A Review of Pharmaceuticals and Endocrine-Disrupting Compounds: Sources, Effects, Removal, and Detections
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- Tijani, J.O., Fatoba, O.O. & Petrik, L.F. Water Air Soil Pollut (2013) 224: 1770. doi:10.1007/s11270-013-1770-3
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There are growing concerns about the increasing trends of emerging micropollutants in the environment due to their potential negative impacts on natural ecosystems and humans. This has attracted attention from both governmental and non-governmental organisations worldwide. Pharmaceuticals, personal care products, and endocrine disruptors are continuously being released consciously or unconsciously into water sources due to poor regulatory frameworks especially in the developing countries. The effects of these contaminants are poorly known. They are not easily biodegradable and have become an environmental nuisance and public health issue. This has heightened the risk of exposure to their deleterious effects in such countries where the majority of the population are still struggling to have access to good quality drinking water supplies and better sanitation. With the rising fear of short- and long-term impacts of the ever-increasing number of persistent recalcitrant organic compounds accumulating in the environment, their removal is gradually becoming an issue to the water treatment industry. Hence, there is a need to develop functional techniques for the management of water contaminated by these emerging contaminants so as to increase the availability and access to safe and good-quality drinking water. We conducted a narrative review on these emerging micropollutants and examined their various documented sources, effects, as well as recent techniques for their effective removal. This becomes necessary due to the increasing occurrence of these pollutants in the aquatic and terrestrial environment. These levels are expected to further increase in the coming years as a consequence of the ever-increasing population density which undoubtedly characterizes developing economies. Our findings show that the present reported treatment techniques in the literature such as biological oxidation/biodegradation, coagulation/flocculation, ozonation, electrodialysis, reverse osmosis, sedimentation, filtration, and activated carbon were not designed for removal of these newly identified contaminants, and as such, the techniques are not sufficient and unable to completely degrade the compounds. We therefore recommended the need for concerted efforts to develop better techniques, especially combined advanced oxidative methods to address the shortcomings of and growing challenge to current practices.
KeywordsPharmaceutical and personal care productsEndocrine disrupting compoundsCoagulation/flocculationOzonationElectrodialysisReverse osmosisSedimentationFiltrationActivated carbonAdvanced oxidation process
The importance of good-quality, potable water to human existence cannot be underestimated, and life will be practically impossible or difficult without the availability of safe drinking water (Swaminathan et al. 2013; Cortés Muñoz et al. 2013). Over the last few decades, there have been tremendous improvements in the supply of safe drinking water to growing populations. This is through various governmental interventions especially in developing countries that want to meet the Millennium Development Goal target of 2015 adopted by United Nations in 2000. This mandate is in line with European Commission Water Framework Directive article 4 that states that all community waters such as surface, ground, transitional, and coastal must be in good status by 2015 (Belgiorno et al. 2007). However, good-quality drinking water is being threatened as a consequence of various natural and anthropogenic inputs involving the discharge of effluents containing priority and emerging micropollutants that are ubiquitous in different receiving water sources. The emerging contaminants include persistent organic pollutants, micro-constituents, cyanotoxins, pesticides and herbicides, disinfection by-products, endocrine-disrupting chemicals (EDCs), pharmaceuticals and personal care products (PPCPs), and a host of other compounds. The rapid technological advancement, rapid increase in human population, inadequate wastewater treatment facilities, poor agricultural practices, inadequate land-use acts, and rapid urbanization coupled with drought occasioned by global climate change have created water pollution problems on a level that have not been seen before (Mara 2003; Moore et al. 2003; Montgomery and Elimelech 2007; Johnson et al. 2008). Almost every industrial process involves the production and release of huge quantities of half a dozen or more of those mentioned pollutants in one form or the other into the aquatic and terrestrial environment. The most frequent occurrence is the increasing accumulation of pharmaceuticals and endocrine-disrupting compounds that have overloaded and tainted the various receiving water bodies (Lindqvist et al. 2005; Kasprzyk-Hordern et al. 2009). The build-up and detection of high concentrations of these micropollutants in aquatic environments has made water unsafe for aquatic animals and for human consumption (Moldovan 2006; Roberts and Thomas 2006; Tamtam et al. 2008; Liu et al. 2009). The number of these emerging contaminants is still expected to increase in treated wastewater due to higher population density. This has resulted in growing public health risks in the developing countries where impacts have been noted such as endocrine conduit disturbance in aquatic organisms, brain damage, convulsion, carcinogenic diseases, reproductive behavioral disorder, cardiovascular disease, liver damage, and lung defects as well as disturbances in gene expression resulting in feminization of aquatic organisms (Mastin 2005; Samir et al. 2006; Campbell et al. 2006; Ha et al. 2007; Diamanti-Kandarakis et al. 2009). The current scenario has become worse due to the lack of adequate and efficient water treatment facilities capable of effectively eliminating and degrading the toxic and recalcitrant compounds to harmless substances (Nakada et al. 2006; Gulkowska et al. 2008; Sui et al. 2010). The deterioration of water quality seriously affects economic progress and impacts negatively on the sustainable development of a nation (Rahman et al. 2009a, b). Therefore, in order to have a healthy population and better society with a consequential effect on the economy growth and sustainability, access to safe drinking water and functional wastewater treatment facilities should not be compromised.
Within the last 10–15 years, different analytical techniques for the detection and quantification of persistent organic contaminants at environmentally low concentrations have been developed (Ternes and Joss 2006). This was as a consequence of the increasing discovery of PPCPs/EDCs in environmental matrices (Kolpin et al. 2002; Tixier et al. 2003; Ashton et al. 2004). Authors such as (Yoon and Lueptow 2005) and (Yoon et al. 2006a, b) separately observed that high pollution levels in the aquatic environment were majorly attributed to the presence of low concentrations of EDCs and PPCPs (nanograms per liter). Although the effect of these compounds on human beings is still under debate, their presence called for caution regarding their chemical perseverance, microbial resistance, and synergistic effects. The degradation and removal of PPCPs/EDCs in aqueous solution therefore requires different techniques which are very diverse. Different destructive methods that allow the efficient elimination of the pollutants from an aqueous form could be chosen. But recent reports suggested that more than one treatment technique may be required to degrade these compounds, depending on the nature of target compounds. The risk posed by these emerging micropollutants on humans and the environment requires further investigation. Therefore, it becomes imperative to improve on the existing techniques used to remove and degrade these chemicals in the natural ecosystems, most essentially in drinking water, as fast as possible. To date, a review of the related literature indicated that there is little or no information regarding the use of combined advanced oxidation processes to remove pharmaceuticals and endocrine disrupting compounds from aqueous solutions.
