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Stabilized landfill leachate treatment using heterogeneous Fenton and electro-Fenton processes
Accepted Manuscript Stabilized Landfill Leachate Treatment using Heterogeneous Fenton and ElectroFenton Processes
T. Sruthi, R. Gandhimathi, S.T. Ramesh, P.V. Nidheesh PII:
S0045-6535(18)31247-5
DOI:
10.1016/j.chemosphere.2018.06.172
Reference:
CHEM 21704
To appear in:
Chemosphere
Received Date:
27 April 2018
Accepted Date:
28 June 2018
Please cite this article as: T. Sruthi, R. Gandhimathi, S.T. Ramesh, P.V. Nidheesh, Stabilized Landfill Leachate Treatment using Heterogeneous Fenton and Electro-Fenton Processes, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.06.172
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ACCEPTED MANUSCRIPT Stabilized Landfill Leachate Treatment using Heterogeneous Fenton and ElectroFenton Processes T. Sruthi1, R. Gandhimathi1,*, S. T. Ramesh1, P. V. Nidheesh2,*
1 Department
2CSIR-
of Civil Engineering, National Institute of Technology, Tiruchirappalli, Thuvakudi, Tamil Nadu, India 620 015.
National Environmental Engineering Research Institute, Nagpur, Maharashtra, India, 440020.
*Corresponding author (R. Gandhimathi): Tel.: +91 431 2503171; Fax: +91 431 2500133. Email address: [email protected] *Corresponding author (P. V. Nidheesh): [email protected]; Phone: +918380095670
1
Email:
[email protected],
ACCEPTED MANUSCRIPT 1
Abstract
2
In the present study, stabilized landfill leachate treatment by heterogeneous Fenton and
3
electro-Fenton (EF) was carried out. Iron-manganese binary oxide loaded zeolite (IMZ) was
4
used as a catalyst for generating hydroxyl radicals in the acidic medium. Heterogeneous
5
Fenton process was capable of removing 88.6% COD from landfill leachate at the optimal
6
conditions, while 87.5% COD removal was observed at optimal EF treatment conditions.
7
Biodegradability of landfill leachate was increased significantly from 0.03 to 0.52 after
8
Fenton treatment. The prepared heterogeneous catalyst was found reusable with a reduction
9
in COD removal rate. Even though, both the processes are efficient for leachate treatment, the
10
low catalyst dosage requirement in case of EF process justifies that it is more feasible than
11
Fenton process.
12
Keywords: Landfill leachate; Fenton; Advanced oxidation processes; Heterogeneous
13
catalyst; Iron-manganese binary oxide loaded zeolite
14 15
2
ACCEPTED MANUSCRIPT 16
1. Introduction
17
Leachate generated from the municipal solid waste dumping site leads to further
18
environmental pollution (Kumar and Alappat, 2003; Nagarajan et al., 2012; Maiti et al., 2016;
19
Naveen et al., 2017). Factors like, surface runoff, precipitation, solid waste degradation, etc.
20
affect the production of leachate. Leachate percolates through the soil and contaminates the
21
groundwater. For example, groundwater near the Ariyamangalam dumping site,
22
Tiruchirappalli, Tamilnadu, India has been severely polluted due to the leachate generated
23
(Kanmani and Gandhimathi, 2013). The groundwater has a very high concentration of total
24
dissolved solids, and the chloride concentration itself is in the range of g L-1. The authors also
25
observed toxic heavy metals such as lead, zinc, copper, manganese, cadmium, etc. in leachate
26
contaminated groundwater. Therefore, treatment of leachate is a serious matter of study.
27
Based on the age, leachate is classified into three categories: young, medium and old or
28
stabilized leachate. Leachate less than one-year-old is categorized as young leachate, while
29
medium and stabilized leachates are 1-5 y old and more than five years old respectively.
30
Among all the three leachates, treatment of stabilized leachate is quite difficult since it has
31
low biodegradability, while young and medium leachates can be treated effectively by
32
biological methods (Kumari et al., 2016; Lakshmikanthan and Sivakumar Babu, 2017). On
33
the other hand, physicochemical wastewater treatment methods are found to be effective for
34
the treatment of stabilized leachate (Gandhimathi et al., 2013, 2015). But, disposal of sludge
35
or concentrate generated after the leachate treatment is also a great challenge.
36
In the recent years, water and wastewater containing non-biodegradable pollutants are being
37
treated effectively by advanced oxidation processes (AOPs) which is gaining a lot of
38
attention nowadays. Hydroxyl radicals generated in AOPs have higher oxidation potential,
39
which is in the range of 2.85 V. The hydroxyl radicals generated in the process can degrade
40
organic pollutants via dehydrogenation, redox reaction and electrophilic addition reactions 3
ACCEPTED MANUSCRIPT 41
(Oturan, 2000; Oturan et al., 2000). Fenton process is widely accepted because it is one of the
42
most efficient AOPs for removing various pollutants (Karthikeyan et al., 2012;
43
Babuponnusami and Muthukumar, 2013; Nidheesh et al., 2013; Nidheesh, 2015; Xavier et
44
al., 2015). In Fenton process, the reaction between ferrous ion and hydrogen peroxide leads to
45
the production of hydroxyl radicals under acidic conditions as in Eq. (1). Ferric ion generated
46
via Fenton reaction, reacts further with H2O2 and regenerates ferrous ions as in the Eq. (2)
47
and (3). Even though the Fenton process is efficient but the factors like H2O2 requirement,
48
low ferrous ion regeneration rate, sludge generation, the increment in solution pH with
49
reaction time, etc. lead to the invention of extended Fenton processes (Nidheesh et al., 2013).
50
Electro-Fenton (EF) process is the best example for this. EF process is an extended Fenton
51
process in which H2O2 is generated electrolytically via two electron cathodic reduction of
52
oxygen in acidic medium, as in Eq. (4) (Brillas et al., 2009; Nidheesh and Gandhimathi,
53
2012). Apart from this, EF process nullifies other disadvantages of Fenton processes such as
54
slow ferrous regeneration rate, sludge generation and increase in solution pH with reaction
55
time (Nidheesh et al., 2014a). Ferric ions undergo cathodic reduction immediately as they are
56
generated via Fenton reactions. Cathodic oxidation of water molecules (Eq. (5)) produces
57
sufficient protons in the aqueous medium, which neutralizes the hydroxyl ions generated
58
from Fenton reactions, and it also brings the solution pH near to its initial condition (Chen
59
and Lin, 2009).
60
Fe
61
Fe
62
Fe
63
O2 + 2H
3+
-
2+
+ H2O2→Fe
3+
2+ + • + H2O2⟶Fe + H + HO2
3+
• 2+ + + HO2⟶Fe + O2 + H +
•
(1)
+ OH + HO
+ 2e ⟶H2O2
(2) (3) (4)
4
ACCEPTED MANUSCRIPT +
-
(5)
64
2H2O→O2 + 4H
65
Inability to regenerate the catalyst is the main drawback of conventional Fenton process,
66
which is overcome by using heterogeneous catalyst instead of ferrous salts (Nidheesh et al.,
67
2014b; Nidheesh, 2015; Nidheesh and Rajan, 2016). The pollutant absorbing properties of
68
nano scale particles can also be effectively utilized in the treatment of stabilized leachate
69
(Pavithra and Shanthakumar, 2017). Large specific areas of nano-scale sorbents and
70
widespread zeolites are commonly used as adsorbents, because of their three-dimensional
71
porous structures (Kong et al., 2014). Fenton and electro-Fenton process along with the
72
catalyst, iron-manganese binary oxide loaded zeolite (IMZ), and its recyclability is
73
investigated in this study. The catalyst, IMZ has already been utilized in the treatment of
74
groundwater, and it has been found to be very effective in removal of arsenate and humic
75
acid from the groundwater (Kong et al., 2014).
+ 4e
76 77
2. Materials and Methods
78
2.1.
79
Leachate was collected from the Ariyamangalam dumpsite, Tiruchirapalli where the solid
80
inflow rate is 410 metric tons per day. The samples were collected in 5 L plastic cans and
81
were stored at 4◦C to minimize further oxidation. The landfill leachate used in the study was
82
subjected to preliminary characterization. The initial pH, turbidity, alkalinity, total suspended
83
solids, total dissolved solids, total solids, biochemical oxygen demand (BOD5,20), chemical
84
oxygen demand (COD), etc. were analyzed. The standard methods (APHA, 2012) were
85
followed throughout the process for landfill leachate characterization.
Sample collection and characterization
86 87 5
ACCEPTED MANUSCRIPT 88 89
2.2.
