5.1 Removal of alkylphenols by membrane-based processes

During the three last decades, membrane separation processes have evolved from simple laboratory methods to large-scale operations with a significant technical and commercial impact (Cicek, 2003). Nowadays, membrane processes are considered as an efficient technology for water treatment because of the possibility to obtain better water quality in a more compact WWTP, easier to automate with less sludge production and greater cost-effectiveness as compared to conventional processes (coagulation/flocculation/sedimentation/filtration combined with a carbon step and/or a chemical oxidation). Compared to other methods, their remarkable advantages are the high quality of effluent from a chemical point of view and the removal of microbes and viruses without chemical disinfection. Membrane filtration also provides the potential to reuse the wastewater. Drinking water is potentially the major application of membrane-based process mainly due to the recently more stringent drinking water regulations which make conventional treatments inadequate for treating emerging contaminants. However, recent studies have also shown that industrial wastewaters can also be treated efficiently using membrane technology (Crini and Badot, 2007). The technologies proposed for alkylphenol treatment include membrane treatment using biological (membrane bioreactors) or physical processes (membrane filtration such as nanofiltration).

As already mentioned, the performance of an activated sludge process is variable. An additional tertiary treatment can then be added to achieve lower pollutant levels in the liquid stream in terms of total suspended solids (SS) and chemical oxygen demand (COD). Another solution consists of using membrane bioreactor (MBR) technology and the quality of effluents obtained in these plants is similar to that obtained in a combined treatment by a conventional ASP and a tertiary step (Cicek, 2003; Cases et al., 2011). In recent years, particular attention has been paid to the impact of MBR technology on emerging pollutants. MBRs are able to operate at a higher biomass concentrations obtaining a better permeate quality due to membrane separation by filtration (microfiltration or ultrafiltration). MBR systems are also considered to be among the most promising technologies in microbiological wastewater treatment due to the fact that the long life of the sludge gives the bacteria the time to adapt to the treatment-resistant substances.

The results published by Li et al. (2000) showed that, compared to WWTPs, membrane-assisted biological treatment improved the efficiency of eliminating NP and NPEOs. Similar conclusions were recently reached by Cases et al. (2011) for 4-OP and 4-t-OP removal. Previous work in lab-scale activated sludge units by Tanghe et al. (1998) showed that NP at an initial concentration of 8.33 mg L⁻¹ was almost totally removed and biodegraded. The results were temperature dependent: by lowering the temperature, elimination capacities decreased. Aeration was also a key factor in the fate of NP, and removal was shown to be highly dependent on the absence of anoxic zones.

Wintgens and co-workers (Wintgens et al., 2002, 2003, 2004; Gallenkemper et al., 2003; Lyko et al., 2005) carried out various studies to challenge the fact that membrane-based technology was an effective barrier to endocrine disrupting compounds including APs. The removal techniques studied were MBR, nanofiltration (NF), reverse osmosis (RO), activated carbon adsorption and ozonation (Wintgens et al., 2003, 2004). Investigations were conducted on effluents from landfill leachate treatment plants where input NP concentrations were detected in high mg L⁻¹ ranges. MBRs were capable of removing more than 80% of the NP load (Wintgens et al., 2003). RO treatment proved to be less effective in NP removal than MBR. In another study of the removal of endocrine disrupters present in the outflow from a landfill leachate treatment plant, using an MBR system that comprises three bioreactors and an external ultrafiltration unit followed by granular activated carbon adsorption, they reported the elimination of more than 90% of NP (Wintgens et al., 2002). The MBR was the most effective process step while the carbon treatment, applied downstream, was a further polishing step. The choice of membrane type was crucial. Studying eleven different membranes with the laboratory set-up, they observed that retentions for NP and bisphenol A ranged between 70% and 100%. Gallenkemper et al. (2003) also demonstrated that NF provided high quality permeates in water and wastewater treatment containing NP and bisphenol A with similar retention. Lyko et al. (2005) reported that the characteristics of influents (landfill leachate or municipal wastewater) had an effect on the composition of the activated sludge and thus had an associated impact on the efficiency of the membrane unit on micropollutant rejection. When the MBR effluent was compared to a conventional effluent loaded with suspended matter, improved elimination can be expected due to the particle-free effluent quality. Wintgens and co-workers concluded that MBRs were a promising process combination of activated sludge and membrane filtration for wastewater treatment and reuse, and biological degradation was the most important step in the process. A vast number of options can be envisaged, e.g., artificial groundwater recharge, indirect potable reuse, and industrial process water production.

Camacho-Munoz et al. (2012) studied the effectiveness of MBRs as the advanced treatment in the removal of NP and NPEOs in WWTPs during a one-year monitoring study. The system studied operated with flat sheet and hollow fiber membranes in two parallel lines. Moreover, a RO module connected in series after the hollow fiber membrane was evaluated for one month. MBR and conventional ASP were also compared, as well as the influence of the physicochemical properties of the compounds on the removal rates achieved. No significant difference of effectiveness was found between flat sheet and hollow fiber membranes. However, an improvement was obtained with the addition of a RO module. Biodegradation has been shown to be the main route involved in the removal of organic compounds during both technologies.

Nanofiltration and reverse osmosis for AP removal were also proposed by Kiso et al. (2000), Agenson et al. (2003) and Jung et al. (2007). For example, Kiso and co-workers (Kiso et al., 2000) studied the behavior of ten kinds of APs adsorbed on the membrane polymer (polyamide). Their results showed that desalting membranes, including reverse osmosis membranes, were necessary for the effective removal of the APs. As expected, the adsorption was mainly controlled by the hydrophobic interaction.

