Embed Size (px)
Citation preview
Effect of ammonia on degradation of
benzoic acid by the UV/chlorine process
Ehsan Aghdam, Yingying Xiang, Chii Shang
Department of Civil and Environmental Engineering
Hong Kong University of Science and Technology
1
Outline
• Introduction
• AOPs, UV/Cl2 process, breakpoint chlorination, UV/breakpoint, objectives
• Materials and methods
• Results and discussion
• Kinetics of BA degradation at pH 7
• Kinetics of BA degradation at pH 6
• Conclusion
2
Introduction
3
Advanced oxidation processes (AOPs)
Hydroxyl radical-based AOPs
Fenton process
UV/H2O2 process
Ozone/UV process
2+ 3+ -
2 2Fe + H O Fe + OH + HO
2 2H O 2HOh
3 2 2 2 2
2 2
O + H O H O + O
H O 2HO
h
h
•,HOE = 1.89 2.72 V
Neta et al., JPCRD (1988)
4
The UV/Cl2 process
HOCl / OCl-
UV
HO•/O•-
Cl• and Cl2•-
Higher quantum yield than H2O2 (Watts and Linden, 2007)
Multi-barrier disinfection and residual protection
Unknown reactivity toward micropollutants
Unknown byproducts
Selective
Nonselective
2O,Cl O,ClE = 2.4 V and E = 2 V
Fang et al., 2014
5
Breakpoint chlorination
• Presence of ammonia in water due to agricultural activities, wastewater discharge
and eutrophication
3 2 2
2 2 2
2 3 2
NH +HOCl NH Cl+H O
NH Cl+HOCl NHCl +H O
NHCl +HOCl NCl +H O
• Formation of nitrogenous radicals
6
+ -
2 2 2
+ -
2 2
+ -
3
2
2
hυ • • •-
hυ • •
2 2
NH Cl + NOH N + H O + H + Cl
NHCl + NOH N + HOCl + H + Cl
2 HOCl + NOH NO + 3H + 2Cl
NOH + O ONOOH
2ONOOH OONOH NO OH
HOCl/OCl Cl + HO / O
NH Cl Cl + NH
Formation of reactive species during the UV/breakpoint chlorination
Jafvert and Valentine, (1992); Saunier and Selleck, (1979); Schreiber and Mitch, (2007);
Li and Blatcheley, (2009); Fnag et al., (2014)7
Objectives
•To investigate the kinetics of BA degradation by the UV/breakpoint process
at pH 6 and 7
•The effect of initial ammonia concentration on BA degradation during the
UV/breakpoint process at pH 7
•The effect of initial chlorine concentration on BA degradation during the
UV/breakpoint process at pH 6 and 7
8
Materials and methods
Experimental setup
Volume = 250 mL
UV fluence rate = 40 mW/cm2
(NH4)2SO4 as source of ammonia
Analytical methods:
UPLC for BA measurement
9
Abbreviations
brk: breakpoint chlorination with known residual chlorine
UV/brk: UV irradiation accompanied by the breakpoint chlorination
UV/Cl2: coexposure of chlorination and UV irradiation
UV/NH2Cl: coexposure of chloramination and UV irradiation
10
Results and discussion
11
Degradation of BA by brk, UV/brk, UV/Cl2 and UV/NH2Cl at pH7
• 22% Degradation of BA during the brk probably
by the reactive species such as •OH, NOH,
peroxyl radicals and peroxynitrite.
• No further radical formation after approximately
5 min of brk.
• Two stages of degradation by UV/brk.
• 34% degradation of BA during the first stage of
UV/brk due to the increased formation of radicals
by photolysis of free and combined chlorine
species.
• Comparable degradation of BA by UV/Cl2 and
UV/NH2Cl at pH 7.
Conditions: pH = 7, [BA]0 = 5 µM. For the brk and UV/brk: [N –
NH3]0 = 1 mg/L and [Cl2]0 = 12.6 mg/L. For the UV/Cl2: [Cl2]0 = 5
mg/L. For the UV/NH2Cl: [N – NH3]0 = 1 mg/L and [Cl2]0 = 5 mg/L.
