Modified natural zeolite as heterogeneous Fenton catalyst in treatment of recalcitrants in industrial effluent [original]

HOSTED B Y
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Progress in Natural Science: Materials International
jour nal hom epa ge: www.elsevier.com/locate/pnsmi
Original Research
Modi ﬁ ed natural zeolite as heterogeneous Fenton catalyst in treatment of
recalcitrants in industrial e ﬄ uent
Milton M. Arimi
a , b
a
Technische Universität Berlin, Department of Environmental Technology, Chair of Environmental Process Engineering, Secr. KF 2, Straße des 17. Juni 135,
D-10623 Berlin, Germany
b
Moi University Main Campus, Faculty of Technology, P.O. Box 3900, Eldoret, Kenya
ARTICLE INFO
Keywords:
Fenton
Heterogeneous
Wastewater
Zeolite
Treatment
ABSTRACT
Industrial e ﬄ uents with high recalcitrants should undergo post-treatment after biological treatment. The aim of
this study was to use cheap and abundantly available natural materials to develop heterogeneous Fenton
catalysts for the removal of colored recalcitrants in molasses distillery wastewater (MDW). The pellets of zeolite,
which is naturally available in many countries, were modi ﬁ ed by pre-treatment with sulphuric acid, nitric acid
and hydrochloric acid, before embedding on them the ferrous ions. The e ﬀ ects of pH and temperature on
heterogeneous Fenton were studied using the modi ﬁ ed catalysts. The sulphuric acid-ferrous modi ﬁ ed catalysts
showed the highest a ﬀ ectivity which achieved 90% color and 60% TOC (total organic carbon) removal at 150 g/
L pellet catalyst dosage, 2 g/L H
2
O
2
and 25 °C. The heterogeneous Fenton with the same catalyst caused
improvement in the biodegradability of anaerobic e ﬄ uent from 0.07 to 0.55. The catalyst was also applied to
pre-treat the raw MDW and increased it's biodegradability by 4%. The color of the resultant anaerobic e ﬄ uent
was also reduced. The kinetics of total TOC removal was found to depend on operation temperature. It was best
described by simultaneous ﬁ rst and second order kinetics model for the initial reaction and second order model
for the rest of the reaction.
1. Introduction
Many industrial processes generate a lot of wastewater which
cannot be disposed o ﬀ into natural bodies without causing pollution
even after biological treatment due to high concentration of recalci-
trants. This necessitates appropriate secondary treatment after the
primary digestion. The molasses distillery wastewater (MDW) is among
the e ﬄ uents which require post-treatment after biological digestion.
This is because of its high recalcitrant COD (chemical oxygen demand)
( > 1.5 g/L). This COD is caused by melanoidins and related com-
pounds. Melanoidins are the dark colored recalcitrants formed by the
reaction between sugars and amino acids at medium temperature ( >
50 °C) and in basic pH medium [1] . These conditions are prevalent in
sugar production process where by-products of molasses are generated.
The MDW is produced in large quantities as the e ﬄ uent of ethanol
distilleries which use molasses as their substrates [1] . For example a
ﬁ rm producing bioethanol from molasses distillery wastewaters will
release ten liters of e ﬄ uent called molasses distillery wastewater
(MDW) for every liter of bioethanol produced. The volume can be
further increased by another tenfold if dilution with fresh water is done
before MDW is anaerobically digested [2] . This high water requirement
in such processes can be o ﬀ set by reusing the treated e ﬄ uent.
However, the possible reuse of anaerobically digested MDW as dilution
water is only tenable if the recalcitrants are ﬁ rst removed or their
biodegradability increased.
One of the methods commonly applied to increase the biodegrad-
ability of the recalcitrants in wastewater is by the use of Advanced
Oxidation Processes (AOPs): Ozone [3] , UV/H
2
O
2
[4] , ultrasonic [5] ,
Fenton [6] , electrochemical [7] , photocatalysis [8] and ultraviolet
oxidation [9] . The use of ozonation as post-treatment has an advantage
of low sludge formation but its application is limited by the high
installation and operation costs. The problem becomes even more
complicated if the process involves high daily volumes of e ﬄ uents.
Another limitation of ozonation process is the low COD removal
especially where the in ﬂ uent has reasonably high COD. The UV and
photocatalysis processes are also costly because of the high energy
requirements. Moreover, the later process is in research stages and its
industrial application in complex wastewater is not yet optimized. The
http://dx.doi.org/10.1016/j.pnsc.2017.02.001
Received 18 December 2015; Received in revised form 17 February 2017; Accepted 28 February 2017
Peer review under responsibility of Chinese Materials Research Society.
E-mail addresses: [email protected] , [email protected] .
Progress in Natural Science: Materials International 27 (2017) 275–282
Available online 28 March 2017
1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
MARK

Fenton process is simple to operate and moderately e ﬀ ective. However,
the process operates at very low pH values (2 – 3) and the oxidation
chemicals are costly. The electrochemical process has the challenge of
application in complex e ﬄ uents because there are many types of
particles, compounds, cations and anions which interfere with the
process. The application ultrasonic oxidation in treatment of complex
e ﬄ uent is limited by the high installation/process costs as well as low
COD removal.
