Characterization of Iron Phosphate Precipitation
Reinier Hernandez, Prabhakar Pant, PhD.
Applied Research Center, Florida International University
Abstract Results Conclusions
Nowadays heavy metals and specifically uranium contamination, The results for both titrations were recorded simultaneously. The From the results given in Figure 3, the iron (III) and phosphate This current work complements a prior study performed on a
two curves are shown in Figures 1 and 2. ratio at which the maximum precipitation is attained, can be similar system. Some of the findings, such as those obtained with a
has become a major environmental concern. Within the frame
Fig 1. Conductivity curve of iron-monophosphate complex formation determined. These results are in good accordance with those radiotracer experiment, were corroborated by using conventional
of implementation of polyphosphate technology for uranium 2.9
obtained by the radioactive tracer precipitation experiment. analytical techniques. Additional observations were made which
decontamination, the formation of iron (III) phosphate 2.7
enhanced the understanding of this particular system.
The SEM analysis allowed us to analyze the surface properties
Specific Conductivity (uS/cm)
complexes as a possible source of interference has been widely 2.5
and elemental composition of the samples.
The stoichiometry and reaction mechanism of iron phosphate
analyzed. This work intends to further previous studies on iron 2.3
Fig 4. X-ray fluorescence spectrum of Fe phosphate precipitate
2.1 formation was determined.
(III) phosphate complex formation. The reactions of Fe3+ with
1.9 The stoichiometric ratio Fe/PnOm in the precipitate follows the
different phosphate species (mono, pyro, tri) were characterized 1.7 order: Fe/PO43- > Fe/P2O54- > Fe/P3O75- .
by electrometric and atomic absorption methods. An elemental 1.5
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Elemental analysis using X-ray fluorescence yielded a varied
and surface analysis was performed in the precipitates obtained. Volume KH2PO4(uL)
Experimental Procedures The hydrolysis process of the polyphosphate species plays an
Ion Fe3+ Cl- Na+ H2PO4- H+
Conductometric/Potentiometric Titration Ionic Molar conductivity
important role in determining the precipitate composition.
68 76.31 50.08 36 349.65
The titrations (potentiometric and conductometric) were performed Precipitate surfaces were found very similar in all studied cases.
simultaneously using a set up similar to Figure 1. A 400 ml beaker was The shape of the conductivity curve is explained by the ion
used as a conductivity/potentiometric cell. 100 mL of deionized water was balance presented in the reaction. The conductivity raises The composition of the precipitate is given by the ratio between References
added to the titration cell followed by 3 mL of Fe3+ 0.1 M stock solution. because of the increment on the H+ species in solution; notice the peaks corresponding to each element. Corbridge, D.E.C. “Phosphorus: An Outline of its Chemistry,
The mixture was titrated using NaHPO4 0.1 M at a stirring rate of 2 rps. the high molar conductivity of H+. Table 3. Precipitate composition Biochemistry, and Technology”. 4th ed. Elsevier, Amsterdam,
The data was collected manually and analyzed in an Excel spread sheet. Fig 2. pH curve of iron-monophosphate complex formation Sample P/Fe ratio O/Fe ratio O/P ratio precipitate composition
mono 3 1.96 9.1 4.64
2.7 De Jager, H., and A.M. Heyns. “Study of the hydrolysis of sodium
FAA & SEM Analysis of Iron-Polyphosphate Precipitation mono 4 0.95 2.98 3.15
pyro 5 1.96 6.91 3.52 polyphosphate in water using Raman spectroscopy”. Applied
A ten point iron calibration curve (0.5 ppm, 0.75 ppm, 1.0 ppm, 1.5 ppm, 2.6
pyro 6 1.67 4.75 2.85
Spectroscopy, 52:808-814, 1998.
2.0 ppm, 2.5 ppm, 3.0 ppm, 3.5 ppm, 4.0 ppm, 5.0 ppm) was prepared tri 5 1.54 5.75 3.73
tri 6 2.3 7.8 3.39
Ed. David R. Lide. "Equivalent Conductivity of Electrolytes In
in each case. A total of 26 precipitation experiments were prepared by 2.45
Aqueous Solution." CRC Handbook of Chemistry and Physics
The variation in precipitate composition corresponds to the
mixing in a 50 mL centrifuge tube, different volumes of iron stock (25
90th Edition Internet Version 2010. 2010. Web.
presence of different phosphate species within the crystal.
mM) and the respective phosphate species for a given molar fraction.
0 5000 10000 15000 20000 Fig 5. SEM images of Fe/phosphate precipitates.
Phosphate molar fraction KH2PO4 Volume (uL) (A,B) Mono 500X. (C,D) Pyro 500X. (E) Tri 150X. (F) Tri 1000X. Knox, A.S., R.L. Brigmon, D.I. Kaplan, and M.H. Paller, 2008.
Sample monophosphate pyrophosphate triphosphate
1 0.83 0.83 0.83 “Interactions among phosphate amendments, microbes and
2 0.8 0.8 0.8
3 0.75 0.75 0.75
The pH trend presented in Figure 2 supports the reaction uranium mobility in contaminated sediments”. Science of the
4 0.66 0.66 0.66 mechanism proposed before in ec. 1. Total Environment 395:63–71.
5 0.5 0.5 0.5
6 0.33 0.33 0.33 Fig 3. pH curve of iron-monophosphate complex formation
9 0.16 0.16 I would like to acknowledge the contribution to this work from Ms.
After 72 hours, the samples were centrifuged at 4500 rpm for 15 minutes 80 Alejandro Amor and Dr. Prabhakar Pant (Mentor).
and the precipitate separated from the supernatant. The precipitates Special thanks to the DOE/FIU Science and Technology Workforce
% of Iron remaining
were dried in a vacuum oven at 130 C and kept in a desiccator. The 60
Development Program and all of its members for their guidance
supernatants were filtered through a 0.45 µm membrane filter and pyrophosphate
E F and support.
diluted accordingly. Iron concentration was measured using a Flame
Atomic Absorption Spectrophotometer and measuring the absorbance at
248.3 nm; deuterium lamp background correction was used.
Two precipitate samples of each phosphate were analyzed for elemental 0
0.0 0.2 0.4 0.6 0.8 1.0
composition and surface properties in the Scanning Electron Microscope. Phosphate molar fraction