2 Pharmaceutical and Personal Care Products
Some representative classes of EDCs and PPCPs reported in surface waters, wastewater, and sewage treatment plants
Butylbenzylphthalate, di-(2-ethylhexyl) phthalate, and di-n-butylphthalate
They are found in detergents, resins, some addictives, and monomers used in the production of plastics
2,2-bis-p-Chlorophenyl-1,1-dichloroethylene (DDE), 2,2-bis-p-chloro-phenyl-1,1,1-trichloroethane(DDT), deltamethrin, carbofuran, atrazine, lindane, vinclozolin, carbendazim, and tributylin
Extensively used in agriculture. Insecticides, herbicides, and fungicides are included in this class
Tributyltin and triphenyltin
Compounds used in antifouling paints on ships
Nonylphenol, nonylphenol etoxylate, octylphenol, octylphenol etoxylate
They are used during the production of phenol resins, as plastic additives, emulsifiers, in agricultural and industrial applications
Bisphenol A is used in the manufacture of polymers (polycarbonates and epoxy resins), flame retardants and rubber chemicals
Pharmaceuticals (synthetic steroids)
Diethylstilbestrol and 17-ethinylestradiol
Synthetic steroids mainly consist of oral contraceptives as well as steroids used for substitution therapy during menopause
Estrogens naturally and daily excreted in human urine and animals
Acetaminophen (analgesic), diclofenac, ibuprofen
NSAIDs are the most used and abused drugs in the world today
Ketoprofen, naproxen, phenazone, indomethacine
All NSAIDs have analgesic, antipyretic and anti-inflammatory effect
Triclosan and thymol
Antiseptics are chemical agents that slow or stop the growth of microorganisms (germs) on external surfaces of the body and help prevent infections. Antiseptics should be distinguished from antibiotics that destroy microorganisms inside the body and from disinfectants, which destroy microorganisms found on inanimate (non-living) objects
Sulfonamides, fluoroquinolones, trimetoprim, chlortetracycline, erythromycin, lincomycin, oxytetracycline, tetracycline, roxithromycin, tylosin
Used for the treatment of bacterial infections in both humans and animals
Antiepileptics are commonly used in medicine to stop, prevent or control seizures (convulsions, partial seizures, generalized tonic–clonic seizures, etc.)
Anti-anxiety/hypnotic agents are used to relieve anxiety, nervousness, and tension associated with anxiety disorders
Lipid regulators may be used to lower cholesterol and triglyceride (fat-like substances) levels in the blood
2.1 Sources of Pharmaceuticals, Personal Care Products, and Endocrine Disruptors
In emerging and underdeveloped nations such as those in Africa and Asia, careless refuse dumping and untreated industrial effluent discharge into waterways are very common. This also contributes to the high level of PPCPs and EDCs in aquatic ecosystems (Olujimi et al. 2010). The discharge and exposure to small concentration of PPCPs and EDCs were reported to be allied with endocrine gland malfunction, chronic toxicity, and pathogen resistance (Rosal et al. 2010). The coverage pattern of PPCPs and EDCs manufactured for human and animal also differs. The human PPCPs and EDCs behave as pseudo-persistent when the effluents were discharged into aquatic systems despite their short environmental half-life and are likely to go back into the urban water cycle (Daughton 2002, 2004; Ankley et al. 2007) while animal drugs are transported to surface waters during rainfall or discharged openly in a single quantity to aquatic systems. Some of the PPCPs and EDCs later find their way into receiving water systems (Boyd et al. 2003; Barnhoorn et al. 2004; Carballa et al. 2004). For instance, in China, municipal wastewater discharge was identified as the major contributor of EDCs and PPCPs to riverine water (Peng et al. 2008). Also, more than 80 PPCPs and EDCs that were detected according to Heberer (2002) originated from reclaimed water used for artificial groundwater recharge. Kolpin et al. (2002) in his study established that close to 80 % of 139 streams in US were contaminated with antimicrobial agents, fire retardants, steroidal hormones, as well as emerging pollutants. Thus, the growing re-use of reclaimed domestic wastewaters for irrigation could also be central in low-level PPCPs escaping into urban storm water runoff from the irrigated area. Also, Dean (2007) and Lyons (2008) reported independently that once the emerging micropollutants are released into the environment their harmful effects on the local ecosystems and human residents are unlimited. Yu and Chu (2009) attributed the wide occurrence of some PPCPs and EDCs such as ibuprofen, bisphenol A, caffeine, triclosan, and bis (2-ethylhexyl) phthalate along the West Prong Little Pigeon River, in the USA, to the wastewater discharge into the river. Ortiz de García et al. (2013) linked the presence of PPCPs and EDCs in the natural environment to many factors among which were the consumption patterns and water use as well as the peculiarity of the wastewater treatment plants. Jin et al. (2004) reported that there have been reports of high concentrations of bisphenol A (BPA) in the rivers and marine environments in China, although the assessment of BPA has not been extensive especially in terms of monitoring data for drinking water. Peng et al. (2008) observed wide occurrence of 19 endocrine disrupting pharmaceuticals in urban riverine water of the Pearl River Delta, South China. The concentrations of the micropollutants after analysis were discovered to be high. The authors ascribed the large presence of pharmaceutically active compounds in the river to careless refuse dump and municipal wastewater discharge.
2.2 Environmental Fate and Behavior of PPCPs and EDCs
if Log KOW < 2.5, then it signifies low sorption potential;
if 2.5 < log Kow < 4.0, it means medium sorption potential;
- (3)Log KOW >4.0 may be equivalent to high sorption ability (Jones-Lepp and Stevens 2007; Caliman and Gavrilescu 2009). Therefore, the environmental distribution, bioavailability, reactivity, transport phenomenon, and toxicity potential of PPCPs and EDCs in natural and artificial ecosystem depends largely on the hydrophilicity and hydrophobicity sorption coefficient value of the contaminants (Suarez et al. 2008). Some indicative parameters such as estrogenic activity potency (relative to estradiol), environmental concentration, half-life, sorption Koc for the environmental fate, and behavior of several EDCs and PPCPs are shown in Table 2, which lists some information concerning the environmental fate as well as the effects of EDCs and PPCPs.Table 2
Properties and processes that govern the fate of endocrine disruptor PPCPs in the environment
Types of EDCs/environmental fate/effects
Ionic, water-soluble ECDs; increased transport and dispersion/dilution
Some pesticides and industrial EDCs; global redistribution by evaporation and atmospheric transport
Many organic halogenated compounds; persistence in the environment
Non-polar EDCs that bind to solids and lipids; partition into soil/sediment and biota (solid/liquid/gas)
Affects ionic EDCs, e.g., phenolics, and cellular reaction media
Osmoregulation; affects also fate of ionic species in coast gradients
Governs reactions; extremes (cold) increase stress; adapted hormonal functions
Marked effects on many hormonal cycles (diurnal, lunar, annual)
Binder, carrier, reaction site, etc.
