Kriveshini Pillay

Multi-walled carbon nanotubes as adsorbents for the removal of parts per billion levels of hexavalent chromium from aqueous solution

The adsorption capabilities for the removal of parts per billion levels (ppb) of hexavalent chromium by three adsorbents namely activated carbon, functionalized multi-walled carbon nanotubes (MWCNTs) and unfunctionalized multi-walled carbon nanotubes were investigated as a function of contact time, initial solution pH, initial Cr(VI) concentrations and the presence of competing anions. The unfunctionalized MWCNTs showed the highest adsorption capability with up to 98% of a 100 ppb Cr(VI) solution being adsorbed. Both functionalized and non-functionalized MWCNTs showed a superior adsorption capability to that of activated carbon. The removal of Cr(VI) was higher at lower pH. Furthermore, the uptake of Cr(VI) was hindered by the presence of the competing anions, Cl(-) and SO(4)(2-). Both Langmuir and Freundlich isotherms have been used to describe the Cr(VI) adsorption process. The major mechanisms for Cr(VI) removal have been identified as an ion exchange mechanism, intraparticle diffusion and electrostatic interactions. The adsorbed Cr(VI) could also be desorbed readily from the MWCNTs surface at high pH.

Ageing of chromium(III)-bearing slag and its relation to the atmospheric oxidation of solid chromium(III)-oxide in the presence of calcium oxide

Slag arising in ferrochromium and stainless steel production is known to contain residual levels of trivalent chromium. As the chromium is normally bound in the slag matrix in various silicate or spinel phases, and hence not easily mobilised, utilisation or controlled disposal of such slag is generally considered unproblematic. Experimental test work with a number of slag materials indicates, however, that very gradual oxidation of trivalent to hexavalent chromium does occur when the slag is exposed to atmospheric oxygen, rendering a quantifiable but small portion of chromium in this much more mobile and toxic form. Mechanisms and rates of the oxidation reaction were investigated in a number of long-term studies using both original slag materials and artificial mixes of chromium and calcium oxides. Powders of these materials, some of them rolled into balls, were left to age under different conditions for periods of up to 12 months. In the slag samples, which contained between 1 and 3 wt.% chromium, 1000-10000 microg Cr(VI) were found per gram of chromium within 6-9 months of exposure to an ambient atmosphere. The rate of the oxidation reaction decreased exponentially, and the reaction could generally be said to have ceased within 12 months. In mixtures of calcium and chromium oxides the oxidation reaction is presumed to occur at the boundaries between chromium oxide and calcium oxide phases through diffusion of oxygen along the grain boundaries and of Cr(3+) across the boundaries, resulting in the formation of calcium chromate. In the slags, where calcium and chromium oxide can form a solid solution, the oxidation is likely to occur at the exposed surface of grains containing this solution.

Development of a polyaniline-lignocellulose composite for optimal adsorption of Congo red

A polyaniline lignocellulose composite (PLC) was synthesized and used in the removal of Congo red (CR) from aqueous solution. The adsorption process showed good fits to both the pseudo-second-order and pseudo-first-order models and the Redlich Peterson isotherm. Boundary layer diffusion was the rate-limiting step. The adsorption was spontaneous and endothermic. The combined effect of pH and initial dye concentration was antagonistic; the combined effect of initial dye concentration and temperature was synergistic, while the combined effect of pH and temperature was reciprocal. The maximum CR adsorption capacity of PLC was evaluated as 1672.5 mg g(-1). The optimal removal was calculated as 99.85% at pH 4.29, initial dye concentration of 28.5 mg L(-1) and adsorbent dosage of 0.69 g L(-1). The predicted removal capacity showed a good correlation to the experimental results. PLC has demonstrated a superior adsorption capacity to many other adsorbents reported and could be used as an efficient adsorbent for CR removal from industrial wastewater.

Nanosponge cyclodextrin polyurethanes and their modification with nanomaterials for the removal of pollutants from waste water: A review

Water is a worldwide vital resource for sustaining life and due to the pollution of water by different classes of pollutants (inorganic, organic and pathogens), many ongoing studies in water purification remain a critical issue to governments, scientists and industries. The challenge is to develop a water purification technology which will be effective at removing these contaminants simultaneously and reducing their concentrations to ultra low levels from waste water. This review article serves to give an overview on cyclodextrin nanosponge adsorbents which have already been used for water treatment. The modification of these cyclodextrin nanosponges with existing adsorbent nanomaterials (carbon nanotubes, TiO2 and silver nanoparticles) and the factors affecting the adsorption capacity of these nanosorbents are discussed. The nanotoxicity of these engineered nanosorbents material is also addressed since nanotoxicity is a major concern to human health and environment.

