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The chemistry, analysis, treatment, and occurrence of boron and chromium are of high interest since they are under consideration for revised MCLs in potable water. As a starting point to guide regulatory decision-making, a comprehensive review of boron in relation to potable water was undertaken. That work demonstrated that there were not proven cost-effective options for boron treatment. In preparation for a national survey of boron and chromium occurrence, it was discovered that existing analytical protocols sometimes '"missed" much of the total chromium that was present in water. It was determined that this was due to incomplete dissolution of particulates during routine Standard Method analysis of drinking water at pH 2.0. A more rigorous hydroxylamine digestion was developed and applied to circumvent this limitation. In relation to treatment, it was determined that sodium carbonate softening at pH 10.3 is a viable method of removing various inorganic contaminants including chromium from drinking water sources. The nationwide survey revealed that removals varied widely and were dependent on solution composition. Median removal of chromium was 92%. Linear and nonlinear empirical models were fit to crudely estimate the removal of various contaminants in the presence of other elements that are typically removed in the softening process (i.e. calcium, magnesium, silicon, iron, and aluminum). Boron was removed to a much lesser extent (median removal 2%) in this study, consistent with the general result of the literature review that enhanced treatments will be required for this contaminant. Modified precipitative softening was examined as a potentially attractive option to remove boron from natural waters. It was discovered that in some cases when magnesium and silicon were present, and if the pH was 10.8 ± 0.2, very high levels of boron removal (up to 90%) could be achieved versus 10% typically observed for conventional processes. This can be exploited to remove boron in waters naturally containing high levels of magnesium and silicon, or by adding supplemental amounts of either magnesium or silicon when one constituent is deficient. / Ph. D.

The removal of toxic and heavy metal contaminants from aqueous waste streams and industrial effluents is one of the most important environmental issues being faced the world over. In order to combat this problem, the commonly used procedures for removing metal ions from dilute aqueous streams include chemical precipitation, ion exchange, reverse osmosis and solvent extraction. However, these techniques have certain disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal. The hazardous wastes generated from metal mining and smelting operations also need to be decontaminated before entering the ecosystem. Chromium contamination of soil and ground water is a significant problem worldwide. The extensive distribution of this pollutant is due to its numerous industrial applications such as metal plating, alloying, leather tanning and wood industry. Cr (VI) is toxic and carcinogenic in nature while Cr (III) is innocuous. Conventional chromium removal techniques involve reduction to the Cr (III) form and subsequent precipitation as its hydroxide. However, disposal of the solid sludge remains a problem. The search for alternative and innovative treatment techniques has focussed attention on the metal uptake capacities of various microorganisms such as yeast, algae, fungi and bacteria. It is well documented that microbial biomass is capable of adsorbing metal ions from aqueous solution even when the cells have been killed. In the present investigation, the potential of utilising a gram positive, neutrophilic, facultative anaerobe like Bacillus polymyxa, in the bioremoval of Cr (VI) and Cr (III), has been assessed under different conditions. The growth of Bacillus polymyxa has been studied in the presence of varying concentrations of chromium ions. Subsequently, adaptation of the bacteria to Cr (VI) and Cr (III) has also been carried out. The biological reduction of Cr (VI) and its biosorption have been monitored during the growth of the unadapted and 2 ppm Cr (VI) adapted strains. The bioremoval of Cr (VI) and Cr (III) has also been assessed using the metabolic products obtained during bacterial growth. Detailed investigations have been carried out to determine the bioremoval efficiencies of both living and non-living cells of Bacillus polymyxa, with respect to Cr (III) and Cr (VI). The various parameters influencing the bioremoval of chromium by the cells, such as time, pH, wet biomass loading and initial metal concentration, have been studied. Electrokinetic studies on the bacterial cells, before and after interaction with Cr (VI) and Cr(III)have been carried out. The morphological changes induced in the bacterial strains consequent to interaction with Cr (III) and Cr (VI) have been examined by scanning electron microscopy. The results of the present investigation revealed that bioreduction of Cr (VI) was feasible during the growth of both adapted and unadapted bacteria. The time taken for 90% bioremoval was 72 h in the case of the unadapted strain, whereas with the adapted strain only around 48 h were required to achieve comparable results. The metabolic products obtained by enzymatic bacterial action were also found to be efficient in bringing about the bioremoval of Cr (VI). The bioremoval efficiency was marginally better when a lower concentration of Cr (VI) was used. Over 80% bioremoval was achieved in about 10 h using 2 ppm Cr (VI) while almost 48 h were necessary for a similar amount of removal to be effected using 5 ppm Cr (VI). In the case of the metabolite obtained from the adapted strain, complete removal of 2 ppm Cr (VI) was possible in 24 h. The living cells of Bacillus polymyxa were not only able to accumulate Cr (VI) but were also capable of bioreduction to the Cr (III) form, when the pH was in the range of 1.5 to 4. The maximum bioremoval of about 75% of Cr (VI) was observed at pH 2, with 45% being attributed to bioreduction, with an equilibration time of 48 h. In the case of Cr (III) nearly 90% uptake could be achieved at a natural pH of 5.5, equilibration time of 24 h and using 1 g of wet biomass. Biosorption was the only method of removal present in the non-living system. In the case of nonliving biomass, the optimum conditions for maximum Cr (VI) removal (65%) were pH 2, equilibration time of 12 h and a biomass loading of 1 g, whereas for Cr (III), the maximum uptake of about 97% occurred at an initial pH of 5, equilibration time of 12 h and 0.4 g wet biomass. The non-living cells showed a better efficiency in removing Cr (III), while the living cells exhibited a greater tendency towards the bioremoval of Cr (VI) than the non-living ones. Electrokinetic measurements revealed that consequent to interaction with Cr (VI) or Cr (III), significant surface modification was brought about on the cells of Bacillus polymyxa. Further, the isoelectric point was found to be shifted towards less acidic values after interaction with Cr (III) or Cr (VI). The probable mechanisms of the bioremoval processes are highlighted.

