Adjustable N:P2O5 Ratio Urea Phosphate Fertilizers for Sustainable

Dec 19, 2016 - Department of Physical and Inorganic Chemistry, Kaunas University of Technology, Radvilenu st. 19, LT-50254 Kaunas, Lithuania. ABSTRACT...
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Adjustable N:P2O5 Ratio Urea Phosphate Fertilizers for Sustainable Phosphorus and Nitrogen Use: Liquid Phase Equilibria via Solubility Measurements and Raman Spectroscopy Criztel Navizaga,† Jennifer Boecker,† Alfredas Martynas Sviklas,† Jolanta Galeckiene,‡ and Jonas Baltrusaitis*,†

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Department of Chemical and Biomolecular Engineering, Lehigh University, B336 Iacocca Hall, 111 Research Drive, Bethlehem, Pennsylvania 18015, United States ‡ Department of Physical and Inorganic Chemistry, Kaunas University of Technology, Radvilenu st. 19, LT-50254 Kaunas, Lithuania ABSTRACT: Design and use of the adjustable N:P2O5 ratio fertilizers is crucial in proper nutrient management if sustainable phosphorus use is to be ensured. Overfertilization with phosphorus can lead to its fixation in soil, as well as the unwanted environmental phenomena, such as eutrophication. Urea phosphate, CO(NH2)2·H3PO4, based liquid fertilizers were synthesized in this work, and their resulting physicochemical properties were determined. For this purpose, phase composition information on the CO(NH2)2·H3PO4−CO(NH2)2−H2O ternary system was analyzed, and critical points on the polytherm were determined. Liquid fertilizer compositions were determined and their corresponding physicochemical properties established. Raman spectroscopy showed that CO(NH2)2·H3PO4 partially retains its strong bonding interactions between both molecular adducts in aqueous solutions suggesting their improved nitrogen management efficiency in soils. Effect of these acidic pH fertilizer solutions on the pH of soil was determined and was found negligible. The potential of these fertilizers for reducing the loss of nitrogen from the wet soil is also discussed. KEYWORDS: Urea phosphate, Polytherm, Phase composition, Liquid fertilizer, Raman

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finite. Importantly, while phosphorus is the 11th most abundant element in the Earth’s crust, its practically useful deposits are geographically concentrated in only a few countries.3 While the potential and impacts of phosphorus recovery from waste, both liquid and solid,6−8 are currently being investigated [only about 10% of human waste phosphorus is returned to agriculture with one of the methods involving extracting struvite (magnesium ammonium phosphate) at wastewatertreatment plants9 or from sewage sludge10], sustainable phosphorus use needs to be established by optimizing the land use, as well as by improving fertilizer formulations11 and their application techniques.5 Thus, one of the major sustainable phosphorus (in conjunction with nitrogen) control points currently is in creating fertilizers with an adjustable nitrogen to phosphorus (N:P2O5) ratio, so eutrophication is reduced.12 Ideal N:P ratios range from 10:1 to 20:1 on a mass basis (4.4:1 to 8.7:1 as in N:P2O5), but the exact magnitude for actual plants and ecosystems varies considerably.13 Nitrogen and phosphorus containing fertilizers that are currently used, mono- and diammonium phosphates

INTRODUCTION Phosphorus is one of the four major nutrients (nitrogen, phosphorus, potassium, and sulfur) necessary for plants to sustain their biological activity, as it is involved in an array of processes, such as photosynthesis, respiration, energy generation, and nucleic acid biosynthesis.1 Phosphorus also is an integral component of several plant structures, such as phospholipids.1 It is considered to be a “limiting nutrient” since approximately half of the world’s agricultural lands are deficient in phosphorus.2 There is no substitute for phosphorus in crop growth since phosphorus cannot be manufactured (or destroyed) and, unlike other major life elements (carbon, nitrogen, oxygen, and hydrogen), has no gaseous phase compound significant enough to allow for its sustainable circulation via the atmosphere.3 Additionally, some phosphorus forms, such as apatitic phosphate, are highly immobile in soil because they react with many chemical and biological soil constituents, so efficient crop uptake requires P fertilizer to be placed close to crop seeds or roots.4 It has been shown that phosphorus is not being used sustainably; e.g., its footprint is unilaterally directed from the phosphorus rich mineral mines to crop production, processing, and consumption locations, where it becomes inactive and unsuitable for recycling and may cause contamination.5 This compromises sustainable phosphorus use since phosphate rock reserves are © 2016 American Chemical Society

