Aerogel: Vinegar
Week #03: How To Remove Heavy Metal Ions From The 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 :-)
A Few Knowledge Drops… 💧
Our filter is designed to remove and adsorb heavy metal ions like Lead (II), Cadmium (II), Copper (I), and Mercury because of the nitrogen atoms in our mesh bag water filter.
But a typical design mission statement is that our filters have regenerative power with most properties remaining stable after >30 adsorption-desorption cycles.
To remove heavy metal ions from the filter, one simply adds the filter to a tray full of acetic acid; also known as vinegar.
Vinegar is a low-cost bulk chemical and it can be produced in all locations across Latin America.
But Why Vinegar? Why Not Bleach? 🍼
Vinegar is made by combining acetic acid (CH3COOH) and water (H2O). Acetic acid is produced by oxidation (loss of electrons) of ethanol (C2H5OH) by acetic acid bacteria. In most countries, commercial production involves a double fermentation where ethanol is produced by the fermentation of sugars by yeast.
The quality of alcohol (ex: ethanol)/acetic acid to water is crucial for effective heavy metal adsorption removal. Why?
Vinegar can create an ion-exchange effect.
Ion-Exchange Effect → The process of extracting undesirable ionic contaminants that are removed from wastewater.
In ion exchange, a safe ionic element/substance (*that must enter the water*) is exchanged for a deadly heavy metal ion. It’s a trade deal.
Stronger binding ions (like lead, mercury, etc.) displace weaker bonding ions and are removed from the water. Typical alginate filters will be doing cation exchange. The ions entering the water in exchange for the heavy metal ions will be either H+ ions or Ca2+ ions.
But how effective is vinegar at removing heavy metal ions, Carlos?
There is one hydrogen atom electron per molecule of vinegar (CH3COOH) that wants to give away its electron and leave the vinegar molecule as H+, (H+ is a hydrogen atom with no electron, only a proton)
Therefore, vinegar would be left with an anionic charge (CH3COO-). This is known as an acetate ion.
Anion (-) → More electrons than protons
Cation (+) → More protons than electrons
What Would Happen If Vinegar & Lead (II) React? ⚛️
Vinegar speeds up rusting because it contains a dilute form of acetic acid; positive hydrogen ions in the acid remove electrons from iron, ionizing it and making it susceptible to rust.
With heavy metal ions, the removal of electrons forces the metal ion to “attach” to the material. In other words, the heavy metal ion would prefer to hang around with the vinegar than the alginate aerogel beads.
It is also how the bond between alginate aerogel beads and the heavy metal ions interact.
But if the beads & vinegar have the same interaction, why does vinegar win?
A mesh bag filter will contain approximately 50 beads. There will be hundreds of metal ion-adsorbing hydroxyl (-OH) groups and carboxyl (-COOH) groups.
But a small tray full of vinegar will contain thousands of metal-adsorbing -OH groups & -COOH groups. There will be a stronger bond when there is more.
Debate: Nitric Acid Vs. Acetic Acid 🥊
Wait, what does nitric acid have to do with all this?
Nitric Acid is acetic acid’s stronger, more powerful (acidic) brother.
Technically speaking: Nitric Acid releases an H+ ion and a nitrate ion; The pH of nitric acid is 2.04 at 10 mM/L (pKa is -1.3)
Acetic acid is kinda weak because it does not completely dissociate (“convert”) into H+ & CH3COO- in a dilute aqueous solution (“dilute aqueous” means that there is little water present).
The pH of acetic acid is 3.39 at 10 mM/L (pKa is 4.76)
Dissociate → An acid dissolves in water; the acid transfers a proton to one of the water’s lone electron pairs
Note: The lower the pKa number is, the measured acid can be very strong and have a higher chance of “giving away” its protons. The strongest acids will have a pKa less than zero.
But looking at the problem in Latin America, we realize that acetic acid (despite this disadvantage) is found in the local markets of any village. So reusability and recyclability of the filter ARE possible even in the most remote village in Mexico.
Acetic Acid Wins.
© 2023 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.
