Photocatalytic degradation of polychlorinated biphenyls (PCBs) in seawater was successfully achieved at laboratory level with UV light and at pilot-plant scale under natural solar radiation using carbon-modified titanium oxide (CM-n-TiO2) nanoparticles. The photocatalytic performance of CM-n-TiO2 was comparatively evaluated with reference n-TiO2 under identical conditions. As a result of carbon incorporation, significant enhancement of photodegradation efficiency using CM-n-TiO2 was clearly observed. To optimize the operating parameters, the effects of catalyst loading and pH of the solution on the photodegradation rate of PCBs were investigated. The best degradation rate was obtained at pH 5 and CM-n-TiO2 loading of 0.5 g L−1. The photodegradation results fitted the Langmuir-Hinshelwood model and obeyed pseudo-first-order reaction kinetics.
Polychlorinated biphenyls (PCBs) are a class of persistent organic pollutants that are ubiquitous in the environment. USEPA has classified PCBs as compounds with significant human health risk, due to their toxicity, carcinogenicity, and bioaccumulation nature [
Incineration is the main remediation technology for PCBs. However, it demands expensive facilities and high temperatures of more than 1200°C [
Although photocatalytic remediation of contaminated water has been extensively studied using UV/n-TiO2 at laboratory scale, relatively few attempts have been carried out at pilot-plant scale under natural sunlight. To the best of our knowledge, no study has been focused specifically on the photocatalytic removal of PCBs from seawater using CM-n-TiO2 at solar-driven pilot-plant scale. Therefore, it could be of interest for environmental and economic considerations to evaluate the possibility of exploitation of the renewable solar energy through the utilization of pilot plant for the purification of polluted water.
In this context, this work aimed at studying the degradation efficiency of polychlorinated biphenyls (PCBs) in seawater using CM-n-TiO2 nanoparticles at laboratory scale under artificial UV light as well as evaluating the viability and the performance of the pilot plant for the remediation of these compounds under natural sunlight. The experimental conditions including CM-n-TiO2 loading and solution pH were optimized at the lab scale and were applied for the solar pilot plant.
CM-n-TiO2 nanoparticles were fabricated by sonicated sol-gel method using titanium(IV) isopropoxide as a titanium and carbon-containing precursor. The preparation and characterization of CM-n-TiO2 have been reported in detail elsewhere [
Clean seawater samples, collected from Sharm Obhur, Jeddah Red Sea coast, were spiked with various concentrations of PCBs (Aroclor 1254 and Aroclor 1260). Photocatalytic degradation experiments were performed at laboratory level with UV light and at pilot-plant scale with natural solar radiation. The effects of operating parameters including CM-n-TiO2 loading and pH of the solution on the photodegradation rate of PCBs have been investigated first at lab scale to reach the optimum conditions and then were applied at pilot-plant scale.
For lab scale photocatalytic experiments, a 500 mL Pyrex glass reactor was used as a batch reactor under illumination of UV light. Both contaminated samples and the photocatalyst were loaded inside the photocatalytic reactor and continuously stirred for uniform mixing. Prior to light irradiation, the suspensions were equilibrated for 30 min in the dark. Then, the photoreactor was irradiated with low pressure UV fluorescent lamp (Upland, 15 W of wavelength 365 nm) placed inside Fluorescence Cabinet (CC-80, Spectroline).
Solar pilot-plant scale reactor, so-called Solar Falling Film Reactor (SFFR), was designed and built. The performance of the CM-n-TiO2/SFFR system was evaluated towards the photocatalytic removal of PCBs under real sunlight illumination. The SFFR consists of flat tray, top distributor, bottom collector, a pump (Pedrollo, Italy, model: PKm 60-BR, 550 W), and a batch tank (equipped with electric mixer to allow homogenization) located underneath the flat tray (Figure
Photograph of the Solar Falling Film Reactor (SFFR).
Treated PCBs solution was sampled at regular irradiation intervals. The samples containing photocatalyst were centrifuged for 5 minutes and then the supernatant was shaken with 2 mL of a mixture of hexane and dichloromethane (1 : 1) for 15 min. Using a nitrogen evaporator, extracted samples were concentrated to 0.5 mL and then transferred to screw capped vials and stored at 4°C before analysis. The concentration of PCBs (Ar 1254 and Ar 1260) was measured using gas chromatograph coupled with 63Ni electron capture detector (GC-ECD, Shimadzu 2010). Rxi-XLB capillary column (30 m × 0.32 mm × 0.5
In our previous work [
Optical properties of CM-n-TiO2 and n-TiO2 nanoparticles.
