Waste-derived bioorganic substances for light-induced generation of reactive oxygenated species

Article abstract of DOI:10.1002/cssc.201000237

Urban waste-derived bioorganic substances (UW-BOS) have shown promise as chemical auxiliaries for a number of technological applications in the chemical industry and in environmental remediation. In this study, the application of these substances in the photodegradation of organic pollutants is addressed. The experimental work is specifically focused on the photolysis mechanism promoted by AC8, a UW-BOS isolated from a 2:1 w/w mixture of food and green residues, composted for 110 days, using 4-chlorophenol (4-CP) as probe molecule. The production of OH and the ¹O₂ is monitored by EPR spectroscopy. The correlation between radical species evolution and photodegradation of 4-CP is investigated. The effect of ¹O ₂ and OH scavengers on the 4-CP degradation process is also checked. The results suggest that the role of these species in the photodegradation of 4-CP depends on AC8 concentration. AC8 is thereby proven to be a photosensitizer for applications in environmental remediation. The results on AC8 further support the use of urban bio-waste as a versatile source of chemical auxiliaries of biological origin for use in diversified applications. An offer you can't refuse: A bioorganic substance isolated from a composted urban refuse (AC8) is applied as sensitizer in the photodegradation of 4-chlorophenol. Production of OH and ¹O₂ species is monitored by EPR spectroscopy and the data suggest that the role played by these species in the photodegradation process is strictly related to AC8 concentration. AC8 is thereby proven to be a photosensitizer for applications in environmental remediation.

Full text of DOI:10.1002/cssc.201000237

DOI: 10.1002/cssc.201000237
Waste-Derived Bioorganic Substances for Light-Induced
Generation of Reactive Oxygenated Species
Alessandra Bianco Prevot,*[a] Paola Avetta,[a] Debora Fabbri,[a] Enzo Laurenti,[b]
Tatiana Marchis,[b] Daniele G. Perrone,[c] Enzo Montoneri,[c] and Vittorio Boffa[c]
Urban waste-derived bioorganic substances (UW-BOS) have
shown promise as chemical auxiliaries for a number of techno-
logical applications in the chemical industry and in environ-
mental remediation. In this study, the application of these
substances in the photodegradation of organic pollutants is
addressed. The experimental work is specifically focused on
the photolysis mechanism promoted by AC8, a UW-BOS
isolated from a 2:1 w/w mixture of food and green residues,
composted for 110 days, using 4-chlorophenol (4-CP) as probe
EPR spectroscopy. The correlation between radical species evo-
lution and photodegradation of 4-CP is investigated. The effect
·
of 1O2 and OH scavengers on the 4-CP degradation pro-
cess is also checked. The results suggest that the role of these
species in the photodegradation of 4-CP depends on AC8 con-
centration. AC8 is thereby proven to be a photosensitizer for
applications in environmental remediation. The results on AC8
further support the use of urban bio-waste as a versatile
source of chemical auxiliaries of biological origin for use in
diversified applications.
·
1
molecule. The production of OH and the O2 is monitored by
Introduction
Urban bio-wastes (UW) have been reported to contain soluble
bioorganic substances (BOS) that show promise as chemical
auxiliaries for a number of technological applications in the
chemical industry and in environmental remediation.[1–6] As
part of a project[7] aimed at promoting sustainable products
and processes from UW at industrial level, we report herein the
potential of urban refuse-derived bioorganic substances
(UW-BOS) as photosensitizers for environmental applications.
