Aerogel: Modification
Week #02: Requirements For A Functioning Aerogel Filter
Preface ✨
Hello everyone!
My intentions for writing these articles are:
- Explain technical knowledge about aerogels in simple terms (to the public)
- Store information and habits for my future self and others (in <7 minutes)
Coolio? Sweet. Enjoy the series :-)
Alginate Aerogel: New Information 📃
Alginate aerogels mainly consist of macroporous beads. The holes in the material are larger than 50 nanometers.
Alginate is a prime material because:
- Alginate hydrogel perlas are a highly dispersed (spread out) structure. The reason is that the material can contain up to 98% water. It allows the alginate chemical groups to create the 3-D structure/network of a perla.
- But why? The main chemical groups in alginate are hydroxyl (-OH) & carboxyl (-COOH) groups. These groups are negatively charged. Heavy metal ions tend to be positively charged. Just like a magnet: Opposites attract.
But here is the new problem with -OH & -COOH groups:
- These groups have the natural tendency to be hydrophilic. This means they LOVE water to be near them. So they will attempt to bring as many water molecules near them as possible.
- Heavy metal ion bonds could be displaced by the water and the adsorption uptake fails to be sustained.
- The pH is also a concern for real-world applications. The adsorption rate of the perlas goes up as long as the pH goes up.
Alginate’s tendency is to effectively adsorb heavy metal ions. This tendency cannot be allowed in our filters.
So, Carlos, how do you fix this dilemma?
The Solution: Cross-Linking ⛓️
Cross-Linking is the action of a chemical group linking/combining two different polymer groups. These links are either covalent bonds or ionic bonds.
Covalent Bonds → Two atoms share their electrons equally.
Ionic Bonds → Two atoms share their electrons unequally; One atom has both electrons, and the other has lost one.
Our aerogels must have a covalent bond to optimize their use for real-world applications.
Our objective to optimize applications of alginate perlas via cross-linking is as follows:
- Clear visual improvement of adsorption capability/capacity compared to the raw material
- Increasing the pH range for the application of the perlas
- Strengthen the stability of the perlas
- Increase the preservation of the perlas during the manufacturing process
But why?
- Modification of the raw material can improve adsorption, desorption, recycling, etc. of the perlas
- The pH of water across Latin America ranges from pH 4 - pH 6
- The perlas must resist outside forces like squeezing, running water, etc.
- We would not have the desired properties of the filter if we fail to preserve the perlas during manufacturing
But what are these desired properties you want in your filter? What are your idea’s values?
Requirements 🖋️
An adsorbent that is effective in the treatment of various pollutants should meet several requirements:
- Effective at removing a variety of heavy metal ion pollutants.
- An effective adsorption rate of >95%
- A high selectivity at different concentrations to examine removal efficiency for one metal versus another.
- The Perla size can offer a surface area >85 m²/g and a porosity level >92%
- A satisfactory physical, mechanical, and thermal resistance.
- Regenerative power with most properties remaining stable after >30 adsorption-desorption cycles.
- A high tolerance to a large variety of operating parameters (strong acidic or basic medium water and temperature of water).
- The basic/raw structure is rich in active functions that are capable to remove heavy metal ions.
- Inexpensive raw materials and cheap modification and manufacturing process
- Desorption acids must be inexpensive and abundant throughout Latin America.
How To Accomplish Our Mission Statement 💡
These are the fundamental chemistry questions that demonstrate the mission statement is feasible for aerogel Perlas:
Question 1: How does molecular weight affect the polymer properties?
Answer 1: As molecular weight increases, the mechanical properties of the polymer increase as well. Every polymer has an ideal molecular weight at which the balance of different properties is optimized.
Commercial available sodium alginate is between 32,000 - 200,000 g/mol. (Source)
Question 2: How does the pH of water affect the perla’s properties?
Answer 2: Polyacids (like algin/alginate) accept protons at low pH values (pH levels like pH 3 or pH 4). But the perlas look deflated; almost like a flat tire.
The polymers deprotonate (let go of hydrogen ions) and become negatively charged at higher pH values. This creates the shape of the perlas to look round because the material is swelling up like a balloon! This also allows for the heavy metal ions to be adsorbed instead of the free-hanging hydrogen ions.
Swelling behavior is noticed when the pH is greater than the pKa of the polymer. The pKa of carboxylic acids (like alginate) is 3 - 4.22. The pH has to be greater than 3 for swelling to commence.
Question 3: What the hell is pKa??
Answer 3: pKa is a number that describes how strong or weak an acid is. Specifically, pKa measures the strength of an acid by how tightly a proton is held (by a Bronsted acid).
The lower the pKa number is, the measured acid can be very strong and the higher chance to “give away” its protons. The strongest acids will have a pKa less than zero.
Alginate has two functional groups: M-acid groups & G-acid groups.
- The pKa of M-acid groups is 3.4
- The pKa of G-acid groups is 3.6
The pH (of the water) has to be over 3.6 (minimum!) to notice swelling of the perlas and heavy metal ion adsorption.
BUT…
The pH of the desorption source (e.g. vinegar) has to be under 3.4 (minimum!) to remove the heavy metal ions from the perlas.
Question 4: So, Carlos, that would depend on what chemical the ions attach to. What is the chemical/compound that will do the adsorption?
Answer 4: It is the solvent that is responsible for the majority of the perla’s properties.
Solvent → The liquid in which a solute is dissolved to form a solution.
The main characteristic that the solvent needs to accomplish our mission statement is that the material must have amine groups.
Amine groups are compounds and functional groups that contain a basic nitrogen atom with a lone pair. The lone pair could be hydrogen, hydrocarbons, etc.
Some of the most effective amine groups (considering that alginate has naturally-hydrophilic -OH & -COOH groups) are NH or -NH2.
Our aerogel perlas will contain the -NH2 group using a chemical known as EDTA. The technical name is Ethylenediaminetetraacetic acid (and good luck saying that lol)
But why NH & -NH2 groups, Carlos?
These groups have a tendency to adsorb heavy metals from the Classification B group and some heavy metal ions from the Classification Borderline group.
The heavy metal ions that alginate should adsorb in the Class B Group are Lead (II), Cadmium (II), Copper (I), and Mercury.
The heavy metal ions alginate should adsorb in the Class Intermediate Group are Chromium (III), Zinc (II), Cobalt (II), Nickel (II), Iron (II), Copper (II), Arsenic (III), and Lead (IV).
© 2023 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.
