Unlocking Clean Water, the Purification Mechanism of Graphene Oxide-Based Membranes 🌊

Hello guys, As you know, graphene oxide is an advanced material that can be used in the water purification industry. What is meant by water scarcity and contamination? According to UNISEF, even in countries with sufficient water resources, water scarcity remains a challenge. This issue often arises from factors such as inadequate infrastructure, contamination, conflict, and climate change, as well as human factors and poor management. 

In this previous series of articles, we have discussed the basics of graphene oxide (GO) and what makes graphene oxide a key material in modern research. Graphene oxide is used as a filter membrane (with a suitable substrate) for water purification, especially in waste water threats. So, in this article, we will discuss the purification mechanism of graphene oxide-based membranes in waste water purification. 

Figure 01: Industrial Wastewater

First of all, what is waste water, and why is waste water purification important? Waste water is water that has been contaminated by human activities, including domestic, industrial, commercial, and agricultural processes. It contains a variety of pollutants, such as organic matter, nutrients, pathogens, and chemicals, making it unsuitable for reuse without proper treatment. Rapid industrial growth in industries like batteries, electroplating, mining, circuit boards, refineries, fertilizers, textiles, and paper production generates large amounts of wastewater contaminated with heavy metals, causing environmental issues. Wastewater purification is crucial for several reasons, such as

• Environmental Protection
• Public Health
• Conservation of Water Resources

When we consider the structure of the graphene oxide, it is single-atom-thick sheets of carbon with oxygen-containing functional groups. In this case, we will explore the mechanisms used in heavy metal (or other cations) removal using graphene oxide membranes. GO membranes reject heavy metal ions through three main mechanisms. They are 

• Size exclusion
• Adsorption
• Electrostatic interactions

Figure 02: Size Exclusion, Electrostatic interaction and Adsorption mechanisms

Let's discuss these mechanisms one by one.

 

1. Size Exclusion

Graphene Oxide (GO) membranes are made up of multiple layers of GO nanosheets stacked on top of each other. The Modified Hummers Method is essential for producing highly oxidized, thin, and hydrophilic GO sheets that can be easily dispersed, and Sonication further reduces the GO sheet size, prevents aggregation, and improves dispersion, leading to well-defined nanochannels in GO membranes.

Figure 03: Schematic diagram of metal ions with two hydration shells 

going through the graphene membrane channels

When these layers are assembled, they naturally form ultrathin nanochannels between them. These nanochannels act as precise filtration pathways for water molecules while blocking larger contaminants. The interlayer spacing between GO sheets can be engineered to exclude ions based on size. As an example, let's consider Cd²⁺ ions. According to references in the aqueous solutions, ions are hydrated and expand their ionic radii according to the below graph. Normally, due to the functional group amount, the interlayer space can be varied in the range of 0.6 nm to 1.4 nm. Simply, if the hydrated heavy metal ions are larger than the interlayer spacing of GO nanosheets, then these ions would get rejected. 

But what about water molecules? Water molecules are around 0.275 nm in size. So, the interlayer spaces are enough to allow water molecules to pass through the membrane while blocking the lager contaminants.

Figure 04: Variation in hydrated radius of ions

We can refine and reduce the interlayer space by chemical methods and thermal methods, but we have to consider other performances as well rather than the rejection rate. Figure 04 shows how other performances vary with the interlayer spacing of the GO membrane. 

 Figure 05: (a) Effect of different layers separation and water pressure on water flux; (b) effect of different layers separation on membrane permeability (Molecular Dynamic simulation results)

2. Electrostatic Interaction

Graphene oxide (GO) has functional groups such as hydroxyl (-OH), carboxyl (-COOH), and epoxide (-C-O-C-) on its surface, which can carry a negative charge. In an aqueous environment, Cd²⁺ exists as a positively charged ion. The negatively charged oxygen-containing groups on GO (mainly the carboxyl and hydroxyl groups) create an electrostatic attraction with the Cd²⁺ ions. This interaction results in Cd²⁺ being drawn to the GO surface, where it is adsorbed. The stronger the negative charge on the GO surface, the stronger the electrostatic interaction with positively charged Cd²⁺ ions. 

3. Adsorption

Adsorption is the process by which ions (like Cd²⁺) adhere to the surface of the GO material. This process can occur via several types of bonding, including physical adsorption and chemical adsorption. When we consider the physical adsorption, the Cd²⁺ ions are attracted to the surface of GO via weak forces like van der Waals interactions, without any chemical reaction. In the chemical adsorption, the functional groups on GO (such as carboxyl groups) may form stronger bonds with the Cd²⁺ ions. 

The adsorption process results in Cd²⁺ ions being removed from the water and immobilized onto the surface of GO. The capacity of GO to adsorb cadmium depends on factors such as the surface area, the amount of functional groups, and the pH of the solution. 

If you want to understand more about how charged particles interact with membranes, including electrostatic repulsion and selective ion adsorption, you should explore the Donnan exclusion theory.

In the next article, we will discuss another interesting topic. Thank you for reading. Have a great day.