Carbon-/Zeolite-Supported TiO2 for Sorption/Photocatalysis Applications in Water Treatment

The role of various carbon forms, i.e., activated carbon and carbon nanotubes/nanofibers as support for TiO2 in drinking water treatment, is discussed. Also, TiO2 supported onto zeolite that acts bifunctionally as a sorbent/photocatalyst for drinking water treatment is presented. The main contaminants of natural organic matter (NOM), arsenic species, and nitrogen compounds from drinking water sources by the type of groundwater and surface water can be removed/degraded by sorption/photocatalysis using TiO2 supported onto carbon and/or zeolite. TiO2 supported on powdered activated carbon (PAC-TiO2), granular activated carbon (GAC-TiO2), and zeolite (Z-TiO2), namely, supported TiO2, was synthesized through the sol-gel method, and TiO2 and multiwall carbon nanotubes/ carbon nanofibers dispersed within epoxy matrix (CNT-TiO2-Epoxy, CNF-TiO2-Epoxy), namely, TiO2 composite, were obtained through the two-roll mill method. Kinetic study results through specific mathematic models allowed to elucidate some mechanistic aspects for sorption and photocatalysis for the application in drinking water. The intercalation of the carbonand zeolite-supported TiO2 layers into a filtering system allows to develop a self-cleaning filtering system in drinking water.


Introduction
Water pollution is a concern for the European population, and the quality objectives for the water protection are set through the EU Water Framework Directive. Also, one of the targets of the Clean Water and Sanitation Goal within the Millennium Sustainable Development Goals of the 2030 Agenda is to achieve universal and equitable access to safe and affordable drinking water for all by 2030 [1].
The challenges in treating drinking water are dependent on the water sources, mainly consisted of surface and groundwater. Problematic substances in the drinking water sources can include organic matter and/or different inorganic contaminants, e.g., nitrite, nitrate, and ammonium.
One of the common and advanced unitary processes used in drinking water technology is sorption designed as activated carbon-or zeolite-based filtering. Due to more hydrophobic nature of the activated carbon, it has been recognized for elimination of a broad range of hazardous materials belong to the organic class from aqueous solutions. Activated carbon is less efficiently applied for removal of simple inorganic metallic ions or small-size anions [2].
Zeolite is also a known filtering material that exhibits a high sorption capacity [3] characterized by a selectivity degree in relation with the type of zeolite based on its specific properties of molecular sieve and ionic exchanger [4][5][6]. The main disadvantage is represented by the fouling of the filtering material surface.
Photocatalysis process is considered very promising for advanced water treatment based on the oxidation and reduction reaction for in situ generation of highly reactive transitory species (i.e., H 2 O 2 , OH, O 2 , O 3 ) for mineralization of organic compounds and disinfection by-products [7][8], through the electron-hole pair formation under UV or solar irradiation.
Among various semiconductors that have been investigated in photocatalysis application, titanium dioxide (TiO 2 ) has attracted much attention due to its physical and chemical stability, negligible toxicity, the resistance to corrosion, redox selectivity, high photostability, and easy preparation [3,[9][10]. By UV irradiation onto TiO 2 surface, under conditions of photon energy (hv) greater than or equal to the bandgap energy of TiO 2 , the electron will be photoexcited from the valence band to the empty conduction band leading to an empty unfilled valence band that corresponded to the hole and thus creating the electron-hole pair. The electron-hole pair is involved within various oxidative/reductive reactions including the degradation of organics.
For the drinking water treatment, the main disadvantage of TiO 2 -based photocatalysis is given by the necessity of the further separation phase to remove TiO 2 from water, in order to avoid the loss of catalyst particles and introduction of the new pollutant of contamination of TiO 2 in the treated water [11]. A solution to avoid the introduction of a new separation phase includes catalyst fixation onto various supports, e.g., activated carbon [12] and mesoporous clays [13]. The catalyst immobilization on different supports allows getting catalyst composites, which are considered a new generation of catalyst with different properties in relation with those of solely TiO 2 . According to the obtaining methods, there are a large variety of composites [14], from which in this chapter will be discussed: • Composites with TiO 2 coated (supported) on the support of activated carbon by the type of powder and granular and zeolite [15][16][17][18][19][20][21] • Hybrid composites, which comprise a matrix material of polymer (epoxy) in which TiO 2 and the support of carbon nanotubes or nanofibers are dispersed [22][23][24] Taking into account that the catalyst represents the key of the performance of the photocatalytic application, the catalyst-based composite should exhibit various effects related to the components and the obtaining methods [14], such as simple supporting effect, stabilizing the microstructure or active components, formation of new compounds which act as active components or stabilizers, having two or more functions, controlling of redox performances, and influencing the kinetics of adsorption/desorption and diffusion of molecules.
In this chapter, two types of composites obtained by two different methods and applied in the drinking water treatment are presented: • TiO 2 supported on powdered activated carbon (PAC-TiO 2 ), granular activated carbon (GAC-TiO 2 ), and zeolite (Z-TiO 2 ), namely, supported TiO 2 .