Retention and transport
On humus/minerals; on aerosol particles; on plants and other biological tissues
In sorbing particles or as salts; dynamic equilibria with resuspension
With other ligands e.g., in humus
With humus; may enhance transport of, e. g., heavy metals in soil
In free air, surface water, ground water (porous soil)
Impermeable soils; slow but may be important overall
Of polar EDCs in particular
Important for many persistent organics, e.g., PCBs
Important for many persistent organics, e.g., PCBs
Depends on molecular structure and environment (humidity, pH, nutrients, redox)
Photo-decay in atmosphere and surface water
For some EDCs/conditions
Bio-accumulating EDCs in food-chains; in tissues (lipid, bone)
In (deep non-turbulent) sediments and soils
2.3 Effects of Pharmaceutical and Personal Care Products
Many nations in the world today are highly concerned about the impending environmental crisis and substantial risk occassioned by the occurrence of PPCPs in the environment (Bound and Voulvoulis 2005; Glassmeyer et al. 2005; Heberer and Feldman 2005; Lishman et al. 2006; Larsson et al. 2007). Although many drugs are fashioned to respond to quite a large range of health challenges at low physiological doses (milligrams per kilogram), some are more dynamic and can perform best at nanograms-per-kilogram concentrations. After accomplishing the specific functions in the body, some still interact even at low quantity with multiple non-therapeutic receptors over the long-term, resulting in potentially harmful effects on non-targeted receptors in both aquatic and terrestrial wildlife. Strauch (2011) asserted that the effects of PPCPS on human toxicity irrespective of their concentration in the water supply are yet to be ascertained. Research conducted by Ternes et al. (2004) and Topp et al. (2008)) discovered that certain PPCPs, such as estrogenic compounds, have a very high bioaccumulation potential. This environmental bioaccumulation exacerbates the abnormal hormonal control causing reproductive impairments, decreased fecundity, increased incidence of breast and testosterone cancers, and persistent antibiotic resistance. This severe, eco-toxic, and prolonged toxicity experienced due to the presence of PPCPs in the environment is similar to that of health effects caused by the presence of metallic elements detected at concentration below 1 mg/L in which the endocrine system are grossly affected (Sharpe and Irvine 2004; Xia et al. 2005; Bisceglia and Roberts 2006). Lear et al. (2002) and Hileman (2005) both submitted that contamination by certain PPCPs, especially antibiotics, at trace level in water equally contribute to the widespread impact and frequent occurrence of antibacterial resistance. This rise in antibacterial resistance has overstretched the medical costs of communities due to re-occurrence of hospitalization as well increased treatment cost. This is even worse in the developing countries where income per day is mostly less than 1 US dollar. Lin et al. (2008) revealed that certain fractions of partially metabolized drugs were found in waste stream, animal waste, and pharmaceutical effluents, and some of these drugs later undergo various degrees of transformation in the aqueous media.
During this period, entirely new products that differ with diverse ecotoxicological activity from the starting material are formed. The main characteristic property and processes linked to the occurrence and fate of the endocrine-disrupting pharmaceuticals are summarized in Table 2 below. The fate and behavior of these emerging micropollutants are quite diverse and complex depending on the environmental factors. The knowledge on the environmental fate of EDCs and PPCPs are still limited.
3 Endocrine-Disrupting Compounds
Some compounds do obstruct or mimic or antagonize the endocrine system, and these substances are known as endocrine-disrupting chemicals. Endocrine systems control a quite a large number of body physiological activities such as reproductive processes like embryonic development, sex differentiation, and metabolic development (Flint et al. 2012). Endocrine-disrupting compounds may block the hormonal growth in the endocrine system and affect the physiological activities of endogenous hormones (Schug et al. 2011). They are alternatively called environmental hormones that cause adverse effects on aquatic and terrestrial organisms through altering the metabolism of natural hormones, modifying hormones receptor in a cell, interference, or binding to receptors of the endocrine system (Jiao and Cheng 2008; Olujimi et al. 2010). EDCs can be natural or man-made. Others include alkyl-phenols, dioxins, various drugs, synthetic birth control pills, pesticides, plasticizers, and phenolic products (Jackson and Sutton 2008). The exposure of humans to endocrine disruptors in recent years has received increased attention due to the high exposure rate especially among the aquatic organisms. Ever since, there has been increasing concern with regard to the likely impacts of exposure to chemical compounds with endocrine-disrupting activity in the environment (Segner 2005; Sumpter 2005; Game et al. 2006; Mauricio et al. 2006; Xue and Xu 2006; Moder et al. 2007; Shin et al. 2007; Hecker and Giesy 2008). In addition, high tendencies of multiplicative and evolving abnormalities in infants and children, temporal descending trends in semen quality and testosterone levels, and worldwide increased rates of testicular and thyroid cancers among adult male populations have been attributed to EDCs exposure (Swan et al. 2000; Huyghe et al. 2003; Davies and Welch 2006; Travison et al. 2007; Andersson et al. 2007). These reported health cases alerted the scientific and public communities regarding the potential risk of environmental EDCs to men’s health. Although the effect of EDCs on human health and wildlife biology were first documented in the 1930s, the real research progressed in the late 1970s and early 1980s (Vogel 2004; Trenholm et al. 2006; Moder et al. 2007). Human beings exposure to EDCs could be through dermal absorption or contaminated media such as water, food, air, and soil. To date, no comprehensive list of EDCs exists, because most of the new chemicals are being manufactured continuously. There is a very limited and incomplete evidence of endocrine-disrupting activity because more than 87,000 new chemicals in the market have not been tested for their endocrine toxicity (Snyder et al. 2006; Kim et al. 2007). Presently, more than 38,000 chemicals and potentially toxic elements have been identified as potential endocrine-disrupting pharmaceuticals. This is an indication that more chemicals may be recognized as endocrine disrupters as the sum of compounds surges with unanticipated effects (Ferraz et al. 2007; Fatoki and Opeolu 2009). This will require identification and elimination of these compounds from the water system. Yet again, this well-documented knowledge of the hazards has yet to trigger an adequate precautionary principle approach to control and monitoring new compounds before their release into market place and from there into the receiving environment.
3.1 Effects of Endocrine-Disrupting Compounds
Some endocrine-disrupting chemicals, hormone target, and animals affected
Mammals, birds, fish
Butylbenzyl phthalate (BBP), di(2-ethylhexyl) phthalate (DEHP), di-n-butylphthalate ( D-n-BP)
Mammals, birds, fish, reptiles, amphibians
Polychlorinated biphenyl (PCBs), dioxins, pentachlorophenol (PCP), polychlorinated dibenzofurans (PCDFs)
Flame retardants, unintended by-products during incineration
Mammals, birds, fish, reptiles, amphibians
2,2-bis-p-chloro-phenyl-1,1,1-trichloroethane (DDT), 2,2-bis-p-chlorophenyl-1,1-dichloroethylene (DDE), DDE, chlordane dieldrin, heptachloro, lindane, endosulfan, oxychlordane, etc.
Estrogens and androgens
Mammals, birds, fish, reptiles, amphibians
There are diverse opinions on the effects of EDCs on wildlife and humans; some researchers are of the view that there is a link between exposure and effects of EDCs in animals and humans while some disputed the claims (Xu et al. 2008). However, studies by Michael (2001) discovered that exposure to EDCs have consequential health effects on many aquatic species which includes a low sperm count, increasing ovarian, prostate, testicular, breast cancer, diabetes/metabolic syndrome, and reproductive failures. These effects of EDCs on the natural ecosystem depend on the lipophilicity increase, exposure time, and biotransformation mechanism. However, some EDCs fail to degrade directly, but instead were metabolized into other by-products, even more toxic than the original chemicals, thereby increasing their effects on the ecosystem. Information on the aquatic organisms affected by exposure to EDCs and PPCPs are shown in Table 3. Human beings and aquatic organisms get exposed to PPCPs or EDCs through water, food, soil, or dermal adsorption (Olujimi et al. 2010). Burkhardt-Holm et al. (2005) and Kidd et al. (2007) attributed the declining rate of fish in freshwater systems to the widespread presence of estrogenic chemicals. Safe (2000) also perceived that exposure of aquatic organisms to organochlorines led to feminizations of fishes and gulls, and sexual irregularities in alligators. In addition, there are increasing occurrences of reproductive and developmental abnormalities among infants and children, pregnancy loss, birth defects, growth retardation, ovarian failure, low semen quality, and increased rates of testicular and thyroid cancers (Swan et al. 2000; Huyghe et al. 2003; Davies and Welch 2006; Travison et al. 2007; Andersson et al. 2007; Enewold et al. 2009). These deficiencies occurred as a result of exposure to EDCs through oral ingestion such as breast milk, infant formula, contaminated water, food, surface, and carpet dust (Olujimi et al. 2010). This has generated concern regarding the potential risk of environmental EDCs to men’s health. Mackenzie et al. (2005) and Safe (2000) ascribed the decline in sex ratios in Canada and the United States to over exposure to EDCs. With the recently reported cases of adverse health implication due to exposure to EDCs, the development of highly efficient technology is necessary in order to maintain public health and ecosystem stability (Lee et al. 2003; Braga et al. 2005).