5.2 Removal of APs by biotechnological-based methods

Another interesting biologically-based technology uses packed bed biofilm reactors which maintain a high biomass concentration and activity during the treatment minimizing, at the same time, start up operations and cell washout (Soares et al., 2003a,b, 2006). In addition, bioreactors display high tolerance versus very high and variable organic loads. Soares et al. (2003a) tested a packed bed bioreactor, with 170 ml glass bead carriers and 130 ml medium for NP removal with a Sphingomonas sp. The bioreactor was first continuously fed with the medium saturated with NP in an attempt to simulate the groundwater pollution. At best, nonylphenol was degraded by 99.5% at a feeding rate of 69 ml h⁻¹ and a removal rate of 4.3 mg nonylphenol day⁻¹, resulting in a 7.5-fold decrease in effluent toxicity according to the Microtox test. The bioreactor was then fed with soil leachates at 69 ml h⁻¹ from artificially contaminated soil (1 g nonylphenol kg⁻¹ soil) and a real contaminated soil (0.19 g nonylphenol kg⁻¹ soil). Nonylphenol was always completely removed from the leachates of the two soils. It was removed to 99% from the artificial soil but only 62% from real contaminated soil after 18 and 20 d of treatment, respectively, showing limitation due to NP adsorption. The authors demonstrated that it was possible to develop a continuous biofilm reactor with a Sphingomonas sp. strain capable of removing 4-NP from groundwater (Soares et al., 2003b). In another study, the authors set up a mixed packed bed reactor capable of removing 4-NP mixtures at temperatures below 15 °C (Soares et al., 2006). This was the first reported case of cold-adapted microorganisms capable of mineralizing NP.

Other interesting results using biotechnological systems for NP removal have been reported. Fujii et al. (2003) developed a lab-scale packed-column reactor inoculated with a bacterial consortium for NP elimination. Sphingomonas cloacae S-3T was isolated from wastewater from a sewage treatment plant in Tokyo in 2000. In flask-scale experiments, S-3(T) cells immobilized on porous polypropylene carriers (beads) efficiently degraded NP to concentrations routinely measured in aquatic environments [a few parts per billion (ppb), or micrograms per liter]. The system worked properly and consistently removed several hundred ppb of NP to ecologically safe concentrations of less than 10 ppb in industrial wastewater without the addition of nutrients. The effect of wastewater pH on the system performance was also evaluated and wastewater samples with pH values of 6 or 8 were treated efficiently without pH adjustment. These results suggest that a biotreatment system using NP-degrading bacteria can efficiently remediate industrial wastewater and contribute to the preservation of aquatic environments.

Bertin et al. (2007) and Di Gioia et al. (2008) developed a set of parallel batch immobilized cell bioreactors, packed with different materials (silica beads, granular activated carbon, and glass spheres), and inoculated with a Pseudomonas sp. strain for the treatment of water artificially contaminated with NPEO (synthetic solutions at concentrations in the range 30–90 mg L⁻¹). The results indicated a remarkable decontamination potential and microbial stability along with biodegradation performances higher than those displayed by freely suspended cells of the same strain. The three biofilm reactors, tested under batch conditions, showed comparable degradation capabilities and specificities, being able to remove from 77% to 99% of the load after 9 days of batch treatment. Biofilm grew well and homogeneously on the packed beds of the three reactors. NPEOs were mostly removed through biodegradation, as suggested by the accumulation of two metabolites typical of NPnEO aerobic biodegradation, i.e., phenol and 4-nonylphenol, and by the low amounts of NPEO recovered from the reactors at the end of the study. However, NPEO degradation was not complete and trace amounts of 4-NP persisted in the reactors at the end of the treatment. In order to use more efficient biocatalysts, the same group proposed to use an aerobic bacterial consortium obtained from textile wastewater capable of degrading both 4-NP and NPEOs (Di Gioia et al., 2004, 2008, 2009). The consortium was composed mainly of Alfa and Gammaproteobacteria, with a high percentage of members of the Pseudomonas genus among the latter group (Di Gioia et al., 2008). Packed bed reactors were developed at the lab-scale by immobilizing the consortium on silica beads or granular activated carbon. NP concentration at 100 mg L⁻¹ was the sole carbon and energy source in the system. The consortium was able to colonize both supports and, in continuous mode, the films that formed showed a remarkable stability. Interesting degradation percentages were obtained (80–97%). Both reactors displayed comparable NP mineralization under batch and continuous conditions. The immobilization support influenced the overall biodegradation potential of the immobilized culture. The authors concluded that biotechnological processes could represent an effective alternative to physical (adsorption) and/or chemical (ozonation, photolysis) treatments for the decontamination of effluents and solutions containing NPs and related compounds.

Cabana et al. (2007b,c) proposed cross-linked laccase aggregates for the elimination of NP, bisphenol A and triclosan. This type of immobilization involves the precipitation of the enzyme and the chemical cross-linking of the protein using bifunctional compounds. Cross-linking prevented the dissolution and possible loss of aggregates after removing the precipitating agent. This resulted in laccase stabilization against thermal and chemical denaturation and in modifications of the kinetic behavior. Their results showed that a 100-ml reactor with 0.5 mg of material operated continuously at a hydraulic retention time of 150 min at room temperature and pH 5 could remove all three EDCs from a 5 mg L⁻¹ solution. The authors noted that the use of immobilized enzymes such as laccase operating in a continuous fluidized bed reactor was an effective biotechnological way of eliminating NP from aqueous waste. In another study (Cabana et al., 2009), laccase from a strain of the white rot fungus Coriolopsis polyzona was immobilized covalently on the diatomaceous earth support Celite using different strategies. The first approach involved the sequential activation of the support surface with gamma-aminopropyltriethoxysilane followed by the reaction of the functionalized surface with glutaraldehyde (GLU) or glyoxal (GLY) and the immobilization of laccase on the activated surface. Another strategy tested the simultaneous internal cross-linking of the protein with GLU or GLY and the immobilization of the laccase on the silanized surface. The two strategies were modified to test the impact of the concomitant addition of bovine serum albumin (BSA) as a stabilizing agent during the immobilization steps. The highest laccase activity and the greatest degree of activity recovery (tested using 2,2′-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) as the substrate) were achieved by the sequential immobilization procedure using GLU as the cross-linking agent. The solid catalysts featuring internal cross-linking of the protein showed significantly a higher stability against several denaturants. The biocatalyst formed using GLU in the sequential procedure was applied in a packed bed reactor for the continuous treatment of 5 mg L⁻¹ solutions of NP, bisphenol A (BPA) and triclosan (TCS) through repeated batch treatments. All of these EDCs were eliminated at a contact time of less than 200 min by using 3.75 units of laccase activity for BPA and TCS and 1.88 U for NP. These elimination performances were maintained over five consecutive treatment cycles using the same biocatalyst. This system was also able to remove the EDCs from 100 mg L⁻¹ solutions. Kinetic measurements showed a decreasing affinity of the solid biocatalyst for NP, TCS and BPA in that order. The authors concluded that laccase immobilization was a promising tool to increase the applicability and reusability of laccase in biotechnology for EDC removal.