0 5 10 15 20 25 30 35 400.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Brk
UV/brk
UV/Cl2
UV/NH2Cl
[BA
] /
[BA
] 0
Time (min)
12
Effect of initial ammonia concentration on the BA degradation by UV/brk
• Comparable efficiency of UV/Cl2 in absence
of ammonia and initial ammonia
concentration of 0.2 mg/L as N.
• Faster formation and consumption of the
radicals in the stage I of the UV/brk with
increasing initial ammonia concentration.
Conditions: pH = 7, [BA]0 = 5 µM, [N – NH3]0 = 0, 0.2, 1, 3
and 5 mg/L as N, target residual chlorine after the
breakpoint = 5 mg/L as Cl2.
0 10 20 30 40
0.0
0.2
0.4
0.6
0.8
1.0
[N-NH3]0 = 0 mg/L, [Cl
2]0 = 5 mg/L
[N-NH3]0 = 0.2 mg/L, [Cl
2]0 = 6.25 mg/L
[N-NH3]0 = 1 mg/L, [Cl
2]0 = 12.6 mg/L
[N-NH3]0 = 3 mg/L, [Cl
2]0 = 27.8 mg/L
[N-NH3]0 = 5 mg/L, [Cl
2]0 = 43 mg/L
[BA
] /
[BA
] 0
Time (min)
13
0
20
40
60
80
100
BA
deg
rada
tion
(%
)
Initial ammonia concentration (mg/L as N)
UV/brk stage II
UV/brk stage I
Brk
0.2 1 3 5
• At the stage I, enhancing effect of UV irradiation
on BA degradation at initial ammonia
concentrations of 0.2 and 1 mg/L as N.
• Higher formation of reactive species at higher
initial ammonia concentrations at stage I.
• Higher degradation rates during the stage II of
UV/brk at higher initial ammonia concentrations.
0.017
0.019
0.021
0.023
0.025
0.027
0 2 4
Pseudo f
irst-
ord
er
degra
dation r
ate
(m
in-1
)
Initial ammonia concentration (mg/L as N)
Conditions: pH = 7, [BA]0 = 5 µM, [N – NH3]0 = 0, 0.2, 1, 3 and
5 mg/L as N, target residual chlorine after the breakpoint = 5
mg/L as Cl2. Stage I and II took 5 min and 35 min respectively.
Effect of initial ammonia concentration on the BA degradation by UV/brk
14
Effect of initial chlorine concentration on BA degradation by UV/brk at pH 7
Conditions: pH = 7, [N-NH3]0 = 1 mg/L, [Cl2]0 = 7.6, 9.6,
12.6 and 17.6 mg/L, [BA]0 = 5 µM.
• At the stage I, lower BA degradation
percentage with increasing initial chlorine
concentration probably due to the radical
scavenging of the chlorine species.
• Continuous degradation of BA after the
stage II with the initial chlorine of 7.6 mg/L
which did not have free residual chlorine
during the stage II.
0 5 10 15 20 25 30 35 40
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
[N-NH3]0 = 1, [Cl
2]0 = 7.6 mg/L with UV
[N-NH3]0 = 1, [Cl
2]0 = 9.6 mg/L with UV
[N-NH3]0 = 1, [Cl
2]0 = 12.6 mg/L with UV
[N-NH3]0 = 1, [Cl
2]0 = 17.6 mg/L with UV
[BA
] /
[BA
] 0
Time (min)
15
0
20
40
60
BA
de
gra
datio
n (
%)
Initial chlorine concentration (mg/L as Cl2)
UV/brk stage II
UV/brk stage I
Brk
7.6 9.6 12.6 17.6
0.015
0.017
0.019
0.021
0.023
0.025
7 12 17
Pseudo f
irst-
ord
er
degra
dation r
ate
(m
in-1
)
Initial chlorine concentration (mg/L as Cl2)
• Lower formation of reactive species at higher initial
chlorine concentrations at pH 7 in the stage I probably
due to the scavenging effect of OCl-.
• Higher degradation rate in the stage II with increasing
the initial chlorine dosage probably due to the higher
chlorine residual.
