photoreduction of hg (ii) in the presence of so42- under artificial solar radiation
TRANSCRIPT
Photoreduction of Hg(II) in the presence of SO42- under artificial solar
radiation
Xijia Li1, a, Dingyong Wang2,b,*, Kun Yang3,c, Zixuan Zhong4,d
1Department of Resources and Environment, Southwest University, Chongqing, 400715, China
2Chongqing Key Laboratory of Agricultural Resources and Environment, Chongqing, 400715, China
3Department of Resources and Environment, Southwest University, Chongqing, 400715, China
4Department of Resources and Environment, Southwest University, Chongqing, 400715, China
Keywords: Mercury reduction, Photochemistry, Sulfate, Artificial radiation.
Abstract: Photoreduction of Hg(II) is of great importance but remains not fully understood especially
in the presence of SO42-
. In this study, laboratory experiments were conducted to investigate the
reduction of Hg(Cl)2 with various SO42-
concentrations under artificial solar radiation. The whole
process was tracked by changing Hg(0) concentrations in argon; the rate constants were calculated by
trial method and were compared with other experiment. The results show the reaction rate decreased
with increasing SO42-
concentrations (0-20 mg L-1
) and the cause of inhibitory effect is assumed with
two explanations. The concentration of Hg(0) in argon increased firstly and decreased later in each
treatment, since the main reactions in rising and dropping period are different. The comparison
indicates that reduction rate is influenced by combined factors such as the form of mercury, the
quantity of DOM and TSSs, depth of water and quality of light source.
1. Introduction
Mercury, one of the most toxic heavy metal in environmental system, has toxicity accumulation in
both inorganic and organic form [1,2]. Besides, mercury can emit from earth’s surface to atmosphere
in the form of Hg(0), which could remain in atmosphere for a long time (0.5-2a) and then subsides
into the surface after a transition over great distances, resulting in the pollution of a large scale [3,4].
The transformation of Hg(II) has two competitive reaction pathways—methylation and reduction
[5]. Methylation produces Methylmercury (MeHg) of severer toxicity [6], but reduction reduces the
quantity of Hg(II) available for methylation [7]. Of various biotic and abiotic processes which are
responsible for mercury reduction in aquatic system, photochemical reaction is widely believed to be
distinctively significant [4,5]. Despite numerous field and lab observations, the direct mechanism of
mercury photoreduction still remains to be unclear.
Meanwhile, recent studies indicate that interfering ions play crucial roles in this process. For
example, Si and Ariya (2008), Allar and Arsenie (1991) observed that Cl- significantly hinders
photoreduction through binding of Hg(II) in solution [8,9]. Besides, Zhang et al. (2012) determined
that NO3- inhibits the photoreduction of mercury by producing ·OH to re-oxidize dissolved gaseous
mercury (DGM) [10]. However, the effect of SO42-
on photoreduction of Hg(II) was still not
published in paper. Therefore, the role of SO42-
in photoreduction of aquatic mercury warrants more
attention.
From the research gap presented above, Hg(II) in aqueous solutions containing SO42-
concentration
gradient were exposed to the simulated natural light and the effect of SO42-
was estimated by the
mercury flux. The variation trend of concentration of Hg(0) in argon was clarified and the cause of
relatively higher rate constant was discussed.
Advanced Materials Research Vols. 821-822 (2013) pp 917-921Online available since 2013/Sep/18 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.821-822.917
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2. Materials and Methods
2.1 Preparation of solutions
Stock solutions of 10 µg Hg(II) L-1
HgCl2 and 0.5, 2, 10, 20 g SO42-
L-1
Na2SO4 were prepared by
dissolving solid chemicals with ultra-pure water (resistivity > 18.2 MΩ cm-1
), and they were stored in
brown quartz bottles which kept in a 4°C refrigerator. The solution samples were obtained by diluting
stock solutions in the reactor in each experiment.
TEFLON pipes were used to minimize Hg(0) absorption; disposable gloves (Deli, Shanghai
China) were worn throughout the procedures to meet the required clean standard. All the experiments
were performed in a room with stationary temperature of 20°C ± 0.7°C.
