstable structures would be formed. These would have to
transform to the stable forms before further polymerization is
possible.
This hypothesis can explain several features of the condensa-
tion curves (Figure 3). Here, the rate-limiting step of reaction
would be the dissociation of OH-groups from the phosphates.
In the presence of excess phosphate, this would show zero-
nFe(H2PO4)3fFenH(4n+2)P3nO(11n+1) +(n-1)H2Of
Fen(PnO3n)3+(2n+1)H2O+[(n-1)H2O] (4)
nMn(H2PO4)4fMnnH(6n+2)P4nO(15n+1) +
(n-1)H2OfMnn(PnO3n)4+(3n+1) H2O+
[(n-1)H2O] (5)
nMg(H2PO4)2fMgnH(2n+2) P2nO(7n+1) +
(n-1)H2OfMgn(PnO3n)2+(n+1)H2O+[(n-1)H2O]
(6)
Table 1. Chemical Composition and Solubility Characteristics of Fe-Mn Polyphosphate Fertilizersa,b
% solubility in
water 0.33 M citric acid 0.1 M (2%) citric acid 0.005 M DTPA
% chemical
composition Fe Mn P Fe Mn P Fe Mn P Fe Mn P % polymerization &
number-average chainlength (n)
Fe2O3)4.76 (0.02)
MnO2)2.71 (0.04)
MgO )2.57 (0.01) ND ND (0.02) 7.5 91.1 (0.03) 81.1 (0.05) 94.3 (0.02) 85.0 (0.03) 81.3 (0.06) 79.8 (0.01) 87.6 (0.01) 86.8 (0.05) 85.7 (0.02) 21.2 (0.5) & 3.34 (0.05)
P2O5)47.23 (0.05)
NH4+)14.79 (0..1)
aNumbers in parentheses are deviations of the data for duplicate observations. bMean values are reported here.
2874 Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007
order kinetics, that is, a straight line in the kinetic curve. As
the structure becomes larger and unstable intermediates are
formed, condensation would slow down to allow for structural
rearrangements. Polymerization would subsequently continue
with the same zero-order kinetics until more unstable structures
are formed. Therefore, there would be another slowing down
of the rate, resulting in plateau formation.
3. Characterization and Testing of the Compound. Table
1 shows the chemical analysis data, number-average chain
length, and solubility characteristics of the iron-manganese
compound prepared in the bench scale. The polyphosphate was
a short-chain polymer with a number-average chain length of
3.34. Water solubility of Fe or Mn from the fertilizer was too
low to be detectable (<0.1%). Solubility over a period of time
(0-21 days) showed no further release of cations from the
compound. The hydrolysis rate of the polyphosphate appeared
to be slow, and the compound was quite stable in water.
Solubility of the compound in citrate and DTPA was, however,
very high. About 80-90% of the Fe and Mn were solubilized
in 0.33 M and 0.1 M (2%) citric acid and 0.005 M DTPA (Table
1). Solubility in these reagents is an index of availability of
nutrients to crops.,9,14 thus the plant availability of the fertilizer
Fe and Mn is expected to be high.
Room-temperature ESR spectrum showed a broad band with
ag-value of 1.986 that corresponds to Fe(III) and Mn(IV). A
weak Mn(II) band was also observed at a g-value of 2. IR
spectrum (Table 2) showed absorptions at 1074 cm-1that may
be attributed to P-O ionic stretching16 and at 919 cm-1due to
P-O-P vibrations. Whereas long-chain polyphosphates, as
reported in the literature, show absorptions at 1250 cm-1, this
is shifted to lower wavenumbers for a short-chain polyphos-
phate;16 the data, therefore, suggest that the iron-manganese
polyphosphate contained mostly short-chain P-O-P. Similar
absorptions had been observed5,6 for two other short-chain
polyphosphates of zinc (at 1100 cm-1) and copper (at 1150-
1050 cm-1). Weak absorption at 1291 cm-1however, implies
some long-chain P-O-P. The presence of hydrogen-bonded
OH groups is also indicated by the strong bands at around 3236
cm-1.
Although the compound was a short-chain polyphosphate,
XRD indicated that it was fairly well crystallized (Table 2).
In order to determine reflections due ammonium polyphos-
phates, XRD of a sample containing no ammonium polyphos-
phate (i.e., a sample neutralized with MgO) was also studied.
