Synthesis, characterization and evaluation of ionic liquids and polymeric ionic liquids/functionalized multiwalled carbon nanotubes for Cr(VI) adsorption.

Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.In this study, a series of imidazolium and pyridinium-based ionic liquids (ILs), polymeric ionic liquids (PILs), and their carbon nanotubes-functionalized composites were synthesized, characterized and applied as potential adsorbents for hexavalent Cr(VI). Polymeric ionic liquids of different polymerizable moieties (vinyl and styrenic moieties) were studied. Furthermore, multi-walled carbon nanotubes (MWCNTs) were synthesized, characterized and dispersed on both imidazolium and pyridinium-based ILs and PILs, respectively. Thermal studies revealed that vinyl pyridinium PILs possess good thermal stability than the vinyl imidazolium counterparts. The size of the counter-anions bromide (Br-), hexafluorophosphate (PF6-), and bis(trifluoromethanesulfonyl) imide (TFSI-) and the charge delocalization in cationic rings greatly influenced the glass transition temperatures of PILs. Expectedly, pyridinium and imidazolium-based PILs with hexafluorophosphate ions showed poor solubility in polar protic solvents (water, methanol) and good solubility in polar aprotic solvents (DMSO, DMF, THF) except acetone (a dipolar aprotic solvent). The as-synthesized ILs/MWCNT composites were characterized using FTIR spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermal analysis. The results obtained indicate that the pyridinium-based ILs exhibited higher decomposition temperatures (above 400 °C) compared to imidazolium-based ILs counterparts (onset decomposition at 250 °C) with poor water-solubility and their glass transition temperatures were dependent on ion mobility. The effect of the alkyl lateral chain (propyl and isopropyl) at the first and third position of imidazolium and N-position of pyridinium cationic rings towards their thermal stability, conductivity, and solubility of the ionic liquids was investigated. Their solubilities in different polar and non-polar solvents were also investigated. Spectroscopic and microscopic analyses confirmed the formation of the ILs/MWCNT composites with new functionalities and unaltered surface morphology of carbon nanotubes. Pyridinium and imidazolium-based PILs/MWCNT composites were characterized by thermal, spectroscopic, and electron microscopy techniques. It was observed that the composites were thermally stable compared to the corresponding precursors and were insoluble in polar aprotic solvents. For application, solid-liquid adsorption process was used in the adsorption of Cr(VI) from aqueous solution using the as-synthesized ILs/MWCNT and PILs/MWCNT composites as adsorbents. Under batch adsorption experiments, the effect of solution pH, contact time and initial concentration of Cr(VI) were investigated. It was established that the adsorption of Cr(VI) took place under acidic conditions (pH=2-3), thereby confirming significant adsorption of dichromate (Cr2O7-) and hydrochromate (HCrO4-) anions. At lower pH values, the ionic and π-anionic electrostatic interactions between the positively-charged regions of the composites and Cr(VI) were believed to have facilitated the adsorption of anionic (Cr2O7-) and (HCrO4-). Adsorption results obtained based on contact time showed that increase in contact time gradually increases the adsorption of Cr(VI) within 2 h. However, further increase in experimental contact time above 2 h insignificantly affected the adsorption of Cr(VI) due to early or quick oversaturation of the surface active sites on the adsorbents. The adsorption of Cr(VI) onto ILs/MWCNT and PILs/MWCNT composites fitted well into both Langmuir and Freundlich adsorption isotherms. However, the homogeneity/heterogeneity nature of the adsorbents relied on the diversified nature of the composites, which includes bulky pyridinium and imidazolium organic cations with delocalized charges, some large counter anions and the graphitic functional carbon groups. In order to understand the mechanisms of the adsorption of Cr(VI) onto ILs/MWCNT and PILs/MWCNT composites, pseudo-second-order kinetic model was employed. The results obtained showed that the calculated maximum adsorption capacities (qecal) and experimental maximum adsorption capacities (qe.exp) depict high correlation co-efficiencies (R2>0.99) confirming the applicability and feasibility of pseudo-second-order model on the adsorption of Cr(VI) in this study.Abstract available in pdf

