Ntebogeng S. Mokgalaka

Multielement analysis of tea leaves by inductively coupled plasma optical emission spectrometry using slurry nebulisation

In order to evaluate the slurry nebulization method as an alternative method for the analysis of tea, the metal ion content of tea samples of different origins was determined. The concentrations of six elements, Al, Ba, Ca, Mg, Mn and Zn, were determined by introducing tea as a slurry into an inductively coupled plasma optical emission spectrometer (ICP-OES). Calibration was performed by using either aqueous standard solutions or by using the simplified generalized standard additions method (GSAM). The simplified GSAM is an expansion of the conventional standard additions method, based on the principle of varying both the sample mass and the amount of standard solution added. Initially the tea was ground for 3 h prior to slurry preparation. A carbonization procedure was included before the sample was ground. The grinding time was reduced to only 30 min. For comparison, tea was also digested by boiling in acid and being introduced into the plasma as an aqueous solution. The results obtained using slurry analysis were much higher than the results obtained from digested tea using aqueous standards for calibration. The metal content of the tea samples was found to be suitable for differentiating amongst the tea samples studied. The accuracy of the method was checked by analyzing a tea certified reference material. The Students t-test showed that values obtained using slurry nebulization were close to the certified values at a 95% confidence level. The simplified GSAM was applied to further confirm the accuracy of the slurry technique, and the values obtained using this method were comparable with those obtained using the slurry method with aqueous calibration. The slurry method can, therefore, be successfully applied to the analysis of tea leaves without the need to pre-dissolve them. This could avoid the use of hazardous chemicals, incomplete dissolution and loss of volatile analytes.

Reductive separation and slurry nebulization of converter matte for ICP-OES analysis of some platinum group metals and gold

A new method that involves slurry nebulization–ICP-OES analysis and permits accurate estimation of platinum group metals (PGMs) and gold in converter matte has been developed. The method involves reduction of the PGMs and Au with SnCl2 in concentrated HCl medium by heating on a laboratory hotplate and subsequent filtration of the hot mixture through a 0.45 µm membrane filter to separate the accompanying base metals. The resulting metallic precipitate of Au and PGMs was quantitatively transferred into a micronizing mill, ground for 30 min, diluted to 1000 ml with distilled-de-ionized water, and placed in an ultrasonic bath for 15 min to ensure homogeneity. An aliquot of this concentrated slurry was diluted to give a slurry of 0.005% m/v concentration prepared in 5.0% v/v hydrochloric acid and 1.0% m/v Triton X-100 as a dispersant and was directly nebulized into the ICP-OES for the determination of Au, Ir, Pd, Pt, Rh and Ru. The concentrations of Au and PGMs determined using ICP-OES were validated employing the standard additions method and by comparing with literature values that were obtained utilizing reductive separation and conventional dissolution procedure. The method, which allows separation of the PGMs and Au from a complex matrix offers the advantage of reduced analysis time (by ∼60%) and minimized contamination from additional sample preparation steps involved in the traditional procedure.

Adaptation and detoxification mechanisms of Vetiver grass (Chrysopogon zizanioides) growing on gold mine tailings

Vetiver grass (Chrysopogon zizanioides) was investigated for its potential use in the rehabilitation of gold mine tailings, its ability to extract and accumulate toxic metals from the tailings and its metal tolerant strategies. Vetiver grass was grown on gold mine tailings soil, in a hothouse, and monitored for sixteen weeks. The mine tailings were highly acidic and had high electrical conductivity. Vetiver grass was able to grow and adapt well on gold mine tailings. The results showed that Vetiver grass accumulated large amounts of metals in the roots and restricted their translocation to the shoots. This was confirmed by the bioconcentration factor of Zn, Cu, and Ni of >1 and the translocation factor of <1 for all the metals. This study revealed the defense mechanisms employed by Vetiver grass against metal stress that include: chelation of toxic metals by phenolics, glutathione S-tranferase, and low molecular weight thiols; sequestration and accumulation of metals within the cell wall that was revealed by the scanning electron microscopy that showed closure of stomata and thickened cell wall and was confirmed by high content of cell wall bound phenolics. Metal induced reactive oxygen species are reduced or eliminated by catalase, superoxide dismutase and peroxidase dismutase.

Green Chemistry in action: towards sustainable production of Gold nanoparticles

Abstract This study reports the synthesis of gold nanoparticles from a gold precursor salt (HAuCl4·3H2O) using Moringa oleifera bark broth, a cheap renewable material, without adding external surfactant, capping agent or template. Biomolecules responsible for reducing Au3+ to Au0 and stabilization of the resulting nanoparticles were extracted from the bark, and the synthesis was monitored for precursor concentration, percentage broth, pH of reaction media and reaction time. The biosynthesized nanoparticles were characterized using spectroscopic (FTIR and UV-Vis) techniques, advanced microscopic imaging (HRTEM, SEM/EDS), and Zeta potential measurements. Distinct color change from yellow to wine red was observed, indicative of the formation of gold particles at nanoscale. The SPR band was found at around 550 nm, in agreement with conventional synthetic protocols. The particles were stable with a net negative surface charge (–20 mV), a contribution associated with the protein nature of the broth. Addition of Pb2+ to the polydisperse nanoparticle suspension resulted in a color shift, to a faint blue color, coupled with a corresponding SPR shift to higher wavelengths, depending on the concentration of Pb2+ added. This color change is attributed to the aggregation of the colloidal particles due to complexation effects of the metal ions with the biomolecules on the surface of the nanoparticles.

Speciation Methods for the Determination of Organotins (OTs) and Heavy Metals (MHs) in the Freshwater and Marine Environments

Our primary goal for the development of analytical methods is their application in environmental monitoring to achieve good assessment of the contamination situation in freshwater and marine environments. As clearly stated in the endocrine disrupting contaminant (EDCs) program strategic plan for health related water issues (HRWI) of the Republic of South Africa (Version 1.2B, 7/02/2001), one of the objectives in the water research field is to protect aquatic ecosystems and human health based on sound science and defensible data through developing and validation of appropriate methods and by investigating the sources, persistence and effects of potential EDCs in water to support the risk assessment process and contribute towards a trustworthy environmental policy for endocrine disrupting contaminants.