Catalyst preparation and characterization
90
The catalyst used for the study was iron-manganese binary oxide loaded zeolite (IMZ). 200
91
mL of 0.23 M FeSO4.7H2O was added dropwise to 200 mL of 0.75 M of KMnO4 solution
92
which had been mixed with 10 g of zeolite under constant stirring. The resulting mixture was
93
stirred for two hours over magnetic stirrer and aged at room temperature for 12 h, pH was
94
maintained in the range of 7 to 8 using 5 M NaOH and the suspension was filtered and dried
95
at 105◦C for 4 h. Then the sample was calcined at 600◦C and then the dried catalyst was
96
crushed and stored in a desiccator before use. Catalyst characterization was performed using
97
Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM).
98 99
2.3.
Heterogeneous Fenton treatment
100
The experiment was performed in a batch reactor consisting of 1000 mL borosil glass beaker.
101
In a typical run, about 200 mL of sample was taken to which predetermined amount of H2O2
102
and catalyst were added. The reactants were well mixed using a magnetic stirrer at 100 to 200
103
rpm, at room temperature. Leachate samples were taken at regular time intervals for the
104
estimation of remaining COD. Then the different experimental conditions like catalyst
105
dosage, pH and H2O2 dosage were optimized.
106 107
2.4.
Heterogeneous electro-Fenton treatment
108
The experiment was performed in a batch reactor consisting of 1000 mL borosil glass beaker.
109
In a typical run, about 400 mL of leachate sample was taken to which predetermined amount
110
of catalyst (IMZ) was added. The reactants were well mixed using a magnetic stirrer at 100 to
111
200 rpm, at room temperature. Graphite electrodes were used as both anode and cathode and 6
ACCEPTED MANUSCRIPT 112
the external voltage was supplied to the electrolytic cell at room temperature. Samples were
113
withdrawn at regular time intervals for COD estimation. Then the different operating
114
conditions like catalyst dosage, pH, and voltage were optimized.
115
COD reduction by heterogeneous Fenton and heterogeneous EF processes were expressed as
116
a ratio of COD remaining at time ‘t’ (Ct) to COD of raw leachate (C0).
117 118
2.5.
Recyclability study
119
The sludge from the Fenton and the electro-Fenton process under optimized conditions was
120
incubated at 150◦ C overnight, calcined at 600◦ C and was ground into a fine powder. This
121
recycled catalyst was again used for the sample treatment in both Fenton and electro-Fenton
122
processes under optimized conditions and the results obtained were recorded and analyzed.
123 124
3. Results and Discussion
125
3.1.
126
The leachate characteristics are given in Table.1. The results are comparable with our results
127
reported previously (Gandhimathi et al., 2013; Venu et al., 2014; Gandhimathi et al., 2015;
128
Asha et al., 2016; Krupa et al., 2016; Venu et al., 2016). From the Table 1, it can be seen that
129
the leachate has an alkaline pH and the BOD5,20 to COD ratio is 0.03 which suggests that the
130
leachate cannot be treated using biological treatment methods. Therefore, physico-chemical
131
method, especially AOPs are preferred as they lead to complete mineralization of complex
132
organic matter.
Characteristics of leachate
133 134
7
ACCEPTED MANUSCRIPT 135 136
3.2.
Catalyst characterization
137
Scanning Electron Microscopy was used to study surface morphology of IMZ catalyst. The
138
SEM result (Fig. 1a) indicated that the surface of synthesized IMZ catalyst had bead shaped
139
appearance. The SEM image of IMZ catalyst reveals that on the surface of zeolite, iron and
140
manganese were evenly distributed.
141
Fourier Transform Infrared (FTIR) spectrum of IMZ catalyst is shown in Fig. 1b. The
142
functional group of the IMZ catalyst has been investigated using FTIR spectrum. The plotted
143
curve shows the bands at 3442.68 cm-1 are assigned to the vibration of O-H stretching due to
144
the deformation vibration of water molecules indicate the presence of physically adsorbed
145
water on IMZ (Kong et al., 2014). The characteristic peak at 1595.15 cm-1 reflects the O-H
146
bonding vibrations combined with Mn atoms. The peak at 1007.74 cm-1 corresponds to the
147
bonding vibration of the hydroxyl group (Fe-OH) (Kong et al., 2014).
148 149
3.3.
Landfill leachate treatment by heterogeneous Fenton process
150
The efficiency of Fenton process for the removal of COD from landfill leachate was
151
optimized by studying the effect of various operational parameters such as initial catalyst
152
dosage (Fig. 2a), H2O2 concentration (Fig. 2b) and solution pH (Fig. 2c). Initially the
153
experiments were performed at solution pH 3, because this is a well-accepted optimal pH for
154
the effective Fenton oxidation process. Effect of catalyst dosage on the performance of
155
Fenton process was studied by changing the initial catalyst dosage, as 500, 700, 900 and 1100
156
mg L-1 without changing the initial pH of solution and H2O2 concentrations (Fig. 2a). Initial
157
catalyst concentration was considered high, because the optimal catalyst dosage in Fenton
158
process falls in the range of g L-1 (Nidheesh et al., 2013). Laiju et al. (2014) used iron-loaded 8
ACCEPTED MANUSCRIPT 159
mangosteen as heterogeneous catalyst in Fenton oxidation of landfill leachate and found the
160
optimal catalyst dosage as 1750 mg L-1. COD removal efficiency of Fenton process increased
161
with increase in catalyst dosage from 500 to 700 mg L-1. COD reduction was noted as 51.6%
162
and 81.2%, respectively for initial catalyst dosages of 500 and 700 mg L-1, after 10 min of
163
oxidation. Even though, there was a significant difference in COD reduction at the initial
164
stages, COD removal at the completion of 90 min was observed to be the same for added
165
catalyst concentrations. Thus, COD removal rate was higher for 700 mg L-1 catalyst at the
166
initial stages of Fenton treatment. This indicates the better performance of catalyst at 700 mg
167
L-1 than 500 mg L-1. The decrement in the efficiency of the landfill leachate treatment was
168
observed due to further increase in the catalyst concentration in heterogeneous Fenton
169
process. Reduction in COD removal efficiency of Fenton process was not much for the
170
catalyst concentration of 900 mg L-1, while a significant reduction was observed in the case
171
of 1100 mg L-1. Final COD removal efficiency of Fenton process declined to 66.75% for the
172
initial catalyst dosage of 1100 mg L-1, compared to 88.6% COD removal efficiency, when the
173
catalyst dosage was 700 mg L-1, after 90 min of oxidation. This reduction is mainly attributed
174
to the scavenging reactions which occur at higher catalyst concentrations. Hydroxyl radicals
175
generated via Fenton reactions, reacts with excess ferrous ions present and gets converted
176
into its ionic form. Similar results were reported by Laiju et al. (2014). Therefore, the optimal
177
catalyst dosage was taken as 700 mg L-1 and further experiments were carried out with this
178
optimised catalyst dosage.
179
The concentration of H2O2 is another parameter which influences the pollutant degradation
180
ability of Fenton process. Effect of H2O2 concentration on COD removal from landfill
181
leachate by Fenton oxidation was studied at pH 3 and optimal catalyst dosage of 700 mg L-1,
182
by varying the concentration of hydrogen peroxide from 0.003 to 0.043 M (Fig. 2b). Initial
183
concentration of 0.003 M of hydrogen peroxide showed 80.2% COD removal from landfill 9
ACCEPTED MANUSCRIPT 184
leachate after 90 min of Fenton oxidation. Increase in H2O2 concentration increased the
185
efficiency of Fenton process. After 10 min, COD removal efficiency corresponding to 0.003
186
and 0.033 M H2O2 were observed as 69.8% and 83.3%, respectively. This enhanced COD
187
removal efficiency with increase in H2O2 concentration is mainly attributed to the accelerated
188
Fenton reactions and subsequent hydroxyl radical generations. Decrement in COD removal
189
efficiency of Fenton process was observed with further increase in concentration of hydrogen
190
peroxide from 0.033 M. After 90 min of Fenton process, COD removal efficiency for 0.043
191
M hydrogen peroxide was found to be 65.7%, while that of 0.033 M was 88.5%. Decrease in
192
the treatment efficiency was observed at higher concentration of hydrogen peroxide, because
193
of increased reactions between hydrogen peroxide and hydroxyl radicals as in Eq. (6) (Panda
194
et al., 2011). Recombination of hydroxyl radicals at elevated hydrogen peroxide
195
concentration (Eq. (7)) is the other scavenging reaction which retards the efficiency of Fenton
196
process. Similar result was observed by Laiju et al. (2014), Nidheesh and Rajan (2016) and
197
Xavier et al. (2015).
