O efeito de substâncias húmicas na transferência de massa de oxigênio na interface ar-água

Abstract

The oxygen transfer through the air-water interface is an important process that occurs in natural flows such as rivers, streams and lakes. It can be numerically represented by the volumetric oxygen transfer coefficient (KLa). In the literature there are a large number of studies showing how turbulence and anthropic substances affect KLa. However, there is a gap in how organic compounds, such as humic substances (HS) naturally found in water bodies, can interfere with the KLa estimative. In order to contribute to this gap in the literature, the present study aims to investigate the influence of HS on KLa under different turbulence intensities at the air-water interface. The experiments were carried out in an oscillating grid tank which provided three levels of turbulence intensities (Reynolds numbers of 5,116, 10,316 and 15,433). For each turbulence level, 15 different HS concentrations values were tested and the KLa was estimated. The results showed that, regardless of the water turbulence level in the tank, HS reduces KLa. The maximum reduction was 11%, 16% and 17%, for Reynolds numbers of 5,166, 10,316 and 15,433, respectively. The results also show that the presence of HS molecules also reduces the horizontal and vertical turbulent velocity components, along the air-water interface. The barrier effect, produced by the occupation of HS molecules at the interface, was the main phenomenon responsible for the reduction in the oxygen transfer and also in the turbulent velocity components. A relationship between the oxygen mass transfer velocity and interfacial turbulent kinetic energy showed that in the presence of HS the air-water interface behaves like a solid wall, reducing the oxygen diffusion and surface renewal. The results show that water bodies with identical hydraulic conditions may have diferrent rates of oxygen mass transfer, even when they are not polluted. This occurs due to the presence of humic substances, wgich have surfactant characteristics.