operation oaxaca: pass the juice.

desorption of metal ions w/ lemons (015)

Carlos Manuel Jarquín Sánchez
8 min readFeb 19, 2024

this is carlos.

we just finished a deal with a lab.

processing has begun.

entry to the lab will be this week, if God permits.

and now, we mentioned that the status quo of metal ion filtration cannot recycle all materials, chemical-intense, non-environmentally friendly, and not an abundant organic material (economical).

our way to the new way of water filtration is on adsorption.

the only roadblocks left for an adsorption-based material are:

  • low selectivity
  • production of waste products
  • scaling it up into industry-standard

number two is resolved when we use mother nature;

the waste products are biodegradable (the mango peels).

now i’ll tackle the “low selectivity” problem.

and it’s time to pour me some juice…

lemon juice.

’cause operation oaxaca has some ingredients from its motherland that can help us…

in removing ions from water & sludge.

(metal ions typically go to sludge treatment/disposal, but with peels, we have a more economical method).

sip the juice and sit back.


sippin da lean juice.

since the mango peel will adsorb metals (instead of ion exchange or chelation),

we will need to have a statement.

this wet-lab filter prototype must handle 10 adsorption-desorption cycles…

with an adsorption of >95% and desorb without structure deformation per cycle.

i’m not trying to use a metal ion treatment facility.

i’m sticking to this rule:

use what you got.

and i have mother nature’s gifts.

that’s all i’ll need.

and guess what?

the two household appliances in your home, that you use to remove hard water stains from your bathroom…

can also be used to desorb other metal ions!!!


so, what are they? the cost? the chemistry? effectiveness?

those are white vinegar and lemon juice.

specifically, acetic acid and citric acid.

citric acid / lemon juice.

lemon juice is made from citric acid, the compound that makes this all work.

the juice of a lemon (fresh off a tree) is about 5% to 6% citric acid.

so water and citric acid are the big components of lemon juice.

vinegar combines citric acid (C6H8O7) and water (H2O).

while citric acid is produced naturally in fruits like lemons, limes, oranges, berries, etc.

all other citric acid is manufactured.

and to develop it, we need black mold (Aspergillus niger fungus).

the black mold efficiently converts sugars into citric acid.


this time, imma just let chatgpt answer, i’m not concerned about the how, as i’ll be using natural sources.

chatgpt says:

substrate selection: a carbohydrate source is provided to the aspergillus niger culture. this substrate is typically a sugar-containing material such as glucose, sucrose, or molasses.

growth and metabolism: aspergillus niger metabolizes the carbohydrates present in the substrate through enzymatic processes. enzymes produced by the mold break down the complex carbohydrates into simpler sugars, such as glucose and fructose.

citric acid formation: the metabolism of the sugars by aspergillus niger leads to the production of citric acid as a metabolic byproduct. this occurs through a series of biochemical reactions within the fungal cells, including the citric acid cycle (also known as the krebs cycle or tca cycle).

optimization: the fermentation process is optimized for citric acid production by controlling factors such as ph, temperature, oxygen levels, and nutrient availability. these conditions are adjusted to promote the growth of aspergillus niger and maximize citric acid yield.

harvesting and processing: once the fermentation is complete and citric acid has been produced, the mixture undergoes harvesting and processing steps to isolate and purify the citric acid. this typically involves separation techniques such as filtration, centrifugation, and precipitation to remove the fungal biomass and other impurities.

recovery and drying: the citric acid is recovered from the solution using methods such as crystallization or ion exchange. the resulting citric acid crystals are then dried to remove any remaining moisture, resulting in a powdered form of citric acid suitable for commercial use.

ok, so we know that for mango peels to attach with the metal ions, we need the gallic acid groups to have their carboxyl groups locked and loaded.

carboxyl (-COOH) & hydroxyl groups (-OH) are anionic, negatively charged.

metal ions are cationic, positively charged.

opposites attract.

but now, to desorb…

what must happen for the metal ion to adsorb onto the acetate group (COO)?

two conditions:

  • pKa
  • pH

technical terms: pKa measures the strength of an acid by how tightly a proton is held (by a bronsted acid).

bronsted-lowry acids → describe their ability to donate protons in reactions with bases. when an acid donates a proton to a base, it forms its conjugate/joint base.


the pH measures the concentration of hydrogen ions in a solution.

pKa measures the strength of an acid in a solution.

the lower the pKa number is, the measured acid can be very strong and the higher the chance to “give away” its protons.

