Ozone and ozone/vacuum-UV degradation of diethyl dithiocarbamate collector: Kinetics, mineralization, byproducts and pathways

Article abstract of DOI:10.1039/c9ra04127c

The diethyl dithiocarbamate (DDC) collector, a precursor of toxic N-nitrosamines, is detected in flotation wastewaters usually at the ppm level. In this study, the O₃ and O₃/Vacuum-UV (O₃/VUV) processes were compared to investigate the efficient removal of DDC with a low risk of N-nitrosamine formation. The results showed that 99.55% of DDC was removed at 20 min by O₃/VUV, and the degradation rate constant was 3.99 times higher than that using O₃-alone. The C, S and N mineralization extents of DDC using O₃/VUV reached 36.36%, 62.69% and 79.76% at 90 min, respectively. O₃/VUV achieved a much higher mineralization extent of DDC than O₃-alone. After 90 min of degradation, O₃/VUV achieved lower residual concentrations of CS₂ and H₂S, and released lower amounts of gaseous sulfur byproducts compared to O₃-alone. The solid phase extraction and gas chromatography-mass spectrometry (SPE/GC-MS) analysis indicated that the main byproducts in O₃/VUV degradation of DDC were amide compounds without the detection of N-nitrosamines. The avoidance of N-nitrosamine formation might be attributed to exposure of UV irradiation and enhanced formation of OH radicals in the O₃/VUV system. The degradation pathways of DDC were proposed. This work indicated that O₃/VUV was an efficient alternative treatment technique for the removal of DDC flotation collector with low risk of N-nitrosamine formation.

Full text of DOI:10.1039/c9ra04127c

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Ozone and ozone/vacuum-UV degradation of
diethyl dithiocarbamate collector: kinetics,
mineralization, byproducts and pathways†
Cite this: RSC Adv., 2019, 9, 23579
*
Pingfeng Fu,
Yanhong Ma, Huifen Yang, Gen Li and Xiaofeng Lin
The diethyl dithiocarbamate (DDC) collector, a precursor of toxic N-nitrosamines, is detected in flotation
wastewaters usually at the ppm level. In this study, the O3 and O3/Vacuum-UV (O3/VUV) processes were
compared to investigate the efficient removal of DDC with a low risk of N-nitrosamine formation. The
results showed that 99.55% of DDC was removed at 20 min by O3/VUV, and the degradation rate
constant was 3.99 times higher than that using O3-alone. The C, S and N mineralization extents of DDC
using O3/VUV reached 36.36%, 62.69% and 79.76% at 90 min, respectively. O3/VUV achieved a much
higher mineralization extent of DDC than O3-alone. After 90 min of degradation, O3/VUV achieved lower
residual concentrations of CS2 and H2S, and released lower amounts of gaseous sulfur byproducts
compared to O3-alone. The solid phase extraction and gas chromatography-mass spectrometry (SPE/
GC–MS) analysis indicated that the main byproducts in O3/VUV degradation of DDC were amide
compounds without the detection of N-nitrosamines. The avoidance of N-nitrosamine formation might
be attributed to exposure of UV irradiation and enhanced formation of cOH radicals in the O3/VUV
system. The degradation pathways of DDC were proposed. This work indicated that O3/VUV was an
efficient alternative treatment technique for the removal of DDC flotation collector with low risk of N-
nitrosamine formation.
Received 31st May 2019
Accepted 24th July 2019
DOI: 10.1039/c9ra04127c
rsc.li/rsc-advances
DDC can signicantly increase the bioaccumulation of heavy
metals in Daphnia magna and Rainbow trout.7,8 Moreover, DDC
1. Introduction
can even affect the biological behaviors of heavy metals accu-
mulated within mice and rats. The metals such as nickel9 and
mercury10 can be partly redistributed from traditional metal
enriching organs to the brain and adipose tissue of mice. In
addition, due to its instability in acidic medium, DDC can be
hydrolyzed to diethylamine (DEA) with a rate constant of 9.2 ꢀ
10ꢁ4 sꢁ1 with a half-life of 0.2 h at pH 5.11 However, the
hydrolysis rate constant of DDC may become as low as 1.0 ꢀ
10ꢁ7 sꢁ1 with a half-life of 1905.5 h at pH 9.11 Subsequently, N-
nitrosodiethylamine (NDEA) can be generated in drinking water
while DEA is oxidized by water disinfection oxidants like Cl2 and
O3.11 NDEA is a typical nitrogenous disinfection byproduct with
probable cytotoxicity, genotoxicity and carcinogenesis. It has
been included in the 3rd version of the Contaminant Candidate
List (CCL 3) of the US EPA for drinking water. Based on these
facts, DDC is an important indirect precursor of toxic NDEA.
Therefore, special attention should be paid for the removal of
DDC and its N-nitrosamine byproducts from the aquatic
system.