Catalyst | Crystal phase | Crystalline size (nm) | Bandgap (eV) | Atomic % | ||
---|---|---|---|---|---|---|
Ti | O | C | ||||
CM-n-TiO2 | Anatase | 31.4 | 1.8 | 29.81 | 61.21 | 8.98 |
n-TiO2 | Anatase | 41.5 | 2.99 | 36.54 | 63.46 | 00.00 |
The effect of CM-n-TiO2 dose on the photocatalytic degradation of a mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm) in seawater under illumination of UV light was studied to attain the optimum catalyst loading (Figure
Effect of catalyst dose on the photocatalytic degradation of mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm) in seawater under illumination of UV light.
The effect of pH on the photodegradation of PCBs in seawater under illumination of UV using CM-n-TiO2 was studied at three different pH values 5, 7, and 9. As clearly shown in Figure
Effect of pH on the photocatalytic degradation of mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm) in seawater using 0.5 g L−1 of CM-n-TiO2 under illumination of UV light.
In order to examine the photocatalytic efficiency of CM-n-TiO2, comparison with unmodified n-TiO2 was performed under the same optimum experimental conditions (Figure
Photocatalytic degradation of a mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm) in seawater using n-TiO2 and CM-n-TiO2 under illumination of UV light.
To evaluate the viability and the performance of the solar pilot plant (SFFR), the photocatalytic degradation of PCBs (1.0 ppm) in seawater was examined at the optimal conditions, obtained from laboratory scale experiments, of pH 5 and 0.5 g L−1 of CM-n-TiO2 (Figure
Photocatalytic degradation of PCBs (1.0 ppm) in seawater using CM-n-TiO2 at lab scale and pilot-plant scale.
On the other hand, a comparison with regular n-TiO2 was performed under the same experimental conditions in order to assess the photocatalytic performance of CM-n-TiO2 in the SFFR for the degradation of Ar 1254 (0.5 ppm), Ar 1260 (0.5 ppm), and a mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm). As a result of carbon incorporation, remarkably higher photodegradation efficiency of CM-n-TiO2 is clearly noted, reflecting the capability of CM-n-TiO2 to harvest maximum solar light photons and hence enhance the degradation efficiency. After 60 min of solar irradiation, PCBs were easily degraded with efficiencies of 92.1% for Ar 1254 (Figure
Photocatalytic degradation of (a) Ar 1254 (0.5 ppm), (b) Ar 1260 (0.5 ppm), and (c) mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm) in seawater at the optimal conditions of pH 5 and 0.5 g L−1 of the photocatalyst (n-TiO2 and CM-n-TiO2) under illumination of natural sunlight using the pilot plant (SFFR).
To depict the kinetics of photocatalytic reactions of aqueous organics, the Langmuir-Hinshelwood (L-H) model was employed [
By plotting
Apparent rate constants (
PCBs | CM-n-TiO2 | n-TiO2 | ||||
---|---|---|---|---|---|---|
|
|
|
|
|
| |
Ar 1254 (0.5 ppm) | 0.0416 | 16.66 | 0.9854 | 0.0218 | 31.80 | 0.9759 |
Ar 1260 (0.5 ppm) | 0.0509 | 13.62 | 0.9818 | 0.0231 | 30.01 | 0.985 |
Ar 1254 (0.5 ppm) + Ar 1260 (0.5 ppm) | 0.0482 | 14.38 | 0.9909 | 0.0225 | 30.81 | 0.9983 |
Kinetic analysis for the photocatalytic degradation of (a) Ar 1254 (0.5 ppm), (b) Ar 1260 (0.5 ppm), and (c) mixture of Ar 1254 (0.5 ppm) and Ar 1260 (0.5 ppm) in seawater using the pilot plant (SFFR).
Unique carbon-modified titanium oxide (CM-n-TiO2) nanoparticles were successfully fabricated via sonicated sol-gel method using titanium(IV) isopropoxide as Ti and a carbon-containing precursor. Comparative evaluation of the photocatalytic performance of carbon-modified and regular titanium oxide towards the photocatalytic removal of PCBs was performed. The bandgap energy has been reduced from 2.99 eV for n-TiO2 to 1.8 eV for CM-n-TiO2, which in turn improved the photocatalytic performance of CM-n-TiO2 by absorption of more light photons. The results showed that the removal rate of PCBs was favorable at catalyst dosage of 0.5 g L−1 and pH 5. The photodegradation kinetics of PCBs using CM-n-TiO2 followed a pseudo-first-order reaction. The photocatalytic degradation of PCBs in seawater has been successfully achieved using CM-n-TiO2 nanoparticles at laboratory level with UV light and at pilot-plant scale (SFFR) under natural solar radiation. Furthermore, the results obtained evidenced the validity of CM-n-TiO2/SFFR system as an attractive and promising technique for the remediation of polluted water.
The authors declare that they have no competing interests.
This project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award no. 12-NAN2241-03. The authors also acknowledge with thanks the Science and Technology Unit, King Abdulaziz University, for technical support. The authors are thankful to Mr. Kazem Sultan and Mr. Yasar N. K. for their appreciable help in the experimental analysis.