- Email: [email protected]
Degradation of Acid Orange 7 through radical activation by electro-generated cuprous ions
Journal Pre-proof Degradation of Acid Orange 7 through radical activation by electro-generated cuprous ions Zuo Tong How, Daniel John Blackwood
PII:
S2213-3437(19)30573-1
DOI:
https://doi.org/10.1016/j.jece.2019.103450
Reference:
JECE 103450
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
24 June 2019
Revised Date:
27 August 2019
Accepted Date:
30 September 2019
Please cite this article as: How ZT, Blackwood DJ, Degradation of Acid Orange 7 through radical activation by electro-generated cuprous ions, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103450
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Degradation of Acid Orange 7 through radical activation by electro-generated cuprous ions Zuo Tong How and Daniel John Blackwood*
Department of Material Science and Engineering, National University of Singapore, Singapore, Singapore 119077
ro
of
*Corresponding author: Daniel John Blackwood, Email: [email protected]
Jo
ur na
lP
re
-p
Graphical abstract
1
of ro -p re lP ur na
Highlight:
Degradation of AO7 by PDS activated from in situ electrochemically generated Cu+
EC-Cu-PDS have higher degradation efficiency of 70% as compare to 56% of EC-Fe-
Jo
PDS
EE/O of the EC-Cu-PDS processes ranged from 0.008 to 2.42 kWh m-3
Optimal operation condition was pH 2 with cathode to anode area of 4:1
SO4•− generated by both electrochemically at the anode and Cu+ activation of PDS
2
Abstract In this paper, the degradation of Acid Orange 7 (AO7), by persulfate activated from in situ electrochemically generated Cu+ (EC-Cu-PDS) was studied. The influence of key parameters such
of
as pH, current density, copper concentration and electrode size were investigated, with the optimal operation condition being pH 2 with cathode to anode area of 4:1. The degradation efficiency of
ro
the EC-Cu-PDS after 30 min was compared to the efficiency of other electrocatalysis system using
-p
iron, cobalt or silver. The proposed EC-Cu-PDS process has a higher degradation efficiency of 70%, as compare to 56% of the EC-Fe-PDS process. Although the EC-Cu-PDS process has a lower
re
degradation efficiency compared to 100% of Co-PMS and 80% of the EC-Ag-PDS processes, the EC-Cu-PDS process would be cheaper and safer to operate than the latter processes. The estimated
lP
electrical energy consumption of the EC-Cu-PDS processes was significantly lower than other electrocatalysis processes, ranging from 0.008 to 2.42 kWh m-3. The degradation of AO7 by the
ur na
EC-Cu-PDS process was via oxidization of AO7 by both hydroxyl radicals generated at the anode
Jo
and sulfate radicals generated by both electrochemically at the anode and Cu+ activation of PDS.
Keywords: Acid orange 7, Persulfate, Sulfate radical, Electrochemical advanced oxidation process, In situ chemical oxidation
3
1. Introduction Azo dye such as Acid Orange 7 (AO7, also known as Orange II) (Fig. S1) is a synthetic dye widely used in numerous applications [1, 2]. The azo dye is designed to be resistant to biological, chemical and photochemical degradation [1, 3, 4], being hardly degraded by conventional wastewater treatment processes [5], resulting in adverse impact to the environment and public health [6]. Advanced oxidation processes (AOPs) that utilized the highly reactive hydroxyl radical (•OH),
of
which is more efficient and non-selective as compared to conventional oxidants, have been
ro
investigated for their removal of persistent organic contaminants, including azo dye [1, 2, 7-9]. The sulfate radical-based oxidation processes is an emerging AOP that has received
-p
attention due to its effective destruction of persistent organic contaminants [10-12]. Sulfate
re
radicals (SO4•−) are more effective in the mineralization of organic acids than hydroxyl radicals, due to their higher redox potential; SO4•− (E◦= 2.60–3.10 V vs SHE), •OH (E◦= 1.90–2.70 V vs
lP
SHE) [13, 14]. Table S1 shows a list of relevant standard reduction potentials and chemical reactions.
ur na
Generally, SO4•− is generated from either peroxymonosulfate (PMS) or peroxydisulfate (PDS) (commonly known as persulfate) ions, through thermal activation [15], UV activation [1618], sonication [19] or transition metals activation [20-22]. Among these methods, the transition metal activation of SO4•− from PMS ions has demonstrated the highest efficiency for contaminant
Jo
degradation [23]. Transition metal activation of PMS and PDS ions is achieved in homogeneous and heterogeneous systems through a Fenton like reaction, similar to production of •OH from hydrogen peroxide [20] (Eq. 1-3). Mn+ + H2O2 → M(n+1)+ + •OH + OH-
(1)
Mn+ + S2O82- → M(n+1)+ + SO4•− + SO42-
(2)
4
Mn+ + HSO5- → M(n+1)+ + SO4•− + OH-
(3)
Among the transition metals, iron, as Fe2+ ions, is most commonly used for •OH generation from hydrogen peroxide and SO4•− from PMS or PDS. However, iron salts are only soluble in acidic conditions and the by-product from the activation are insoluble, having to be removed from the treatment system, so fresh Fe2+ ions are needed for the reaction to continue [24]. To reduce Fe2+ consumption, it can be regenerated by reduction of Fe3+ to Fe2+ using electrochemical methods
of
[25, 26].
ro
Anipsitakis and Dionysiou (2004) [20] found that while silver was the most effective transition metal for activation PDS to generate of SO4•−, cobalt was the most for PMS activation.
-p
A further benefit of using cobalt is that Co2+ could be regenerated from Co3+ in an aqueous system,
re
which minimized the concentration of Co2+ required, it is also far cheaper than silver. However, cobalt ions are classified as carcinogens and therefore have to be remove from the treated water
lP
before discharge [27].
Another transition metal that has been investigated for the generation of •OH or SO4•−, is
ur na
copper. Most studies on the use of copper for the generation of SO4•− were on heterogeneous activation of PMS or PDS by copper-based oxides, with the oxidation state of copper at either +1 or +2 [12, 22, 28, 29]. In addition, aqueous Cu2+ [30] and metallic Cu nanoparticles [31] have been found to activate PMS/PDS. Although Cu+ can effectively activate PDS, it is rarely studied,
Jo
especially for homogenous activation, as it is easily oxidized by dissolved oxygen or other mild oxidants [32]. Possible methods to overcome the instability of Cu+ ions, include producing it in situ by reducing Cu2+, either by a reducing agent, such as hydroxylamine [32] or electrochemically [33-35]. However, to date, Cu+ produced in situ electrochemically has only been attempted for the activation of H2O2, but not the more powerful oxidizers PDS and PMS.
5
In this paper, we investigate the degradation of AO7 using sulfate radicals generated by the homogenous activation of PDS or PMS using Cu+ produced in situ from the cathodic reduction of Cu2+. The mechanism of the process is investigated and the charge efficiency determined. The difference in the removal efficiency of AO 7 between homogenous Cu2+, Fe2+, Co2+ and Ag+ with
of
and without electrolysis are also presented.
ro
2. Materials and Methods 2.1 Experimental procedures and analysis.
-p
The test solutions consisted of AO7 with initial concentration of 0.1 mM. To this was added 1 mM of metallic salt, being either iron(II) sulfate, copper(II) sulfate, cobalt(II) chloride or silver(I)
re
nitrate along with 4 mM of either potassium persulfate (PDS) or potassium peroxymonosulfate
lP
(PMS, commonly known as Oxone™). The solution was buffered with 10 mM phosphate (K2HPO4/KH2PO4), with sulfuric acid or sodium hydroxide used to adjust to the required pH of 2,
ur na
3, 7 or 10.
Batch experiments were carried out in an undivided three electrode electrochemical cell containing 100 mL of electrolyte. Carbon cloth (2225 Type 900 Activated Carbon Fabric, Spectracarb) of dimensions of 1 × 1 cm, unless otherwise stated, was used as both the working and
Jo
counter electrodes. Carbon cloth was chosen as it is inert, has a large overpotential for water splitting and is cheaper than other carbon-based electrodes. The reference electrode was a saturated calomel electrode (+0.241 V vs SHE, CH Instrument), but all potentials reported have been converted to the standard hydrogen electrode (SHE) scale. The experiments were performed in duplicate with errors within 5%.
6
Experiments were performed at 22.5 ±1 °C and a direct current density of 1 mA cm-2, useless stated otherwise, was applied via a galvanostat (SP-200, Bio-logic). The reaction solutions were continuously stirred using magnetic stirrer. Aliquots (200 µL) were withdrawn at t = 0 and fixed intervals of 3 min until 30 min. The AO7 concentration was measured by UV-visible absorbance at wavelength = 485 nm (UV-1800, Shimazu UV-vis spectrometer). The carbon cloth electrodes were rinse with deionized water after every batch and dried in an oven at 60 ◦C for 45
ro
of
min.
-p
3. Results and Discussion 3.1 Effect of current density.
re
Table 1 shows the removal efficiency (%) of AO7 after 30 min at different current densities for a
lP
test solution at pH 3 with and without Cu2+ and peroxydisulfate. In the absence of both these reagents, i.e. with just anodic oxidation, the removal efficiency of the AO7 after 30 min decreased with decreasing current density; from 47% at 20 mA cm-2 down to 19% at 0.25 mA cm-2. This
ur na
reduction of removal efficiency was expected as anodic oxidation of the AO7 can occur at the anode’s surface either directly by electron transfer or indirectly by •OH weakly physiosorbed at the anode surface [36]. The reaction rates of both routes were proportional to the applied current
Jo
density. The potential of the anode also dropped from 4.2 V vs SHE at 20 mA cm-2 to 0.7 V vs SHE at 0.25 mA cm-2, which would also have reduced the number of •OH radicals formed. When Cu2+ was added into the system, but not peroxydisulfate, at 20 mA cm-2 the removal
efficiency after 30 min reduced to only 3% (Table 1). This reduction of removal efficiency is likely due to the consumption of most of the reactive •OH radicals generated at the anode by the Cu+ generated at the cathode [34, 35], thus the •OH were no longer available to oxidize the AO7. When 7
both Cu2+ and PDS was added into the system, the removal efficiency improved at all current densities, being 55% at 20 mA cm-2, 61% at 1 mA cm-2, 54% at 0.5 mA cm-2 and 51% at 0.25 mA cm-2 (Table 1). When compared to the removal efficiencies without either of these reagents, the improvements are 8%, 24%, 22% and 32% for 20, 1, 0.5 and 0.25 mA cm-2 respectively. The smaller improvement at 20 mA cm-2, reflects the fact that at this very high current density electrolysis of water occur, with oxygen being generated at the anode which obtained a potential
of
of 4.2 vs SHE. As the generated oxygen oxidize Cu+ to Cu2+, there will be less Cu+ available for
ro
the activation of PDS. No oxygen was evolved at the lower current densities, where the anode potentials were 1.5, 1.1 and 1.0 V vs SHE for 1.0, 0.5 and 0.25 mA cm-2 respectively. Although
-p
thermodynamically oxygen can be evolved at 1.11 V vs SHE, carbon is a poor catalyst for this
re
reaction with typically an overpotential of 500 mV, so significant electrolysis would not be expected below 1.6 V vs SHE [25].
lP
The charge efficiency was calculated using: 𝜂=
Fn 𝑞
(4)
ur na
where η is the charge efficiency (%), F is the Faraday constant 96485.33289 C mol-1, n is the number of equivalent moles of AO7 removal per mole of electrons and q is electrical charge (C). Equation (5) shows that four electrons are required to break the azo bond in AO7 to produce
Jo
sodium sulphanilamide and 1-amino-2-naphthol, such that n has a value of 4 (Fig. S1). Note that although it takes 84 electrons to fully mineralize one AO7 molecule (Eq. 6) the UV-visible absorbance technique is effectively monitoring the number of azo bonds broke, C16H11N2O4SNa + 4H+ + 4e- → C6H6NO3SNa + C10H9NO
(5)
C16H11N2O4SNa → CO2 + Na+ + SO42- + 2NO3- + 87H+ + 84e-
(6)
8
Table 1 shows that although the overall removal efficiency of AO7 decreased with decreasing current density, the charge efficiency had the opposite trend, i,e, it increased with decreasing current density, being only 5.9% at 20 mA cm-2 but an impressive 436% at 0.25 mA cm-2 . Charge efficiencies above 100% are possible due to auto oxidation by the radical intermediates form during the oxidation of AO7 by the SO4•− [36]. The low charge efficiency at the highest current density indicates the occurrences of side reactions, such as oxygen evolution,
of
which limit the availability of Cu+ for PDS activation.
ro
A higher charge efficiency could indicate lower energy was required to remove the same percentage of AO7. Therefore, for a persulfate activated from in situ electrochemically generated
-p
Cu+ (EC-Cu-PDS) system the optimal condition for the removal of AO7 would be a balance
re
between higher current density to provide good anodic oxidation for overall removal efficiency and a as low as possible current density to minimize the required energy. These experiments show
lP
that while a lower current density would generate more Cu+ for the activation of PDS, it would also result in reduced anodic oxidation reducing the overall removal efficiency of AO7.
ur na
Furthermore, carbon cloth is able to activate the PDS directly for the removal of AO7 (discussed in detail at section 3.2), a process that would mask the electrochemical route if too low a current density is applied.