Diethyl dithiocarbamate (DDC), one of the widely used dithio-
carbamates (DTCs), has a strong chelating capacity with metal
species to generate insoluble and stable metallic complexes.1
Acting as a superior ligand, an extremely large quantity of DDC
is used as a otation collector in mining, as a fungicide in
agriculture, and as a functional additive in rubber and lubri-
cants. In the mining industry, to separate base metal suldes or
platinum minerals from ores, the DDC collector can react with
mineral surfaces to render them hydrophobic, and subse-
quently to form stable particle-bubble agglomerates.2,3 In
China, the DDC collector even gets a trivial name, SN-9, due to
its wide usage in the separation of Cu–Fe and Pb–Zn minerals.4,5
Owing to the ubiquitous applications of DDC in various elds, it
has been frequently detected in surface water and municipal
wastewater at the ppb level.6
It has been documented that DDC and its N-nitrosamine
byproducts are extremely toxic to aquatic animals and human
beings. For example, the hydrophobic metallic complexes of
Currently, N-nitrosamines in drinking water and wastewater
were of great concern. Some methods including advanced
oxidation processes (AOPs),12 integrated membrane system13
and nanoscale zero-valent iron,14 have been developed to
remove toxic NDEA. However, the information on how to
School of Civil and Resources Engineering, University of Science and Technology
Beijing, Beijing 100083, China. E-mail: pffu@ces.ustb.edu.cn; Fax: +86 10
82385795; Tel: +86 13520202167
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra04127c
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remove NDEA's precursors such as DDC in aqueous solutions is
very limited. This may be attributed to very low concentration
(ppb level) of DDC in water because most of the DDC used as
fungicides and additives are discharged with a fugitive emission
mode.15 Nevertheless, the DDC concentration in the mineral
otation wastewaters is as high as ppm level, e.g., 10–
100 mg Lꢁ1, because of the high dosage of DDC collector (30–
200 g tꢁ1 (ore)) in mining industry.16,17 To avoid toxic effects of
DDC collector to aquatic organisms and reduce the release of N-
nitrosamines into aquatic system, it is extremely necessary to
remove DDC collector with ppm level before the mineral ota-
tion wastewaters are discharged into rivers.
Recently, biodegradation,18,19 ozonation11,20 and chemical
oxidation with hypochlorite21 have been investigated to degrade
DDC collector. Although the DDC can be removed by the
biodegradation, the half-life of DDC biodegradation was as high
as 36.96 h.18 The chemical oxidation can rapidly decompose
DDC. For instance, 91.28% removal of DDC was achieved even
within 1 min using 400 mg Lꢁ1 hypochlorite.21 However, it has
been reported that the oxidation of DDC with free chlorine and
hypochlorite can form N-nitrosamine byproducts.11 In our
previous work, 100 mg Lꢁ1 of DDC solution was effectively
decomposed by the ozonation with the half-life of 10.09 min.20
Nevertheless, the formation of N-nitrosamines is also observed
in the ozonation of DDC.11 The degradation of NDEA by O3-
alone is usually less effective with a second-rate constant of only
0.08 Mꢁ1 Sꢁ1, much lower than that of 7 ꢀ 108 Mꢁ1 Sꢁ1 by
hydroxyl radicals (cOH).12,22 Thus, the combined processes such
as the O3/H2O2 and O3/UV254nm have been developed to remove
NDEA from aquatic system.12,23,24 Therefore, a novel process
with a high yield of cOH radicals can provide the benets of
effective removal of DDC collector with low risk of N-nitrosa-
mine formation.
O3 + H2O + hn254nm / H2O2 + O2
H2O2 + 2O3 / 2cOH + 3O2
H2O2 + hn254nm / 2cOH
(3)
(4)
(5)
In this study, the degradation of DDC collector at initial
concentration of 100 mg Lꢁ1 is compared by the O3 and O3/VUV
at low O3 dosage of below 1.27 mg minꢁ1 Lꢁ1. The objectives of
this work are (1) to compare the removal efficiencies and
mineralization extents of DDC by the O3 and O3/VUV; (2) to
investigate the generation of CS2 and H2S byproducts; (3) to
identify organic byproducts and propose degradation pathways
of DDC. This work may be helpful to explore a method to
effectively remove DDC collector from mineral otation
wastewaters.
2. Materials and methods
2.1 Chemicals and VUV lamp
Sodium diethyl dithiocarbamate trihydrate (C5H10NS2Na$3H2O)
with analytical grade was purchased from Shanghai Aladdin
Bio-Chem Technology Co., Ltd, China. Fig. S1 (see in the ESI†)
shows the molecular structure of DDC. Other chemicals such as
cupric acetate, lead acetate, diethylamine, silver sulfate and
mercury sulfate were of analytical grade and purchased from
Sinopharm Chemical Reagent Co., Ltd, Beijing, China. Deion-
ized water was used in the degradation experiments. A 40 W
VUV lamp was purchased from Bright Star Light & Electricity
Co., Ltd, Guangdong, China. The lamp emitted about 10% of
185 nm VUV radiation and 90% of UV254nm radiation (see in
Fig. S2†). To conduct the O3/UV254nm and UV254nm photolysis
experiments, the UV254nm lamp, with same electric power and
size to the VUV lamp, was purchased.
In various AOPs, the O3/vacuum-UV (O3/VUV) has the higher
yield of cOH radicals than O3, O3/UV254nm and O3/H2O2 per mass
of oxidants.25 The VUV lamp, namely, low pressure Hg lamp
2.2 The degradation procedures
covered with high pure quartz glass, can emit about 10% radi- The DDC degradation was conducted in a jacket glass reactor
ation at 185 nm and 90% radiation at 254 nm. In the O3/VUV connected with a thermostatic bath. Fig. 1 showed the sche-
system, the 185 nm VUV irradiation of water can in situ generate matic diagram of experimental setup. The cylindrical reactor
cOH radicals (eqn (1) and (2)). By combining UV254nm light had a 1240 mm height and 53 mm internal diameter. The VUV
emitted from VUV lamp and dosed O3, the O3/UV254nm can be or UV254nm lamp was inserted into a quartz tube installed at the
formed with the generation of cOH radicals via eqn (3)–(5). axis of the reactor. Ozone was generated from an O3 generator
Therefore, three oxidation processes, i.e., VUV photolysis, O3/ (SW-004 model, Qingdao West Electronic Puriers Co., Qing-
UV254nm and ozonation, may be included in the O3/VUV dao, China) using air as the source. The O3 stream was intro-
system.26 Thus, previous studies had demonstrated that the O3/ duced into the reactor through a porous glass plate.
