Soils and Environmental Pollution Chapter 18

Of these 5 fates, inorganics are subject to 4 –all but degradation. The notion is that the inorganic contaminant is actually an element. As such, it may appear in one of many molecules or ions but it does not degrade unless it is radioactive. Another point, too, is that phytoremediation is a type of bioremediation. Also, there are cases in which an organic contaminant is removed by use of plants. So bioremediation applies also to inorganic and phytoremediation to organics.

By 1 kg, what is meant is active ingredient, and that is a lot. The benefits are obvious, however, the detriments are probably mostly unknown.

Lethal dose 50 = kills ½ the population. Lethal concentration 50 is analogous.

Comparing the mammalian toxicities of 2,4-D and atrizine, the latter is much less toxic. However, it is much more toxic to fish, no?

↓ ↑ The basic idea is that the more of the substance (in a volume of soil) that is associated with the solids, the less there is in solution, so there is less mobility.

It’s an organic cation so it is adsorbed onto – charged sites on soil particles and strongly so. Look back at 2,4-D. It’s an anion (on dissociation of the acid), so just the opposite, right? Hydrophobic means low water solubility. Couple this with the concept of like dissolves like and you get.

Two differences between the Dundee and Sharkey soils is that the Sharkey has a higher content of clay (= more surface area for adsorption) and more organic matter. Thus, the compound (a derivative of the herbicide bentazon) is more highly adsorbed in the Sharkey. The plot is called an isotherm and relates the concentration of a chemical associated with solids to the concentration of the chemical in solution. The terms adsorbed and sorbed are not necessarily synonymous. The former implies chemical adherence to a surface, while the latter is broader, also including precipitation. Use of adsorbed on the y-axis of the graph would be more apt.

After 1 half-life, ½ remains, after 2 half-lives, ¼, etc. Strictly speaking, the concept of a half-life is applicable only to a degradation process that follows 1 st -order kinetics, i.e., the rate of degradation is directly proportional to the mass of the chemical at all times. Often, degradation does not follow 1 st -order kinetics so use of half-life in these cases is meaningless. Even so, the word half-life is often used.

Degradation of compound X can be abiotic, biotic or both. Usually the latter is more important, i.e., most of the degradation that occurs is biologically mediated.

For most organic chemicals, degrada- tion is relatively fast. Personally, I don’t trust one of these data sets.

First, water movement is faster. Second, where this occurs, the soil is macroporous, perhaps sandy, and if sandy, there is little surface area or organic matter for adsorption. Compare the mobility (mass and velocity) of compounds A, B, C and D that differ in the extent to which they are adsorbed and rate at which they are degraded.

A contaminant may move in runoff as the dissolved chemical species or bound to suspended solids

Adding N, P, K, etc. adds essential elements needed by the population of microorganisms that degrade the hydrocarbons. They can’t live by C and H alone (see earlier discussion on C / N ratio effect). Better aeration = faster metabolism. Ditto for nutrients.

Early work of this type led to isolation of organisms that had a great capacity to degrade a specific contaminant but that when introduced into soil at the contaminated site, died off. Isolation of superior degraders from the site worked.site worked.

Some sources of inorganic contaminants.

A mitigating fact is that inorganics tend to exist in forms that are the least soluble, hence, least mobile and plant-available. These are the general forms, and their relative occurrence and mobility.

This is what you want to do and often simple prescriptions are effective, like See next slide for common effect of soil pH on the solubility of many metals. Recall discussion on metal micronutrient solubility. Perhaps surprising but true, the chemically reduced form is more soluble at any pH than the oxidized form. Thus, wet, reducing conditions in the soil favor solubility, mobility and uptake. So, improve aeration at the site. As above, recall related discussion on redox metal micronutrients.

Curves like these, relating extent of adsorption to pH, are called adsorption edges (the figure doesn’t look very edgy but imagine the pH range was expanded and the spacing shortened –more edgy). You get the point, though, decreasing mass in solution with increasing pH.

OK, first, really, the phytoremediating plant must survive, i.e., not experience toxicity. Beyond this minimum, the plant must take up the contaminant in high amounts (high tissue concentration) and produce a lot of biomass. What is removed is tissue concentration x tissue mass per area, per time. Make sense?