198
H2O2 + OH →H2O + HO2
199
HO + HO ⟶H2O2 (7)
200
Solution pH conditions play a major role in the oxidation of Fenton. It is commonly accepted
201
pH, as the optimal condition for the successful radical production in Fenton processes is 3.
202
The results of the current study also show the same (Fig. 2c). Finally, pH 3 was found to be
203
optimum pH for COD removal for heterogeneous Fenton process. At pH lower than 3,
204
efficiency of the process is reduced to 71.94% after 90 min of oxidation. Formation of
205
oxonium ion is the main reason behind lowering of efficiency at higher acidic conditions.
206
H2O2 reacts with protons present in the water at higher acidic conditions and form oxonium
207
ions. This reaction reduces the net H2O2 concentration useful for the generation of hydroxyl
●
•
●
(6)
•
10
ACCEPTED MANUSCRIPT 208
radicals. Similarly, lower efficiency of Fenton process at higher pH is mainly due to the
209
instability of hydrogen peroxide. COD removal efficiency is reduced to 37.6% at pH 8 after
210
90 min of oxidation. Similar result was observed by Laiju et al. (2014), and Xavier et al.
211
(2015). COD removal efficiency of heterogeneous Fenton process using iron loaded
212
mangosteen was optimal at pH 3 (Laiju et al., 2014).
213
Experiments were also conducted to study the contribution of hydrogen peroxide (oxidation
214
of pollutant by hydrogen peroxide) and percentage of adsorption on catalyst during
215
heterogeneous Fenton reactions (Fig. 2d). Pollutant removal by hydrogen peroxide was
216
investigated by adding 0.033 M hydrogen peroxide at pH 3. Experiment was performed in
217
batch mode for a working volume of 200 mL. Results show that hydrogen peroxide was able
218
to remove 64.1% of COD from the landfill leachate after 90 min. Efficiency of COD removal
219
solely in the presence of H2O2 was observed to increase with time. This indicates that
220
hydrogen peroxide exists in the solution till 90 min and the results observed for the
221
heterogeneous Fenton reactions were due to hydroxyl radicals generated in the water
222
medium.
223
Catalyst used in the study is amorphous in nature and can adsorb the pollutants over its
224
surface. Adsorption test was carried out for the catalyst dosage (700 mg L-1) corresponding to
225
the optimal heterogeneous Fenton reaction condition. In 200 mL of pH regulated landfill
226
leachate, 700 mg L-1 of IMZ was added and stirred continuously. Prepared catalyst turned out
227
to be a good adsorbent as well. COD removal efficiency of the catalyst increased with time
228
and reached to 79.74% after 90 min. Thus in heterogeneous Fenton reactions, both adsorption
229
and H2O2 oxidation are responsible for COD removal from the system.
230
Pollutants adsorbed over the surface of catalyst were found to degrade due to the attack of
231
hydroxyl radicals generated during the process. Pollutant removal efficiency obtained for
232
heterogeneous Fenton process is not due to the sorption of pollutant over the catalyst surface, 11
ACCEPTED MANUSCRIPT 233
which is very clear from the results observed at 30 min reaction time and rate of pollutant
234
removal. For the Fenton process, removal of COD is very rapid, as compared to the sorption
235
process. Fenton oxidation of pollutant occurs at the initial stages of the process and
236
subsequent removal after this is negligible. For example, 83.3% of pollutant was removed
237
after 10 min of Fenton oxidation and reached only to 88.5% after 90 min, at the optimal
238
condition. At the same time, COD removal efficiency of adsorption process gradually
239
increased and reached to 79.74% at 90 min, from 62.6% at 30 min.
240 241
3.4.
Heterogeneous EF oxidation of leachate
242
Electrochemical methods are found to be very effective for the treatment of landfill leachate
243
(Fernandes et al., 2015; Mandal et al., 2017) and use of solid catalysts improved the
244
performance of electrochemical processes significantly (Ganiyu et al., 2018). Effect of
245
catalyst dosage (Fig. 3a), solution pH (Fig. 3b) and applied voltage (Fig. 3c) on efficiency of
246
COD removal of EF process was performed and the operational parameters were optimised.
247
Optimal catalyst required for effective operation of EF process is extremely low, compared to
248
conventional Fenton process (Nidheesh et al., 2013). For example, EF process was able to
249
remove 97% of color, 64% of COD and 47.7% of TOC from real textile wastewater after 60
250
min of electrolysis operated at an applied voltage of 7 V with ferric ion concentration of 10
251
mg L-1 (Nidheesh and Gandhimathi, 2014a). EF process was able to reduce 37% COD and
252
67.7% of color from textile wastewater operated at applied voltage of 5 V, pH 3 with ferric
253
ion concentration of 5 mg L-1, and mode of operation was continuous with a flow rate of 10
254
mL min-1 (Nidheesh and Gandhimathi, 2015a). Compared to optimal catalyst concentration of
255
homogeneous Fenton process, heterogeneous Fenton process requires slightly higher catalyst
256
dosage for its optimal operations (Xavier et al., 2015). Based on this, heterogeneous EF
257
oxidation of landfill leachate was carried out with catalyst dosages ranging from 15 to 35 mg 12
ACCEPTED MANUSCRIPT 258
L-1 (Fig. 3a). Heterogeneous EF process efficiency increased with increase in catalyst dosage
259
from 15 to 25 mg L-1 as observed in heterogeneous Fenton process. Further increase in
260
catalyst dosage decreased the COD removal efficiency of heterogeneous EF process and was
261
mainly due to the excessive scavenging reactions as explained earlier. Thus, 25 mg L-1 of
262
catalyst was considered as optimal for heterogeneous EF operations.
263
Since, the optimal condition for the effective operation of EF process was found to be pH 3,
264
effect of pH on landfill leachate treatment by heterogeneous EF process was studied by
265
conducting experiments at pH 2, 3 and 4. Present study also reveals that pH near 3 is the
266
optimal condition for heterogeneous EF process (Fig. 3b). Compared to heterogeneous
267
Fenton process, higher removal efficiency was observed at pH 2 in heterogeneous EF
268
process. This indicates the continuous formation of H2O2 at the cathode surface and this
269
nullifies the oxonium generation in the water medium at lower pH conditions. Similar result
270
was observed by George et al. (2013, 2014) for the removal of salicylic acid from water
271
medium. The authors observed higher pollutant oxidation at pH 2.5.
272
To find out the voltage level required for the optimal operation of heterogeneous EF process,
273
experiments were carried out at three different voltages, as 3, 4 and 5 V (Fig. 3c). Increase in
274
voltage from 3 to 4 V, increased the performance of heterogeneous EF process and further
275
increase in voltage to 5 V decreased its COD removal efficiency. In heterogeneous EF
276
process, increase in the voltage from 3 V is mainly attributed to the increase in H2O2
277
generation and subsequent formation of hydroxyl radical leading to enhancement in the
278
efficiency of the process. Reformation of ferrous ions was found, which increased with
279
increase in applied voltage. Decrement in the efficiency of COD removal at higher voltages is
280
due to the increase of side reactions such as oxygen and hydrogen evolution as in Eq. (8) and
281
(9). Similar result was observed by Nidheesh and Gandhimathi (2014b, 2015b) for the
282
removal of rhodamine B dye from water medium. 13
ACCEPTED MANUSCRIPT 283
+ 2H2O2⟶4H + O2 + 4e (8)
284
2H
285
COD removal efficiency of heterogeneous EF process under optimal conditions was
286
compared with landfill leachate treatment by aeration and electrolysis (Fig. 3d). Aeration of
287
landfill leachate removed 60.2% of its initial COD. This removal was rapid at the initial
288
stages and gradually increases with aeration time. Electrolysis of landfill leachate
289
significantly reduced its 73.5% of COD. This reduction may be due to the EF or EF like
290
reactions occurring in the presence of dissolved heavy metals in the leachate (Kanmani and
291
Gandhimathi, 2013). COD removal efficiency of electrolysis is significantly lesser than that
292
of heterogeneous EF process. This indicates the significance of external catalyst addition in
293
EF process. Thus the removal efficiency observed for heterogeneous EF process is mostly
294
due to the hydroxyl radicals generated in the presence of heterogeneous catalyst.
295
Biodegradability enhancement of treated wastewater is the main advantage of every AOPs.
