Water purification in a solar reactor incorporating TiO₂ coated mesh structures Open Access

Heterogeneous solar photocatalysis uses solar UV light in the range between 300 nm and 400 nm to photo-excite the catalyst in contact with water and in the presence of oxygen (Malato et al. 2009) to generate hydroxyl radicals (^•OH). TiO₂ is the most-used photocatalyst because of its biological and chemical inertness, photo-stability, availability, and nontoxicity (McCullagh et al. 2011). The largest exposed active catalyst surface area, relative to the reactor volume, can be obtained in a so-called dispersed-phase reactor. However, a complex and expensive separation step is required to remove the fine catalyst particles from the water. This can be avoided by using immobilized photocatalyst reactors (Feitz et al. 2000; Malato et al. 2009; McCullagh et al. 2011). For efficient contaminant degradation such reactors require a large reactor surface with low throughput because of their low catalyst surface to reactor volume ratio. The degradation potential can be increased by changing the configuration of such reactors. Zhang et al. (2004) reported that the rate of degradation with a corrugated plate was a factor 1.5 faster than with the flat-plate for the same exposed reactor area. More complex configurations can be used to increase even further the catalyst surface to reactor volume ratio. However, although the catalyst surface to reactor volume ratio may increase, light penetration in the reactor can then become a limiting factor (Feitz et al. 2000; Dijkstra et al. 2001).

TiO₂ coatings were successfully applied on various materials of different surface areas such as ordinary glass and borosilicate glass (Parra et al. 2004; Fujishima et al. 2008; Zhang et al. 2012), cellulose fibres (Goetz et al. 2009), and stainless steel (Yanagida et al. 2006). To optimize the active catalyst surface to reactor volume ratio as well as light penetration and light distribution, a novel reactor was developed containing a stack of TiO₂ coated meshes (El-Kalliny et al. 2014). Stainless steel woven meshes were used as catalyst support material and TiO₂ coatings were applied by electrophoretic deposition (EPD) using a commercial sol gel (O5100) as a stable suspended electrolyte. In this paper, the TiO₂ coated mesh reactor was used to study the adsorption and solar photocatalytic degradation efficiency of humic acids (HA) as a model compound for surface water contaminants. The performance of the catalyst structure was investigated at different scales, including a batch system in a beaker and a recirculating flow fixed-bed reactor. Results were compared with those obtained with a similarly coated flat-plate reactor of the same size.

Immobilization of the TiO₂ photocatalyst on grade 304 stainless steel woven meshes and flat plates was done using the EPD technique described in El-Kalliny et al. (2014). Meshes had a wire diameter of 0.355 mm and an aperture diameter of 0.915 mm, resulting in an open area of 52%. Flat plates were 2 mm thick and had the same dimensions as the rectangular mesh sheets (32 cm × 19 cm).

The batch experiments with small circular meshes (ø 5.5 cm) were carried out in 250 mL Pyrex glass beakers (internal diameter 7 cm) using a magnetic stirrer (IKA, RT15) with a stirring rate of 550 rpm. The meshes were placed on top of each other and separated by 2-mm-thick silicone rubber rings. The temperature was kept at 32 ± 1 °C by using a recirculation cooler (Julabo, FL300). This temperature is a simulation for the temperature of the system subjected to solar light. Working solutions of HA were prepared from a 1,000 mg·L⁻¹ stock solution of HA sodium salt (Sigma Aldrich). The HA sodium salt was dissolved in aerated deionized water and filtered through a 0.45 μm syringe-driven filter unit (Millex) to remove suspended solids (0.03 g·L⁻¹).

The schematic view of the fixed-bed photocatalytic reactor is presented in the Supplementary Data (Figure S1), available with the online version of this paper. The body of the reactor was made of Perspex and the cover was 4-mm-thick borosilicate glass. UV solar light (300–400 nm) transmission of this cover, measured by Xenocal UV-sensor (Atlas), was 92.9%. Four mesh layers (32 cm × 19 cm) were placed in the reactor, separated by 2-mm-thick silicone rubber spacers. Water recirculated and entered into the reactor from the short side (19 cm width) by a peristaltic pump (Minipuls 3, Gilson) with a maximum flow rate of 200 mL·min⁻¹, corresponding to a hydraulic retention time through the reactor of 3.25 min. These conditions showed that there were no dead zones in the reactor with a good mixing as clarified by the color injection method. This method showed that mixing occurred within 10 s, which is a very short time compared with the reaction retention time of about 1 hour. Mixing of outlet water was done in a separate container by a magnetic stirrer. In the flat plate reactor experiment, the flat plate of 32 cm × 19 cm coated with TiO₂ was placed instead of mesh layers. The chamber of a SUNTEST XXL+ (Atlas) equipped with three Xenon lamps irradiating solar light (65 J·m⁻²·s⁻¹ intensity in the range of 300–400 nm) was used for both systems.