VUV could achieve much higher removal efficiencies of n-butyl
100 mg Lꢁ1 of DDC solution was prepared by dissolving
xanthate,24 natural organic matter27 and phenols28 than O3- 0.265 g DDC with trihydrate in 2 L deionized water. The initial
alone. For the degradation of N-nitrosamines, simple UV254nm pH was adjusted to 10.0 using 0.05 mol Lꢁ1 NaOH or HCl
photolysis can remove N-nitrosamines effectively due to their solution. While 2 L of DDC solution was put into the reactor, the
strong UV absorption.29,30 Therefore, it can be expected that the ozonation of DDC was conducted by introducing the O3 stream.
degradation of DDC by the O3/VUV would achieve higher The O3/VUV and O3/UV254nm degradation of DDC was per-
removal of DDC with low risk of N-nitrosamine formation than formed by introducing O3 stream and turning on the VUV and
O3-alone.
UV254nm lamps, respectively. The O3 dosage rate was controlled
to be 0.62–1.27 mg minꢁ1 Lꢁ1. To perform the VUV and UV254nm
photolysis of DDC, an air stream was introduced at a ow rate of
1.67 L minꢁ1, and the VUV and UV254nm lamps were simulta-
H2O + hn185nm / cOH + cH
(1)
(2)
H2O + hn185nm / cOH + H+ + eeq
ꢁ
neously turned on, respectively. In this study, the O3/UV254nm
,
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determined at 430 nm using dehydrated alcohol as a reference
solution. The aqueous H2S concentration was determined by an
iodometric method (HJ/T 60-2000). The suldes in the treated
DDC solution should be transformed into ZnS by adding
Zn(CH3(COO)2) and NaOH solution before the analysis.
2.3.3 Measurement of the amount of gaseous CS2 and H2S
emitted into air. The amount of gaseous CS2 emitted into air
was measured by a diethylamine spectrophotometric method
(GB/T 14680-93). As shown in Fig. 1, the emission gas was
allowed to pass through a lead acetate coated cotton tube to
remove the H2S, and then introduced into the CS2 absorption
liquid. The amount of gaseous H2S was determined by a meth-
ylene blue spectrophotometric method (GB/T 11742-89). To
collect all of gaseous CS2 and H2S, the emission gas continu-
ously passed through the CS2 and H2S absorption liquid for
90 min, respectively.
2.3.4 SPE/GC-MS analysis of organic byproducts. The solid
phase extraction and gas chromatography-mass spectrometry
(SPE/GC-MS) was developed to determine organic byproducts in
the O3/VUV degradation of DDC. The SPE 24-port vacuum
manifold system (Phenomenex, U.S.) was used to extract
byproducts from water. The Strata™-X (10 g, code: 8B–S029-
MFF, Phenomenex) was employed as the SPE sorbent. Firstly,
the SPE cartridges were preconditioned with 2 mL of acetoni-
trile, followed by 1 mL of ultrapure water. Aerwards, 50 mL of
acidied samples were allowed to pass through the cartridges at
a ow rate of 3 mL minꢁ1. Then, the cartridges were rinsed with
5 mL ultrapure water, and further dried under vacuum for
10 min to removal residual water. The analytes were eluted with
3 mL of acetonitrile. The organic extracts were transferred into
300 mL GC vials for analysis.
Fig. 1 Schematic diagram of experimental setup ((1) ozone generator;
(2) flow meter; (3) column reactor; (4) sampling valve; (5) quartz tube;
(6) VUV lamp; (7) absorbent cotton with lead acetate; (8) CS2
absorption liquid; (9) H2S absorption liquid; (10) activated carbon
column).
VUV photolysis and UV254nm photolysis experiments were per-
formed for the comparative purposes. To fully evaluate the
mineralization of DDC, the degradation time was extended to
90 min although nearly 100% of DDC could be removed within
20 min. All experiments were conducted at 25 ꢂ 2 ꢃC. Aqueous
samples were taken at predetermined time intervals to measure
2ꢁ
ꢁ
the concentrations of DDC, TOC, SO4
respectively.
and NO3 anions,
The SPE extracts were analyzed by the GC-MS (Shimadzu,
GCMS–QP2010 SE, Japan), equipped with a capillary column
(Zebron ZB–FFAP, 30 m ꢀ 0.32 mm ꢀ 0.25 mm, Phenomenex,
U.S.). The oven temperature program was as follow: an initial
2.3 Analysis and calculation
2.3.1 Determination of the concentrations of DDC, TOC, oven temperature was 80 ꢃC (held foꢁr15 min), then increased to
2ꢁ
ꢁ
SO4 and NO3 ions. The concentration of DDC was deter- 220 ꢃC (held for 10 min) at 8 ꢃC min , and nally increased to
mined by a UV-vis spectroscopic method since the DDC had 300 ꢃC (held for 1 min) at 30 ꢃC minꢁ1. High purity He gas
a maximum absorption peak at 256 nm. The absorbance of (purity $ 99.999%) was used as carrier gas at ow rate of 1
aqueous samples was recorded at 256 nm by a UV–vis spectro- mL minꢁ1. The ion source of mass spectra was operated in the
photometer (UV–5500PC, Shanghai Metash Instruments Co. electron impact mode (EI mode, electron energy 70 eV, 220 ꢃC).