Wastewater treatment by the Fenton process involves oxidation of
COD by the highly reactive hydroxyl radicals formed by the reaction of
ferrous ions and hydrogen peroxide. The enhancement of biodegrad-
ability of wastewater after the treatment with Fenton process has been
reported before in treatment of chip board e ﬄ uent where the BOD
5
/
COD ratio was raised from 0.09 to 0.33 [10] . Another study with
Fenton pre-treatment process reported some improvements in COD
removal from 90% to 99% in poultry manure wastewater after
anaerobic digestion with upward anaerobic sludge blanket (UASB)
reactor [11] . In addition to improving biodegradability of the e ﬄ uent,
the Fenton process removes the remnant COD, toxicity and the color of
the e ﬄ uent. The Fenton process combined with coagulation was
reported to e ﬀ ectively remove the textile color [6] . Another study on
Fenton oxidation process with some pharmaceutical wastewater ob-
served 45 – 65% reduction of COD by the oxidation step; the overall
COD removal was 98% when a biological step was added [12] . This
demonstrates that the toxic wastewater had clearly become biodegrad-
able after Fenton oxidation. The electro-Fenton pre-treatment has also
been applied in olive oil mill wastewater pre-treatment where more
than 65% polyphenols were removed; this improved the subsequent
anaerobic digestion [13] . Similar reports on enhanced biodegradability
of land ﬁ ll leachate after pre-treatment with Fenton process have also
been documented [14] . Fenton oxidation has also been applied in the
removal of lignin from cellulosic biomass before anaerobic digestion
and thereby increasing its methane yield [15] .
The classical Fenton process is the simplest and the oldest form of
Fenton processes and has been used in treatment of various waste-
waters [9,16] . The process has several limitations which include: high
sludge formation, operation at adverse pH values (normally 2 – 3), high
remnant metal ions in the sludge and treated e ﬄ uent, inactivation of
heavy metal ions in the sludge by formation of hydroxide complexes
and the need to separate the catalyst after the process. In pursuit of
overcoming these limitations, several other processes have been
developed from the classic Fenton process. The ﬁ rst process was to
couple light energy from ultraviolet source or emissions from the sun in
a process called photo-Fenton oxidation [17] . The extra energy helps in
dissociation of the hydrogen peroxide molecules for easy reactivity. It
also helps to convert ferric ions back to ferrous ions catalyst after the
reaction. The process has been instrumental in improving the kinetics
and the performance of Fenton process including: decreasing the
demand for the catalyst and improving the color removal [18,19] .
There are also reports of enhanced performance in COD removal,
increase in biodegradability [20] and detoxi ﬁ cation of toxic e ﬄ uents
[21,22] after treatment with photo-Fenton oxidation. However, the
photo-Fenton process does not address the other limitations especially
the low pH operation and the problem of high heavy metal ions in the
ﬁ nal e ﬄ uent. The addition of energy requirement also adds to the
process cost.
Another key modi ﬁ cation to the classic Fenton process aims at
eliminating the limitations of low pH operation and recovery of spent
catalyst from the e ﬄ uent by the use of heterogeneous Fenton pro-
cesses. It entails embedment of the ferrous catalyst and the acid group
on a carrier material. By limiting the catalyst supply in the e ﬄ uent, the
method ensures that minimal sludge is formed by coagulation process.
In addition, the amount of heavy metal ions in the sludge and treated
e ﬄ uent is minimized. The catalyst ions are supposed to be slowly
released from the embedment where they react with the hydrogen
peroxide to form the radicals. The need for extremely low operation pH
can also be avoided by immobilising the acid on the carrier material.
After the process, the heterogeneous catalysts are easily separated from
the e ﬄ uent and sludge for reuse.
Many groups have reported di ﬀ erent materials as possible carrier
medium for heterogeneous Fenton oxidation: synthetic zeolite [23] ,
clay [24] , activated carbon [25] and modi ﬁ ed iron-carbon catalyst [26] .
The main limitations of the most suggested material is either low
e ﬀ ectiveness or the high costs. Activated carbon and synthetic zeolites
adsorbents are the most studied heterogeneous carrier materials
because of their high e ﬀ ectiveness. The former is limited by its high
costs as well as the production of large amount of sludge. Zeolites are
the compounds of aluminosilicates and can be arti ﬁ cially synthesized
by reacting sodium aluminate with sodium silicate. The ratio of silica to
the alumina determines the type (X or Y) of the synthetic zeolite. The Y
type of synthetic zeolite is the most commonly applied type in
preparation of heterogeneous Fenton catalysts [23,27,28] . One way
of producing heterogeneous Fenton catalyst from synthetic zeolite is by
impregnation of ferric ions followed by calcinations [29] . Another
process is by the ion exchange for example where the sodium in zeolite
containing high sodium content is replaced with ferric ions [28] . The
synthetic zeolites have been used as heterogeneous material for Fenton
oxidation in wastewater treatment where high e ﬀ ectiveness ( > 80%
TOC and 100% color removal) was reported [23] . It was also observed
that the catalyst was able to work well at pH 5 and the accumulation of
heavy metals in the treated e ﬄ uent was reduced by 17 fold compared
to homogeneous Fenton process [23] .
