Gerald K. Muthakia

Fluoride adsorption onto an acid treated lateritic mineral from Kenya: Equilibrium studies

Adsorption of fluoride (F) ions from water using acid treated lateritic mineral (LM-1) from Kenya was studied by batch experiments. The effect of acid-treatment of adsorbent and change in temperature, mass of LM-1, pH and selected competing ions was evaluated. The adsorption process was strongly influenced by temperature, pH and adsorbent dosage. The percentage F removal increased the presence of the nitrate and the chlorate ions but decreased the presence of sulphates, chloride and phosphate ions. Adsorption isotherms were classified according to Giles’ classification and the adsorption data validated using Langmuir and Freundlich isotherms. The data correlated to both the Langmuir and Freundlich isotherms although the data fit to the Freundlich model was somehow better. This showed that F adsorption onto LM-1 followed a mixed adsorption mechanism in which physisorption reactions involving intra-particle diffusion of F into mesoporous sites in LM-1 became increasingly important at higher concentrations and temperatures whereas ion-exchange mechanism involving surface OH- appear to dominate at low surface coverage in more alkaline conditions. With maximum adsorption capacity of 10.5 mg/g, LM-1 could be used to remove F water. Key words: Equilibrium analysis, fluoride adsorption, Langmuir and Freundlich isotherms, Lateritic mineral adsorbent, low-cost adsorbents.

Novel chromium(III)/(VI) adducts of XPS-determined mixed valence, from electroreduced chromium(VI)

SummaryConditions of chromium(VI) and acidity have been found at which a novel brown deposit from electroreduction. Its 1∶1 chromium(III):chromium(VI) composition, and the 2∶1 ratio of the alkali-leached product, were established by XPS. The composition bounds for electrodeposition of the 1∶1 solid and the competing insulative chromium(III) film were established electrochemically. Earlier reported chromium(III) and chromium(VI) solid or gels, some deposited from alkali, differ in colour and composition properties.

A kinetic study of the oxidation of indigo carmine with acidic bromate

The oxidation kinetics of indigo carmine (disodium 3,3′-dioxobi-indolin-2,2′-ylidene-5,5′-disulphonate) with potassium bromate have been studied in aqueous sulphuric acid, by monitoring the absorbance of indigo carmine (IC) at 610 nm. The reaction involves competitive and consecutive reaction steps—an initial slow step followed by a rapid one for depletion of IC. For the initial stages the reaction order is four- first-order with respect to IC and bromate ion and second order with H+ ion. For the fast reaction step the studies are limited to qualitative treatment due to the complex nature of the reaction. The rate of depletion of IC increased with time and with the increase in HOBr concentration. Hypobromous acid, the reaction intermediate, may possibly compete with bromate ion for IC to give an intermediate, which is further oxidised to yield the final product, isatin-5-monosulphonic acid. The stoicheiometric ratio of IC to bromate is 3:2. The dual role of bromine ion as an inhibitor and autocatalyst in the reaction mechanism is discussed. Computer simulations were performed using the proposed mechanism. The results of the computer simulations are similar to the experimental observations.

Electrical conductivities of manganate-permanganate mixtures: new mixed-valence manganate(VI)-(VII) salts Ba 3 (MnO 4 ) 4 and BaK(MnO 4 ) 2

Mixed-valent salts Ba3(MnO4)4 and BaK(MnO4)2 are indicated by maximal 1:1 mixed-component conductivities, as governed by Marcus reorganisation energies.

A kinetic study of the oxidation of 2,4-dinitrophenylhydrazine with acidic bromate

The kinetics of the reaction of 2,4-dinitrophenylhydrazine with bromate ion in sulphuric acid solution have been studied by spectrophotometric techniques. In the fast initial step, 2,4-dinitrophenylhydrazine (dnph) is oxidised to the diazonium ion (dnp [graphic omitted]2). The reaction is of first-order with respect to dnph, bromate, and H+. The stoicheiometric ratio of dnph to bromate ion is 1 : 1. The intermediate diazonium ion gives the final products, including 1,3-dinitrobenzene, in a slow reaction.