Ltd, China). The TOC was measured using a Shimadzu TOC–V Full-scan mass (m/z 40–500) was recorded to identify organic
organic carbon analyzer.
byproducts. According to fragmentation rules of organic
The SO42ꢁ concentration was analyzed by a barium chromatꢁe species under electron ionization conditions, the byproducts
spectrophotometry method (HJ/T 342–2007). The NO3
were identied by comparing mass spectra with National
concentration was determined by using an ion chromatograph Institute of Standards and Technology (NIST) mass spectral
(Metrohm 792 Basic, Switzerland). Anionic species were sepa- library data.
rated by a Star-Ion A300 anionic column using a carbonate
eluting phase consisted of 2.4 mmol Lꢁ1 NaHCO3 and 2.5 mmol
3. Results and discussion
3.1 Degradation of DDC by the O3, UV254nm, VUV, O3/
Lꢁ1 Na2CO3 at ow rate of 1 mL minꢁ1
.
2.3.2 Analysis of the concentrations of aqueous CS2 and
H2S. The aqueous CS2 concentration was measured by a dieth-
UV254nm and O3/VUV
ylamine cupric acetate spectrophotometric method (GB/T The degradation performances of DDC by the O3, UV254nm
,
15504-1995). 50 mL of aqueous sample was purged with a 100 VUV, O3/UV254nm and O3/VUV processes were compared, and
mL minꢁ1 N2 stream for 1 h to volatilize CS2 which was subse- the results are shown in Fig. 2 and listed in Table 1. The O3
quently absorbed with a diethylamine and cupric acetate mixed dosage rate was 1.16 mg minꢁ1 Lꢁ1 and initial pH of DDC
liquid. Then, the absorbance of CS2 absorption liquid was solution was 10.0. In this work, the removal of DDC by O3,
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was still the main mechanism for the DDC removal in the
VUV photolysis.
The kapp achieved by the O3/UV254nm was 0.219 minꢁ1, much
higher than that by UV254nm photolysis and O3-alone, respec-
tively. As mentioned above, the UV254nm photolysis of dissolved
O3 can yield H2O2, which in turn reacts with O3 or UV254nm
radiation to generate cOH radicals as specied in eqn (3)–(5).32
The formation of cOH radicals plays a key role in DDC degra-
dation by the O3/UV254nm. In this case, the O3/UV254nm could be
also formed in the O3/VUV system by combining dosed O3 and
UV254nm radiation emitted from the VUV lamp. As given in Table
1, the O3/UV254nm and VUV photolysis had achieved much
higher kapp value compared to O3-alone, revealing that two
former processes were main mechanisms for the DDC removal
in the O3/VUV system.
For all processes tested, the O3/VUV could achieve the
highest degradation rate of DDC. Specially, the kapp obtained
by the O3/VUV was 3.99 times higher than that by O3-alone.
Since the UV254nm and VUV lamps used in this work had same
electric power, the O3/VUV and O3/UV254nm had same energy
consumption in DDC removal. However, it can be seen that
the O3/VUV had the higher DDC degradation rate as given in
Table 1. Consequently, it was quite evident that the O3/VUV
was more suitable for the DDC removal compared to the O3/
UV254nm. In the previous biodegradation of DDC, the degra-
dation rate constant of 0.45 dayꢁ1 (3.125 ꢀ 10ꢁ4 minꢁ1) was
reported, suggesting that the removal of DDC by the
biodegradation was extremely low.18 Although the DDC was
found to be effectively oxidized by hypochlorite, the high
dosage of hypochlorite, for example 400 g Lꢁ1, might result in
potential secondary pollution of free chlorite and high
process cost.21 Based on above results, it was considered that
the O3/VUV could be used to remove high concentration DDC
collector from otation wastewaters with fast reaction
kinetics.
Fig. 2 The variations of DDC concentration with degradation time in
the O3, UV254nm, VUV, O3/UV254nm and O3/VUV process. The inset
figure was the pseudo-first-order kinetic fitting of ln(C/C0) versus time
t.
UV254nm, VUV, O3/UV254nm and O3/VUV could be described
with a pseudo-rst order kinetic equation (eqn (6)),
ln(C/C0) ¼ ꢁkapp
t
(6)
where, C and C0 (mg Lꢁ1) were the DDC concentration at
degradation time t and initial, respectively, kapp (minꢁ1) was the
pseudo-rst order rate constant, and t (min) was degradation
time.
As shown in Fig. 2 and Table 1, the UV254nm photolysis had
achieved the higher DDC removal and larger degradation rate
constant (kapp) than O3-alone. It seemed that the DDC could
be readily degraded by absorbing UV radiation. As indicated
in our previous work, the DDC had a maximum UV absorp-
tion at 256 nm.20 Thus, the degradation may be initiated by
exciting DDC molecules with the absorption of UV254nm light.
As given in Table 1, the kapp obtained by VUV photolysis was
1.75 times higher than that by UV254nm photolysis, revealing
that cOH radicals generated by 185 nm VUV irradiation of
H2O (eqn (1) and (2)) had promoted the DDC degradation.31
However, it should be noted that the degradation initiated by
direct absorption of UV radiation (UV254nm and 185 nm VUV)
3.2 Effect of O3 dosage
The O3 dosage is one of key parameters for O3-based AOPs. It
determines both the removal efficiency of pollutants and the
treatment cost. In this work, the O3 dosage rate was chosen
to be as low as 0.62–1.27 mg minꢁ1 Lꢁ1 to investigate the
removal of DDC by the O3/VUV. As shown in Fig. 3, when the
O3 dosage rate increased from 0.62 to 1.27 mg minꢁ1 Lꢁ1, the
kapp for the O3 and O3/VUV rose from 0.0199 to 0.0826 minꢁ1
and from 0.211 to 0.308 minꢁ1, respectively. These results
indicate that the DDC degradation could be signicantly
enhanced at higher O3 dosage. It can be attributed to the
higher yield of cOH radicals by combining O3 and UV254nm
radiation together. However, as shown in Fig. 3, the kapp
obtained by VUV photolysis had reached 0.201 minꢁ1, much
higher than that (0.0826 minꢁ1) by O3-alone even at the
highest O3 dosage rate of 1.27 mg minꢁ1 Lꢁ1. As discussed
above, the O3/UV254nm and VUV photolysis were two main
mechanisms for DDC removal in the O3/VUV system. Thus,
the O3/VUV in this case can achieve high efficiency of DDC
removal even at low O3 dosage.