4 Removal of Pharmaceuticals and Endocrine-Disrupting Compounds—An Overview
Despite appropriate legislation framework especially in the advanced nations to check inflow of endocrine disruptors and pharmaceuticals into drinking water, the problem still persist (Focazio et al. 2008; Swati et al. 2008; Rahman et al. 2009a, b). Efforts were made and still ongoing to check mate this ugly trend towards removing or degrading the persistent refractory pharmaceuticals and endocrine-disrupting compounds in water and wastewater. A number of the conventional wastewater treatment technologies have so far been reported in the literature. However, these technologies are not without defects in the removal of pharmaceuticals and endocrine disruptors from contaminated water. For instance, physicochemical treatment such as activated carbon adsorption (Nowotny et al. 2007; Snyder et al. 2007; Yu et al. 2008), reverse osmosis, nanofiltration and (Snyder et al. 2007) and coagulation–flocculation, and membrane filtration (Snyder et al. 2007; Yoon et al. 2007) were discovered not to perfectly remove EDCs and PPCPs. Instead, they end up generating other toxic by-products. Nevertheless, an appreciable success was recorded when biological and physical treatments were combined, including less generation of by-products (Zhang et al. 2010). This was evidenced in the study conducted by Al-Rifai et al. (2011) when combined activated sludge, microfiltration, and reverse osmosis were used to remove 11 pharmaceuticals and two endocrine disruptors in a water recycling process. The authors established 97 % removal rate when water concentration was below 0.1 μg/L for most compounds except bisphenol A with concentration above 0.5 μg/L. The authors concluded that the combined system did not only efficiently remove the micropollutants, but also succeeded in reducing the concentrations of some physicohemical parameters. Hence, in order to reduce their concentration to a less harmful level, a more sensitive and reliable technology is needed. To date, little or no extensive compilation of papers published on the use of combined techniques to degrade pharmaceuticals and endocrine-disrupting compounds from wastewater have been reviewed. Suffice to mention that most of the reported methods of removing emerging contaminants are categorized into three approaches according to Liu et al. (2009); these include physical methods, biodegradation, and advanced oxidation processes. This review focused most importantly homogeneous and heterogeneous advanced oxidation processes.
4.1 Advanced Oxidation Processes
Advanced oxidation processes (AOPs) are very effective techniques capable of completely mineralizing the EDCs and PPCPs (Mendez-Arriaga et al. 2008). AOPs are chemical oxidation techniques encompassing organic pollutant removal and degradation under the influence of highly reactive hydroxyl radicals (Saien and Soleymani 2007; Saien and Khezrianjoo 2008; Saien et al. 2011). The hydroxyl radical is highly reactive and non-selective against refractory organic pollutants with subsequent conversion of the pollutants to CO2, H2O, and mineral acids. In the last couple of years, AOPs have been extensively exploited and reported in the literature for the degradation of different wastewater containing contaminants such as pesticides, pharmaceuticals, endocrine disruptors, and dye molecules (Stasinakis 2008; Saien et al. 2011; Machulek Jr et al. 2013). AOPs could be generally classified into (1) homogeneous AOPs and (2) heterogeneous AOPs. The two categories are reviewed as follows:
Heterogeneous Advanced Oxidation Processes
This is defined as a reaction process in which semiconductor catalyst dissociated under the influence of photon energy. In this case, the catalyst and the reactant exist in different phases. Among the heterogeneous advanced oxidation processes is the solid semiconductor TiO2 photocatalyst reviewed below.
4.1.1 Heterogeneous Photocatalysis
Semiconductor photocatalysis recently emerged technique as one of the advanced oxidation processes responsible for degrading recalcitrant, hazardous, and non-biodegradable substances in wastewater and drinking water into relatively less risky products such as CO2, H2O, and inorganic ions. Despite these useful functions, the post-separation of the used TiO2 photocatalysts from wastewater treatment remains a major challenge. Many efforts were geared towards addressing this problem, and such efforts included immobilization of TiO2 upon different supports such as silica, perlite, activated carbon, clay, zeolites, and many others (Yao et al. 2010). Most of these support materials are chosen because of their higher surface area. This is to guarantee highly active reaction centers as well as improved light penetration and reflection on the support pores (Friedmann et al. 2010; Czech and Rubinowska (2013). Several studies have considered the use of titania photocatalysts or the immobilized form of titania to degrade and mineralize organic compounds (Nakashima et al. 2003). Some of these are summarized as follows so as to give comparative data to the scholars in this area. Due to the enormous availability of treatment techniques for contaminants of emerging concern, it is not possible to review all, but the review will try to capture advanced oxidation techniques involving singular and combined photocatalytic degradation of emerging contaminants.
Chong and Jin (2012) explored the photocatalytic ability of a titanium dioxide (TiO2) nanofiber as a pre-treatment technique to degrade carbamazepine using simulated wastewater. The findings revealed that 78 % of CBZ, 40 % of chemical oxygen demand (COD) and 23 % of PO4 concentrations from influent contaminated water were removed during the 4-h reaction time. After 0.5-h treatment, the analysis by high-performance size exclusion chromatography (HPLC) revealed a dramatic shift of molecular weight from higher portion (>10–1,000 kDa) to a lower level (<10 kDa). Tsai et al. (2009) described the photocatalytic degradation of bisphenol A conducted using a TiO2 suspension reactor. The degradation capability was evaluated with respect to initial BPA concentration, TiO2 dosage, initial pH, and temperature. The obtained first-order reaction constants (kobs) validated the degradation of BPA during the experiments. The photocatalytic degradation of diclofenac, metoprolol, estrone, and chloramphenicol was conducted in a tube reactor using different commercially available TiO2 by Czech and Rubinowska (2013). The prepared photocatalysts were characterized using BET, X-ray diffraction (XRD), and scanning electron microscopy (SEM), and it was observed that anatase TiO2 possessed the highest activity in the photocatalytic oxidation of diclofenac, chloramphenicol, and estrone whereas the presence of rutile only enhanced the photooxidation of metoprolol. The anatase TiO2 with 21 nm crystals was highly efficient for COD reduction in the treated water and the photooxidation of all studied pollutants followed pseudo-first-order kinetics. The photocatalytic degradation of endosulphan and lindane (endocrine-disrupting compounds) in the presence of titania was also reported by Ashiya and Guatam (2011). The study revealed that the presence of acetone in the reaction mixture slowed the reaction competition for hydroxyl radicals between pesticides and acetone. The degradation efficiency of 95 % (endosulphan) and 98 % (lindane) coincided with the zero point charge of the catalyst at pH 7. Chong et al. (2011) opined that immobilized anatase titanate nanofiber and mesoporous TiO2 infused kaolinite catalysts in a photoreactor (SB-ASP) were responsible for the CBZ elimination from municipal wastewater. The experiment was investigated by varying the following parameters, batch reactor cycles, effluent organic matter, nitrate, and phosphate. The analyzed results as obtained from high-performance size exclusion chromatography revealed that the carbamazepine degradation was achieved.
