2023 VT Nanoscience Professional Development Workshop for High School Teachers

Hands-On Activity: Nano-Enabled Water Treatment

Introduction: Due to their fundamental physical and chemical characteristics, as well as their unique properties and behaviors, nanomaterials are increasingly recognized as valuable tools for water treatment. Here are several reasons why nanomaterials are useful in water treatment:

Increased Surface Area: Nanomaterials possess a high surface area-to-volume ratio. This characteristic allows for more contact points between the nanomaterial and the water, facilitating enhanced adsorption and catalytic reactions.
Enhanced Adsorption Capacity: Nanomaterials can be engineered to have specific surface properties that promote the adsorption of contaminants and functionalized to selectively target and remove pollutants such as heavy metals, organic compounds, and even pathogens from water.
Improved Catalytic Activity: Some nanomaterials exhibit excellent catalytic properties, which can be utilized to generate reactive oxygen species, such as hydroxyl radicals, which can effectively degrade organic pollutants in water.
Antibacterial Properties: Certain nanomaterials possess inherent antibacterial properties, which can be beneficial for water disinfection.
Reactivity Towards Emerging Contaminants: Nanomaterials offer potential solutions for removing emerging contaminants that conventional water treatment methods struggle to address such as pharmaceuticals, personal care products, and microplastics.

Goal: To demonstrate how basic nanomaterial characteristics influence properties and behaviors, making them useful for water treatment. This activity will use a Martian soil simulant to show how a small amount of a natural nanomaterial (known as ferrihydrite) can effectively remove organic "pollutants" from simulated wastewater. An overview of the procedure is given below along with key concepts for achieving learning objectives.

This activity is supported by pre-recorded lectures and slides on Water Treatment and Martian (Nano)Mineralogy.

Setting up the Activity

Challenge Scenario for Students: Imagine you are an astronaut living on Mars similar to Matt Damon in The Martian and your water purification system has failed. You have been collecting and storing wastewater but have no freshwater left, so you need to act quickly.

After introducing nano-related concepts such as increased surface area, enhanced surface reactivity, amphoterism, protonation/deprotonation, and surface charge, students can collaborate to devise methods for wastewater treatment using Martian soil that contains a natural nanomaterial, ferrihydrite.

Step 1: Prepare a Simulated Wastewater

We are using a blue Neon Food Coloring purchased from Kroger. This particular color is made from water, propylene glycol, and propylparaben. The images one the left show the color from one drop of food coloring mixed with 40 mL water (stock solution). We diluted the solution by mixing 10 mL of stock with 40 mL water.

Key Concept: Dyes and other types of organic molecules (natural or anthropogenic source), as well as oxyanions e.g., sulfate, phosphate, and nitrate have a net charge to to protonation and deprotonation reactions. Food coloring dyes tend to have a negative charge in solution at near-neutral pH.

Step 2: Prepare Soil Samples

A Martian Soil Simulant was purchased from the Exolith Lab and the University of Central Florida. The Mars Global (MGS-1) High Fidelity Martian Dirt Simulant contains 11 minerals including 3.5 wt.% ferrihydrite. Ferrihydrite, a ferric hydroxide, is a highly reactive natural nanomaterial that is ubiquitous in Mars soil, as well as in many surface environments on Earth. Play sand was obtained from a local home and garden store.

Weight approximately 8-9 grams of Martian simulant and 8-9 grams of play sand and transfer into separate containers that are transparent (images show 50 mL Falcon tubes).

Key Concept: The surfaces of ferrihydrite and other transition metal oxides and hydroxides are amphoteric, which means their surfaces undergo pH-dependent protonation and deprotonation reactions. Metal oxides typically have oxygen atoms on their surfaces, which can bind with protons (H+) from an acidic solution, leading to surface protonation. The proportions of OH2+, O2-, and OH (neutral) result in the net surface charge. The surfaces of ferrihydrite are positive at pH <8 (the point of zero charge, PZC) and negative at pH >8.

Step 3: Mix Simulated Wastewater with Soils

Transfer 50 mL of the simulated wastewater (diluted dye solution) into each of the containers containing soil samples and mix thoroughly (shake gently for 15-20 seconds). Place the containers side by side and allow to wait approximately 10-15 minutes for particles to settle at static conditions.

Step 4: Compare Mars Soil and Play Sand

Notice how the blue color is removed from the water mixed with the Mars simulant (MGS, right) but retained in the case of the play sand (PS, left). If available a centrifuge can help to remove the fine particles that remain suspended in the water overlying the sediments.

Key Concept: The organic dye molecules are removed from solution in the MGS sample due to chemical or physical adsorption to the mineral surfaces. The interaction is driven by the opposite charges of the dye molecules (negative) and the mineral surfaces, especially ferrihydrite which is net positive at near neutral pH. Although ferrihydrite is only 3.5 wt.% of the sample it has an outsized effect due to its nanosized particle size, which results in a large amount of reactive surface area (100's meters squared per gram). This means that a small amount of a nanomaterial can have a large impact on the dynamics of pollutants in chemical systems.

Alternative: Column Experiments

This video link shows how column experiments can be used in place of mixing the soil and wastewater in batch reactors (as described above). Note that the video shows the addition of dryer lint to simulate microplastics in wastewater. The dryer lint particles also have a net negative charge so they are removed by adsorption and due to physical trapping in the sediments.

Supply List (materials shown in video):

Large syringes (link)
Luer lock valves (link) (link)
Simulated Martian Exolith (link)
Play sand as control filter media (no NPs)
Food coloring (simulate contaminant)
Nylon mesh (link) or coffee filter
Ring stand with clamps
Large glass or plastic beaker or container (transparent)