Jo
3.2 Electrical energy consumption estimation. In order to further evaluate the electrical energy efficiency of the electrochemical plus Cu2+ plus PDS (EC-Cu-PDS) process, to allow comparisons with similar processes, an estimation of the
9
electrical energy consumption per order of magnitude (90%) removal in 1 m3 of contaminated water (EE/O) in term of kWh m-3 was conducted using the following formula [1]: 𝐸𝐸/𝑂 =
𝑈𝐶𝑒𝑙𝑙 𝐼𝑡
(7)
𝐶 𝑉𝑙𝑜𝑔( 0 ) 𝐶
where Ucell is the average cell voltage (V), I is the electrical current (A), t is the reaction time (hours) and V is the volume (liters).
of
Table 2 presents the estimated electrical energy required for the present EC-Cu-PDS process and similar techniques in the literature for the removal of AO7. This shows that the EE/O
ro
for the EC-Cu-PDS process decreases linearly with decreasing current density (j), from 2.42 kWh
-p
m-3 order-1 at j = 20 mA cm-2 to 0.008 kWh m-3 order-1 at j = 0.25 mA cm-2. Table 2 reveals that even the least efficient EE/O for the EC-Cu-PDS process, 2.42 kWh m-3 order-1 at j = 20 mA cm-2,
re
is significantly lower than similar systems reported in the literature; such as 19.7 kWh m -3 order-1 for electro-oxidation with iron (II,III) oxide (EC-Fe3O4) [37], 8.69 kWh m-3 order-1 for electro-
lP
oxidation with iron (II,III) oxide and persulfate (EC-Fe3O4-PDS) [1] and 7.84 kWh m-3 order-1 for electro-oxidation with manganese (IV) oxide and persulfate (EC-MnO2-PDS) [38]. One reason for
ur na
the better EE/O in the EC-Cu-PDS process is likely to be because it is a homogenous system, where the electron transfer between the electrodes, copper ions and PDS would more efficient as compared to the heterogeneous literature systems, where electron transfer involves insoluble metal
Jo
oxides.
Another plausible reason for the difference in EE/O is the choice of anode materials (Table
2). In this study, carbon cloth electrodes were used rather than the Ti/ RuO2-IrO2 used in literature processes. Carbon cloth has a higher oxygen evolution overpotential (0.5 V) as compared to Ti/ RuO2-IrO2 (0.2 V) [39] and it has been reported that electrodes with a higher oxygen evolution
10
overpotential generally have a higher anodic oxidation efficiency [40] and therefore required lower electrical energy to achieve higher degradation of AO7. It should be noted that although the processes used by Xu et al. 2017 [38] had a faster AO7 degradation rate of 8.8 × 10-4 s-1, compared to 5.7 × 10-4 s-1 of this study, their EE/O was more than
of
an order of magnitude higher for similar conditions, i.e. the present work is far more efficient.
3.3 Effect of copper concentration and cathode size.
ro
In order to investigate the impact of copper concentration on the kinetics and efficiency of AO7degradation using EC-Cu-PDS system, a blank and five different concentration of copper (II)
-p
sulfate: 0.0, 0.05, 0.1, 0.5, 1.0 and 4.0 mM, were used. The current density (1 mA cm-2), pH 3 and
re
persulfate concentration (4 mM) were kept constant. Table S2 shows that the pseudo-first order rate constant for the degradation of AO7 increased with increasing Cu2+ concentration, being 2.70
lP
× 10-4 s-1 for the control and 6.20 × 10-4 s-1 with 4 mM, at the same time the removal efficiency after 30 mins increased from 37% to 65% (Fig. 1a).
ur na
The removal efficiency with the lowest Cu2+ concentration (0.5 mM) at 37% is the same as the control with no copper ions. A possible reason is that all the Cu+ ions generated were reoxidized back to Cu2+ ions by dissolved oxygen (~ 0.25 mM). Therefore, when 0.05 mM of Cu was added into the system, insufficient Cu+ survived long enough to activate any significant
Jo
concentration of SO4•−. The more favorable reaction with dissolved oxygen than PDS would explain the sharp increase in reaction rate from 2.78 × 10-4 s-1 at 0.05 mM of Cu2+ to 4.04 × 10-4 s1
at 0.1 mM of Cu2+. At 0.1 mM the Cu+ generated would first be consumed by the dissolved
oxygen, and the excess Cu+ available would activate the PDS and thus a sharp increase in reaction rate and removal efficiency were observed.
11
The initial dissolved oxygen content would be virtually identical in all the test solutions under the same pH, temperature and pressure. Therefore, once enough Cu2+ was available to generate sufficient Cu+ ions to overcome the oxygen’s influence, the reaction rate increased with Cu2+ concentration, until it started to plateau at 1.0 mM, with the removal efficiency only increasing by a further 4% at 4.0 mM (Table S2). The plateau in the removal efficiency was possibly limited by the generation of Cu+ from
of
the cathodic reduction of Cu2+. To test this, the influence of the cathode size was investigated. Four
ro
different cathode sizes of 1, 1.69, 2.25 and 4 cm2 were tested at a constant current of 1 mA (i.e. 1 mA cm-2 with respect to the anode that was maintained at 1 cm2) and same initial Cu2+, AO7 and
-p
PDS concentrations of 1, 0.1 and 4 mM respectively. Fig. 1b shows that the rate of removal of
re
AO7 increase linearly with increasing cathode size, reaching 99% efficiency by 30 min with the 4 cm2 cathode. Since the anode size was kept constant at 1 cm2 the increase in removal efficiency of
lP
AO7 on increasing the cathode size cannot be from an increase of anodic oxidation but instead from the increase in cathodic reduction of Cu2+ to Cu+, which in turn increased the rate of SO4•−
ur na
generation.
3.4 Effect of pH and reaction mechanism. Fig. 2 shows the influence of pH on the removal efficiency of AO7 by anodic oxidation at an
Jo
anodic current density of 1 mA cm-2 in the absence and presence of both PDS and Cu2+. In the absence of these two reagents the removal efficiency of AO7 at pH 7 was 35%, which is similar to the 37% reported above for pH 3 (Table 1) and 32% for pH 2, but at pH 10 the removal efficiency dropped to only 16%. This drop in removal efficiency might be due to the lower oxidizing strength of •OH in basic media [41, 42]. Therefore, the observation that in the absence of Cu2+ and PDS the
12
removal efficiency is almost independent of pH over the range pH 2 to pH 7, but drops significantly at pH 10 suggests that the main mechanism of the anodic oxidation of AO7 at a carbon cloth anode is the indirect oxidation by •OH radicals, rather than the direct electron transfer from the AO7 at the anode surface. When Cu2+ and PDS was added into the system (Fig. 2b), a stronger pH dependency of the removal efficiency of AO7 was observed. As the pH was increased from pH 2, pH 3, pH 7 to
of
pH 10 the removal efficiency decreased in the order 70%, 61%, 52% down to 6%. The small
ro
decline from pH 2 to pH 7 might be partly due to the precipitation of insoluble copper (II) hydroxide (Cu(OH)2: solubility =1.722 × 10-6 g·mL-1 at 20 ◦C) but more likely, a lowering of the
-p
electrochemical activation of PDS is responsible (see section 3.3). The dramatic fall in the removal
re
efficiency that occurred on going from pH 7 to pH 10 is due to the formation of insoluble Cu(OH)2, such that there are insufficient Cu+ ions for the activation of the PDS to generate the SO4•−required
lP
to oxidize the AO7. Although alkaline activation of PDS has been reported [43, 44], this was at pH 12 or 1.5 to 3 M of sodium hydroxide with 0.5 M PDS, where the hydroxyl and PDS
ur na
concentration would be two to four magnitudes times higher than the 4 mM PDS and 0.1 mM sodium hydroxide (pH 10) solution used in this study. Closer inspection of Fig. 2b reveals that at pH 10 there is an initial drop in the AO7 concentration and its concentration increases during the first 6 min; the t = 0 is actually less than
Jo
the starting concentration, i.e. some of the AO7 was lost as soon as the test solution was poured into the electrochemical cell. This initial drop in AO7 concentration is thought to be cause by the coagulation of AO7 by Cu(OH)2 that readily forms at pH 10. However, on application of the current the electrical static interactions between Cu(OH)2 and AO7 are broken and so the AO7 is released back into the system resulting in an apparent increase in AO7 concentration during the
13
first 6 minutes. Note that although the increase in pH had an adverse impact on the degradation of AO7 by the EC-Cu-PDS system, the formation of Cu(OH)2 at neutral and basic conditions could be utilize for the removal of copper from the system when eventually required. It has been reported by Yang et al. (2011) [45] that carbonaceous activation of PDS is possible, so this may occur directly at the carbon cloth electrodes without the need to involve Cu+ ions. Fig. 2c thus shows the influence of pH on the removal of AO7 at carbon cloth in the presence
of
of 4 mM PDS, but without the addition of any Cu2+ ions nor the application of any current density.
ro
Fig. 2c shows that the AO7 removal efficiency of the PDS activated by the carbon cloth increased with decreasing pH. The increased in PDS activation by the carbon cloth as the solution becomes
-p
more acidic might be due to the increased in hydrophilicity of the solution and thus having a shorter
re
retention on the hydrophobic carbon cloth, allowing a faster exchange of the PDS on the surface. This is aligned with observations by Yang et al., (2011) [45] that an increase in absorptivity of the
lP
solution reduced the activation of PDS. The mechanism of AO7 oxidation with PDS and carbon cloth without electrolysis is illustrated in Fig. 3a; the AO7 is oxidized by radicals that are
ur na
generated by the dissociation of the PDS at the carbon cloth. Figure 2d shows the same set of experiments as in Figure 2c, except now a current density of 1 mA cm-2 was applied to the system. It can be seen that although the current density improved the removal efficiency at pH 10, this is not the case for the neutral and acidic solution where the
Jo
removal efficiency declined significantly. From example, at pH 2 the removal efficiency declined from 63% without electrolysis to 43% with electrolysis. A plausible explanation for the difference in removal efficiency is that electrolysis modifies the interactions between the carbon cloth and the PDS in the solution. The application of the electrical current would cause the carbon cloth electrodes to be charged (regardless of whether these are the anode or the cathode) and thus become
14
more hydrophilic, which in turn would increase the retention of the solution at the carbon cloth resulting in lower PDS activation and AO7 removal efficiency due to lower diffusion of the PDS. However, if the difference in removal efficiency was only due to the change in hydrophobicity of the carbon cloth, a reduction in removal efficiency would also have been expected at pH 10. Since PDS can be activated by electrolysis [46, 47], when the electrical current is applied the PDS is likely to be electrochemically activated to SO4•− radicals at the anode alongside
of
electrochemically generated •OH. The electrochemical activation of PDS and direct generation of •
ro
OH radicals would explain the higher removal efficiency in acidic media than in basic media, as
the oxidation power •OH radicals is known to decrease with increasing pH13. However, under the
-p
acidic conditions the •OH and SO4•− radicals generated at the anode are not sufficient to replace
re
the loss in SO4•− generated by the direct activation of the PDS at the carbon cloth that occurs without applied current density, so application of the current still leads to an overall decrease in
lP
AO7 removal efficiency. With respect to the pH 10 solution, because the direct activation of the PDS at the carbon cloth is so much less efficient under basic conditions (Fig. 2c), it is easier for
ur na
the SO4•− radicals generated at the anode to make up for its loss. Chen et al., (2018) [48] reported that SO4•− generation by electrolysis is more efficient at under basic conditions, so application of the current leads to an overall increase in AO7 removal. The results therefore suggest that when electrical current was applied, the carbonaceous activation of PDS was reduced and
Jo
electrochemical activation of PDS became the dominant pathway for the activation of PDS. The mechanism of AO7 oxidation with EC-PDS is illustrated in Fig. 3b; the AO7 is oxidized by •OH and SO4•− radicals generated directly at the anode. The mechanism of AO7 oxidation by the EC-Cu-PDS process (Fig. 3c) would be a combination of the EC-PDS process with transition metal activation of PDS. That is the AO7 is
15
oxidized by •OH and SO4•− generated directly at the anode and by SO4•− generated by the activation of PDS by the Cu+ generated from the cathodic reduction of Cu2+. This combination of multiple oxidation pathways in the EC-Cu-PDS process resulted in the highest removal efficiency of 70% (at pH 2) as compared to the highest of 40% for anodic oxidation, 60% for carbon cloth (carbonaceous) activation of PDS and 45% for EC-PDS process. It would be interesting to ascertain the contribution rates of the species involved in the multiple pathways towards AO7
of
degradation, but the in situ electrochemical EPR experiments (necessary to achieve this are
ro
difficult to perform) [49].