The high cost of the synthetic zeolites is the main limitation in its
application especially in the developing countries. Natural zeolites
occur on earth surface in many parts of the world. They are used
commercially as adsorbents to remove dyes [30] and heavy metals [31] .
The natural zeolite materials are cheap due to their abundance and
have been tried before as adsorbent for the removal of color in
industrial e ﬄ uent but not with great results [32] . The main limitation
of natural zeolite as a catalyst carrier in Fenton process is its low
e ﬀ ectiveness especially in the e ﬄ uents loaded with high organic matter.
The current study seek to develop an e ﬀ ective heterogeneous Fenton
catalysts which will overcome the limitation of the classic Fenton
oxidation by operating at high pH, reducing the sludge formation,
minimizing the remnant heavy metal ions in the treated e ﬄ uent, and
being recoverable after use for future reuse. In addition, the catalyst
should be cost e ﬀ ective for application in treatment of various e ﬄ uents.
The investigations tested several modi ﬁ cations of natural zeolite as the
ﬁ rst steps in attempt to develop a cheap and more e ﬀ ective carrier
material for heterogeneous Fenton process. Moreover, the kinetics of
the heterogeneous Fenton process and the e ﬀ ect of the temperature
and pH were also investigated for the selected methods of zeolite
modi ﬁ cation.
2. Materials and methods
2.1. Materials
The Nordzucker AG, Braunschweig, Germany provided the raw
MDW which had been pre-concentrated to 600 g/L and packaged in
20 l plastic containers. The natural zeolite purchased from Egezeolit
Company, Turkey. Hydrogen peroxide (w/v 35%), sodium hydrogen
sulphite (w/v 39%), and hydrochloric acid (w/v 32%) were all
purchased from Merck KGaA, Darmstadt, Germany. Other chemicals
also purchased from the same company include: Nitric acid, sulphuric
acid, sodium hydroxide, trace element compounds and ferrous chloride
heptahydrate (FeSO
4
·7H
2
O) which were of laboratory grade.
2.2. Anaerobic digestion
The pr e-co ncen trat ed MD W wa s dil uted to CO D 10 – 12 g/ L with tap
wate r. The tr ace nut rien ts were ad ded, i nclud ing ca lciu m, nick el, co balt,
M.M. Arimi Progress in Natural Science: Materials International 27 (2017) 275–282
276

moly bd ate, zin c, mang anes e, co pper sa lts, an d pH was adju sted to 7 wi th
sodi um hydr ox ide. Th e subs tr ate was tr ansfe rred to a 3-l ta nk with a
Rush to n st irr er as an anae robi c bior eact or. The in ocul ums pel let slud ge
was ob tai ned fr om a br ewer y wast ewat er trea tm ent pl ant an d was ad ded
to 30 % of the bi or eact or volu me. Th e temp erat ur e of the bi orea ct or was
cont rol led at 35 °C by a wa te r jacke t. Th e reac tor wa s op erat ed in a
sequ ence ba tch mo de wi th a dail y subs trat e fe ed of 1. 4 l. The ana ero-
bica lly di gest ed e ﬄ ue nt was deca nted af ter sett ling an d stor ed at 4 °C for
furt he r test s. Its ma in para mete rs incl ude CO D 100 0 – 1 300 mg/L , BOD
5
60 – 130 mg /L, cond ucti vi ty 6 – 8m S / c m a n d p H 7 – 8. T he othe r char -
acte rist ic s of the e ﬄ ue nt have be en docu ment ed [33] .
2.3. Modi ﬁ cation of natural zeolite pellets
Natural zeolite pellets were washed with tap water and rinsed with
distilled water before drying overnight in an oven at 105 °C. Various
treatments were carried out on the pellets as preparation for embed-
ding ferrous ions on it. For each treatment, 200g zeolite pellets were
put in a 500 mL labelled beaker and 100 mL of 2 M of the treating
solution (sulphuric acid, nitric acid, hydrochloric acid or sodium
chloride) added. The mixtures in the beakers were shaken at
120 rpm for 6 h by a rotor shaker which was placed inside an oven
maintained at 60 °C. The excess treating solution was then poured out
and the zeolite pellets dried overnight in the oven at 100 °C. After
cooling, 100g samples of this zeolite were weighed into separate
beakers clearly marked and 100 mL of 0.2 M FeSO
4
solution added
into each beaker. For control experiment, no acid/base pre-treatment
was done but the catalyst was embedded directly on the pellets as
described [27] . The beakers were shaken by a rotor shaker placed in the
oven at 60 °C at 120 rpm for 6 h. The excess solution was kept for
analysis and the pellets washed ten times with excess tap water to
remove unbound ferrous ions. The pellets were rinsed twice with
distilled water and dried overnight in oven at 105 °C. They were then
cooled and weighed before use. All the water from the washings was
added to the unused FeSO
4
solution from the oven. The solution was
mixed to homogeneity and a sample was drawn for iron content
analysis by the AAS machine.
