Nanocatalysts in Fenton Based Advanced Oxidation ...

RESEARCH ARTICLE

Copyright © 2016 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Bionanoscience Vol. 10, 356–368, 2016

Nanocatalysts in Fenton Based Advanced Oxidation Process for Water and Wastewater Treatment Govindaraj Divyapriya, Indumathi M. Nambi∗ , and Jaganathan Senthilnathan Environmental and Water Resources Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600036, India Fenton based advanced oxidation process is a promising technology for the complete mineralization of persistent organic pollutants over the traditional physicochemical and biological treatment processes. Application of nanomaterials in the recent advancement of Fenton based water and wastewater treatment processes are reviewed in this study. Usage of active and stable nanocatalyst such as metal oxides, graphene/graphene oxide/carbon nanotubes supported metal oxide, clay based nanocomposites in Fenton, photo-Fenton, electro-Fenton and photoelectro-Fenton processes are discussed in detail. This review also highlights the application of nanocomposite based electrodes in electrochemical oxidation process for treating the bio-refractory compounds.

Keywords: Advanced Oxidation Processes, Fenton, Nanocatalyst, Nanocomposite. Delivered by Ingenta to: York University Libraries IP: 79.110.18.104 On: Thu, 06 Apr 2017 18:38:34 1. INTRODUCTION 2. PRESENCE Copyright: American Scientific PublishersOF EMERGING CONTAMINANTS IN WATER AND This chapter gives an overview of the application of nanoWASTEWATER materials in advanced oxidation process (AOPs) and its In the past two decades, several research articles have current advancement in the area of water and wastewater been described the existence of new chemical substances treatments. AOPs comprise of efficient and eco-friendly considered as “emerging contaminants” in water and methods that have been developed over periods of time to wastewater systems. Emerging contaminants can enter in principally remove emerging contaminants and persistent to receiving water bodies (surface and ground water) organic pollutants (POPs) from waters and wastewaters.70 through point sources such as discharge of partially The principles involved in the AOPs, their advantages treated/untreated domestic sewage, industrial effluent, farm and applications of nanomaterials have compared and effluents and landfill leachate and non-point sources such discussed in detail. In treatment of emerging contamias agricultural runoff and wash out from roadways. Presnants and POPs from water and wastewater, nanomaterience of these emerging contaminants in aquatic systems als offer greater removal efficiency and display enhanced potentially causes serious threat to humans and the eco performance when compared to the conventional chemisystem.10 Furthermore, they do not have any acceptable cal and biological process currently being used. Recent regulatory standards for drinking water and their long term advancements in the fabrication of nanomaterials and/or impacts on human and wildlife are yet to be evaluated nanosheets for the treatment of drinking water and indusfurther.18 trial wastewater in different oxidation and advanced oxidaMost of the emerging contaminants present in water tion process are discussed. In addition, the recent advances and wastewater are persistent in nature and existing conin the application of nanomaterial-based electrodes for ventional treatment systems are not adequate to treat the formation of hydroxyl radicals in electrochemical and/or remove such contaminants. In addition, presence of treatment techniques are also discussed. Furthermore, the low concentration (g/L to ng/L) of these contaminants latest research trends and future prospects are briefly in drinking water sources provides additional burden to highlighted. the existing conventional treatment systems. Presence of

∗

Author to whom correspondence should be addressed.

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emerging contaminants like endocrine disrupting chemicals (EDCs), pharmaceutically active compounds (PhACs), personal care products (PCPs), pesticides, surfactants, 1557-7910/2016/10/356/013

doi:10.1166/jbns.2016.1387

Divyapriya et al.

Nanocatalysts in Fenton Based Advanced Oxidation Process for Water and Wastewater Treatment

and electrostatic interaction mechanisms which make the system to work more effectively towards the removal of emerging contaminants.10 61 Similarly, adsorption of contaminants by using activated carbon involves just phase transferring mechanism from liquid to solid resulting in generation of toxic sludge without any degradation.89 The major setback of physical treatments is that complete degradation of contaminant is not possible and higher operation cost hinders the practical applicability.