296
BOD5, 200 and COD of leachate were observed as 185 and 6160 mg L-1, respectively before
297
treatment and 402 and 768 mg L-1, respectively after treatment. Biodegradability of raw
298
landfill leachate was very less (0.03) and it increased to 0.52 after Fenton treatment. Thus,
299
biological process can be used further for the removal of pollutants remaining in the leachate.
+
-
+ 2e →H2
(9)
300 301
3.5.
Recycling of catalyst
302
Reusable nature of catalyst is one of the main benefits of heterogeneous Fenton systems over
303
homogeneous systems (Nidheesh, 2015). The Fenton catalyst used in the study was also
304
reused for Fenton and EF oxidation of landfill leachate (Fig. 4). Recycled catalyst dosage was
305
considered same as that of optimal catalyst dosage and the efficiency of this process was
14
ACCEPTED MANUSCRIPT 306
compared with Fenton experiments carried out using freshly prepared catalyst. Efficiency of
307
Fenton and EF process using recycled heterogeneous catalyst matched the efficiency of both
308
process when carried out using raw catalyst, after 90 min of reaction. But, the rate of
309
pollutant removal is higher for the processes carried out using fresh catalyst. This is mainly
310
due to the deactivation of heterogeneous catalyst (Nidheesh, 2015). Impurities sorbed over
311
the surface of catalyst may hinder the exposure of iron species. This reduces the effective
312
reactions between H2O2 and catalyst; and subsequent hydroxyl radical generation. Similar
313
results were observed for iron loaded mangosteen as a heterogeneous catalyst for landfill
314
leachate treatment by Fenton process (Laiju et al., 2014). The authors observed COD removal
315
after 60 min as 81% and 59% respectively for raw and recycled heterogeneous catalyst.
316 317
4. Conclusions
318
From the study, it can be concluded that the use of IMZ as a catalyst in Fenton and EF
319
process is suitable for the mineralization of organic matter in stabilized landfill leachate.
320
Catalyst dosage, solution pH and hydrogen peroxide concentration were influenced the
321
performance of heterogeneous Fenton process significantly. At optimum conditions of
322
heterogeneous Fenton process (700 mg L-1 Catalyst, 0.033 M H2O2 and pH 3), 88.6% COD
323
removal was obtained. COD removal efficiency of heterogeneous EF process was dependent
324
on the catalyst dosage, solution pH and applied voltage. At optimum conditions of
325
heterogeneous electro-Fenton process (25 mg L-1 catalyst, pH 3, voltage 4 V) 87.5% of COD
326
removal was obtained. Recycled catalyst in Fenton and electro-Fenton process gave almost
327
the same removal efficiency as that of the raw catalyst. But the COD removal rate reduced
328
significantly with the use of recycled catalyst, and is mainly attributed to the poisoning of
329
catalyst surface. Biodegradability of stabilized landfill leachate was increased from 0.03 to
330
0.52 after heterogeneous Fenton treatment. Compared to Fenton process, electro-Fenton 15
ACCEPTED MANUSCRIPT 331
process requires less catalyst concentration which implies the later process is more feasible
332
for
the
stabilized
leachate
16
treatment.
ACCEPTED MANUSCRIPT 1
References
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3 4
Methods . ISBN 9780875532356 Asha, T.T., Gandhimathi, R., Ramesh, S.T., Nidheesh, P. V, 2016. Treatment of stabilized
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and thermodynamic studies. Environ. Sci. Pollut. Res. 19, 1828-1840.
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Kong, S., Wang, Y., Zhan, H., Liu, M., Liang, L., Hu, Q., 2014. Competitive adsorption of
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humic acid and arsenate on nanoscale iron-manganese binary oxide-loaded zeolite in
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groundwater. J. Geochemical Explor. 144, 220–225.
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Krupa, J.T., Gandhimathi, R., Ramesh, S.T., Nidheesh, P.V., Anantha Singh, T.S., 2016.
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Investigation of biobarrier for leachate containment through batch and continuous flow
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studies. J. Environ. Eng. 142, C4015006.
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Kumari, M., Ghosh, P., Thakur, I.S., 2016. Landfill leachate treatment using bacto-algal co-
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culture: An integrated approach using chemical analyses and toxicological assessment. 18
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Ecotoxicol. Environ. Saf. 128, 44–51. Laiju, A.R., Sivasankar, T., Nidheesh, P. V., 2014. Iron-loaded mangosteen as a
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Lakshmikanthan, P., Sivakumar Babu, G.L., 2017. Performance evaluation of the bioreactor
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electrochemical oxidation: Drawbacks, challenges and future scope. Waste Manage. 69,
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250-273.
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Nagarajan, R., Thirumalaisamy, S., Lakshumanan, E., 2012. Impact of leachate on
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groundwater pollution due to non-engineered municipal solid waste landfill sites of
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Naveen, B.P., Mahapatra, D.M., Sitharam, T.G., Sivapullaiah, P.V., Ramachandra, T.V.,
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2017. Physico-chemical and biological characterization of urban municipal landfill
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leachate. Environ. Pollut. 220, 1-12.
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Nidheesh, P.V., Gandhimathi, R., 2015a. Textile wastewater treatment by electro-fenton
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process in batch and continuous modes. J. Hazardous, Toxic, Radioact. Waste 19,
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04014038.
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Nidheesh, P.V., Gandhimathi, R., 2015b. Electro Fenton oxidation for the removal of Rhodamine B from aqueous solution in a bubble column reactor under continuous mode. 19
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Desalin. Water Treat. 55, 263–271. Nidheesh, P.V., Gandhimathi, R., 2014a. Effect of solution pH on the performance of three
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electrolytic advanced oxidation processes for the treatment of textile wastewater and
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sludge characteristics. RSC Adv. 4, 27946–27954.
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Nidheesh, P.V., Gandhimathi, R., 2014b. Comparative removal of rhodamine B from
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aqueous solution by electro-Fenton and electro-Fenton-like processes. Clean - Soil, Air,
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Water 42, 779–784.
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Nidheesh, P.V., Gandhimathi, R., Ramesh, S.T., 2013. Degradation of dyes from aqueous solution by Fenton processes: A review. Environ. Sci. Pollut. Res. 20:2099–2132. Nidheesh, P. V., 2015. Heterogeneous Fenton catalysts for the abatement of organic pollutants from aqueous solution: a review. RSC Adv. 5, 40552–40577. Nidheesh, P. V., Gandhimathi, R., 2012. Trends in electro-Fenton process for water and wastewater treatment: An overview. Desalination 299, 1-15. Nidheesh, P. V., Gandhimathi, R., Sanjini, N.S., 2014a. NaHCO3 enhanced Rhodamine B
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removal from aqueous solution by graphite-graphite electro Fenton system. Sep. Purif.
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Technol. 132, 568–573.
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Nidheesh, P. V., Gandhimathi, R., Velmathi, S., Sanjini, N.S., 2014b. Magnetite as a
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heterogeneous electro Fenton catalyst for the removal of Rhodamine B from aqueous
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solution. RSC Adv. 4, 5698–5708.
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Nidheesh, P. V., Rajan, R., 2016. Removal of rhodamine B from a water medium using
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hydroxyl and sulphate radicals generated by iron loaded activated carbon. RSC Adv. 6,
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5330–5340.
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generated hydroxyl radicals for in situ destruction of organic pollutants: Application to
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Oturan, M.A., Peiroten, J., Chartrin, P., Acher, A.J., 2000. Complete destruction of p-
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3474–3479.
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Panda, N., Sahoo, H., Mohapatra, S., 2011. Decolourization of Methyl Orange using Fentonlike mesoporous Fe2O3-SiO2 composite. J. Hazard. Mater. 185, 359-365. Pavithra, S., Shanthakumar, S., 2017. Removal of COD, BOD and color from municipal solid waste leachate using silica and iron nano particles - a comparative study. Glob. Nest J. 19, 122–130. Venu, D., Gandhimathi, R., Nidheesh, P. V., Ramesh, S.T., 2014. Treatment of stabilized landfill leachate using peroxicoagulation process. Sep. Purif. Technol. 129, 64–70. Venu, D., Gandhimathi, R., Nidheesh, P. V, Ramesh, S.T., 2016. Effect of solution pH on
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leachate treatment mechanism of peroxicoagulation process. J. Hazardous, Toxic,
15
Radioact. Waste 20, 4–7.
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Xavier, S., Gandhimathi, R., Nidheesh, P.V., Ramesh, S.T., 2015. Comparison of
17
homogeneous and heterogeneous Fenton processes for the removal of reactive dye
18
magenta MB from aqueous solution. Desalin. Water Treat. 53, 109–118.