Received: October 17, 2016 Revised: December 12, 2016 Published: December 19, 2016 1747

DOI: 10.1021/acssuschemeng.6b02511 ACS Sustainable Chem. Eng. 2017, 5, 1747−1754

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stoichiometric ratios of the reactants. Chemicals used, urea (CO(NH2)2, analytical grade) and phosphoric(V) acid (H3PO4, 85% weight in H2O), were obtained from Sigma-Aldrich and were ACS reagent grade. Additionally, for Raman experiments high purity urea phosphate (Sigma-Aldrich, ≥ 98%) was obtained. Distilled water was used as a solvent where necessary. Synthesis was performed by carefully dosing H3PO4 under vigorous stirring while cooling down the reaction vessel. Solid urea has a melting point of 133 °C, and with slow heating it begins decomposing at ∼80 °C.26 For this reason, the synthesis temperature was maintained below 60 °C at all times. Typical H3PO4 dosing took 5−7 min, after which the resulting solution was left to settle down. The product obtained was recrystallized in H2O and dried at 60 °C until a constant mass was obtained. Calculated synthesis yield was 95%. XRD analysis of the thus synthesized crystalline CO(NH2)2·H3PO4 material yielded a diffraction pattern with the peaks, analogous as those reported in the literature for CO(NH2)2·H3PO4.27−29 Solubility Experiments. Solubility measurements of binary CO(NH2 )2 ·H3PO4−H2 O and ternary CO(NH2)2 ·H3 PO4 −CO(NH2)2−H2O mixtures were performed using the visual polythermal method. It is based on the observation of the surface of the liquid. The liquidus temperature is the temperature at which the first crystals appear during cooling and the last crystal disappears during heating. The average value is then taken as the liquidus temperature. Typical cooling agents used were chosen depending on the crystallization temperature of the solids and were as follows:

and polyphosphates, possess varying chemical properties, as well as the corresponding nutrient amounts and ratios. In particular, monoammonium phosphate (NH4H2PO4) is the most thermally stable, not releasing any NH3 up to 100−110 °C. Diammonium phosphate ((NH4)2HPO4) starts decomposing and releasing NH3 at 70 °C forming monoammonium phosphate. The least stable is ammonium phosphate ((NH4)3PO4) which releases NH3 at 30−40 °C.14 For these reasons, N:P2O5 fertilizers typically are composed of both monoammonium phosphate and diammonium phosphate. They are usually phosphorus rich (theoretical N and P2O5 amounts of 12:62, 21:54, and 29:48 for mono- and diammonium phosphates and ammonium phosphate, respectively) and typically are made of gaseous NH3 and wet-process H3PO4. Fertilizer grades containing both mono- and diammonium phosphates are also produced, such as 13−52−0 and 16−48−0.14 The N:P2O5 ratio in solid single chemical compound containing fertilizers is typically fixed so there is a clear need for obtaining adjustable ratio fertilizers for sustainable phosphorus use. This can be done by creating liquid fertilizers of required composition.15−17 Creating liquid N:P2O5 fertilizers, however, requires determining a precise binary phase composition of the components involved. Urea phosphate, an adduct of CO(NH2)2 and H3PO4, was first reported by Matignon and Dode,18 and its potential as a N:P2O5 fertilizer has already been recognized.19−21 It is of particular interest in adjustable N:P2O5 ratio liquid fertilizer design since its total nutrient mass fraction is up to 60%. An added benefit of these compounds is that a single crystal is obtained with OC(NH2)2 and H3PO4 in the proximity of each other. This has been shown to significantly improve overall fertilizer release properties by retarding enzymatic hydrolysis of CO(NH2)2 by soil urease and reduction of gaseous N losses as NH3.22,23 The molecular structure of these adducts in aqueous solutions, however, has not been investigated. A reasonable approach to increasing the nitrogen content of these fertilizers is dissolving additional amounts of CO(NH2)2, solubility permitting. In the literature, CO(NH2)2·H3PO4− H3PO4−H2O ternary system solubility has been explored,24 as well as that of CO(NH2)2·H3PO4−NH4H2PO4−H2O,25 and the corresponding phase diagrams were constructed on the basis of solubility data. However, urea phosphate has a relatively low thermal stability and can easily decompose into phosphoric acid, carbon dioxide, and ammonia with further generation of NH4H2PO4.22,25 In order to adjust the N:P2O5 ratio, we investigated physicochemical properties of the ternary CO(NH2)2·H3PO4−CO(NH2)2−H2O system with the goal of determining critical points on the corresponding polytherm, as well as establishing the crystallographic and chemical composition of the compounds crystallized out of this system at 0 °C, the temperature used in designing liquid fertilizers. For this purpose, we investigated and devised a phase diagram of CO(NH2)2·H3PO4 in phase equilibrium with H2O and used Raman spectroscopy to obtain knowledge about the molecular bonding state therein. We then explored phase equilibria of aqueous CO(NH2)2·H3PO4 solutions obtained using CO(NH2)2·H3PO4 and urea and explored their resulting compositional and crystalline phase landscapes in order to establish the concentration and temperature boundaries, suitable for stable liquid fertilizers.