-p
3.5 Comparison of four transition metals for electrocatalysis persulfate system.
re
To understand the suitability of using copper for the activation of persulfate, as compare to the normally used iron, cobalt (reported to be the most effective in activation of PMS) [20] or silver
lP
(reported to be the most effective in the activation of PDS) [20], a series of experiments were carried out using these four transition metals as Fe2+, Co2+, Ag+ and Cu2+ (1mM); and PDS or PMS
ur na
with and without electrolysis at 1 mA cm-2 in pH 2 solutions for the removal AO7 were conducted. Figs 4a and 4b show that when the four transition metals were used to activate PDS. No degradation of AO7 was observed within 30 min when only PDS was added (control, Fig. 4a). It can be seen that cobalt performed the worst, with an AO7 removal efficiency of only 1% without
Jo
electrolysis and 40% when coupled with electrolysis. In the absence of an applied current, Fe2+ initially rapidly degraded AO7 achieving 40% removal efficiency by 3 min, then followed by a slow degradation reaching 50% by 30 min. The slow degradation observed after the third minute is likely due to most of the reactive Fe2+ having being oxidized to Fe3+. The application of a current to the ferrous system had only a minor impact, slightly improving the removal efficiency to 56%
16
by 30 min. This is because Fe2+ cannot be cathodically regenerated from Fe3+ efficiently due to precipitation of insoluble ferric sulfate (Fe2(SO4)3) and ferric hydroxide (Fe(OH)3). In addition, the fact that little removal of AO7 was observed after 3 min indicates the formation of the insoluble salts had a negative impact on the anodic oxidation of AO7. This might be due to fouling of the carbon cloth electrode by the Fe2(SO4)3, thus the removal of AO7 was by the slower electrocoagulation rather than anodic oxidation.
of
When Ag+ was used, without applying a current density the removal efficiency increased
ro
almost linearly with time (Fig. 4a), but was only 36% after 30 min as compared to 50% when using iron. However, if a longer time had been used it is likely that the silver would overtake the
-p
iron as the rate of degradation using Ag+ increases with time (Fig. 4a). When 1 mA cm-2 was
re
applied to the silver system it gave the best performance at all times, reaching a removal efficiency of 80% at 30 mins. Finally, when copper was used, the removal efficiency was only 3% by 30 min,
lP
but when combined with electrolysis the efficiency increased to 70% after 30 min, second only to the silver system. The mechanism for the copper system has already be discussed.
ur na
Figs. 4c and 4b show the results obtained when PMS was used to generate the SO4•− radicals, again without and with the application of 1 mA cm-2. No degradation of AO7 was observed within 30 min when only PDS was added (control, Fig. 4c).The findings are consistent with the literature [14, 20], with cobalt being very effective in the removal of AO7 even without
Jo
the application of an electrical current. The cobalt system reached 50% removal within 6 min and 100% by 15 min, due to the continuous regeneration of the active Co2+ from Co3+. When cobalt was used with electrolysis, the time required fully degrade the AO7 reduced from 15 min to 6 min, reaching 50% removal within a minute.
17
When iron was used for the activation of PMS, the results were similar to that when PDS had been used, the AO7 degraded rapidly within the first 3 min to 50% without electrolysis and 55% with electrolysis and a slow removal rate after the first 3 min reaching 54% without electrolysis and 61% with electrolysis. When silver was added with PMS, only 2% removal of AO7 was observed, which was expected as Ag+ has been reported to be not effective in the activation of PMS.18 When the silver was coupled with electrolysis the removal efficiency was
of
increased to 30% after 30 mins. For the copper PMS system, the 30 min removal efficiency was
ro
only 1% without the application of an electrical current, but increased to 64% wen the 1 mA cm-2 was applied.
-p
Overall, Fig. 4 shows that although the EC-Cu-PDS process has it disadvantage of having
re
a slower reaction rate as compared to EC-Fe-PDS, the copper system have a higher removal efficiency (70%) per mole of transition metal used as compared to the iron system (56%).
lP
Furthermore, the copper system continues to be active beyond the 30 min mark, whereas the iron systems removal capabilities have been exhausted. Likewise, when operated at its optimal
ur na
condition, the EC-Cu-PDS system has comparable removal efficiency to EC-Ag-PDS, but with the obvious advantage of copper being much cheaper than silver. Finally, even though the CoPMS system was significantly more efficient than EC-Cu-PDS system, the copper system is a much safer option, as cobalt is classified as carcinogen, plus copper is cheaper than cobalt.
Jo
Furthermore, the cost of PDS is lower than PMS [27].
4. Conclusion This study demonstrated that an EC-Cu-PDS process, where the active Cu+ is generated in situ at a carbon cloth cathode by reduction of Cu2+, was effective in activating PDS to SO4•− radicals for 18
the subsequent degradation of AO7. The optimum concentration of Cu2+ was found to be 1 mM and highest removal efficiencies were achieved under acidic conditions. With respect to current density, high values improved the removal efficiency but decreased the charge efficiency, with 1 mA cm-2 being found to be a good compromise. The removal efficiency increased with cathode area, reaching a maximum removal efficiency with a cathode to anode area ratio of 4:1. Although the removal efficiency of the EC-Cu-PDS process, at 70%, was lower than the
of
Co-PMS and of the EC-Ag-PDS processes, at 100% and 80% respectively, the new process would
ro
be cheaper to operate. In addition, cobalt is classified as carcinogen, while copper is not classified as a hazardous metal. Another advantage of the proposed EC-Cu-PDS process is that it is
-p
potentially cheaper than other electrocatalysis or electrochemical advance oxidation processes due
re
to its lower electrical energy consumption of less than 2.42 kWh m-3 order-1 (Table 2) and the used
Acknowledgements
lP
of relatively inexpensive and inert carbon cloth electrodes.
ur na
This research grant was supported by the Singapore National Research Foundation under its Environmental & Water Research Programme (Project Ref No. 1301-IRIS-33) and administered
Jo
by PUB, Singapore’s national water agency.
References
[1] H. Lin, H. Zhang, L. Hou, Degradation of C. I. Acid Orange 7 in aqueous solution by a novel electro/Fe3O4/PDS process, J. Hazard. Mater. 276 (2014) 182191.https://doi.org/10.1016/j.jhazmat.2014.05.021 [2] A. Özcan, M.A. Oturan, N. Oturan, Y. Şahin, Removal of Acid Orange 7 from water by electrochemically generated Fenton's reagent, J. Hazard. Mater. 163 (2009) 12131220.https://doi.org/10.1016/j.jhazmat.2008.07.088 19
Jo
ur na
lP
re
-p
ro
of
[3] H. Zhang, Y. Lv, F. Liu, D. Zhang, Degradation of C.I. Acid Orange 7 by ultrasound enhanced ozonation in a rectangular air-lift reactor, Chem. Eng. J. 138 (2008) 231238.https://doi.org/10.1016/j.cej.2007.06.031 [4] A. Sharma, M. Verma, A.K. Haritash, Degradation of toxic azo dye (AO7) using Fenton's process, Adv. Environ. Res. 5 (2016) 189-200.10.12989/AER.2016.5.3.189 [5] R. Ganesh, G.D. Boardman, D. Michelsen, Fate of azo dyes in sludges, Water Res. 28 (1994) 1367-1376.https://doi.org/10.1016/0043-1354(94)90303-4 [6] A. Babuponnusami, K. Muthukumar, A review on Fenton and improvements to the Fenton process for wastewater treatment, J. Environ. Chem. Eng. 2 (2014) 557572.https://doi.org/10.1016/j.jece.2013.10.011 [7] S. Bahnmüller, C.H. Loi, K.L. Linge, U.v. Gunten, S. Canonica, Degradation rates of benzotriazoles and benzothiazoles under UV-C irradiation and the advanced oxidation process UV/H2O2, Water Res. 74 (2015) 143-154.https://doi.org/10.1016/j.watres.2014.12.039 [8] D.P. Mohapatra, S.K. Brar, R.D. Tyagi, P. Picard, R.Y. Surampalli, Analysis and advanced oxidation treatment of a persistent pharmaceutical compound in wastewater and wastewater sludge-carbamazepine, Sci. Total Environ. 470-471 (2014) 5875.https://doi.org/10.1016/j.scitotenv.2013.09.034 [9] M. Zhou, Q. Yu, L. Lei, G. Barton, Electro-Fenton method for the removal of methyl red in an efficient electrochemical system, Sep. Purif. Technol. 57 (2007) 380387.https://doi.org/10.1016/j.seppur.2007.04.021 [10] H.V. Lutze, J. Brekenfeld, S. Naumov, C. von Sonntag, T.C. Schmidt, Degradation of perfluorinated compounds by sulfate radicals – New mechanistic aspects and economical considerations, Water Res. 129 (2018) 509-519.https://doi.org/10.1016/j.watres.2017.10.067 [11] J. Wu, H. Zhang, J. Qiu, Degradation of Acid Orange 7 in aqueous solution by a novel electro/Fe2+/peroxydisulfate process, J. Hazard. Mater. 215-216 (2012) 138145.https://doi.org/10.1016/j.jhazmat.2012.02.047 [12] Y. Zhang, Q. Zhang, J. Hong, Sulfate radical degradation of acetaminophen by novel iron– copper bimetallic oxidation catalyzed by persulfate: Mechanism and degradation pathways, Appl. Surf. Sci 422 (2017) 443-451.https://doi.org/10.1016/j.apsusc.2017.05.224 [13] P. Neta, R.E. Huie, A.B. Ross, Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution, J. Phys. Chem. Ref. Data 17 (1988) 1027-1284.10.1063/1.555808 [14] W.-D. Oh, Z. Dong, T.-T. Lim, Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects, Appl. Catal. B Environ. 194 (2016) 169-201.https://doi.org/10.1016/j.apcatb.2016.04.003 [15] Y. Ji, C. Dong, D. Kong, J. Lu, Q. Zhou, Heat-activated persulfate oxidation of atrazine: Implications for remediation of groundwater contaminated by herbicides, Chem. Eng. J. 263 (2015) 45-54.https://doi.org/10.1016/j.cej.2014.10.097 [16] Y.-H. Guan, J. Ma, X.-C. Li, J.-Y. Fang, L.-W. Chen, Influence of pH on the Formation of Sulfate and Hydroxyl Radicals in the UV/Peroxymonosulfate System, Environ. Sci. Technol. 45 (2011) 9308-9314.10.1021/es2017363 [17] M. Nihemaiti, D.B. Miklos, U. Hübner, K.G. Linden, J.E. Drewes, J.-P. Croué, Removal of trace organic chemicals in wastewater effluent by UV/H2O2 and UV/PDS, Water Res. 145 (2018) 487-497.https://doi.org/10.1016/j.watres.2018.08.052 [18] M. Verma, A.K. Haritash, Degradation of amoxicillin by Fenton and Fenton-integrated hybrid oxidation processes, J. Environ. Chem. Eng. 7 (2019) 102886.https://doi.org/10.1016/j.jece.2019.102886 20