19
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ACCEPTED MANUSCRIPT 1
List of Figures
2
Fig. 1 (a) SEM image and (b) FTIR spectra of IMZ
3
Fig. 2 Heterogeneous Fenton oxidation of landfill leachate (a) Effect of catalyst dosage on
4
the performance of heterogeneous Fenton process at solution pH 3 and hydrogen peroxide
5
concentration of 0.013 M (b) Variation of COD reduction with hydrogen peroxide
6
concentration at constant solution pH 3 and catalyst dosage of 700 mg L-1 (c) heterogeneous
7
Fenton oxidation efficiency at different initial pH conditions for constant catalyst dosage of
8
700 mg L-1 and hydrogen peroxide concentration of 0.033 M (d) pollutant removal by
9
hydrogen peroxide oxidation (denoted as hydrogen peroxide), adsorption on IMZ (indicated
10
as catalyst) and heterogeneous Fenton oxidation (represented as Fenton) at solution pH 3,
11
catalyst dosage of 700 mg L-1 and hydrogen peroxide concentration of 0.033 M
12
Fig. 3 Landfill leachate treatment by heterogeneous EF process (a) Effect of catalyst dosage
13
on COD removal at constant solution pH 3, applied voltage 5 V, inner electrode spacing of 3
14
cm and electrode area of 25 cm2 (b) Variation of COD removal with changes in initial pH
15
conditions at catalyst concentration of 25 mg L-1, applied voltage 5 V, inner electrode spacing
16
of 3 cm and electrode area of 25 cm2 (c) Heterogeneous EF oxidation efficiency at various
17
applied voltages for constant catalyst concentration of 25 mg L-1, solution pH 3, inner
18
electrode spacing of 3 cm and electrode area of 25 cm2 (d) Landfill leachate treatment
19
efficiencies of aeration, electrolysis and heterogeneous EF processes
20
Fig. 4 Reusability of heterogeneous catalyst in (a) Fenton process and (b) EF process
21 22
ACCEPTED MANUSCRIPT Fig. 1 a
ACCEPTED MANUSCRIPT Fig. 2
ACCEPTED MANUSCRIPT Fig. 3
ACCEPTED MANUSCRIPT Fig. 4
ACCEPTED MANUSCRIPT Highlights
Effective treatment of stabilized landfill leachate by Fenton and electro-Fenton Iron-manganese binary oxide loaded zeolite is an efficient heterogeneous catalyst Enhancement in biodegradability of leachate after treatment
ACCEPTED MANUSCRIPT Table 1. Characteristics of stabilized landfill leachate Parameters pH
Value 8.18
Alkalinity (mg L-1 as CaCO3) 10000 Total solids (mg L-1)
17000
TSS (mg L-1)
7500
TDS (mg L-1)
9500
TVS (mg L-1)
1120
TFS (mg L-1)
15880
COD (mg L-1)
6160
BOD5,20 (mg L-1)
185
BOD5,20/COD
0.03
T. Sruthi, R. Gandhimathi, S.T. Ramesh, P.V. Nidheesh PII:
S0045-6535(18)31247-5
DOI:
10.1016/j.chemosphere.2018.06.172
Reference:
CHEM 21704
To appear in:
Chemosphere
Received Date:
27 April 2018
Accepted Date:
28 June 2018
Please cite this article as: T. Sruthi, R. Gandhimathi, S.T. Ramesh, P.V. Nidheesh, Stabilized Landfill Leachate Treatment using Heterogeneous Fenton and Electro-Fenton Processes, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.06.172
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ACCEPTED MANUSCRIPT Stabilized Landfill Leachate Treatment using Heterogeneous Fenton and ElectroFenton Processes T. Sruthi1, R. Gandhimathi1,*, S. T. Ramesh1, P. V. Nidheesh2,*
1 Department
2CSIR-
of Civil Engineering, National Institute of Technology, Tiruchirappalli, Thuvakudi, Tamil Nadu, India 620 015.
National Environmental Engineering Research Institute, Nagpur, Maharashtra, India, 440020.
*Corresponding author (R. Gandhimathi): Tel.: +91 431 2503171; Fax: +91 431 2500133. Email address: [email protected] *Corresponding author (P. V. Nidheesh): [email protected]; Phone: +918380095670
1
Email:
[email protected],
ACCEPTED MANUSCRIPT 1
Abstract
2
In the present study, stabilized landfill leachate treatment by heterogeneous Fenton and
3
electro-Fenton (EF) was carried out. Iron-manganese binary oxide loaded zeolite (IMZ) was
4
used as a catalyst for generating hydroxyl radicals in the acidic medium. Heterogeneous
5
Fenton process was capable of removing 88.6% COD from landfill leachate at the optimal
6
conditions, while 87.5% COD removal was observed at optimal EF treatment conditions.
7
Biodegradability of landfill leachate was increased significantly from 0.03 to 0.52 after
8
Fenton treatment. The prepared heterogeneous catalyst was found reusable with a reduction
9
in COD removal rate. Even though, both the processes are efficient for leachate treatment, the
10
low catalyst dosage requirement in case of EF process justifies that it is more feasible than
11
Fenton process.
12
Keywords: Landfill leachate; Fenton; Advanced oxidation processes; Heterogeneous
13
catalyst; Iron-manganese binary oxide loaded zeolite
14 15
2
ACCEPTED MANUSCRIPT 16
1. Introduction
17
Leachate generated from the municipal solid waste dumping site leads to further
18
environmental pollution (Kumar and Alappat, 2003; Nagarajan et al., 2012; Maiti et al., 2016;
19
Naveen et al., 2017). Factors like, surface runoff, precipitation, solid waste degradation, etc.
20
affect the production of leachate. Leachate percolates through the soil and contaminates the
21
groundwater. For example, groundwater near the Ariyamangalam dumping site,
22
Tiruchirappalli, Tamilnadu, India has been severely polluted due to the leachate generated
23
(Kanmani and Gandhimathi, 2013). The groundwater has a very high concentration of total
24
dissolved solids, and the chloride concentration itself is in the range of g L-1. The authors also
25
observed toxic heavy metals such as lead, zinc, copper, manganese, cadmium, etc. in leachate
26
contaminated groundwater. Therefore, treatment of leachate is a serious matter of study.
27
Based on the age, leachate is classified into three categories: young, medium and old or
28
stabilized leachate. Leachate less than one-year-old is categorized as young leachate, while
29
medium and stabilized leachates are 1-5 y old and more than five years old respectively.
30
Among all the three leachates, treatment of stabilized leachate is quite difficult since it has
31
low biodegradability, while young and medium leachates can be treated effectively by
32
biological methods (Kumari et al., 2016; Lakshmikanthan and Sivakumar Babu, 2017). On
33
the other hand, physicochemical wastewater treatment methods are found to be effective for
34
the treatment of stabilized leachate (Gandhimathi et al., 2013, 2015). But, disposal of sludge
35
or concentrate generated after the leachate treatment is also a great challenge.
36
In the recent years, water and wastewater containing non-biodegradable pollutants are being
37
treated effectively by advanced oxidation processes (AOPs) which is gaining a lot of
38
attention nowadays. Hydroxyl radicals generated in AOPs have higher oxidation potential,
39
which is in the range of 2.85 V. The hydroxyl radicals generated in the process can degrade
40
organic pollutants via dehydrogenation, redox reaction and electrophilic addition reactions 3
ACCEPTED MANUSCRIPT 41
(Oturan, 2000; Oturan et al., 2000). Fenton process is widely accepted because it is one of the
42
most efficient AOPs for removing various pollutants (Karthikeyan et al., 2012;
43
Babuponnusami and Muthukumar, 2013; Nidheesh et al., 2013; Nidheesh, 2015; Xavier et
44
al., 2015). In Fenton process, the reaction between ferrous ion and hydrogen peroxide leads to
45
the production of hydroxyl radicals under acidic conditions as in Eq. (1). Ferric ion generated
46
via Fenton reaction, reacts further with H2O2 and regenerates ferrous ions as in the Eq. (2)
47
and (3). Even though the Fenton process is efficient but the factors like H2O2 requirement,
48
low ferrous ion regeneration rate, sludge generation, the increment in solution pH with
49
reaction time, etc. lead to the invention of extended Fenton processes (Nidheesh et al., 2013).