Patureau et al. (2008) studied several treatment processes of mixed sludge naturally contaminated with nonylphenol ethoxylate (NPEO) in order to evaluate their efficiency for the removal of these substances. Anaerobic and aerobic treatments were carried out in continuous stirred tank reactors, operated separately or combined together, at mesophilic and thermophilic temperatures and with or without ozone post-treatment. Anaerobic mesophilic removal of NPE led to complete removal of NP2EO, incomplete removal of NP1EO and non stoichiometric production of NP, with consequent NPEO removal of 25%. At thermophilic temperature, anaerobic digestion led to an increase in total solid removal efficiency, while improving NPEO degradation (30%). Under thermophilic aerobic conditions, the three compounds were removed simultaneously with a NPEO removal efficiency higher than under anaerobic conditions (39%). This removal was always well correlated to the removal of total solids meaning that bioavailability remained the main limiting factor. Combination of either thermophilic aerobic-mesophilic anaerobic or mesophilic anaerobic-ozonation treatments enhanced NPEO removal by comparison to single systems (45% and 48%, respectively). These results confirmed the high potential of existing and up-graded sewage sludge treatments to degrade such refractory and aged compounds.

Numerous studies reviewed by Corvini et al. (2006) concerning NP biodegradation using microbiological systems were carried out at lab scale and under real conditions. The authors discussed NP biodegradation by axenic cultures of microorganisms and complex microbial communities, with specific attention to the structure of the alkyl chain. Among the bacteria able to degrade NP, sphingomonads were very interesting microorganisms for metabolic studies due to their unusual mechanisms involved in the degradation pathway.

5.3 Removal of APs by adsorption-oriented processes

Adsorption on synthetic or natural adsorbents has also been proposed for AP removal. These include carbons (Paune et al., 1998; Abe, 1999; Iwasaki et al., 2002; Choi et al., 2005; Yu et al., 2008, 2009a,b; Liu et al., 2009; Xing et al., 2009), silica gels (Inumaru et al., 2000; Nikolenko et al., 2002), clays (Barhoumi et al., 2003; Nagasaki et al., 2003; Espantaleon et al., 2003), hypercrosslinked resins (Yang and Ren, 2010; Fan et al., 2011, 2012), cyclodextrins (Murai et al., 1996, 2006; Aoki et al., 2003, 2004; Pluemsab et al., 2007; Bonenfant et al., 2009, 2010; Sancey and Crini, 2012), biomass (Lang et al., 2009; Gao et al., 2011a), biosorbents such as chitosan (Yamada et al., 2006, 2009), alginate (Pluemsab et al., 2007; Gao et al., 2011b), and algae (Gao et al., 2011a,b; Peng et al., 2012), and magnetic particles (Kurinobu et al., 2007; Niu et al., 2012).

Iwasaki et al. (2002) studied the adsorption of APs with alkyl chains consisting of 5–9 carbon atoms onto microporous activated carbons (AC) prepared from coconut shells under different activation times. They showed that all APs were strongly adsorbed by all microporous carbons. The results indicated that adsorption ability increased mainly with activation time. The amount of 4-NP adsorbed increased with the porosity and specific surface area of the carbon used. In either APs with linear alkyl chains (heptyl > hexyl > pentyl) and those with branched alkyl chains (nonyl > octyl), the amount adsorbed increased with the number of carbon atoms. This was attributed to a molecular sieving effect. Abe (1999) and Choi et al. (2005) reported a similar efficiency of AC for NP removal. Removal performances were also studied using a lab-scale carbon fixed bed. Data showed that NP with a high K_ow value could be effectively removed.

Xing et al. (2009) proposed ordered mesoporous carbons with varying pore sizes synthesized using ordered mesoporous silica as hard templates for NPEO removal. As evidenced by adsorption tests, the isotherms of NPEO on material were well simulated by a Langmuir adsorption model. The surface area of a pore larger than 1.5 nm, was found to be a crucial factor for the adsorption capacity of NPEO, while the most probable pore diameter of a material was found to be vital to the adsorption rate of NPEO. Results also showed that adsorption temperature had larger effects on adsorption rate than on adsorption capacity. Theoretical studies show that the adsorption kinetics can be well depicted using a pseudo-second-order kinetic model.

Yu et al. (2008) studied the adsorption of carbamazepine and NP on activated carbon. The results showed that, at low concentrations (10–800 ng L⁻¹), removal of the target compounds was contrary to expectations based on their hydrophobicity. Nonylphenol (logK_ow 5.8) was most poorly adsorbed, whereas carbamazepine (logK_ow 2.45) was most adsorbable. In other works (Yu et al., 2009a,b), the same group investigated the adsorption of NP on granular activated carbons with the objective to investigate preloading effects by natural organic matter (NOM) on adsorption capacity and kinetics under conditions and concentrations (i.e., ng L⁻¹) relevant for drinking water treatment. Isotherms demonstrated that NP was significantly negatively impacted by NOM fouling. The carbon with the widest pore size distribution had considerably greater NOM loading, resulting in a lower adsorption capacity. Model predictions and experimental data were in good agreement for NP compounds, which demonstrated the effectiveness and robustness of the pore and surface diffusion model used in combination with the time-variable parameter approach for predicting removals at environmentally relevant concentrations (Yu et al., 2009b).

Inumaru et al. (2000) proposed octylsilane-grafted hexagonal mesoporous silica for low-concentration nonylphenol removal. They showed that the adsorbent exhibited high efficiency comparable to that of activated carbon.

Espantaleon et al. (2003) proposed a method of oriented-adsorption onto clays to reduce the pollution of effluents produced in tanning. Natural bentonite activated with 0.5 M H₂SO₄ was the most effective adsorbent for NPEO. The authors concluded that the process with clays could constitute a simple, selective and economical alternative to conventional physical–chemical treatments.

