Carbon nanomaterials for the removal and recovery of metal ions from aqueous solutions

Carbon nanomaterials are a group of materials which have been gaining increasing recognition for their applications in environmental remediation. Studies on the use of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) for the remediation of various contaminants including metal ions like Cr(VI) and Hg(II) have already been undertaken. However, these studies have mostly focussed on the uptake of parts per million (ppm) levels of contaminants. The uptake of parts per billion (ppb) levels of such contaminants have not been extensively examined. Studies on the effect of sulphur doping on the uptake of Hg have not been undertaken. Competitive adsorption studies in which competing cations and anions are present have also been scarce. The effect of surface modifications such as acid and base-treatment and the effect of nitrogen doping have not been extensively studied. This study was therefore undertaken to address the above issues. It was found that unfunctionalised MWCNTs could adsorb 98% of a 100 ppb Cr(VI) solution. These materials had superior adsorption capabilities to that of activated carbon and functionalised MWCNTs. The difference in adsorption capabilities of these materials was attributed to the differences in points of zero charge (pHpzc). The Langmuir and Freundlich adsorption isotherm models were also used to describe the adsorption process. Sulphur-containing MWCNTs (S-MWCNTs) showed the highest uptake capacities for both 100 ppb solutions of Hg(II) and organic forms of mercury respectively. This was attributed mainly due to mercury’s high affinity for sulphur and evidence of a chemisorption process was presented. The uptake capacity of sulphur-containing activated carbon (S-AC) was inferior to that of the S-MWCNTs but this was attributed mainly to a difference in sulphur content. The only advantage that S-MWCNTs presented over S-AC was a greater selectivity in the presence of SO2. The S-MWCNTs were also highly selective to the uptake of Hg in the presence of competing cations and in a chlor-alkali effluent where a complex chemical matrix was noted. The Freundlich adsorption isotherm model best described the uptake of Hg. Both Cr(VI) and Hg were efficiently desorbed in 0,1 M NaOH and 0,5% thiourea in 0,05 M HCl solutions respectively. This implied that the MWCNTs could be reused and regenerated and this could address cost issues. Results from the surface modification studies showed that acid-treatment (both strong and weak) resulted in oxygen-containing functional groups which lowered the point of zero charge of the MWCNTs thereby rendering these suitable for cation uptake. The effect of base-treatment depended on the type of base used. The strong base KOH had a similar effect to that of the acid-treated MWCNTs. The weak base NH3 on the other hand resulted in the presence of quartenary nitrogen which increased the point of zero charge and made the MWCNTs more suitable for anion uptake. Similar observations were made for nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) and nitrogen doped carbon spheres (N-CSs) . The effect of nitrogen doping does however depend on the form in which nitrogen is present. In this case the predominant form of nitrogen was quartenary nitrogen. Carbon nanomaterials have therefore demonstrated a great ability to extract a variety of anions and cations from aqueous solution and thus show potential for industrial applications especially since they show superior adsorption capabilities to that of activated carbon. The selectivity of the unfunctionalised MWCNTs and MWCNTs treated with acid and base was however, poor when competing anions and cations were present. This suggested that the selectivity of these materials needs to be improved upon in further studies before these are used in industrial applications

A kinetic and mechanistic study on the oxidation of chromium oxide in pure chemicals and in ferrometallurgical slags

Bibliography: leaves 103-108.Chromium can exist in a number of oxidation states but the environmentally stable forms are trivalent (Cr(III)) and hexavalent (Cr(VI)) chromium. These two forms are noted for their different degrees of toxicity and mobility. Hexavalent chromium is more toxic and mobile and has been responsible for a number of illnesses in humans (Sheehan et. al., 1991). Elemental chromium and its compounds have a variety of uses in the industrial sector. The ferrometallurgical industry in particular makes use of chromium since this element imparts properties such as hardness and strenght to stainless steel. However, this industry also produces slags that contain residual amounts of chromium oxide (Cr[2]O[3]) along with other constituents such as calcium oxide (CaO). Thermodynamic data from the literature, has shown that Cr[2]O[3] can undergo oxidation at ambient temperature when in contact with CaO and atmospheric oxygen (Kilau and Shah, 1984; Hattori et al., 1978). Furthermore, the oxidation of Cr[2]O[3] in the presence of Ca(OH)[2] has also been observed at ambient temperature (Petersen, 1998)

Heavy-metal spent adsorbents reuse in catalytic, energy and forensic applications- a new approach in reducing secondary pollution associated with adsorption

Heavy metal ions are toxic causing a variety of health complications. Heavy metal ions mainly include Cr(VI), Hg(II), Pb(II), Cd(II), Cu(II), Fe(III) and Co(II). The adsorption technique is the most practical in removing metal ions from water since it is cost effective, simple to operate and has high removal efficiency. After heavy metals adsorption the spent adsorbent need to be disposed. The approach to focus on heavy metal laden adsorbents is because these present a greater challenge towards potential secondary pollution since the heavy metal contaminants cannot be degraded. If improperly disposed, heavy metals spent adsorbent can led to secondary environmental pollution. Many options are being used to reduce secondary pollution such as spent adsorbent regeneration and recycling with the aim of reusing the adsorbent for adsorption. In this review various methods of adsorbent regeneration are discussed highlighting their limitations. The aim of this review is to discuss other novel alternative approaches of dealing with heavy metals spent adsorbents. The new strategies include heavy metals spent adsorbents reuse in catalytic, energy and forensic applications. These approaches are new and very few reports are available. However, from the results of the reports, the aforementioned methods promise to be a viable option and more research needs to be directed towards them