50
Electro-Fenton (EF) process is the best example for this. EF process is an extended Fenton
51
process in which H2O2 is generated electrolytically via two electron cathodic reduction of
52
oxygen in acidic medium, as in Eq. (4) (Brillas et al., 2009; Nidheesh and Gandhimathi,
53
2012). Apart from this, EF process nullifies other disadvantages of Fenton processes such as
54
slow ferrous regeneration rate, sludge generation and increase in solution pH with reaction
55
time (Nidheesh et al., 2014a). Ferric ions undergo cathodic reduction immediately as they are
56
generated via Fenton reactions. Cathodic oxidation of water molecules (Eq. (5)) produces
57
sufficient protons in the aqueous medium, which neutralizes the hydroxyl ions generated
58
from Fenton reactions, and it also brings the solution pH near to its initial condition (Chen
59
and Lin, 2009).
60
Fe
61
Fe
62
Fe
63
O2 + 2H
3+
-
2+
+ H2O2→Fe
3+
2+ + • + H2O2⟶Fe + H + HO2
3+
• 2+ + + HO2⟶Fe + O2 + H +
•
(1)
+ OH + HO
+ 2e ⟶H2O2
(2) (3) (4)
4
ACCEPTED MANUSCRIPT +
-
(5)
64
2H2O→O2 + 4H
65
Inability to regenerate the catalyst is the main drawback of conventional Fenton process,
66
which is overcome by using heterogeneous catalyst instead of ferrous salts (Nidheesh et al.,
67
2014b; Nidheesh, 2015; Nidheesh and Rajan, 2016). The pollutant absorbing properties of
68
nano scale particles can also be effectively utilized in the treatment of stabilized leachate
69
(Pavithra and Shanthakumar, 2017). Large specific areas of nano-scale sorbents and
70
widespread zeolites are commonly used as adsorbents, because of their three-dimensional
71
porous structures (Kong et al., 2014). Fenton and electro-Fenton process along with the
72
catalyst, iron-manganese binary oxide loaded zeolite (IMZ), and its recyclability is
73
investigated in this study. The catalyst, IMZ has already been utilized in the treatment of
74
groundwater, and it has been found to be very effective in removal of arsenate and humic
75
acid from the groundwater (Kong et al., 2014).
+ 4e
76 77
2. Materials and Methods
78
2.1.
79
Leachate was collected from the Ariyamangalam dumpsite, Tiruchirapalli where the solid
80
inflow rate is 410 metric tons per day. The samples were collected in 5 L plastic cans and
81
were stored at 4◦C to minimize further oxidation. The landfill leachate used in the study was
82
subjected to preliminary characterization. The initial pH, turbidity, alkalinity, total suspended
83
solids, total dissolved solids, total solids, biochemical oxygen demand (BOD5,20), chemical
84
oxygen demand (COD), etc. were analyzed. The standard methods (APHA, 2012) were
85
followed throughout the process for landfill leachate characterization.
Sample collection and characterization
86 87 5
ACCEPTED MANUSCRIPT 88 89
2.2.
Catalyst preparation and characterization
90
The catalyst used for the study was iron-manganese binary oxide loaded zeolite (IMZ). 200
91
mL of 0.23 M FeSO4.7H2O was added dropwise to 200 mL of 0.75 M of KMnO4 solution
92
which had been mixed with 10 g of zeolite under constant stirring. The resulting mixture was
93
stirred for two hours over magnetic stirrer and aged at room temperature for 12 h, pH was
94
maintained in the range of 7 to 8 using 5 M NaOH and the suspension was filtered and dried
95
at 105◦C for 4 h. Then the sample was calcined at 600◦C and then the dried catalyst was
96
crushed and stored in a desiccator before use. Catalyst characterization was performed using
97
Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM).
98 99
2.3.
Heterogeneous Fenton treatment
100
The experiment was performed in a batch reactor consisting of 1000 mL borosil glass beaker.
101
In a typical run, about 200 mL of sample was taken to which predetermined amount of H2O2
102
and catalyst were added. The reactants were well mixed using a magnetic stirrer at 100 to 200
103
rpm, at room temperature. Leachate samples were taken at regular time intervals for the
104
estimation of remaining COD. Then the different experimental conditions like catalyst
105
dosage, pH and H2O2 dosage were optimized.
106 107
2.4.
Heterogeneous electro-Fenton treatment
108
The experiment was performed in a batch reactor consisting of 1000 mL borosil glass beaker.
109
In a typical run, about 400 mL of leachate sample was taken to which predetermined amount
110
of catalyst (IMZ) was added. The reactants were well mixed using a magnetic stirrer at 100 to
111
200 rpm, at room temperature. Graphite electrodes were used as both anode and cathode and 6
ACCEPTED MANUSCRIPT 112
the external voltage was supplied to the electrolytic cell at room temperature. Samples were
113
withdrawn at regular time intervals for COD estimation. Then the different operating
114
conditions like catalyst dosage, pH, and voltage were optimized.
115
COD reduction by heterogeneous Fenton and heterogeneous EF processes were expressed as
116
a ratio of COD remaining at time ‘t’ (Ct) to COD of raw leachate (C0).
117 118
2.5.
Recyclability study
119
The sludge from the Fenton and the electro-Fenton process under optimized conditions was
120
incubated at 150◦ C overnight, calcined at 600◦ C and was ground into a fine powder. This
121
recycled catalyst was again used for the sample treatment in both Fenton and electro-Fenton
122
processes under optimized conditions and the results obtained were recorded and analyzed.
123 124
3. Results and Discussion
125
3.1.
126
The leachate characteristics are given in Table.1. The results are comparable with our results
127
reported previously (Gandhimathi et al., 2013; Venu et al., 2014; Gandhimathi et al., 2015;
128
Asha et al., 2016; Krupa et al., 2016; Venu et al., 2016). From the Table 1, it can be seen that
129
the leachate has an alkaline pH and the BOD5,20 to COD ratio is 0.03 which suggests that the
130
leachate cannot be treated using biological treatment methods. Therefore, physico-chemical
131
method, especially AOPs are preferred as they lead to complete mineralization of complex
132
organic matter.
Characteristics of leachate
133 134
7
ACCEPTED MANUSCRIPT 135 136
3.2.
Catalyst characterization
137
Scanning Electron Microscopy was used to study surface morphology of IMZ catalyst. The
138
SEM result (Fig. 1a) indicated that the surface of synthesized IMZ catalyst had bead shaped
139
appearance. The SEM image of IMZ catalyst reveals that on the surface of zeolite, iron and
140
manganese were evenly distributed.
141
Fourier Transform Infrared (FTIR) spectrum of IMZ catalyst is shown in Fig. 1b. The
142
functional group of the IMZ catalyst has been investigated using FTIR spectrum. The plotted
143
curve shows the bands at 3442.68 cm-1 are assigned to the vibration of O-H stretching due to
144
the deformation vibration of water molecules indicate the presence of physically adsorbed
145
water on IMZ (Kong et al., 2014). The characteristic peak at 1595.15 cm-1 reflects the O-H
146
bonding vibrations combined with Mn atoms. The peak at 1007.74 cm-1 corresponds to the
147
bonding vibration of the hydroxyl group (Fe-OH) (Kong et al., 2014).
148 149
3.3.
Landfill leachate treatment by heterogeneous Fenton process
150
The efficiency of Fenton process for the removal of COD from landfill leachate was
151
optimized by studying the effect of various operational parameters such as initial catalyst
152
dosage (Fig. 2a), H2O2 concentration (Fig. 2b) and solution pH (Fig. 2c). Initially the
153
experiments were performed at solution pH 3, because this is a well-accepted optimal pH for
154
the effective Fenton oxidation process. Effect of catalyst dosage on the performance of
155
Fenton process was studied by changing the initial catalyst dosage, as 500, 700, 900 and 1100
156
mg L-1 without changing the initial pH of solution and H2O2 concentrations (Fig. 2a). Initial
157
catalyst concentration was considered high, because the optimal catalyst dosage in Fenton
158
process falls in the range of g L-1 (Nidheesh et al., 2013). Laiju et al. (2014) used iron-loaded 8
ACCEPTED MANUSCRIPT 159
mangosteen as heterogeneous catalyst in Fenton oxidation of landfill leachate and found the
160
optimal catalyst dosage as 1750 mg L-1. COD removal efficiency of Fenton process increased
161
with increase in catalyst dosage from 500 to 700 mg L-1. COD reduction was noted as 51.6%
162
and 81.2%, respectively for initial catalyst dosages of 500 and 700 mg L-1, after 10 min of
163
oxidation. Even though, there was a significant difference in COD reduction at the initial
164
stages, COD removal at the completion of 90 min was observed to be the same for added
165
catalyst concentrations. Thus, COD removal rate was higher for 700 mg L-1 catalyst at the
166
initial stages of Fenton treatment. This indicates the better performance of catalyst at 700 mg
167
L-1 than 500 mg L-1. The decrement in the efficiency of the landfill leachate treatment was
168
observed due to further increase in the catalyst concentration in heterogeneous Fenton
169
process. Reduction in COD removal efficiency of Fenton process was not much for the
170
catalyst concentration of 900 mg L-1, while a significant reduction was observed in the case
171
of 1100 mg L-1. Final COD removal efficiency of Fenton process declined to 66.75% for the
172
initial catalyst dosage of 1100 mg L-1, compared to 88.6% COD removal efficiency, when the
173
catalyst dosage was 700 mg L-1, after 90 min of oxidation. This reduction is mainly attributed
174
to the scavenging reactions which occur at higher catalyst concentrations. Hydroxyl radicals
175
generated via Fenton reactions, reacts with excess ferrous ions present and gets converted
176
into its ionic form. Similar results were reported by Laiju et al. (2014). Therefore, the optimal
177
catalyst dosage was taken as 700 mg L-1 and further experiments were carried out with this
178
optimised catalyst dosage.