Nagasaki et al. (2003) studied the adsorption of NP on Na-montmorillonite using a batch experiment. They compared the results with those obtained using α-SiO₂, α-Al₂O₃ and gibbsite. It was found that NP adsorption on Na-montmorillonite reached the equilibrium state within 24 h, and that the interlayer width of Na-montmorillonite did not change before and after the sorption. These data suggest that NP was adsorbed on the outer surface of Na-montmorillonite. The amount of NP adsorbed on the surface of octahedral sheets of Na-montmorillonite was found to be identical to that adsorbed to gibbsite. On the other hand, adsorption on α-Al₂O₃ was smaller, and that on α-SiO₂ was very low. Adsorption amount was found to increase monotonously with pH. It was noted that NP was adsorbed on the broken edges of the octahedral alumina sheets of Na-montmorillonite.

Yang and Ren (2010) proposed hypercrosslinked polymers for the adsorption of NP10EO from aqueous solutions. The results showed that the amount adsorbed depends on the specific surface area, the pore size of the polymer, and also the temperature of the solution. Thermodynamic analysis indicated that the adsorption process was characterized by an interaction of the hydrophobic part of the surfactant molecule with the surface of the polymer and by the formation of micelles.

Fan et al. (2011) recently evaluated synthetic porous resins as adsorbents for phenol, bisphenol A and NPEO removal from aqueous solutions. The resins used were hypercrosslinked aminated polymers. Several interactions were proposed to explain the adsorption mechanism, namely hydrophobic effects, π–π bonds, hydrogen bonding, steric effects, and electrostatic interactions. However, in the case of NP10EO adsorption onto polymeric adsorbents, the mechanism was mainly dependent on the pore size and molecular morphology. The structure, surface area and functional groups of the polymer did not appear to play a role in the adsorption process. In another work, Fan et al. (2012) showed that the adsorption of NPnEO with shorter EO chains was greater than that of molecules with longer EO chains on the surfaces of resins, and the strength of the adsorption was affected by resin surface-surfactant and surfactant-surfactant lateral interactions. Lateral interactions were endothermic, and contributed more to the adsorption process on aminated resins than on hypercrosslinked resins. The surface interactions were mainly due to bonding between the hydrophobic moiety of the surfactant and hydrophobic surfaces of the resins without electrical interactions or hydrogen bonding, even on aminated resins.

Many reports also deal with the interaction of APs in solution with cyclodextrins (CDs), natural molecules derived from starch. These substances have a remarkable capacity to form inclusion complexes in solution or in the solid state with other molecules through host–guest interactions (Morin-Crini and Crini, 2013). Murai et al. (1996) previously studied the adsorption of nonionic surfactants and ionic surfactants on cyclodextrin-based gels. They showed that one gram of material was able to adsorb 85% of APEO from a 150 ml solution containing 200 ppm of nonionic surfactant. In another work, Murai et al. (2006) showed that according to the adsorption isotherms of 4-NP and 4-NPEO, their adsorption onto cyclodextrin polymer was dependent on the length of the ethoxylates in the NPEO. As the ethoxylate chain of NPEO became longer, the efficiency of adsorption onto materials decreased. Column adsorption and desorption tests revealed that the compounds adsorbed were successfully released by various kinds of aqueous alcohol solutions.

Aoki et al. (2003, 2004) proposed an insoluble cross-linked chitosan bearing cyclodextrin moieties for the adsorption of p-nonylphenol and bisphenol A. The material proposed adsorbed bisphenol A quicker than p-nonylphenol and interesting adsorption capacities were reported (Aoki et al., 2003). Almost all of the bisphenol A and 4-nonylphenol adsorbed were released from the beads by treatment with 50% aqueous ethanol solution (Aoki et al., 2004).

Bonenfant et al. (2009, 2010) showed that NP removal by cyclodextrin was rapid and the polymer exhibited high nonylphenol 9-ethoxylate (NP9EO) adsorption capacities (1.1–6.8 mg of NP9EO per g of polymer). They concluded that cyclodextrin-based gels were interesting adsorbents for the removal of nonionic surfactants such as APEO from aqueous solutions.

There is no doubt that the use of advanced technologies based on chemical oxidation (namely AOPs for advanced oxidation processes) is considered a highly competitive wastewater treatment technology for the removal of those organic compounds not treatable by conventional techniques due to their high chemical stability and/or low biodegradability (Gültekin and Ince, 2007; Belgiorno et al., 2007; Rizzo, 2011). However, treatment for complete mineralization is generally expensive because the oxidation intermediates formed during chemical oxidation tend to become increasingly resistant to their complete chemical degradation. One attractive potential alternative is to apply AOPs in a pre-treatment to improve conventional treatment of industrial wastewater. Sancey and Crini (2012) recently showed that, an AOP pre-treatment combined with an adsorption-oriented process using a cyclodextrin derivative, was a highly efficient treatment for industrial wastewater containing recalcitrant molecules such as NP. The combined process consisting of cyclodextrin-based adsorption of pre-oxidized discharge water was interesting for the removal of recalcitrant organic compounds present in metal surface-treatment wastewaters.

Pluemsab et al. (2007) proposed calcium alginate beads covalently linked with alpha-cyclodextrin for their ability to serve as a supporting matrix for bacterial degradation of NP. The results from column chromatographic experiments showed that the material exhibited a strong affinity for NP adsorption. Although addition of cyclodextrin to the culture broth of S. cloacae retarded NP degradation, the bacteria immobilized on the cyclodextrin-alginate beads were effective for the degradation. Batch degradation tests using the bead-immobilized bacteria showed 46% NP recovery after a 10-day incubation at 25 °C, and recovery reached about 17% when wide shallow incubation tubes were used to facilitate uptake of the viscous liquid NP on the surface of the medium. Scanning electron microscopic photographs revealed that multiplying bacteria were present both on the surface and inside the beads but that the cyclodextrin-alginate matrix was stable and suitable for 10-day incubation.