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• • • •

ice + KCl (cooling down to −11.0 °C) ice + NH4NO3 (down to −17.3 °C) ice + NaCl (down to −21.2 °C) dry ice + ethanol (down to −72.0 °C)

For the solubility measurements, 5 g of the solution was used, and temperature was measured using a Hg thermometer with a measurement error of ±0.1 °C. Chemical Analysis. Total nitrogen, as well as urea nitrogen, were determined using Kjeldahl digestion. Briefly, urea is transformed quantitatively into ammonia by boiling in the presence of sulfuric acid. The obtained ammonia is distilled from an alkaline medium, the distillate being collected in an excess of standard sulfuric acid. The excess acid is titrated by means of a standard alkaline solution.30 The phosphorus amount, expressed as P2O5, was determined using a photocalorimetric method via complexation with ammonium vanadate and ammonium molybdate.31 Soil pH Determination. pH values of the soil (Lithuanian loam) subjected to the treatment with the liquid urea phosphate, as well as diurea sulfate fertilizers, were determined using the extraction method.32 In particular, liquid fertilizer treated circumneutral (pH = 6.62) soil samples were subjected to the extraction with distilled H2O at the water-to-soil ratio of 5:1 for 1 h. Initial soil characteristics, such as total organic carbon (TOC) and moisture content, were determined gravimetrically at 520 and 100 °C, respectively, and the corresponding values determined were determined to be 4.39% and 45.70%. Instrumental Analysis. XRD analysis was performed using a DRON-6 instrument equipped with a Cu anode (λ = 0.154 nm). Diffraction patterns were acquired for 2Θ angles from 4° to 70°. The d-values of the synthesized urea phosphate, obtained from the XRD data of the crystallized material, were compared against those available in the literature.27−29 Differential thermal analysis was performed using a Du Pont Instruments 990 thermal analyzer operating at 10 mV/cm sensitivity and 10 °C/min temperature ramp time. Aluminum oxide was used as the inert material. Raman spectra were acquired using WITec alpha300R confocal Raman microscope using 532 nm laser and ×20 objective. Laser intensity at the sample was ∼54 mW.

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RESULTS AND DISCUSSION CO(NH2)2·H3PO4 Synthesis, Structural/Thermal Properties, and CO(NH2)2·H3PO4−H2O Binary System Phase Composition. It has long been known that urea exothermally

EXPERIMENTAL DETAILS

CO(NH2)2·H3PO4 Synthesis. Urea phosphate (1:1 molecular ratio of urea to H3PO4) was synthesized using the corresponding 1748

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Figure 1. Recorded XRD and TGA curves (inset) of the synthesized CO(NH2)2·H3PO4 crystalline materials. Calculated interplanar spacings, nm, are also shown.

