International Journal of Applied Environmental Sciences

ISSN 0973-6077 Volume 12, Number 8 (2017), pp. 1509-1519

© Research India Publications

http://www.ripublication.com

A Review: Catalysts for Organic Heterogen

Wastewater Treatment

Uus Supriatna 1, Dadan Sumiarsa2, Akhmad Zainal Abidin3, Yayat Dhahiyat4

1 Doctoral Program of Environmental Science

Postgraduate School Universitas Padjajaran

Jl. Dipati Ukur 35 Bandung, Indonesia.

2Faculty of Mathematics and Natural Sciences Universitas Padjadjaran

Jl. Raya Bandung-Sumedang km.21 Jatinangor, Indonesia 45363.

3 Department of Chemical Engineering

Bandung Institute of Technology

Jl. Ganesha 10 Bandung, Indonesia.

4Faculty of Fishery and Marine Sciences Universitas Padjadjaran

Jl. Raya Bandung-Sumedang km.21 Jatinangor, Indonesia 45363.

Abstract

The treatment for organic heterogen wastewater commonly uses conventional

technologies, which unfortunately are inefficient due to its high cost.

Therefore, a new inexpensive method for the treatment of industrial

wastewater is always interesting to be researched. Photocalatytic is an

advanced oxidation process for wastewater decontamination. It uses oxide

semiconductor materials as photocatalysts which are promising materials for

water purification. Titanium dioxide (TiO2) is the most prominent candidate

for high-performance photocatalysis, due to its low cost and ability to absorb

solar energy. The development of photocatalysis has been continued by

researchers, and the name of the new methode of photocatalysis is

sonophotocatalysis. It is a combination of photocatalytic reaction with

ultrasonic irradiation or the simultaneous irradiation of ultrasound and light

with a photocatalyst. An example of this method is by treating organic

1510 Uus Supriatna, Dadan Sumiarsa, Akhmad Zainal Abidin, Yayat Dhahiyat

heterogen wastewater using 2,4,6 trichlorophenol (TCP) as the model and

degrading it with ultrasonic or ultraviolet irradiation or a combination of both

(rutile or anatase for sonocatalysis, photocatalysis, or sonophotocatalysis,

respectively), in the presence of TiO2. The results suggested that rutile worked

well under sonocatalysis; whilst anatase was the preference for photocatalysis;

whereas a combination of rutile and anatase mixture is better for

sonophotocatalysis.

Keywords: anatase, oxide semiconductor, rutile, photocatalytic, sonocatalytic,

sonophotocatalytic.

INTRODUCTION

The treatment for organic heterogen wastewater commonly uses conventional

technologies, which unfortunately are inefficient due to its high cost. There are three

categories of this conventional technology: 1. physical treatment, e.g. reverse

osmosis, activated carbon adsorption (Joseph et al, 2009a), biosorption (Sharain-Liew

et al, 2011), solvent extraction and hyperfiltration; 2. chemical treatments, such as

chemical degradation, chemical oxidization incineration, wet oxidation, hypercritical

oxidation, high-pressure impulsive discharge and low temperature plasma;

3. biological treatment, such as activated sludge, membrane separation technique and

aerobic/anaerobic methods. The toxicity of wastewater organic heterogen such as

2,4,6-trichlorophenol (TCP) rendered biological treatments to be ineffective in

decomposing TCP and other chlorophenols. This has resulted that biodegradation

treatment is ineffective (Wang et al, 2000; Krishnaiah, 2003; Jung et al, 2001; Aksu

and Yener, 2001). New inexpensive methods for the treatment of industrial

wastewater are always interesting to be researched. In this article, we review three

methods for treating organic heterogen wastewater, that are sonocatalysis,

photocatalysis, and sonophotocatalysis.

An advanced oxidation process for water purification and recovery is defined as

sonocatalysis. This method employs ultrasound irradiation with catalyst (Aatral et al,

2014). Other oxidation method, e.g. heterogeneous photocatalysis, can be used to

treat such pollutant due to its efficiency and low cost (Kavitha and Palanisamy,

2011). Photocalatytic is an advanced oxidation process for wastewater

decontamination. It uses oxide semiconductor materials as the photocatalysts, which

are promising materials for water purification, due to their low cost and the ability to

absorb solar energy. Titanium dioxide (TiO2) is the most prominent candidate for

high-performance photocatalysts . It has more advantages over other oxide

semiconductors, e.g. its relatively high reactivity with both light and water as well as

organic solutes dissolved in water. At the same time, in aqueous medium, TiO2

exhibits an outstanding resistance to corrosion and photocorrosion (Bak et al, 2010;

A Review: Catalysts for Organic Heterogen Wastewater Treatment 1511

Maldotti and Molinari, 2011). The combination of photocatalytic reaction with

ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with a

sonocatalyst is named sonophotocatalysis (Harada, 2001).