179
The concentration of H2O2 is another parameter which influences the pollutant degradation
180
ability of Fenton process. Effect of H2O2 concentration on COD removal from landfill
181
leachate by Fenton oxidation was studied at pH 3 and optimal catalyst dosage of 700 mg L-1,
182
by varying the concentration of hydrogen peroxide from 0.003 to 0.043 M (Fig. 2b). Initial
183
concentration of 0.003 M of hydrogen peroxide showed 80.2% COD removal from landfill 9
ACCEPTED MANUSCRIPT 184
leachate after 90 min of Fenton oxidation. Increase in H2O2 concentration increased the
185
efficiency of Fenton process. After 10 min, COD removal efficiency corresponding to 0.003
186
and 0.033 M H2O2 were observed as 69.8% and 83.3%, respectively. This enhanced COD
187
removal efficiency with increase in H2O2 concentration is mainly attributed to the accelerated
188
Fenton reactions and subsequent hydroxyl radical generations. Decrement in COD removal
189
efficiency of Fenton process was observed with further increase in concentration of hydrogen
190
peroxide from 0.033 M. After 90 min of Fenton process, COD removal efficiency for 0.043
191
M hydrogen peroxide was found to be 65.7%, while that of 0.033 M was 88.5%. Decrease in
192
the treatment efficiency was observed at higher concentration of hydrogen peroxide, because
193
of increased reactions between hydrogen peroxide and hydroxyl radicals as in Eq. (6) (Panda
194
et al., 2011). Recombination of hydroxyl radicals at elevated hydrogen peroxide
195
concentration (Eq. (7)) is the other scavenging reaction which retards the efficiency of Fenton
196
process. Similar result was observed by Laiju et al. (2014), Nidheesh and Rajan (2016) and
197
Xavier et al. (2015).
198
H2O2 + OH →H2O + HO2
199
HO + HO ⟶H2O2 (7)
200
Solution pH conditions play a major role in the oxidation of Fenton. It is commonly accepted
201
pH, as the optimal condition for the successful radical production in Fenton processes is 3.
202
The results of the current study also show the same (Fig. 2c). Finally, pH 3 was found to be
203
optimum pH for COD removal for heterogeneous Fenton process. At pH lower than 3,
204
efficiency of the process is reduced to 71.94% after 90 min of oxidation. Formation of
205
oxonium ion is the main reason behind lowering of efficiency at higher acidic conditions.
206
H2O2 reacts with protons present in the water at higher acidic conditions and form oxonium
207
ions. This reaction reduces the net H2O2 concentration useful for the generation of hydroxyl
●
•
●
(6)
•
10
ACCEPTED MANUSCRIPT 208
radicals. Similarly, lower efficiency of Fenton process at higher pH is mainly due to the
209
instability of hydrogen peroxide. COD removal efficiency is reduced to 37.6% at pH 8 after
210
90 min of oxidation. Similar result was observed by Laiju et al. (2014), and Xavier et al.
211
(2015). COD removal efficiency of heterogeneous Fenton process using iron loaded
212
mangosteen was optimal at pH 3 (Laiju et al., 2014).
213
Experiments were also conducted to study the contribution of hydrogen peroxide (oxidation
214
of pollutant by hydrogen peroxide) and percentage of adsorption on catalyst during
215
heterogeneous Fenton reactions (Fig. 2d). Pollutant removal by hydrogen peroxide was
216
investigated by adding 0.033 M hydrogen peroxide at pH 3. Experiment was performed in
217
batch mode for a working volume of 200 mL. Results show that hydrogen peroxide was able
218
to remove 64.1% of COD from the landfill leachate after 90 min. Efficiency of COD removal
219
solely in the presence of H2O2 was observed to increase with time. This indicates that
220
hydrogen peroxide exists in the solution till 90 min and the results observed for the
221
heterogeneous Fenton reactions were due to hydroxyl radicals generated in the water
222
medium.
223
Catalyst used in the study is amorphous in nature and can adsorb the pollutants over its
224
surface. Adsorption test was carried out for the catalyst dosage (700 mg L-1) corresponding to
225
the optimal heterogeneous Fenton reaction condition. In 200 mL of pH regulated landfill
226
leachate, 700 mg L-1 of IMZ was added and stirred continuously. Prepared catalyst turned out
227
to be a good adsorbent as well. COD removal efficiency of the catalyst increased with time
228
and reached to 79.74% after 90 min. Thus in heterogeneous Fenton reactions, both adsorption
229
and H2O2 oxidation are responsible for COD removal from the system.
230
Pollutants adsorbed over the surface of catalyst were found to degrade due to the attack of
231
hydroxyl radicals generated during the process. Pollutant removal efficiency obtained for
232
heterogeneous Fenton process is not due to the sorption of pollutant over the catalyst surface, 11
ACCEPTED MANUSCRIPT 233
which is very clear from the results observed at 30 min reaction time and rate of pollutant
234
removal. For the Fenton process, removal of COD is very rapid, as compared to the sorption
235
process. Fenton oxidation of pollutant occurs at the initial stages of the process and
236
subsequent removal after this is negligible. For example, 83.3% of pollutant was removed
237
after 10 min of Fenton oxidation and reached only to 88.5% after 90 min, at the optimal
238
condition. At the same time, COD removal efficiency of adsorption process gradually
239
increased and reached to 79.74% at 90 min, from 62.6% at 30 min.
240 241
3.4.
Heterogeneous EF oxidation of leachate
242
Electrochemical methods are found to be very effective for the treatment of landfill leachate
243
(Fernandes et al., 2015; Mandal et al., 2017) and use of solid catalysts improved the
244
performance of electrochemical processes significantly (Ganiyu et al., 2018). Effect of
245
catalyst dosage (Fig. 3a), solution pH (Fig. 3b) and applied voltage (Fig. 3c) on efficiency of
246
COD removal of EF process was performed and the operational parameters were optimised.
247
Optimal catalyst required for effective operation of EF process is extremely low, compared to
248
conventional Fenton process (Nidheesh et al., 2013). For example, EF process was able to
249
remove 97% of color, 64% of COD and 47.7% of TOC from real textile wastewater after 60
250
min of electrolysis operated at an applied voltage of 7 V with ferric ion concentration of 10
251
mg L-1 (Nidheesh and Gandhimathi, 2014a). EF process was able to reduce 37% COD and
252
67.7% of color from textile wastewater operated at applied voltage of 5 V, pH 3 with ferric
253
ion concentration of 5 mg L-1, and mode of operation was continuous with a flow rate of 10
254
mL min-1 (Nidheesh and Gandhimathi, 2015a). Compared to optimal catalyst concentration of
255
homogeneous Fenton process, heterogeneous Fenton process requires slightly higher catalyst
256
dosage for its optimal operations (Xavier et al., 2015). Based on this, heterogeneous EF
257
oxidation of landfill leachate was carried out with catalyst dosages ranging from 15 to 35 mg 12
ACCEPTED MANUSCRIPT 258
L-1 (Fig. 3a). Heterogeneous EF process efficiency increased with increase in catalyst dosage
259
from 15 to 25 mg L-1 as observed in heterogeneous Fenton process. Further increase in
260
catalyst dosage decreased the COD removal efficiency of heterogeneous EF process and was
261
mainly due to the excessive scavenging reactions as explained earlier. Thus, 25 mg L-1 of
262
catalyst was considered as optimal for heterogeneous EF operations.
