interaction between gypsum and a p-bearing aqueous solution

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LABORATORY METHODS – PRACTICAL ASSIGNEMENT
INTERACTION BETWEEN GYPSUM AND A P-BEARING AQUEOUS SOLUTION
INSTRUCTIONS: Please follow the guidelines described in the tasks. Your assignment
should be delivered in hardcopy, including your name, student ID number and the
answers clearly listed by task. Failure to comply with communicated deadlines will
involve a -10% per delayed day penalty on assignment mark.
Experimental device
The followed experimental procedures consisted on the immersion of fragments of
exfoliation of natural gypsum in 100 ml of a 75 mM phosphoric acid (H 3PO4) aqueous
solution, previously neutralized with sodium hydroxide (NaOH) until the desired initial
pH value of 6 was attained. The experiments took place inside polypropylene batches,
closed with Parafilm film in order to avoid dissolution evaporation, and were maintained
at constant temperature of 25 ± 0.1 ºC and 1 atm of pressure. The dissolutions remained
at constant 25 rpm stirring during the experiments, using floating stirrers covered with
Teflon. Figure 1 shows a schematic representation of the used experimental device.
These experiments were designed to study the evolution of the aqueous solution
chemistry during the interaction process. In this case, the grain sizes were selected to
have diameters ranging from 1.0 to 1.5 mm. The duration of these experiments was of
72 hours, and dissolution samples were collected at 2, 4, 6, 8, 10, 12, 24, 36, 48, 60, 72
hours of elapsed time after inserting the gypsum grains in the reaction batch. Finally,
each experiment was repeated three times in order to examine the reproducibility of
the process.
100 ml of H3PO4 at 75 mM
Figure 1. Experimental device used for interaction experiments.
I – ANION CONCENTRATIONS
IA. Analytical methods
The measure of total phosphate and sulphate concentrations in the aqueous dissolution
was carried out by means of ionic chromatography, using a Metrohm Advanced
Compact 861 IC chromatographer. Samples collected during the experimental stage
were diluted and compared with six reference dissolutions with different
concentrations. These dissolutions were prepared using standard dissolutions of Na 2SO4
and KHPO4, both having 1000 ± 2 ppm (Panreac) concentration.
An anion column (6.1006.520 Metrostep A SUPP 5 – 150) was used and the applied
eluent was a dissolution of 15 mM of NaOH and 2.0 mM Na2CO3, with a flux velocity of
1.0 ml/min. and 5 MPa of pressure. The suppressor consisted of a solution with 100 mM
of H2SO4.
IB. Calibration lines
In ion chromatography, aqueous concentrations are measured in a chromatogram of
retention time versus detector response (Figure 2). In this graphical representation, the
retention time enables the identification of the component in question, while its
concentration is proportional to the corresponding peak’s area. For more information
on ion chromatography, please consult Harvey, 2000.
Figure 2. Schematical representation of a typical chromatogram (CHROMacademy.com).
In the present case, we intend to measure sulphate and phosphate concentrations, and
the results obtained from standard solutions are presented in table 1.
Table 1 – Obtained chromatographic retention peak areas for standard
phosphate and sulphate aqueous solutions.
Phosphate (ppm)
10.2250
20.1456
40.0920
60.0871
79.7783
99.9175
Sulphate (ppm)
10.2723
19.9471
40.0898
60.0498
80.4411
99.4190
Run 1
24.3419
49.6771
105.6500
167.8710
232.9210
301.2880
Run 1
76.2839
148.6670
319.1340
501.0630
688.3830
865.2370
Peak Area
Run 2
24.2887
49.4130
106.0560
168.0300
232.6120
301.7820
Run 2
75.4431
149.0850
319.7000
501.9080
688.9800
863.6050
Run 3
24.1368
49.6346
106.4140
168.2220
232.9320
302.0426
Run 3
76.0087
149.4590
320.2160
501.1660
690.7540
863.2410
TASK 1: Using the average value for the obtained areas, plot the results versus the
concentrations of each standard solution and extract the corresponding trendline
equations for sulphate and phosphate calibrations. What is the precision (in % of the
mean) of the method in both cases? Don’t forget to present the calculations.
