Tam Tran

VETIVER GRASS, VETIVERIA ZIZANIOIDES: A CHOICE PLANT FOR PHYTOREMEDIATION OF HEAVY METALS AND ORGANIC WASTES

Glasshouse and field studies showed that Vetiver grass can produce high biomass (>100t/tha−1 year−1) and highly tolerate extreme climatic variation such as prolonged drought, flood, submergence and temperatures (−15°–55°C), soils high in acidity and alkalinity (pH 3.3–9.5), high levels of Al (85% saturation percentage), Mn (578 mg kg−1), soil salinity (ECse 47.5 dS m−1), sodicity (ESP 48%), and a wide range of heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Se, and Zn). Vetiver can accumulate heavy metals, particularly lead (shoot 0.4% and root 1%) and zinc (shoot and root 1%). The majority of heavy metals are accumulated in roots thus suitable for phytostabilization, and for phytoextraction with addition of chelating agents. Vetiver can also absorb and promote biodegradation of organic wastes (2,4,6-trinitroluene, phenol, ethidium bromide, benzo[a]pyrene, atrazine). Although Vetiver is not as effective as some other species in heavy metal accumulation, very few plants in the literature have a wide range of tolerance to extremely adverse conditions of climate and growing medium (soil, sand, and tailings) combined into one plant as vetiver. All these special characteristics make vetiver a choice plant for phytoremediation of heavy metals and organic wastes.

Solution chemistry of iodide leaching of gold

Abstract The thermodynamic equilibria and kinetic aspects of gold dissolution in iodide electrolytes have been studied with emphasis on the effect of different oxidants on the system. In conjunction with kinetic measurements, the chemix computer program was used to predict the concentration profiles of the predominant species at equilibrium in different solution conditions for the systems Au-I−-I2-H2O and Au-I−-OCl−-H2O. The thermodynamic study showed that I3− is the predominant oxidants species in both systems. However, if the concentrations of OCl− and I− are equal, solid iodine is formed. In these systems iodide (I−)_is used to form I3− (responsible for the gold oxidation) and more free iodide needed for the gold complexation is destroyed in the I−-OCl− system than the I−-I2 system. The formation of solid AuI also explains the lower rate of gold dissolution determined for certain conditions in the kinetic study. The thermodynamic modelling supports the kinetic measurements which show that, although the I−-OCl− system has a higher oxidation capacity, it does not extract gold as well as the I−-I2 system. In all cases there exist optimum oxidant/iodide ratios for achieving maximum gold extraction rates. A mixture which has the highest I3− and free I− concentration will attain the best gold extraction rate.

Froth flotation of monazite and xenotime

Abstract The flotation of monazite and xenotime using sodium oleate as collector has been investigated using a modified Hallimond tube and also by a technique which measures particle-bubble adhesion strength. The zeta potential of both minerals was also measured. There has been a wide range of values reported in the literature for the point of zero charge (pzc) of monazite, the results here indicate the pzc occurs at pH 5.3. For xenotime the pzc was found to occur at about pH 3.0, there appear to be no other values reported. Maximum floatability of both minerals occurred at pH values greater than 7, where the mineral surfaces were negatively charged suggesting that the anionic oleate collector is not physically adsorbed. The flotation results were found to correlate well with the distribution of Re(OH) 2+ species in aqueous solution, suggesting the collector is chemisorbed. The results also confirm previous indications that xenotime is slightly more floatable than monazite.

Gold dissolution in iodide electrolytes

Abstract Several oxidants (hypochlorite, iodine and hydrogen peroxide) were used to evaluate the dissolution of gold in iodide electrolytes at ambient temperatures. Evans diagrams constructed for the two half cells involved in the dissolution process show that hydrogen peroxide is not a suitable oxidant for the iodide system. The gold dissolution rate in an iodine-iodide mixture is dependent upon the concentration of iodide, iodine and the solution pH. An optimum iodine-iodide mole ratio of 0.35–0.4 was found for 0.1 M KI solutions, with pH ranging from 2.7 to 11.5; these are capable of dissolving gold at a maximum rate of 17 mg/cm2h. If a small addition of hypochlorite is made (

Kinetics and mechanism of the preparation of Raney® copper

Raney® copper is an active hydrogenation catalyst formed by the selective dissolution of aluminium from a Cu–Al alloy. The structure of Raney® copper is presented in a series of images taken using a focussed ion beam miller (FIB). The images show a structure consisting of a uniform three-dimensional network of fine copper ligaments. A rotating disc electrode, used to control the diffusion layer, enabled a study of the kinetics of the leaching reaction at 269–303u2009K in 2–8u2009m NaOH. Under these conditions, the reaction rate was constant and independent of hydroxide concentration. The activation energy for leaching was determined as 69±7u2009kJu2009mol−1. The mixed corrosion potential of the dissolving alloy has been related to the exposed copper surface area, which in turn is dependent on the leaching rate and the mechanism of rearrangement. The overall mechanism of formation/rearrangement of the Raney® copper structure was found to be mainly dissolution/redeposition of copper atoms, with surface or volume diffusion, or possibly both, playing a minor role.