2.4. Heterogeneous Fenton process
The pH of anaerobically digested MDW was adjusted to a set value
(1.5, 2, 3, , 4, 6 or 7) by use of 10 M HCl. Di ﬀ erent amounts of
sulphuric acid modi ﬁ ed zeolite (3 – 20g) were placed in the beaker
before adding 100 ml of pH adjusted MDW. This was followed by
adding 500 mL of 35% H
2
O
2
while shaking at 120 rpm with a rotor
shaker. The samples (2 mL each) were taken at di ﬀ erent time interval
and their pH was increased beyond 10 by adding 2 drops of 10 M
NaOH to stop any further reaction and induce coagulation. The
experiment was repeated without taking samples and the ﬁ nal treated
e ﬄ uent was coagulated by adjusting its pH to 4 – 5 using NaOH and
HCL. The mixture in the beaker was allowed to clarify by allowing it to
sit still for 20 min. The clari ﬁ ed top e ﬄ uent was pipetted out and used
for BOD and remnant iron content analysis. The experiment was
repeated using catalyst pellets modi ﬁ ed by other modifying solutions.
The best performing modi ﬁ cations in terms of color and TOC
removal were H
2
SO
4
and HNO
3
; the two were used to perform further
investigations. The optimal catalyst dose was determined by carrying
out the experiment with various catalyst dosages and optimal H
2
O
2
:
COD ratio of 2 [34] . The optimal reactants conditions were used to
study the e ﬀ ects on the temperature by operating at di ﬀ erent tem-
peratures (20 – 60 °C) and pH values (2.5 – 7). All the tests were
repeated twice or thrice for duplicity purposes.
The optimal dosage of sulphuric acid modi ﬁ ed zeolite catalyst
(0.5 g/l) was used to study the biodegradability changes with time.
The laboratory beakers; 200 mL capacity were ﬁ lled with 100 ml MDW
which had pH adjusted to 4. Into each beaker, 15 g of the modi ﬁ ed
catalyst was added before pouring in 500 ml of 35% H
2
O
2
to start o ﬀ
the reaction. Two beakers were withdrawn from the shaker at set time
(5 min, 1 h, 2 h, 4 h and 18 h). The reaction in these beakers was
stopped by adding 500 ml of sodium hydrogen sulphite solution. The
pH was adjusted to 4 – 5 using sodium hydroxide solution. It was
allowed to sit still for an hour for clari ﬁ cation to occur. The top
clear e ﬄ uent was pipetted out and used for TOC, COD and BOD
5
analysis.
The kinetic tests on TOC removal by heterogeneous Fenton were
done with 500 ml beaker ﬁ lled with 200 ml MDW at pH 3.5. Twenty
grams of modi ﬁ ed zeolite (sulphuric acid or nitric acid) was added in
each beaker before pouring in 1 ml of 35% H
2
O
2
to start o ﬀ the
reaction. Two samples of 1.5 ml each were drawn from each beaker
after a set time and placed into eppendorf tubes using a pipette. The
reaction in the tubes was immediately stopped by adding into each tube
two drops of concentrated NaOH solution. The tubes were shaken and
allowed to sit still for twenty minutes for sedimentation to take place.
The top clear portion of the e ﬄ uent was pippeted out and used for TOC
analysis.
2.5. Pre-treatment of raw MDW by heterogeneous Fenton
The test on enhancement of biodegradability was carried out with
100 mL raw MDW. The e ﬄ uent with TOC (4 – 5) g/L and unaltered pH
(4.8) were placed in labelled beakers in a shaker and various weights of
sulphuric modi ﬁ ed zeolite (1 – 16) g/L added. Various dosages of
hydrogen peroxide were ﬁ nally added to start o ﬀ the reaction. The
beaker contents were shaken at 120 rpm for 24 h. The end samples
were taken without allowing the beaker contents to settle and their
TOC and biodegradability analyzed.
2.6. Analysis
The total organic carbon (TOC) and dissolved organic carbon
(DOC) were analyzed by Analytik Jena Multi N/C 3100 while the
turbidity was analyzed by Hach 2100 AN turbidometer. The measure-
ment of the pH was done by a WTW microprocessor pH meter. The
COD was analyzed using a Hach Lange kit and the color was quanti ﬁ ed
with a Hach Lange DR 500 spectrophotometer. The remnant iron ions
in the treated e ﬄ uent were analyzed by AAS while the BOD was
measured with an OxiTop system, according to the German DIN EN
9408. The iron bound on zeolite was calculated by subtracting the
unbound iron from the total iron supplied. The unbound iron was
analyzed by AAS machine from the homogenized solution containing
unused ferrous solution and all the washing water for the pellets after
iron embedment. The di ﬀ erence between the ferrous ions supplied and
total amount of unbound iron quanti ﬁ ed with AAS was used to
calculate the ferrous ions embedded on the modi ﬁ ed zeolite pellets.
