DOI: 10.14457/tu.the.2016.1327
title: MWCNTs embedded in SiO2 particles as a solid-phase extraction adsorbent for chlorinated phenolic compounds

description: Multi-walled carbon nanotubes (MWCNTs) are promising materials widely used for environmental analysis because of their high adsoptivity and stability. To further enhance adsorptivity limited by agglomeration of MWCNTs, they were embedded in SiO2 particles through the FCCVD method. The parameters which are reaction time and flow rate of the carbon source (CH4) were studied to obtain optimum conditions for MWCNTs synthesis. The obtained MWCNTs were characterized by TEM, SEM, Raman, and FTIR spectroscopy to confirm their morphology and crystallinity. The optimum conditions were a CH4 flow rate of 100 mL/min, H2 flow rate of 200 mL/min, Ar flow rate of 300 mL/min and a reaction time of 20 min. Under these conditions, MWCNTs with average outer and inner diameters of around 50 and 10 nm, respectively, were obtained. SiO2 particles with embedded MWCNTs were studied for their adsorption of gaseous chlorinated phenolic compounds (CPCs), with emphasis on the effect of number of chlorine substituents. The CPC compounds of 2-chlorophenol (CP) and 2,4-dichlorophenol (DCP) were compared against phenol (P). Adsorption of P and CPCs on the particles fit well the Langmuir isotherm. The adsorption capacities of P, CP, and DCP on SiO2 particles with embedded MWCNTs were found to be 3.12, 13.83, and 44.25 mg/g, respectively. Desorption activation energy was determined by thermogravimetric analysis. Chlorine substitution on P changed the adsorption process from physical to chemical adsorption. MWCNTs embedded in SiO2 particles were investigated on dynamic adsorption through breakthrough curve using packed bed adsorption. Both flow rate and initial concentration were studied on effects of breakthrough curve. Increase of either flow rate or initial concentration led a faster saturation point because of increase of mass flow rate through the column that improved mass diffusion between CPCs and MWCNTs. DCP showed the highest adsorption capacity following by CP and P, respectively. It is because DCP had higher adsorption ability on MWCNTs surface than others. Considering bed capacity, it dropped with increasing of flow rate or initial concentration because of difficulty of mass diffusion into channels or pores of MWCNTs. MWCNTs embedded in SiO2 particles showed high potential for use as a pre-concentration unit of solid-phase extraction for CPCs. ATD equipped with GC-MS was performed in this study using the particles as an adsorbent. Desorption temperature, desorption time and amount of adsorbent were also studied to obtain the optimum condition of operation. The optimum condition was received at 300oC of desorption temperature, 20 min of desorption time and 300 mg of adsorbent amount. MWCNTs/SiO2 particles showed high desorption recovery (92-97 %) and high R2 (0.99) in calibration curve. High limit of detection (LOD) and limit of quantification (LOQ) were obtained in range of 38 - 87 µg/L and 116-263 µg/L, respectively. MWCNTs embedded in SiO2 particles were tested the performance against commercial adsorbents, Tenax TA and Carbopack B. Tenax TA shows highest response area which also indicated highest adsortivity, followed by MWCNTs/SiO2 and carbopack B, respectively.