In the last thirty years, great attention has been devoted to
light-induced oxidation processes for environmental remedia-
tion,[8–12] particularly for the treatment of waste waters. These
techniques are based on the generation by light energy of
products fitting a wide range of uses (Figure 1). UW-BOS are
chemically similar to SW-BOS. They are described[7] as likely
mixtures of substances with apparent molecular weights in the
range 1–3ꢀ105 Da, formed by long aliphatic carbon chains
substituted by aromatic rings and functional groups such as
COOH, CON, C=O, PhOH, OÀalkyl, OÀaryl, OCO, OMe, and
NRR’, (R and R’=alkyl or H). These organic moieties, represent-
ed in the virtual molecular fragment shown in Figure 1, are
most likely remnants of the main constituents of the source
bioorganic refuse matter, which are not completely mineralized
by microbial degradation. The proposed mechanism for SW-
BOS photoactivity can thus also be proposed for UW-BOS; it is
based on the production, under UV/Vis irradiation, of excited
triplet states that can in turn react with organic substrates by
two main mechanisms: hydrogen transfer and energy
transfer.[18,20,21]
·
highly reactive species, mostly OH radicals, able to promote
the degradation of organic pollutants into nontoxic or less
toxic products, and ultimately their mineralization. Natural
organic matter present in water and soil contains bioorganic
substances (SW-BOS) that have light-absorbing power. They
have been demonstrated to be able to promote photo-
chemical reactions and have been widely studied in order to
understand water auto-purification mechanisms.[13–18] In princi-
ple SW-BOS could be recovered from soil and then added to
specific polluted wastewaters to enhance and/or promote the
photodegradation of contaminants. However, due to their
poor concentration in soil and water, isolation of SW-BOS
would be rather costly. Recent work[5,19,20] has demonstrated
that UW-BOS also have good photosensitizing properties. Con-
trary to soil and water, urban wastes are a rather appealing
source of photosensitizers for several reasons. As result of in-
creased production due to urbanization, urban wastes are con-
centrated in confined areas by municipal collection. In addi-
tion, depending on the type of treatment and on composition,
they may provide high yields of a large variety of bio-based
A group of UW-BOS was previously shown to perform effi-
ciently as sensitizers for the photodegradation of azo-dyes.[5]
The presence of dissolved oxygen was also found to favor the
degradation process. Hydrogen transfer was hypothesized as
the main mechanism responsible for the photosensitization;
generated hydrogen atoms can be transferred to the dissolved
[a] Prof. A. Bianco Prevot, Dr. P. Avetta, Dr. D. Fabbri
Dipartimento di Chimica Analitica, Universitꢀ di Torino
Via P.Giuria 5, 10125 Torino (Italy)
Fax: (+39)116707615
E-mail: Alessandra.biancoprevot@unito.it
[b] Dr. E. Laurenti, T. Marchis
Dipartimento di Chimica IFM, Via P.Giuria 7, 10125 Torino (Italy)
[c] D. G. Perrone, Prof. E. Montoneri, Dr. V. Boffa
Dipartimento di Chimica Generale e Chimica Organica, Universitꢀ di Torino
Corso Massimo d’Azeglio 48, 10125 Torino (Italy)
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85

A. Bianco Prevot et al.
1
·
the study of O2 and OH forma-
tion and evolution has been
studied mainly by using probes/
scavengers or by EPR spectros-
copy.[16,23,24] In the first case, suit-
able molecules react selectively
with the species under investiga-
tion, yielding a specific reaction
product (in the case of probes)
or inhibiting the kinetics of the
photodegradation process of a
target organic molecule. In the
second case, the detection and
identification process involves
the formation of a persistent
spin-adduct radical from a com-
pound acting as spin-trap and
the target species (Schemes 1
and 2).
These adducts have distinctive
EPR spectra in which the signal
intensities allow estimation of
the amount of trapped radicals.
However, both approaches suffer
from possible drawbacks of indi-
rect methods. Scavengers are
not usually specific for a single
radical. Spin-adduct formation
reactions may be affected by
low yield and/or insufficient
product stability.
In the present research, the
photosensitizing properties of
one type of UW-BOS, namely
AC8, were studied using 4-chlor-
ophenol (4-CP) as probe mole-
·
cule. The production of OH and
1O2 was monitored by EPR spec-
troscopy. The correlation be-
tween the evolution of radical
species and the photodegrada-
tion of 4-CP was investigated. Fi-
Figure 1. Sourcing bio-refuse, isolated soluble bioorganic substances (BOS; R=alkyl, X=H or C, N, O atoms of
other molecular fragments) and related bio-based products (BBP); virtual molecular fragment represents the
identified C moieties in BOS (see Experimental Section).
