Open Science Repository Chemistry
Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension
Khulood A. A. AbuAlola
Chemistry Department, Community College for Girls in Hyanakiyah, Taibah University, Al-Madinah Al-Munawarah, Saudi Arabia
Behavior of the chromate ions and their removal efficiency in CaO-Al₂(SO₄)₃ suspensions with molar ratio 5:1 was studied. The suspension was prepared by agitating with a magnetic stirrer for constant 3 hours at room temperature. Run products were collected by filtration, washing and air-dry and evaluated by XRD, SEM and DTA. According to XRD and SEM results the major product in this suspension was ettringite with minor amounts of gypsum and calcite. DTA results showed that the presence of the chromate ions in the suspension retards the formation of the products and decreases the degree of their crystallinity. However, there is high removal of the chromate ions by the formed suspension products. This is related to the substitution of the chromate ions by sulfate in the ettringite crystals.
Keywords: CaO-Al₂(SO₄)₃, suspensions, chromate ions, removal.
Citation: AbuAlola, K. A. A. (2013). Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension. Open Science Repository Chemistry, Online(open-access), e70081936. doi:10.7392/Chemistry.70081936
Received: February 2, 2013
Published: March 6, 2013
Copyright: © 2013 AbuAlola, K. A. A. Creative Commons Attribution 3.0 Unported License.
The presence of heavy
metals in the environment has been of great concern because of their growing
discharge, toxicity and other adverse effects on receiving waters. Heavy metals
contaminants in soils originate in the spreading of inorganic fertilizers,
sewage sludge and industrial wastes. One of those toxic species is chromium
and its chromate derivatives, which, if present in levels greater than
the permissible limit, become long-term hazardous contaminants because of their
high toxicity, causing severe damage to the kidneys and nervous system. In
alkaline solutions, Cr (VI) primarily occurs as CrO42-
ions are somewhat soluble and can escape into aqueous leaching solutions. The
problems of determination and reduction of soluble chromates are of great
interest, and this can be verified looking at the several technical papers and
patents that have been presented on this topic during the last years (2,
Suspensions of saturated
solutions of calcium oxide with aluminium sulfate will result in different
products and depend on many factor like the molar ratio of calcium oxide to
aluminium sulfate, initial pH of the solution and concentration of sulfate ions (4)in the suspensions. If the calcium oxide:aluminium sulfate molar ratio is more
than 4, a product called ettringite, (3CaO.Al2
O), is formed. Ettringite is a naturally occurring mineral
found in Germany for the first time (5,6). This mineral is
characterized by the very high content of water molecules and is very important
to the cement technology, since it appears as an early hydration product for the
first stage of hydration of Portland cement. Recently, ettringite attracted a
special attention in view of environmental issues, specifically in sub-surface
geology concerning SO3
-comprising waste dumping and fluorine sorption from
contaminated waste waters as well as underground waters (7-9).
Chromate ions which may present as a contaminate in the waste water can
substitute sulfate ions in the crystal structure of ettringite and, for this,
ettringite represents a good reagent for their removal(10-12).
In the present study,
removal behavior of chromate ions by CaO-Al2
is studied. The characteristics of the products after various time intervals were studied using X-ray diffraction (XRD), scanning electron
microscope (SEM) and differential thermal analysis (DTA).
2.1. Preparation of the suspensions
Reagents of Ca oxide, CaO,
special grade of Sigma Aldrich, BET specific surface area 13.37 m2
and Al-sulfate hydrate, Al2
were used. Primarily, Al-sulfate solution was prepared by diluting with
deionized water to obtain 0.01mol/L solution as Al2
which mixed with 100 ml water containing 0.05 mol/L CaO in presence of various concentrations of chromate ions. Different
mixes were prepared designated as E0, E1, E2, E3, E4 and E5 with
concentrations of CrO42-
ions 0, 0.01, 0.02, 0.03 and
0.05 molar/l respectively.
Each suspension was agitating by a magnetic
stirrer for constant 3 hours at room temperature. After 1, 2, 4, 6 and 24 hours of
mixing process, filtration was carried out and the filtrate was stored for
determination of the remains of CrO42-
The precipitation remained in the filter paper was washed out by distilled
water and dried in air for 24 hours. After drying, it was stored in a desiccator
containing silica gel for analysis. The precipitated was evaluated by XRD, SEM
2.2 Evaluation of the products
The products produced from
the calcium oxide, aluminum sulfate suspension were evaluated by using X-ray diffraction analysis (XRD), scanning electron microscope
(SEM) and differential
thermal analysis (DTA). X-ray
examination was carried out by employing Mac Science MXP3
under 40kV–20mA CuK
α radiation. The
growth of the products crystals was studied by SEM examination. A JEOL-JSM-5400
high resolution scanning electron microscopy was used (Shimadzu Co., Japan). For DTA test, differential
thermal analyzer was used at heating rate of 20°C/min. The measurements were
made in N2
atmosphere using Shimadzu DTA – 50H. The sample chamber
was purged with nitrogen at a flow of 30 ml/min.