Treatment of emerging contaminants in water by combined advanced oxidation processes
Type of water
Combined treatment technique
Results and comments
Low-pressure lamp (17 W, 254 nm); CH2O2 = 5 and 20 mmol/L, pH 2.0–7.0
Complete removal of paracetamol with mineralization between 21 % to 40 % by using H2O2 concentration of 5.0 and 20 mmol/L
Clofibric acid (PPCPs)
Low-pressure lamp (17 W, 254 nm); Co = 1.0 mmol/L, CH2O2 = 1.0 mmol/L, pH 5, Tr = 60 min
Almost complete removal of clofibric acid in 60 min with low concentration
17β-Estradiol, estrone and 17α-ethinyestradiol (EDCs)
Sterile double-distilled water
UV and and UV/TiO2
High-pressure Hg lamp (152 W): C0 = 10 μg/L; TiO2 immobilized: UVA lamp (photolysis)
Very fast removal of estrogenic activity of the three compounds tested by photocatalysis. The 50 % of estrogenic activity was removed in 10 min. Removal of 100 % was reached in 1 h of treatment. Photolysis needs more time to remove estrogenic activity
Coleman et al. (2004)
Antibiotic (metronidazole) (PPCPs)
UV photo-Fenton, UV/H2O2
Low-pressure lamp: UV = 0-600 mJ/cm2, concentration of H2O2 = 25–50 mg/L, antibiotics (Co) = 6 μmol/L; pH (UV) = 6 pH(photo-Fenton) = 3.5
UV provides low degradation compared with UV/H2O2. Photo-Fenton gives 20 % higher removal than Fenton.
Shemer et al. (2006)
Laboratory-grade water, surface water of low alkalinity and high alkalinity
Low-pressure UV and medium-pressure UV, 10−5 mM H2O2
Reduction in estrogenic activity occurs faster in laboratory water than in surface water indicating scavenging effects and also dependence on water alkalinity. In most cases, complete removal occurs at ≤600 mJ/cm2 fluence
Linden et al. (2007)
Naproxen, iohexol, carbamazepine, clofibric acid (PPCPs)
Laboratory-grade water, surface water
Medium-pressure UV 200–300 nm at pH = 7 1–3 μM H2O2
Moderate degradation at 100 mJ/cm2 fluence and >99 % at 600–1,700 Mj/cm2 depending on the drug. Lower degradation with direct photolysis. Rates decrease in surface water compared to laboratory water
Pereira et al. (2007a)
Ultrasonic irradiation with TiO2, SiO2, SnO2, TiO2/SiO2
Sonication at 216, 617,850 KHz, diclofenac concentration 50–100 mg/L
90 % degradation after 60 min at 216 or 617 kHz and 20 % at 850 kHz without particles. Particles enhance degradation
Hartmann et al. (2008)
Petrochemical industrial wastewater
TiO2 nanoparticle dosage (initial phenol concentration (30 to 110 mg/L), pH of solution (3 to 11), contact time (0 to 300 min), UV irradiation duration
The highest removal efficiency of phenol occurred at contact time (300 min) and solution pH (3) and initial phenol concentration (50 mg/L). Combined UV/TiO2 improved the removal efficiency
Ali Nickheslat et al. (2013)
Phenol and chlorophenol
pH of solution, initial hydrogen peroxide concentration, TiO2 dosage, UV irradiation duration
Within 90 min irradiation time, 74.6 % and 79.8 % phenol and chlorophenol were destroyed. The optimum removal conditions were pH 4, 0.5 g/L TiO2, 90 min irradiation time, 75 ml/L H2O2. UV/H2O2/TiO2 combined system was more efficient than UV/H202 and UV/TiO2
Dixit et al. (2010)
Initial pH of solution, temperature, ozone dosage, initial H2O2 concentration, rotation speed
The degradation rate increases with increasing solution pH and rotation speed. 100 % complete degradation of 40 mg/L phenol was achieved when O3/H2O2 were combined.
Zeng et al. (2013)
Photocatalytic, sonolytic, sonophotocatalytic
Artificial UV lamp 125 W, Solution pH, catalyst dosage, oxidant concentration
80 % degradation was achieved in 2 h for photocatalytic treatment. With the integration of ultrasound, 96 % destruction rate was achieved within 1 h 30 min. The trends of degradation was sonophotocatalysis > photocatalysis > sonocatalytic > sonolysis
Verma et al. (2013)
Diethyl phthalate (DEP)
Single (sonolysis, photolysis, photocatalysis) and combined (sonophotolysis, sonophotocatalysis)
UV irradiation time, photocatalyst dosage variation ultrasonic frequency variation
There was a rapid degradation and mineralization of DEP with the sonophotocatalytic processes more than singular sonolysis and photolysis. The study demonstrated the combined process decomposed DEP faster than single advanced oxidation process.
Naa et al. (2012)
Di-(2-ethylhexyl) phthalate (DEHP)
Synthetic and real petrochemical wastewater
H2O2 concentration, Fe2+ concentration, and solution pH, contact time
85.6 % degradation of DEHP was noticed within 60 min at pH 3. The study also suggested that the optimum concentration condition were 90 mg/L H2O2, 5 mg/L Fe2+, and 20 mg/L DEHP in the solution at pH 3.
Esmaeli et al. (2011).
