Equations and their physical interpretation in numerical modeling of heavy metals in fluvial rivers

Based on the previous work on the transport-transformation of heavy metal pollutants in fluvial rivers, this paper presented the formulation of a two-dimensional model to describe heavy metal transport-transformation in fluvial rivers by considering basic principles of environmental chemistry, hydraulics, mechanics of sediment transport and recent developments along with three very simplified test cases. The model consists of water flow governing equations, sediment transport governing equations, transport-transformation equation of heavy metal pollutants, and convection-diffusion equations of adsorption-desorption kinetics of particulate heavy metal concentrations on suspended load, bed load and bed sediment. The heavy metal transport-transformation equation is basically a mass balance equation, which demonstrates how sediment transport affects transport-transformation of heavy metals in fluvial rivers. The convection-diffusion equations of adsorption-desorption kinetics of heavy metals, being an extension of batch reactor experimental results and a major advancement of the previous work, take both physical transport, i.e. convection and diffusion and chemical reactions, i.e. adsorption-desorption into account. Effects of sediment transport on heavy metal transport-transformation were clarified through three examples. Specifically, the transport-transformation of heavy metals in a steady, uniform and equilibrium sediment-laden flow was calculated by applying this model, and results were shown to be rational. Both theoretical analysis and numerical simulation indicated that the transport-transformation of heavy metals in sediment-laden flows with clay-enriched riverbed possesses not only the generality of common tracer pollutants, but also characteristics of transport-transformation induced by sediment motion. Future work will be conducted to present validation/application of the model with available data.

EXPERIMENTS ON SELF-PURIFICATION OF HEAVY METALPOLLUTANTS IN NATURAL RIVERS

This paper is concerned with the experiments of simulating self-purification of heavy metal pollutants in natural rivers, such as tributaries polluted by heavy metal polutants joining a main stream, or an uncontaminated tributary with much sediment entering a main stream polluted by heavy metal pollutants, and the experiments on the effect of sediment movement on the adsorption of heavy metal pollutants. Further-more, the effect of turbulence, sediment supply conditions and sediment motion patterns on the adsorption of heavy metal pollutants are studied in a device, in which turbulence is generated by a harmonically vibrating set of grids. The results indicate that sediment motion pattern has no effect on the characteristic parameters in the adsorption isotherm, such as the saturated adsorption content by unit weight of seidment and the constant of adsorption-desorption rate in the Langmuir adsorption isotherm, but it has significant effect on the coefficient of adsorption rate (or the coefficient of desorption rate). When the turbulent intensity of flow is low and all sediment exists as bottom mud, the coefficient of adsorption rate of sediment (or the coefficient of desorption rate of sediment) is very small, thus descending rate of concentration of dissolved heavy metal pollutant concentration is small, and when the turbulent intensity of flow is high, sediment supply condition, that is, whether sediment exists on the bed or not, influences greatly the variation of dissolved heavy metal pollutant concentration. When the sediment on the bed could be lifted up, descending rate of dissolved heavy metal pollutant concentration is large. In other wards, the coefficient of adsorption rate (or the coefficient of desorption rate) by unit weight of suspended sediment is much larger than that by unit weight of bottom mud.