TiO 2 -based composites obtaining and morphostructural properties
The granular, powdered activated carbon and natural zeolite were functionalized with TiO 2 using titanium tetraisopropoxide (TTIP) as TiO 2 precursors by sol-gel method (Figure 1).  For the synthesis process of the composite materials consisting of carbon nanotubes or carbon nanofibers mixed with TiO 2 particles within epoxy matrix, the two-roll mill technique was applied (Figure 2).  The morphology of all synthesized materials was observed by a scanning electronic microscope (SEM, Inspect S PANalytical model) coupled with the energy-dispersive X-ray analysis detector (EDX). Figure 3a-c presents SEM images for PAC-TiO 2 , GAC-TiO 2 , and Z-TiO 2 , which show the layered structure of activated carbon and a nonuniform distribution and more or less agglomeration of TiO 2 nanoparticles onto the surface. It can be also seen that TiO 2 only adhered to the surface of zeolite without the insight of the inside of the zeolite pores.
The results of semiquantitative elemental analysis of the synthesized material surface were presented by EDX spectra indicating Ti and C presence for PAC-TiO 2 and GAC-TiO 2 and, also, the presence of Ti on the natural zeolite (Figure 4). Small amounts of Al, Si, and S were identified within the activated carbon as both powdered and granular as impurities and the presence of K and Ca within the zeolite composition were found.
For composite materials that used epoxy as matrix, it can be noticed that TiO 2 nanoparticles were attached on the nanostructured carbon surface, and, also, a well dispersion of carbon and TiO 2 nanoparticles within epoxy matrix is noticed (Figure 5a,b). A slight more uniform distribution of TiO 2 nanoparticles seems to be for CNF in comparison with CNT [22]. The presence of TiO 2 particles in composite materials was confirmed by EDX spectra (Figure 6).    X-ray diffraction measurements were carried out to determine the crystal phase composition using a PANalytical X'PertPRO MPD diffractometer. Figure 7 presents the XRD patterns of GAC-TiO 2 , PAC-TiO 2 , and Z-TiO 2 . The anatase form of TiO 2 is the predominant phase, identified by diffraction lines at 2θ of 25.3, 38.6, 48, 54, and 62.97° [25]. Also, the XRD results revealed that the major component of natural zeolite used in this study is clinoptilolite (2θ ~ 10°; 12°; 22.5°; 30°) [26].

TiO 2 -based composite application for sorption and photocatalysisunitary processes in drinking water treatment
Various types of carbon and zeolite are reported as good sorbents for various types of pollutants from water [2,[15][16][17][18][19][20][21][22][23][24]. The easy separation of these materials from water and their good sorption capacity that is important in the first stage of the overall photocatalysis process make them very promising as support for the TiO 2 immobilization.
Humic acids (HAs) represent a main component of natural organic matter that gives the organic loading of drinking water sources, which must be removed or destroyed because of water quality regulations. Also, the presence of HAs before the chlorination step as disinfection could lead to trihalomethane and other toxic by-product generation. The efficiency of supported TiO 2 materials for the sorption and the photocatalysis of HA from water, expressed as HA removal efficiency, is presented comparatively in Figure 9. The HA removal efficiency was calculated using the following equation: where C 0 and C t are the concentrations of HA in aqueous solution in terms of A 254 at initial time and at any time t, respectively (mg·L −1 ).
The sorption was assessed as the preliminary and compulsory step of the overall photocatalysis process taking into consideration also the possibility to use the supports as simple sorbents and only for self-cleaning to apply the photocatalysis. The sorption capacity for HA is better for PAC-TiO 2 , while the photocatalytic activity is better for GAC-TiO 2 . This should explain the morphological structure and the sizes of the particles. The lower particle size leads to the higher sorption capacity, while for the photocatalysis application the size of support decreased the photocatalytic activity may be due to the finer suspension should hinder the UV irradiation penetration to the TiO 2 surface.The sorption and the photocatalysis capacities of TiO 2 composite materials assessed in terms of HA removal efficiency showed the superiority of CNT-TiO 2 -Epoxy versus CNF-TiO 2 -Epoxy. It can be noticed that a slight photocatalytic activity was found for both materials (Figure 10). These behaviors should be explained by the available sites of the carbon in the sorption process and the available sites of TiO 2 in the photocatalysis process, which are in direct relations with the morphology, size, and the bonding type between TiO 2 , carbon, and zeolite support. Also, epoxy resin hindered the available sites, which decreased both sorption capacity and photocatalytic activity of TiO 2 composite materials.
In order to determine some mechanism aspects regarding the sorption and photocatalysis processes, several kinetic models must be checked to find the optimum one. Two different kinetic models are used to fit the experimental data, i.e., pseudo-first-order and pseudosecond-order kinetic models [27].

Pseudo-first-order kinetic model
The pseudo-first-order kinetic adsorption was suggested by Lagergren (1898) for the adsorption of solid/liquid systems. It can be expressed in integrated form as shown in Eq. (2): where, k 1 is the rate constant of adsorption (min −1 ) and q t is the adsorption loading of dye (mg·g −1 ) at time t (min).