Visible light degradation of ibuprofen using PANI coated WO3@TiO2 photocatalyst

A PANI-coated heterojunction of WO3@TiO2 nanocomposite was fabricated in three stages. The performance evaluation of the prepared photocatalyst for the degradation of ibuprofen was performed under visible light. Characterization of the photocatalyst using X-ray diffraction (XRD) analysis showed that the TiO2 prepared constituted of the anatase phase. Furthermore, results from in situ XRD analysis of WO3 show that it consisted of monoclinic and orthorhombic crystalline structures. These phases were not affected by the incorporation of PANI as revealed by XRD analysis. Results from Transmission electron microscopy (TEM) examination showed that sphere-like WO3 and TiO2 nanorods of different sizes were prepared In addition, fabrication of a heterojunction of WO3@TiO2 wrapped in PANI was shown by TEM analysis. Results from photoluminescence studies indicate that coupling TiO2 with WO3 enhanced the charge separation and the degradation performance of the nanocomposite. Supporting the heterojunction on PANI enhanced the degradation efficiency as indicated during the performance evaluation process. Diffuse reflectance spectra (DRS) calculations of the PANI/WO3@TiO2 catalysts showed that they can be used under visible light. The experimental results of X-ray Photon Spectroscopy (XPS) analysis showed the presence of elements W, C, O, Ti, and N. Solution pH influenced the degradation process and the maximum degradation efficiency was attained at pH 9. The degradation followed the Langmuir-Hinshelwood kinetic model with a Kinetic constant of 3.5 × 10−2. The rate of degradation increased in the presence of bicarbonate/carbonate ions and persulfate ions

Preparation of carbon-aerogel polypyrrole composite for desalination by hybrid capacitive desalination method

Capacitive Deionization (CDI) is an emerging technology with great potential applications. Most researchers view it as a viable water treatment alternative to reverse osmosis. This research reports the preparation and application of a carbon aerogel polypyrrole (CA-PPy) composite for the desalination of NaCl solution by the hybrid CDI method. The carbon aerogel (CA) was prepared from a Resorcinol / Formaldehyde precursor by the sol–gel method. The aerogel obtained from the sol–gel was then pyrolysed in a tube furnace to form CA. Polypyrrole (PPy) was prepared by the Oxidative chemical polymerisation of pyrrole, ferric chloride hexahydrate (oxidant), and sodium dodecyl sulfate (dopant). A composite of CA and PPy was then prepared and used to modify carbon electrodes. The CA-PPy composite was characterised to verify its composition, morphology, thermal properties, and functional groups. The electrochemical properties of the material were determined by Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) tests. The electrochemical tests were done using a GAMRY potentiostat electrochemical workstation, a 1.0 M KCl was used as the electrolyte, and the applied potential window was (-0.2 to + 0.6) V for the CV test. The EIS test was done with the same concentration of KCl electrolyte at an applied potential of 0.22 V and at a frequency range of (0.1 – 100, 000) Hz. The optimal specific capacitance of the CA is 115F/g, and that of the composite is 360.1F/g, they were both obtained at a scan rate of 5 mV/s. The CDI desalination study of the CA-PPy composite showed a salt adsorption capacity (SAC) of 10.10 mg/g (300 mg/L NaCl solution) – 15.7 mg/g (800 mg/L NaCl solution) at 1.2 V applied voltage. The salt recovery efficiency of the electrode material in the 300 mg/L solution is 27 %, in the 500 mg/L solution, it is 20.12 %, and in the 800 mg/L solution, it is 15.41 %. The electrode material also showed good electrochemical stability after nine cycles of ion adsorption/desorption study

Electrochemical detection of Hg(II) in water using self-assembled single walled carbon nanotube-poly(m-amino benzene sulfonic acid) on gold electrode

This work reports on the detection of mercury using single walled carbon nanotube-poly (m-amino benzene sulfonic acid) (SWCNT-PABS) modified gold electrode by self-assembled monolayers (SAMs) technique. A thiol containing moiety (dimethyl amino ethane thiol (DMAET)) was used to facilitate the assembly of the SWCNT-PABS molecules onto the Au electrode surface. The successfully assembled monolayers were characterised using atomic force microscopy (AFM). Cyclic voltammetric and electrochemical impedance spectroscopic studies of the modified electrode (Au-DMAET-(SWCNT-PABS)) showed improved electron transfer over the bare Au electrode and the Au-DMAET in [Fe (CN)6]3−/4− solution. The Au-DMAET-(SWCNT-PABS) was used for the detection of Hg in water by square wave anodic stripping voltammetry (SWASV) analysis at the following optimized conditions: deposition potential of −0.1 V, deposition time of 30 s, 0.1 M HCl electrolyte and pH 3. The sensor showed a good sensitivity and a limit of detection of 0.06 μM with a linear concentration range of 20 ppb to 250 ppb under the optimum conditions. The analytical applicability of the proposed method with the sensor electrode was tested with real water sample and the method was validated with inductively coupled plasma – optical emission spectroscopy. Keywords: Self-assembly, Gold electrode, Carbon nanotubes, Electrochemical detection, Mercur