Lang et al. (2009) studied NP biosorption on chitosan-immobilized dead fungal beads. Chitosan, a biopolymer produced on an industrial scale by the deacetylation of chitin (the second most abundant biopolymer in nature) is commonly used as support material for biomass immobilization (Crini and Badot, 2008). Biosorption is the passive uptake of substances by non-growing or non-living microbial biomass such as bacteria, fungi and algae. In general, better removal of organic compounds was obtained with dead biomass than with live biomass. Lang et al. (2009) showed that the use of fungal biomass of Rhizopus arrhizus immobilized on chitosan beads enabled the effective and innovative treatment of recalcitrant pollutants such as NP. A maximum monolayer capacity of the beads was over 312 mg g⁻¹ and the process was rapid (40 min of contact time). The regeneration of used dead beads with methanol was effective for at least 4 batch cycles. The technology is also interesting because the biomass could be readily prepared on a commercial scale at a very low cost.

Different Chlorella species presented the ability to remove NP rapidly and in large quantities as reported by Gao et al. (2011a). Among the species, Chlorella vulgaris enabled the highest NP elimination: nearly all NPs were removed from the medium and more than 80% of NP were degraded after a short exposure time (other conditions: 25 °C with light illumination, initial biomass between 0.5 and 1 mg chlorophyll L⁻¹). The mechanisms included initial rapid adsorption and uptake, followed by accumulation and biodegradation. The biodegradation ability was species-specific and dependent significantly on growth conditions, particularly light. The authors concluded that, among different microalgae, the Chlorella genus was able to efficiently remove NP from water and degrade it. In another study, Gao et al. (2011b) demonstrated that alginate-immobilized C. vulgaris beads were more effective than freely suspended cells in removing NP from wastewater. The spiked NP, at an initial concentration of 1 mg L⁻¹, was almost completely removed (>98%) under an optimal algal bead concentration (4 beads ml⁻¹) after 96 h of exposure. The NP biodegradation efficiency was significantly enhanced through optimization of the cell density per algal bead and/or algal bead concentration per unit volume of medium. The method based on immobilization allows higher biomass concentrations and facilitates separation and non-destructive recovery of biomass from pollutant-bearing solutions.

Peng et al. (2012) also studied the adsorption characteristics of NP by algal C. vulgaris suspensions. Experimental parameters affecting the adsorption process, such as pH, contact time and the effect of solvent were studied. The results showed that the data were well described by the Langmuir adsorption model and indicated that NP adsorption by biosorbent was high during the first 30 min, and equilibrium was achieved after a contact time of 60 min. Adsorption of NP was pH-dependent: as the pH decreased, the adsorption capacity increased. The authors also showed that the fluorescence intensity of NP decreased and that the fluorescence peak of NP moved toward the infrared when algae were added.

Kurinobu et al. (2007) proposed the use of magnetic photocatalyst particles with a core–shell structure of three layers coating Fe₃O₂ core particles with SiO₂ and TiO₂ for NP removal and degradation. NP adsorption on the magnetic photocatalyst particles was found to be effective. As the photocatalyst particles were magnetic, they were easily recovered.

Niu et al. (2012) prepared core/shell structured carbon-encapsulated magnetic nanoparticles (CMNPs) with a simple method using inorganic iron salt and glucose solution as precursor substance for 4-n-NP and 4-OP removal. The synthesis procedure did not require the use of organic solvents. The materials were efficiently adsorbed for AP removal from water samples. The mechanism of adsorption was mainly based on π–π stacking interactions, hydrophobic interactions and hydrogen bonding between substances and graphitic carbon.

Yamada et al. (2006, 2009) proposed enzymatic removal of APs from aqueous solutions through a two-step approach conversion of APs to quinines with mushroom tyrosinase. Subsequently the enzymatically generated quinine derivatives were adsorbed on chitosan beads at pH 7 and 45 °C as the optimum conditions. This process was quite effective for 4-NP and 4-n-OP with removal values of 97–100%. Mushroom tyrosinase can effectively oxidize linear and branched APs into quinine derivatives (quinine conversion of branched APs was slower than that of linear compounds). The use of porous chitosan beads led to an increase in adsorption of enzymatically generated quinine derivatives, because quinine adsorption occurred heterogeneously at the solid–liquid interface. The use of melB tyrosinase was interesting because it offered quinine conversion without requiring H₂O₂.

5.4 Removal of alkylphenols by advanced chemical oxidation-based technologies

Destruction processes using various oxidation systems appear to be more interesting. They are based on chemical techniques involving oxidation reactions. Chemical oxidation is increasingly used in the treatment of water. It is necessary when the water to be processed contains non-biodegradable pollutants refractory to conventional decontamination treatment. The treatment can be carried out either by standard oxidation procedures (chemical or catalytic, electrochemical oxidation), or using advanced oxidation processes (AOPs), which have appeared in recent years. In water and wastewater treatment, oxidation is useful as it enables partial destruction with standard procedures (resulting in COD and TOC reduction) or total destruction with AOPs of the organic matter (mineralization).

Classical chemical oxidation techniques are based on the use of various powerful oxidants such as ozone, in the presence of a catalyst or not. Ozonation is commonly used in a large number of water treatment plants not only as a clarifying and disinfecting agent (Gültekin and Ince, 2007), but also as an oxidizing agent with the aim to degrade/mineralize the organic matter. For instance, ozone is a strong oxidant and reacts easily with organic molecules which have sites with high electron densities involving double bonds – the preferred sites for ozone attack. This high reactivity then leads to rapid oxidation kinetics. The mechanism in ozonated waters is either direct oxidation by molecular ozone or indirect oxidation by hydroxyl radicals that are formed by the decomposition of ozone in alkaline conditions. However, ozone-based processes have become the object of increasingly strict levels of legislation, especially their use in the final stages of mains water treatment (drinking water). Owing to its very strong oxidant activity, ozone has also become the most frequently used oxidant for the decontamination of industrial effluent - only fluorine has a higher oxidation potential. It enables the transformation of industrial compounds that are difficult to degrade into molecules much easier to deal with biologically for example. Another advantage of ozone is that it can be used to process large amounts of effluent with little or no formation of by-products (structures conjugated by ozone become non-toxic). However, the process does require a complex set-up. Ozone is an unstable oxidant with a short lifetime and as such must be used soon after it has been synthesized. It is produced on-site form oxygen using electric discharge. In addition, as ozone is expensive to produce, it is generally used on effluent that has been pre-treated in a biological or a physicochemical process. Its use can also be associated to that of other oxidants (such as H₂O₂), catalysts (metal oxides) or to other methods (UV irradiation, membrane filtration) in order to increase its oxidation throughput capacity. Hydrogen peroxide has also proved to be an excellent oxidant, without impact on the environment, and can be used in conjunction with ozone (Snyder et al., 2006).