2.2 Equipment and setup
As shown in Fig.1, 150 mL bottle made of borosilicate glass was used as the reactor. A gold trap
amalgam was placed to remove the trace mercury in Argon. The Hg(0) produced was purged with
Argon and dried by soda lime and then analyzed by RA-915+ multifunctional mercury analyzer
(Lumex Ltd., Russia), which is linked to the computer. 35 W xenon lamp (Jiri, Tianjin, China) of
280-800 nm (Natural light) used as point light source was supplied to the reactor. When samples were
treated with artificial solar light, the distance between xenon lamp and reaction solution was 10 cm.
Fig. 1. Schematic of reaction system
2.3 Experimental method
Firstly, open RA-915+ multifunctional mercury analyzer, then bulk ultra-pure water into the reactor.
Secondly, connect all the equipment according to Fig. 1 and purge the argon into the reaction system
at a regulated rate, 10 mL s-1
. The ultra-pure water was purged in the same condition as that of later
experiment until the concentration of Hg(0) measured by RA-915+
analyzer was under 5 ng m-3
and
kept steady for more than 30 min. Finally, add the solution into the reactor to begin measurement.
Hg(II) (10 ng L-1
) in HgCl2 solution with 0, 0.5, 2, 10, 20 mg SO42-
L-1
Na2SO4 solution spiked in it
respectively was exposed to simulated natural light for 120 min in each experiment and data was
collected at every second and mean was calculated at every 5 s. Duplicated experiments were
conducted for each treatment.
918 Advances in Textile Engineering and Materials III
Hg(0) flux was obtained using the equation [10]: F = c__
× v × t, where F is mercury release flux
(ng), c__
is mean concentration in argon (ng m-3
) within detecting time, v is flow rate of purge gas (m3
min-1
) and t is detecting time (min). Reaction rate constants were calculated by trial method.
3 Results and discussion
3.1 Effect of SO42-
on the photoreduction of Hg(Cl)2
Fig.2. Hg(0) flux from HgCl2 (a) and Hg(0) concentrations in argon (b) with various SO42-
concentrations, calculated at 15 min intervals in 120 min under simulated natural light. Initial Hg(II) concentration was 10 ng L
-1.
Previous studies have shown that photoreduction of Hg(II) in water decreases in the presence of Cl-
and NO3- [8,9,10], we observed Hg(II) photoreduction rate also decreased with increasing
concentration of SO42-
(Fig. 2 (a)).
It can be established from Fig. 2 (a) that the total Hg0
yield for 120 min was 0.716, 0.644, 0.497,
0.457, 0.394 ng when the concentration of SO42-
was 0, 0.5, 2, 10, 20 mg L-1
respectively. It was
found that 89% of the Hg(II) in the solution without SO42-
was reduced to Hg(0) after it was
photo-exposed, while only 49% of the Hg(II) was reduced in the absence of 20 mg L-1
SO42-
.
Correlation analysis showed photoreduction rate and Hg(0) flux are negatively correlated to the
concentration of SO42-
.
Since the published paper about this issue was not available, we hypothesized the causes of this
inhibition from basic part. The whole process could be described with Eq. (1) [10].
Hg(II) DGM Emission
→ Hg(0) (1)
It could be deduced from Eq. (1) that Hg(0) in carrier gas was determined by DGM. Considering
the production and consumption pathways of DGM, there may be two explanations for the inhibition.
One is that the SO42-
may compete for complexation with Hg(II) and hence reduces the quantity of
Hg(II) which is available for reduction. An alternative explanation for the inhibitory effect of SO42-
is
probably its role in promoting the reverse reaction, that is, DGM reoxidation stimulates the
Hg(II)/Hg(0) shift towards the Hg(II) production.
The explanations for the inhibition mechanism were assumed but no conclusion can be reached
only from this study. Further research is needed to look into the possibility of these two explanations
presented above.
3.2 Variations of Hg(0) concentration in argon
The concentration of Hg(0) in argon measured by RA-915 analyzer (Lumex Ltd., Russia) reflects the
emission rate of DGM, which was determined by the reduction degree of Hg(II) in solution (Eq(1)).
Advanced Materials Research Vols. 821-822 919
According to the curve of Hg(0) concentrations changed with time (Fig. 2 (b)), photoreduction of
Hg(II) can be described qualitatively and the kinetic process can be analyzed quantitatively.