3. Results
3.1. Color removal by heterogeneous Fenton system
The preliminary tests on color removal by heterogeneous Fenton
with sulphuric acid modi ﬁ ed zeolite catalysts produced the highest
color removal. At room temperature, the catalyst was able to achieve
almost 90% color removal with initial pH of e ﬄ uent at 4 ( Fig. 1 a). The
modi ﬁ cation of natural zeolites by treatment with NaOH produced no
improvement in the color removal and so this experiment was not
continued. Among the preliminary tests made, the natural zeolite pre-
treatment by hydrochloric acid had the lowest color removal at 25 °C
and pH 4. For all the zeolite treatment methods tested, the optimal
color removal occurred between zeolite concentrations 100 – 150 g/L
( Fig. 1 a). The choice of hydrogen peroxide concentration 2 g/L used in
this experiment is based on optimal H
2
O
2
/COD ratio of 2 reported
elsewhere [34] . When the recovered catalysts pellets were reused
M.M. Arimi Progress in Natural Science: Materials International 27 (2017) 275–282
277

without re-modi ﬁ cation treatment, the process was able to remove
more than 80% of the original color ( Fig. 1 b).
3.2. TOC removal by heterogeneous Fenton process and the pH e ﬀ ects
of the process
The recalcitrant TOC after biological digestion of MDW is between
1 – 2 g/L. The goal of the post-treatment is either to eliminate the TOC
or make it more biodegradable especially if the e ﬄ uent is to undergo
some reuse or disposal in productive land. The survey of the modi ﬁ ca-
tion process indicated that the sulphuric acid/ferrous ion pre-treat-
ment produced the best results in TOC removal with almost 60%
removal at pH 3 – 4 and 298 K ( Fig. 2 a). The reuse of the same catalyst
on the similar process removed more than 40% TOC. Nitric acid
modi ﬁ cation produced catalysts with had the second best performance.
However, this was only 41% TOC removal in the ﬁ rst run ( Fig. 2 a).
The performance of the modi ﬁ ed zeolite catalysts were dependent
on the initial pH of the substrate. At pH ≤ 2, the nitric acid modi ﬁ ed
zeolite gave the highest performance in terms of TOC removal.
However, at pH ≥ 3, the sulphuric acid modi ﬁ ed zeolite catalyst showed
the highest TOC removal ( Fig. 2 b). The ability of sulphuric acid
modi ﬁ ed zeolite to perform better in TOC removal at slightly higher
pH values is possibly due to its ability to adsorb and retain some acid
on its surface.
3.3. Heterogeneous Fenton and the temperature e ﬀ ects of the process
The two best modi ﬁ ed zeolite catalysts (sulphuric acid and nitric
acid) were used to test the e ﬀ ect of the temperature on the MDW color
removal ( Fig. 3 a) and TOC removal ( Fig. 3 b). For both sulphuric acid
and nitric acid pretreated zeolite pellets, the highest temperature tested
(333 K), produced the highest color removal which was over 90%. The
results indicate that the removal of MDW color by heterogeneous
Fenton is dependent on the operation temperature where the increase
in temperature increases the color removal.
However, the e ﬀ ect of the temperature on overall TOC removal for
sulphuric acid and nitric acid treated zeolite had di ﬀ erent pattern,
Fig. 3 b. The highest TOC removal was at 303 K for the sulphuric acid
modi ﬁ ed zeolite. For the zeolite pretreated by nitric acid, the increase
in temperature from 293 K to 323 K increases the performance. The
e ﬀ ect of temperature on TOC removal in heterogeneous Fenton can be
Fig. 1. Heterogeneous Fenton color removal at room temperature with 2 g/L H
2
O
2
,
150 g/L modi ﬁ ed catalyst for: di ﬀ erent heterogeneous catalyst dosages (a) catalyst reuse
experiment, (b).
Fig. 2. The TOC removal by heterogeneous Fenton at room temperature, 150 g/L
modi ﬁ ed catalyst, 2 g/L H
2
O
2
for ﬁ rst use and reuse of catalyst (a) and the pH e ﬀ ects of
TOC removal by the process at room temperature, 150 g/L modi ﬁ ed catalyst, 2 g/L H
2
O
2
(b).
Fig. 3. The temperature e ﬀ ects at optimal pH (3 or 4) of the color (a) and overall TOC
(b) removal by the heterogeneous process at room temperature, 150 g/L modi ﬁ ed
catalyst, 2 g/L H
2
O
2
.
M.M. Arimi Progress in Natural Science: Materials International 27 (2017) 275–282
278

explained by the two main processes involved; oxidation and coagula-
tion. High temperature increases the kinetic energy and collision
frequency of particles which favors the rate of oxidation. However,
the e ﬀ ect is not the same with coagulation process. The overall TOC
removal was highest at medium temperature because of the e ﬀ ect of
temperature on coagulation. High temperatures favour the kinetics of
dissociation of particles coalescing together which prevents the forma-
tion of big particles. The room temperatures are best suited for high
TOC removal because bigger particles can be formed by coagulation.
Very Low temperatures may not favour the process because of the slow
kinetics. The sensitivity of coagulation rate by ferric ions to increased
temperatures has been reported elsewhere [35] . This explains the slight
decrease in performance of the zeolite treated by sulphuric acid with
higher temperatures.