Jo
ur na
lP
re
-p
ro
of
[19] C. Cai, H. Zhang, X. Zhong, L. Hou, Ultrasound enhanced heterogeneous activation of peroxymonosulfate by a bimetallic Fe–Co/SBA-15 catalyst for the degradation of Orange II in water, J. Hazard. Mater. 283 (2015) 70-79.https://doi.org/10.1016/j.jhazmat.2014.08.053 [20] G.P. Anipsitakis, D.D. Dionysiou, Radical Generation by the Interaction of Transition Metals with Common Oxidants, Environ. Sci. Technol. 38 (2004) 3705-3712.10.1021/es035121o [21] G. Lente, J. Kalmár, Z. Baranyai, A. Kun, I. Kék, D. Bajusz, M. Takács, L. Veres, I. Fábián, One- Versus Two-Electron Oxidation with Peroxomonosulfate Ion: Reactions with Iron(II), Vanadium(IV), Halide Ions, and Photoreaction with Cerium(III), Inorg. Chem. 48 (2009) 17631773.10.1021/ic801569k [22] H.-y. Liang, Y.-q. Zhang, S.-b. Huang, I. Hussain, Oxidative degradation of p-chloroaniline by copper oxidate activated persulfate, Chem. Eng. J. 218 (2013) 384391.https://doi.org/10.1016/j.cej.2012.11.093 [23] P. Devi, U. Das, A.K. Dalai, In-situ chemical oxidation: Principle and applications of peroxide and persulfate treatments in wastewater systems, Sci. Total Environ. 571 (2016) 643657.https://doi.org/10.1016/j.scitotenv.2016.07.032 [24] S. Malato, P. Fernández-Ibáñez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends, Catal. Today 147 (2009) 1-59.https://doi.org/10.1016/j.cattod.2009.06.018 [25] J.D. Benck, B.A. Pinaud, Y. Gorlin, T.F. Jaramillo, Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte, PLOS ONE 9 (2014) e107942.10.1371/journal.pone.0107942 [26] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton’s Reaction Chemistry, Chem. Rev. 109 (2009) 65706631.10.1021/cr900136g [27] T. Zhang, Y. Chen, Y. Wang, J. Le Roux, Y. Yang, J.-P. Croué, Efficient Peroxydisulfate Activation Process Not Relying on Sulfate Radical Generation for Water Pollutant Degradation, Environ. Sci. Technol. 48 (2014) 5868-5875.10.1021/es501218f [28] X. Du, Y. Zhang, I. Hussain, S. Huang, W. Huang, Insight into reactive oxygen species in persulfate activation with copper oxide: Activated persulfate and trace radicals, Chem. Eng. J. 313 (2017) 1023-1032.https://doi.org/10.1016/j.cej.2016.10.138 [29] Y. Feng, J. Liu, D. Wu, Z. Zhou, Y. Deng, T. Zhang, K. Shih, Efficient degradation of sulfamethazine with CuCo2O4 spinel nanocatalysts for peroxymonosulfate activation, Chem. Eng. J. 280 (2015) 514-524.https://doi.org/10.1016/j.cej.2015.05.121 [30] C.S. Liu, K. Shih, C.X. Sun, F. Wang, Oxidative degradation of propachlor by ferrous and copper ion activated persulfate, Sci. Total Environ. 416 (2012) 507512.https://doi.org/10.1016/j.scitotenv.2011.12.004 [31] P. Zhou, J. Zhang, J. Liu, Y. Zhang, J. Liang, Y. Liu, B. Liu, W. Zhang, Degradation of organic contaminants by activated persulfate using zero valent copper in acidic aqueous conditions, RSC Adv. 6 (2016) 99532-99539.10.1039/C6RA24431A [32] P. Zhou, J. Zhang, J. Liang, Y. Zhang, Y. Liu, B. Liu, Activation of persulfate/copper by hydroxylamine via accelerating the cupric/cuprous redox couple, Water Sci. Technol. 73 (2015) 493-500.10.2166/wst.2015.509 [33] S. Jacqueline George, R. Gandhimathi, P.V. Nidheesh, S.T. Ramesh, Optimization of salicylic acid removal by electro Fenton process in a continuous stirred tank reactor using response surface methodology, Desalin. Water Treat. 57 (2016) 4234-4244.10.1080/19443994.2014.992970
21
Jo
ur na
lP
re
-p
ro
of
[34] P.V. Nidheesh, R. Gandhimathi, Comparative Removal of Rhodamine B from Aqueous Solution by Electro-Fenton and Electro-Fenton-Like Processes, CLEAN – Soil, Air, Water 42 (2014) 779-784.10.1002/clen.201300093 [35] N. Oturan, M. Zhou, M.A. Oturan, Metomyl Degradation by Electro-Fenton and ElectroFenton-Like Processes: A Kinetics Study of the Effect of the Nature and Concentration of Some Transition Metal Ions As Catalyst, J. Phys. Chem. A 114 (2010) 10605-10611.10.1021/jp1062836 [36] M. Panizza, G. Cerisola, Direct And Mediated Anodic Oxidation of Organic Pollutants, Chem. Rev. 109 (2009) 6541-6569.10.1021/cr9001319 [37] H. Lin, L. Hou, H. Zhang, Degradation of Orange II in aqueous solution by a novel electro/Fe3O4 process, Water Sci. Technol. 68 (2013) 2441-2447.10.2166/wst.2013.513 [38] Y. Xu, H. Lin, Y. Li, H. Zhang, The mechanism and efficiency of MnO2 activated persulfate process coupled with electrolysis, Sci. Total Environ. 609 (2017) 644654.https://doi.org/10.1016/j.scitotenv.2017.07.151 [39] F.C. Moreira, R.A.R. Boaventura, E. Brillas, V.J.P. Vilar, Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters, Appl. Catal. B Environ. 202 (2017) 217-261.https://doi.org/10.1016/j.apcatb.2016.08.037 [40] C.A. Martínez-Huitle, L.S. Andrade, Electrocatalysis in wastewater treatment: recent mechanism advances, Química Nova 34 (2011) 850-858 [41] J.M. Campos-Martin, G. Blanco-Brieva, J.L.G. Fierro, Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process, Angew. Chemie Int. Ed. 45 (2006) 69626984.10.1002/anie.200503779 [42] J. Wang, S. Wang, Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants, Chem. Eng. J. 334 (2018) 15021517.https://doi.org/10.1016/j.cej.2017.11.059 [43] O.S. Furman, A.L. Teel, R.J. Watts, Mechanism of Base Activation of Persulfate, Environ. Sci. Technol. 44 (2010) 6423-6428.10.1021/es1013714 [44] S. Furman Olha, L. Teel Amy, M. Ahmad, C. Merker Marissa, J. Watts Richard, Effect of Basicity on Persulfate Reactivity, J. Environ. Eng. 137 (2011) 241-247.10.1061/(ASCE)EE.19437870.0000323 [45] S. Yang, X. Yang, X. Shao, R. Niu, L. Wang, Activated carbon catalyzed persulfate oxidation of Azo dye acid orange 7 at ambient temperature, J. Hazard. Mater. 186 (2011) 659666.https://doi.org/10.1016/j.jhazmat.2010.11.057 [46] H. Song, L. Yan, J. Jiang, J. Ma, Z. Zhang, J. Zhang, P. Liu, T. Yang, Electrochemical activation of persulfates at BDD anode: Radical or nonradical oxidation?, Water Res. 128 (2018) 393-401.https://doi.org/10.1016/j.watres.2017.10.018 [47] H. Song, L. Yan, J. Jiang, J. Ma, S. Pang, X. Zhai, W. Zhang, D. Li, Enhanced degradation of antibiotic sulfamethoxazole by electrochemical activation of PDS using carbon anodes, Chem. Eng. J. 344 (2018) 12-20.https://doi.org/10.1016/j.cej.2018.03.050 [48] L. Chen, C. Lei, Z. Li, B. Yang, X. Zhang, L. Lei, Electrochemical activation of sulfate by BDD anode in basic medium for efficient removal of organic pollutants, Chemosphere 210 (2018) 516-523.https://doi.org/10.1016/j.chemosphere.2018.07.043 [49] R.G. Compton, C.R. Greaves, A.M. Waller, A wall-jet electrode for in-situ electrochemical ESR, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 277 (1990) 8392.https://doi.org/10.1016/0022-0728(90)85092-J
22
of ro -p re lP ur na
Jo
Table 1: Removal efficiency (%) of AO7 after 30 min, the apparent rate constant for the degradation of AO7 and the charge efficiency with different current density (20, 1.0, 0.5 and 0.25 mA cm-2) at pH 3 with and without the reagents Cu2+(1 mM) and peroxydisulfate (4 mM). Charge efficiencies of more than 100% are possible due to auto oxidation by the radical intermediates form during the oxidation of AO7 by the SO4•−34. Samples condition Removal Degradation rate Charge efficiency efficiency after constant (%) -4 -1 30 min (%) (× 10 ) (s ) 20 mA No addition Cu2+ + PDS 47 3.46 23
3 55
0.20 3.90
5.9
37 61
2.70 5.73
131
32 54
1.92 4.32
232
19 51
1.13 3.73
436
ur na
lP
re
-p
ro
of
Cu2+ Cu2+ + PDS 1 mA No addition Cu2+ + PDS Cu2+ + PDS 0.5 mA No addition Cu2+ + PDS Cu2+ + PDS 0.25 mA No addition Cu2+ + PDS Cu2+ + PDS
Jo
Table 2: Comparison electrical energy consumption of different processes Method Electrode Operating parameters EE/O (kWh m-3) materials 2+ EC-Cu -PDS Cathode and AO7 = 0.1 mM, PDS = 4 Anode: mM, Cu2+ = 1 mM, Carbon cloth H2PO4/HPO4 = 10mM j = 20 mA cm-2, pH = 3 2.42 j = 1 mA cm-2, pH = 2 0.029 j = 1 mA cm-2, pH = 3 0.037 j = 1 mA cm-2, pH = 7 0.047 -2 j = 0.5 mA cm , pH = 3 0.016 j = 0.25 mA cm-2, pH = 3 0.008 24
Reference This study
EC-Fe3O4PDS
AO7 = 0.05 mM, Fe3O4 = 0.5 g L-1, j = 8.4 mA cm2 , pH 3.0, Na2SO4 = 50 mM
19.7
Lin et al., 201335
AO70 = 0.05 mM, PDS = 10 mM, Fe3O4 = 0.5 g L1 , j = 8.4 mA cm-2, pH 6.0, Na2SO4 = 50 mM
8.69
Lin et al., 20141
AO7 = 0.14 mM, PDS = 4.2 mM, MnO2 = 0.6 g L1 , j = 12 mA cm-2, pH 6.0, Na2SO4 = 50 mM
7.84
Xu et al., 201736
Jo
ur na
lP
re
-p
ro
EC-MnO2PDS
Cathode: Stainless steel plate Anode: Ti/ RuO2-IrO2 Cathode: Stainless steel plate Anode: Ti/ RuO2-IrO2 Cathode: Ti plate Anode: Ti/ RuO2-IrO2
of
EC-Fe3O4
Figure 1: a) Degradation of AO7 (0.1 mM) by different copper concentration with current density 1 mA cm-2 at pH 3 and persulfate (4 mM). b) Degradation of AO7 (0.1 mM) with different cathode size with current density of 1 mA cm-1 at anode, at pH 3, copper (1mM) and peroxydisulfate (4 mM). The anode size remained as 1 cm2 for all conditions.
25
of ro -p re lP
Jo
ur na
Figure 2: Degradation of AO7 (0.1 mM) after 30 min at pH 2,3,7 and10 at current density of 1 mA cm-1 and at different conditions; a) anodic oxidation; b) peroxydisulfate (PDS) (4mM) activation by electrochemically generated Cu+ from Cu2+ (1mM), EC-Cu-PDS; c) carbon cloth activation of PDS (4mM); d) electrochemical activation of PDS (4mM), EC-PDS.
26
of ro -p re lP
Jo
ur na
Figure 3: Reaction mechanism in a) carbon cloth with peroxydisulfate (PDS) system b) EC- PDS c) EC-Cu-PDS. Carbon cloths were used as both the cathode and anode.