Compared to some other analysis [4,11,12], it is more accurate and convenient.
It can be seen in Fig.2 (b) that in each treatment the concentration of Hg(0) increased firstly and
then decreased later and the extent of variation turned to be gentle finally (after 80 min). In the very
beginning (t=0), the photochemical reaction could not be excited since solution did not absorb
sufficient photo energy, so the concentration of Hg(0) was 0. With time accumulated, the photo
energy absorbed became more and more until the electronic excitation occurred. From Eq. (1), Hg(II)
was photo reduced gradually, accordingly the concentration of DGM and Hg(0) became higher and
higher. In this period, the production rate of DGM was greater than the emission rate of DGM, which
was shown to be the rising curve in Fig.2 (b). The concentration continued to increase with the
reaction proceeded further until t≈18 min, when the curve reached the peak. At this moment, the
production rate equaled the emission rate of DGM. Afterwards, the production of DGM and
concentration of Hg(0) decreased. It can be concluded that in the rising period, the main reactions in
the solution were the photoreduction of Hg(II) and the emission of Hg(0) while in the dropping period
the main process was the emission of Hg(0).
3.3 The discussion of rate constant
The lowest rate constant in this study was 0.0051 min-1
with maximum SO42-
, however, it was higher
than those in field observation of Zhang and Lindberg (2001), for the reduction rate constant of Hg(II)
was 0.0017 min-1
in pondwater and 0.0033 min-1
in lakewater respectively in their study [13]. There
are some reasons which can be attributed to the discrepancy. Firstly, the reaction solutions in their
study were from natural waters including pond and lake, with some ligands such as dissolved organic
matter (DOM) and undoubtedly not all the mercury was in reducible form, while in this study, the
solutions were prepared with ultra-pure water and thus almost all the mercury was in reducible form.
Secondly, according to the phenomenon that a coastal shelf site with higher total suspended solids
(TSSs) had a lower reduction rate constant [14] combined with the fact that there was no particulates
in solutions of this study, it was hypothesized that no radiation scattering, refraction and absorption
resulting from TSSs in this study ultimately leaded to higher rate constants. Thirdly, based on the
equation 0dK D
dI I e− ×
= using Beer-Lambert law [15], we deduced the intensity of radiation decreases
exponentially with depth increasing in water column. The reactor in their study had a diameter of 8 cm
and a height of 20 cm while it was 4 cm and 16 cm in this study, associated with the fact that volume
of their solution was larger, the radiation attenuation arisen from water should be considered to make
an exhaustive conclusion. There is a possibility that longer time might be taken with same photo
received because energy was lost by water in the process of transmission. Finally, the source of light
was natural solar radiation in their study while it was the xenon lamp in this study, combination of the
quality and intensity of radiation could also make a difference.
4 Conclusions
a) The effects of increasing SO42-
concentrations resulting in the decreasing photoreduction of
Hg(Cl)2 suggest that the coexisting sulfate ion is of importance to mercury reduction in water. The
cause of that inhibitory effect was assumed with two explanations, which were complexation
assumption and reoxidation assumption.
b) The concentration of Hg(0) increased first and decreased later in each treatment. The main
reactions in the rising period were photoreduction of Hg(II) and emission of Hg(0) while the main
process in the dropping period was emission of Hg(0).
c) The reasons for relatively higher rate constant clarified from several aspects claims the reductions
of Hg(II) in water are results of combined environmental factors.
d) The responsible mechanisms of Hg(II) photoreduction with SO42-
presented remain to be fully
uncovered; sufficient attention and further studies are needed to reveal the details.
920 Advances in Textile Engineering and Materials III
Acknowledgements
This study was supported by the Major State Basic Research Development Program of China (973
Program) (No. 2013CB430004) and the National Natural Science Foundation of China (No.
41173116) and the National College Students’ Innovation Training Program (No. 1210635018). I
thank our group members for working together regardless of the difficulties we met. I thank Dr. Sun
Rongguo of the department of Resources and Environment at Southwest University for his help
through the experiments. Especially, the encouragement, guidance and inspirations from my
teacher—Professor Dingyong Wang during the whole process is greatly appreciated.
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