operation oaxaca: tastes sour.
desorption of metal ions w/ white vinegar (014)
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.
CJ
cookin up.
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!!!
brilliant!
so, what are they? the cost? the chemistry? effectiveness?
those are white vinegar and lemon juice.
specifically, acetic acid and citric acid.
acetic acid / white vinegar.
white vinegar is made from acetic acid, the compound that makes this all work.
typical vinegar is ~4% acetic acid by volume.
so water and acetic acid are the big components of vinegar.
vinegar combines acetic acid (CH3COOH) and water (H2O).
acetic acid is produced by oxidation (loss of electrons) of ethanol (C2H5OH) by acetic acid bacteria.
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.
stronger binding ions (like lead, mercury, etc.) displace weaker bonding ions and are removed from the water.
so for a mango peel, it would be better if i could use H+ (hydrogen ion) as the ion that would go into the water.
but why do i say that?
because there is one hydrogen atom electron per molecule of vinegar (CH3COOH).
that hydrogen atom wants to lose its electron and leave the vinegar molecule as H⁺.
(H⁺ is a hydrogen atom with no electron, only a proton)
that means vinegar would be left with a negative/anionic charge (CH3COO⁻). this is known as an acetate ion.
anion (-) → more electrons than protons.
cation (+) → more protons than electrons.
what must happen for the metal ion to desorb off of the acetate group (COO⁻)?
two conditions:
- pKa
- pH
pKa → a number that describes how strong or weak an acid is.
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.
example: when acetic acid (CH3COOH) donates a proton to water (H2O)… it forms the acetate ion (CH3COO⁻) and a hydronium ion (H3O⁺).
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:
C = carbon
H = hydrogen
O = oxygen
since gallic acid 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)
the pKa for the carboxyl groups is between 4.4 - 4.5 for the acetate group to dissociate.
dissociate → the process when a compound breaks apart into simpler molecules if certain conditions are met (thermal, pH, radiation, pKa, etc.)
and the other condition is the solution that our mango peel will be in:
the vinegar.
and then, there’s pH.
the pH of acetic acid is 3.39 at 10 mM/L (pKa is 4.76)
and this will reflect itself in vinegar.
and it will work to desorb metals!!
why?
the pKa of gallic acid is ~4.4…
so the pH of the desorption source (e.g. vinegar) must be lower than the pKa value of the functional groups of the mango (gallic acid) for the vinegar to be effective.
so it will work!!
gallic acid (pKa of 4.4) > vinegar (pH of 3.39)
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 vinegar.
but why tho?
at lower pH, there exist more H⁺ ions in the vinegar… because acetic acid breaks into smaller molecules…
but if the pH does not surpass the pKa, then the acetic acid stays the same.
all this can happen if we change the pH.
this is known as hydrolysis, btw.
and because acetic acid is a weak acid, not all of the acetic acid molecules in the vinegar will dissociate into acetate with a hydrogen ion…
BUT!!
there’s a quick fix to that problem.
the pH of the vinegar 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.
why?
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’s more hydrogen ions (H⁺) in the solution compared to the concentration when the functional groups (gallic acid) are 50% dissociated.
because when pKa number > pH number, the acetic acid will be protonated. (left side of pic)
and when pH number > pKa number, the acetic acid will become an acetate ion. (left side of pic)
remember:
the pH measures the concentration of hydrogen ions in a solution.
pKa measures the strength of an acid in a solution.
mini example of what the pH & pKa relation has to do with all of this.
the pKa of acetic acid is between 4.4 - 4.5.
and the pH of vinegar is about 3.39.
we want pKa > pH… so that the acetate groups are protonated…
and go from:
CH3COO⁻ + H⁺ → CH3COOH (IF pKa > pH stays like this)
in english: acetate ion + hydrogen ion → acetic acid because acetate ions now have their “long-lost proton” 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 vinegar can attach themselves to the acetate ion that’s bonded to the metal ion.
…and the hydrogen ion takes its place and kicks the metal ion into the pool of vinegar.
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
since this was a long one, we’ll discuss about citric acid in the other article.
after that, we’ll continue the discussion on mango peel tannins, gallic acid…
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.
