operation oaxaca: ancient scrolls.

what have others attempted so far? (021)

Carlos Manuel Jarquín Sánchez
5 min readFeb 26, 2024

this is carlos.

the usual is going on:

preparing for lab entry.

yet, i talked with the department head of engineering.

and he brought up how economics always comes first.

hell yes, always.

that’s how i decided the sludge treatment process contains all the valuable plus toxic metals.

go there and remove them.

but first, i need to know the treatment process like the palm of my hand.

so what you waitin’ for?

nose-dive in.


run the sludge, part ii.

if you didn’t read part i, no worries.

all you need to know is the intention of this article.

in the last one, i went in-depth about how sludge treatment works, and where the gaps are.

from papers dating back to 1974–1977:

the investigators knew that certain heavy metal ions were harder to remove from industrial waste (fertilizers, mining, etc.), known as sludge.

normally, it takes two rounds to attempt extracting >80% of metal ions.

but there’s too much remaining before it’s adequate to discharge into landfills or for agricultural applications.

so the question/dilemma we will resolve today is:

“if physical methods such as ion exchange, (activated carbon) adsorption, or electrolysis are to be used;

they would best be employed after the primary clarifiers to minimize equipment clogging.

primary clarifiers → (primary wastewater treatment)

this would again inhibit the metals from reaching aerobic microorganisms (i.e. aerobic fermentation, secondary wastewater treatment).”

because all heavy metals metal are concentrated in the sludge.

so some method should be developed to reclaim the metals from the sludge before it is recirculated into the environment by land disposal.

the common method for the treatment & disposal of sewage sludge (ss) in united states.

low removal efficiency of copper is attributable to its insoluble forms existing in sludge and the stronger binding of copper to sludge biomass.

and i did find some papers that used citric acid to remove metal ions from the sludge.

from their words:

“the extraction of heavy metals with citric acid alone, the extraction percentages of all the tested heavy metals increased slowly and eventually attained equilibrium within 24 h.”

those heavy metal ions were chromium, nickel, copper, and zinc.

and the best concentration of citric acid to extract heavy metals was 0.2M.

M → moles; ‘x’ amount of moles of solute per liter of solution.

solute → the substance that’s dissolved in the liquid/solution.


but increasing the citric acid concentration only slightly increased the heavy metal ion removal.

even if citric acid concentration goes up so that we can remove more heavy metals from sludge…

we must consider the rise of processing costs and the difficulty of pH readjustment for sludge composting.

zinc was the easiest to filter out with the 0.2M of citric acid concentration.

the hardest one to filter out was copper.

in order, from easiest to hardest to filter, it went:

Zn > Cr > Ni > Cd > Pb > Cu

here are some reasons why this occurs:

citric acid behaves like a polydentate organic ligand (chelator).

ligand → a molecule/ion that binds to a central metal atom or ion to form a coordination complex (i.e. covalent bonds)

polidentate organic ligand → a type of ligand that can bind to a metal ion via two or more coordinate covalent bonds.

^and these ligands form stable complexes (bonds) with metal ions due to their ability to ‘bond’ with the metal at multiple points simultaneously.


this is important because the citric acid (or organic ligand/chelator) must form ‘stronger + stable’ bonds to the metal ions than to the biosolid functional groups (in the sludge).

this way, the chelator (citric acid) can remove the ions from the biosolid sludge waste.

since citric acid is a weak acid, it will take more concentration/liquid to get an acidic pH… and that gives the metal ions more location to bond.

additionally, since citric acid is tricarboxylic, each molecule has three spots for metal ions to adsorb onto.

chelation 101.

chelation a process that bonds the functional groups and metal ions in a specific arrangement. this arrangement requires three or more atoms attaching to the central heavy metal ion.


a fluid that can create stronger bonds with heavy metal ions than the filter is necessary to regenerate the filter for multiple adsorptions.

the fluid/chemical must have a stronger bond than carboxyl groups [cooh], hydroxyl groups [oh], and amines [-nh, nh2].

the determining factor is the pka.

the lower the pka number is, the measured acid can be very strong and have a higher chance of “giving away” its protons.

giving away its protons results in a stronger attraction to heavy metal ions.

and the strongest acids will have a pka less than zero.

pKa → it measures the strength of an acid by how tightly a proton is held (by a bronsted acid).

and the main caveat for chelation is the pH.


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

pKa measures the strength of an acid in a solution.

the pH level must be between a pH of 4 — 6 because the optimized adsorption rates are located in this range.

cationic heavy metal ions will be adsorbed more than anionic heavy metal ions within a pH of 4–6.

but this only applies to specific cases of citric acid.

we are focusing on adsorption, not chelation.

remember, all we have is mother nature and its tools.

use what we got.

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