27
of ro -p re lP
Jo
ur na
Figure 4: Degradation of AO7 (0.1 mM) after 30 min using either Co2+, Fe2+, Ag+ or Cu2+ (1mM) at pH 2 with a) peroxydisulfate (PDS) (4mM); b) PDS with electrolysis at j = 1 mA cm -1; c) peroxymonosulfate (PMS) (4mM); d) PMS with electrolysis at j = 1 mA cm-1
28
PII:
S2213-3437(19)30573-1
DOI:
https://doi.org/10.1016/j.jece.2019.103450
Reference:
JECE 103450
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
24 June 2019
Revised Date:
27 August 2019
Accepted Date:
30 September 2019
Please cite this article as: How ZT, Blackwood DJ, Degradation of Acid Orange 7 through radical activation by electro-generated cuprous ions, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103450
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Degradation of Acid Orange 7 through radical activation by electro-generated cuprous ions Zuo Tong How and Daniel John Blackwood*
Department of Material Science and Engineering, National University of Singapore, Singapore, Singapore 119077
ro
of
*Corresponding author: Daniel John Blackwood, Email: [email protected]
Jo
ur na
lP
re
-p
Graphical abstract
1
of ro -p re lP ur na
Highlight:
Degradation of AO7 by PDS activated from in situ electrochemically generated Cu+
EC-Cu-PDS have higher degradation efficiency of 70% as compare to 56% of EC-Fe-
Jo
PDS
EE/O of the EC-Cu-PDS processes ranged from 0.008 to 2.42 kWh m-3
Optimal operation condition was pH 2 with cathode to anode area of 4:1
SO4•− generated by both electrochemically at the anode and Cu+ activation of PDS
2
Abstract In this paper, the degradation of Acid Orange 7 (AO7), by persulfate activated from in situ electrochemically generated Cu+ (EC-Cu-PDS) was studied. The influence of key parameters such
of
as pH, current density, copper concentration and electrode size were investigated, with the optimal operation condition being pH 2 with cathode to anode area of 4:1. The degradation efficiency of
ro
the EC-Cu-PDS after 30 min was compared to the efficiency of other electrocatalysis system using
-p
iron, cobalt or silver. The proposed EC-Cu-PDS process has a higher degradation efficiency of 70%, as compare to 56% of the EC-Fe-PDS process. Although the EC-Cu-PDS process has a lower
re
degradation efficiency compared to 100% of Co-PMS and 80% of the EC-Ag-PDS processes, the EC-Cu-PDS process would be cheaper and safer to operate than the latter processes. The estimated
lP
electrical energy consumption of the EC-Cu-PDS processes was significantly lower than other electrocatalysis processes, ranging from 0.008 to 2.42 kWh m-3. The degradation of AO7 by the
ur na
EC-Cu-PDS process was via oxidization of AO7 by both hydroxyl radicals generated at the anode
Jo
and sulfate radicals generated by both electrochemically at the anode and Cu+ activation of PDS.
Keywords: Acid orange 7, Persulfate, Sulfate radical, Electrochemical advanced oxidation process, In situ chemical oxidation
3
1. Introduction Azo dye such as Acid Orange 7 (AO7, also known as Orange II) (Fig. S1) is a synthetic dye widely used in numerous applications [1, 2]. The azo dye is designed to be resistant to biological, chemical and photochemical degradation [1, 3, 4], being hardly degraded by conventional wastewater treatment processes [5], resulting in adverse impact to the environment and public health [6]. Advanced oxidation processes (AOPs) that utilized the highly reactive hydroxyl radical (•OH),
of
which is more efficient and non-selective as compared to conventional oxidants, have been
ro
investigated for their removal of persistent organic contaminants, including azo dye [1, 2, 7-9]. The sulfate radical-based oxidation processes is an emerging AOP that has received
-p
attention due to its effective destruction of persistent organic contaminants [10-12]. Sulfate
re
radicals (SO4•−) are more effective in the mineralization of organic acids than hydroxyl radicals, due to their higher redox potential; SO4•− (E◦= 2.60–3.10 V vs SHE), •OH (E◦= 1.90–2.70 V vs
lP
SHE) [13, 14]. Table S1 shows a list of relevant standard reduction potentials and chemical reactions.
ur na
Generally, SO4•− is generated from either peroxymonosulfate (PMS) or peroxydisulfate (PDS) (commonly known as persulfate) ions, through thermal activation [15], UV activation [1618], sonication [19] or transition metals activation [20-22]. Among these methods, the transition metal activation of SO4•− from PMS ions has demonstrated the highest efficiency for contaminant
Jo
degradation [23]. Transition metal activation of PMS and PDS ions is achieved in homogeneous and heterogeneous systems through a Fenton like reaction, similar to production of •OH from hydrogen peroxide [20] (Eq. 1-3). Mn+ + H2O2 → M(n+1)+ + •OH + OH-
(1)
Mn+ + S2O82- → M(n+1)+ + SO4•− + SO42-
(2)
4
Mn+ + HSO5- → M(n+1)+ + SO4•− + OH-
(3)
Among the transition metals, iron, as Fe2+ ions, is most commonly used for •OH generation from hydrogen peroxide and SO4•− from PMS or PDS. However, iron salts are only soluble in acidic conditions and the by-product from the activation are insoluble, having to be removed from the treatment system, so fresh Fe2+ ions are needed for the reaction to continue [24]. To reduce Fe2+ consumption, it can be regenerated by reduction of Fe3+ to Fe2+ using electrochemical methods
of
[25, 26].
ro
Anipsitakis and Dionysiou (2004) [20] found that while silver was the most effective transition metal for activation PDS to generate of SO4•−, cobalt was the most for PMS activation.
-p
A further benefit of using cobalt is that Co2+ could be regenerated from Co3+ in an aqueous system,
re
which minimized the concentration of Co2+ required, it is also far cheaper than silver. However, cobalt ions are classified as carcinogens and therefore have to be remove from the treated water
lP
before discharge [27].
Another transition metal that has been investigated for the generation of •OH or SO4•−, is
ur na
copper. Most studies on the use of copper for the generation of SO4•− were on heterogeneous activation of PMS or PDS by copper-based oxides, with the oxidation state of copper at either +1 or +2 [12, 22, 28, 29]. In addition, aqueous Cu2+ [30] and metallic Cu nanoparticles [31] have been found to activate PMS/PDS. Although Cu+ can effectively activate PDS, it is rarely studied,
Jo
especially for homogenous activation, as it is easily oxidized by dissolved oxygen or other mild oxidants [32]. Possible methods to overcome the instability of Cu+ ions, include producing it in situ by reducing Cu2+, either by a reducing agent, such as hydroxylamine [32] or electrochemically [33-35]. However, to date, Cu+ produced in situ electrochemically has only been attempted for the activation of H2O2, but not the more powerful oxidizers PDS and PMS.
5
In this paper, we investigate the degradation of AO7 using sulfate radicals generated by the homogenous activation of PDS or PMS using Cu+ produced in situ from the cathodic reduction of Cu2+. The mechanism of the process is investigated and the charge efficiency determined. The difference in the removal efficiency of AO 7 between homogenous Cu2+, Fe2+, Co2+ and Ag+ with
of
and without electrolysis are also presented.
ro
2. Materials and Methods 2.1 Experimental procedures and analysis.
-p
The test solutions consisted of AO7 with initial concentration of 0.1 mM. To this was added 1 mM of metallic salt, being either iron(II) sulfate, copper(II) sulfate, cobalt(II) chloride or silver(I)
re
nitrate along with 4 mM of either potassium persulfate (PDS) or potassium peroxymonosulfate
lP
(PMS, commonly known as Oxone™). The solution was buffered with 10 mM phosphate (K2HPO4/KH2PO4), with sulfuric acid or sodium hydroxide used to adjust to the required pH of 2,
ur na
3, 7 or 10.
Batch experiments were carried out in an undivided three electrode electrochemical cell containing 100 mL of electrolyte. Carbon cloth (2225 Type 900 Activated Carbon Fabric, Spectracarb) of dimensions of 1 × 1 cm, unless otherwise stated, was used as both the working and
Jo
counter electrodes. Carbon cloth was chosen as it is inert, has a large overpotential for water splitting and is cheaper than other carbon-based electrodes. The reference electrode was a saturated calomel electrode (+0.241 V vs SHE, CH Instrument), but all potentials reported have been converted to the standard hydrogen electrode (SHE) scale. The experiments were performed in duplicate with errors within 5%.
6
Experiments were performed at 22.5 ±1 °C and a direct current density of 1 mA cm-2, useless stated otherwise, was applied via a galvanostat (SP-200, Bio-logic). The reaction solutions were continuously stirred using magnetic stirrer. Aliquots (200 µL) were withdrawn at t = 0 and fixed intervals of 3 min until 30 min. The AO7 concentration was measured by UV-visible absorbance at wavelength = 485 nm (UV-1800, Shimazu UV-vis spectrometer). The carbon cloth electrodes were rinse with deionized water after every batch and dried in an oven at 60 ◦C for 45
ro
of
min.
-p
3. Results and Discussion 3.1 Effect of current density.
re
Table 1 shows the removal efficiency (%) of AO7 after 30 min at different current densities for a
lP
test solution at pH 3 with and without Cu2+ and peroxydisulfate. In the absence of both these reagents, i.e. with just anodic oxidation, the removal efficiency of the AO7 after 30 min decreased with decreasing current density; from 47% at 20 mA cm-2 down to 19% at 0.25 mA cm-2. This
ur na
reduction of removal efficiency was expected as anodic oxidation of the AO7 can occur at the anode’s surface either directly by electron transfer or indirectly by •OH weakly physiosorbed at the anode surface [36]. The reaction rates of both routes were proportional to the applied current
Jo
density. The potential of the anode also dropped from 4.2 V vs SHE at 20 mA cm-2 to 0.7 V vs SHE at 0.25 mA cm-2, which would also have reduced the number of •OH radicals formed. When Cu2+ was added into the system, but not peroxydisulfate, at 20 mA cm-2 the removal
efficiency after 30 min reduced to only 3% (Table 1). This reduction of removal efficiency is likely due to the consumption of most of the reactive •OH radicals generated at the anode by the Cu+ generated at the cathode [34, 35], thus the •OH were no longer available to oxidize the AO7. When 7
both Cu2+ and PDS was added into the system, the removal efficiency improved at all current densities, being 55% at 20 mA cm-2, 61% at 1 mA cm-2, 54% at 0.5 mA cm-2 and 51% at 0.25 mA cm-2 (Table 1). When compared to the removal efficiencies without either of these reagents, the improvements are 8%, 24%, 22% and 32% for 20, 1, 0.5 and 0.25 mA cm-2 respectively. The smaller improvement at 20 mA cm-2, reflects the fact that at this very high current density electrolysis of water occur, with oxygen being generated at the anode which obtained a potential
of
of 4.2 vs SHE. As the generated oxygen oxidize Cu+ to Cu2+, there will be less Cu+ available for
ro
the activation of PDS. No oxygen was evolved at the lower current densities, where the anode potentials were 1.5, 1.1 and 1.0 V vs SHE for 1.0, 0.5 and 0.25 mA cm-2 respectively. Although
-p
thermodynamically oxygen can be evolved at 1.11 V vs SHE, carbon is a poor catalyst for this
re
reaction with typically an overpotential of 500 mV, so significant electrolysis would not be expected below 1.6 V vs SHE [25].
lP
The charge efficiency was calculated using: 𝜂=
Fn 𝑞
(4)
ur na
where η is the charge efficiency (%), F is the Faraday constant 96485.33289 C mol-1, n is the number of equivalent moles of AO7 removal per mole of electrons and q is electrical charge (C). Equation (5) shows that four electrons are required to break the azo bond in AO7 to produce
Jo
sodium sulphanilamide and 1-amino-2-naphthol, such that n has a value of 4 (Fig. S1). Note that although it takes 84 electrons to fully mineralize one AO7 molecule (Eq. 6) the UV-visible absorbance technique is effectively monitoring the number of azo bonds broke, C16H11N2O4SNa + 4H+ + 4e- → C6H6NO3SNa + C10H9NO
(5)
C16H11N2O4SNa → CO2 + Na+ + SO42- + 2NO3- + 87H+ + 84e-
(6)
8
Table 1 shows that although the overall removal efficiency of AO7 decreased with decreasing current density, the charge efficiency had the opposite trend, i,e, it increased with decreasing current density, being only 5.9% at 20 mA cm-2 but an impressive 436% at 0.25 mA cm-2 . Charge efficiencies above 100% are possible due to auto oxidation by the radical intermediates form during the oxidation of AO7 by the SO4•− [36]. The low charge efficiency at the highest current density indicates the occurrences of side reactions, such as oxygen evolution,
of
which limit the availability of Cu+ for PDS activation.
ro
A higher charge efficiency could indicate lower energy was required to remove the same percentage of AO7. Therefore, for a persulfate activated from in situ electrochemically generated
-p
Cu+ (EC-Cu-PDS) system the optimal condition for the removal of AO7 would be a balance
re
between higher current density to provide good anodic oxidation for overall removal efficiency and a as low as possible current density to minimize the required energy. These experiments show
lP
that while a lower current density would generate more Cu+ for the activation of PDS, it would also result in reduced anodic oxidation reducing the overall removal efficiency of AO7.
ur na
Furthermore, carbon cloth is able to activate the PDS directly for the removal of AO7 (discussed in detail at section 3.2), a process that would mask the electrochemical route if too low a current density is applied.
