Recent Developments in Environmental Photocatalytic Degradation of Organic Pollutants: The Case of Titanium Dioxide Nanoparticles—A Review

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2015, Article ID 790173, 29 pages http://dx.doi.org/10.1155/2015/790173 Review Article Recent Developments in Environmental Photocatalytic Degradation of Organic Pollutants: The Case of Titanium Dioxide Nanoparticles—A Review Mphilisi M. Mahlambi,1 Catherine J. Ngila,1 and Bhekie B. Mamba2 1 Department of Applied Chemistry, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, South Africa Nanotechnology and Water Sustainable Unit, College of Science, Engineering and Technology, University of South Africa, Florida Campus, Johannesburg 17025, South Africa 2 Correspondence should be addressed to Mphilisi M. Mahlambi; Received 29 April 2015; Revised 29 July 2015; Accepted 5 August 2015 Academic Editor: Xin Zhang Copyright © 2015 Mphilisi M. Mahlambi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The presence of both organic and inorganic pollutants in water due to industrial, agricultural, and domestic activities has led to the global need for the development of new, improved, and advanced but effective technologies to effectively address the challenges of water quality. It is therefore necessary to develop a technology which would completely remove contaminants from contaminated waters. TiO2 (titania) nanocatalysts have a proven potential to treat “difficult-to-remove” contaminants and thus are expected to play an important role in the remediation of environmental and pollution challenges. Titania nanoparticles are intended to be both supplementary and complementary to the present water-treatment technologies through the destruction or transformation of hazardous chemical wastes to innocuous end-products, that is, CO2 and H2 O. This paper therefore explores and summarizes recent efforts in the area of titania nanoparticle synthesis, modifications, and application of titania nanoparticles for water treatment purposes. 1. Introduction The South African National Water Act (Act number 36 of 1998) specifically states that water resources must remain fit for use on a sustainable basis and that their quality must be constantly monitored [1]. Therefore the availability of water should be based not only on the quantity but also on the quality of the available water [2]. However, due to agricultural, industrial, and domestic activities the quality of river water or groundwater continues to deteriorate due to pollution by hazardous materials [3, 4]. Water pollution is defined as the direct or indirect introduction of substances into the water bodies. These pollutants may be harmful to human health or the quality of aquatic ecosystems thus affecting the use of amenities and other legitimate uses of water [1]. The sources of water pollution are categorised as either a point source or nonpoint source (diffuse sources). Point source water pollution occurs when the polluting substance is emitted directly into the water system, for example, a pipe that spews sewage directly into a river, while nonpoint source (NPS) pollution refers to diffuse contamination which occurs when pollutants enter a water system through runoff, for example, when fertiliser is washed into a river by surface runoffs. Water pollutants can be classified as physical (odour, colour, solids, or temperature), biological (pathogens), or chemical (organic or inorganic compounds) [1, 2, 5–9]. Organic pollutants are of more concern than the other types because of their carcinogenic and mutagenic effects even after exposure to minute concentrations [10, 11]. 1.1. Organic Pollutants. Organic contaminants have become of more concern due to the inability of conventional water-treatment technologies to completely decompose these contaminants in aqueous media [12, 13]. The ubiquitous appearance of organic contaminants in sewage effluents, groundwater, drinking water, and sludge poses a significant 2 Journal of Nanomaterials threat to humans and aquatic organisms [14]. Volatile organic compounds (VOCs) are known to be toxic and carcinogenic and have been implicated in the depletion of the stratospheric ozone layer while also contributing to global warming [10, 15]. These pollutants have been reported as being mutagenic and hence are responsible for the emergence of antibiotic resistance bacteria and genes [16]. Some organic pollutants are referred to as persistent organic pollutants (POPs) because when they enter the environment, they do not readily break down and may remain there for very long periods of time, for example, polychlorinated biphenyls (PCBs), and may enter the food chains and accumulate to levels detrimental to organisms that are high up in the food chain [17]. Also organic pollutants are a serious threat because they can be transported from the source of contamination through air as vapour or as dust particles by water currents or sediments and released in a new environment [17]. Some of these organic pollutants eventually contaminate groundwater and surface waters; however, groundwater contamination is likely to be the primary source of human contact with these toxic chemicals [18]. Generally, exposure to organic contaminants could be through breathing, through ingestion, through drinking, or by skin contact. aesthetic and malodour problems in water. The organic acids that result from the oxidation of NOM have the capability to corrode turbines and engineering systems and this affects transportation of contaminants [19, 22, 27, 28]. Thus, understanding the impact of NOM in water-treatment processes is vital for human health and water-treatment plants as well as industrial processes where pure water is a prerequisite. 1.2. Natural Organic Matter. Natural organic matter (NOM) is an agglomeration of organic compounds that naturally occur when animal and plant material break down [19–21]. NOM consists of a wide range of compounds with diverse chemical properties (due to geographic origin and age of the decomposing organism) and occurs in all natural water sources [20, 22, 23]. NOM components are a heterogeneous mixture of complex organic materials which consists of both hydrophilic and hydrophobic components. The hydrophilic components are microbial by-products and contain a higher proportion of aliphatic carbon and nitrogenous compounds with relatively high charge density such as amino acids and proteins as well as polysaccharides [22, 24, 25]. Humic substances (HS) constitute the more hydrophobic fraction of NOM and exhibit relatively high specific ultraviolet absorbance (SUVA) values due to the presence of a relatively large proportion of aromatic carbon, phenolic structures, and conjugated double bonds [5, 19, 21, 22, 24, 25]. Due to the complexity of NOM no single tool can give its definitive structural or functional information. Nondestructive spectroscop (...truncated)