3.4. The TOC removal kinetics by heterogeneous Fenton
The determination of Fenton reactions kinetics is usually di ﬃ cult.
This is because these processes are complex in nature which includes
the presence of many reacting species and occurrence of simultaneous
oxidation and coagulation processes. This determination is even more
di ﬃ cult where the e ﬄ uent has a complex matrix like in MDW. The
overall TOC removal was used to gauge the process kinetics. The results
from these studies indicate that the process generally follows second
order kinetics ( Fig. 4 ). This is in agreement with previous studies with
homogeneous Fenton which reported the same [34] .
The kinetics of overall TOC removal was ﬁ tted into the ﬁ rst order
model Eq. (1) .
dc
dt
kt =−
t
(1)
The data was ﬁ tted into the model by plotting the equation of ln (c
t
/
c
o
) against time but the data did not agree with the model. The data was
also ﬁ tted into the second order kinetics model described by the Eq.
(2) :
dc
dt
kc =−
t
t
2

(2)
The model was applied by plotting 1/c
t
versus time. The data for the
time period after ten minutes ﬁ tted more accurately into the second
order kinetics model for both modi ﬁ ed zeolite catalysts. Moreover, the
data was also tested with the model equation for the simultaneous ﬁ rst
and second order reactions model proposed by Emami et al. 2010 [36] ,
which is described by the following equation with C=c
t
/c
o
. It was
plotted using Eq. (3) .
⎛
⎝
⎜ ⎞
⎠
⎟
C
C
k
t

ln
1−
=
(3)
The results ﬁ tted in the simultaneous ﬁ rst and second order model
well for the data from zeolite catalyst modi ﬁ ed with nitric acid for the
initial time period (less than 10 min) where the k values were 0.032
(min)
− 1
, 0.038 (min)
− 1
and 0.049 (min)
− 1
, for the reaction at (293 K),
(303 K) and (323 K) respectively. The reaction rate for the rest of the
reaction was however reversed with the reaction at highest temperature
(323 K) having the smallest k value, 0.005 L/(g-min)
− 1
and (303 K)
with the highest value, 0.007 L (g-min)
− 1
from the second order model.
3.5. Heterogeneous Fenton and biodegradability
The heterogeneous catalyst optimization at pH 3.5 and after 4 h
indicated that 0.45 g/L dosage of embedded ferrous ions was optimal
for the COD removal and biodegradation ( Fig. 5 a). Though an increase
in catalyst beyond the optimal value slightly increases the COD
removal, the biodegradability is not enhanced because excess ferrous
Fig. 4. The kinetics of TOC elimination by heterogeneous Fenton with 2 g/L H
2
O
2
and 150 g/L Zeolite at temperatures (293 K, 303 K and 323 K), pH 3.5 for sulphuric acid modi ﬁ ed
zeolite with its second order kinetics (a) and nitric acid modi ﬁ ed zeolite with simultaneous ﬁ rst and second order kinetics, pH (b).
M.M. Arimi Progress in Natural Science: Materials International 27 (2017) 275–282
279

ions scavenge on the radicals formed. It also results in formation of
ferric ions which may form iron oxyhydroxides and deposit producing
unnecessary sludge [14] .
The tests performed using modi ﬁ ed zeolite catalyst as heteroge-
neous Fenton agree with those from classic Fenton process that the
biodegradability of treated e ﬄ uent increase with time ( Fig. 5 b). The
results indicate that the classic Fenton had higher biodegradability
increase compared to the heterogeneous process for similar duration of
reaction time.
3.6. Pre-treatment of raw MDW by heterogeneous Fenton process
The pre-treatment of raw MDW was done with controlled reactants
in heterogeneous Fenton process with the ratio of hydrogen peroxide to
in ﬂ uent COD of 1:36. The process resulted in more than 4% increase in
biodegradability which was accompanied by less than 4% loss of COD
( Fig. 6 ). Moreover, the pre-treatment of the raw MDW before anaerobic
digestion reduced the color of the e ﬄ uent after anaerobic digestion by
almost 30%. It was also observed that the optimum heterogeneous
Fenton pH with natural zeolite catalyst pretreated by sulphuric acid
was 4 – 5. This implies that the raw MDW before anaerobic digestion
which has pH 4.8 can be pretreated using the zeolite modi ﬁ ed catalyst
in heterogeneous Fenton preoxidation without the need to pre-adjust
its pH.
4. Discussion
Fenton process is an established advanced oxidation technology
and its use of in removal of color and organic matter from various
wastewaters has been reported [6,11,16,17,19,37] . The main purposes
of Fenton oxidation process in post-treatment of wastewater include
the removal of COD, color and toxicity before the e ﬄ uent disposal.
