Aerogel: Oaxaca & Mangoes

Week #36: Use What We Got

Carlos Manuel Jarquín Sánchez

5 min readAug 14, 2023

Preface! ✨

Hello everyone!

My intentions for writing these articles are:

Explain technical information about my project in simple terms to you
Store information for my future self & others (in <7 minutes)

Coolio? Sweet. Enjoy the series :-)

Anyone Hungry? 🥭

There is a reason why people commit one of the two errors to mastery:

Not starting
Not going all the way

Because to start, one must use what’s at their disposal.

And to keep going, one must use what’s at their disposal.

Nothing changes…

Except for this:

As you get closer to the objective,

One collects more resources.

So one must use THOSE tools too.

And in Oaxaca’s case,

To resolve the water contamination crisis:

I took a deep dive into products that can fulfill the task.

And I DID find something:

FRUIT.

They have the chemistry required to remove heavy metal ions from wastewater.

And one of the top fruits produced in Oaxaca is…

Mangoes.

After conversations with locals outside of San José Del Progreso, Oaxaca, México,

They were also on board with the concept of using what’s available.

Now, onto the research.

We’ll win this war.

Peeling The Truth… 🌫️

Mexico mango exports:

468,420 tons of mango were exported from Mexico in 2020.

The top customer of mangoes is the USA.

64 out of 100 mangoes imported by the USA came from Mexico.

Other top mango importer countries include:

Canada
Japan
Spain
The Netherlands
France

2021: 59 million boxes of mango was shipped to the USA

Peak months of mango exportation are:

May
June
July (most popular)

Top Mexican states to produce mango are:

Michoacán
Nayarit
Sinaloa
Oaxaca
Chiapas
Jalisco

The top varities of mango produced in Mexico are:

ATAULFO → Small, flat, green-to-yellow mango.

CRIOLLO / PETACON → Large, sweet, round, reddish-orange version of mango.

TOMMY ATICINS & THE KENT → Very large fruit, almost as big as a small melon.

Pricing Of Mangoes:

$5.50 - $6.50 (large fruit)

$3.50 - $4.00 (10" fruit)

$3.50 - $3.75 (12" fruit)

Mangoes tend to have a self-regulating ceiling.

If variety is right & the size is very large,

A retailer can get as much as $1.99 for a conventional piece of fruit.

But sales do make an impact when a mango price moves above $1.

All prices are in USD.

Mango Chemistry… ⚗️

Tannins

Tannins are polyphenolic compounds that are found in many compounds… including mangoes.

Polyphenol → A category of plant compounds.
These plant compounds contain more than one hydroxyl group (-OH).
A polyphenol looks like this:

Tannins are responsible for the astringent taste of mangoes & other fruits.

They also be hydrolyzable or condensed.

Hydrolyzable → Made up of gallic acid units that are linked together by sugar molecules.

Condensed → Smaller units of gallic acid that are linked together with sugar molecules + other compounds present.

“Compounds” is synonymous with the word “molecules”, for this article.

Hydrolyzable tannins are more soluble in water & broken down more easily by enzymes.

Condensed tannins are more diffcult to break because of more bonds.

Gallic Acid → A monomeric phenolic compound solid (aka a plant chemical).
It’s colorless, it’s bound (linked) to sugars, & belongs to groups of hydrolyzable tannins.
Chemical formula: C6 H2 (OH)³ COOH
(Yes, it’s mixed in with water to a degree)

Tannins are also found in the peel, flesh, & seed of mangoes.

The concentration of tannins is the highest in the peel & it decreases as you move towards the flesh.

The seeds have the 2nd largest concentration of tannins.

Metal Ion Removal Processes

The tannins in mango peels bind to metal ions through a process called chelation.

Chelation is a chemical reaction in which a metal ion is surrounded by a molecule or ion that contains multiple electron-donating groups.

The electron-donating groups form bonds with the heavy metal ion… and the groups effectively remove the metal ion from the solution (aka water).

In the case of mango peels:

The tannins contain multiple hydroxyl groups (-OH) that can donate electrons to metal ions.

The metal ions then form bonds with the hydroxyl groups, which effectively removes them from the solution.

The specific mechanism of chelation by tannins is not fully understood.

But it is thought to involve the formation of a “matrix / complex structure” between the metal ion & the tannin molecule.

The chelation of metal ions in tannins is a reversible process.

And the metal ions can be released from the material if the pH of the solution is changed.

Typically, the acidity must go from a pH 4 to a pH 6.
(The pH of water is 7, btw.)
And depending on what metal ion one’s trying to remove, the pH must go up to pH 9 for >80% metal ion removal from water. (Ex: Iron)

This means that the tannins in mango peels can be or used to remove metal ions from water.

But the metal ions can also be used / released back into the water if the pH of the water has changed.

For the above to occur, the pH must drop back down.
Why?
When the pH is lower than 7, there’s more anionic (negative) charge.
The tannin material has an anionic charge.
The metal ions have a cationic (positive) charge.
And like magnets, opposites attract.
But how will a metal ion stick to the mango tannin peel if all of the water is negatively charged?
That’s why when we raise the pH of water past 7, the water becomes alkaline, or positively charged.
Now, only few anionic charges are left, so higher chance of metal ions bonding to the tannins.
And if you wanna remove the metal ions from the tannin peel:
Add more negative charge.
You do that by dropping the pH below 7, so that there’s a sea of negative charges for the small positive charges of the metal ion to float around.

The effectiveness of chelation by tannins depends on a number of factors.

This includes:

The metal ion type
The concentration of the tannins (molecules)
The pH of the solution (water)