Fenton and photo-Fenton
Fe2+/H2O2/Sunlight at pH 6.5, Diclofenac concentration 50 mg/L
Fast drug degradation was accompanied by slower mineralization
Perez-Estrada et al. (2005a)
TiO2 photocatalysis and ozonation
Variation of TiO2 concentration, ozone dose, UV irradiation time and solution pH
The results revealed that increase in ozone dosage up to 0.48 g/h and a decreased of TiO2 above 1 g/L at optimum pH = 6, accelerate the removal rate. 88 % chemical oxygen demand (COD) removal in 3 h by the combined photocatalytic ozonation process was noticed, whereas 54 % and 67 % COD were individually removed by photocatalysis and ozonation process. Hence, carbendazim was effectively mineralized by the combined photocatalytic ozonation process. The study revealed that combined TiO2-based photocatalysis and ozonation significantly overwhelmed the individual treatment technique ozonation and TiO2 photocatalysis
Rajeswari and Kanmani (2009)
Treatment of emerging contaminants in water by single advanced oxidation processes
Type of water
Single treatment technique
Results and comments
17β-Estradiol, estrone (EDCs)
Deionized water and treated effluent from a sewage treatment plant
Immobilized TiO2/UV-black fluorescent lamp, 250 μg/L
Fast degradation of both estrogens following first-order kinetics. Rate increases with increasing catalyst surface area and temperature decreasing initial concentration. Degussa is generally more active
Nakashima et al. (2003)
Clofibric acid, carbamazepine, iomeprol, iopromide (PPCPs)
Suspended Degussa or Hombikat TiO2/artificial sunlight at pH = 3.4–6.5, 0.5–10 mg/L
First-order kinetic rates increase with increasing catalyst loading and decreasing initial concentration. Degussa is generally more active than Hombikat. Determination of by-products and pathways
Doll and Frimmel (2004)
UV irradiation time variation, H2O2 concentration, initial concentration of abamectin
70 % and 60 % removal and mineralization rate of abamectin was achieved within 60 as well as 180 min UV irradiation, respectively. The use of advanced oxidation process such as photo-Fenton assisted in degrading abamectin
Matos et al. (2012)
Suspended Degussa TiO2/UV-254 nm or UV-365 at pH = 3.5–11, 2–10 mM
First-order rate under UV short wave irradiation is much faster than under UV long wave. Several factors such as initial drug, catalyst, and oxygen concentrations, pH, and light intensity are tested concerning drug degradation. Identification of by-products
Yang et al. (2008)
Gemfibrozil, tamoxifen (EDCs)
Suspended Degussa or anatase TiO2/UV-360 nm at pH = 10, 2.5–47 mg/L
Gemfibrozil undergoes only photocatalytic degradation but not photolytic. Tamoxifen undergoes direct photolysis but its metabolites undergo photocatalytic degradation. Degussa is more active than anatase. Mineralization takes much longer than drug removal
Yurdakal et al. (2007)
Suspended Degussa or anatase TiO2 or ZnO/UV-(320–400 nm) at pH = 5, 10–80 mg/L
First-order rate increases with increasing drug concentration and catalyst loading and adding H2O2. Degussa TiO2 and ZnO are equally active. Complete elimination of drug activity after 90 min corresponding to 70 % mineralization
Chatzitakis et al. (2008)
Diclofenac, naproxen, ibuprofen (PPCPs)
Suspended Degussa TiO2/artificial sunlight, 25–200 mg/L
Diclofenac and naproxen are susceptible to photolysis but not ibuprofen. By-products of low biodegradability accompany diclofenac and naproxen degradation
Mendez-Arriaga et al. (2008)
In another study, photodegradation assisted by TiO2 was conducted for degrading estrone and 17β-estradiol using sunlight and UV irradiation (Zhang and Zhou 2008). The study confirmed the use of UV photodegradation to be far more efficient than solar irradiation, and this was ascribed to the vigorous absorbance of ultraviolet energy by EDCs, although the method did not depend on initial chemical concentration or sample matrix. Thus, heterogeneous photocatalysis was confirmed as a very effective method for removal of EDCs from water. Sin et al. (2012) in his review article submitted that heterogeneous photocatalysis in the presence of UV, solar light, and oxygen is highly effective for the removal and decomposition of EDCs in water. The authors established that the degradation efficiency is influenced by solution pH, reaction temperature, light intensity and wavelength, catalyst loading, and initial substrate concentration.The study on the adsorption and photocatalytic degradation of 15 different pharmaceuticals by titanium dioxide (TiO2) anatase and rutile nanowires together with commercially available TiO2 (P25) nanoparticles in the presence or absence of UV were investigated by Hu et al. (2012). The UV-exposed anatase nanowires effectively mineralized venlafaxin, atorvastatin, ibuprofen, naproxen, and gemfibrozil while the effective removal of fluoxetine and norfluoxetine was by rutile-phase nanowires. The exposure to 265 nm UV effectively decayed sulfamethoxazole and atrazine. The kinetic studies showed that the removals of the pollutants were completed within 20–40 min. Rengaraj and Li (2011) established the photocatalytic degradation of bisphenol A under UV illumination. Ultrasonic-assisted sol–gel method was used to synthesize and characterize Ag-doped titania. The characterization was done using SEM, EDX, XRD, XPS, and UV-visible absorption spectroscopy. The loading of Ag on TiO2 catalysts significantly hastened the photocatalytic oxidation of BPA in aqueous suspension. The reaction mechanism specified that the nanosized Ag doped on TiO2 nanocomposites acted as the center for electron capture where the reduction of oxygen took place. This reduction of oxygen by means of the separation in Ag–TiO2 nanocomposites was responsible for the faster BPA degradation rate compared with pure TiO2 particles. Also, Fabiola et al. (2008) revealed that the degradation of naproxen in a 0.078 L Duran reactor equipped with a 1 kW Xe-lamp using TiO2 as photocatalyst did not only confirm photocatalysis treatment as a highly efficient technique but recommend further improvement in the biodegradability under the operational conditions tested.
However, Zhang et al. (2010) appraised the photocatalytic oxidation of endocrine-disruptors, Escherichia coliform (Escherichia coli) as well as the estrogenic activity of a secondary effluents obtainable from a wastewater treatment plant using a photocatalytic reactor coated with TiO2. Filtration followed by photocatalysis was used to compare the reduction rate of estrogenic activity and E. coli in both filtered and unfiltered wastewater samples. The inactivation rate of E. coli and estrogenic activity improved considerably after filtration which was ascribed to the partial removal of suspended solids which thus enhanced the photocatalytic oxidation. Combined photocatalysis with filtration pretreatment was reported to be effective. Lin et al. (2013) utilized immobilized TiO2 photocatalyst supported on a fiberglass to photo mineralize ibuprofen. The authors established direct relationship between photocatalytic dosage and contaminant degradation rate. Addition of H2O2 significantly enhanced the decomposition rate. In the same vein, Madhavan et al. (2010) investigated the combined effect of sonolysis, photocatalysis, and sonophotocatalysis on the degradation of ibuprofen. The authors discovered that the obtained degradation rate from sonophotocatalysis was significantly higher than the individual advanced oxidation processes studied. The synergy index value of combined advanced oxidation was 0.3 greater than individual sonolytic and photocatalytic process. Equally, Frontistis et al. (2012) reported the efficiency of the combined photocatalysis and ultrasonic irradiation regarding 17α- ethynylestradiol (EE2) degradation in aqueous solution. The study established a synergistic effect rather than a cumulative action between the two combined systems. In fact, increase in catalyst dosage and photon flux together with a decrease matrix complexity was responsible for estrogen concentration changes. So far, this review has shown that photocatalysis using titania or its immobilized form may be useful in degrading and completely mineralizing some of the PPCPs and EDCs in contaminated water.
4.1.2 Homogeneous Advanced Oxidation Processes
Homogeneous advanced oxidation process is a reaction process in which both the reactant and photocatalysis are in the same phase. Most of the commonly used homogeneous photocatalysis are ozone, metal oxide, metal oxide/hydrogen peroxide, etc. The mineralization and decomposition of emerging micropollutants by homogeneous advanced treatment therefore exploits ozone, hydrogen peroxide, ultraviolet light, or combined process using a combination of ozone and hydrogen peroxide, hydrogen peroxide, and ultraviolet light, ozone/ultraviolet light/ferrous ions or ferric ions, ferrous or ferric ions/hydrogen peroxide, ferrous or ferric ions/ultraviolet light to degrade toxic pollutants. This section review some of the previous research conducted using either single or combined system to destroy the contaminants of emerging concern. Xu et al. (2007) demonstrated ozone oxidation technique to investigate the degradation of endocrine disruptor bisphenol A in drinking water. The effects of different ozone dosages, BPA initial concentrations, and ozone adding time were monitored in a reactor. The outcome of the study revealed that ozone dosage plays a dominant role during the process of BPA degradation, while the impact of the contact time could be ignored, although high ozone dosage, and dissolved ozone concentration greatly contributed to the extent of BPA degradation. Thus, the O3-H2O2 processes proved to have similar effects on the degradation of BPA by ozone oxidation. Li et al. (2011) reported the pentachlorophenol (PCP) removal under xenon lamp irradiation in presence of metal oxide bismuth silicate (BSO). A mineralization rate of 99.1 % after 120 min was observed. Decrease in pH was observed from pH 6.2 to 4.6. The electron spin resonance results indicated that the degradation mechanism was largely due to the presence of superoxide radical (O2−), and the GC-MS analysis confirmed the presence of some intermediates, mainly aromatic compounds and aliphatic carboxylic acids. Table 2 captures other single treatment techniques used to remove and degrade emerging micropollutants.