Table 1 The removal efficiency at 20 min, degradation rate constant
(kapp), half-life (t1/2) and correlation coefficient (R2) in the degradation
of DDC by the O3, UV254nm
respectively
, VUV, O3/UV254nm and O3/VUV,
Removal efficiency
at 20 min (%)
Process
kapp (minꢁ1
)
t1/2 (min)
R2
O3
UV254nm
VUV
O3/UV254nm
O3/VUV
80.25
90.96
98.45
98.94
99.55
0.0687
0.115
0.201
0.219
0.274
10.09
6.03
3.45
3.17
2.53
0.992
0.984
0.991
0.996
0.998
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2ꢁ
ꢁ
2ꢁ
SO4 ꢁ or NO3 anion than O3-alone, indicating more SO4 and
NO3 anions were accumulated in the O3/VUV system.
Fig. 5 showed the relative concentrations of DDC and TOC,
2ꢁ
ꢁ
and the changes of SO4 2ꢁand NO3 concentrations. The
ꢁ
concentrations of TOC, SO4 and NO3 at 90 min were also
presented in Table 2. As shown in Fig. 5, with the rapid decrease
of the C/C0 of DDC, the TOC/TOC0 was also reduced with the
2ꢁ
ꢁ
increase of SO4 and NO3 concentrations, revealing the C, S
and N elements of DDC could be mineralized by the O3, VUV
and O3/VUV, respectively. However, as given in Table 2, the O3/
VUV obtained higher concentrations of SO42ꢁ and NO3ꢁ anions
with lower TOC at 90 min than O3-alone and VUV photolysis.
In this work, to quantitatively describe the mineralization of
DDC, the mineralization extents of C, S and N elements were
calculated. The mineralization extent of C was expressed by eqn
(7),20
TOC0 ꢁ TOC
gC
¼
ꢀ 100%
(7)
TOC0
Fig. 3 The rate constants (kapp) in the O3 and O3/VUV degradation of
DDC.
where, gC was the mineralization extent of C, TOC0 and TOC
were the TOC of DDC solution at initial and 90 min, respec-
tively. The mineralization extent of S or N element was dened
by eqn (8):
3.3 Mineralization of C, S and N elements of DDC
171
Canion
C0
As shown in Fig. S1 (see in the ESI†), the DDC molecule contains
g ¼
ꢀ
ꢀ 100%
(8)
2ꢁ
n ꢀ Manion
functional S and N atoms. Thus, inorganic anions such as SO4
ꢁ
and NO3 may be generated in the degradation of DDC.20 As
where, g was the mineralization extent of S (gS) or N (gN)
element of DDC, n was the number of S or N atom of DDC,
shown in Fig. 4, the ion chromatographic analysis exhibited that
both sulfate (SO42ꢁ) and nitrate (NO3ꢁ) anions were generated
aer the oxidation of DDC by the O3 and O3/VUV, respectively.
2ꢁ
ꢁ
Manion was the relative molecular weight of SO4 or NO3ꢁ
anion, Canion (mg Lꢁ1) was the concentration of SO42ꢁ or NO3
at 90 min, and C0 (mg Lꢁ1) was initial concentration of DDC.
The mineralization extents of C, S and N elements were
summarized in Table 2. It can be found that the gC, gS and gN at
90 min obtained by the O3/VUV were 1.72, 3.71 and 4.24 times
higher than that by O3-alone, respectively. The higher extents of
mineralization well revealed that both DDC and its byproducts
were removed by the O3/VUV with higher degradation rates
compared to O3-alone. However, in this work, the VUV photol-
ysis achieved the higher mineralization extents of C, S and N
elements of DDC than O3-alone at the O3 dosage rate of 1.16
mg minꢁ1 Lꢁ1, revealing that the VUV photolysis had made
great contribution in further degradation of various byproducts
2ꢁ
ꢁ
Other ionic species such as SO32ꢁ, S2O3 and NO2 were not
detected, suggesting that these species with low valences of S
2ꢁ
ꢁ
and N were further oxidized into SO4 and NO3 anions.33,34 It
well demonstrated that the complete mineralization of DDC was
feasible for both the O3 and O3/VUV process. However, as shown
in Fig. 4, the O3/VUV obtained higher peak intensity of either
for the O3/VUV process. As mentioned above, the O3/UV254nm
,
VUV photolysis and the ozonation could be responsible for the
generation of cOH radicals in the O3/VUV system. Thus the O3/
VUV could obtain higher yields of oxidative species than O3-
alone at same O3 dosage. As listed in Table 2, the mineralization
extent of C, S and N elements by the O3/VUV increased as
following: gC < gS < gN. The results suggested that the S and N
elements of DDC were relatively easier to be mineralized
compared to C element. Accordingly, it can be reasonably
inferred that some organic byproducts without the S or N
element might be accumulated in treated DDC solution.
3.4 Generation of CS2 and H2S byproducts
Fig. 4 Ion chromatographic analysis of 100 mg Lꢁ1 DDC solution
It has been well documented that sulfur byproducts, such as
treated at 0 min (1), 90 min by the O3 (2) and O3/VUV process (3).
carbon disulde (CS2), carbonyl sulde (COS), hydrogen sulde
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2ꢁ
ꢁ
Fig. 5 The relative concentrations of DDC and TOC, and changes of SO4 and NO3 concentrations in the O3 (a), VUV (b) and O3/VUV (c)
degradation of DDC.