263
Since, the optimal condition for the effective operation of EF process was found to be pH 3,
264
effect of pH on landfill leachate treatment by heterogeneous EF process was studied by
265
conducting experiments at pH 2, 3 and 4. Present study also reveals that pH near 3 is the
266
optimal condition for heterogeneous EF process (Fig. 3b). Compared to heterogeneous
267
Fenton process, higher removal efficiency was observed at pH 2 in heterogeneous EF
268
process. This indicates the continuous formation of H2O2 at the cathode surface and this
269
nullifies the oxonium generation in the water medium at lower pH conditions. Similar result
270
was observed by George et al. (2013, 2014) for the removal of salicylic acid from water
271
medium. The authors observed higher pollutant oxidation at pH 2.5.
272
To find out the voltage level required for the optimal operation of heterogeneous EF process,
273
experiments were carried out at three different voltages, as 3, 4 and 5 V (Fig. 3c). Increase in
274
voltage from 3 to 4 V, increased the performance of heterogeneous EF process and further
275
increase in voltage to 5 V decreased its COD removal efficiency. In heterogeneous EF
276
process, increase in the voltage from 3 V is mainly attributed to the increase in H2O2
277
generation and subsequent formation of hydroxyl radical leading to enhancement in the
278
efficiency of the process. Reformation of ferrous ions was found, which increased with
279
increase in applied voltage. Decrement in the efficiency of COD removal at higher voltages is
280
due to the increase of side reactions such as oxygen and hydrogen evolution as in Eq. (8) and
281
(9). Similar result was observed by Nidheesh and Gandhimathi (2014b, 2015b) for the
282
removal of rhodamine B dye from water medium. 13
ACCEPTED MANUSCRIPT 283
+ 2H2O2⟶4H + O2 + 4e (8)
284
2H
285
COD removal efficiency of heterogeneous EF process under optimal conditions was
286
compared with landfill leachate treatment by aeration and electrolysis (Fig. 3d). Aeration of
287
landfill leachate removed 60.2% of its initial COD. This removal was rapid at the initial
288
stages and gradually increases with aeration time. Electrolysis of landfill leachate
289
significantly reduced its 73.5% of COD. This reduction may be due to the EF or EF like
290
reactions occurring in the presence of dissolved heavy metals in the leachate (Kanmani and
291
Gandhimathi, 2013). COD removal efficiency of electrolysis is significantly lesser than that
292
of heterogeneous EF process. This indicates the significance of external catalyst addition in
293
EF process. Thus the removal efficiency observed for heterogeneous EF process is mostly
294
due to the hydroxyl radicals generated in the presence of heterogeneous catalyst.
295
Biodegradability enhancement of treated wastewater is the main advantage of every AOPs.
296
BOD5, 200 and COD of leachate were observed as 185 and 6160 mg L-1, respectively before
297
treatment and 402 and 768 mg L-1, respectively after treatment. Biodegradability of raw
298
landfill leachate was very less (0.03) and it increased to 0.52 after Fenton treatment. Thus,
299
biological process can be used further for the removal of pollutants remaining in the leachate.
+
-
+ 2e →H2
(9)
300 301
3.5.
Recycling of catalyst
302
Reusable nature of catalyst is one of the main benefits of heterogeneous Fenton systems over
303
homogeneous systems (Nidheesh, 2015). The Fenton catalyst used in the study was also
304
reused for Fenton and EF oxidation of landfill leachate (Fig. 4). Recycled catalyst dosage was
305
considered same as that of optimal catalyst dosage and the efficiency of this process was
14
ACCEPTED MANUSCRIPT 306
compared with Fenton experiments carried out using freshly prepared catalyst. Efficiency of
307
Fenton and EF process using recycled heterogeneous catalyst matched the efficiency of both
308
process when carried out using raw catalyst, after 90 min of reaction. But, the rate of
309
pollutant removal is higher for the processes carried out using fresh catalyst. This is mainly
310
due to the deactivation of heterogeneous catalyst (Nidheesh, 2015). Impurities sorbed over
311
the surface of catalyst may hinder the exposure of iron species. This reduces the effective
312
reactions between H2O2 and catalyst; and subsequent hydroxyl radical generation. Similar
313
results were observed for iron loaded mangosteen as a heterogeneous catalyst for landfill
314
leachate treatment by Fenton process (Laiju et al., 2014). The authors observed COD removal
315
after 60 min as 81% and 59% respectively for raw and recycled heterogeneous catalyst.
316 317
4. Conclusions
318
From the study, it can be concluded that the use of IMZ as a catalyst in Fenton and EF
319
process is suitable for the mineralization of organic matter in stabilized landfill leachate.
320
Catalyst dosage, solution pH and hydrogen peroxide concentration were influenced the
321
performance of heterogeneous Fenton process significantly. At optimum conditions of
322
heterogeneous Fenton process (700 mg L-1 Catalyst, 0.033 M H2O2 and pH 3), 88.6% COD
323
removal was obtained. COD removal efficiency of heterogeneous EF process was dependent
324
on the catalyst dosage, solution pH and applied voltage. At optimum conditions of
325
heterogeneous electro-Fenton process (25 mg L-1 catalyst, pH 3, voltage 4 V) 87.5% of COD
326
removal was obtained. Recycled catalyst in Fenton and electro-Fenton process gave almost
327
the same removal efficiency as that of the raw catalyst. But the COD removal rate reduced
328
significantly with the use of recycled catalyst, and is mainly attributed to the poisoning of
329
catalyst surface. Biodegradability of stabilized landfill leachate was increased from 0.03 to
330
0.52 after heterogeneous Fenton treatment. Compared to Fenton process, electro-Fenton 15
ACCEPTED MANUSCRIPT 331
process requires less catalyst concentration which implies the later process is more feasible
332
for
the
stabilized
leachate
16
treatment.
ACCEPTED MANUSCRIPT 1
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List of Figures
2
Fig. 1 (a) SEM image and (b) FTIR spectra of IMZ
3
Fig. 2 Heterogeneous Fenton oxidation of landfill leachate (a) Effect of catalyst dosage on
4
the performance of heterogeneous Fenton process at solution pH 3 and hydrogen peroxide
5
concentration of 0.013 M (b) Variation of COD reduction with hydrogen peroxide
6
concentration at constant solution pH 3 and catalyst dosage of 700 mg L-1 (c) heterogeneous
7
Fenton oxidation efficiency at different initial pH conditions for constant catalyst dosage of
8
700 mg L-1 and hydrogen peroxide concentration of 0.033 M (d) pollutant removal by
9
hydrogen peroxide oxidation (denoted as hydrogen peroxide), adsorption on IMZ (indicated
10
as catalyst) and heterogeneous Fenton oxidation (represented as Fenton) at solution pH 3,
11
catalyst dosage of 700 mg L-1 and hydrogen peroxide concentration of 0.033 M
12
Fig. 3 Landfill leachate treatment by heterogeneous EF process (a) Effect of catalyst dosage
13
on COD removal at constant solution pH 3, applied voltage 5 V, inner electrode spacing of 3
14
cm and electrode area of 25 cm2 (b) Variation of COD removal with changes in initial pH
15
conditions at catalyst concentration of 25 mg L-1, applied voltage 5 V, inner electrode spacing
16
of 3 cm and electrode area of 25 cm2 (c) Heterogeneous EF oxidation efficiency at various
17
applied voltages for constant catalyst concentration of 25 mg L-1, solution pH 3, inner
18
electrode spacing of 3 cm and electrode area of 25 cm2 (d) Landfill leachate treatment
19
efficiencies of aeration, electrolysis and heterogeneous EF processes
20
Fig. 4 Reusability of heterogeneous catalyst in (a) Fenton process and (b) EF process
21 22
ACCEPTED MANUSCRIPT Fig. 1 a
ACCEPTED MANUSCRIPT Fig. 2
ACCEPTED MANUSCRIPT Fig. 3
ACCEPTED MANUSCRIPT Fig. 4
ACCEPTED MANUSCRIPT Highlights
Effective treatment of stabilized landfill leachate by Fenton and electro-Fenton Iron-manganese binary oxide loaded zeolite is an efficient heterogeneous catalyst Enhancement in biodegradability of leachate after treatment
ACCEPTED MANUSCRIPT Table 1. Characteristics of stabilized landfill leachate Parameters pH
Value 8.18
Alkalinity (mg L-1 as CaCO3) 10000 Total solids (mg L-1)
17000
TSS (mg L-1)
7500
TDS (mg L-1)
9500
TVS (mg L-1)
1120
TFS (mg L-1)
15880
COD (mg L-1)
6160
BOD5,20 (mg L-1)
185
BOD5,20/COD
0.03