1.1. Interphase Mass Transfer – Application to Environmental Interfaces

Today, we will continue with the discussion of the fundamentals of transport. So, last class, we had discussed the issue of molecular diffusion, the flux of any species A generally is the combination of velocity and the concentration.

In this section, we are revisiting the fundamental concept of mass transfer, particularly how molecular diffusion and bulk flow contribute to the transport of substances in the environment. Fluids like air and water carry species A (like pollutants) where their movement is characterized by both their speed (velocity) and how concentrated they are (concentration).

Imagine you have a balloon filled with air. Over time, the air molecules will diffuse through the balloon's material, and the speed at which they escape depends on how concentrated they are inside the balloon compared to outside.

So if you are looking at the concentration, so we will make it a generic thing, so we call it as j. Generally, the flux, the molecular diffusion is jA term, bulk flow essentially means there is a velocity that is carrying the concentration.

The overall 'flux' (denoted as j) describes the flow of species A, which is influenced by both molecular diffusion (jA) and bulk flow. Bulk flow is basically the transport of concentration due to the movement of the fluid itself, while molecular diffusion is the movement of molecules from areas of high concentration to low concentration.

Think about a tea bag in a cup of hot water. Initially, the tea is concentrated in the bag, but as time passes, the tea molecules diffuse into the water. Meanwhile, if you stir the water with a spoon, you're creating a bulk flow that helps to mix the tea more quickly.

So, there are certain scenarios in the environment where the bulk flow is negligible or advection is negligible and there are scenarios where the bulk flow is much greater than molecular diffusion.

Two specific scenarios are outlined regarding mass transfer: 1) When bulk flow is negligible, molecular diffusion will dominate, and 2) When bulk flow is strong, it overshadows diffusion. In environments like deep lakes, diffusion would prevail in areas where there’s no significant flow.

Picture a still pond. In the deeper parts where there’s little movement (no wind), substances will slowly mix due to diffusion. In contrast, if you splash water in the pond, you create a strong flow that quickly disperses particles, allowing bulk flow to dominate.

Is there any system that the first one is true? Any environmental system where the advection or bulk flow is very small, the diffusion is what primarily drives mass transfer?

The discussion shifts to identifying environmental systems where diffusion might be the main mode of mass transfer. For example, in lakes where deep waters do not circulate actively, diffusion becomes vital for mixing chemicals or pollutants.

Imagine throwing a piece of food into a very deep, still aquarium. The food will slowly diffuse and break down over time, but without active water movement, the process is slow and reliant on diffusion.

When thermal convection indicates that this mixing is driven by density, density differences driven. So, when will there be thermal convections?

Thermal convection occurs when temperature differences create density variations in fluids. Warmer water is less dense and will rise, while cooler water sinks, creating a mixing effect. This process can occur in seasons such as winter when the surface cools significantly.

In winter, when outdoor temperatures drop, the surface of a lake cools. The colder denser water sinks while warmer water rises, causing mixing similar to how hot air rises in a room which leads to overall air circulation.

Groundwater essentially means it is soil, we are talking about soil systems. So, soil systems, there is a water table and there is an aquifer, these are porous media.

Groundwater movement involves both flow (which can be quite slow) and diffusion. The interaction between the water table, aquifers, and soil composition influences how pollutants are transported in groundwater systems.

Think of a sponge soaked in water. As you apply pressure, the water flows out slowly, but some water is still trapped in the sponge while it diffuses through small pores, similar to how chemicals move through soil and groundwater.

Where will there be mass transfer in the unsaturated zone? Will there be mass transfer in the unsaturated zone, this red chemical can it go into the unsaturated zone?

In unsaturated zones, mass transfer occurs primarily through diffusion since there isn’t enough water to create flow. This implies that chemicals or pollutants can move slowly through the air spaces in the soil, driven by concentration gradients.

Imagine sand in a water filter. If you pour colored water on top, the color takes time to seep down through the sand because there is limited water flow. The color spreads slowly through diffusion instead of quickly flowing down like it would in a pool.

In the sediment side, nothing is moving, everything is fixed, there is no flow, usually there is no flow.

In sediments, pollutants are likely to move mainly through diffusion since sediment typically lacks active flow. Over time, pollutants that have been deposited in sediments can take years to be observed due to the slow nature of diffusion.

Think of a pond where fish waste settles to the bottom. Over time, those waste particles gradually break down and disperse through the sediment, but this happens very slowly, similar to how stored energy depletes gradually over time.

--