SONOCATALYSIS

There has been an increasing interest in the use of ultrasound to destroy organic

contaminants present in waste water since 1990. The high power ultrasonic irradiation

has many advantage such as safety, high penetrability in water medium, high

degradation efficiency, and energy coservation without any generation of secondary

pollutan. Aatral and colleagues (2014) concluded that ultrasonic irradiation process

are capable of degrading various recalcitrant organic compounds (Aatral et al, 2014).

The ultrasonic process is one of advanced oxidation process. It also involves pyrolysis

phenomena (thermal degradation) and the generation of hydroxyl radical (•OH),

which has a very high oxidation potential and able to oxide almost all organic

pollutans and volatile matter such as NH3 (Mahvi et al, 2009).

In sonocatalytic process, accoustic cavitations are created when ultrasonic waves

reach the refraction cycle. It occurs where a negative accoustic pressure is sufficiently

large to pull the water molecules from each other, and resulted voids in the liquid. On

the other hand, during compression cycle of ultrasonic wave which pushes molecules

apart, the accoustic pressure is positive (Aatral et al, 2014).

Shimizu and colleagues (2007) investigated about sonocatalytic degradation of

methylene blue with TiO2 pellets in water. In conclusion, ultrasound irradiation of

TiO2 in aqueous solution resulted in significant generation of hydroxyl radicals, and

this process may have potential for the treatment of organic dyes in wastewater

(Shimizu et al, 2007), whilst other researchers reported that sonocatalytic method

adopting mixed crystal TiO2 powder as sonocatalyst, was an advisable choice for the

treatment of non or low transparent wastewaters in the future (Wang et al, 2008).

Other experiment was reported about semiconductor metal oxide nanoparticles

assisted sonocatalytic process for the degradation of an organic dye. In conclusion,

sonocatalytic appears to be an effective method of degrading organic coumpounds.

The present of catalyst under ultrasonic irradiation enhanced the percentage of dye

degradation (Aatral et al, 2014).

PHOTOCATALYSIS

Photocatalytic techniques have been one of the most interesting processes for

wastewater treatment because of its many advantages over other traditional

techniques, such as convenient in handling, quick and low concentration (ppb level)

oxidation, and none of high toxic products i.e., polycyclic aromatic compound are

1512 Uus Supriatna, Dadan Sumiarsa, Akhmad Zainal Abidin, Yayat Dhahiyat

observed after photocatalytic process has reached. Wastewater treatment by

photocatalytic has been widely reported (Qaradawi and Salman, 2002; Wang et al,

2004; Liu et al, 2005, 2006; Baiocchi et al, 2002; Kansal et al, 2007; Bianco-Prevot et

al, 2004; Rizzo et al, 2009; Giraldo et al, 2010; Lin and Lee, 2010; Selli et al, 2008;

Sanchez et al, 2011; Son et al, 2009; Petrik and Kimmel, 2010; Elmolla and

Chaudhuri, 2010; Yang et al, 2008).

The catalyst TiO2 as well as ZnO, have been considered as the promising

photocatalyst due to their high photocatalytic activity, photostability, and wide-band

gap, moreover these catalysts have less toxicity. Significant quantum efficiency of

ZnO was reported higher than that of TiO2 (Mai et al, 2008). The better activity of

ZnO than TiO2 was reported in some cases (Chen, 2007). The use of ZnO

in photocatalytic process mediated was proven successful to organic pollutant

degradation (Height et al, 2006; Akyol et al, 2004). This catalyst is available at low

cost and absorbs over larger fraction of the solar spectrum than TiO2 (Christoskova

and Stoyanova, 2001), thus ZnO is considered as suitable material for photocatalytic

degradation of organic pollutants than TiO2. However, many studies have reported

about the photocatalytic activity of TiO2 for either dye degradation or antimicrobial

activity (Fatimah et al, 2009; Zulfakar et al, 2011; Qourzal et al, 2006; Tchatchueng

et al, 2009; Desai and Kowshik, 2009).

