Wang M, Orr AA, He S, Dalaijamts C, Chiu WA, Tamamis P, Phillips TD. Montmorillonites Can Tightly Bind Glyphosate and Paraquat Reducing Toxin Exposures and Toxicity. ACS Omega. 2019 Oct 17;4(18):17702-17713. PMC free article
Biology and Chemistry Introduction
The changing pH of our GI tract
Our stomach can be pretty low, acidic, owing to the secretion of hydrochloride acid, the same as we use to keep the pH of our swimming pools a bit on the acidic side.
It is common Internet knowledge that the pH of our gastrointestinal tract changes from compartment to the next.
The “bulk” pH in the compartments will affect the charge of the soluble compounds and the surface charges on the insoluble clay.
The chemistry of solutes and surfaces
Left panel, solutes, something that is dissolved
Right panel, on the X-axis we have pH. pH is the -log10 the H3O+ concentration. This is a proper chemistry of saying H+ because H+ don’t wander around free. They are always attached to a H2O. Anyhow, pH 1 is 0.1 MH3O+and pH 8 is 0.0000001M H3O+. and pH On the Y-axis we have the fraction of total glycine. At pH 2 H3O+ dumps H+ any place it sees a negative charge. At pH 2 every atom in Gly2 that can take an extra H has one. Even the N has its maximum of three H. At pH 2.72 half of the red OH in the carboxyl group has a H. This is the definition of pK. At pH 6 we’ve got pretty much 100% GlyH with an H on the carboxyl -OH and no extras on the Nitrogen. As we increase the pH we by definition totally decrease the concentration of H3O+ . We’ve got loads of H2O thinking they’d like to grab a H+ and be H3O+ instead. At pH 10 about half the glycine is Gly and the other half GlyH. Note that the second pK is 9.8.
Right panel, charged surfaces
On the right hand side we have a mixed metal oxide, or a clay. Two different metal atoms are connected by two oxygen atoms. IEP are are the pH at which the material has no net charge or that the negative charges equal the positive charges.. According to the Wikipedia IEP page alumina may have two IEP at pH 7-8 and pH 8-9. Silica has an IEP of 2 ish.
- At a solution pH below the IEP the two positively charged metal ions have waters stuck on them. One of the connecting oxygens also has an H+ stuck on it.
- At the IEP solution pH the connecting O don’t have any H+ stuck on them. It’s sort of implied that the Me with an -OH still has a + charge to balance out the Me with an O–. This also implies that the IEP of alumina might be different from an alumina silicate.
- At a solution pH above the IEP there are not H to be found.
Binding of paraquat and glyphosate to a solid surface is a trade off between solution pH and the charges on the solutes and the surfaces.
Kitty Cat metaphor, Gibbs energy: enthalpy of binding
This is a terribly confusing topic. Weebly is a good place to start…
Kitty Cat in the tree still has some entropy. She can twitch her tail or lick her whiskers. Kitty Cat playing on sleeping on the bench has some enthalpy; it would take a little bit of energy to get her off. The energy of taking Kitty from the tree to the ground, or vice versa, is Gibbs Free energy. ΔG=ΔH-ΔTS can be kind of confusing when we lose track of the direction we are going. If the final energy is less than the initial energy, rock initially sitting at the top of a hill versus a final resting point at the bottom, then the reaction is spontaneous. Let’s journey with Kitty Cat very negative ΔH. By the same token Kitty Cat loses S in going from the grass to the tree branch. It is possible that -ΔTS
|ΔH>0||Only spontaneous are really high temperatures. Kitty must have her claws in the grass and less motion on the tree limb||never happen spontaneously Kitty is prancing on the tree limb, sluggish on the grass, and claws into the grass too.|
|ΔH<0||Spontaneous at low temperatures too cold for Kitty to scamper around at top speed in the grass.||Spontaneous always. Kitty for some reason is less fidgety in the grass than the tree yet still clawing into the tree.|
Ca2+ montmorillonite binds paraquat and glyphosate
This study came out of Texas A&M University. The authors mention that workers exposed to paraquat and glyphosate may have detectable levels of these agents in their blood and urine. We can see where the very pre clinical studies may be heading. The analogy of this study is Kitty Cat (glyphosate) going from playing in the grass (solution that the clay is in) to the tree (the clay)
Figure 1 Now we get to pKa
Figure 1A Left Glyphosate has three domains of binding interest: (1) A phosphate group (2) a nitrogen, (3) a carboxyl group. All of these may carry a charge depending on the pH of the bulk solution. Right Paraquat has four domains of interest (1) The C on the N is really a methyl group on a (2) nitrogen that may sometimes have a positive charge. Some part of the ring structures may have a (3) single carbon bonds or a set of (4) two double carbon carbon bonds. Also note the alumina (pink balls) silicate (yellow balls) nature of montmorilonite. The oxygens are red balls. Note the Nitrogens of paraquat over the monmorillonite structure.