·
nally, the effect of 1O2 and OH
scavengers on the 4-CP degrada-
tion process was analyzed.
oxygen with the formation of various reactive oxygenated spe-
cies (ROS) that, in turn, contribute to the degradation of the
·
1
probe molecule. Particularly, OH and O2, are of prime impor-
tance for their contribution to degradation of organic pollu-
tants in irradiated aqueous solutions.[22] However, direct evi-
dence of this reaction mechanism was not obtained, although
knowledge of the reaction mechanism was considered impor-
tant to optimize and address the use of UW-BOS in specific
applications.
1
In previous studies concerning photo-assisted degradation
processes in water in the presence of natural organic matter,
Scheme 1. Formation of spin-adduct 4-oxo-TMPÀ O2 (4-oxo-TEMPO; 4-oxo-
TMP=2,2,6,6-tetramethyl-4-piperidone hydrochloride).
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Bioorganic Substances for Generation of Reactive Oxygenated Species
Scheme 2. Formation. of spin-adduct DMPOÀOH (DMPO=5,5-dimethyl-1-
pyrroline-N-oxide).
Results and Discussion
AC8 chemical structure
In the present study, a UW-BOS (AC8) was isolated from food
and green wastes mixed in a 1:2 weight ratio. The refuse
mixture was piled and periodically turned to promote aerobic
biodegradation of the bioorganic matter. After 110 days, the
biomass was reduced to one third of its initial volume. Indeed,
during this biodegradation treatment, part of the refuse organ-
ics was completely mineralized, whereas lignin and other recal-
citrant biopolymers underwent structural modifications
becoming soluble in alkali. AC8 was extracted from the residu-
al refuse by a facile procedure (see Experimental Section).
1
Figure 3. EPR spectrum of 4-oxo-TMPÀ O2 adduct. AC8 concentra-
tion=2000 mgLÀ1; TMP concentration=45 mm; irradiation time=15 min.
EPR analysis of AC8
Figure 3 shows a typical EPR spectrum of the 4-oxo-TMP-1O2
spin adduct (Scheme 1) obtained by irradiating a 2000 mgLÀ1
aqueous solution of AC8 in the presence of 45 mm TMP. The
spectrum shows the typical triplet, with a 16.1 G hyperfine
splitting constant. To optimize the irradiation time before the
EPR measurement, two sets of experiments were performed at
20 and 150 mgLÀ1 AC8 concentration and irradiation times
ranging from 0 to 120 min.
Once isolated, AC8 was characterized according to
a
previously reported procedure.[2] The AC8 functional group
distribution, the relative ratios of which are represented in the
virtual molecular fragment in Figure 2, are given in Table 1.
Figure 4 shows that the signal intensity for both AC8 con-
centration values increases upon increasing the irradiation
time to reach a maximum value at 15 min irradiation and then
decreases. This behavior is likely to arise from a progressive
degradation of 4-oxo-TMP and/or of 4-oxo-TEMPO, due to
their reactions with other reactive species produced during the
AC8 irradiation. For this reason, the successive EPR experi-
ments were performed after 15 min of irradiation. Figure 4 also
shows that the 4-oxo-TEMPO signal intensity at 150 mgLÀ1
AC8 concentration is higher than that at 20 mgLÀ1 over the
whole irradiation-time experimental range. Thus, to assess the
Figure 2. Virtual molecular fragment fitting analytical data from Table 1.
Table 1. AC8 functional group distribution.
Functional group
Concentration/meqgÀ1
aliphatic C
OMe
17.1
0.0
2.9
4.0
1.3
ammine C (NC)
OÀalkyl C (OR)
di-OÀalkyl C (ROCOR)
aromatic and/or olefinic C (C=C), excluding PhO 6.8
phenol (PhOH)
1.3
2.5
3.8
0.0
1.6
phenyl ether (PhOX, X=R, Ar)
carboxylic acid C (COOH)
amide C (CON)
Figure 4. Intensity of 4-oxo-TEMPO EPR spectrum versus irradiation time.