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Physical, chemical and corrosion properties of a hexavalent chromium conversion coating (CCC) and that of a commercial third generation trivalent chromium system; Tripass LT1500, on zinc electrodeposited steel has been studied. Moreover, the role of additives has been studied to elucidate film formation and corrosion resistance mechanisms. Micro-cracking and self-repair corrosion protection behaviour commonly associated with hexavalent CCCs has also been investigated. Scanning Electron Microscopy (SEM) studies showed that for both hexavalent and trivalent CCCs were in general, flat with a spherical-like structure and in the case of the former microcracked beyond 122 nm conversion coating thickness. In general, the micro-crack pattern observed e.g. a dense crack network, depended upon the underlying zinc substrate morphology. The study has also demonstrated the effect of SEM imaging and prior specimen preparation conditions on hexavalent CCC micro-cracking and blistering. X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and Infrared (IR) data has indicated that the hexavalent CCC film formation appears to be a electrochemical/sol-gel mechanism given the lack of zinc content at surface and subsurface regions within the conversion coating, presence of H2O and in particular the contribution of Cr(OH)3 as opposed to Cr2O3. An alternative film formation mechanism may exist for trivalent CCC given a higher proportion of zinc at surface and sub-surface regions, IR data analysis indicating that chromium is possibly deposited from a chromium (III) complex ion such as [CrC2O4(H2O)4] + , moreover as Cr(OH)3 and Cr2O3 compounds as indicated by XPS data analysis. The role of cobalt nitrate during film formation is unclear given that cobalt was not detected within the trivalent CCC from XPS and AES data. Electrochemical LPR measurements, polarisation curves and XPS data has shown in general, self-repair corrosion protection properties for hexavalent CCC to be lacking. Instead, it is proposed that the corrosion protection behaviour for hexavalent and trivalent CCC to be barrier. Polarisation curves and LPR data showed that the corrosion resistance performance for trivalent CCC was higher than hexavalent CCC, in general. LPR data showed that the omission of cobalt nitrate and increased addition of sodium molybdate content within the Tripass LT1500 treatment solution formulation was found to overall decrease corrosion resistance within the trivalent CCC. In addition, silica based topcoat and black trivalent CCCs was also investigated and characterised using AES, SEM and LPR. Zinc whiskers was also observed from zinc electrodeposits following exposure to thermal treatment (150°C for 1 h). Elemental analysis and grain pattern investigations failed to help determine the cause of zinc whisker initiation. Zinc whiskers was seen to protrude out of hexavalent and trivalent CCCs, with the latter requiring a longer thermal exposure time.

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