Selective removal of toxic Cr(VI) from aqueous solution by adsorption combined with reduction at a magnetic nanocomposite surface

The adsorption of toxic hexavalent chromium (Cr(VI)) and its reduction to trivalent chromium (Cr(III)) are important processes for the treatment of industrial wastewater. Conducting polymers can adsorb and reduce Cr(VI) to less toxic Cr(III) but have low adsorption capacities due to agglomeration of particles and are difficult to separate from treated water. In this study, magnetic polypyrrole (PPy)-polyaniline (PANI)/iron oxide (Fe3O4) nanocomposite was synthesized for the selective removal of Cr(VI) in aqueous solution. PPy-PANI/Fe3O4 nanocomposite was characterized using various techniques including ATR-FTIR, FE-SEM, HR-TEM, EDX, TGA, XRD, VSM and XPS analyses. PPy-PANI/Fe3O4 nanocomposite (0.05g) removed 99% of Cr(VI) from aqueous solution (100mg/L, pH 2). Speciation studies confirmed Cr(VI) adsorption and reduction to Cr(III) by the PPy-PANI/Fe3O4 nanocomposite in solutions with initial pH of 2 and 3 and that no Cr(VI) reduction occurred at pH values of 4 and above. The Langmuir maximum adsorption capacity for Cr(VI) removal by PPy-PANI/Fe3O4 nanocomposite at pH 2 was 303mg/g at 25°C. PPy-PANI/Fe3O4 nanocomposite was highly selective for Cr(VI) removal and could be used for three consecutive treatment cycles without loss of adsorption capacity. Moreover, the magnetic nanocomposite could be separated from the reaction fluid using an external magnet. PPy-PANI/Fe3O4 nanocomposite is therefore a promising magnetic adsorbent for the treatment of industrial wastewater.

Rapid and efficient removal of fluoride ions from aqueous solution using a polypyrrole coated hydrous tin oxide nanocomposite.

Polypyrrole/hydrous tin oxide nanocomposites (PPy/HSnO NC 1, 2, 3, 4 and 5) were synthesized through encapsulating HSnO by the PPy via an in situ polymerization for fluoride removal. The optimized adsorbent i.e. PPy/HSnO NC 3 was characterized using FE-SEM, HR-TEM, ATR-FTIR, XRD, BET, TGA and zeta sizer. Microscopic images revealed the encapsulation of HSnO by precipitating PPy during polymerization. The FTIR and XRD studies confirmed the presence of both constituents. The BET surface area and pHpzc of the adsorbent were estimated to be 65.758m(2)/g and 7.6, respectively. The fluoride adsorption followed pseudo-second-order model and was commendably rapid. The monolayer adsorption capacity was found to be 26.16-28.99mg/g at pH 6.5±0.1. The thermodynamic parameters indicated the sorption of F(-) was spontaneous, endothermic and that physisorption occurred. The calculated activation energy (Ea∼20.05kJ/mol) provided further evidence of a physisorption mechanism. Moreover, the adsorbent performed very well over a considerably wide pH range of 3.5-8.5 and in the presence of other co-existing ions. The regeneration of the F(-) laden PPy/HSnO NC 3 showed a high desorption efficiency of 95.81% up to 3 cycles. Ground water tested results also demonstrate the potential utility of the PPy/HSnO NC as an effective adsorbent.

Selective removal of Cr(VI) from aqueous solution by polypyrrole/2,5-diaminobenzene sulfonic acid composite

A polypyrrole/2,5-diaminobenzenesulfonic acid (PPy/DABSA) composite, synthesised by the in situ oxidative polymerization of pyrrole in the presence of DABSA, was studied as an adsorbent for the removal of Cr(VI) from aqueous solution. The structure and morphology of the composite were investigated by ATR-FTIR, FE-SEM, EDX, TGA, XRD and XPS studies. The adsorption of Cr(VI) by PPy/DABSA composite was highly pH dependent and optimum removal was achieved at pH 2. Adsorption of Cr(VI) was confirmed by EDX and XPS studies. The isotherm data fitted the linear Langmuir model well, with a maximum adsorption capacity of 303mg/g at 25°C. Thermodynamic parameters (ΔG°, ΔH° and ΔS°) were calculated using isotherm data and confirmed that the adsorption process was spontaneous and endothermic. Adsorption kinetics was best described by the pseudo-second-order model. The activation energy of the adsorption process suggested that Cr(VI) was chemisorbed by PPy/DABSA composite. PPy/DABSA composite could be used for three consecutive adsorption-desorption cycles without loss of its original adsorption capacity. Highly selective removal of Cr(VI) was observed even when co-existing ions such as Cu(2+), Zn(2+), Ni(2+), Cl(-), SO4(2)(-) and NO3(-) were present in the solution. In summary, the potential of PPy/DABSA composite for remediating industrial wastewater contaminated by Cr(VI) has been demonstrated.

