Dion Ewing Giles

Review of fluoride removal from drinking water

Fluoride in drinking water has a profound effect on teeth and bones. Up to a small level (1-1.5mg/L) this strengthens the enamel. Concentrations in the range of 1.5-4 mg/L result in dental fluorosis whereas with prolonged exposure at still higher fluoride concentrations (4-10mg/L) dental fluorosis progresses to skeletal fluorosis. High fluoride concentrations in groundwater, up to more than 30 mg/L, occur widely, in many parts of the world. This review article is aimed at providing precise information on efforts made by various researchers in the field of fluoride removal for drinking water. The fluoride removal has been broadly divided in two sections dealing with membrane and adsorption techniques. Under the membrane techniques reverse osmosis, nanofiltration, dialysis and electro-dialysis have been discussed. Adsorption, which is a conventional technique, deals with adsorbents such as: alumina/aluminium based materials, clays and soils, calcium based minerals, synthetic compounds and carbon based materials. Studies on fluoride removal from aqueous solutions using various reversed zeolites, modified zeolites and ion exchange resins based on cross-linked polystyrene are reviewed. During the last few years, layered double oxides have been of interest as adsorbents for fluoride removal. Such recent developments have been briefly discussed.

Iron and aluminium based adsorption strategies for removing arsenic from water

Arsenic is a commonly occurring toxic metal in natural systems and is the root cause of many diseases and disorders. Occurrence of arsenic contaminated water is reported from several countries all over the world. A great deal of research over recent decades has been motivated by the requirement to lower the concentration of arsenic in drinking water and the need to develop low cost techniques which can be widely applied for arsenic removal from contaminated water. This review briefly presents iron and aluminium based adsorbents for arsenic removal. Studies carried out on oxidation of arsenic(III) to arsenic(V) employing various oxidising agents to facilitate arsenic removal are briefly mentioned. Effects of competing ions, As:Fe ratios, arsenic(V) vs. arsenic(III) removal using ferrihydrite as the adsorbent have been discussed. Recent efforts made for investigating arsenic adsorption on iron hydroxides/oxyhydroxides/oxides such as granular ferric hydroxide, goethite, akaganeite, magnetite and haematite have been reviewed. The adsorption behaviours of activated alumina, gibbsite, bauxite, activated bauxite, layered double hydroxides are discussed. Point-of-use adsorptive remediation methods indicate that Sono Arsenic filter and Kanchan™ Arsenic filter are in operation at various locations of Bangladesh and Nepal. The relative merits and demerits of such filters have been discussed. Evaluation of kits used for at-site arsenic estimation by various researchers also forms a part of this review.

Cyanide thermodynamics 2. Stability constants of copper(I) cyanide complexes in aqueous acetonitrile mixtures

The stability (formation) constants of the binary Cu(I)-CN(-) complexes have been measured in five aqueous mixtures containing from 10% to 70% v/v acetonitrile (MeCN) by glass electrode potentiometry at 25 degrees C and an ionic strength of l M (NaClO(4)). The constants show monotonic increases with MeCN concentration, the changes being greatest for the higher order complexes, consistent with the unfavourable solvation of CN (-) in these mixtures. The sparing solubility of CuCN(s) prevented determination of the stability constant for CuCN (0) (soln.) at low MeCN concentrations.

The kinetics of dissolution of slaked lime

The rates of dissolution of Ca(OH)2 in the form of rotating discs have been studied in water and in aqueous calcium nitrate and sodium hydroxide solutions, and have been found to be consistent with the rates of combined slaking and dissolution of CaO discs. A similar consistency has been found between rates of dissolution of powdered Ca(OH)2 and of powdered CaO in water. Our results support a mechanism for CaO slaking in which under moderate agitation the overall rate is controlled by diffusion of Ca2+ and OH− ions from the reacting surface, and under more vigorous agitation the dissolution of Ca(OH)2 at the surface controls the rate.

Reactions of lime with carbonate-containing solutions

In the Bayer process for refining alumina, lime (either quicklime or slaked lime) is added to the process liquor to precipitate calcium carbonate and restore alkalinity to the solution. The work described in this paper was undertaken to elucidate the mechanism of this precipitation reaction about which surprisingly little is known. In the first part of the study, the composition of calcium oxide disks after reacting with sodium carbonate solutions was examined by XRD and TGA. TGA results show that the amount of calcium carbonate formed on the disks increases with sodium carbonate concentration but the amount of calcium hydroxide reaches a minimum at a solution concentration of about 0.05 M carbonate and then increases at higher carbonate concentrations. The kinetics of the formation of calcium carbonate were also followed using HPLC to monitor the change in bulk carbonate concentration due to the reaction of powdered lime in sodium carbonate solutions. The effect of the agitation speed and temperature on the reaction rate indicates that the formation of calcium carbonate is a diffusion-controlled process. To account for these findings it is suggested that at concentrations above 0.05 M carbonate a more porous heterogeneous CaCO3–Ca(OH)2 layer appears owing to direct attack on the surface by carbonate before a layer of calcium hydroxide can form. Scanning electron micrographs of disks after reaction in carbonate solutions support this suggestion. The second part of this study deals with the dissolution of lime in the presence of sodium carbonate. The concentration of Ca2+ from dissolution of calcium hydroxide disks was determined by AAS. It was found that the dissolution process is slowed by the formation of calcium carbonate, and is diffusion controlled. The reaction of powdered calcium oxide in the sodium carbonate solution was followed by calorimetry. It was found that the relationship between the extent of dissolution and the carbonate concentration is similar to that obtained by TGA for the formation of calcium carbonate, suggesting the same mechanism.

