Abatement of fluoride from water using manganese dioxide-coated activated alumina.

Batch adsorption of fluoride onto manganese dioxide-coated activated alumina (MCAA) has been studied. Adsorption experiments were carried out at various pH (3-9), time interval (0-6 h), adsorbent dose (1-16 g/l), initial fluoride concentration (1-25 mg/l) and in the presence of different anions. Adsorption isotherms have been modeled using Freundlich, Langmuir and Dubinin-Raduskevich isotherms and adsorption followed Langmuir isotherm model. Kinetic studies revealed that the adsorption followed second-order rate kinetics. MCAA could remove fluoride effectively (up to 0.2 mg/l) at pH 7 in 3h with 8 g/l adsorbent dose when 10mg/l of fluoride was present in 50 ml of water. In the presence of other anions, the adsorption of fluoride was retared. The mechanism of fluoride uptake by MCAA is due to physical adsorption as well as through intraparticle diffusion which was confirmed by kinetics, Dubinin-Raduskevich isotherm, zeta-potential measurements and mapping studies of energy-dispersive analysis of X-ray.

Chlorination byproducts, their toxicodynamics and removal from drinking water.

No doubt that chlorination has been successfully used for the control of water borne infections diseases for more than a century. However identification of chlorination byproducts (CBPs) and incidences of potential health hazards created a major issue on the balancing of the toxicodynamics of the chemical species and risk from pathogenic microbes in the supply of drinking water. There have been epidemiological evidences of close relationship between its exposure and adverse outcomes particularly the cancers of vital organs in human beings. Halogenated trihalomethanes (THMs) and haloacetic acids (HAAs) are two major classes of disinfection byproducts (DBPs) commonly found in waters disinfected with chlorine. The total concentration of trihalomethanes and the formation of individual THM species in chlorinated water strongly depend on the composition of the raw water, on operational parameters and on the occurrence of residual chlorine in the distribution system. Attempts have been made to develop predictive models to establish the production and kinetics of THM formations. These models may be useful for operational purposes during water treatment and water quality management. It is also suggested to explore some biomarkers for determination of DBP production. Various methods have been suggested which include adsorption on activated carbons, coagulation with polymer, alum, lime or iron, sulfates, ion exchange and membrane process for the removal of DBPs. Thus in order to reduce the public health risk from these toxic compounds regulation must be inforced for the implementation of guideline values to lower the allowable concentrations or exposure.

Adsorption of Co2+, Ni2+, Cu2+ and Zn2+ from 0.5 M NaCl and major ion sea water on a mixture of delta-MnO2 and amorphous FeOOH.

The pH(pzc) values of several mechanical mixtures of amorphous hydrous oxides of iron (amorphous FeOOH) and manganese (delta-MnO2) have been determined using the solid addition method. While the pH(pzc) of delta-MnO2 remains almost unchanged, the corresponding value for amorphous FeOOH tends to increase with increased proportion of delta-MnO2 in the mixtures. The adsorption behavior of Co2+, Ni2+, Cu2+, and Zn2+ with respect to pH on a mechanical mixture of 70% delta-MnO2 and 30% amorphous FeOOH from 0.5 M NaCl and major ion sea water has been studied. Since delta-MnO2 is much more active adsorbent than amorphous FeOOH at pH below 6.5, the adsorption data on mixture have not only been normalized with respect to the mass of delta-MnO2 in the mixture, but also compared with adsorption data on delta-MnO2 alone. It is interesting to note that though each trace metal behaves in a different way from the other especially with respect to the nature of electrolyte medium, it is generally observed that the adsorption on the mixed oxide system is higher than that on delta-MnO2 alone under similar condition. It is also observed that adsorption in major ion sea water at a particular pH value is lower than in 0.5 M NaCl solution.

Adsorption of Co, Ni, Cu, and Zn on hydrous manganese dioxide from complex electrolyte solutions resembling sea water in major ion content.

Adsorption of Co, Ni, Cu, and Zn onto a poorly crystalline hydrous manganese dioxide (delta-MnO2) has been studied in complex electrolyte solutions such as (a) 0.5 M NaCl+0.054 M MgCl2, (b) 0.5 M NaCl+0.028 M Na2SO4, and (c) artificial sea water prepared according to the standard literature method. These three solutions allow us to identify the specific effect of major cations, major anions, and the mixture of major cations and anions (including carbonate and bicarbonate) that is present in real sea water. The adsorption isotherm in major ion sea water at pH 7.25 indicates that while Co and Zn exhibit increases in adsorption with increase in concentration, Ni shows relatively poor adsorption, reaching a plateau at 0.075 mM concentration. The three trace metals (Co, Ni, and Zn) show Langmuirian behavior for adsorption at low concentration. It is generally observed that the fractional adsorption vs pH curve shifts to higher pH either in the presence of 0.054 M MgCl2 or in sea water. In the presence of 0.028 M Na2SO4 the fractional adsorption vs pH curve remains almost unchanged with respect to a 0.5 M NaCl solution. The competitive adsorption of one trace metal in the presence of other three in major ion sea water indicates that this phenomenon is more predominant with Ni and Zn than with Co and Cu.

Adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide from simple (1-1) electrolyte solutions.

The adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide (delta-MnO2) has been studied using two methods, viz., isotherms at constant pH in the presence of buffer solution and pH variation in the absence of buffer solution from a fixed metal ion concentration. While the adsorption isotherm experiments were carried out in 0.5 M NaCl only, pH variation or batch titration experiments were carried out in 0.5 M NaCl, 0.01 M NaCl, and 0.01 M KNO3 solutions. The complex nature of adsorption isotherms at constant pH values indicates that adsorption of all the cations is non-Langmuirian (Freundlich) and takes place on the highly heterogeneous oxide surface with different binding energies. The proton stoichiometry derived from isotherms at two close pH values varies between 0.3 and 0.8. The variation of fractional adsorption with pH indicates that the background electrolyte solution influences the adsorption of cations through either metal-like or ligand-like complexes with Cl-, the former showing a low adsorption tendency. The proton stoichiometry values derived from the Kurbatov-type plot varies not only with the electrolyte solution but also with the adsorbate/adsorbent ratio. The variation of fractional adsorption with pH can be modeled either with the formation of the SOM+ type or with a combination of SOM+ and SOMOH type complexes, depending upon the cation and electrolyte medium. The equilibrium constants obtained from Kurbatov-type plots are found to be most suitable in these model calculations. Adsorption calculated on the basis of ternary surface metal-chlorocomplex formation exhibits very low values.

The Electrical Double Layer at Hydrous Manganese Dioxide/Electrolyte Interface.

The interfacial properties of two hydrous oxides of manganese, namely an amorphous variety and a poorly crystalline synthetic birnessite, in different electrolyte media, have been studied by using a batch potentiometric titration procedure with a 72-h equilibration period. The data obtained by this procedure are more relevant to a natural water system than those obtained by the rapid titration method as reported earlier [J. Colloid Interface Sci. 131, 103 (1989).]. Although the points of zero charge do not show any major variations, the surface dissociation and complexation (with Na(+) in NaCl) constants differ with the method of titration. Both of the samples are characterized by their high surface charge (low DeltapK(a)) and surface potential, and as a result, complexation takes place possibly within the inner helmholtz plane. It is concluded that the triple-layer model is not applicable to this colloid system and that a basic Stern model would be more suitable in this case. Copyright 2001 Academic Press.