(H2S), sulte (SO32ꢁ) and thiosulfate (S2O32ꢁ), are generated
Fig. 6 showed the concentrations of aqueous CS2 and H2S in
during the degradation of xanthate collectors by the the O3 and O3/VUV degradation of DDC, respectively. During the
AOPs.26,33,35,36 In this case, the DDC collector had same func- degradation of DDC, the CS2 concentration increased up to
tional –CSSꢁ group as the xanthates. Thus, above-mentioned a maximum value, and then decreased at longer time for both
sulfur byproducts might be also generated while the DDC was the O3 and O3/VUV process. At the initial stage, more CS2 was
degraded. However, to the best of our knowledge, there is no generated by the O3/VUV compared to O3-alone. However, at
report on the formation of sulfur byproducts during the DDC 90 min, the CS2 concentration was reduced to an extremely low
degradation by the AOPs. Because the CS2 and H2S are typical level of 0.10 mg Lꢁ1 for the O3/VUV, but was still as high as
toxic sulfur contaminants, the concentrations of aqueous CS2 0.89 mg Lꢁ1 for O3-alone. In general, aqueous H2S concentra-
and H2S were determined while the DDC was degraded by the tions had similar change behaviors to CS2 as shown in Fig. 6(b).
O3 and O3/VUV, respectively.37,38 Since both the CS2 and H2S are Specially, when the H2S concentration by the O3/VUV was
highly volatile, the amounts of gaseous CS2 and H2S emitted decreased aer 20 min, it still increased signicantly by O3-
into air were also measured to investigate the emission behav- alone. Therefore, a low H2S concentration of 0.16 mg Lꢁ1 for the
iors of CS2 and H2S from treated DDC solutions.
O3/VUV was obtained, but the H2S concentration was as high as
3.34 mg Lꢁ1 for O3-alone aer 90 min of DDC degradation.
Consequently, although sulfur byproducts could be further
oxidized by the O3 and O3/VUV, the O3/VUV achieved much
lower residual concentrations of CS2 and H2S, attributing to the
enhanced yields of cOH radicals in the O3/VUV system.
Fig. 7 depicts the amounts of gaseous CS2 and H2S emitted
into air. The amount of emitted CS2 for the O3/VUV was slightly
higher than that for O3-alone. As the CS2 was a volatile
contaminant, the higher concentration of aqueous CS2 at initial
stage (Fig. 6(a)) would release more CS2 into gas phase in the O3/
VUV system. As shown in Fig. 7, the amount of emitted H2S for
Table 2 The concentrations of TOC, SO42ꢁ, and NO3 anions at
90 min and the mineralization extents of C, S and N elements of DDC
by the O3, VUV and O3/VUV, respectively
ꢁ
Concentration (mg Lꢁ1
)
Mineralization extent (%)
2ꢁ
ꢁ
Process
TOC
SO4
NO3
C
S
N
O3
VUV
O3/VUV
54.91
51.67
45.21
18.66
60.42
69.18
6.82
9.07
28.92
18.85
24.45
32.36
16.91
57.69
62.69
18.81
25.02
79.76
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Fig. 6 The concentrations of aqueous CS2 (a) and H2S (b) byproducts in the O3 and O3/VUV degradation of DDC, respectively.
byproducts were summarized in Tables 3 and 4 with retention time
and other special characteristics. The mass spectrums of identied
byproducts were shown in Fig. S3 (see in the ESI†).
As shown in Fig. 8 and Table 3, six amide byproducts were
detected at 20 min, and the N,N-diethylformamide (peak 1) was
the main byproduct of DDC degradation. Since the removal of
DDC at 20 min had reached 99.85% as given in Table 1, the DDC
was not detected. By extending the degradation time from 20 to
90 min, the relative abundance of N,N-diethylformamide (peak
1) was remarkably decreased. Meanwhile, the peaks of N,N-
dimethylacetamide (peak 2), N-ethylacetamide (peak 3) and N-
methylacetamide (peak 4) disappeared completely. The results
indicated the further decomposition of these amines by the O3/
VUV. Nevertheless, the relative abundance of 5,6-dihydro
thymine (peak 5) and N-formyl-N-methylformamide (peak 6)
increased, and the diacetamide (peak 7) appeared as shown in
Fig. 8. Because almost 100% removal of DDC was achieved aer
Fig. 7 The amounts of gaseous CS2 and H2S emitted into air for
90 min in the O3 and O3/VUV degradation of DDC.
O3-alone was 12.24 times higher than that for the O3/VUV. The
results were well in accordance with much higher concentration
of aqueous H2S for O3-alone as indicated in Fig. 6(b). By
considering the emission of CS2 and H2S together, the amount
of emitted CS2 and H2S for the O3/VUV was just 0.685 mg, much
lower than that (2.949 mg) for O3-alone. In summary, in
comparison with O3-alone, the O3/VUV achieved lower aqueous
concentrations and less emitted amounts of sulfur byproducts
(CS2 and H2S) during the degradation of DDC. The result
revealed that the release of toxic CS2 and H2S could be inhibited
by the O3/VUV degradation of DDC.
3.5 Byproducts identied by SPE/GC-MS
To better understand the degradation mechanism of DDC, the
SPE/GC-MS analysis was employed to identify organic byproducts
generated in the O3/VUV degradation of DDC. All the peaks with
Fig. 8 GC-MS total ion chromatograms of DDC solutions treated at
a response on mass spectrometry were marked in Fig. 8. Identied 20 and 90 min in the O3/VUV degradation of DDC.