The high efficiency of AP removal during ozonation is well documented. Petrovic et al. (2004) studied the oxidative degradation of NPEOs in water using ozonation and electron beam irradiation with and without the addition of ozone as treatment processes. It was found that both ozonation and the two AOPs applied were able to decompose not only the NPEOs but also the polyethyleneglycols formed as by-products from NPEO degradation to residual concentrations below the limit of detection. Moreover, the treatment processes were also used to study the oxidative degradation of nonylphenoxy acetic acid (NPEC) and of NP which were formed as by-products from biodegradation of NPEOs.

Ning et al. (2007a,b) proposed that ozonation can be used to degrade NP and OP. Ozone has been found to react readily with both these APs, mainly in acid conditions (pH = 2). Studying the reaction mechanism between ozone and these two alkylphenols, they suggested that an initial product of ozonation was hydroxyl-alkyl phenol.

Both NP and NPEO oxidation performances and kinetics were evaluated by Hyunook et al. (2007) under different operating conditions created by varying ozone doses, solution pH and molar ratios of H₂O₂/O₃. For example, they demonstrated that at pH 7, more than 90% NP (initial NP concentration: 0.51 mg L⁻¹) can be removed by 4 mg L⁻¹ O₃. Over the pH range 5–9, NP was also effectively oxidized. NP destruction was also significantly increased when H₂O₂ was applied along with ozone. The authors concluded that ozonation could be very effective in oxidizing NP and NPEOs.

Sun et al. (2008) investigated the ozonation mechanism of 4-NP. Using GC–MS analyses, the following substances were identified: mixtures of 4-nonylbenzoquinone homologs, 4-nonylcatechol homologs and the carboxylic compounds formed. The estrogenic activity of ozonation by-products of 4-NP was studied using a recombinant yeast bioassay. The mixtures of 4-nonylcatechol homologs had a higher estrogenic activity, while mixtures of 4-nonylbenzoquinone homologs had a lower estrogenic activity than 4-NP. However, the estrogenic activity of the 4-nonylphenol ozonation solution was ultimately eliminated. The removal of the estrogenic activity of 4-NP was enhanced by the addition of humic acid, indicating the favorable effect of the presence of natural organic materials in water treatment.

Ozone oxidation was proven by Baig et al. (2008) to be an effective solution for the degradation of selected estrogenic active substances detected in secondary wastewaters such as 4-t-OP and 4-iso-NP, up to their limit of detection. The matrix-effect of wastewater was investigated performing ozone experiments under batch mode and continuous mode using drinking water and wastewater from a local plant both spiked with the non-detected substances. The results obtained indicated that the wastewater matrix greatly affected the kinetics of ozone reaction with these substances but did not really change the related reactivity scale. The ozone dose corresponding to the full conversion of target substances consequently increased as their oxidation took place competing with reactions of background pollutants represented by the COD content. However, a usual dose close to 12 mg L⁻¹ was found sufficient to provide high degradation yields for all substances studied while 35% of the COD was removed.

Zhang et al. (2008) studied the ozonation of NP and OP in aqueous solutions containing acetonitrile as a cosolvent. Batch experiments were carried out to test the effects of initial pH values, ozone dosages, and initial APs concentration on degradation efficiency. At pH 9.0, the two APs were completely degraded after a period of 6 min. After 6 min for ozonation, the degradation efficiency of NP was nearly 42% with the ozone dosage of 0.13 mg min⁻¹ at pH 6.5, but with the ozone dosage of 0.38 mg min⁻¹, the degradation efficiency reached 96%. The effect of ozone dosage on degradation of OP showed that the higher the initial AP concentration, the lower the AP degradation efficiency. During the ozonation process, the rapid decrease in the pH and the sharp increase in the electrical conductivity indicated the formation of by-products with a higher polarity, which was confirmed by the HPLC analysis. The authors concluded that ozonation was a feasible process for removing APs from water.

Kim et al. (2005) compared ozonation and electrochemical degradation of NPs. Ozonation was efficient but they showed that electrochemical oxidation was also an interesting option for the treatment of NPEOs. The principal advantages were easy implementation, the possibility to operate in a wide range of treatment conditions and the fact that there was no need to store and dispense treatment reagents.

Bertanza et al. (2010a,b) reported that NP removal by conventional WWTPs is recognized but in order to reach very low concentrations, further treatment such as chemical oxidation might be required. They studied the ozonation of an effluent from an Italian WWTP (Verona, Italy) and the reaction was monitored using chemical and microbiological analyses. The authors concluded that ozonation was an economically feasible option as tertiary step.

Tehrani-Bagha et al. (2012) recently studied the efficiency of different UV-enhanced ozonation processes for degradation of a nonylphenol ethoxylate with 40 oxyethylene units (200 mg L⁻¹) in lab scale experiments at ambient temperature. The reaction was monitored by the reduction of COD and TOC and by the absorbance band of the aromatic ring of the molecule. The results showed that a combination of UV irradiation and ozonation was considerably more efficient than the individual processes (at least twice as efficient in terms of COD and TOC reductions). The synergistic effect of ozonation and UV irradiation was particularly pronounced when medium-pressure UV irradiation was used. By adding alkali to the solution, the efficiency of UV-enhanced ozonation increased with respect to COD reduction but decreased with respect to TOC reduction. This indicates partial oxidation with a lower degree of mineralization of the surfactant.

AOPs have also been widely explored for the decontamination of water polluted mainly by organic substances. These processes are used to complete the degradation initiated by biological treatment or as a pretreatment to enhance the biodegradability of recalcitrant or treatment-inhibiting compounds present in wastewaters. This enhancement is achieved by the partial oxidative degradation of the molecules to give intermediate reaction products that can be degraded by microorganisms in a biological post-treatment. AOPs can also be used as a treatment step to mineralize the compounds. For instance, in proper operation conditions, it is possible to mineralize the target pollutants to CO₂, which is the most stable end product of chemical oxidation (Gültekin and Ince, 2007).