Interdisciplinary fields of science that was originated from the intersection of

Chemistry and Physics, and to some extent Photobiology (natural photosynthesis),

proposed heterogeneous photocatalysis, which is based on four basic pillars: (i)

heterogeneous catalysis; (ii) photochemistry; (iii) molecular spectroscopy of adsorbed

molecules and solid-state spectroscopy; together with (iv) materials science and

surface science of semiconductors and insulators (Emeline et al in Suib, 2013).

Heterogeneous photocatalytic reaction cycle starts from the absorption of quanta light

by the solid photocatalyst and culminates. It works with the chemical transformations

of molecules on the surface ultimately evolving reaction products into either gaseous

or liquid phase. The role of the photocatalyst and corresponding photophysical events

which take place in solids are often treated in a rather simplistic manner. An ensemble

of particles are absorb photons. Furthermore, they are considered to be a light

harvesting system. The photocatalyst particle plays the role of sensitizer and the

source of intermediates—i.e. photoelectrons and photoholes (reducing and oxidizing

agents, respectively). The solids are considered to be of primordial importance in

photocatalysis for intrinsic or fundamental absorption of light. That is a reasonable

approach in a majority of studies oriented on mechanistic investigations of chemical

reactions or on practical applications of heterogeneous photocatalysis. At the same

time, the complexity and variety of photophysical processes in solid photocatalysts

and the interdependence between physical and chemical events at the surface of

micro- and nano-particles should always be kept in mind, even in applied

A Review: Catalysts for Organic Heterogen Wastewater Treatment 1513

heterogeneous photocatalysis. Accordingly, at the surface of a photocatalyst particle

now describes some of the important photophysical events preceding the chemical

reactions.(Emeline et al in Suib, 2013).

The illumination of a semiconductor photocatalyst with ultraviolet (UV) radiation

activates the catalyst, establishing a redox environment in the aqueous solution during

the photocatalytic process. Sensitizers for light induced redox processes due to their

electronic structure was caused by semiconductors, which is characterized by a filled

valence band (VB) and an empty conduction band (CB). The energy difference

between the VB and CB is called the band gap. The photons absorbed by

semiconductor photocatalyst with energies equal to or higher than its band gap or

threshold energy. Each photon of requisite energy hits an electron in the octubeied

valence band of the semiconductor atom. It can elevate that electron to the

unoctubeied conduction band leading to excited state conduction band electrons and

positive valence band holes. Different paths may took the fate of these charge

carriers (Fig.1). Firstly, they can get trapped either in shallow traps (ST) or in deep

traps (DT). Then, they can recombine nonradiatively or radiatively, dissipating the

input energy as heat. Finally, electron donors or acceptors adsorbed on the surface of

the photocatalyst reacted with them. In fact, it was recently shown that any photo-

redox chemistry occurring at the particle surface emanates from trapped electrons and

trapped holes rather than from free valence band holes and conduction band electrons

(Bak T et al, 2010).

Fig 1. The process involved in semiconductor particles upon bandgap excitation (Bak

et al, 2010)

1514 Uus Supriatna, Dadan Sumiarsa, Akhmad Zainal Abidin, Yayat Dhahiyat

TiO2 is certainly the most investigated semiconductor in photocatalysis due to its

uniqueness and attractive characteristics, such as high photocatalytic activity,

stability, environmental tolerance (Maldotti and Molinari in Bignozi, 2011), strong

oxidizing power, non-toxic, and low cost (Orang and Abdollahi, 2011; Gaya, 2014),

availability, chemostability, reusability, outstanding electronic, and optical

characteristics (Gaya, 2014).

TiO2 has been applied to a variety of problems of environmental in addition to water

purification (Bak et al, 2010). TiO2 is suitable for decomposition of organic and

inorganic compounds at very low concentrations ranging from 0.01 to 10 ppm

because of its highly oxidizing effect (Castellote and Bengtsson in Ohama and Van

Gemert, 2011). Hence, TiO2 can be used in self-cleaning glasses and in the

mineralisation of aqueous or air borne pollutants. The disinfection of water and air

from microorganisms also can be achieved by TiO2 photocatalysis (Gaya, 2014).

SONOPHOTOCATALYSIS

Organic heterogen wastewater type such as TCP was degraded with ultrasonic or

ultraviolet irradiation or a combination of both, in the presence of TiO2 semiconductor

catalyst (anatase and/or rutile), in order to study the effectiveness of sonocatalysis,

photocatalysis and sonophotocatalysis oxidation in a batch sonophotoreactor system.