Among the limitations of the classic Fenton processes, the requirement
of operation at low pH (2 – 3) is the main problem to most complex
e ﬄ uents. For example, most of the cost involved in treatment of
molasses distillery wastewater by the Fenton method goes to adjust the
pH prior to the oxidation process and neutralisation of treated e ﬄ uent
[38] . The high remnant metal contents in the e ﬄ uent treated by classic
Fenton process ( > 30 g/L) [38] is also very problematic. This is
because most environmental authorities worldwide have the limit of
iron below 10 g/L for the safe disposal. The development of an e ﬀ ective
heterogeneous Fenton catalyst has the potential to solve these limita-
tions. This study was aimed at using an abundantly available material,
natural zeolite to develop a heterogeneous Fenton catalyst. The
material is cheap and freely available on earth surface in many parts
of the world. There is no documentation of a cheap and e ﬀ ective
modi ﬁ cation of natural zeolite material as heterogeneous Fenton
catalyst [27] .
The Fenton oxidation is e ﬀ ected by the reaction of hydroxyl radicals
which oxidize and/or mineralize the substrate aided by their high
reactivity. The process is a chain of reactions which begins by
generation of the radicals as shown in Eq. (4) ;
Fe
2+
+H
2
O
2
→ Fe
3+
+
•
OH+OH
−
(4)
The reaction of radical is complex because it can attack any ion
species in the e ﬄ uent thereby producing di ﬀ erent product for each
reaction. In presence of the organic matter at optimal substrate/
reactants ratio the following reaction takes place;
•
OH+RH → R
•
+H
2
O (5)
The catalyst is regenerated as follows;
Fe
3+
+H
2
O
2
→ Fe
2+
+O
2
•
+H
2
O (6)
In case of excess ferrous catalyst the radicals are scavenged as
follows;
Fe
2+
+
•
OH → Fe
3+
+OH
−
(7)
It is thus important that in homogeneous Fenton processes, the
presence of the optimal ratio of the reactants is maintained. In
heterogeneous Fenton, the catalyst is not freely available in the solution
but embedded on a carrier material. This makes the process more
ﬂ exible in terms of optimum reactants ’ ratio dependence. However, the
process may have slower kinetics compared to homogeneous processes.
This is because the catalysts ions are embedded on the carrier material
which provides less contact area for the oxidants/catalyst compared to
the homogeneous processes. This study showed that the reaction would
proceed beyond 200 min compared to less 120 min reported for room
temperature in homogeneous Fenton process [34] . The heterogeneous
Fenton has additional advantage in that the catalyst embedment
ensures that there is no excess catalyst in the solution hence no
possibility of high sludge formation due to coagulation or deposition of
iron oxyhydroxides [14] .
The optimal H
2
O
2
:COD ratio used in heterogeneous Fenton with
anaerobically digested MDW was 2.12. It was reported earlier for
Fig. 5. The COD and biodegradability changes in anaerobically digested MDW after
heterogeneous Fenton with sulphuric acid modi ﬁ ed zeolite catalyst, 2 g/L H
2
O
2
at pH 2.5
after 5 h (a) and comparison of biodegradability changes with time with 2 g/L H
2
O
2
,p H
2.5, 170 mg/L Fe
2+
for classic Fenton instead of 0.65 Fe
2+
g/L in modi ﬁ ed zeolite (b).
Fig. 6. The variation of the COD and biodegradability of raw MDW after pretreatment
with sulphuric acid modi ﬁ ed zeolites at room temperature, 20 g/L catalyst pellets, 2 g/L
H
2
O
2
and pH 4.8.
M.M. Arimi Progress in Natural Science: Materials International 27 (2017) 275–282
280

homogeneous Fenton oxidation [34] . This study also observed that the
optimal ratio of the ferrous ions embedded on sulphuric acid modi ﬁ ed
zeolite pellets to COD was (1:3). The similar ratio of ferrous ions to
COD in homogeneous Fenton processes is much higher (1:20) [34] .
This is however case dependent and will be determined by how ferrous
ions were embedded on the zeolite and the washing done to remove
unbound or loosely bound ferrous ions. The heterogeneous process
achieved up to 60% COD removal and increased the biodegradability of
digested MDW from 0.07% to 0.5% at room temperature conditions.
In addition to TOC and color removal, the heterogeneous Fenton
process can be applied to pre-treat the raw wastewater and increase its
biodegradability before biological treatment. The improvement of
MDW biodegradability is caused by oxidation of recalcitrant to more
biodegradable products and their removal through coagulation process.
Our previous studies indicated that the improvement of biodegrad-
ability after coagulation process was negligible [39] . This implies that
the oxidation of recalcitrant contributes more to improving biodegrad-
ability than the coagulation process associated with the Fenton process.
Another disadvantage of the classic Fenton is the high remnant metal
ions. The remnant iron concentration in treated e ﬄ uent for classic
Fenton can go beyond 50 mg/L [38] but with heterogeneous Fenton,
the same was less than 10 mg/L. The maximum iron allowed in most
European countries is between 3 – 10 mg/L, which implies that the
heterogeneous Fenton has an edge over homogeneous Fenton in this
regard.
Previous studies have reported that the increase in temperature
improves the TOC removal by conventional Fenton process [34,36] .I n
this study, the e ﬀ ects of temperature on TOC and color removal were
di ﬀ erent. The higher temperature favors the color removal for the
experiment with catalyst as modi ﬁ ed zeolites. However, the TOC
removal had completely opposite e ﬀ ects especially with the sulphuric
acid-ferrous modi ﬁ ed catalyst. This can be explained by the two
processes; coagulation and oxidation, which are involved in the
elimination of TOC and color in the Fenton processes. Oxidation
involves conversion of ferrous ions into ferric ions and the vice versa.