AC8 concentration=20 mgLÀ1 (&) and 150 mgLÀ1 (*); signal intensity
calculated as average of the triplet spectral lines.
keto C (C=O)
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87

A. Bianco Prevot et al.
signal intensity showed a similar behavior in a time range of
3–60 min. Moreover, there was a significant increase in the
background spectral intensity arising from the solvent, leading
to a worsening of the signal/noise ratio. The irradiation time
occurring between the spin-trap addition and the EPR
measurement was thus fixed at 3 min.[16]
Figure 7 shows the intensity of the EPR spectrum of the
DMPOÀOH adduct with respect to AC8 concentration. In con-
trast to 4-oxo-TEMPO (Figure 5), the DMPOÀOH adduct spec-
trum intensity reaches a maximum value at about 20 mgLÀ1
and then decreases at higher AC8 concentrations. This profile
can be explained hypothesizing that two different processes,
·
both production and scavenging of OH by irradiated AC8,
occur in this case.
Figure 5. Intensity of 4-oxo-TEMPO EPR signal, after background subtraction,
versus AC8 concentration. 4-oxo-TMP concentration=45 mm; irradiation
time=15 min. Linear fit of data; slope=0.3198, intercept=1.7913,
r2 =0.9920.
1
AC8 concentration effect on O2 production, further irradiation
experiments were performed in the 0–2000 mgLÀ1 AC8 con-
centration range.
The observed trend of the signal intensity (Figure 5)
suggests that 1O2 production is directly proportional to AC8
concentration. Other experiments were performed to evidence
Figure 7. Intensity of DMPOÀOH EPR signal intensity versus AC8 concentra-
tion. DMPO concentration=17.4 mm; irradiation time=3 min.
Photodegradation of AC8 is proven by the significant
decrease of the UV/Vis absorbance with irradiation time
(Figure 8). This process involving radical species may well con-
tribute to the decrease of hydroxyl radicals available for the
Figure 6. EPR spectrum of DMPOÀOH adduct EPR. AC8 concentra-
tion=30 mgLÀ1; DMPO concentration=17,4 mm; irradiation time=3 min.
the formation of OH radicals. The typical EPR spectrum of the
DMPOÀOH spin-adduct (Figure 6) formed according to
Scheme 2 is a 1:2:2:1 quartet, with a 14.8 G hyperfine splitting
constant. Due to the overall poor signal/noise ratio, for quanti-
tative analysis the line showing the highest signal/noise ratio
was chosen as an analytical tool and the number of accumulat-
ed scans was increased without changing the scan rate or re-
cording time. This procedure led to a significant improvement
of the signal/noise ratio (Figure 6, inset).
Figure 8. UV/Vis spectrum evolution of AC8 (150 mgLÀ1) at different irradia-
tion times recorded after diluting the samples by a factor of 10. Inset: AC8
abatement versus irradiation time.
1
Compared to the O2 detection (Figure 4), for hydroxyl radi-
cal determination, the effect of irradiation time on DMPOÀOH
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Bioorganic Substances for Generation of Reactive Oxygenated Species
DMPOÀOH adduct formation and thus cause the behavior
observed in Figure 7.
4-Chlorophenol photodegradation
4-CP was chosen as a model compound to investigate the exis-
tence of a correlation between the kinetics of its photodegra-
1
dation, the O2 and OH radical production, and the addition of
1O2 and OH radical scavengers.
Preliminary experiments performed on 4-CP aqueous solu-
tions, irradiated up to six hours in the absence of AC8, showed
that direct photolysis of 4-CP does not occur to an appreciable
level (Figures 9 and 10). Afterwards, the substrate (5 mgLÀ1)
Figure 10. Degradation of 4-CP (5 mgLÀ1) in the presence of AC8
(1200 mgÀ1), 2-propanol (0.01m) and sodium azide (0.03m).
·
1
could be due to both OH and O2 (Figures 5 and 7). However,
only sodium azide seemed to significantly inhibit the photode-
gradation process, whereas the addition of 2-propanol had no
appreciable effect at this concentration. The sodium azide
1
effect seems consistent with the high O2 production, about
40 times higher than that at 100 mgLÀ1 AC8 concentration.
The negligible effect of 2-propanol at 1200 mgLÀ1 AC8 concen-
·
tration may suggest that, in this case, OH does not play a sig-
nificant role in 4-CP degradation and indirectly confirms that
singlet oxygen is mainly responsible for 4-CP degradation.
These results suggest that the role played by OH radicals and
singlet oxygen in 4-CP degradation is strictly related to their
relative production and depends on the AC8 concentration.