• Comparable BA degradation percentages at all initial
chlorine concentrations after 40 min of UV/brk
Conditions: pH = 7, [N-NH3]0 = 1 mg/L, [Cl2]0 = 7.6, 9.6,
12.6 and 17.6 mg/L, [BA]0 = 5 µM.
Effect of initial chlorine concentration on BA degradation by UV/brk at pH 7
16
Degradation of BA by brk, UV/brk, UV/Cl2 and UV/NH2Cl at pH 6
0 5 10 15 20 25 30 35 400.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Brk
UV/brk
UV/Cl2
UV/NH2Cl
[BA
] / [B
A] 0
Time (min)
Conditions: pH = 6, [BA]0 = 5 µM. For the brk and UV/brk: [N –
NH3]0 = 1 mg/L and [Cl2]0 = 12.6 mg/L. For the UV/Cl2, [Cl2]0 = 5
mg/L. For the UV/NH2Cl, [N – NH3]0 = 1 mg/L and [Cl2]0 = 5 mg/L.
• Higher efficiency of UV/brk than
UV/Cl2 during the stage I.
• Higher efficiency of UV/Cl2 than
UV/NH2Cl due to higher radical
formation during the UV/Cl2 at pH 6.
17
Effect of initial chlorine concentration on BA degradation by UV/brk at pH 6
0
10
20
30
40
50
60
70
80
90
100
BA
de
gra
datio
n (
%)
Initial concentration of chlorine (mg/L as Cl2)
Degradation during stage I
Degradation during stage II
7.6 9.6 12.6 17.6
Conditions: pH = 6, [N-NH3]0 = 1 mg/L, [Cl2]0 = 7.6, 9.6,
12.6 and 17.6 mg/L, [BA]0 = 5 µM.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
7 9 11 13 15 17
Pseudo f
irst-
ord
er
degra
dation
rate
(m
in-1
)
Initial chlorine concentration (mg/L as Cl2)
• Higher formation of reactive species at higher
initial chlorine concentrations at pH 6.
• Higher degradation rate with increasing the
chlorine dosage in the stage II probably due to
the higher chlorine residual.
• Higher BA degradation percentages by
increasing initial chlorine concentrations after 40
min of UV/brk
18
Effect of pH on the UV/Cl2 process in presence of ammonia
0
20
40
60
80
100
BA
de
ga
rdatio
n (
%)
Degradation during stage I at pH 6
Degradation during stage II at pH 6
Degradation during stage II at pH 7
Degradation during stage I at pH 7
UV/Cl2
UV/NH2Cl
Initial concentration of Cl2 in presence of [N-NH
3]0 = 1 mg/L as N
7.6 9.6 12.6 17.6
• Generally, higher efficiency of
UV/Cl2 at pH 6 than that at pH 7
• Different dependency of the UV/brk
on the initial chlorine concentration
at pH 6 and 7 due to different
concentrations of OCl- and HOCl at
pH 6 and 7.
Conditions: pH = 7, [BA]0 = 5 µM. For the UV/brk: [N – NH3]0 = 1 mg/L and
[Cl2]0 = 7.6, 9.6, 12.6 and 17.6 mg/L. For the UV/Cl2: [Cl2]0 = 5 mg/L. For the
UV/NH2Cl: [N – NH3]0 = 1 mg/L and [Cl2]0 = 5 mg/L.
19
Conclusions
• UV irradiation enhanced the BA degradation during the brk.
• With the same amount of residual chlorine at pH 7, increasing [N-NH3]0
enhanced the degradation during the stage I of UV/brk and increased the
pseudo first-order degradation in the stage II.
• 2 mg/L of residual chlorine at pH 7 was the optimum chlorine dosage to
degrade BA by the UV/brk.
• The effect of residual chlorine on BA degradation by UV/brk depended on pH
probably due to different radical scavenging effects of HOCl and OCl-.
• Generally, the UV/Cl2 was more efficient at pH 6 than pH 7.
20
Acknowledgements
This research is supported by the Hong Kong Research
Grants Council under grant number 16208914.
21