A wide variety of AOPs are available for wastewater treatment based on photocatalytic systems (TiO₂/UV, Fe/UV–Vis) or reactions that produce hydroxyl radicals by the photolysis of hydrogen peroxide and ozone (H₂O₂/UV, O₃/UV, H₂O₂/O₃/UV). Photocatalysis is an oxidation process in which a metal oxide semiconductor (e.g., TiO₂) immersed in water and irradiated by near UV light enables the formation of free hydroxyl radicals. TiO₂ is the most widely used catalyst because of its non-toxicity, low-cost, photo-stability and water insolubility under most environmental conditions (Belgiorno et al., 2007). Photocatalytic reactions can be divided into heterogeneous and homogeneous processes that use either catalyst particles or coatings such as TiO₂, or soluble catalysts or redox systems like iron ions or iron complexes. The light energy necessary can either be provided by lamps or solar radiation. Photocatalysis processes have been found to be effective for the degradation of EDCs as covered in the following reviews which can be consulted (Gültekin and Ince, 2007; Belgiorno et al., 2007).

Sherrard et al. (1996) previously reported the degradation of NPE in aqueous solution (2 L of aqueous solution containing 2 g L⁻¹ NPE and 0.1% w/v TiO₂ catalyst) using heterogeneous photocatalysis. From their GC–MS, ESMS and NMR data, they concluded that photocatalysis was an efficient method. Intermediates were identified and suggested that the ethylene oxide chain was more susceptible to degradation than aliphatic or aromatic moieties.

Castillo et al. (2001) investigated the photodegradation of non-ionic surfactants (nonylphenol- and alcohol-polyethoxylates, NPEOx and CnEOx) in different waters (deionized water and industrial effluent) with and without a photoinducter (Fe(III)). The intermediates detected included NP2EO and nonylphenol ethoxy acetic acid (NPE2C). Much smaller amounts of NP9EO degradation products having only the alkyl chain carboxylated were also formed in the photocatalysis experiment. The C10EO6 photoproducts identified included fatty alcohols and acids. PEGs were also formed. Photodegradation in wastewater samples was more efficient than in deionized water.

Yamazaki et al. (2008) investigated photocatalytic degradation of 4-OP by recirculating the aqueous solution through a packed bed reactor with TiO₂. The kinetics of degradation was independent of the flow rate but a decrease in TOC became smaller as the flow rate increased. Under illumination for 6 h at a flow rate of 28.5 ml min⁻¹, 83.2% of 4-OP was degraded but 60.7% of the initial TOC remained. LC–MS measurements using electrospray ionization revealed the formation of byproducts. The degradation rate was remarkably accelerated by the addition of K₂S₂O₈: 4-OP completely disappeared under irradiation for 2–4 h and in the presence of S₂O₈²⁻, 4-OP was degraded without TiO₂, which was attributable to reactions involving SO₄⁻ radicals. The UV/TiO₂/S₂O₈²⁻ system was more suitable than UV/S₂O₈²⁻ for the decontamination of water containing 4-OP.

The applicability of different photochemical AOPs, namely, direct UV-C photolysis, heterogeneous photocatalysis, and photo-Fenton reactions for the degradation of 300 mg L⁻¹ NP9EO in water was studied by de la Fuente et al. (2010). Toxic 4-nonylphenol was never found as a byproduct of the degradation after any of the treatments. Aldehydes were formed in all of the processes, but they appeared at low levels in photo-Fenton reactions. Photo-Fenton treatments were considered to be the best degradation processes.

Zhang et al. (2012) recently studied the application of TiO₂ photocatalysis in advanced wastewater treatment to simulate the removal of endocrine disrupting compounds (bisphenol A, NP, estrone, and 17 alpha-estradiol) and estrogenic activity from secondary effluents of a municipal wastewater treatment plants, aiming to evaluate the possibility of using photocatalysis for advanced wastewater treatment. The removal efficiencies of alkyl-hydroxybenzenes (bisphenol A, NP) by traditional processes in wastewater treatment were more sensitive to temperature than the removal of steroid hormones (estrone, and 17 alpha-estradiol), resulting in a risk of effluent insecurity. All the targeted EDCs in secondary effluent were totally degraded by TiO₂ photocatalysis within 60 min except BPA (92% removal), following pseudo first-order kinetics. The estrogenic activity was also totally removed: when EDCs reacted with hydroxyl radicals, aromatic rings were cleaved, resulting in a decreased estrogenic activity. Their findings suggested that TiO₂ photocatalysis is an effective method for the removal of certain EDCs and estrogenic activity from secondary effluents and can be used for advanced wastewater treatment.

Although photocatalysis by TiO₂ was thought to be an effective method for the degradation of organics, some researchers were concerned about the practicality of the process related to the relative difficulty of separating the catalyst powders and/or crystals from solution at the end of treatment (Gültekin and Ince, 2007). Different combined treatments have also been proposed. Ike et al. (2002) reported that the effectiveness of ozonation and ultraviolet photocatalytic treatment in the degradation of NPEO metabolites, using lab-scale reactors, follows the order: nonylphenoxy carboxylic acid >> NP > nonylphenol monoethoxylate. The degradation rate of NP by UV/TiO₂ was the highest among the metabolites tested, while NP1EO showed the lowest degradation rate. Acidic metabolites were completely degraded within 4–6 min (initial concentration 0.4–1 mg L⁻¹), the NP concentrations were reduced by 75–80% in 6 min, while only 25–50% of NP1EO was eliminated at the same time. NP was efficiently degraded by ozonation, however, it was much less effective for NP1EO decomposition.

Balabanic et al., 2012 evaluated the treatment performance of different wastewater treatment procedures (biological treatment, filtration, and AOPs) for the reduction of COD and seven EDCs including NP from pulp and paper wastewaters. Two pilot plants were running in parallel and the following treatments were compared: (i) anaerobic biological treatment followed by aerobic biological treatment, UF and RO, and (ii) anaerobic biological treatment followed by MBR and RO. Moreover, at the lab-scale, four different AOPs (Fenton reaction, photo-Fenton reaction, photocatalysis with TiO₂, and ozonation) were applied. The authors showed that concentrations of the EDCs studied in paper mill wastewaters were effectively reduced (100%) by both combinations of pilot plants and photo-Fenton oxidation (98%), while Fenton process, photocatalysis with TiO₂ and ozonation were less effective (70–90%, respectively).