3.3. Chemical Processes Coagulation, chlorination and oxidation are some of the important chemical treatment techniques for water and wastewater. Coagulation involves the addition of alum/ferric salts, results in generation of metal hydroxide flock. Coagulation shows very poor removal efficiency against dissolved organic contaminant.99 Chlorination is capable of oxidizing the most of the organic contaminants; formation of organochlorine by-products or intermediate makes this process undesirable to use for water and wastewater treatment application.42 AOPs involve in-situ generation of highly reactive hydroxyl radical (OH ), mainly focusing on complete oxidation and destruction of recalcitrant organic contaminants in short period. AOPs can act as better alternatives to the existing treatment tech3. EXISTING TREATMENT TECHNIQUES nique, which is currently being used as pre or post treatFOR WATER AND WASTEWATER Delivered by Ingenta to: York University ment option forLibraries the complete mineralization of emerging Conventional water and wastewater treatment techniques IP: 79.110.18.104 On: Thu,contaminants. 06 Apr 20178918:38:34 are intended to remove traditional and/or well-established Copyright: American Scientific Publishers waste or contaminants, which are having regulatory limits. The most important and well-established water and 4. NANOTECHNOLOGY IN CONTAMINANTS wastewater treatment methodologies are physical, chemiREMOVAL cal and biological treatment processes. Recent advances in nanotechnology potentially resolve the water quality issues and enhance the performance 3.1. Biological Processes of existing treatment processes. Innovative applications Activated sludge process (ASP) which is currently being of nanomaterials in the field water treatment are more used in conventional treatment involves conversion of disexciting and promising development of nanotechnology solved organic matter into colloidal biomass and separated in toxic contaminant removal. Enhanced chemical and by clarifiers from treated effluent. Although ASP is effiphysical properties of nanomaterials greatly influence the cient in removing dissolved organic compounds, it exhibits water treatment processes such as nanosorption, nanofilpoor efficiency in removing emerging contaminants owing tration, degradation and decontamination. Nanometals and to their persistence and low concentration in aqueous metal oxides show remarkable results against heavy metenvironment.61 Membrane biological reactors show betals, persistent organic pollutants, radionuclides, bacteria ter efficiency than ASP particularly in removing emerging and viruses. Figure 1 represents the application of nancontaminants.63 otechnology in contaminants removal, respective nanomaterials and their properties. 3.2. Physical Processes Physical methods such as membrane treatment, nanofil5. ADVANCED OXIDATION tration, reverse osmosis and adsorption are some of the PROCESSES (AOPs) important treatment techniques for the removal of emergAOPs are promising methods, widely used for the coming contaminants from water and wastewater. Removal plete remediation of water contaminated with toxic, recalof emerging contaminants by membrane filtration is citrant organic compounds as well as microorganisms. one of the promising techniques to get high quality The principle of AOPs is the generation of highly reacwater by removing wide range of organic contamitive oxygen species such as hydroxyl radicals, hydrogen nants like bacteria and viruses without chemical treatperoxide (H2 O2 , and superoxide (O2 ) to oxidize organic ments. Nanofiltration process involves adsorption, sieving J. Bionanosci. 10, 356–368, 2016

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phthalates, polycyclic aromatic hydrocarbons (PAHs), etc., in water and wastewater systems are extensively reported and well documented in the literature.18 26 78 Identification of EDCs in water and wastewater system is one of the upmost priorities for the modern water treatment system. Even low concentration of EDCs can disturb the reproductive system of mammals by interfering with their endocrine (hormone) system. Furthermore, chemicals released from pharmaceutical industries, pesticides and chemicals from other sources identified as EDCs, which are able to mimic hormones.10 PhACs and PCPs are widely used chemicals in human and veterinary medicines.26 Similarly, personal care products that include soap, toothpaste, sunscreen, insect repellent, cosmetics and fragrances are made up of variety of chemicals that can enter in to the water bodies through the release of domestic effluents during bathing and washing activities.16 Another important contamination is pesticides from agricultural runoff. Presence of several organochlorine and organophosphorous pesticides like methyl parathion, malathion, endosulfan, DDT, lindane, 2,4-dichlorophenoxyacetic acid, etc. have been identified in water sources with exceeding permissible limits.5 23


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Nanotechnology in contaminant removal

Adsorption

Application

1.Nanocarbon (carbon nanotubes (CNTs), graphene and graphene oxide) 2.Nanometal oxides (iron oxide, titanium dioxide and alumina)

1.Nanostructured membranes (CNTs, polymers, ceramics and dendrimeres) 2.Nanoreactive membranes (Metal nanoparticles such as titanium dioxide and silver nano particles)

1.Photocatalysis (Metal oxides such as titanium dioxide, zinc oxide) 2.Fenton and Fenton like reactions (Nano zerovalent iron and iron oxides) 3.Nano metals (gold and silver nanoparticles)

High specific surface area and adsorption sites, specific interactions with contaminants, reusability

Sieving and electrostatic interactions, antimicrobial activity, selectivity and hydrophilicity

Stable catalytic ability, generation of radical species, reusability

Removal of recalcitrant organics, heavy metals and radionuclides

Removal of recalcitrant organics, heavy metals, salts, bacteria and viruses

Removal of recalcitrant organics, bacteria and viruses

Respective Nanomaterial

Required properties

Target Contaminants

Degradation and disinfection

Membrane filtration

Fig. 1. Application of nanotechnology for contaminants removal from water and wastewater.