The leaching of copper from sulphur activated chalcopyrite with cupric sulphate in nitrile-water mixtures☆

If chalcopyrite is roasted with sulphur at 400–450°C pyrite and idaite or bornite are produced. Bornite plus pyrite are also prepared by roasting a 1:1 mixture of chalcopyrite and covellite. These copper-iron sulphides were leached with acidified aqueous cupric sulphate solutions containing acetonitrile or hydracrylonitrile and the results are compared with leaching with acidified cupric chloride in brine. The nitrile route has the advantage of a less corrosive sulphate medium for subsequent copper recovery processes. Bornite appears to be the most attractive product from the roasting of sulphur and chalcopyrite because much of its copper can be readily leached. Iron reports to the solution only in the latter stages of extraction. Up to 80% of the copper in this bornite is leached with CuSO4/RCN/H2O at 60°C. Copper is recovered from the resulting cuprous sulphate solution by electrowinning with inert anode. The products are copper cathodes and cupric sulphate, which is recycled. The leach residue may be used to reactivate further chalcopyrite or is leached of its copper by established routes.

The kinetics of copper dissolution in acetonitrile-water copper(II) solutions

The rate of dissolution of rotating copper discs in acidified copper(II) solutions prepared in an acetonitrile-water solvent (mole fraction acetonitrile = 0.·25) was measured in the temperature range 270–360 K. The rate of dissolution was found to be proportional to the square root of the rotation speed and to have an activation energy of 18 ± 2kJ mol−1. The reaction is therefore considered to be diffusion controlled. The reduction of copper(II) to copper(I) on a rotating platinum electrode, and the oxidation of copper(O) to copper(I) on a rotating copper electrode, were investigated. These two reactions together make up the dissolution reaction. From the intersection of the polarization curves, it was confirmed that the dissolution reaction was diffusion controlled. Dissolution rates calculated from the electrochemical measurements agreed with the kinetic measurements. Estimates of some electrochemical parameters in the acetonitrile mixed solvent were made. The surface characteristics of annealed and unannealed copper discs which had been etched in the copper(II) solution are discussed briefly.

Cuprous hydrometallurgy Part IV. Rates and equilibria in the reaction of copper sulphides with copper(II) sulphate in aqueous acetonitrile

Copper(II) sulphate in solutions of aqueous acetonitrile leaches copper from copper sulphides to form stable copper(I) sulphate solutions. Covellite and chalcopyrite are oxidised and leached more rapidly in the early stages of leaching with acidic CuSO4/CH3CN/H2O than with acidic iron(III) sulphate in water. A redox equilibrium between copper(I) sulphate, copper(II) sulphate and partially leached solid copper sulphide, CuxS, is established. The equilibrium concentration of Cu2SO4 and the value of x in CuxS, in solutions saturated with CuSO4, are interdependent if the acetonitrile concentration is constant. This behaviour is considered in terms of the structural and electrochemical changes which occur, in the solids Cu2S and CuS, as leaching proceeds. According to the activities of acetonitrile, of copper(I) sulphate and of copper(II) sulphate, i.e. according to the redox potential of the solution, CuS either can be oxidised by copper(II) sulphate to a less copper-rich copper sulphide and even to sulphur, or reduced by copper(I) sulphate to a more copper-rich sulphide, up to Cu2S, in acidic solutions containing CuSO4, Cu2SO4, CH3CN and water. This observation leads to an easy method of generating Cu2S from CuS or from sulphur.

Cyanide thermodynamics. 1. Stability constants of cadmium(II) and zinc(II) cyanide complexes in aqueous acetonitrile mixtures

Abstract Stability constants for the cyanide complexes of zinc(II) and cadmium(II) have been determined in five acetonitrile-water mixtures containing up to 70% (v/v) MeCN. The constants were measured at 25°C and an ionic strength of 1M NaClO4 by high precision glass electrode potentiometry. Complexes containing up to four cyanide ions have been detected for both Zn(II) and Cd(II) although the lower order complexes ZnCN+ and Zn(CN)2° are difficult to quantify because of the sparing solubility of the zinc cyanide salt. The stability constants for both systems increase monotonically with MeCN concentration and show a striking similarity. Evidence for the formation of ternary hydroxocyano-metal ion complexes was obtained at high pH values but their stability constants could not be evaluated because of the lack of quantitative information for the binary M(II)-OH− species.