Experimental determination and CFD modeling of diffusion and advection of naphthenic acids in water

• Experimentally derived diffusion coefficients and advection velocities for Naphthenic acids • CFD simulations of laboratory-scale research through flume and tank experiments • Providing transport parameters for the initial fate of Naphthenic acids of oil spills in surface waters Naphthenic acids (NAs) are a group of polar carboxylic acids that naturally occur in crude oil and are considered to be the major source of environmental pollution due to the extraction of oil sands. While there have been many modeling studies focused on their transport in complex matrices, there is still no experimentally derived molecular diffusion coefficients and advection velocities for density-driven commercial NAs in pure water. This laboratory study helps fill this void by using a recirculating flume for longitudinal transport and a static tank for vertical sinking, with changes in concentration monitored using chemical oxygen demand (COD) and electrical conductivity (EC). The outcomes demonstrate an initial sinking to the bottom, controlled by very small density differences or injection effects, after which buoyancy, driven upward migration, and surface spreading take place. The molecular diffusion coefficient was used as 0.69 × 10⁻⁹ m²/s, while the advection velocities that were obtained were checked against the two, dimensional CFD simulations in Ansys Fluent 2021 R2 (RMSE = 2. 1 mg/L for outlet concentrations; NSE = 0. 82). These standard parameters set the stage for basic understanding of the early life of the low, solubility heavy oil pollutants in water, thereby showing how critical the role of subtle buoyancy effects is.