Because of the grid structure of the mesh, there was wire overlap at the contact points. From basic geometrical considerations it can be calculated that for the given wire diameter and mesh aperture the overlap was 16.25% of the total wire surface. The exposed coated surface area was therefore corrected accordingly.

The HA concentration was determined by the UV absorption at 254 nm, which is representative of the aromatic moieties. This was done with a Hach Lange DR 5,000 spectrophotometer as described in El-Kalliny et al. (2014).

Figure 1 shows (mg·m⁻²) as a function of contact time for the small and large mesh structures in the batch system and in the recirculating flow fixed-bed reactor, respectively. The data show a strong increase in during the first 20 minutes, levelling off thereafter to a saturation value at equilibrium. Figure 1 shows that the adsorption capacity increased with increasing ⁠. This is an indication of the presence of sufficient active sites per unit volume that can acquire and adsorb HA molecules with increasing ⁠. The values of ⁠, ⁠, and (from Equation (6)) were obtained by least-square-fitting of the experimental measurements and the resulting curves are drawn for different C₀ in Figure 1. The good fit shows that the adsorption kinetics of HA onto the TiO₂ film coated on the meshes are well-described by the combination of the pseudo-first-order kinetic model and the Freundlich adsorption model (Equation (6)). Therefore, probably a multilayer of adsorbed molecules was formed on the catalyst surface and saturated the adsorption sites. The values of the constants and the corresponding 95% confidence level intervals are inserted in Figure 1. The fit has R² of 0.964 and 0.961 with the root-mean-square deviation equal to 1.43 and 1.17 for the small meshes and the large meshes, respectively. The for the recirculating flow fixed-bed reactor was slightly higher than for the batch system. This reflected a difference range of 7%–14% of calculated values (Supplementary Data, Table S1, available with the online version of this paper). It was found that the calculated (via Equation (5)) for small meshes ranged from 44.12 to 30.05 mg·m⁻² for small meshes and ranged from 37.78 to 27.74 mg·m⁻² for large meshes with values of 14–8 mg·L⁻¹. The similarity between the two systems in the behaviour of HA adsorption over the meshes suggests that up-scaling of the mesh structure is feasible. Further, the higher value of the kinetic constant k₁ suggests that there were no mass transfer limitations in the recirculating flow fixed-bed reactor compared with the well-stirred batch system.

Before starting photocatalytic degradation experiments, the systems were preconditioned for 2 h in the dark to attain adsorption equilibrium. When exposure to solar light started, the HA concentration was equal to the C_e. Therefore, in Figure 2, the decrease of HA concentration due to exposed solar energy Q (in kJ·L⁻¹) is given as C/C_e for different C₀ values. An additional photolysis experiment was done in the batch reactor by irradiating a HA solution of 10 mg·L⁻¹ without any photocatalyst present. The results in Figure 2(a) show that no noticeable degradation of HA occurred up to 12.5 kJ·L⁻¹ of solar energy irradiation without the catalyst. However, when applying the small coated meshes, HA concentrations were reduced to less than half of those when solar irradiation started. By using a flat plate in the fixed bed reactor at an initial HA concentration of 10 mg·L⁻¹ the solar photocatalytic degradation of HA was 3.4 times lower than in the case of the coated mesh structure at the same exposed energy (Figure 2(b)). Even though the light intensity was not homogeneously distributed over the expanded surface area (⁠ for four coated meshes was increased from 1 m² to 3 m², see above), the attenuation factor was still higher than 3, indicating that a more spatial distribution of light over the surface was beneficial. Not only were more active sites of TiO₂ photocatalyst available on the mesh structure than on the flat plate for the same volume of water, but these sites were also more efficient due to better light distribution. Fujishima et al. (2008) already showed that the catalyst efficiency increases by immobilization on a large surface area support, due to an increase in the surface-area-to-volume ratio (Fujishima et al. 2008). That the mesh structure allowed a high light penetration and distribution in the reactor was reported earlier (El-Kalliny et al. 2014), but now it is also shown that this translates into increased photodegradation of HA.