reacts with inorganic acids making stable complexes.18,33 Urea phosphate has been reported by Matignon and Dode,18 and thermal characteristics in the CO(NH2)2·H3PO4−CO(NH2)2 binary system were determined with the eutectic point of 71.0 °C at 37% of CO(NH2)2. From the molecular point of view, intermolecular O−H···O and N−H···O interactions between CO(NH2)2 and H3PO4 have been postulated from X-ray and neutron diffraction data.27,28,34 The CO(NH2)2· H3PO4 obtained in this work was a crystalline material with the XRD pattern shown in Figure 1. Peaks observed were indexed against the available literature data,27−29 and their corresponding interatomic distances were calculated to be 0.878, 0.570, 0.546, 0.481, 0.440, 0.383, 0.379, 0.374, 0.353, 0.350, 0.322, 0.298, 0.285, 0.273, 0.260, 0.232, 0.220, 0.195, and 0.184 nm. The most pronounced peak at 2Θ = 10.1° with the interatomic distance of 0.878 nm is due to the diffraction from the (200) plane in the Pbca space group unit cell of CO(NH2)2·H3PO4.29 These results were later used as a reference to confirm the identity of the solid compounds, crystallized from a complex CO(NH2)2·H3PO4−CO(NH2)2−H2O system. Further, TGA analysis of the crystalline material synthesized was performed to identify the important thermal transitions. The first peak observed at 115 °C was attributed to the melting of the parent compound, which coincides with the decomposition onset into its constituents, CO(NH2)2 and H3PO4, followed by the exothermal transition at 160 °C due to the hydrolysis of CO(NH2)2. The resulting NH3 neutralizes H3PO4 to form NH4(H2PO4) which at 185 °C forms more complex forms of ammonium phosphates. In agreement, complex chemical transitions were observed by 31P NMR spectroscopy during the pyrolysis of NH4(H2PO4) and (NH4)2(HPO4) in the presence of urea at 120 °C to go through a sequence of reactions involving di- and triphosphates and eventually leading to ammonium hexacyclophosphate (NH4PO3)6.35 The corresponding concentrations of the solid and liquid phases formed in the −8.0 to 40 °C temperature range for the CO(NH2)2·H3PO4−H2O mixture were determined using a visual polythermal method and shown in Figure 2. It can be seen that CO(NH2)2·H3PO4 dissolves very well in H2O; e.g., 65.00% of the solid compound can be dissolved at 37.4 °C. The polytherm exhibits a single critical point at −8.0 °C with the composition of the liquid of 30.00% of CO(NH2)2·H3PO4 and 70.00% of H2O and the solid phase composed of ice and CO(NH2)2·H3PO4. Before the critical point, the solid phase is only composed of ice, whereas at higher CO(NH2)2·H3PO4

Figure 2. Measured polytherm of CO(NH2)2·H3PO4 dissolution in H2O. Several different solid phases were detected, namely, Ice* (ice), UP* (urea phosphate), and Ice* + UP* (ice + urea phosphate). One critical point was found at −8.0 °C.

concentrations in solution, CO(NH2)2·H3PO4 is the solid phase and crystallizes at increasing temperatures. CO(NH 2 ) 2 ·H 3 PO 4 −CO(NH 2 ) 2 −H 2 O Ternary System Phase Composition. CO(NH2)2·H3PO4 has a N:P2O5 ratio of 1:2.5. For an increase in the nitrogen amount needed to be closer to that accommodated by plants, CO(NH2)2 can be added, but the resulting three component system solubility and phase chemical analysis need to be performed, and the literature data is not available. While CO(NH2)2·H3PO4 dissolves well in pure H2O, the presence of an additional CO(NH2)2 phase will distinctly change its solubility properties. These new solubility conditions will also dictate the largest amount of nitrogen that can be present in the liquid fertilizers. Thus, in the next step, solubility of the ternary CO(NH2)2·H3PO4− CO(NH2)2−H2O phase was explored. Nine sets of experiments were conducted as shown in Table 1. In particular, a fixed CO(NH2)2·H3PO4 to H2O ratio was used in experiments I through VI while changing the CO(NH2)2 added, whereas a fixed CO(NH2)2 to H2O ratio was used in experiments VII through IX with varying CO(NH2)2·H3PO4 added. Figure 3 shows polytherm curves determined using I through VI solution compositions, and Figure 4 shows those of VII through IX. Tabulated critical point data, including solution phase composition, critical point temperature, as well as the solid phase crystalline phase detected, are summarized in Table 2. It can be seen from Figure 3a that I through III composition solutions have two crystallization regions due to ice and CO(NH2)2. 1749