Figure 9. Degradation of 4-CP (5 mgLÀ1) in the presence of AC8
(100 mgLÀ1), 2-propanol (0.01m) and sodium azide (0.03m).
was irradiated in the presence of AC8 at two different concen-
1
Conclusion
trations; 100 mgLÀ1, whereby low O2 production but the high-
·
est OH production take place (Figures 5 and 7, respectively),
Information about radical species involved in the AC8 photo-
sensitizing mechanism and evidence of its good performance
as a photosensitizer in 4-CP photodegradation have been ob-
tained. The results offer scope for extending the investigation
to other BOS materials obtained from different UW sources
(Figure 1) to assess possible source–property relationships of
BOS photosensitizers, and thus to help focus product applica-
tion to specific cases. Together with previous results,[1–5] this
study further supports the use of urban bio-wastes as a
versatile source of chemical auxiliaries of biological origin for
use in diversified applications.
1
·
and 1200 mgLÀ1, whereby the ratio between O2 and OH pro-
duction is greatly increased. At 100 mgLÀ1 AC8 concentration,
in the absence of scavengers, after 24 h irradiation about 65%
·
of 4-CP is degraded (Figure 9). Based on the relatively high OH
production (Figure 7) and low 1O2 production (Figure 5), it
·
could be hypothesized that OH is mainly responsible for the 4-
CP degradation.
·
The addition of 2-propanol, a OH scavenger, led to signifi-
cant reduction of the 4-CP abatement from 65% to 33%, in
agreement with the previous hypothesis. However, in the pres-
ence of sodium azide, a 1O2 scavenger, the 4-CP abatement
decreased from 65% to 15%. Since sodium azide is mainly re-
1
Experimental Section
ported as s highly selective O2 scavenger but is also known to
react with OH radicals, this result could be explained if both
Materials
1
·OH and O2 play an active role in the 4-CP degradation, and
5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), 2,2,6,6-tetramethyl-4-
piperidone hydrochloride (4-oxo-TMP), 4-CP, 2-propanol, sodium
azide, acetonitrile, and potassium hydroxide were purchased from
Aldrich. Ultrapure water was obtained from a Millipore Milli-QTM
system. The AC8 bioorganic substance was extracted as previously
reported[8] by aqueous alkali followed by precipitation at pH<1.5
with HCl from refuse supplied by ACEA Pinerolese. This company
runs a waste treatment plant performing anaerobic digestion and/
the higher effect of sodium azide could thus be explained on
·
the basis of its additional OH-scavenging property.
The 24 h irradiation of 4-CP in the presence of 1200 mgLÀ1
AC8 concentration and in the absence of scavengers, yielding
a 75% abatement, is slightly more efficient than that at the
lower AC8 concentration (Figure 10). In agreement with the re-
sults obtained with 100 mgLÀ1of AC8, the 4-CP degradation
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89

A. Bianco Prevot et al.
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supplier by 35.2% humic and 46.0% w/w (referred to dry matter)
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All samples (5 mL) were placed in Pyrex closed cells and irradiated,
under continuous stirring, with a 1500 W xenon lamp (Solarbox,
Co. Fo. Megra, Milan, Italy) equipped with a 340 nm cut-off filter.
EPR spectra were registered at room temperature with a Bruker
ESR 300E spectrometer operating at X-band and equipped with a
flat quartz EPR cell. The acquisition parameters were as follows:
Frequency=9.69 GHz, microwave power=5.024 mW, center field=
3450 G (1O2) or 3440 G (·OH), sweep width=80 G (1O2) or 20 G
(·OH), number of scans=62 (1O2) or 120 (·OH), receiver gain=1ꢀ
105, modulation amplitude=0.52 G, conversion time=40.96 ms
(1O2) or 10.24 ms (·OH). 4-oxo-TMP (45 mm) and DMPO (17.4 mm)
·
were employed as trapping agent for 1O2 and OH study, respec-
tively (Schemes 1 and 2). All experiments were carried out by
adding the spin trap to the cell before irradiating AC8. EPR spectra
were acquired immediately after the irradiation. The degradation
of 4-CP was monitored by HPLC, employing a Merck-Hitachi instru-
ment, equipped with Lichrospher RP-C18 (125 mmꢀ4 mm i.d., par-
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