Crystallization and characterization of struvite crystals through gel growth technique
Crystal growth or crystallization is defined as controlled phase or state change to solid state. This state transition might happen from any state of matter, liquid, gas or solid to solid state which are crystals. Crystalline materials produced through chemical reaction in any media can readily b...
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| Format: | Thesis |
| Language: | English |
| Published: |
2017
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| Online Access: | http://psasir.upm.edu.my/id/eprint/71204/1/FK%202017%2068%20-%20IR.pdf http://psasir.upm.edu.my/id/eprint/71204/ |
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| Summary: | Crystal growth or crystallization is defined as controlled phase or state change to solid
state. This state transition might happen from any state of matter, liquid, gas or solid to
solid state which are crystals. Crystalline materials produced through chemical reaction in
any media can readily be controlled in order to increase the crystal size, yield and change
the crystals morphology. However, the gel media is known to be one of the best media for
investigating the effects of different operational parameters on crystals size, morphology,
yield and purity so far.
Magnesium ammonium phosphate hexahydrate (MgNH4PO4.6H2O) which is better
known as struvite, is one of the most fascinating inorganic phosphate compounds. There
are several reasons associated with the study of struvite crystals. First of all, it is one of
most popular scale deposits in wastewater treatment plants. Second of all, struvite is
known as one of the major components of urinary calculi.
Crystallization and characterization of struvite crystals grown in gel media has been
carried out in the current study. Struvite crystals were grown in two different gels of gelatin
and silica gel using both single and double diffusion crystallization techniques. The pH
ranged from 6 to 9 for silica gel and gelatin. Different gel densities were practiced ranging
from 1.04 to 1.08 for silica gel and 1.04 to 1.11 for gelatin. Totally the crystals grown in
silica gel had more variety of morphologies and the silica gel itself demonstrated less
number of limitations compare to gelatin gel. Crystals with prismatic, pyramidal,
dendritic, star shaped and branched star shaped morphologies were grown through this
technique mainly in silica gel. The grown crystals in both gel media have been
characterized by SEM, EDX and FTIR sets. The SEM test revealed the inner morphology,
crystal size and the volume of cracks inside the crystals. EDX test proved the existence of
comprising elements, namely phosphorous, oxygen and magnesium. The FTIR test
revealed the internal chemical bonds of N-H, O-H, P-O, water of crystallization and metal oxygen bonds. The FTIR results are presented and compare to other studies carried out
formerly.
The harvested struvite crystals grown within different operational conditions were
analyzed by powder X-ray diffraction (XRD) set. The crystal size was calculated based on
Scherrer equation which ranged from 27.9124 to 32.5919 nm. It has been found that
struvite crystallizes in the orthorhombic (single-phase) Pmn21 space group with unit cell
parameters: a = 6.9550 Å, b = 6.1420 Å, c = 11.218 Å and α = β = ϒ = 90°. Totally, the
crystals grown in pH of 8 had larger crystal size. This was also approved by the SEM
analysis showing less amount of cracks happening at this pH value.
Struvite crystal habits were analyzed using optical microscopy. All three crystals
diaphaneity, transparent, translucent and opaque were noticed in the crystallization of
struvite crystals in silica gel. The crystals formed in the acidic pH values (6, 6.5)
demonstrated high transparency while the crystals formed at high alkaline pH values had
more translucent and opaque diaphaneity. The high alkaline pH value (9) lead to the
formation of Liesegang rings which is described in detail in the context.
The solubility of produced crystals were analyzed through solubility test. Proportional
portions of the harvested crystals were stirred well for one day at constant rate of 400 RPM
and after that dried out in the oven at 35 °C. The solubility results revealed that the
minimum solubility happens for the crystals grown in gel with pH of 8 at 392 and 474
mg/l for gel SG of 1.07 and 104 respectively.
An optimization study based on Taguchi model of experiment design has been carried out
and the results were announced. The effects of different combination of four process
parameters of gel density, gel pH, magnesium molarity and ADP molarity on struvite
crystal size were studied and the results were reported. The results of the ANOVA analysis
revealed that pH was the most influential parameter in sense of producing bigger crystals
with contribution percent of over 65% followed by magnesium dosage (25%), ADP
molarity (7%) and gel density (1%).
The kinetics and growth mechanism of struvite crystallization in gel media has been
studied in this thesis. For this purpose, both pseudo first and second-order kinetic models
were applied to the experimental data. The tests were carried out on the amount of
magnesium concentraton changes over time and rate of struvite crystals formation
assuming equamolar reaction (consumption) of 1:1:1 for Mg:NH4:PO4. The magnesium
concentration was measured at certain intervals and based on the concentration reduction
of magnesium, the kinetic rate constants (K1, K2) and rates of reaction (R) for samples
grown in various operational conditions were calculated and reported. Based on the results
of the kinetic study the experimental data better fits with second-order kinetic model with
minimum and maximum R2 over 97% and 99% compare to first-order kinetic model with
minimum and maximum R2 of 89% and 95% respectively. The obtained pseudo first and
second-order rate constants can be used to explain the struvite crystallization behavior. |
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