[BelindaM]

Hello all,

I hope I'm in the right subforum for this; I should mention first and foremost that I have no experience with chemistry, at all. Nothing except the high school basics, of which by now I remember maybe 10% on a good day. I really know.. well, effectively nothing.

I'm following a college course about data analysis which took an unexpected chemical turn, and now I'm here trying to make sense of a soil problem they gave us. They ask us about solid-solution partitioning of heavy metals, among many other things, but the problem is the literature they provided to explain this assumes the reader to be familiar with (basic) chemistry. There's three sentences before they dive into speciation, Freundlich models and ionic strength, and I have no idea -- no idea at all -- what they mean with anything. Neither google, nor wikipedia, nor their own source material has been able to help me out. I just don't understand.

Hence I was wondering if anyone here would be able to help me out here? To explain in the most basic way what solid-solution partitioning of metals is? There are a billion more questions that I have, but it would be a start.. and maybe I could go from there.

Any help is greatly appreciated!

[Corribus]

I would be helpful to see the actual problem.

But a general answer to your question is that substances diffuse spontaneously from regions of high concentration to regions of low concentration until equilibrium is reached. Equilibrium is often (though not always) defined by a state where the concentration is the same everywhere in the system. We tend to think of diffusion in cases where a solid/liquid solute is dissolved in a liquid solvent (a drop of dye added to a glass of water) or a gas/vapor dissolved in a gas (someone lights a scented candle in one corner of a big room). But diffusion also happens in solids, although the process is often slower. So, if a metal (solid) is deposited at high concentration in a part of solid (say, rock), the metal will gradually diffuse so that it is lower concentration everywhere. Bear in mind, that diffusion process may take years, decades, centuries, eons.

Partitioning in chemistry typically refers to a solute moving from one region to another across some kind of interface/boundary, often between two different phases having very different base properties. For instance, if a gas dissolved in a liquid is transferred to the atmosphere, we would usually call that partitioning of the gas between the two phases. It comes from the idea of "part" of the gas transfers from one phase to the other.

But again, it helps to see the actual problem to provide a more specific answer.

[BelindaM]

Thank you so much for your answer! The examples really help a lot! So if I'm understand correctly, partitioning is comparable to diffusion (substance moving from areas with a high concentration to areas with a low concentration, until its more or less evened out), but generally includes this substance moving from one phase or material to another?

To give some more details about the exact problem they've given us, it's about metal contamination of a floodplain system. A river passes by a brick factory and transports metals such as copper and zinc, then drops them off downstream as sediments. There, the metals mix into the soil solution and contaminate the area, impacting the health of local plants and wildlife. They also go into how these metals can bind into the soil in different ways, e.g. under aerobic and anaerobic circumstances, and how these all affect the resulting toxicity, but this I don't fully understand yet.

Where solid-solution partitioning comes into play is here:
"For the risk assessment of contaminated soils, information on the solid–solution partitioning and speciation of contaminant metals is necessary as they both affect metal mobility and bioavailability." I believe they want us to understand how it impacts concentrations of copper and zinc, soil toxicity, and consequently the system as a whole.

[Corribus]

Thank you so much for your answer! The examples really help a lot! So if I'm understand correctly, partitioning is comparable to diffusion (substance moving from areas with a high concentration to areas with a low concentration, until its more or less evened out), but generally includes this substance moving from one phase or material to another?

Yes. This is still part of the diffusion process, but there is usually a resistance (of a kind) of the substance moving from one phase to another. For example, suppose you have a glass that is half-filled with water and half-filled with olive oil. The water and oil obviously don't mix, so the two liquids are different phases and the boundary between them is an interface. Now suppose the water layer has some sugar dissolved in it, so that the concentration of sugar is uniform in the water. What happens? The sugar will diffuse from the water layer and (partition) into the oil layer because the oil layer has less concentration than the water layer, but sugar is also not very soluble in oil. So even at equilibrium (after a long time), the concentration in the water will be much higher than in the oil - not much sugar partitions into the oil despite the tendency of diffusion to occur toward regions of lower concentration. The degree of partitioning (embodied in a partition coefficient) is dependent not only on the relative concentrations that drive diffusion, but also the solubility of the substance in each phase

So, to sum:

Diffusion is the movement of substance from a region of high concentration to a region of low concentration.
Partitioning is the movement of a substance from one phase to another across an interface.

Diffusion is primarily determined by a concentration gradient. Partitioning is determined additionally by the thermodynamic factors in each phase that would determine how happy a substance is to be in that phase.

To give some more details about the exact problem they've given us, it's about metal contamination of a floodplain system. A river passes by a brick factory and transports metals such as copper and zinc, then drops them off downstream as sediments. There, the metals mix into the soil solution and contaminate the area, impacting the health of local plants and wildlife. They also go into how these metals can bind into the soil in different ways, e.g. under aerobic and anaerobic circumstances, and how these all affect the resulting toxicity, but this I don't fully understand yet.

Ok. In this case you have not only passive diffusion, but also active transport of the substance (due to the current of the water).

Where solid-solution partitioning comes into play is here:
"For the risk assessment of contaminated soils, information on the solid–solution partitioning and speciation of contaminant metals is necessary as they both affect metal mobility and bioavailability." I believe they want us to understand how it impacts concentrations of copper and zinc, soil toxicity, and consequently the system as a whole.

So partitioning in this case probably refers to metals initially dissolved in the water transferring to (partitioning into) the solid soil phase before they are carried off to wherever by the motion of the water (e.g.the ocean). So if you start with a certain mass/concentration of metal in water that is carried through the soil, a certain portion of the metals will end up in the ocean, and a certain portion will be retained by the soil. That's the partitioning process. Speciation refers to understanding the form of the metal (either different ion like chromium(III) versus chromium (VI), or complexation of the metals with other organic or inorganic substances in soil. It's important because metals have different toxicity or bioavailability (ability to be taken up and metabolized by living creatures) depending on their form.

Hope that helps.

[BelindaM]

That actually helps so much! The water/oil example is one I've also found online, but it was never explained in a way that I understood. Now it makes sense though.

Honestly I can't thank you enough. I felt kind of hopeless with how they just dumped all these chemistry terms on us, but a few concrete explanations like this make all the difference. This is infinitely more understandable than any of their literature. I'm so happy you included speciation as well; that was going to be my next headache, haha.

You really helped me out here -- I don't think I would've gotten very far otherwise. Thanks a bunch!

[Corribus]

Any time. Glad it was helpful.