(53z) Carbon Nanotubes for Environmental Applications: Solid-Phase Extraction and Gas Sensing for Chlorinated Phenolic Compounds

Chlorinated phenols are the most direct precursors of dibenzo-p-dioxins and dibenzofurans (PCDD/PCDFs) which are highly carcinogenic and persistent organic pollutants (POPs) found in exhaust gases emitted from municipal solid waste incinerators and their emissions are an important environmental issue. CNTs materials have been attracted multi-faceted interests because of their unique characteristics and potential application, used for environmental detection. The graphitic structure of carbon nanotubes (CNTs) can induce interaction with aromatic compounds through the sp² hybridization state of CNTs due to Ï€-Ï€ bonds interaction. Therefore, the potential of CNTs adsorption targeting chlorophenols should be explored.

The aims of this research are study of feasibility of chlorinated phenols gas sensing and solid phase extraction applications using different carbon based materials (CNTs membrane and CNTs embedded on SiO₂ particles). CNTs membrane was prepared from commercial MWCNT by vacuum filtration. Meanwhile, the CNTs embedded on SiO₂ were prepared through floating-catalyst chemical vapor deposition (FCCVD) method. The adsorption characteristics of chlorinated phenol compounds (phenol, mono- and di-chlorophenol) on both of carbon-based materials were studied in gas phase. The adsorption energy was investigated using the thermogravimetric (TGA) technique presenting in term of activation energy for desorption (E_d).

SiO₂ particles embedded MWCNTs were studied on the gaseous adsorption of chlorinated phenolic chemicals (CPCs). 2-chlorophenol (CP) and 2,4- dichlorophenol (DCP), as CPCs probes, were tested against phenol (P). The adsorptions of P and CPCs on SiO₂ particles embedded MWCNTs were well fit with Langmuir isotherm. The adsorption capacity when equilibrium concentration equals unity of SiO₂particles embedded MWCNTs for P, CP and DCP was found to be 3.12, 13.83 and 44.25 mg/g, respectively. The activation energy of their desorptions (Ed) was ranked in order from high to low; DCP (62.2 kJ/mol) > CP (49.6 kJ/mol) > P (37.2 kJ/mol). The phenomenon of phenol adsorption onto MWCNTs was physisorption, while the adsorption of CP and DCP behaved in chemisorptions. The chlorine substitution on P affected the phenomena shifted from physical to chemical adsorption. MWCNTs embedded on SiO₂ particles showed high potential as a pre-concentration unit for solid-phase micro extraction (SPME).

CNT membrane were prepared by vacuum filtration and utilized as gas sensor. The electrical resistivity of the CNT membrane surface was measured by the four-point probe technique. The CNT membrane responded quickly upon the exposure of the target gases and the sensitivity was directly proportional to the degree of chlorination in the target CPCs. The CNT membrane gas sensor showed good sensitivity to all gaseous CPCs through the concentration range of 10-100 ppm. The sensitivity values obtained were 13.778×10^-4, 8.835×10^-4 and 7.122×10^-4 for 2,4-dichlorophenol, 2-chlorophenol and phenol, respectively. It is noteworthy that the adsorption capacity and sensitivity of the CNT membrane to the target gases increased with the increase in the number of chlorine atoms in the phenolic compounds. After 4 consecutive cycles test, the CNT membrane still gave unchanged responses indicating satisfied stability and repeatability. The results suggest that the acid-functionalized CNT membrane is a promising material for a simple detection of CPCs in air.