Remediation of metal ions in aqueous solution using activated carbon from Zea may stem

Zea mays stem and its activated carbon were prepared through chemical activation method using four different activating reagents (NaOH, H₃PO₄, H₂SO₄, KOH) and were used as adsorbents for the removal of Pb(II), Cu(II), Hg(II) and Cr(III) from aqueous solution. The results shows that activated carbon has high surface area and pore volume compared to the powdered raw Zea mays stem. Prepared activated carbon was characterized using physico-chemical properties such as carbon yield, iodine number, moisture content, percentage adsorption, and analytical instruments such as Fourier transform Infrared spectroscopy (FTIR), atomic absorption spectrometer (AAS), scanning electron microscopy (SEM), Energy Dispersive X-ray analysis (EDS), powder X-ray Diffraction (pXRD). The adsorption of Pb(II), Cu(II), and Cr(III) ions were pH, contact time, and concentration dependent. Based on the results obtained from the batch experiments, activated carbon prepared from Zea mays stem is not good enough for the removal of Hg(II) from aqueous solution. Adsorption ability was calculated and found to be 66.67% for activated carbon obtained from H₂SO₄, 21.21% for activated carbon obtained from KOH, and 20% for activated carbons obtained from NaOH and H₃PO₄. The pH 5-6 was chosen for all experiments, contact time was 2 hours, and adsorbent dosage was 2 g, initial concentration range from 200, 400, 600, 800, and 1000 ppm at room temperature. The metal ion removal trend was found in the order Pb(II)>Cu(II)>Cr(III)>>Hg(II). The Langmuir model fitted well in most of the cases with > 0.99. Consequently, the adsorption of Pb(II) and Cu(II) followed Langmuir isotherm model while that of Cr(III) best fitted the Freundlich isotherm model. The results indicated that the adsorption process followed two possible mechanisms. (I) Metal ion – adsorbent complex model and (II) Metal ion – ion-exchange adsorbent complex model

REMOVAL OF METAL IONS FROM AQUEOUS SOLUTIONS USING ACTIVATED CARBON PREPARED FROM ZEA MAYS STEM

Activated carbon prepared from Zea mays stem using chemical activation was used as adsorbent for the removal of metal ions from aqueous solution. The results indicate that the adsorption of Pb(II), Cu(II) and Cr(III) ions is pH, contact time, and concentration dependent while the adsorption of Hg(II) is very different. The maximum removal percentages obtained at pH 5-6 were 97.44 % for Pb(II), 98.37 % for Cu(II), 89.37 % for Cr(III) and there was no adsorption for mercury ions. The metal ion removal trend follows the order Pb(II) > Cr(III) > Cu(II). The results show that Langmuir model fitted well in most cases with R2 > 0.99. Consequently, the adsorption of Pb(II) and Cu(II) followed Langmuir isotherm model while that of Cr(III) best fitted on Freundlich isotherm model

Activated Carbons from Waste Tyre Pyrolysis: Application

The development of better and efficient methods of consuming less and/or wasting little resource materials is becoming more important. In this study, pyrolytic waste tyre carbon black residue and commercial grade activated carbon were characterized and evaluated against adsorption of mercury vapor. The performance of the raw carbon black residue and the activated carbon against mercury vapor generated in the laboratory was determined using a designed reactor system. The adsorption of Hg+ was investigated at temperatures ranging from 200 to 280°C for 6 hours. Batch experiments were conducted for the different carbon residue samples and characterization analysis were done before and after adsorption using the spectroscopic, microscopic, and structural techniques to elucidate the structural arrangements and properties of the carbonaceous materials. Spectroscopic analysis of these carbonaceous residues showed a C=C stretching vibration attributed to the lignocellulose aromatic ring at 1657–2000 cm−1. Comparatively, it was also observed that the Infrared spectrum of raw carbon black exhibits less functional groups as compared to the H2SO4-AC and H2O2-AC carbonaceous residues prepared