Thereby, reflection at 1.79 Å could be attributed to ammonium
polyphosphate. Other ammonium phosphates were also con-
firmed by their reflections at 3.71 Å in the ammoniated sam-
ple and its absence in the Mg-neutralized sample. On com-
parison with XRD data for similar short-chain zinc and cop-
per polyphosphates,5,6 several common reflections are ob-
served. These reflections at around 3.04 and 2.0 Å are present
in all these polyphosphates, and they have been attributed to
a periodicity in the polyphosphate chain.5,6 Reflections at 5.2
Å could be due to ammonium pyrophosphate and that at
3.7 Å (Table 2) to ammonium hydrogen phosphates.17 All of
the reflections, however, cannot be interpreted since the
compound is new and necessary data are not available in the
literature.
Chillies (Capsicum frutescens) grown on a black soil showed
excellent response to slow-release Fe-Mn fertilizer (Table 3).
At 2 kg/ha Fe and 1 kg/ha Mn, yield increased by 179%
compared to the control (no Fe-Mn fertilization). An increase
Table 2. XRD, IR, and ESR Data
XRD spectra (Aa) XRD spectra (Bb) IR absorptions (A)
d(A°) intensity assignmentc,e d(A°) intensity assignmentc,e wavenumber (cm-1) assignmentd
1.67 32 Fe-Mn pp 1.68 33 Fe-Mn pp 3236 OH
1.80 34 2370 P-OH
1.79 22 App 1.93 35 Mg pp 2344 P-OH
2.05 33 pp 1687 Mg-O-P
2.00 98 pp 2.20 34 1443 NH4
2.40 45 Mg pp 1404 NH4
2.56 21 Fe-Mn pp 2.55 41 Mg pp 1291 P-O
2.59 57 Fe-Mn pp 1074 P-O
2.63 26 pp 2.73 60 pp 919 P-O-P
2.88 54 Mg p 541 Fe-O
3.04 100 pp, MAP 3.04 95 pp Mn-O
3.10 100 pp
3.71 40 MAP, DAP 3.46 96
4.19 53 Mg p
5.21 39 App, pp 4.53 50 Mg pp
4.62 45 pp
4.84 77 Mg pp
5.37 56 pp
5.94 86 Mg pp
aIron-manganese-magnesium-ammonium polyphosphate. bIron-manganese-magnesium polyphosphate fertilizer. cRef 17. dRef 16. eApp: ammonium
polyphosphate; pp: polyphosphate; MAP: monoammonium phosphate; DAP: diammonium phosphate; Mg p :magnesium phosphates;
Table 3. Results of Plant Experiment with Chilli
average yield of chilli pods (g/pot) total uptake of Fe by fruits (mg/pot) total uptake of Mn by fruits (mg/pot) vitamin C (mg/1000 g)
Fe-Mn dose
(kg/ha) water-soluble
fertilizers slow-release
fertilizer water-soluble
fertilizers slow-release
fertilizer water-soluble
fertilizers slow-release
fertiliz er water-soluble
fertilizers slow-release
fertiliz er
0-0 2.47 2.47 0.30 0.30 0.12 0.12 2000 2000
2-1c6.90ac0.50 c0.24 c3566
4-2 4.57 c0.95 c0.34 c3566 c
6-3b7.18a12.5a0.85 2.95 0.27 0.61 2573 2710
8-4 6.27a7.20a0.68 1.51 0.27 0.32 1791 1959
aSignificant with respect to control at 5% level (LSD0.05); CD at 5% is 3.73; Standard Error is 1.75. bOptimal dose. cData not available due to crop
failure by pest attack.
Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007 2875
in yield of 406% was observed (significant also at P )1% level)
at a dose of 6 kg/ha Fe and 3 kg/ha Mn. Conventional fertilizers
also increased yields, but the extent of increase was less (Table
3). Average Fe uptake by chillies was observed to be higher in
slow-release fertilizer treatments. Chillies were also enriched
in vitamin C in slow-release fertilizer treatments (Table 3), with
an increase of 78% observed at an application rate of 2 kg/ha
Fe. Due to large variance caused by pest attack, the data for
Fe, Mn, and vitamin C contents did not show significance at
the 5% level.
Good fertilizer response obtained with chillies suggests that
other crops with high iron or manganese requirements would
also be responsive to this slow-release fertilizer.
Conclusion
We were successful in developing a new polymeric compound
of iron-manganese that would be an effective slow-release
fertilizer material.18 The compound is a partially polymerized
phosphate that is produced by heating a mixture of oxides and
phosphoric acid at around 200 °C to an optimum degree of
polymerization of around 21%. This is a polycondensation
reaction with multistage kinetics and complex reaction routes.
The polyphosphate itself is liquid and acidic; neutralization to
an optimum pH 5.6-6.0 converts it to a form that can be dried
and ground to a fine powder. The final product is free-flowing
and nonhygroscopic.