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ACS Sustainable Chemistry & Engineering Table 1. CO(NH2)2·H3PO4−CO(NH2)2−H2O Ternary Phase Composition Used in Analysisa number I II III IV V VI VII VIII IX a

mass composition (5 (5 (5 (5 (5 (5 (5 (5 (5

− − − − − − − − −

x)[10% x)[20% x)[30% x)[40% x)[50% x)[60% x)[10% x)[20% x)[30%

CO(NH2)2·H3PO4 + 90% H2O] + xCO(NH2)2 CO(NH2)2·H3PO4 + 80% H2O] + xCO(NH2)2 CO(NH2)2·H3PO4 + 70% H2O] + xCO(NH2)2 CO(NH2)2·H3PO4 + 60% H2O] + xCO(NH2)2 CO(NH2)2·H3PO4 + 50% H2O] + xCO(NH2)2 CO(NH2)2·H3PO4 + 40% H2O] + xCO(NH2)2 CO(NH2)2 + 90% H2O] + x[CO(NH2)2·H3PO4] CO(NH2)2 + 80% H2O] + x[CO(NH2)2·H3PO4] CO(NH2)2 + 70% H2O] + x[CO(NH2)2·H3PO4]

Total of 5 g of material was used, and x varied from 0 to 5.

Figure 3. Measured polytherms of ternary CO(NH2)2·H3PO4−CO(NH2)2−H2O varying composition system crystallization for (a) I, II, and III and (b) IV, V, and VI solution compositions, shown in Table 1. Fixed mass ratio of CO(NH2)2·H3PO4 to H2O was used at each data point, and CO(NH2)2 was added to obtain the total analyte mass of 5 g. Red points on the curve minima show the corresponding temperature values as well as the solid phase crystalline composition.

of the solutions with compositions II and III follow a similar pattern with the critical point temperature slightly decreasing in each case (−14.5 and −15.5 °C, respectively). Crystallization from the solution compositions of IV through VI shown in Figure 3b also proceeds in two distinct regions. Initially, at lower CO(NH2)2 concentrations, CO(NH2)2·H3PO4 is the crystalline phase obtained, and after the critical point, CO(NH2)2. At the 50:50 and 60:40 compositions of CO(NH2)2·H3PO4 and H2O, a sharp increase in the critical point temperatures can be observed with the measured values of 11.5 and 22.0 °C (compositions V and VI in Figure 3b). Similarly, crystallization curves were obtained for the compositions VII though IX and shown in Figure 4. Here, the solid phase is composed of ice at lower CO(NH2)2·H3PO4 concentrations and of CO(NH2)2·H3PO4 at those above the critical point. Critical point temperatures measured were −9.5, −12.0, and −14.7 °C for the compositions VII though IX, respectively. Phase diagrams of the ternary CO(NH2)2·H3PO4−CO(NH2)2−H2O system were constructed using the data obtained from the solubility experiments shown in Figures 2−4. Three distinct crystalline phase regions, Ice* (ice), U* (urea), and UP* (urea phosphate), can be distinguished by the black polytherm curve connecting the eutectic points measured (* signifies the crystalline phase present). It can be seen that the crystallization temperature at the critical points changes from −15.5 to 22.0 °C. The absolute lowest crystallization temperature determined was −15.5 °C with the solution composition of 30.0% CO(NH2)2, 21.00% CO(NH2)2·H3PO4, and 49.00% H2O. The saturated solution crystallization isotherm at 0 °C, corresponding to that typically used in liquid fertilizers, is

Figure 4. Measured polytherms of ternary CO(NH2)2·H3PO4− CO(NH2)2−H2O varying composition system crystallization for VII, VIII, and IX solution compositions, shown in Table 1. Fixed mass ratio of CO(NH2)2 to H2O was taken at each data point, and CO(NH2)2· H3PO4 was added to obtain total analyte mass of 5 g. Red points on the curve minima show the corresponding temperature values as well as the solid phase crystalline composition.

Before the critical point, ice is the only solid phase that crystallizes with the decreasing crystallization temperature as CO(NH2)2 concentration increases. The composition I critical point appears at −11.5 °C with a corresponding solution composition of 32.50% CO(NH2)2, 6.75% CO(NH2)2·H3PO4, and 60.75% H2O. The solid phase contains both ice and CO(NH2)2. Upon a further increase of the CO(NH2)2 concentration, crystallization temperature starts increasing, and the obtained solid phase changes to CO(NH2)2. Crystallization out 1750

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Table 2. CO(NH2)2·H3PO4−CO(NH2)2−H2O Critical Points with the Corresponding Solution Composition and Solid Phase Detected solution composition, wt % sample

CO(NH2)2 (U*)

2CO(NH2)2·H3PO4 (UP*)

