Wellington Masamba

Mechanisms of heavy metal sorption on alkaline clays from Tundulu in Malawi as determined by EXAFS

Chromium(III), copper(II), zinc(II), cadmium(II), mercury(II) and lead(II) cations are among the most common heavy metal pollutants in industrial waste waters. In our continued work on cost effective wastewater heavy metal removal agents and methods using local material, this study examines the interactions of chromium(III), copper(II), zinc(II), cadmium(II), mercury(II) and lead(II) cations with natural mixed clay minerals from Tundulu in Malawi using extended X-ray absorption fine structure (EXAFS) spectroscopy. The mixed clays were previously characterised and found to contain illite, low ordered kaolinite, mixed layer minerals and the non-clay mineral carbonate fluoroapatite with a mean pH(PZC) of 9.63. The EXAFS analyses provided qualitative evidence that oxygen atoms occupy the first coordination shells in all the studied central atoms. The metal species on the clay mineral surfaces seem to be adsorbates and/or precipitates of hydrolysis products. Chromium(III) forms a polynuclear hydrolysis complex on the mineral surface with Cr-O bond and Cr...Cr distances of 2.00 and 3.03 A, respectively, which is indicative of a chain structure with edge sharing CrO(6) octahedra. Copper(II) is bound to phosphate groups on the surface at low pH and has a first shell of coordinated oxygen atoms with Jahn-Teller distortion as revealed by different Cu-O bonds of 1.96 A for the equatorial ones, at 2.30 and 2.65 A for the axial oxygens, and a Cu-P distance at 3.29 A is distinguished as well. Upon treatment at neutral pH copper(I) oxide seems to be the main precipitation product on the clay surface. At neutral pH zinc(II) forms also polynuclear hydrolysis complexes with Zn-O bond and Zn...Zn distances of 2.01 and 3.11A, respectively, which shows the presence of edge sharing ZnO(4) tetrahedra. Cadmium(II) is adsorbed to the clay surfaces as a six-coordinated CdO(6) complex in octahedral fashion, but it is not possible to distinguish if cadmium is hydrated or partly hydrolysed. Mercury(II) is present as linear O-Hg-O units but without any observable Hg...Hg distance at high pH showing that mercury(II) is hydrolysed but not present as mercury(II) oxide. At low pH, linear O-Hg-Hg-O units are present showing that mercury(II) is reduced to mercury(I). No precise chemical environment around the lead(II) could be obtained for the lead(II) treated clays due to formation of different hydrolysis structures with multiple coordination numbers by lead(II) salts on the mixed clays.

Dissolved Organic Matter Accumulation, Reactivity, and Redox State in Ground Water of a Recharge Wetland

Ground water beneath the seasonal swamp of the Okavango Delta, a recharge wetland in northwestern Botswana, is known to be a sink for solutes. In this study, measurements of organic carbon and inorganic ion concentrations, as well as UV-visible and fluorescence spectroscopy, were used to examine dissolved organic matter (DOM) storage and redox state of fulvic acids in ground water beneath an island and riparian woodland. Increasing dissolved organic carbon (DOC) concentrations along the ground-water flowpath suggests an accumulation of DOM in ground water, especially beneath island centers. However, the increase in DOC concentration was relatively less than the increase in chloride and sulfate concentrations, indicating non-conservative behavior of DOM in ground water beneath wetland islands. In combination with a decrease in fulvic acid content and specific UV absorbance, this result suggests that preferential sorption or destabilization of more aromatic organic compounds may be occurring under conditions of high pH and salinity. Finally, the increase in reduced fluorescence components (semiquinone- and hydroquinone-like components) along the ground-water flowpath strongly supports the transition to reduced fulvic acids in ground water of island centers. The reactivity and potential electron-shuttling function of fulvic acids may play an important role in the dissolution of metal oxides and associated DOM-iron-arsenic interactions in ground water of this recharge wetland.

Spatial and Seasonal Variability in Surface Water Chemistry in the Okavango Delta, Botswana: A Multivariate Approach

The annual flood pulse in the Okavango Delta (Botswana), has a major influence on water chemistry and habitat. We explore spatial and temporal patterns in a suite of chemical variables, analysed from 98 sample points, across four regions, taken at different stages of the flood cycle. The major pattern in water chemistry is characterised by an increasing gradient in ionic concentration from deep-water sites in the Panhandle to more shallow, distal regions to the south. Concentrations of cations, anions, dissolved organic carbon, and SiO2 are significantly higher in the seasonally inundated floodplains than in permanently flooded regions. Several variables (including Na and total nitrogen) significantly increase from low flood to high flood, while others (including HCO3, SiO2, and Cl) increase in concentration, initially between low flood and flood expansion, before declining at maximum flood extent. Redundancy analysis (RDA) revealed that hydrological variables (water depth, flow velocity, flood frequency, and hydroperiod class) significantly explain 17% variation in surface water chemistry. Predictions of increasing flood volume in the near future may result in a decline in alkalinity and dilution of DOC. Our study provides an important baseline from which to monitor future change in the Delta.

Effect of Particle Size on Loading Capacity and Water Quality in Water Defluoridation with 200°C Calcined Bauxite, Gypsum, Magnesite and their Composite Filter

Water defluoridation research has been going on world over in an attempt to mitigate dental and skeletal fluorosis. The effects of high fluoride ingestion on teeth and bone are well documented (Varol et al., 2010; Smittakorn et al., 2010; Castillo et al., 2007). Fluoride is known to cause browning and mottling of teeth and weakening of the bones leading to crippling conditions (Vasudevan et al., 2009; Feenstra et al., 2007). A number