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Table 3 GC-MS retention time and special characteristics of identified byproducts in the O3/VUV degradation of DDC at 20 min
Molecular
No.
formula
Compounds
Retention time (min)
Similarity (%)
Main characteristic ions (m/z)
1
2
3
4
5
6
C5H11NO
C4H9NO
C4H9NO
C3H7NO
C5H8N2O2
C3H5NO2
N,N-diethylformamide
N,N-dimethylacetamide
N-ethylacetamide
N-methylacetamide
5,6-Dihydro thymine
N-formyl-N-methylformamide
4.099
6.185
6.406
6.607
7.697
8.084
99
91
96
97
99
98
58, 72, 86, 101, 102
58, 72, 73, 87
72, 87
58, 73
127, 128, 129
59, 87
20 min, these three amides should be generated from certain have three pathways. As shown in Fig. 8, the N,N-dieth-
intermediates. Consequently, the number of organic byprod- ylformamide was the primary byproduct of DDC degradation.
ucts of DDC could be reduced by extending the reaction time.
Therefore, the main pathway (pathway 1) may be the oxidation
In this work, main byproducts in the O3/VUV degradation of of the –CSSꢁ group of DDC into –CHO group to form the N,N-
DDC were amide compounds without the generation of N-nitro- diethylformamide. Simultaneously, the S2ꢁ ions are released
samines such as NDEA. However, the formation of NDEA was (eqn (9)). Another two pathways of DDC degradation may be
previously observed in the oxidation of DDC with Cl2, NaClO2 and related to the generation of the diethylamine (DEA). Although
O3, respectively.11 In the O3/VUV degradation of DDC, two the DEA was not detected by SPE/GC-MS, the formation of DEA
mechanisms, i.e., UV photolysis and enhanced cOH radical was previously reported through the hydrolysis or the oxidation
formation, may contribute to the inhibition of N-nitrosamine of DDC.11 As mentioned above, the DDC can be easily hydro-
formation. Because N-nitrosamines have a strong UV absorption lyzed to DEA with the release of CS2 (eqn (10)).40 But the
via the homolytic cleavage of the N–N bond, the UV photolysis hydrolysis of DDC can be inhibited in alkaline pH. For example,
was proved to degrade N-nitrosamines effectively through N–N the hydrolysis rate constant of DDC at pH 5 is 9.2 ꢀ 10ꢁ4 sꢁ1
,
bond ssion.29,39 The rate constant for NDEA degradation under but the rate constant at pH 9 is remarkably reduced to 1.0 ꢀ
UV irradiation reached 2.60 ꢀ 10ꢁ2 and 1.62 ꢀ 10ꢁ3 mmol 10ꢁ7 sꢁ1
.
In this work, initial pH of DDC solution was 10.0,
11
Lꢁ1 minꢁ1 at pH 6.0 and 10.0, respectively.39 In this O3/VUV meaning that the DEA formation from the hydrolysis of DDC
system, the UV (UV254nm + 185 nm VUV) photolysis may promote (pathway 2) may be low efficient. Thus, the yield of DEA
to the degradation of possibly generated N-nitrosamines.
(pathway 3) should be mainly attributed to the breaking of N–C
Previous studies had concluded that an oxidation system with bond of DDC by cOH radicals (eqn (11)).
the higher yield of cOH radicals could achieve higher degradation
(CH3CH2)2NCSSꢁ + cOH / (CH3CH2)2NCHO + S2ꢁ (9)
rate of NDEA.23,24 For instances, compared to O3-alone, the UV/O3
could inhibit the regeneration of NDEA effectively via the inter-
action of NDEA and cOH radicals.23,24 As mentioned above, the
rate constant of NDEA degradation with cOH radicals is nearly 9
orders higher than that with the O3.22 Therefore, cOH radicals are
the key oxidant for NDEA degradation. In this case, three
Hydrolysis: (CH3CH2)2NCSSꢁ / (CH3CH2)2NH + CS2 (10)
Oxidation: (CH3CH2)2NCSSꢁ + cOH / (CH3CH2)2NH
+ CS2
(11)
mechanisms, i.e., the ozonation, VUV photolysis and O3/UV254nm
,
had made contributions to the formation of cOH radicals in the
O3/VUV system, resulting in much higher yield of cOH radicals
compared to O3-alone. Therefore, the degradation of possible N-
nitrosamines would be enhanced, and the N-nitrosamine
formation was inhibited by the O3/VUV.
The DEA can be effectively oxidized by cOH radicals.41,42 The
H-atom abstraction by cOH radicals from –NH and CH2 groups
of DEA may occur (eqn (12) and (13)).41 The N-ethylacetamide
may be generated from eqn (13) and (14). Then, the N-ethyl-
acetamide can be further oxidized to diacetamide, N-methyl-
acetamide and N,N-dimethylacetamide. As shown in Fig. 8, the
relative abundance of 5,6-dihydro thymine and N-formyl-N-
methyl formamide increased with the degradation of the N,N-
diethyl formamide while the degradation time rose from 20 to
3.6 Possible degradation pathways of DDC by the O3/VUV
In this work, the degradation pathways of DDC by the O3/VUV
were proposed as shown in Fig. 9. The degradation of DDC may
Table 4 GC-MS retention time and special characteristics of identified byproducts in the O3/VUV degradation of DDC at 90 min
Molecular
No.
formula
Compounds
Retention time (min)
Similarity (%)
Main characteristic ions (m/z)
1
5
6
7
C5H11NO
C5H8N2O2
C3H5NO2
C4H7NO2
N,N-diethylformamide
5,6-Dihydro thymine
N-formyl-N-methylformamide
Diacetamide
4.101
7.692
8.075
9.767
96
95
99
94
57, 58, 86, 101
102, 127, 128, 129
59, 87
59, 60, 73
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Fig. 9 Proposed degradation pathways of DDC by the O3/VUV process.