In order to investigate the dependence of photocatalytic degradation on the adsorption step by checking the applicability of the LH model, it is essential to calculate the apparent rate constants (k_app) for the removal of HA for different initial HA concentrations (Equation (12)). The relations between ln(C) and the exposed energy (Q), including light absorption, resulted in straight lines for different ⁠. The slopes of these lines represent the apparent rate constants k_app. These pseudo-first-order rate constants are inserted in Figure 2 together with their correlation coefficient R², which is always greater than 0.98. The k_app was calculated for the first stage of the photocatalytic reaction. In this stage, the concentration of HA is predominant relative to the by-products. Accordingly, the rate of reaction depends on the HA concentration, not on by-products. Therefore, the simplified form of the LH model (Equation (12)) was used. Consequently, the experimental data are fitted to the pseudo-first-order kinetic model, as shown in Figure 2. It is noticeable that k_app for the degradation of HA in the fixed bed reactor is higher than that in the glass beaker reactor, as is shown in Figure 2. This can be attributed to wall effects of the beaker from absorbing or transmitting part of the incident solar UV light, resulting in a lower captured light energy than calculated. The experimental results show that the k_app decreases with increasing of HA. This is consistent with what was reported previously by Li et al. (2002) for the degradation of HA by a TiO₂ suspension. As the amount of light absorbed by the HA solution was taken into consideration (see Equations (9) and (10)), therefore, this behaviour could be due to the higher number of adsorbed HA molecules (i.e. ⁠) at a high HA ⁠. These HA molecules may block some active sites on the TiO₂ surface, which are responsible for the production of active radicals. Hence, the number of produced ^•OH decreases with increasing HA ⁠, leading to a slower rate of photodegradation. This indicates that availability of ^•OH is rate-limiting by hindering the reaction of HA with the produced ^•OH.

The k_app for the degradation of HA by coated meshes was 3.4 times higher than that over a flat plate at the same absorbed energy, the same projected area, and at the same initial HA concentration of 10 mg·L⁻¹.

The validation of LH kinetics for photodegradation of HA with the glass beaker reactor and with the fixed-bed solar reactor can be done by checking the linearity of the relation of 1/r₀ versus 1/C₀. The high correlation coefficients (R² = 0.969 and 0.998 for small and large meshes, respectively) show that the photocatalytic degradation of HA using TiO₂ coated over the woven meshes obeyed the LH model. According to this model, the rate of reaction (r) is proportional to the fraction of the catalyst surface that is covered by HA. This gives an indication that HA molecules are first adsorbed onto the photocatalyst surface and then decomposed (i.e., the photocatalytic reaction mainly occurs on the catalyst surface and not in the bulk of the solution), which is consistent with previous studies (Bekbolet et al. 2002; Li et al. 2002). The actual rate constant (⁠⁠) of the reaction, as obtained from Equation (13), for large coated meshes in the recirculating flow fixed-bed reactor (1.495 mg·kJ⁻¹) was close to that for small coated meshes in the batch system (1.462 mg·kJ⁻¹), which proves that up-scaling was feasible.

• The adsorption kinetics of HA onto TiO₂ film coated over stainless steel woven meshes obeyed pseudo-first-order adsorption kinetics according to the Freundlich adsorption isotherm.

• The degradation kinetics of HA using small coated meshes in the batch reactor and large coated meshes in the recirculating flow fixed-bed solar reactor obeyed the LH kinetic model.

• Up-scaling of mesh size was possible as was shown by the comparable adsorption and photodegradation kinetics for the two investigated reactor sizes.

• The photocatalytic degradation of HA in a solar reactor with four superimposed stainless steel meshes was 3.4 times faster than the flat-plate reactor. This brought the photocatalytic efficiency of such reactors closer to that of dispersed-phase reactors without the complex separation of the TiO₂ photocatalyst.

This research work has been carried out via the PhD program (El-Kalliny 2013) within the framework of INNOWATOR project, financed by Agentschap NL, the Organization for Prohibition of Chemical Weapons (OPCW), and Egyptian Cultural Affairs & Missions Sector (the Egyptian Government).