The compound is crystalline and shows XRD patterns that
are not characteristic of any known substance. ESR shows Mn
in IV and II states and Fe in the III state. Experiments with a
test crop (chillies) produced a 179% increase in yield at a dose
of 2 kg/ha Fe-1 kg/ha Mn as slow-release fertilizer. This also
resulted in increased uptake of Fe and an increase in vitamin C
content of the fruit. Considering all performance parameters
measured (i.e., yield, Fe uptake, and vitamin C content), an opti-
mum application rate of 6 kg/ha Fe was observed for the
fertilizer.
Use of such slow-release fertilizers could drastically cut down
micronutrient dosages, which are at present very high because
of low use-efficiency of the soluble salts such as ferrous and
manganous sulfates. This could also lead to more favorable
economics of fertilizer use and thereby encourge micronutrient
usage. In view of the simplicity of the synthesis route, attractive
chemical properties of the compound (low water solubility and
high bio-availability of nutrients), easy application, and appar-
ently good fertilizing efficiency, this slow-release iron-man-
ganese fertilizer could provide an environment-friendly alterna-
tive for effective fertilization. Similar compounds could be
synthesized on the basis of this concept, to develop slow-release
fertilizers for other micronutrients and their combinations.
Acknowledgment
We are grateful to the Department of Science & Technology,
Government of India, for financial support. We thank Professor
P. Ray, Department of Chemical Engineering, Calcutta Uni-
versity, for his suggestions.
Literature Cited
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New Jersey, 1968.
(2) Shaviv, A. Advances in controlled-release fertilizers. AdV. Agron.
2001,71,1.
(3) Singh, K.; Sharma, H. C.; Singh, C. S.; Singh, Y.; Nishizawa, N.
K.; Mori, S. Effect of polyolefin resin coated slow release iron fertilizer
and its methods of application on rice production in calcareous soil. Soil
Sci. Plant Nutr. 2004,50, 1037.
(4) Ranney, M. W. Fertilizer AdditiVes and Soil Conditioners; Noyes
Development Corporation: New Jersey, 1978.
(5) Ray, S. K.; Varadachari, C.; Ghosh, K. Novel slow-releasing
micronutrient fertilizers. I. Zinc coumpounds. Ind. Eng. Chem. Res. 1993,
32, 1218.
(6) Ray, S. K.; Varadachari, C.; Ghosh, K. Novel slow-releasing
micronutrient fertilizers. 2. Copper coumpounds. J. Agric. Food Chem. 1997,
45, 1447.
(7) Ray, S. K.; Varadachari, C.; Ghosh, K. Process for producing a slow-
releasing zinc fertilizer. Indian Patent 172,800, 1990.
(8) Ray, S. K.; Varadachari, C.; Ghosh, K. Process for producing a slow-
releasing copper fertilizer. Indian Patent 177,205, 1991.
(9) Mortvedt, J. J.; Giordano, P. M.; Lindsay, W. L. Micronutrients in
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(10) Maxwell, J. A. Rock and Mineral Analysis; Interscience: New York,
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(11) Varadachari, C. An investigation on the reaction of phosphoric
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357.
(12) Ray, S. K.; Chandra, P. K.; Varadachari, C.; Ghosh, K. Removing
micronutrient metal cation interferences prior to titrimetric determination
of polyphosphate chain length. J. Agric. Food Chem. 1998,46, 2222.
(13) Hossner, K. A. Dissolution for total element analysis. In Methods
of Soil Analysis, Part 3; Sparks, D. L., Ed; American Society of
Agronomy: Madison, WI, 1996.
(14) Lindsay, W. L.; Norvell, W. A. Development of a DTPA soil test
for zinc, iron, manganese and copper. Soil Sci. Soc. Am. J.1978,42, 421.
(15) Davies, M. B.; Austin, J.; Partridge, D. A. Vitamin C: Chemistry
and Biochemistry; Royal Society of Chemistry: London, 1991.
(16) Corbrige, D. E. C.; Lowe, E. J. Infrared spectra of some inorganic
phosphorus compounds. J. Chem. Soc.1954, 493.
(17) JCPDS. Powder Diffraction File; International Center for Diffraction
Data: Newtown Square, PA , 1984.
(18) Varadachari, C. A process for the manufacture of bio-release iron-
manganese fertilizer. International Patent Application PCT/IN2004/
000234, 2004.
ReceiVed for reView June 21, 2006
ReVised manuscript receiVed January 9, 2007
Accepted February 12, 2007
IE060787N
2876 Ind. Eng. Chem. Res., Vol. 46, No. 9, 2007
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