Cr2O3 promoted skeletal Cu catalysts for the reactions of methanol steam reforming and water gas shift

Abstract Promotional effects of chromia on the structure and activity of skeletal copper catalysts for methanol steam reforming and water gas shift have been studied. Catalysts were prepared by leaching CuAl2 alloy particles in aqueous NaOH solutions containing sodium chromate at various concentrations. XPS spectra showed that the surface of the resulting catalysts mainly consisted of Cr3+ compounds and Cu0. Cu+ and/or Cu2+ were not observed by XPS. Increasing the concentration of chromate in the leach liquor resulted in decreases in pore diameter and copper crystallite size but significant enhancement of BET surface area was observed while the total pore volume was maintained. The addition of small amounts of chromate to the leach liquor significantly enhanced the Cu surface area. However, higher concentrations of chromate in the leach liquor decreased the Cu surface areas although the total surface areas increased. The activities of Cr2O3 promoted skeletal copper catalysts for both methanol steam reforming and water gas shift reactions were determined separately. The results indicated that deposition of Cr2O3 on skeletal copper catalysts significantly improved the specific activities for these reactions. Chromia is found to act as a structural and catalytic promoter for these reactions.

The use of ion exchange resins for the treatment of cyanidation tailings. Part 1-process development of selective base metal elution

Abstract This work investigates the use of oxidative acid eluents for the elution of base metals from strong base ion exchange resins. Eluents composed of a mixture of H 2 O 2 and H 2 SO 4 were tested for eluting base metals from resins loaded with mixtures of base and precious metal cyanides. This process removed 100% of Cu and Zn loaded on the resin, without affecting the precious metal loading. It was found that copper could be removed separately from the other base metals. The elution technique was not effective for removing iron from the resin. Cyanide associated with base metals was recovered as NaCN. Some oxidation of cyanide was noticed, subject to the elution conditions. This oxidative acid elution process could be used in commercial operations for the selective elution of base metals from a strong base ion exchange resin bed operating in alternative adsorption/base metal elution cycles. Thus, virtually all metal cyanide species could be recovered from cyanide leached solutions or slurries to give relatively clean tailings without compromising precious metal recovery efficiency. The process also caters for cyanide recovery and recycling.

Copper interaction during the dissolution of gold

Abstract A certain amount of gold is unknowingly lost in the presence of native copper, most probably due to a cementation mechanism, during extraction from its ores. In this study, the cementation of gold onto copper was evaluated using suspended particles of synthetic and native copper. It was shown that metallic copper causes the cementation of gold from gold cyanide solution, the rate of which is strongly dependent on temperature and the initial concentration of cyanide and copper. In the presence of excessive cyanide and dissolved oxygen, the cemented gold redissolves. Thermodynamic study, based on the CHEMIX program, showed that the distribution of copper and gold cyanide species in the copper-gold-cyanide-water system is strongly dependent on the solution pH and initial CN − /Cu molar ratio (copper and cyanide concentrations) and confirmed the experimental trends observed.

A kinetic study of the cementation of gold from cyanide solutions onto copper

Abstract The kinetics of cementation of gold onto copper was evaluated using a copper rotating disc and suspended particles systems. It was found that the cementation of gold onto copper is controlled by a kinetic step. For both rotating disc and suspended particles systems, the effects of temperature, cyanide and gold concentration on the rate of cementation are more significant than those of rotation speed and solution pH. The kinetic rate constant of cementation of gold onto copper is comparatively smaller than those found for the gold-zinc-cyanide or silver-copper-chloride systems. The activation energy was found to be 53.4 and 54.1 kJ/mol for copper rotating disc and suspended particles, respectively, which values are close to the measured value of 56.9 kJ/mol found in the electrochemical study. The morphology of the deposits, determined by Field Emission Scanning Electron Microscope, confirmed observations from experimental measurements, showing the passivation or acceleration of cementation due to the different types of deposit formed.

FIB preparation of a sensitive porous catalyst for TEM elemental mapping at high magnifications

Doped skeletal copper catalysts, created by selective dissolution of aluminium from a copper-aluminium alloy by caustic in the presence of an additive, has been imaged at very high magnification using a TEM. The location of the additive species has been determined by elemental mapping using an attached EDS detector. Zinc resides primarily inside the copper ligaments while chromium resides on the outside of the ligaments. Since skeletal copper is highly porous and extremely sensitive, conventional TEM preparation techniques could not be used. The preparation of the TEM samples was carried out using a focussed ion beam (FIB) miller. Several methods are outlined, along with the reasons some of them were unsuccessful for this particular material. The successful result indicates this technique may be used to analyse similar skeletal materials, such as nickel, cobalt, etc., which are otherwise difficult to prepare. It may also provide a basis for high resolution imaging or elemental analysis of other porous and/or highly sensitive materials.