This study suggested that rutile worked well under sonocatalysis, anatase was the

preference for photocatalysis, whilst sonophotocatalysis has obtained benefit from a

combination of rutile and anatase. Sonophotocatalysis oxidation of TCP

has demonstrated a degradation that was higher than sonocatalysis or photocatalysis

individually while the first-order kinetics rate constants indicated that

sonophotocatalysis degradation of TCP was synergistic with a positive value of

0.0203 in the presence of the mixture catalyst (Joseph et al, 2012).

In recent years, the simultaneous use of ultrasound and photocatalysis

(sonophotocatalysis) has been studied regarding process efficiency to degrade various

organics (Orang and Abdollahi, 2011). The interesting advantages at kinetic level was

showed in sonophotocatalytic process, due to the presence of a synergistic effect

between sonocatalysis and photocatalysis. (Matsuzawa et al, 2002). The simultaneous

use of two techniques (sonocatalytic and photocatalytic) was reported more effective

than their sequential combination though leading to just additive effects, and more

effective than sonocatalysis only when employing relatively low ultrasound intensity

(Kavitha and Palanisamy, 2011).

The coupling photocatalysis with sonolysis has been given the beneficial effect may

be attributed to several reasons, namely: (i) increased production of hydroxyl radicals

in the reaction mixture; (ii) enhanced mass transfer of organics between the liquid

phase and the catalyst surface (Bali, 2004; (iii) ultrasound-induced luminescence to

A Review: Catalysts for Organic Heterogen Wastewater Treatment 1515

catalyst exitation which has a wide wavelength range below 375 nm (Shimizu et al,

2007; Wang et al, 2008); and (iv) the ultrasound de-aggregating catalyst particles

increased catalytic activity and thus increasing surface area (Silva AMT et al, 2007)

that the TiO2 surface area increased by about 130% after the sonophotocatalytic

treatment of a phenolic wastewater.

Many studies have been reported on combining ultrasonic with UV light irradiation in

the presence of TiO2, and it shows excellent remove ability. Sonophotocatalytic

process provides an excellent opportunity to reduce reaction time without need

extreme physical conditions (Orang and Abdollahi, 2011; Gaya, 2014). Cheng et al

(2016) studied about sonophotocatalytic degradation and mineralization of bisphenol

A (BPA) in a sequential ultrasound intensified photocatalytic reactor using nanometer

TiO2 as photocatalyst. Exhibition of a synergetic effect and benefits for BPA

mineralization has been shown by combination of sonolysis and photocatalysis of

TiO2 (Cheng et al, 2016). Na et al (2012) has shown that sonophotolytic degradation

of DEP resulted in a significantly faster degradation than the photolytic and sonolytic

processes due to the higher photon energy and higher hidroxyl radical generation by

homolysis of water. Sonophotolytic removing ratio is 90-100%, photolytic 70-100%,

and sonolytic 58% (Na et al, 2012).

Furthermore, sonolytic, sonocatalytic, and sonophotocatalytic degradations of

chitosan in the presence of TiO2 nanoparticles have been studied by Taghizadeh and

Abdollahi (2011). They reported that TiO2 sonophotocatalysis was always faster than

the respective individual processes due to the enhanced formation of reactive radicals

as well as the possible ultrasound-induced increase of the active surface area of the

catalyst (Taghizadeh and Abdollahi, 2011). Other researchers, Verma et al, (2013),

investigated the photocatalytic, sonolytic, and sonophotocatalytic degradation of 4-

chloro-2-nitrophenol (4C2NP) using heterogeneous (TiO2). Following the

degradation, the trends are sonophotocatalysis, photocatalysis, sonocatalytic, and then

sonolysis. The degradation results of sonophotocatalytic of pharmaceutical compound

showed that it could be used as efficient and environmentally friendly technique for

complete degradation of recalcitrant organic pollutants which increases the chances

for the reuse of wastewater (Verma et al , 2013).

1516 Uus Supriatna, Dadan Sumiarsa, Akhmad Zainal Abidin, Yayat Dhahiyat

Fig.1 Residual concentration of TCP after 1 hour of degradation for: (upper)

sonocatalysis, (middle) photocatalysis, and (lower) sonophotocatalysis (Joseph

et al, 2012)

Fig.2 Schematic diagram of the sonophotocatalysis synergistic effect (Joseph

et al, 2009)

A Review: Catalysts for Organic Heterogen Wastewater Treatment 1517

CONCLUSION

Of the three techniques, the combined anatase and rutile mixture catalyst is the best

for treatment of heterogen organic waste water treatment and degradation schemes

has been resulted in synergistic effect.

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