Synthesis of carbon nanotubes on impregnated powdered activated carbon for removal of cadmium from water

Abstract

Water resources are being polluted from various sources. Types of emerging pollutants are also increasing while the traditional pollutants are not solved efficiently yet. Removal of toxic metals is one of the biggest challenges in ensuring safe water for all. Those can be removed by chemical processes, physical processes such as ultra filtration and reverse osmosis, adsorption by polymers and various types of media which exhaust fast and expensive to reactivate. Cadmium (Cd2+) was selected, in the present study, for its potential hazards to living beings at low concentration and difficulties in removing from water. Therefore, due to their high adsorption capacity and high surface area, carbon nanotubes (CNT) were synthesized and used to remove cadmium from water. Fixed catalyst chemical vapor deposition reactor (FCCVD) system was fabricated, upgraded and used to grow carbon nanotubes. Growth parameters (reaction time, reaction temperature, and gas flow rates) for CNT on powdered activated carbon (PAC), as a novel substrate were optimized. The PAC was impregnated with Fe3+ catalyst. Design Expert software was used to design the experimental plan and to determine the optimization parameters for the growth of CNT, considering removal (%) of Cd2+ as a response. Based on the screening test, four samples were selected for detailed study. Selected CNT-PAC samples were characterized using field emission electron microscope (FESEM) and transmission electron microscope (TEM) to confirm the CNT growth as well as to study the microstructure of the nanoscale product. In addition, the selected CNT-PAC samples were further characterized using Fourier transform spectroscopy (FTIR), thermal gravimetric analysis (TGA) and Brunauer, Emmett and Teller (BET) surface area. The surface properties of CNT-PAC were modified by oxidative functionalization using three different methods: sonication with KMnO4, refluxing with HNO3 at 140 oC and in-situ gasification with CO2 at 750 oC. By comparing the performance, the best modified sample was found to be the one, which was functionalized with KMnO4. Design Expert software was also used to optimize the removal of cadmium from water with pH, contact time, adsorbent dose and agitation speed as controlling parameters. KMnO4 treatment of the most promising CNT-PAC sample (B3) enhanced the Cd+2 removal from 38.87% to 98.35%. Tip growth was observed from TEM. The results showed that BET surface area was 974.9 m2/g, Zeta potential was - 46.1 mV and TGA combustion temperature range was between 371 oC and 560 oC. The selected functionalized sample was used to perform optimization study on removal of Cd2+ from water using central composite design (CCD) experimental design. The optimum conditions were pH 4.92, agitation speed 160 rpm, contact time 60 min and adsorbent dose 214 mg/L. Langmuir and Freundlich adsorption isotherms were studied for the novel adsorbent. Langmuir constants were qm = 69.759 mg/g, Kl = 0.223 and R2 = 0.924. The system was found more likely to follow Freundlich model with R2 of 0.961, Kf of 9.215 and n of 3.015. Three kinetic models (pseudo first order, second order and intraparticle adsorption) of adsorption were studied too. The Pseudosecond- order was the best fit with R2 of 0.996 and model constants were qe = 34.29 (mg/g) and K2 = 0.0013 (g/mg.min). Coefficients for the pseudo first order were qe= 21.145 (mg/g) and K1= 0.09 (min-1) with R2 of 0.936. The intraparticle kinetic model exhibited R2 value of 0.812 with Kd = 1.295 (g/mg.min0.5). The CNT-PAC adsorbent after oxidative functionalization with KMnO4 was proven to be an efficient adsorbent for Cd2+ removal from water. Thus this novel nanocomposite material is promising for other adsorption applications for its significant surface properties.