IC. Batch experiment results – phosphate and sulphate concentrations
TASK 2: in the Appendix: Data you may find the measured chromatographic results,
regarding the evolution of sulphate and phosphate concentrations as a function of time,
for the 3 replicated experiments. Determine the missing concentrations in mM ( 103mol/L) applying the expressions of the calibration lines. For each experiment present
the precision as % of the average areas.
II – CALCIUM CONCENTRATIONS
IIA. Analytical methods
The concentrations of total calcium in dissolution were measured with a PYE-UNICAM
SP9 atomic absorption/emission spectrophotometer equipped with an acetylene
burner. The analyses were carried out using a calcium light bulb (422.7 nm of wave
length).
Each sample was analysed eight times and the detection limit for calcium is 2 ppm. The
reference dissolutions applied for all experiments were prepared using a standard
dissolution of 1000 ppm (Panreac).
IIB. Calibration lines
Atomic Absorption Spectrometry (AAS) is a method requiring frequent re-calibration
during operation procedures and for the sake of simplicity you will not be asked to
perform the tedious task of finding a new calibration expression every 10
measurements. The final results for each of the 3 experiments are included in the
Appendix: Data section.
III – INTEGRATION OF RESULTS
TASK 3 – Build tables with the averaged results of mM concentrations of calcium,
sulphate and phosphate for the 3 repeated experiments, including the standard
deviation.
TASK 4 – plot your results in a graph of concentration versus time and include error bars
in your data points.
TASK 5 – plot your pH measurements vs time.
IV – INTERPRETATION AND FOLLOW UP
TASK 6 – explain the variation of concentrations as a function of time in the experiments.
TASK 7 – as the experiments proceeded, gypsum grains were also sampled and imaged
with the aid of a Scanning Electron Microscope (Figure 3). The micrographs reveal a
progressive coating of gypsum by newly formed crystals. In your opinion, what would
be the next analytical steps to be pursued?
Figure 3. Coating of tabular crystals overgrowing a gypsum grain, ~10 hours into the experiment.
APPENDIX: DATA
Experiment B
Experiment A
1. Chromatographic results for phosphate
Sample
P6-A1
P6-A2
P6-A3
U6-A1
U6-A2
U6-A3
U6-A4
U6-A5
U6-A6
U6-A7
U6-A8
Sample
P6-B1
P6-B2
P6-B3
U6-B1
U6-B2
U6-B3
U6-B4
U6-B5
U6-B6
U6-B7
U6-B8
Time (h)
2
4
6
8
10
12
24
36
48
60
72
Time (h)
2
4
6
8
10
12
24
36
48
60
72
Dilution
98.59
99.78
100.42
99.63
99.94
99.05
95.91
100.41
96.75
97.93
97.42
Dilution
100.09
100.48
100.07
100.06
100.25
97.94
96.98
98.63
97.67
99.78
97.46
1
179.375
155.228
55.3493
167.292
166.444
168.109
173.767
161.428
169.279
170.23
168.392
Peak areas
2
180.409
168.873
160.995
167.063
166.631
168.678
174.311
164.881
169.52
168.66
169.294
3
181.727
164.889
162.436
167.607
167.194
168.925
173.734
164.953
170.306
169.024
168.976
Average
180.504
166.881
161.716
mM
66.586
62.635
61.222
STD
1.179
2.817
1.019
1
162.317
154.455
158.453
167.793
168.15
171.916
174.649
171.783
170.994
168.193
173.135
Peak areas
2
169.259
164.506
160.37
167.63
169.492
172.498
174.795
171.891
171.491
167.693
173.951
3
168.961
165.17
161.868
167.864
170.017
172.433
175.102
171.183
171.976
169.052
174.184
Average
169.110
164.838
160.230
mM
63.610
