The study attempts to determine the adsorption characteristics of bed sediments of rivers for the control of metal pollution. In particular, it looks at adsorption of zinc ions on bed sediments for the river Ganga at Hardwar. In the natural conditions of river water, suspended loads and sediments have an important function of buffering higher metal concentrations of water, particularly by adsorption or precipitation.
The effect of various operating variables viz., solution pH, sediment dose, contact time and particle size on the adsorption of zinc have been studied. The optimum contact time needed to reach equilibrium is of the order of 60 minutes and is independent of the initial concentration of zinc ions.
The adsorption curves are smooth and continuous leading to saturation, suggesting the presence of monolayer coverage of zinc ions on the surface of the adsorbent. The extent of adsorption increases with an increase of pH. Furthermore, the adsorption of zinc increases with increasing adsorbent doses and decreases with adsorbent particle size. The important geochemical phases, iron and manganese oxide, act as the active support material for the adsorption of zinc ions. The adsorption data have been analysed with the help of Langmuir and Freudlich adsorption models to determine the mechanistic parameters associated with the adsorption process.
An attempt has also been made to obtain thermodynamic parameters of the process, viz, free energy change, enthalpy change and entropy change. The negative values of free energy change indicated spontaneous nature of the adsorption of zinc on the bed sediments and positive values of enthalpy change suggest the endothermic nature of the adsorption process.
The study of the adsorptive properties of the sediments can provide valuable information relating to the tolerance of the system to the added heavy metal load.
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