contaminants to mineralized products such by as Ingenta CO2 , H2 O sonolysis (ultrasound−US) and various combination of Delivered to: York University Libraries 7 36 94 and inorganic ions (Fig. 2). Figure 3 represents the various these technologies. IP: 79.110.18.104 On: Thu, 06 Apr 2017 18:38:34 American Publishers The hydroxyl radical is a powerful,Copyright: non-selective chem- Scientific AOPs used for the generation of free radicals. ical oxidant, which acts very rapidly with wide range of AOPs have considerable advantage over existing conorganic contaminants. The relative oxidizing powers of difventional treatment methods like ferent oxidizing species is given in Table I. (i) effective against low concentration of pollutants Formation of hydroxyl radicals can be achieved by (ii) no chemical or biological sludge production using (single oxidants and/or combinations) hydrogen peroxide, ozone and UV radiation or combination of hydrogen peroxide with ferrous ions (Fenton’s reagent), Photocatalysis UV/TiO2 electrochemical oxidation, supercritical water oxidation, UV/TiO2/H2O2 Fenton process (Fe2+/H2O2) Fenton based

PhotoFenton (Fe3+/H2O2/UV) SonoFenton (Fe2+/H2O2/US)

Fig. 2. Degradation of organic pollutants in the presence of hydroxide radicals.

US/H2O US/H2O2

Table I. Relative oxidation power of some oxidizing species. AOP

Oxidizing species

Relative oxidation power

Chlorine Hypochlorous acid Permanganate Hydrogen peroxide Ozone Atomic oxygen Hydroxyl radical Positively charged hole on titanium dioxide, TiO+2

100 110 124 131 152 178 205 235

S. no 1 2 3 4 5 6 7 8

Source: From Carey, Water Pollution Research Journal Canada, 27, 1, 1992.

358

Sonolysis

US/O3 US/UV/TiO2 O3/H2O2

Ozone based process

O3/UV O3/UV/H2O2

Electrochemical oxidation Wet oxidation Other AOPs

Supercritical water oxidation Electron beam irradiation Γ-irradiation

Anodic oxidation ElectroFenton 3+ – (Fe /H2O2(e )) PhotoElectocatalysi – s (UV/TiO2/e ) PhotoElectroFenton 3+ – (UV/Fe /H2O2(e )) SonoElectroFenton (US/Fe3+/H2O2(e–))

Fig. 3. Classification of advanced oxidation processes.

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(iii) non-selective (iv) complete mineralization can be achieved.

O2

Conduction band e–

UV irradiation

O2•

Excitation Recombination H2O Valence band

h+

6. PHOTOCATALYST BASED AOPs HO• Photocatalysis is the process of generating reactive species through the irradiation of catalyst with light source by phoFig. 4. Reaction mechanism of photocatalysis. tochemical reaction. When the photocatalyst and chemical reactant are not in the same phase, it is called as 7. FENTON BASED AOPs heterogeneous photocatalysis. Among the various semi7.1. Fenton Process conducting photocatalyst, TiO2 and ZnO are the more Fenton reactions have been extensively studied for the effective against and extensively studied photocatalysts for degradation of wide range of pollutants and emerging con90 the degradation of organic compounds. The band-gap taminants. Fenton reaction is an eco-friendly process and energy of TiO2 (anatase) and ZnO are 3.2 eV and 3.4 eV does not require any harmful reagents.68 HO radicals 16 respectively; hence it needs UV radiation for excitation. are produced during the activation of hydrogen peroxide When catalysts absorb light, transition of electrons from (H2 O2 by Fe2+ ion and forms Fe3+ ion (Fig. 5). The oxithe valence to the conduction band takes place and leavdized Fe3+ ion further reacts with H2 O2 and forms Fe2+ ing holes behind the valence band. Both of these phoion. Generation of HO radicals and conversion of Fe2+ to − togenerated electrons in conduction band ecb and holes Fe3+ ions are represented by following equations. Delivered by Ingenta to: York University Libraries in valence band (h+ vb can undergo oxidation and reducIP: 79.110.18.104 18:38:34 tion in aqueous solution and produce HO radicals. On: HOThu, 06 Apr 2017 (4) Fe2+ + H2 O2 → Fe3+ + OH− + HO Copyright: American Scientific Publishers + are produced due to the reaction between hvb and water Fe3+ + H2 O2 → Fe2+ + HO2 + H+ (5) molecules adsorbed on the surface of the catalyst. Simi− − 2+ 3+ larly, ecb react with molecular oxygen to produce super(6) Fe + HO2 → Fe + HO2 oxide radical (O− 2 ) which undergoes further reaction and Fe3+ + HO2 → Fe2+ + O2 + H+ (7) forms HO radical.65 Reactions occurring on the surface of TiO2 can be written as following: (8) H O + HO → HO + H O 2

TiO2 + h