Jo
3.2 Electrical energy consumption estimation. In order to further evaluate the electrical energy efficiency of the electrochemical plus Cu2+ plus PDS (EC-Cu-PDS) process, to allow comparisons with similar processes, an estimation of the
9
electrical energy consumption per order of magnitude (90%) removal in 1 m3 of contaminated water (EE/O) in term of kWh m-3 was conducted using the following formula [1]: 𝐸𝐸/𝑂 =
𝑈𝐶𝑒𝑙𝑙 𝐼𝑡
(7)
𝐶 𝑉𝑙𝑜𝑔( 0 ) 𝐶
where Ucell is the average cell voltage (V), I is the electrical current (A), t is the reaction time (hours) and V is the volume (liters).
of
Table 2 presents the estimated electrical energy required for the present EC-Cu-PDS process and similar techniques in the literature for the removal of AO7. This shows that the EE/O
ro
for the EC-Cu-PDS process decreases linearly with decreasing current density (j), from 2.42 kWh
-p
m-3 order-1 at j = 20 mA cm-2 to 0.008 kWh m-3 order-1 at j = 0.25 mA cm-2. Table 2 reveals that even the least efficient EE/O for the EC-Cu-PDS process, 2.42 kWh m-3 order-1 at j = 20 mA cm-2,
re
is significantly lower than similar systems reported in the literature; such as 19.7 kWh m -3 order-1 for electro-oxidation with iron (II,III) oxide (EC-Fe3O4) [37], 8.69 kWh m-3 order-1 for electro-
lP
oxidation with iron (II,III) oxide and persulfate (EC-Fe3O4-PDS) [1] and 7.84 kWh m-3 order-1 for electro-oxidation with manganese (IV) oxide and persulfate (EC-MnO2-PDS) [38]. One reason for
ur na
the better EE/O in the EC-Cu-PDS process is likely to be because it is a homogenous system, where the electron transfer between the electrodes, copper ions and PDS would more efficient as compared to the heterogeneous literature systems, where electron transfer involves insoluble metal
Jo
oxides.
Another plausible reason for the difference in EE/O is the choice of anode materials (Table
2). In this study, carbon cloth electrodes were used rather than the Ti/ RuO2-IrO2 used in literature processes. Carbon cloth has a higher oxygen evolution overpotential (0.5 V) as compared to Ti/ RuO2-IrO2 (0.2 V) [39] and it has been reported that electrodes with a higher oxygen evolution
10
overpotential generally have a higher anodic oxidation efficiency [40] and therefore required lower electrical energy to achieve higher degradation of AO7. It should be noted that although the processes used by Xu et al. 2017 [38] had a faster AO7 degradation rate of 8.8 × 10-4 s-1, compared to 5.7 × 10-4 s-1 of this study, their EE/O was more than
of
an order of magnitude higher for similar conditions, i.e. the present work is far more efficient.
3.3 Effect of copper concentration and cathode size.
ro
In order to investigate the impact of copper concentration on the kinetics and efficiency of AO7degradation using EC-Cu-PDS system, a blank and five different concentration of copper (II)
-p
sulfate: 0.0, 0.05, 0.1, 0.5, 1.0 and 4.0 mM, were used. The current density (1 mA cm-2), pH 3 and
re
persulfate concentration (4 mM) were kept constant. Table S2 shows that the pseudo-first order rate constant for the degradation of AO7 increased with increasing Cu2+ concentration, being 2.70
lP
× 10-4 s-1 for the control and 6.20 × 10-4 s-1 with 4 mM, at the same time the removal efficiency after 30 mins increased from 37% to 65% (Fig. 1a).
ur na
The removal efficiency with the lowest Cu2+ concentration (0.5 mM) at 37% is the same as the control with no copper ions. A possible reason is that all the Cu+ ions generated were reoxidized back to Cu2+ ions by dissolved oxygen (~ 0.25 mM). Therefore, when 0.05 mM of Cu was added into the system, insufficient Cu+ survived long enough to activate any significant
Jo
concentration of SO4•−. The more favorable reaction with dissolved oxygen than PDS would explain the sharp increase in reaction rate from 2.78 × 10-4 s-1 at 0.05 mM of Cu2+ to 4.04 × 10-4 s1
at 0.1 mM of Cu2+. At 0.1 mM the Cu+ generated would first be consumed by the dissolved
oxygen, and the excess Cu+ available would activate the PDS and thus a sharp increase in reaction rate and removal efficiency were observed.
11
The initial dissolved oxygen content would be virtually identical in all the test solutions under the same pH, temperature and pressure. Therefore, once enough Cu2+ was available to generate sufficient Cu+ ions to overcome the oxygen’s influence, the reaction rate increased with Cu2+ concentration, until it started to plateau at 1.0 mM, with the removal efficiency only increasing by a further 4% at 4.0 mM (Table S2). The plateau in the removal efficiency was possibly limited by the generation of Cu+ from
of
the cathodic reduction of Cu2+. To test this, the influence of the cathode size was investigated. Four
ro
different cathode sizes of 1, 1.69, 2.25 and 4 cm2 were tested at a constant current of 1 mA (i.e. 1 mA cm-2 with respect to the anode that was maintained at 1 cm2) and same initial Cu2+, AO7 and
-p
PDS concentrations of 1, 0.1 and 4 mM respectively. Fig. 1b shows that the rate of removal of
re
AO7 increase linearly with increasing cathode size, reaching 99% efficiency by 30 min with the 4 cm2 cathode. Since the anode size was kept constant at 1 cm2 the increase in removal efficiency of
lP
AO7 on increasing the cathode size cannot be from an increase of anodic oxidation but instead from the increase in cathodic reduction of Cu2+ to Cu+, which in turn increased the rate of SO4•−
ur na
generation.
3.4 Effect of pH and reaction mechanism. Fig. 2 shows the influence of pH on the removal efficiency of AO7 by anodic oxidation at an
Jo
anodic current density of 1 mA cm-2 in the absence and presence of both PDS and Cu2+. In the absence of these two reagents the removal efficiency of AO7 at pH 7 was 35%, which is similar to the 37% reported above for pH 3 (Table 1) and 32% for pH 2, but at pH 10 the removal efficiency dropped to only 16%. This drop in removal efficiency might be due to the lower oxidizing strength of •OH in basic media [41, 42]. Therefore, the observation that in the absence of Cu2+ and PDS the
12
removal efficiency is almost independent of pH over the range pH 2 to pH 7, but drops significantly at pH 10 suggests that the main mechanism of the anodic oxidation of AO7 at a carbon cloth anode is the indirect oxidation by •OH radicals, rather than the direct electron transfer from the AO7 at the anode surface. When Cu2+ and PDS was added into the system (Fig. 2b), a stronger pH dependency of the removal efficiency of AO7 was observed. As the pH was increased from pH 2, pH 3, pH 7 to
of
pH 10 the removal efficiency decreased in the order 70%, 61%, 52% down to 6%. The small
ro
decline from pH 2 to pH 7 might be partly due to the precipitation of insoluble copper (II) hydroxide (Cu(OH)2: solubility =1.722 × 10-6 g·mL-1 at 20 ◦C) but more likely, a lowering of the
-p
electrochemical activation of PDS is responsible (see section 3.3). The dramatic fall in the removal
re
efficiency that occurred on going from pH 7 to pH 10 is due to the formation of insoluble Cu(OH)2, such that there are insufficient Cu+ ions for the activation of the PDS to generate the SO4•−required
lP
to oxidize the AO7. Although alkaline activation of PDS has been reported [43, 44], this was at pH 12 or 1.5 to 3 M of sodium hydroxide with 0.5 M PDS, where the hydroxyl and PDS
ur na
concentration would be two to four magnitudes times higher than the 4 mM PDS and 0.1 mM sodium hydroxide (pH 10) solution used in this study. Closer inspection of Fig. 2b reveals that at pH 10 there is an initial drop in the AO7 concentration and its concentration increases during the first 6 min; the t = 0 is actually less than
Jo
the starting concentration, i.e. some of the AO7 was lost as soon as the test solution was poured into the electrochemical cell. This initial drop in AO7 concentration is thought to be cause by the coagulation of AO7 by Cu(OH)2 that readily forms at pH 10. However, on application of the current the electrical static interactions between Cu(OH)2 and AO7 are broken and so the AO7 is released back into the system resulting in an apparent increase in AO7 concentration during the
13
first 6 minutes. Note that although the increase in pH had an adverse impact on the degradation of AO7 by the EC-Cu-PDS system, the formation of Cu(OH)2 at neutral and basic conditions could be utilize for the removal of copper from the system when eventually required. It has been reported by Yang et al. (2011) [45] that carbonaceous activation of PDS is possible, so this may occur directly at the carbon cloth electrodes without the need to involve Cu+ ions. Fig. 2c thus shows the influence of pH on the removal of AO7 at carbon cloth in the presence
of
of 4 mM PDS, but without the addition of any Cu2+ ions nor the application of any current density.
ro
Fig. 2c shows that the AO7 removal efficiency of the PDS activated by the carbon cloth increased with decreasing pH. The increased in PDS activation by the carbon cloth as the solution becomes
-p
more acidic might be due to the increased in hydrophilicity of the solution and thus having a shorter
re
retention on the hydrophobic carbon cloth, allowing a faster exchange of the PDS on the surface. This is aligned with observations by Yang et al., (2011) [45] that an increase in absorptivity of the
lP
solution reduced the activation of PDS. The mechanism of AO7 oxidation with PDS and carbon cloth without electrolysis is illustrated in Fig. 3a; the AO7 is oxidized by radicals that are
ur na
generated by the dissociation of the PDS at the carbon cloth. Figure 2d shows the same set of experiments as in Figure 2c, except now a current density of 1 mA cm-2 was applied to the system. It can be seen that although the current density improved the removal efficiency at pH 10, this is not the case for the neutral and acidic solution where the
Jo
removal efficiency declined significantly. From example, at pH 2 the removal efficiency declined from 63% without electrolysis to 43% with electrolysis. A plausible explanation for the difference in removal efficiency is that electrolysis modifies the interactions between the carbon cloth and the PDS in the solution. The application of the electrical current would cause the carbon cloth electrodes to be charged (regardless of whether these are the anode or the cathode) and thus become
14
more hydrophilic, which in turn would increase the retention of the solution at the carbon cloth resulting in lower PDS activation and AO7 removal efficiency due to lower diffusion of the PDS. However, if the difference in removal efficiency was only due to the change in hydrophobicity of the carbon cloth, a reduction in removal efficiency would also have been expected at pH 10. Since PDS can be activated by electrolysis [46, 47], when the electrical current is applied the PDS is likely to be electrochemically activated to SO4•− radicals at the anode alongside
of
electrochemically generated •OH. The electrochemical activation of PDS and direct generation of •
ro
OH radicals would explain the higher removal efficiency in acidic media than in basic media, as
the oxidation power •OH radicals is known to decrease with increasing pH13. However, under the
-p
acidic conditions the •OH and SO4•− radicals generated at the anode are not sufficient to replace
re
the loss in SO4•− generated by the direct activation of the PDS at the carbon cloth that occurs without applied current density, so application of the current still leads to an overall decrease in
lP
AO7 removal efficiency. With respect to the pH 10 solution, because the direct activation of the PDS at the carbon cloth is so much less efficient under basic conditions (Fig. 2c), it is easier for
ur na
the SO4•− radicals generated at the anode to make up for its loss. Chen et al., (2018) [48] reported that SO4•− generation by electrolysis is more efficient at under basic conditions, so application of the current leads to an overall increase in AO7 removal. The results therefore suggest that when electrical current was applied, the carbonaceous activation of PDS was reduced and
Jo
electrochemical activation of PDS became the dominant pathway for the activation of PDS. The mechanism of AO7 oxidation with EC-PDS is illustrated in Fig. 3b; the AO7 is oxidized by •OH and SO4•− radicals generated directly at the anode. The mechanism of AO7 oxidation by the EC-Cu-PDS process (Fig. 3c) would be a combination of the EC-PDS process with transition metal activation of PDS. That is the AO7 is
15
oxidized by •OH and SO4•− generated directly at the anode and by SO4•− generated by the activation of PDS by the Cu+ generated from the cathodic reduction of Cu2+. This combination of multiple oxidation pathways in the EC-Cu-PDS process resulted in the highest removal efficiency of 70% (at pH 2) as compared to the highest of 40% for anodic oxidation, 60% for carbon cloth (carbonaceous) activation of PDS and 45% for EC-PDS process. It would be interesting to ascertain the contribution rates of the species involved in the multiple pathways towards AO7
of
degradation, but the in situ electrochemical EPR experiments (necessary to achieve this are
ro
difficult to perform) [49].
