Dinâmica interfacial da transferência de massa de oxigênio: uma avaliação mecanicista entre surfactantes químicos e biossurfactantes em um canal hidráulico

Abstract

Oxygen mass transfer at the air-water interface, quantified by the k2, is the fundamental mechanism for the self-purification of aquatic ecosystems and the efficiency of sewage treatment processes. Historically, traditional k2 predictive models assume unrealistic hydrodynamic homogeneity and rely almost exclusively on macroscopic parameters, neglecting the physicochemical interactions imposed by surface contaminants. To bridge this gap, this research investigates the mechanistic influence of surfactants on the interfacial dynamics of oxygen transfer, comprehensively evaluating the interactive effect between the nature of the surfactant (chemical versus biological), its concentration, and the modulation driven by the liquid phase temperature. The study was structured along three main axes: initially, a systematic literature mapping highlighted the significant deficit of studies addressing biosurfactants under environmental conditions. Next, a full factorial experimental design was carried out in a recirculating hydraulic flume, evaluating chemical surfactants (SDS and LABSA) and ecological biosurfactants (RHEANCE® One and REWOFERM® SL ONE) at submicellar concentrations of 1 and 2 mg L-1, under isotherms of 20°C and 27°C. Finally, a physicochemical modeling approach supported by a zonal analysis was developed, challenging the use of hydrodynamic averages to assess the spatial decay of the flow. The results proved that adopting global averages masks the true interfacial physics. Through an analysis of covariance (ANCOVA), a highly significant interaction between temperature and concentration was found, revealing that temperature operates in the system as a “thermal switch”. It was observed that, at 20°C, increasing the surfactant concentration promoted an increase in k2, a phenomenon driven by the stabilization of “Marangoni Convection” cells that act as micro-stirrers in low-turbulence zones. In contrast, heating the fluid to 27°C triggered a drop in viscosity and accelerated the adsorption of a stagnant cap, activating the “Barrier Effect” and drastically collapsing k2 for all tested surfactants. Despite the general inhibition at high temperatures, biosurfactants demonstrated a significantly lower negative impact on oxygen transfer than chemical surfactants ones. This structural advantage stems from their “steric porosity”: the bulky hydrophilic heads of biological macromolecules undergo steric hindrance and prevent the dense packing of the monolayer at the interface, ensuring a superior residual permeability to oxygen that does not occur in the rigid films formed by the linear chains of chemical compounds. It is concluded, therefore, that the presence of surfactants, actively modulated by the thermal temperature of the medium, compromises the water reoxygenation capacity. The refinement of water quality models requires the immediate incorporation of rheological and spatial corrections to avoid the dangerous overestimation of self-purification and prevent the formation of hypoxic zones. In this scenario, biosurfactants emerge as a viable and strategic green chemistry technology, mitigating asphyxiation in water bodies and reducing the energy penalty in industrial aeration systems.