Photocatalytic degradation of 2,4-dinitrophenol (DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite in aqueous solution under solar irradiation

Abstract

Nanosized multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite and neat TiO₂ photocatalysts were synthesized by sol–gel technique using tetrabutyl titanate as a precursor. The as prepared photocatalysts were characterized using XRD, SEM, FTIR and UV–vis spectra. The samples were evaluated for their photocatalytic activity towards the degradation of 2,4-dinitrophenol (DNP) under solar irradiation. The results indicated that the addition of an appropriate amount of MWCNTs could remarkably improve the photocatalytic activity of TiO₂. An optimal MWCNTs:TiO₂ ratio of 0.05% (w/w) was found to achieve the maximum rate of DNP degradation. The effects of pH, irradiation time, catalyst concentration, DNP concentration, etc. on the photocatalytic activity were studied and the results obtained were fitted to the Langmuir–Hinshelwood model to study the degradation kinetics. The optimal conditions were an initial DNP concentration of 38.8 mg/L at pH 6.0 with catalyst concentration of 8 g/L under solar irradiation for 150 min with good recyclisation of catalyst. The degree of photocatalytic degradation of DNP increased with an increase in temperature. The MWCNTs/TiO₂ composite was found to be very effective in the decolorization and COD reduction of real wastewater from DNP manufacturing. Thus, this study showed the feasible and potential use of MWCNTs/TiO₂ composite in degradation of various toxic organic contaminants and industrial effluents.

Introduction

Alkyl dinitrophenols are widely acknowledged to be a group of toxic refractory chemicals, which can be detrimental to human health and the environment. 2,4-Dinitrophenol (DNP) is a typical example of this class of toxic compounds. DNP is widely used in petrochemical industry as polymerization inhibitor for vinyl aromatics and in agriculture as a pesticide. Although much benefit is obtained from its uses, DNP has some undesirable side effects, such as toxicity and carcinogenity, and its use and the maximum concentration level in water were restricted by the US Environmental Protection Agency (EPA) (Uberoi and Bhattacharya, 1997). DNP can be hardly destroyed in conventional wastewater treatment, so there is a pressing need to provide a method for the treatment of DNP wastewater which is of low cost and little time consuming.

TiO₂ mediated photocatalytic degradation is a successful and convenient alternative to the conventional methods for the treatment of wastewater containing organic pollutants. TiO₂ has the advantage of good chemical stability, absence of toxicity and relative low cost, but a serious disadvantage is its wide band gap (E_g = 3.2 eV) that requires that UV radiation is used to trigger this attractive photocatalyst, which would greatly hinder the commercialization of TiO₂ photocatalysis. Photocatalytic degradation of organic contaminants using abundant natural solar radiation can be highly economical compared with the processes using artificial UV radiation, which require substantial electrical power input. In regard to this, various attempts have been made to extend the spectral response of TiO₂ into the visible region of the solar spectrum and enhance its photocatalytic activity. Among those attempts, doping non-metal atoms such as nitrogen (Asahi et al., 2001; Nosaka et al., 2005), carbon (Xu et al., 2006, Wang et al., 2005, Janus et al., 2004), sulfur (Ohno, 2004), boron (Zhao et al., 2004) is one of the most efficient methods to narrow band gap energy of TiO₂ and shift the spectral response of TiO₂ from the ultraviolet to the visible light region.

Multi-walled carbon nanotubes (MWCNTs) have attracted considerable attention since their discovery. Taking advantage of the unique electronic and physical properties of the MWCNTs, we expect that the combination of MWCNTs with TiO₂ may induce interesting charge transfer and thus enhance the photocatalytic activity of TiO₂. The application of TiO₂ photocatalysts in the presence of MWCNTs has been reported to enhance chemical reactions (Xia et al., 2007). Although various methods for the preparation of carbon nanotubes (CNTs)/TiO₂ composites have been reported in the literature (Jitianu et al., 2004, Chen et al., 2005), the photocatalytic properties of CNTs/TiO₂ composite photocatalysts remain largely unexplored. In the present work, we have prepared MWCNTs/TiO₂ composite by sol–gel method, which absorbs most of the visible light of the solar spectrum. The photocatalytic degradation efficiency of the composite catalysts for alkyl dinitrophenol compounds was evaluated by using DNP as a probe molecule. The main objective of our work is determining the optimal conditions of experimental parameters for the practical application of toxic organic wastewater by MWCNTs/TiO₂ composite-sunlight process.