UV irradiation (photolysis) is a common practice for drinking water treatment. Photolysis has been one of the most widely investigated AOPs of EDC degradation. Ahel et al. (1994b) previously studied the sunlight photolysis of NP and NPEOs in natural waters. The photochemical oxidation of NPEO was shown to be significantly slower than that of NP. The photochemical degradation of both NP and NPEOs was mainly due to sensitized photolysis while direct photolysis was comparatively slow. Moreover, experiments with D₂O revealed that singlet oxygen was not an important photooxidant of NP at pH values usually found in natural waters.

Neamtu and Frimmel (2006) investigated the photolysis of NP using a solar simulator in the presence/absence of dissolved organic matter (DOM), HCO₃⁻, NO₃⁻ and Fe(III) ions. The effects of different parameters such as initial pH, initial substrate concentration, temperature, and the effect of hydrogen peroxide concentration on photodegradation of nonylphenol in aqueous solution were assessed. The results indicated that the oxidation rate increased in the presence of H₂O₂, Fe(III) and DOM with dissolved organic carbon concentrations not higher than 3 mg L⁻¹. Phenol, 1,4-dihydroxybenzene and 1,4-benzoquinone were identified using an HPLC method as intermediate products of NP photodegradation. Their results also showed a strong decrease in estrogenic activity of NP after 80 h of irradiation in the presence of hydrogen peroxide. The photolysis of NPEOs with an average oligomer length of ten ethoxylate units in aqueous solutions was also studied by Chen et al. (2007). The results showed that the degradation was effective but that the degradation pathway was complex. NPEOs with longer ethoxylate chains were degraded more easily than those with shorter chains. The performance was reduced in the presence of humic acid.

The application of sonochemistry, namely, the physical and chemical effects induced by ultrasonic irradiation in water, was also reported for the elimination of hazardous or toxic environmental contaminants in aqueous solutions. The reaction pathways proposed for the sonolytic degradation of organic compounds are the reaction with hydroxyl radicals (e.g., hydroxyl radical attack and/or addition) and a thermal reaction (e.g., pyrolysis). The hydroxyl radicals are produced by the sonolysis of water molecules inside the collapsing cavitation bubble under extremely high temperature and pressure. The behavior of the hydroxyl radicals generated may either involve the reaction with organic compounds or recombination to form hydrogen peroxide (H₂O₂) inside the cavitation bubble and at its interfacial region. Pyrolysis of volatile organic compounds occurs inside the cavitation bubble or at the interfacial region, where the temperature is sufficiently high for the induction of thermal effects.

Yim et al. (2003) investigated the sonolysis of APs in aqueous solution under a variety of conditions. In particular, the effects of added Fe(II) and Fe(III) during the sonolytic degradation of NP in aqueous solution were explored with a particular focus on degradation enhancement. Their results showed that the sonolytic degradation rate of the APs depended upon their alkyl chain length. Free radicals played a significant role in the sonolytic degradation of APs. The sonochemical effects on the addition of Fe(II) and Fe(III) as catalysts during sonication resulted in a remarkable enhancement of degradation and mineralization.

Ince and co-workers (Gültekin et al., 2009; Ince et al., 2009) also proposed sonochemistry for NP degradation. High frequency irradiation (20 kHz ultrasound) permitted the efficient decomposition of 4-n-NP from water in alkalinized conditions. The rate of reaction increased with increasing concentrations of the substance up to a critical value above which it declined and stabilized (Ince et al., 2009). The impact of pH was such that the rate of degradation was maximum at pH = 10.8, minimum at pH = 3.0 and in between the two extremes at pH = 6.0 (Gültekin et al., 2009). The extreme rapidness of the reaction in alkaline conditions was related to the pKa of NP (10.7) and the facilitation of the covalent bond cleavage between the phenolic and alkyl groups. The fact that the reaction was faster at more acidic pH than at pH 6 attributed to the protonation of the molecule enhancing its migration toward the negatively charged gas bubbles. The reaction rate was also accelerated by bubbling the solution with oxygen gas and by the addition of Fenton and Fenton-like reagents. The enhancement by the latter stresses the role of OH radicals in the overall degradation process. The efficiency of sono-Fenton reactions was significantly improved by the addition of a sufficient dose of H₂O₂ into the reactor.

5.5 Removal of APs by other water and wastewater treatment methods

Wada et al. (1995) reported APS elimination by the formation of water-insoluble precipitates through the reaction of quinine derivatives enzymatically generated from APs with the amino group-containing polymers (chitosan, polyethyleneimine) in solution. Jones and Westmoreland (1998, 1999) treated nonylphenol ethoxylate residues from wool scour effluents by flocculation using a specific flocculant, and reported an uptake >95%. The process transferred the pollutants from the wastewater stream to a spadeable sludge that was used as a starting material to produce high quality compost. During the composting process, the detergents were degraded to below detectable levels.

Ciorba et al. (2002) investigated the behavior of aluminum and carbon steel electrodes in the presence and absence of NPEO and NPEO removal by electrochemically-generated coagulants. The electrode processes were studied by potentiodynamic polarization measurements to determine the influence of surfactant addition (ethylene oxide units and organic load), inorganic composition and pH. The polarization curves showed the extent of the involvement of NPEO in the electrode processes. Their influence appeared clearly in the range of anodic activation, depending on the number of ethoxylate groups. Removal of NPEO from simulated wastewaters was carried out in electrocoagulation cells with vertical or horizontal electrodes. Removal efficiencies calculated from the COD before and after treatment were 30–50% for NP16EO and NP40EO, and 40–80% for NP4EO. The hydrophilic-lipophilic balance number for NP4EO indicated a relative ratio of polar and non-polar groups lower than that for the other two surfactants, which was in favor of a better NP4EO removal. The experiments showed that bulk processes (bridging of electrochemically-generated polymeric species and adsorption of surfactants) were predominant over the involvement of NPEO in the electrode processes, which were only responsible for the electrogeneration of coagulants.