Single stage batch adsorber design for efficient Eosin yellow removal by polyaniline coated ligno-cellulose.

Polyaniline-coated lignin-based adsorbent (PLC) was synthesized and used for uptake of reactive dye eosin yellow (EY) from aqueous solution. The adsorption capability of the adsorbent was found to be more effective than the unmodified adsorbent (LC). In particular, the adsorption capability of the PLC was effective over a wider pH range. This could be owing to its higher point of zero charge, which is more favorable for the uptake of the anionic dye. Adsorption isotherm models suggested a monolayer adsorption was predominant. The mean free energy of adsorption (E(DR)) was found to have values between 8 and 16 kJ mol(-1) which suggests that an electrostatic mechanism of adsorption predominated over other underlying mechanisms. The adsorption process was also found to be spontaneous, with increasing negative free energy values observed at higher temperatures. Chemisorption process was supported by the changes in enthalpy above 40 kJ mol(-1) and by the results of desorption studies. This new adsorbent was also reusable and regenerable over four successive adsorption-desorption cycles. The single stage adsorber design revealed that PLC can be applicable as an effective biosorbent for the treatment of industrial effluents containing EY dye.

Hydrous TiO2@polypyrrole hybrid nanocomposite as an efficient selective scavenger for the defluoridation of drinking water

An adsorptive process for the defluoridation of drinking water was performed using a hybrid nanocomposite of hydrous titanium oxide@polypyrrole (HTiO2@PPy), as a scavenger. The adsorbent was successfully fabricated via facile in situ chemical oxidative polymerization of pyrrole monomer in aqueous media in which HTiO2 nanoparticles were suspended. The developed adsorbent was characterized using various spectro-analytical techniques viz. BET, FTIR, FE-SEM, STEM, EDX, TGA and ZETA SIZER. Relatively high BET surface area (98.17 m2 g−1) and pHpzc (∼8.4) values were obtained for HTiO2@PPy. The synergistic effect of both the counterparts (PPy and HTiO2) of the nanocomposite rapidly enhanced the F− adsorption process. A noteworthy rapid fluoride uptake best described by the pseudo-second-order kinetic model was observed (equilibrium attainment within 5–30 min). The Langmuir model best described the isotherm data with a maximum adsorption capacity of 31.93 mg g−1 at 25 °C and pH 6.5 (±0.2). Thermodynamic and activation parameters provided evidence of the spontaneous, endothermic and physical nature of the adsorption process. The selectivity of HTiO2@PPy for F− sorption was significant in the presence of Cl−, NO3−, HCO3−, SO42− and PO43− co-existing ions and noteworthy reusability for up to three regeneration cycles was achieved. Electrostatic interactions and ion-exchange were proposed to be the possible underlying mechanisms for the adsorption of F− by HTiO2@PPy nanocomposite. Thus, HTiO2@PPy is anticipated to serve as an efficient scavenger for the defluoridation of drinking water.

Epichlorohydrin crosslinked carboxymethyl cellulose-ethylenediamine imprinted polymer for the selective uptake of Cr(VI)

A new ion-imprinted polymer (IIP) was synthesized from sodium carboxymethyl cellulose and used for the adsorption of hexavalent chromium from aqueous solution. Epichlorohydrin was used to stabilize the polymer before ethylenediamine (EDA) ligand and Cr(VI) were introduced. The obtained IIP was characterized with FTIR, XPS, TGA,13C NMR, SEM, EDX, BET and zeta sizer. The kinetics of adsorption followed a pseudo-second-order model while the Langmuir adsorption isotherm provided the best fit with a maximum adsorption capacity of 177.62mg/g at 25°C. The Langmuir adsorption capacity for the non-imprinted polymer (NIP) at 25°C was 149.93mg/g. Thermodynamic parameters indicated an endothermic process and spontaneous adsorption of Cr (VI) on the polymers. IIP adsorbent was successfully recycled 5 times using 0.1M NaOH as a leachate; 98% Cr(VI) was desorbed during the last adsorption-desorption cycle.