H2O

cryst temp, °C

solid phase detected

I II III IV V VI VII VIII IX

32.50 31.20 30.00 30.20 32.50 34.50 6.75 13.98 21.25

6.75 13.76 21.00 27.92 33.75 39.30 32.50 30.10 27.50

60.75 55.04 49.00 41.88 33.75 26.20 60.75 55.92 50.75

−11.5 −14.5 −15.5 −12.5 11.5 22.0 −9.5 −12.0 −14.7

Ice* + U* Ice* + U* Ice* + U* + UP* U* + UP* U* + UP* U* + UP* Ice* + UP* Ice* + UP* Ice* + UP*

aqueous solution at 0 °C is 35.50% CO(NH2)2, 25.80% CO(NH2)2·H3PO4, and 38.70% H2O. That corresponds to the N:P2O5 ratio of 1.8:1 with the total nutrient concentration in these liquid fertilizers of 32.61%. Another conclusion can be drawn from the data presented in Table 3 for 0 °C crystallization liquid fertilizers: the sum of the nutrients does not change systematically with the changes in the corresponding N:P2O5 ratio. Importantly, for N:P2O5 1:1 ratio nutrient concentration is rather large at 30.54%. It is larger than that in the conventional liquid fertilizers obtained neutralizing H3PO4 with NH3 use in obtaining 12:12:0 fertilizers with total nutrient concentration of 24.00%. Flexible N:P2O5 ratios from 2.5:1 to 1:1.8 fertilizers are obtained with the total nutrient concentration of 29.16% and 25.99%, respectively. In addition to 0 °C, other compositions are available for various climate conditions. For example, −10.0 and −5.0 °C crystallization temperature fertilizers have 1.6:1 and 1.7:1 ratios of N:P2O5, whereas in warmer climates those with 5.0, 10.0, and even 15.0 °C crystallization temperature liquid fertilizers can be used with the corresponding 29.74%, 32.35%, and 34.09% of total nutrient concentration. Importantly, liquid fertilizers with N:P2O5 ratio of 7.1:1 and 7.9:1 were also obtained and shown in Table 3, although the total N:P2O5 content was slightly lower at 21.98% and 23.21%, respectively. pH values of these fertilizers were also determined and are also shown in Table 3. It can be seen that it is low and changes very little with the composition from 1.24 to 1.83 for the highest, 41.50%, and the lowest, 18.75%, CO(NH2)2·H3PO4 amount in solution, respectively. Dynamic viscosity and density values of the liquid fertilizers change little and vary from 3.878 to 6.165 mPa s and 1201 to 1241 kg/m3. Very little correlation between the solution concentration and its properties can be observed, chiefly due to the fact that the solution density varies very little. Raman Spectroscopy of Urea and Urea Phosphate Solutions. Raman spectra in the 800−1200 cm−1 region of solid CO(NH2)2 and CO(NH2)2·H3PO4 and their aqueous 5%, 15%, 25%, 35%, and 45% solutions (weight %) were obtained to determine the nature of the CO(NH2)2 complexes that might form and might have implications for liquid fertilizer efficiency. These data are shown in Figure 6. The peak at 1013 cm−1 is attributed to the N−C−N symmetric stretch in CO(NH2)2.36,37 In the presence of aqueous solution the peaks shift toward lower wavenumber for CO(NH2)2 solution at 1006 cm−1 in agreement with the literature data, most likely due to the solvating action of H2O molecules surrounding it.36 Notably, the peak at 1006 cm−1 remained constant regardless of the solvating power of H2O or concentration of CO(NH2)2. CO(NH2)2·H3PO4, on the other hand, showed two peaks at

also shown. Important conclusions can be drawn from the data shown in Figure 5. It can be concluded that, within the ternary

Figure 5. Measured phase diagram of the ternary CO(NH2)2·H3PO4− CO(NH2)2−H2O system. Three distinct crystalline phase regions, Ice* (ice), U* (urea), and UP* (urea phosphate), can be distinguished by the black polytherm curve connecting the eutectic points measured. Crystallization temperatures are shown in red next to the corresponding points. Saturated solution crystallization isotherm at 0 °C corresponding to that of liquid fertilizer is shown in blue. Lines are for eye guidance only. Concentrations are expressed in terms of fraction for brevity.