2.3. Behavior of the
removal of chromate ions
The remains of CrO42-
ions in the filtrate were measured by
colorimetric techniques. This was done using colorimeter at 370 nm, corresponding
to the maximum absorbency of CrO42-
ions. The uptake
percentage was calculated using the following equation:
Removal (molar/l) = [(C0 – Ct) ]
initial concentration of CrO42-
ions insert in
suspensions (molar/l) and Ct
is the remained concentration of CrO42-
ions in the filtrate after various
3. Results and discussion
3.1 X-ray diffraction
XRD patterns of E0, E1, E3
and E4 mixes after 6 hours of mixing are shown in Fig.1. For mix E0, the main
product was ettringite with minor amounts of the gypsum (CaSO4
) phases. This is indicated by the increase in the
intensity of the peaks characterized to ettringite compared to those of the
other products. Calcite, CaCO3
, was also identified in X-ray
patterns. Minor calcite formation was also reported by some authors during
synthesis of ettringite in suspension solutions (8, 13, 14, 15).
Figure 1. X-ray after 6 hours, g-gypsum,
e- ettringite, C- CaO, a- Al2(SO4)3,
Figure 2. X-ray after 24 hours, g- gypsum, e- ettringite, C– CaO
Peaks of unreacted CaO and
) were also identified in E0 patterns. For E1, E3
and E4 suspensions, similar products were obtained but with slight lower
intensities in the peaks characterized to the obtained products (ettringite,
gypsum and calcite), and there is an increase in the intensities of the peaks of
the reactants. This decrease in the intensities of the products peaks accompanied
with increase in the intensities of reactants peaks indicates a retardation to
the reaction between CaO and Al2
) as a result of
presence of chromate ions. This retardation to the main reaction is due to side
reaction, which is the substitution of the chromate ions by the sulfate ions
inside the ettringite crystal.
After 24 hours of mixing
process, XRD patterns of the different mixes were represented in Fig. 2. From
the obtained patterns of E0 suspension, we can notice that the intensity of the
ettringite and the other reaction products were increased while the
intensity of CaO and Al2
) decreased, indicating
progress of the reaction. Besides there are increased intensities of the gypsum phase
in the suspensions containing chromate ions, comparing to E0 suspension. This
indicates an increase in its formation in these suspensions. Here, there is
no significant difference between the intensities of the peaks identified for
the reaction products (gypsum and ettringite) for E0 and the other mixes.
It is obvious that
ettringite formation and its amounts in the suspension, comparing to other
products, depend on molar ratio of calcium oxide to aluminium sulfate in the
starting suspensions. Also, their precipitation depends on pH of the solutions (10)
which is also a function of this molar ratio in suspensions. Strictly saying,
ettringite precipitation may depend on the concentrations of ionic species
dissolved in the suspensions. Here, although the ratio of calcium oxide to
aluminum sulfate used in the suspension was 5:1, not all CaO is soluble.
This permits the formation of other products beside ettringite, like gypsum and
calcite. However, the main reaction product is ettringite, which explains the
high uptake of the chromate ions noticed in the later section.
3.2. Scanning electron microscope
SEM micrographs of the precipitate formed from E0 and E3 mixes after 6 hours of mixing are shown in figures 3 and 4 respectively.
Figure 3. SEM micrographs of E0 suspensions: a- after 6 hours, b- after 24 hours
Figure 4. SEM micrographs of E4 suspensions: a- after 6 hours, b- after 24 hours
Fig. 3 represents SEM micrograph
for E0 mix, after 6 and 24 hours of mixing process. The micrographs showed a
well definite crystal which can be related to ettringite beside to platy
crystals that can be related to CaO. This confirms that the main products of
the reaction of CaO and Al2
) suspensions is the
etringite. After 24 hours of mixing the suspensions, Fig.3- b, there is an
increase in the size and the crystallinity of the formed ettringite.
For E3 suspension after 6
hours, Fig 4 - a, smaller ettringite crystals with a fewer numbers can be
identified in the micrographs after 6 hours of preparing the suspensions. This
confirms that the presence of chromate ions retards the rate of formation of
ettringite. After 24 hours, higher numbers of ettringite crystal can be
observed in SEM micrographs, Fig.4 - b. The reduction in the ettringite crystal
noticed here in the micrographs agrees with the decrease in the intensities of XRD
peaks obtained at the same mixing age and has the same explanation.
