1.1. Soil-Air partition constants

So this issue of log K oc we determine that log K oc and log K ow these are two properties of a chemical which characterise its hydrophobicity?

Log Koc (the organic carbon-water partition coefficient) and Log Kow (the octanol-water partition coefficient) are two key values in environmental chemistry. They help us understand how hydrophobic (water-repelling) a chemical is. A higher log K value typically indicates that the chemical is more hydrophobic, meaning it will prefer to bind to organic materials or sediments rather than remain dissolved in water. This property is crucial for predicting how chemicals behave in the environment, especially regarding their transport and fate in soil and sediments.

Imagine oil and water. Oil has a high hydrophobicity; it floats on water because it does not mix. Similarly, chemicals with a high log Koc prefer to bind with organic materials in soils rather than remain in water, helping us predict where they might travel in the environment.

if you are writing K A32 star, we will write K A32 star as K oc times foc rather than writing it simply like this.

In this equation, KA32* represents the effective partitioning between the chemical, the organic carbon (Koc), and the fraction of organic carbon (foc) present at a site. This reflects that the overall partition constant is influenced by both the inherent property of the chemical and the specific organic carbon content at the site. So, the interaction between the chemical's hydrophobic properties and the organic carbon content in the soil helps determine how that chemical partitions between soil and water.

Think of a sponge soaking up water. The sponge's material (analogous to organic carbon) will affect how much water (the chemical) it can absorb. If the sponge is very porous (high foc), it will hold more water than a denser sponge (low foc), just like high organic carbon soils hold more of certain chemicals.

So for the case of inorganic so we said if inorganic chemicals this doesn’t apply.

Inorganic chemicals do not behave the same way as organic chemicals regarding partitioning. While organic compounds tend to only adhere to organic matter, inorganic compounds are influenced more by the mineral content of soils and their surface charge. This means that when water is present, inorganic compounds may interact differently with soil than organic compounds, as they can bind to the mineral surfaces rather than the organic ones.

Imagine salt (an inorganic compound) being dissolved in water. When it settles, it doesn't stick to the water or the container (like organic compounds), but rather it can bond with the container's surface depending on the conditions, making its behavior distinct.

The rules are based on oxidation; what determines oxidation state of element several things one is pH, one is the dissolved oxygen content.

Redox potential refers to the tendency of a chemical species to acquire electrons and thereby be reduced. In the context of soil chemistry, the oxidation state of inorganic elements (like Chromium) can change based on environmental factors such as pH and the amount of dissolved oxygen. For instance, chromium can exist in multiple oxidation states, where Cr(III) is generally insoluble, while Cr(VI) is soluble and more mobile in water. Changes in oxygen levels can thus affect the solubility and transport of these elements.

Consider how rust (iron oxide) forms when iron is exposed to oxygen and moisture. Just as the presence of oxygen transforms iron, the concentration of dissolved oxygen in soil water can change the oxidation state of certain chemicals, affecting their mobility and reactions in the ecosystem.

Bioavailability means it can be seen in different contexts, from a context of human health and fate and transport.

Bioavailability refers to the proportion of a substance that can enter the body's circulation when introduced into the body and is available to have an active effect. In environmental terms, it determines the extent to which chemicals in soil or sediments can be accessed by organisms, including humans. A chemical being in a soluble form (like Cr(VI)) makes it more bioavailable and therefore potentially more toxic compared to its insoluble counterpart (like Cr(III)).

If you've ever considered how much of a vitamin your body absorbs from food versus a supplement, that's bioavailability in action. In an environmental context, it's similar: the way a chemical is bound in soil or sediment affects how much can be accessed by living organisms. A chemical that's soluble in water can move and affect more life forms compared to one that is stuck in the soil.

If you go and dig it up and you churn it and stir it up. You are introducing oxygen into the system.

When soil is disturbed, like through digging or excavating, it can expose previously buried chemicals to oxygen. This exposure can change the oxidation state of inorganic chemicals, potentially making them more soluble and mobile. For instance, disturbing ground containing Cr(III) that was previously stable can convert it to the more toxic and mobile Cr(VI) form, which can then leach into water sources.

Consider a garden bed you've left undisturbed for a while. When you dig it up, you expose the soil to air, which can change the conditions in the soil and affect the plants' nutrient availability. In a similar way, disturbing contaminated soil can release harmful chemicals into the environment.

--