Mechanism of Chemical Oxo-precipitation to Treat Boroncontaining Solution Using Barium Coagulant
Peter Tsung-Yu Hsieh1, Yu-Jen Shih1, Jui-Yen Lin1, Chiung-Chin Huang1, Yao-Hui Huang*
1*
Department of Chemical Engineering, National Cheng-Kung University, Tainan, Taiwan 701, R.O.C.
*Corresponding author: [email protected]; Tel: +886-6-275-7575 ext62636
INTRODUCTION
In recent years, the authors have developed a “chemical oxo-precipitation (COP)”
process for treating the wastewater containing high boron level [1]. COP based on
chemical precipitation adopted the hydrogen peroxide (H2O2) to enhance the efficiency
of boric acid precipitation. This study focused on exploring the reaction pathway,
behavior of the obtained solid phases and liquid speciation during the COP. The
reaction pathway should obey the Ostwald’s stage rule [2, 3], which describes the
energy stages of the species in the solution. The final precipitates which are the most
thermodynamically stable crystals are characterized to explain the mechanism of COP.
MATERIALS AND METHODS
5.72 g mass H3BO3 (J. T. Baker, ≧99.5%) and 16.95 g mass BaCl2‧3H2O (Panreac,
97%) were dissolved in 500 mL of deionized water, respectively, to yield H2O2/B and
Ba/B in molar ratios of 2 and 0.75. 20.56 mL of 9% H2O2 was also prepared. The pH
values were controlled by adding HCl (Shimakya, 32%) and NaOH (Industrial grade)
during the reaction.
The reaction was carried out in an argon gas purge system. The boron solution was
pretreated with hydrogen peroxide for 20 min at pHp 10.5. After the pretreatment, the
barium solution was immediately added into the pretreated solution. During the
reaction, the pH value (pHr) was controlled at 9.5. 10 mL of samples were withdrawn
from the reactor at specific intervals; the precipitates in samples was separated with a
125 mm filter and dried in an oven (60℃, 1 day), and the filtrates were collected by
centrifugation.
RESULTS AND DISCUSSION
Figure 1 shows the result of COP for treating 1000 ppm boron solution. Obviously, the
whole reaction can be divided into four periods.
1000
Ar-[B]
[B] (ppm)
100
10
1
0
50
100
150
200
250
300
350
time (min)
Figure 1: The result of COP for treating
1000 ppm boron solution.
In the first period, boron level was decreased from 1000 ppm to 30 ppm. Precipitation
acutely generated many amorphous precipitants as identified by XRD (Fig. 2).
In the following period, boron level was decreased from 30 ppm to 20 ppm. XRD (Fig.
3) suggested that the amorphous precipitates would transform into well-crystallized
solid. Meanwhile, the boron level was slightly decreased again. This phenomenon might
result from the formation of thermodynamically stable crystals with lower solubility
than the amorphous precipitates.
2000
12000
Ar_1min
Ar_90min
11000
10000
9000
XRD Intensity
XRD Intensity
8000
1000
7000
6000
5000
4000
3000
2000
1000
0
0
10
20
30
40
50
60
70
80
90
2
Figure 2: Amorphous precipitants
10
20
30
40
50
60
70
80
90
2
Figure 3: Crystal phase
For the third period, the boron level decreased from 20 ppm to around 2 ppm. The
crystal growth in this period might follow the Ostwald ripening, which the larger
crystals grow at the expense of the smaller crystals of same composition and structure.
Moreover, because of the more stable form of crystals with low solubility, the solute
quickly salted out and grew on the crystals.
Finally, the boron level could slide into the range of 1~2 ppm in the fourth period. The
crystals might have the most stable composition and structure in this period.
CONCLUSIONS
During the treatment of COP, the effective boron removal in terms of precipitation and
crystallization of barium perborate was obtained. The mechanism of the overall
crystallization is supposed to be contributed by the phase transition, the Ostwald
ripening and the heterogeneous growth of solute which is salted out. Following the
Ostwald’s stage rule, as the crystals grow, the crystals would “cascade” through
different metastable stages towards the most stable forms. Hence, finally the boron level
was reduced from 1000 ppm to1~2 ppm, while the crystals possessing the most stable
composition and structure could be recovered.
REFERENCES
1. Y.J. Shih, C.H. Liu, W.C. Lan, Y.H. Huang. (2014). “A novel chemical oxoprecipitation (COP) process for efficient remediation of boron wastewater at room
temperature”, Chemosphere, Vol.111, p.232-237.
2. B.L. Joseph D. NG, Jean Witz, Anne Theobald-Dietrich, Daniel Kern, Richard Giege.
(1996). “The crystallization of biological macromolecules from precipitates: evidence
for Ostwald ripening”, Journal of Crystal Growth, Vol.168, p.50-62.
3. Ostwald, W.Z.Z. (1897), Phys. Chem., Vol.22, p.289-330.