The conversion of ferric ions to ferrous ions is slow which makes it the
limiting step of the Fenton process. The increase in temperature
increases the kinetics of this sub-reaction and so the process is faster.
Coagulation on the other hand is a ﬀ ected by the increase in tempera-
ture which provides particles with energy to break o ﬀ from the
coagulated mass and ﬂ ow back into solution. It is thus logical to say
that the color removal in Fenton process is a ﬀ ected more by oxidation
than coagulation of recalcitrant colorants. The TOC loss is however
predominantly caused by the coagulation process.
The increase in biodegradability of digested MDW after treatment
with Fenton and heterogeneous Fenton was found to depend on the
reaction time for both classic and heterogeneous Fenton processes
( Fig. 5 b). It was also observed that the biodegradability after 18 h is far
much higher than that after 30 min for the both processes. This implies
that even though the COD removal is almost instantaneous for classic
Fenton, both processes require long operation time ( > 5 h) for the
enhancement of recalcitrants ’ biodegradability to values above 0.5. The
time requirement for biodegradability enhancement is probably occa-
sioned by the slow oxidation rate of the recalcitrants or their
derivatives to more biodegradable compounds. Similar reports of
changes in biodegradation depending on time after Fenton treatment
have been documented in other compounds [7] .
In pre-treatment processes involving bioenergy recovery, the de-
crease in COD is undesirable because it reduces the process energy
potential. A balance on the need to increase biodegradability and the
loss in COD by pre-treatment is thus necessary. The Fenton process has
been used to pre-treat the lignocellusic biomass before anaerobic
digestion. The pre-treatment served to break down the lignin and
thereby helped to increase the methane yield [15] . This study used the
heterogeneous Fenton to increase the biodegradability of raw MDW
before anaerobic digestion. Of the oxidant dosages tested (0, 0.33, 0.67,
1.0, 1.3) g/L, the minimum COD loss and the maximum biodegrad-
ability increase was with 0.33 g/L H
2
O
2
dosage. The experiment
resulted in about 4% increase in biodegradability with less than 4%
COD loss. However, this occurred at controlled catalyst dose of 0.2 g/L
embedded catalyst. An increase in catalyst dose resulted in high COD
removal because the excess ferrous ions were converted to ferric ions
which caused coagulation. It is important to note that the process was
carried out without alteration of raw MDW pH and at room tempera-
ture for 24 h. This suggests that the modi ﬁ ed catalyst can be used to
solve the limitation of convention Fenton process of low pH operation
in post-treatment and high sludge formation which causes high COD
loss for the pre-treatment process.
5. Conclusion
The most e ﬀ ective modi ﬁ cation of natural zeolite for heterogeneous
Fenton catalyst was sulphuric acid-ferrous treatment which achieved
color and TOC removal above 90% and 60% respectively. The second
performing catalyst was nitric acid-ferrous modi ﬁ ed zeolite. The
e ﬀ ectiveness of the modi ﬁ ed catalysts was found to depend on the
temperature and the pH with the highest temperatures used (333 K)
producing the highest color removal. However, the e ﬀ ect of the high
temperatures on the removal rate of TOC di ﬀ ered for the two modi ﬁ ed
catalysts. The optimum temperatures for overall TOC removal are
303 K and 323 K for sulphuric-ferrous modi ﬁ ed and nitric acid-ferrous
catalysts respectively. Moreover, the optimal pH for sulphuric-ferrous
modi ﬁ ed catalyst is 4 while that of nitric acid-ferrous catalyst is 3. The
biodegradability of the digested MDW treated by heterogeneous
Fenton using modi ﬁ ed catalysts has been found to depend on reaction
time. The value exceeds 0.55 after 18 h.
In addition to posttreatment, the modi ﬁ ed zeolite can be used as a
heterogeneous Fenton catalyst in the pre-treatment of the raw MDW
before anaerobic digestion with the minimal COD loss. The pretreated
e ﬄ uent has the color eliminated by 30% after anaerobic digestion. It is
found that the second order kinetic model describe best the overall
TOC reaction after ten minutes for the sulphuric acid modi ﬁ ed
reaction. The TOC removal data from nitric acid modi ﬁ ed catalyst ﬁ t
in both second order model and simultaneous ﬁ rst and second order
model. However the latter ﬁ ts better for the initial reactions kinetics.
The modi ﬁ cation of natural zeolite by nitric acid and sulphuric acid
treatment has the potential for producing heterogeneous Fenton
catalysts and should be therefore studied further.
Acknowledgement
The support and assistance of Ms Christina Senge and Liane
Kapitzk of Technical University of Berlin in laboratory analysis is
appreciated. The ﬁ nancial support by Deutscher Akademischer
Austausch Dienst and National Commission of Science, Technology
and Innovation Kenya, reference no. 91548162 is also acknowledged.
The analysis equipment from technical environment protection depart-
ment of Technical University of Berlin provided all the analysis
equipments and it is highly appreciated.
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