Pseudo-second-order kinetic model
The pseudo-second-order kinetics, proposed by Ho and Mckay [28], is expressed in Eq. (3): where, k 2 is the rate constant of the pseudo-second-order adsorption kinetics (g·mg −1 ·min −1 ) and q e is the equilibrium adsorption capacity (mg·g −1 ).
Pseudo-first-and pseudo-second-order kinetic models were tested for fitting the sorption and photocatalysis experimental data (Tables 1 and 2). The linear plots of t/qt vs. t show that the experimental data agree with the pseudo-second-order kinetic model for the HA adsorption. The calculated qe values agree very well with the experimental data, and the correlation coefficients for the pseudo-second-order kinetic model are higher than 0.90 in almost all cases. These indicate that the adsorption of HA from water onto materials obeys the pseudo-second-order kinetic model that predicts an exponential decay of concentrations as a function of time. The pseudo-first-order kinetic model was not appropriate for fitting the sorption experimental data, while the photocatalysis experimental data were fitted well with both kinetic models with the correlation coefficient CNT-TiO 2 -Epoxy q e (mg*g −1 ) 0.33 Table 2. Pseudo-first-order kinetic parameters for HA photocatalysis.
higher than 0.90. However, there is a significant difference between calculated and experimental qe using the pseudo-first-order kinetic model that limits its interpretation ( Table 2), and, thus, it means that the pseudo-second-order kinetic model is most appropriate. Besides the kinetic parameter, the pseudo-second-order kinetic model informed about the significance of the sorption step within the overall photocatalysis process. It is well known that the reaction rate and kinetics are influenced by many experimental parameters, e.g., HA concentration, catalyst dose, and pH [22].
Based on the results presented in Table 1, it can be seen that the PAC-TiO 2 exhibited higher sorption capacity and the best kinetics for HA removal and the GAC-TiO 2 exhibited the best photocatalytic kinetics, which are in accordance with the efficiency results. The worse results were achieved for TiO 2 composite materials, which can be explained by the presence of epoxy matrix that reduced the active sites for the sorption and the photocatalysis. Also, the main difference was found between CNT-TiO 2 -Epoxy and CNF-TiO 2 -Epoxy due to the morphology and sizes of the nanostructured carbon component, better for CNT in comparison with CNF. For these types of material, the photocatalytic activity was better also for CNT-TiO 2 -Epoxy.
Arsenic is a common contaminant in drinking water supplies especially for groundwater sources. Inorganic arsenic speciation in water consists of arsenite (AsIII) and arsenate (AsV), but As(III) is more problematic because of its high toxicity and the difficulty to be removed from water [29]. The comparative results regarding As(III) removal in the presence of HA showed a good efficiency for both sorption and photocatalysis using supported TiO 2 material (Figure 11).
Another common problem of groundwater source used for drinking water supply is given by the presence of ammonium, nitrite, and nitrate. Nitrate could not be removed from water on these tested materials, while ammonium and nitrate were removed from Z-TiO 2 (Figure 12). The results showed that ammonium is removed by sorption on the zeolite based on ionic exchange, which is proven by comparison with natural zeolite, while the nitrite removal is based on its oxidation process during UV irradiation, which is enhanced by the presence of the Z-TiO 2 photocatalyst.
Since activated carbon is a versatile sorbent class for the retention of a wide spectrum of organic compounds and arsenic(III) and the zeolite exhibited an adsorption/ion-exchange capacity for ammonium, nitrite, and heavy metals, their common presence as mixed materials broadens the spectrum of contaminants that can be removed from the water.

Conclusions
In this chapter, two types of composites obtained by two different methods and applied in the drinking water treatment were studied: a. TiO 2 supported on powdered activated carbon (PAC-TiO 2 ), granular activated carbon (GAC-TiO 2 ), and zeolite (Z-TiO 2 ), namely, supported TiO 2 , obtained through sol-gel method.
The removal of pollutants/impurities dissolved in water is based, on the one hand, on the sorption capacity of activated carbon and/or zeolite, which exhibited selectivity for certain impurities dissolved under the lamp-off conditions. On the other hand, but taking into account the sorption phase, by starting the lamp, TiO 2 on the active carbon surface and the zeolite acted by photocatalytic activity generating oxidation and reduction processes that cause the degradation and mineralization of the dissolved organic compounds and the transformation of the inorganic contaminants into the compounds that can be retained on the surface material or compounds that do not affect the quality of the water. Also, the oxidation and reduction processes under irradiation conditions allow the destruction of the contaminants and, implicitly, the cleaning of the sorbent surface. A mixture of zeolite and activated carbon as support for TiO 2 could exhibit bifunctionality, depending on the water features. Thus, for low loading of water with contaminants that can be adsorbed on the filter material, the lamp is not switched on during system operation but only in the washing/ regeneration stage, and for a water loaded with contaminants that cannot be adsorbed on the filter material, the system works with the lamp on, ensuring water decontamination and self-cleaning during operation.