the strongest acids will have a pKa less than zero.

so for mango peels, the dominant acid is gallic acid.

gallic acid’s chemical structure is this:


gallic acid for a mango peel has three hydroxyl groups (-OH) and one carboxyl group (-COOH).

so one H⁺ ion can be released as the pKa gets bigger and the pH changes (one H⁺ per carboxyl group).

and the pKa of the gallic acid is between 4.4 - 4.5.

citric acid’s structure doesn’t deviate that far.


citric acid has three carboxyl groups.

and one hydroxyl group.

but the twist is this:

each carboxyl group has its own pKa value.

carboxyl group #1 is 3.128 at 25 °C

carboxyl group #2 is 4.761 at 25 °C

carboxyl group #3 is 6.396 at 25 °C

and the citric acid (or lemon juice) is approximately pH 3 for 1mM aqueous solution.

mM → the number of milli-moles of a dissolved substance per liter of a solution.

mole → a unit to measure a substance amount. the number is 6.022×10²³ particles… aka avogrado’s number.

aqueous → a substance in/dissolved in water

in this case, this means that there is 1/1000 of a mole (or a thousandth of 6.022×10²³ particles) of citric acid in a solution of water… and gives us a pH of 3.

the pH of an aqueous solution of citric acid is 3. (with a pKa of 4.76)

and the pKa of gallic acid is ~4.4.

and it will work to desorb metals!!


because the value of pKa is greater than the value of citric acid.

so we can cause protonation to occur!

protonation → it weakens the binding attraction of gallic acid to metal ions… allowing the ions to desorb from the peels, and be replaced with floating H⁺ ions from the citric acid carboxyl groups (lemon juice).

but why tho?

at lower pH, there exist more H⁺ ions in the vinegar… because that’s what an acidic environment is.

but when the pH goes above one of the carboxyl groups’ pKa… deprotonation occurs.

deprotonation → a functional group (i.e. carboxyl) releasing a hydrogen ion into the solution… in exchange for a metal ion (in our specific case tho)

so what?

carobxyl group #1 is 3.128 at 25 °C

carboxyl group #2 is 4.761 at 25 °C

carboxyl group #3 is 6.396 at 25 °C

the pH is an aqueous solution with citric acid.

if pH is 3.128 < ‘x’ < 4.761… one hydrogen ion is released (H⁺).

if pH is 4.761 < ‘x’ < 6.396… two H⁺ ions are released.

if pH ‘x’ > 6.396 … three H⁺ ions are released. (aka citrate ion)

citrate ion (a proton was released from each carboxyl group)

but if the pH does not go over one, two, or three of these values… then only two, one, or zero H⁺ ions will be released.

and that’s good.

because we want to make sure our citric acid satisfies any of the three constraints.

the pH of the citric acid has to be lower than the pKa of the gallium acid (functional groups)…so that the majority of the gallium acid groups on the mango peel are protonated.


it all comes down to typically protonation (again) and pKa.

the pKa number is the equilibrium constant for an acidic functional group’s dissociation (break into smaller molecules).

when the pH value is less than the pKa constant, there are more hydrogen ions (H⁺) in the solution compared to the concentration when the functional groups (gallic acid) are 50% dissociated.

mini example of what the pH & pKa relation has to do with all of this.

the pKa of citric acid is 4.76.

and the pH of lemon juice is about 3.

we want pKa > pH… so that the acetate groups are protonated…

and go from:

C3H5O(COO)3³⁻ + 3*H⁺ → C6H8O7 (IF pKa > pH stays like this)

in english: citrate ion + hydrogen ions → citric acid because the citrate ion now have their “long-lost protons” back… because at low pH, there’s a lotta hydrogen ions floating around, looking to bond with something.

this means the hydrogen ions from the citric acid (in lemon juice) can attach themselves to the citrate ion that’s bonded to the metal ion.

…and the hydrogen ion takes its place and kicks the metal ion into the pool of lemon juice.

but what are the pKa’s of the metal ions?

magnesium II ion: 11.4

lead II ion: 7.8

chromium III ion: 3.95

copper II ion: 7.34

iron III ion: 2.17

aluminum III ion: 4.85

and that’s it!

we have now covered the basics of desorption.

for the next one, it’ll be a brief economic cost of both of them, and which one is more effective at desorbing ions.

after that, i’ll do a smaller dive into mango peel chemistry.

and then the big article:

how would we scale up the filter AND deliver the technology to thousands of people in usa & mexico?

© 2024–2100 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.