CO(NH2)2·H3PO4−CO(NH2)2−H2O system, CO(NH2)2· H3PO4 slightly decreases the solubility of CO(NH2)2. On the other hand, addition of CO(NH2)2 to the aqueous CO(NH2)2·H3PO4 solution initially increases the solubility of CO(NH2)2·H3PO4. After the amount of CO(NH2)2 added reaches 6.75%, CO(NH2)2·H3PO4 solubility starts decreasing. Liquid Fertilizers from the CO(NH2)2·H3PO4−CO(NH2)2−H2O Ternary System. The CO(NH2)2·H3PO4− CO(NH2)2−H2O ternary phase diagram in Figure 5 also shows the blue points corresponding to the concentrations obtained from the saturated solution curves that intersect 0 °C, shown in Figures 3 and 4. By definition, liquid fertilizer composition is determined by saturated solutions that crystallize at 0 °C. In some instances, depending on the climate and liquid fertilizer use, liquid fertilizers that crystallize out of the saturated solutions at lower or higher temperatures than 0 °C can be used. Thus, the data in Figure 5 were used to formulate the compositions of the corresponding liquid N:P2O5 fertilizers based on CO(NH2)2·H3PO4. The results are summarized in Table 3. It can be seen that the largest nutrient concentration in 1751

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Table 3. Physicochemical Properties of the Proposed Liquid N and P Containing Fertilizers Based on Urea Phosphate solution composition, wt % U*

UP*

H2O

31.50 33.70 37.50 35.50 6.30 19.80 20.25 18.60 17.55 39.00 42.00

27.40 26.52 18.75 25.80 37.00 34.20 32.50 38.00 41.50 6.10 5.80

41.10 39.78 43.75 38.70 56.70 46.00 47.25 43.40 40.95 54.90 52.20

nutrient content, % N + P2O5 nutrient ratio (N:P2O5) 31.80 32.27 29.16 32.61 25.99 30.54 29.74 32.35 34.09 21.98 23.21

1.6:1 1.7:1 2.5:1 1.8:1 1:1.8 1:1 1:1 1:1.1 1:1.1 7.1:1 7.9:1

crystallization temp, °C

pH

dynamic viscosity, mPa s

density, kg/m3

−10.0 −5.0 0.0 0.0 0.0 0.0 5.0 10.0 15.0 0.0 5.0

1.52 1.60 1.83 1.70 1.46 1.62 1.38 1.32 1.24 1.86 1.89

4.016 4.123 5.621 6.165 4.971 4.881 3.878 4.258 4.992 5.342 5.297

1223 1225 1201 1234 1215 1216 1211 1235 1241 1200 1199

Figure 6. Raman spectra of solid CO(NH2)2 and solid CO(NH2)2·H3PO4, as well as their 5%, 15%, 25%, 35%, and 45% weight aqueous solutions.

Figure 7. Measured pH of the soil after its exposure to the 26, 52, 104, 208, 416 mg of CO(NH2)2·H3PO4 and 51, 102, 204, 408, and 816 mg of 2CO(NH2)2·H2SO4 liquid fertilizers per 100 g of soil.

914 and 1033 cm−1. The former is assigned to the symmetric P−O stretch of the PO43− ion while the latter is due to the N−C−N symmetric stretch in CO(NH2)2.38 In the solid material, it is shifted toward higher wavenumbers (1013 to 1033 cm−1) due to the strong hydrogen bonds between both adducts. Hence, the N−C−N symmetric stretch should provide direct insights into the molecular bonding (H2O shell or H3PO4 shell). Indeed, as the CO(NH2)2·H3PO4 concentration is increased from 5% to 45%, two peaks at 1006 and 1019 cm−1 become apparent in the spectra with the

latter becoming more intense at higher CO(NH2)2·H3PO4 concentrations. The peak at 1019 cm−1 can be attributed to solvated CO(NH2)2·H3PO4. This observation has large implications for the proposed aqueous solution fertilizer efficiency. They suggest that even in aqueous solutions the CO(NH2)2 molecule is surrounded by H3PO4 via hydrogen bonds. This, in turn, suggests that aqueous CO(NH2)2·H3PO4 solution can to a large extent possess the same beneficial properties of its solid counterpart due to the close contact between both adduct molecules. 1752

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ACS Sustainable Chemistry & Engineering Soil Acidity Testing. Measured pH values of the liquid CO(NH2)2·H3PO4 fertilizers are low and may affect the pH of the soil, resulting in the undesired side effects for the plants. While certain plants prefer slightly acidic pH soils,39 lowering of the soil pH is generally detrimental. At the very low (