Furthermore, a review conducted by Ning et al. (2007) showed that the combination of ozone and other ozone-based advanced oxidation process were highly efficient in destroying steroidal estrogenic hormones such as estrone (E1), 17b-estradiol (E2), estriol (E3), and 17aethinylestradiol (EE2), except phthalates suspected resist degradation by ozone. Pereira et al. (2007) explored two homogeneous advanced oxidation processes such as UV and UV/H2O2 to degrade pharmaceutically active substances fond in aqeous water. The authors discovered that integration of hydrogen peroxide significantly enhanced the overall degradation efficiency of the pollutants. Hence, combined UV/H2O2 was revealed to be a viable option for degradation of the pollutants. Giri et al. (2010) utilized seven different advanced oxidation methods such as UV, UV/TiO2, UV/H2O2, O3, O3/UV, O3/TiO2, and O3/UV/TiO2 to remove and degrade selected PPCPs in aqueous solution. The authors observed four different removal patterns which include non-degradation by any of the seven techniques, degradation by ozone-based, and not ultraviolet, system, degradation by ultraviolet, and not ozone-based, techniques. In addition, the degradation of the pollutants by combined ozonation and ultraviolet was rapid and efficient. Zelmanov and Semiat (2008) demonstrated that iron(III) oxide nanocatalyst combined with hydrogen peroxide could effectively degraded water contaminated with phenol and ethylene glycol without necessarily exposing the solution to ultraviolet or photolight. The authors concluded that the degradation rate of phenol and ethylene glycol was 35 times and two to four times greater than the literature value reported for Fenton process and UV. Kim et al. (2009) reported that H2O2 addition aided the photodegradation of PPCPs during UV treatment and found that the average first-order rate constant value for all the PPCPs studied increased by a factor of 1.3 when H2O2 was added during UV treatment. The addition of H2O2 improved the photodegradation rates of PPCPs appreciably during UV treatment. Equally, Nakonechnya et al. (2008) explored ozone and hydrogen peroxide combination to degrade steroidal hormone estrone (E1). The authors observed that the E1 degradation was found to depend on pH and O3/H2O2 ratio. Thus, the combination of O3/H2O2 proved to be highly efficient treatment for E1. Lopez et al. (2003) argued that UV and UV/H2O2 treatment methods were able to oxidize pharmaceutical intermediates such as 5-methyl-1,3,4-thiadiazole-2-methylthiol (MMTD-Me) while the direct photodegradation of the two pharmaceutical intermediates have shown that the intermediate with lower concentration degraded faster. Vogna et al. (2004a) proposed diclofenac oxidation using combined UV/H2O2 and ozonation technique and were found to be feasible and viable in bringing about complete mineralization of diclofenac. In another study, the UV/H2O2 method was effective in removing carbamazepine while UV alone was not efficient to reduce the carbamazepine concentration (Vogna et al. 2004b.
However, other AOPs such as ultrasonic irradiation with widespread application of degrading organic pollutants present in wastewater have equally been examined (Shannon et al. 2008). These methods oxidize the complex organic matter found to be extremely difficult to degrade biologically into simpler end products. The reported complete degradation and mineralization potential was ascribed to the reactive nature of the generated free active species most predominantly hydroxyl radical (Giri et al. 2010). The radical was reported to be non-selective and highly reactive with higher oxidation potential of 2.33 V when compared with other conventional oxidizing agents such as H2O2 or KMnO4 (Gogate and Pandit 2004). Xiao et al. (2013) combined continuous wave (CW) and pulsed wave (PW) ultrasound to degrade carbamazepine, ibuprofen, acetaminophen, sulfamethoxazole, ciprofloxacin, propyl gallate, and diethyl phthalate. The findings showed that degradation rates of the pollutants were quicker for smaller compounds but less rapid for larger compounds by PW ultrasound than that achieved under CW ultrasound. The introduction of hydroxyl radical trapping agent, acetic acid, to PPCPs solutions aided in degrading certain fractions within the bulk solution that were positively interrelated with the molar volume of the compound. Chong et al. (2012) joined an integrated membrane bioreactor with titanium dioxide photocatalyst (MBR-TiO2) to mineralize CBZ from pharmaceutical wastewater. The individual systems of MBR and TiO2 photocatalytic technology were monitored separately so as to establish the synergistic operational effects. The respirometric test showed that CBZ introduction acts as a chemical stressor that influenced the biomass metabolism in the MBR which resulted in the reduction of sludge yield. The combined MBR-TiO2 system operated at 4:1 ratio condensed the chemical stressor effects and a maximum of 95 % CBZ removal efficiency was observed. However, despite the success story of efficient degradation of emergent contaminants by credited to various advanced oxidation processes, high operational cost still limits it widely acceptance especially in the emergent nations. Therefore, the integration of less expensive treatment process such as adsorption with advanced oxidation process will further enhance and accelerate the degradation rate of organic pollutants.
4.1.3 Combined Advanced Oxidation Processes with Biological and Physical Treatment
As stated earlier, wastewater treatment containing organic pollutants is complex, expensive, and time-consuming. The survey of literatures revealed that most wastewater treatment plants are not designed for emergent pollutants removal. Therefore, a more practical, versatile, and economical technique is needed. Ever since the identification and detection emergent micropollutants in terrestrial and aquatic environment, numerous treatment processes such as biological and physicochemical methods have been exploited and explored. However, AOPs and other conventional treatment option such as physicochemical treatment have been applied to remove pharmaceuticals and endocrine disruptors from wastewater. Advanced oxidation process coupled with biological process is not often effective against the target pollutants degradation due to the inhibitory effect of hydroxyl radicals by the biomass. The physical treatment such as filtration can be employed before or after the advanced oxidation process. Thus, the application of physical and biological methods helps to remove large insoluble particles, reducing turbidity and interfering ions from the aqueous matrix. Once all these are removed, the degradation efficiency of AOPs will be improved. An overview on the combined AOPs with physical or biological treatment is as follows. Ozonation as one of the AOPs is employed as a pre-oxidation step in a combined treatment line. The combination of ozonation and a moving-bed biofilm reactor to remove estrogenic substances from a conventional activated-sludge treatment was reported by Gunnarsson et al. (2009). Ozonation was used as a pre-treatment step to enhance anaerobic digestion stability of the modelled compounds (Carballa et al. 2007). Not only that, chemical oxygen demand solubilization of 60 % was achieved, thus hastening the destruction of the soluble organic matter during anaerobic digestion. Also, photo-Fenton process was integrated with a biological system to degrade α-methylphenylglycine by Oller et al. (2007a). It was observed that the combined system enhanced the biodegradability and broad elimination of bio-refractory compound (Oller et al. 2007). The coupled technology was able to remove 95 % of total organic carbon (Oller et al. 2011). José et al. (2010) reported that the removal of carbamazepine, clofibric acid, diclofenac, and diazepam from municipal wastewater using biological treatment failed to completely eliminate the pollutants, but incorporation of membrane bioreactor and advanced oxidation processes such as O3, O3/UV, O3/H2O2, and UV/H2O2 effectively achieved complete removal. Nakada et al. (2007) studied the removal of some pharmaceuticals and endocrine disruptors present in municipal wastewater by combined sand filtration and ozonation. It was stressed that the combined efforts of ozone and sand filtration with sludge treatment enhanced the elimination of 80 % of the model pollutants. Ozonation was noted to play the dominant role. In another study, combined advanced oxidation and a biological treatment process was used to remove pesticides from an aqueous matrix. It was observed that the biological treatment assisted in removing the bulk of the chemical oxygen demand while O3/UV enhanced the pesticide decomposition (Lafi and Al-Qodah 2006). Rossnera et al. (2009) utilized three different adsorbents such as activated carbon, carbonaceous resin, and high-silica zeolite to remove EDCs, PPCPs, and flame retardant. Activated carbon was only able to achieve a two-log elimination of the modelled compounds. The incorporation of advanced oxidation processes enhanced the degradation rate. Badawy et al. (2009) utilized combined Fenton and biological treatment process to eliminate some pharmaceuticals from industrial wastewater. The authors revealed that recalcitrant toxic pollutants were partially removed by biological treatment and successfully destroyed by the Fenton process. Thus, the integrated system is more advantageous and capable of completely eliminate hazardous persistent pollutants in water and wastewater.