3.3. Differential thermal
The DTA thermogrames of
precipitates produced from suspensions E0 and E3 after 6 hours of mixing are
shown in Fig. 5. All DSC curves showed three main endothermic peaks located at 105,
490, and 700–780 ºC. The first endotherm located at 105 ºC is mainly due to the
dehydration of the ettringite, which is the main suspension product. The
second peak located at 490 ºC,
the major mass loss, mainly related to the decomposition
(16). We can noticed the increase in the intensity of such
peak in case of suspension E3, which confirms the retardation effect to the
reaction of CaO
results of the presence of chromate ions.
The third endothermic
double peak located at 718–770 ºC is due to the decomposition of
with different degrees of crystallinity (17,18).
The enthalpy of this endotherm varies as a
result of change in the degree
of carbonation of the specimens.
After 24 hours of mixing,
the SDC curves for E0 and E3 are shown in Fig. 6. The same three endothermic
peaks are observed as in the case after 6 hours of mixing. While we can
notice the decrease in the intensity of the second peak characterized by CaO
with an notable increase in the intensity of the first peak characterized by
ettringite, the major reaction products. From that, we can concluded the
progress of the reaction.
Figure 5. DTA thermograms of E0 and E3 suspensions after 6 hours
3.4. Removal behavior of CrO4-2 ions
Figure 6. DTA thermograms of E0 and E3 suspensions after 24 hours
The rate of removal of CrO42-
ions (mole/l) by CaO- Al2
a function of the mixing time was shown in Fig.(7).
The amount of CrO42-
ions removed by all mixes increases
gradually and continuously during the first 6 hours of mixing and nearly 80%
of the chromate ions were removed during this time interval. From 6-24 hours, a
gradual remove was noticed and an equilibrium stage was reached after 24 hours
of the mixing process. For E1 and E2 mixes, nearly all the chromate ions present
removed by ettringite. These indicates the success of the removal of
these anions by the suspension products. For the rest of the mixes, nearly 90%
of the chromate ions were removed by the suspension products. Also, we can note
that the amount removed of CrO42-
ions the suspension
products increased by increasing the initial concentration of the CrO42-
ions present in the suspensions. By
considering that the ettringite is the main suspension product (as we show in
the later sections), we can concluded that overcrowding in the chromate ions
occurred by increasing its concentration, which increases its diffusion and rate
of exchange inside ettringite crystals.
However, from the obtained data all the studied mixes showed high removal of CrO42-
which related to the exchange of the sulfate ions by the chromate ions in the
ettringite crystal. The uptake affinity of the ettringite phase toward the chromate ions is
greater than other products like gypsum and calcite. This high uptake affinity
of ettringite is related to the presence of three exchangeable sites in the
structure of the ettringite.
Figure 7. Removal of chromate ions of different mixes
1 - The products of the
suspension of CaO
molar ratio Ca/Al = 6 is the ettringite, with minor amounts of gypsum and
2 - According to the X-ray
results and SEM micrographs, presence of chromate ions in the suspension
results to slight decrease in the degree of the crystallinity and the size of
the formed ettringite crystals.
3 - DTA results showed that
the presence of chromate ions in that suspension decreases the rate of the
reaction and the formation of the products.
4 - There is high removal of
the chromate ions by CaO-Al2
products, which is related to substitution of chromate ions in the crystal
structure of ettringite.
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Cite this paper
AbuAlola, K. A. A. (2013). Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension. Open Science Repository Chemistry, Online(open-access), e70081936. doi:10.7392/Chemistry.70081936
AbuAlola, Khulood A. A. “Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension.” Open Science Repository Chemistry Online.open-access (2013): e70081936.
AbuAlola, Khulood A. A. “Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension.” Open Science Repository Chemistry Online, no. open-access (March 6, 2013): e70081936. http://www.open-science-repository.com/behavior-of-chromate-ions-in-cao-al2so43-suspension.html.
AbuAlola, K.A.A., 2013. Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension. Open Science Repository Chemistry, Online(open-access), p.e70081936. Available at: http://www.open-science-repository.com/behavior-of-chromate-ions-in-cao-al2so43-suspension.html.
1. K. A. A. AbuAlola, Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension, Open Science Repository Chemistry Online, e70081936 (2013).
1. AbuAlola, K. A. A. Behavior of Chromate Ions in CaO-Al₂(SO₄)₃ Suspension. Open Science Repository Chemistry Online, e70081936 (2013).
Research registered in the DOI resolution system as: 10.7392/Chemistry.70081936.
This work is licensed under a Creative Commons Attribution 3.0 Unported License.