62.356
60.489
STD
0.211
0.470
1.712
Experiment C
Sample
P6-C1
P6-C2
P6-C3
U6-C1
U6-C2
U6-C3
U6-C4
U6-C5
U6-C6
U6-C7
U6-C8
Time (h)
2
4
6
8
10
12
24
36
48
60
72
Dilution
96.33
100.07
99.33
97.73
99.3
100.34
97.96
100.21
98.93
97.17
97.69
1
193.526
162.538
174.18
172.895
170.75
165.649
170.673
167.641
169.647
173.668
172.164
Peak areas
2
190.926
174.462
170.41
172.677
170.813
166.701
171.28
167.58
170.87
174.449
172.566
3
187.894
172.311
162.219
172.406
171.453
166.791
170.652
166.694
170.642
174.491
171.04
Average
190.782
169.770
168.936
mM
68.523
63.828
63.067
Experiment B
Experiment A
2. Chromatographic results for sulphate
Sample
S6-A1
S6-A2
S6-A3
U6-A1
U6-A2
U6-A3
U6-A4
U6-A5
U6-A6
U6-A7
U6-A8
Sample
S6-B1
S6-B2
S6-B3
U6-B1
U6-B2
U6-B3
U6-B4
U6-B5
U6-B6
U6-B7
U6-B8
Time (h)
2
4
6
8
10
12
24
36
48
60
72
Time (h)
2
4
6
8
10
12
24
36
48
60
72
Dilution
20.21
19.8
19.95
99.63
99.94
99.05
95.91
100.41
96.75
97.93
97.42
Dilution
19.95
19.9
19.93
100.06
100.25
97.94
96.98
98.63
97.67
99.78
97.46
1
281.149
638.07
821.308
129.049
143.836
156.579
199.932
197.523
205.054
207.104
205.449
Peak areas
2
281.5
638.493
822.034
129.338
144.105
156.781
200.035
198.061
205.107
206.791
205.821
3
281.747
639.07
823.04
129.349
144.366
157.031
200.048
198.57
205.378
207.312
205.806
Average
281.465
638.544
822.127
mM
7.174
15.198
19.545
STD
0.301
0.502
0.870
1
482.986
684.922
818.39
147.532
162.499
177.197
206.857
207.325
208.192
205.289
210.639
Peak Areas
2
485.712
685.514
821.578
147.928
162.997
177.604
206.971
206.79
208.819
205.528
210.959
3
486.466
686.328
823.942
147.675
163.176
177.593
207.125
207.027
208.728
205.625
211.336
Average
485.055
685.588
821.303
mM
11.775
16.356
19.506
STD
1.831
0.706
2.786
STD
2.819
1.521
2.666
Experiment C
Sample
S6-C1
S6-C2
S6-C3
U6-C1
U6-C2
U6-C3
U6-C4
U6-C5
U6-C6
U6-C7
U6-C8
Time (h)
2
4
6
8
10
12
24
36
48
60
72
Dilution
19.91
20.03
19.98
97.73
99.3
100.34
97.96
100.21
98.93
97.17
97.69
1
231.086
403.425
575.428
163.278
174.149
179.066
202.986
202.663
207.439
211.602
209.022
Peak areas
2
231.587
404.035
575.708
163.329
174.464
179.296
203.374
202.958
207.759
212.106
209.55
3
231.888
404
576.755
163.468
174.192
179.348
203.264
202.429
207.61
211.97
209.295
Average
231.520
403.820
575.964
Experiment B
Experiment A
3. Results for calcium concentrations
Sample
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
A11
Sample
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
t (h)
2
4
6
8
10
12
24
36
48
60
72
t (h)
2
4
6
8
10
12
24
36
48
60
72
Average (mM) Std (mM)
2.93
0.34
5.51
0.30
8.54
0.44
11.29
1.65
12.72
2.01
11.68
2.15
16.53
2.25
16.56
2.07
16.54
2.10
17.08
2.07
16.80
2.29
Average (mM)
4.38
6.70
8.51
12.35
14.81
14.42
16.95
17.26
17.45
17.84
18.19
Std (mM)
0.42
0.15
0.25
1.21
1.36
0.94
1.12
1.14
1.36
1.46
1.43
mM
5.918
9.942
13.891
STD
0.405
0.343
0.699
Experiment C
Sample
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
t (h)
2
4
6
8
10
12
24
36
48
60
72
Average (mM)
2.27
3.62
4.92
7.19
8.72
9.12
14.02
18.13
22.27
26.43
30.54
Std (mM)
1.99
3.29
4.24
5.61
6.82
7.19
11.72
17.45
23.69
30.20
36.87
4. Evolution of pH
Time (h)
2
4
6
8
10
12
24
36
48
60
72
pH
5.95
5.69
5.39
5.07
4.78
4.64
4.35
4.32
4.31
4.30
4.31
References:
Harvey, D. (2000) Modern Analytical Chemistry, McGraw Hill, 795pp.

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