90 min. Therefore, the 5,6-dihydro thymine and N-formyl-N-
methyl formamide may be generated from the degradation of
N,N-diethylformamide. Finally, these amide compounds can be
completely mineralized to CO2, H2O, NO2ꢁ, and NO3ꢁ anions by
cOH radicals.
COS + H2O / H2S + CO2
(17)
(18)
2ꢁ
S2ꢁ + cOH / SO4 + H2O
(CH3CH2)2NH + cOH / (CH3CH2)2Nc + H2O
(12)
4. Conclusions
(CH3CH2)2NH + cOH / (CH3CH2)NHCcHCH3 + H2O (13)
(CH3CH2)NHCcHCH3 + cO / (CH3CH2)NHCHOCH3 (14)
The degradation of DDC collector with initial concentration of
100 mg Lꢁ1 was performed by the O3 and O3/VUV process,
respectively. Compared to O3-alone, the O3/VUV achieved
higher removal efficiency and mineralization extents of DDC.
The rate constant of DDC degradation by the O3/VUV was 3.99
times higher than that by O3-alone. For the O3/VUV, the
removal of DDC reached 99.55% at 20 min, and the mineral-
ization extents of C, S and N elements reached 36.36%, 62.69%
and 79.76% at 90 min, respectively. In the O3/VUV system, the
O3/UV254nm and VUV photolysis were two main mechanisms
for DDC removal. At the end of degradation (90 min), the
residual concentrations of aqueous CS2 and H2S were reduced
to 0.10 and 0.16 mg Lꢁ1 for the O3/VUV, but were still as high
as 0.89 and 3.34 mg Lꢁ1 for O3-alone, respectively. Simulta-
neously, the O3/VUV released lower amount of gaseous CS2
and H2S. The results indicated that the O3/VUV could inhibit
the release of CS2 and H2S byproducts in the degradation of
DDC.
The CS2 byproduct generated via eqn (10) and (11) can react
with cOH radicals to produce dithiocarbonate ions (cCS2OH)
(eqn (15)).43 Subsequently, the cCS2OH is decomposed instan-
taneously to produce hydrogen sulde radicals (cHS) and
carbonyl sulde (COS) (eqn (16)).44 The aqueous COS may be
hydrolyzed to produce hydrogen sulde (H2S) (eqn (17)).45 Then,
S2ꢁ species generated from eqn (9) and (17) were further
2ꢁ
oxidized into SO4 anions (eqn (18)).46 The decreased concen-
trations of aqueous CS2 and H2S well demonstrated the oxida-
tion of CS2 and H2S in the O3/VUV system.
CS2 + cOH / CS2OHc
CS2OHc / COS + HSc
(15)
(16)
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Main byproducts in the O3/VUV degradation of DDC were 19 S. H. Chen, W. Q. Gong, G. J. Mei, L. Xiong and N. A. Xu, Int.
amide compounds such as N,N-diethylformamide. There were J. Miner. Process., 2011, 101, 112–115.
no N-nitrosamines such as NDEA detected by SPE/GC-MS, 20 P. F. Fu, X. F. Lin, G. Li, Z. H. Chen and H. Peng, Minerals,
which may be attributed to UV (UV254nm + 185 nm VUV) 2018, 8, 1–15.
photolysis and enhanced formation of cOH radicals in the O3/ 21 B. Y. Cui, X. T. Wang, Q. Zhao and W. G. Liu, J. Chem., 2019,
VUV system. Furthermore, the degradation pathways of DDC 7038015.
were proposed. The results suggested that the O3/VUV was very 22 S. P. Mezyk, W. J. Cooper, K. P. Madden and D. M. Bartels,
effective in the removal of DDC collector from mineral otation
wastewaters with low risk of N-nitrosamine formation.
Environ. Sci. Technol., 2004, 38, 3161–3167.
23 B. B. Xu, Z. L. Chen, F. Qi, J. Ma and F. C. Wu, J. Hazard.
Mater., 2009, 168, 108–114.
24 B. B. Xu, Z. L. Chen, F. Qi, J. Ma and F. C. Wu, J. Hazard.
Mater., 2010, 179, 976–982.
Conflicts of interest
¨
25 M. G. Gonzalez, E. Oliveros, M. Worner and M. A. Braum, J.
There are no conicts to declare.
Photochem. Photobiol., C, 2004, 5, 225–246.
26 P. F. Fu, J. Feng, H. F. Yang and T. W. Yang, Process Saf.
Environ. Prot., 2016, 102, 64–70.
27 T. Ratpukdi, S. Siripattanakul and E. Khan, Water Res., 2010,
44, 3531–3543.
28 F. J. Beltran, F. J. Rivas and O. Gimeno, J. Chem. Technol.
Biotechnol., 2005, 80, 973–984.
29 C. Lee, W. Choi, Y. G. Kim and J. Yoon, Environ. Sci. Technol.,
2005, 39, 2101–2106.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant number: 51674017). The authors
would greatly appreciate the Experiment Center in School of
Metallurgical and Ecological Engineering, University of Science
and Technology Beijing for providing the SPE/GC-MS analysis.
´
30 M. H. Plumlee, M. Lopez-Mesas, A. Heidlberger, K. P. Ishida
and M. Reinhard, Water Res., 2008, 42, 347–355.
31 N. Quici, M. I. Litter, A. M. Braun and E. Oliveros, J.
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32 J. Swietlik, A. Dabrowska, U. Raczyk-Stanislawiak and
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33 X. H. Chen, Y. H. Hu, H. Peng and X. F. Cao, J. Cent. South
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34 W. X. Lin, J. Tian, J. Ren, P. T. Xu, Y. K. Dai, S. Y. Sun and
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