-p
3.5 Comparison of four transition metals for electrocatalysis persulfate system.
re
To understand the suitability of using copper for the activation of persulfate, as compare to the normally used iron, cobalt (reported to be the most effective in activation of PMS) [20] or silver
lP
(reported to be the most effective in the activation of PDS) [20], a series of experiments were carried out using these four transition metals as Fe2+, Co2+, Ag+ and Cu2+ (1mM); and PDS or PMS
ur na
with and without electrolysis at 1 mA cm-2 in pH 2 solutions for the removal AO7 were conducted. Figs 4a and 4b show that when the four transition metals were used to activate PDS. No degradation of AO7 was observed within 30 min when only PDS was added (control, Fig. 4a). It can be seen that cobalt performed the worst, with an AO7 removal efficiency of only 1% without
Jo
electrolysis and 40% when coupled with electrolysis. In the absence of an applied current, Fe2+ initially rapidly degraded AO7 achieving 40% removal efficiency by 3 min, then followed by a slow degradation reaching 50% by 30 min. The slow degradation observed after the third minute is likely due to most of the reactive Fe2+ having being oxidized to Fe3+. The application of a current to the ferrous system had only a minor impact, slightly improving the removal efficiency to 56%
16
by 30 min. This is because Fe2+ cannot be cathodically regenerated from Fe3+ efficiently due to precipitation of insoluble ferric sulfate (Fe2(SO4)3) and ferric hydroxide (Fe(OH)3). In addition, the fact that little removal of AO7 was observed after 3 min indicates the formation of the insoluble salts had a negative impact on the anodic oxidation of AO7. This might be due to fouling of the carbon cloth electrode by the Fe2(SO4)3, thus the removal of AO7 was by the slower electrocoagulation rather than anodic oxidation.
of
When Ag+ was used, without applying a current density the removal efficiency increased
ro
almost linearly with time (Fig. 4a), but was only 36% after 30 min as compared to 50% when using iron. However, if a longer time had been used it is likely that the silver would overtake the
-p
iron as the rate of degradation using Ag+ increases with time (Fig. 4a). When 1 mA cm-2 was
re
applied to the silver system it gave the best performance at all times, reaching a removal efficiency of 80% at 30 mins. Finally, when copper was used, the removal efficiency was only 3% by 30 min,
lP
but when combined with electrolysis the efficiency increased to 70% after 30 min, second only to the silver system. The mechanism for the copper system has already be discussed.
ur na
Figs. 4c and 4b show the results obtained when PMS was used to generate the SO4•− radicals, again without and with the application of 1 mA cm-2. No degradation of AO7 was observed within 30 min when only PDS was added (control, Fig. 4c).The findings are consistent with the literature [14, 20], with cobalt being very effective in the removal of AO7 even without
Jo
the application of an electrical current. The cobalt system reached 50% removal within 6 min and 100% by 15 min, due to the continuous regeneration of the active Co2+ from Co3+. When cobalt was used with electrolysis, the time required fully degrade the AO7 reduced from 15 min to 6 min, reaching 50% removal within a minute.
17
When iron was used for the activation of PMS, the results were similar to that when PDS had been used, the AO7 degraded rapidly within the first 3 min to 50% without electrolysis and 55% with electrolysis and a slow removal rate after the first 3 min reaching 54% without electrolysis and 61% with electrolysis. When silver was added with PMS, only 2% removal of AO7 was observed, which was expected as Ag+ has been reported to be not effective in the activation of PMS.18 When the silver was coupled with electrolysis the removal efficiency was
of
increased to 30% after 30 mins. For the copper PMS system, the 30 min removal efficiency was
ro
only 1% without the application of an electrical current, but increased to 64% wen the 1 mA cm-2 was applied.
-p
Overall, Fig. 4 shows that although the EC-Cu-PDS process has it disadvantage of having
re
a slower reaction rate as compared to EC-Fe-PDS, the copper system have a higher removal efficiency (70%) per mole of transition metal used as compared to the iron system (56%).
lP
Furthermore, the copper system continues to be active beyond the 30 min mark, whereas the iron systems removal capabilities have been exhausted. Likewise, when operated at its optimal
ur na
condition, the EC-Cu-PDS system has comparable removal efficiency to EC-Ag-PDS, but with the obvious advantage of copper being much cheaper than silver. Finally, even though the CoPMS system was significantly more efficient than EC-Cu-PDS system, the copper system is a much safer option, as cobalt is classified as carcinogen, plus copper is cheaper than cobalt.
Jo
Furthermore, the cost of PDS is lower than PMS [27].
4. Conclusion This study demonstrated that an EC-Cu-PDS process, where the active Cu+ is generated in situ at a carbon cloth cathode by reduction of Cu2+, was effective in activating PDS to SO4•− radicals for 18
the subsequent degradation of AO7. The optimum concentration of Cu2+ was found to be 1 mM and highest removal efficiencies were achieved under acidic conditions. With respect to current density, high values improved the removal efficiency but decreased the charge efficiency, with 1 mA cm-2 being found to be a good compromise. The removal efficiency increased with cathode area, reaching a maximum removal efficiency with a cathode to anode area ratio of 4:1. Although the removal efficiency of the EC-Cu-PDS process, at 70%, was lower than the
of
Co-PMS and of the EC-Ag-PDS processes, at 100% and 80% respectively, the new process would
ro
be cheaper to operate. In addition, cobalt is classified as carcinogen, while copper is not classified as a hazardous metal. Another advantage of the proposed EC-Cu-PDS process is that it is
-p
potentially cheaper than other electrocatalysis or electrochemical advance oxidation processes due
re
to its lower electrical energy consumption of less than 2.42 kWh m-3 order-1 (Table 2) and the used
Acknowledgements
lP
of relatively inexpensive and inert carbon cloth electrodes.
ur na
This research grant was supported by the Singapore National Research Foundation under its Environmental & Water Research Programme (Project Ref No. 1301-IRIS-33) and administered
Jo
by PUB, Singapore’s national water agency.
References
[1] H. Lin, H. Zhang, L. Hou, Degradation of C. I. Acid Orange 7 in aqueous solution by a novel electro/Fe3O4/PDS process, J. Hazard. Mater. 276 (2014) 182191.https://doi.org/10.1016/j.jhazmat.2014.05.021 [2] A. Özcan, M.A. Oturan, N. Oturan, Y. Şahin, Removal of Acid Orange 7 from water by electrochemically generated Fenton's reagent, J. Hazard. Mater. 163 (2009) 12131220.https://doi.org/10.1016/j.jhazmat.2008.07.088 19
Jo
ur na
lP
re
-p
ro
of
[3] H. Zhang, Y. Lv, F. Liu, D. Zhang, Degradation of C.I. Acid Orange 7 by ultrasound enhanced ozonation in a rectangular air-lift reactor, Chem. Eng. J. 138 (2008) 231238.https://doi.org/10.1016/j.cej.2007.06.031 [4] A. Sharma, M. Verma, A.K. Haritash, Degradation of toxic azo dye (AO7) using Fenton's process, Adv. Environ. Res. 5 (2016) 189-200.10.12989/AER.2016.5.3.189 [5] R. Ganesh, G.D. Boardman, D. Michelsen, Fate of azo dyes in sludges, Water Res. 28 (1994) 1367-1376.https://doi.org/10.1016/0043-1354(94)90303-4 [6] A. Babuponnusami, K. Muthukumar, A review on Fenton and improvements to the Fenton process for wastewater treatment, J. Environ. Chem. Eng. 2 (2014) 557572.https://doi.org/10.1016/j.jece.2013.10.011 [7] S. Bahnmüller, C.H. Loi, K.L. Linge, U.v. Gunten, S. Canonica, Degradation rates of benzotriazoles and benzothiazoles under UV-C irradiation and the advanced oxidation process UV/H2O2, Water Res. 74 (2015) 143-154.https://doi.org/10.1016/j.watres.2014.12.039 [8] D.P. Mohapatra, S.K. Brar, R.D. Tyagi, P. Picard, R.Y. Surampalli, Analysis and advanced oxidation treatment of a persistent pharmaceutical compound in wastewater and wastewater sludge-carbamazepine, Sci. Total Environ. 470-471 (2014) 5875.https://doi.org/10.1016/j.scitotenv.2013.09.034 [9] M. Zhou, Q. Yu, L. Lei, G. Barton, Electro-Fenton method for the removal of methyl red in an efficient electrochemical system, Sep. Purif. Technol. 57 (2007) 380387.https://doi.org/10.1016/j.seppur.2007.04.021 [10] H.V. Lutze, J. Brekenfeld, S. Naumov, C. von Sonntag, T.C. Schmidt, Degradation of perfluorinated compounds by sulfate radicals – New mechanistic aspects and economical considerations, Water Res. 129 (2018) 509-519.https://doi.org/10.1016/j.watres.2017.10.067 [11] J. Wu, H. Zhang, J. Qiu, Degradation of Acid Orange 7 in aqueous solution by a novel electro/Fe2+/peroxydisulfate process, J. Hazard. Mater. 215-216 (2012) 138145.https://doi.org/10.1016/j.jhazmat.2012.02.047 [12] Y. Zhang, Q. Zhang, J. Hong, Sulfate radical degradation of acetaminophen by novel iron– copper bimetallic oxidation catalyzed by persulfate: Mechanism and degradation pathways, Appl. Surf. Sci 422 (2017) 443-451.https://doi.org/10.1016/j.apsusc.2017.05.224 [13] P. Neta, R.E. Huie, A.B. Ross, Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution, J. Phys. Chem. Ref. Data 17 (1988) 1027-1284.10.1063/1.555808 [14] W.-D. Oh, Z. Dong, T.-T. Lim, Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects, Appl. Catal. B Environ. 194 (2016) 169-201.https://doi.org/10.1016/j.apcatb.2016.04.003 [15] Y. Ji, C. Dong, D. Kong, J. Lu, Q. Zhou, Heat-activated persulfate oxidation of atrazine: Implications for remediation of groundwater contaminated by herbicides, Chem. Eng. J. 263 (2015) 45-54.https://doi.org/10.1016/j.cej.2014.10.097 [16] Y.-H. Guan, J. Ma, X.-C. Li, J.-Y. Fang, L.-W. Chen, Influence of pH on the Formation of Sulfate and Hydroxyl Radicals in the UV/Peroxymonosulfate System, Environ. Sci. Technol. 45 (2011) 9308-9314.10.1021/es2017363 [17] M. Nihemaiti, D.B. Miklos, U. Hübner, K.G. Linden, J.E. Drewes, J.-P. Croué, Removal of trace organic chemicals in wastewater effluent by UV/H2O2 and UV/PDS, Water Res. 145 (2018) 487-497.https://doi.org/10.1016/j.watres.2018.08.052 [18] M. Verma, A.K. Haritash, Degradation of amoxicillin by Fenton and Fenton-integrated hybrid oxidation processes, J. Environ. Chem. Eng. 7 (2019) 102886.https://doi.org/10.1016/j.jece.2019.102886 20