Figure 2 and 5 enthalphy of binding
Glyphosate may bind to the clay in the stomach but stay bound mostly in the small intestine. The sodium mntmorillonite (SM) seems to have higher affinity than the calcium montmorillonite (XM) at stomach pH but is about the same at small intestine pH 7.
- Red lines (pH 2) run above the blue lines (pH 7)
- There is more separation from the red and blue lines at 26oC (room temperature) vs 37oC (body temp) This is
In this figure we are only looking at pH 7 binding.
|sorbents||pH 2, T1||pH 7, T1||pH 2, T2||pH 7, T2||pH 7, T1||pH 7, T2|
|CM||Qmax = 0.71||Qmax = 0.26||Qmax = 1.27||Qmax = 0.32||Qmax = 0.43||Qmax = 0.29|
|Kd = 1.1 × 105||Kd = 1.5 × 105||Kd = 3.2 × 105||Kd = 2.4 × 105||Kd = 1.5 × 106||Kd = 2.8 × 106|
|rse = 0.037||rse = 0.022||rse = 0.095||rse = 0.056||rse = 0.033||rse = 0.055|
|SM||Qmax = 0.96||Qmax = 0.41||Qmax = 0.9||Qmax = 0.3||Qmax = 0.44||Qmax = 0.44|
|Kd = 1.2 × 105||Kd = 1.4 × 105||Kd = 1.9 × 105||Kd = 2.5 × 105||Kd = 2.0 × 106||Kd = 3.4 × 106|
|rse = 0.11||rse = 0.038||rse = 0.062||rse = 0.022||rse = 0.026||rse = 0.046|
- The larger the binding affinity number the higher the affinity. For those that have forgotten Scientific Notation, Kd = 1.9 × 105 is 190,000. The higher the number the higher the affinity.
- The Qmax is in units of mole of compound to kg clay. Both sodium and calcium bind glyphosate very well at pH 2. Glyphosate has a molecular weight 169.07 grams per mole. 0.96 moles is 162 g or about 0.357 pounds… About a third of a pound of glyphosate pound to 2.2 lb of clay for those that are more familiar with the Imperial System of measurement.
- rse is an error term that we won’t get into.
Following the Baking Soda pesticide wash thing
Organics for Green Living has published a dirty dozen of produce likely to be contaminated with glyphosate. They say that rinsing in a dilute solution of Baking Soda may remove glyphosate. They did not reference their source for such a claim. Ab Internet search was performed to find the original source. The Environmental Health Perspectives branch of the US NIEHS has a good piece on glyphosate induced liver damage and metabolic syndrome in youth link. This author cited the original reference for the baking soda rinse. Further digging was required to unearth the original full article that is public access:
Yang T, Doherty J, Zhao B, Kinchla AJ, Clark JM, He L. Effectiveness of Commercial and Homemade Washing Agents in Removing Pesticide Residues on and in Apples. J Agric Food Chem. 2017 Nov 8;65(44):9744-9752. Sci-Hub free article
about 125 ng/cm2 thiabendazole (Max solubility in water at pH 2.2) and phosmet (soluble in benzene and acetotne) were spotted onto Gala apples obtained at a local store. Chlorox bleach and sodium bicarbonate (baking soda) were also obtained from a local store and used at 10 mg/mL) was used to remove surface thiabendazole and phosmet residues by rinsing apples for 12 and 15 min, respectively. After that, three different washing treatments were examined:
- Clorox Germicidal bleach (25 mg/L for 8 min,
- 10 mg/mL NaHCO 3 solution applied for 8 min,
- tap water applied for 2 min.
The rinse solutions themselves were quickly removed in distilled water. Gold nanoparticles were added for SERS visualization. The exact Surface-enhanced Raman Spectroscopy mechanism of aromatic pesticide communication between gold nanoparticles was never discussed.
PubChem cites many references for water solubility of glyphosate. 10.5 g/Lat pH 1.9 and 20 °C is one of them. At this pH carboxyl groups on the apple may be uncharged. As much as this lovely study is quoted, it is a pity that the authors never tested a mild acid rinse too.
Going back to the Kitty Cat metaphor for partitioning
Environmental Working Group’s “Dirty Dozen” produces items with the most pesticide residue, which include list:
- Sweet bell pepper and Hot peppers
The frustrating thing about this list is that each fruit or vegetable might have its own surface chemistry: Kitty Cat on the bench, Kitty Cat on Joe’s head, Kitty cat, and so on. Getting Kitty Cat mobile on the grass (rinse solution) might require different forms of “encouragement.”
Kitty Cat going back to the bench or Joe’s head might be driven by bonding energy (enthalpy) and a loss of freedom to move around to one degree or another (entropy). If we remove the bench (apple) fast enough, going back will not be an option. This is where our higher affinity clay comes in! If Kitty Cat only takes a short lived prance on the grass (rinse solution) after leaving the bench (apple) landing in a high affinity tree (clay) that she can really sink her claws into may make the return to a prance in the grass (rinse solution) and a return to the bench (apple) very thermodynamically unattractive.