Liu et al. (2009) reviewed the removal mechanism of endocrine disruptors in wastewater treatment using three different methods such as physical, biodegradation, and chemical advanced oxidation processes. The authors recognized that physical and advanced oxidation processes such as ozone/hydrogen peroxide are more advantageous with respect to removal and degradation efficiency while a biodegradation process among which activated sludge treatment was observed to reduce the level of EDCs in surface water. Hence, physical, biological, and chemical advanced oxidation could be combined to improve the removal performance of EDCs.
5 Detection and Quantification of Emerging Micropollutants
The contaminants of emerging concern are structurally complex, and the probability of being highly present in drinking water is increasing due to ever-increasing population density driven by high demand of new chemicals in the market (Marchesini et al. 2007). There is a need to urgently adopt fast and sensitive analytical protocols that can screen, detect, and quantify a large diversity of composites concurrently. This becomes necessary as the various known analytical equipment including liquid chromatography combined with mass spectrometry (LC-MS), high-performance-liquid chromatography (HPLC), liquid chromatography coupled with electrochemical detection (LC-ED), capillary electrophoresis (CE), and gas chromatography coupled with mass spectrometry (GC-MS) used so far are not without some challenges (Gatidou et al. 2007; Comerton et al. 2009; Mottaleb et al. 2009). Presently, most of these instruments are too cumbersome in analyzing samples, and despite the long sample pre-treatment, the equipment can only detect or identify a small range of similar contaminant at a time in the matrices. Apart from this, long pre-concentration steps, lack of rapid processing and detection of sample, and time-consuming analysis negates the continuous use of these analytical instruments. Also, lack of portability and suitability during field work constitute another challenge (Huertas-Perez and Garcia-Campana 2008; Blazkova et al. 2009; Le Blanc et al. 2009). Not only these, the sophisticated and costly nature of the equipment, their labor–energy-intensive nature coupled with the need of the derivatization of samples in the case of (GC-MS) prior to analysis form the basis to develop more simple and robust instrumental techniques that are affordable, durable, and easy to maintain. Sensors and biosensors are presently seen as a replacement to those aforementioned instruments. Iwuoha (2012) established that sensor materials were more sensitive in determining and quantifying the emerging micropollutants than other reported analytical equipment such as GC-MS. Sensors and biosensors, for instance, are presently considered as an alternative to those aforementioned expensive instruments due to the ease of operation and elimination of the pre-treatment steps, lower consumption time, higher sensitivity, and fast responses (Yin et al. 2009). Other benefits include easy determination of pollutants in multifaceted media without rigorous sample preparation (Rodriguez-Mozaz et al. 2006). Presently, large numbers of biosensors developed are still in the laboratory stage and need to be certified before bulk manufacture and use (Rodriguez-Mozaz et al. 2006; Farré et al. 2009). Apart from providing information on the chemical composition of the sample, biological activity such as toxicity and endocrine-disrupting effect can also be obtained. The development of more sensors and biosensors should be the top priority in the nearest future. This should be standardized for easy detection and quantification of emerging micropollutants. Without functional and practicable analytical tools, determination, and quantification of the environmental fate, and behavior of these emergent pollutants will be a mirage. However, despite the simplicity in the use of biosensors to detect EDCs and PPCPs, one major challenge is that biosensors is specific and can only sense a micropollutant at a time. This mean a lot of biosensors need to be developed which will require additional cost and time. Therefore, simple analytical technique such as total organic carbon (TOC) is recommended as a high level of TOC in water or wastewater signifies huge pollutants load. Hence, the water or wastewater necessitates treatment.
6 Conclusion and Recommended Interventions
Advanced oxidation processes can degrade these contaminants without necessarily adding complex chemicals. This is a much green and less complex route than many other options, if energy consumption can be reduced. Although free radicals are short-lived and do the job rapidly without leaving long-term residues when applied properly but need to be managed so as not to leave partially degraded secondary pollutants. The continuous reported adverse health challenges occasioned by poor or partial elimination and degradation of emerging micropollutants by single treatment techniques in drinking water also require urgent interventions. This means that the current single treatment system has failed to do the job and instead of detoxifying the contaminants acts as a point source of environmental contamination. The removal of PPCPs/EDCs from contaminated water is a challenging task due to the complexity and persistence of the pollutants in water. Thus, there is a need to urgently adopt the use of combined advanced oxidation techniques to treat wastewater contaminated by emerging pollutants. Combined advanced oxidation techniques offer the potential to drastically reduce the health and environmental effects of persistent organic pollutants and endocrine disruptors. Hence, the new approaches will help to maintain public health together with environmental sustainability. The combined technology offers the hope that adequate water purification will not only contribute to the production of safer water for drinking purpose but also eliminate the re-surfacing of by-products after single-stage wastewater treatments. This will go a long way to putting to rest the issue of heightened fears concerning the safety of drinking water. Not only this, the task of water regulatory agencies will also become less onerous with regards the treatment of the contaminated water. Emerging micropollutants are now recognized as a problematic issue due to their bioaccumulation in the environment owing to their accidental discharge. A lot still needs to be done, among which is the establishment of monitoring teams to enforce the environmental regulations. There is also a great need to continuously carry out a thorough routine toxicological analysis of drinking water so as to establish the exposure limits to PPCPs/EDCs in humans. Water regulatory agencies need to be informed of the proper treatment technique to adopt while treating tainted wastewater. Future research should also include re-designing and upgrading conventional wastewater treatment plants in urban centers to accommodate the latest technology of degrading and mineralizing emerging pollutants. This will contribute in no small measure to the quality of drinking water. The research on emerging contaminants should be extended to the developing countries where the extent of the deterioration of water sources needs to be established, which will aid the water industry and the policy makers to choose the appropriate advanced water treatment techniques to use for degrading PPCPs/EDCs in water. Biosensors need to be further developed for rapid detection of specific contaminants. In order to have a natural ecosystem that is completely free of pharmaceuticals and endocrine-disrupting compounds, there is a need to introduce green chemistry in the production and development of numerous drugs, medical, and personal care products that are affordable and do not persist in the environment. A regulatory framework based on the precautionary principle should be implemented to screen all compounds for their ecotoxicity prior to approval for use.
The authors acknowledge Environmental and Nano Sciences Research Group, Department of Chemistry, University of the Western Cape, South Africa, for the assistance.