Jo
ur na
lP
re
-p
ro
of
[19] C. Cai, H. Zhang, X. Zhong, L. Hou, Ultrasound enhanced heterogeneous activation of peroxymonosulfate by a bimetallic Fe–Co/SBA-15 catalyst for the degradation of Orange II in water, J. Hazard. Mater. 283 (2015) 70-79.https://doi.org/10.1016/j.jhazmat.2014.08.053 [20] G.P. Anipsitakis, D.D. Dionysiou, Radical Generation by the Interaction of Transition Metals with Common Oxidants, Environ. Sci. Technol. 38 (2004) 3705-3712.10.1021/es035121o [21] G. Lente, J. Kalmár, Z. Baranyai, A. Kun, I. Kék, D. Bajusz, M. Takács, L. Veres, I. Fábián, One- Versus Two-Electron Oxidation with Peroxomonosulfate Ion: Reactions with Iron(II), Vanadium(IV), Halide Ions, and Photoreaction with Cerium(III), Inorg. Chem. 48 (2009) 17631773.10.1021/ic801569k [22] H.-y. Liang, Y.-q. Zhang, S.-b. Huang, I. Hussain, Oxidative degradation of p-chloroaniline by copper oxidate activated persulfate, Chem. Eng. J. 218 (2013) 384391.https://doi.org/10.1016/j.cej.2012.11.093 [23] P. Devi, U. Das, A.K. Dalai, In-situ chemical oxidation: Principle and applications of peroxide and persulfate treatments in wastewater systems, Sci. Total Environ. 571 (2016) 643657.https://doi.org/10.1016/j.scitotenv.2016.07.032 [24] S. Malato, P. Fernández-Ibáñez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends, Catal. Today 147 (2009) 1-59.https://doi.org/10.1016/j.cattod.2009.06.018 [25] J.D. Benck, B.A. Pinaud, Y. Gorlin, T.F. Jaramillo, Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte, PLOS ONE 9 (2014) e107942.10.1371/journal.pone.0107942 [26] E. Brillas, I. Sirés, M.A. Oturan, Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton’s Reaction Chemistry, Chem. Rev. 109 (2009) 65706631.10.1021/cr900136g [27] T. Zhang, Y. Chen, Y. Wang, J. Le Roux, Y. Yang, J.-P. Croué, Efficient Peroxydisulfate Activation Process Not Relying on Sulfate Radical Generation for Water Pollutant Degradation, Environ. Sci. Technol. 48 (2014) 5868-5875.10.1021/es501218f [28] X. Du, Y. Zhang, I. Hussain, S. Huang, W. Huang, Insight into reactive oxygen species in persulfate activation with copper oxide: Activated persulfate and trace radicals, Chem. Eng. J. 313 (2017) 1023-1032.https://doi.org/10.1016/j.cej.2016.10.138 [29] Y. Feng, J. Liu, D. Wu, Z. Zhou, Y. Deng, T. Zhang, K. Shih, Efficient degradation of sulfamethazine with CuCo2O4 spinel nanocatalysts for peroxymonosulfate activation, Chem. Eng. J. 280 (2015) 514-524.https://doi.org/10.1016/j.cej.2015.05.121 [30] C.S. Liu, K. Shih, C.X. Sun, F. Wang, Oxidative degradation of propachlor by ferrous and copper ion activated persulfate, Sci. Total Environ. 416 (2012) 507512.https://doi.org/10.1016/j.scitotenv.2011.12.004 [31] P. Zhou, J. Zhang, J. Liu, Y. Zhang, J. Liang, Y. Liu, B. Liu, W. Zhang, Degradation of organic contaminants by activated persulfate using zero valent copper in acidic aqueous conditions, RSC Adv. 6 (2016) 99532-99539.10.1039/C6RA24431A [32] P. Zhou, J. Zhang, J. Liang, Y. Zhang, Y. Liu, B. Liu, Activation of persulfate/copper by hydroxylamine via accelerating the cupric/cuprous redox couple, Water Sci. Technol. 73 (2015) 493-500.10.2166/wst.2015.509 [33] S. Jacqueline George, R. Gandhimathi, P.V. Nidheesh, S.T. Ramesh, Optimization of salicylic acid removal by electro Fenton process in a continuous stirred tank reactor using response surface methodology, Desalin. Water Treat. 57 (2016) 4234-4244.10.1080/19443994.2014.992970
21
Jo
ur na
lP
re
-p
ro
of
[34] P.V. Nidheesh, R. Gandhimathi, Comparative Removal of Rhodamine B from Aqueous Solution by Electro-Fenton and Electro-Fenton-Like Processes, CLEAN – Soil, Air, Water 42 (2014) 779-784.10.1002/clen.201300093 [35] N. Oturan, M. Zhou, M.A. Oturan, Metomyl Degradation by Electro-Fenton and ElectroFenton-Like Processes: A Kinetics Study of the Effect of the Nature and Concentration of Some Transition Metal Ions As Catalyst, J. Phys. Chem. A 114 (2010) 10605-10611.10.1021/jp1062836 [36] M. Panizza, G. Cerisola, Direct And Mediated Anodic Oxidation of Organic Pollutants, Chem. Rev. 109 (2009) 6541-6569.10.1021/cr9001319 [37] H. Lin, L. Hou, H. Zhang, Degradation of Orange II in aqueous solution by a novel electro/Fe3O4 process, Water Sci. Technol. 68 (2013) 2441-2447.10.2166/wst.2013.513 [38] Y. Xu, H. Lin, Y. Li, H. Zhang, The mechanism and efficiency of MnO2 activated persulfate process coupled with electrolysis, Sci. Total Environ. 609 (2017) 644654.https://doi.org/10.1016/j.scitotenv.2017.07.151 [39] F.C. Moreira, R.A.R. Boaventura, E. Brillas, V.J.P. Vilar, Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters, Appl. Catal. B Environ. 202 (2017) 217-261.https://doi.org/10.1016/j.apcatb.2016.08.037 [40] C.A. Martínez-Huitle, L.S. Andrade, Electrocatalysis in wastewater treatment: recent mechanism advances, Química Nova 34 (2011) 850-858 [41] J.M. Campos-Martin, G. Blanco-Brieva, J.L.G. Fierro, Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process, Angew. Chemie Int. Ed. 45 (2006) 69626984.10.1002/anie.200503779 [42] J. Wang, S. Wang, Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants, Chem. Eng. J. 334 (2018) 15021517.https://doi.org/10.1016/j.cej.2017.11.059 [43] O.S. Furman, A.L. Teel, R.J. Watts, Mechanism of Base Activation of Persulfate, Environ. Sci. Technol. 44 (2010) 6423-6428.10.1021/es1013714 [44] S. Furman Olha, L. Teel Amy, M. Ahmad, C. Merker Marissa, J. Watts Richard, Effect of Basicity on Persulfate Reactivity, J. Environ. Eng. 137 (2011) 241-247.10.1061/(ASCE)EE.19437870.0000323 [45] S. Yang, X. Yang, X. Shao, R. Niu, L. Wang, Activated carbon catalyzed persulfate oxidation of Azo dye acid orange 7 at ambient temperature, J. Hazard. Mater. 186 (2011) 659666.https://doi.org/10.1016/j.jhazmat.2010.11.057 [46] H. Song, L. Yan, J. Jiang, J. Ma, Z. Zhang, J. Zhang, P. Liu, T. Yang, Electrochemical activation of persulfates at BDD anode: Radical or nonradical oxidation?, Water Res. 128 (2018) 393-401.https://doi.org/10.1016/j.watres.2017.10.018 [47] H. Song, L. Yan, J. Jiang, J. Ma, S. Pang, X. Zhai, W. Zhang, D. Li, Enhanced degradation of antibiotic sulfamethoxazole by electrochemical activation of PDS using carbon anodes, Chem. Eng. J. 344 (2018) 12-20.https://doi.org/10.1016/j.cej.2018.03.050 [48] L. Chen, C. Lei, Z. Li, B. Yang, X. Zhang, L. Lei, Electrochemical activation of sulfate by BDD anode in basic medium for efficient removal of organic pollutants, Chemosphere 210 (2018) 516-523.https://doi.org/10.1016/j.chemosphere.2018.07.043 [49] R.G. Compton, C.R. Greaves, A.M. Waller, A wall-jet electrode for in-situ electrochemical ESR, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 277 (1990) 8392.https://doi.org/10.1016/0022-0728(90)85092-J
22
of ro -p re lP ur na
Jo
Table 1: Removal efficiency (%) of AO7 after 30 min, the apparent rate constant for the degradation of AO7 and the charge efficiency with different current density (20, 1.0, 0.5 and 0.25 mA cm-2) at pH 3 with and without the reagents Cu2+(1 mM) and peroxydisulfate (4 mM). Charge efficiencies of more than 100% are possible due to auto oxidation by the radical intermediates form during the oxidation of AO7 by the SO4•−34. Samples condition Removal Degradation rate Charge efficiency efficiency after constant (%) -4 -1 30 min (%) (× 10 ) (s ) 20 mA No addition Cu2+ + PDS 47 3.46 23
3 55
0.20 3.90
5.9
37 61
2.70 5.73
131
32 54
1.92 4.32
232
19 51
1.13 3.73
436
ur na
lP
re
-p
ro
of
Cu2+ Cu2+ + PDS 1 mA No addition Cu2+ + PDS Cu2+ + PDS 0.5 mA No addition Cu2+ + PDS Cu2+ + PDS 0.25 mA No addition Cu2+ + PDS Cu2+ + PDS
Jo
Table 2: Comparison electrical energy consumption of different processes Method Electrode Operating parameters EE/O (kWh m-3) materials 2+ EC-Cu -PDS Cathode and AO7 = 0.1 mM, PDS = 4 Anode: mM, Cu2+ = 1 mM, Carbon cloth H2PO4/HPO4 = 10mM j = 20 mA cm-2, pH = 3 2.42 j = 1 mA cm-2, pH = 2 0.029 j = 1 mA cm-2, pH = 3 0.037 j = 1 mA cm-2, pH = 7 0.047 -2 j = 0.5 mA cm , pH = 3 0.016 j = 0.25 mA cm-2, pH = 3 0.008 24
Reference This study
EC-Fe3O4PDS
AO7 = 0.05 mM, Fe3O4 = 0.5 g L-1, j = 8.4 mA cm2 , pH 3.0, Na2SO4 = 50 mM
19.7
Lin et al., 201335
AO70 = 0.05 mM, PDS = 10 mM, Fe3O4 = 0.5 g L1 , j = 8.4 mA cm-2, pH 6.0, Na2SO4 = 50 mM
8.69
Lin et al., 20141
AO7 = 0.14 mM, PDS = 4.2 mM, MnO2 = 0.6 g L1 , j = 12 mA cm-2, pH 6.0, Na2SO4 = 50 mM
7.84
Xu et al., 201736
Jo
ur na
lP
re
-p
ro
EC-MnO2PDS
Cathode: Stainless steel plate Anode: Ti/ RuO2-IrO2 Cathode: Stainless steel plate Anode: Ti/ RuO2-IrO2 Cathode: Ti plate Anode: Ti/ RuO2-IrO2
of
EC-Fe3O4
Figure 1: a) Degradation of AO7 (0.1 mM) by different copper concentration with current density 1 mA cm-2 at pH 3 and persulfate (4 mM). b) Degradation of AO7 (0.1 mM) with different cathode size with current density of 1 mA cm-1 at anode, at pH 3, copper (1mM) and peroxydisulfate (4 mM). The anode size remained as 1 cm2 for all conditions.
25
of ro -p re lP
Jo
ur na
Figure 2: Degradation of AO7 (0.1 mM) after 30 min at pH 2,3,7 and10 at current density of 1 mA cm-1 and at different conditions; a) anodic oxidation; b) peroxydisulfate (PDS) (4mM) activation by electrochemically generated Cu+ from Cu2+ (1mM), EC-Cu-PDS; c) carbon cloth activation of PDS (4mM); d) electrochemical activation of PDS (4mM), EC-PDS.
26
of ro -p re lP
Jo
ur na
Figure 3: Reaction mechanism in a) carbon cloth with peroxydisulfate (PDS) system b) EC- PDS c) EC-Cu-PDS. Carbon cloths were used as both the cathode and anode.
27
of ro -p re lP
Jo
ur na
Figure 4: Degradation of AO7 (0.1 mM) after 30 min using either Co2+, Fe2+, Ag+ or Cu2+ (1mM) at pH 2 with a) peroxydisulfate (PDS) (4mM); b) PDS with electrolysis at j = 1 mA cm -1; c) peroxymonosulfate (PMS) (4mM); d) PMS with electrolysis at j = 1 mA cm-1
28