Soils and Environmental Pollution

Chapter 18

Organics

Five fates of contaminantsBioremediation

Inorganics

Controlling mobilityPhytoremediation

Of these 5 fates, inorganics are subject to4 –all but degradation. The notion is that the inorganic contaminant is actually anelement. As such, it may appear in one of

many molecules or ionsbut it does not degradeunless it is radioactive.

Another point, too, is that phytoremediation is a type ofbioremediation. Also, there arecases in which an organic contaminant is removed byuse of plants. So bioremediationapplies also to inorganic andphytoremediation to organics.

Organics

Some enter soil

Hurt water qualityBad for soil organisms

Let’s look at pesticides as an example

Use > 1 kg / person annually in US

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

Which of the below is not a class ofpesticide?

Herbicides NematocidesInsecticides RodenticidesFungicides Patricides

Herbicides

Variable toxicity to non-targets

2,4-D

Broadleaf weedsOral LD50 male rats = 764 mg/kgAqueous LC50 trout = 250 ppm (96 hr)

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

Atrazine

Broadleaf weedsOral LD50 male rats = 3090 mg/kgAqueous LC50 trout = 10 ppm (96 hr)

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

Insecticides

Question

DDT has a longer half-life than the fame ofthe dude who discovered its insecticidalproperties (True / False).

Fate of pesticides and other organicchemicals in the soil

VolatilizationSorptionDegradationLeachingRunoff

Volatilization

Sorption

Distribution between solids and solution

Movement with water if sorption

Solution Solids

↓ ↑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.

Depends on solute and soil solids

High / low for cations like paraquat?

High / low for anions like 2,4-D?

High for hydrophobic compounds iforganic matter high / low?

It’s an organic cation so it is adsorbedonto – charged sites on soil particles andstrongly 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. Couplethis with the concept of likedissolves like and you get.

Two differences betweenthe Dundee and Sharkeysoils is that the Sharkeyhas a higher content of clay (= more surface areafor adsorption) and moreorganic matter. Thus, thecompound (a derivative of the herbicide bentazon) is more highly adsorbed in the Sharkey.

The plot is called an isotherm and relates theconcentration of a chemicalassociated with solids tothe concentration of thechemical in solution.

The terms adsorbed and sorbed are not necessarily synonymous. The formerimplies chemical adherence to a surface, while the latter is broader, also includingprecipitation. Use of adsorbed on the y-axis of the graph would be more apt.

Degradation

Rate expressed in half-life

Time required for ½ to be degraded

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

Nonbiological

PhotodegradationChemical degradation

Biological

More important

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 organicchemicals, degrada-tion is relatively fast. Personally, Idon’t trust one of these data sets.

Leaching

Fast / slow in soil with large hydraulicconductivity?

Degradation and sorption reduce leaching

First, water movementis faster. Second, wherethis occurs, the soil is macroporous, perhaps sandy,and if sandy, there is little surfacearea or organic matter foradsorption.

Compare the mobility (mass and velocity) ofcompounds A, B, C and D that differ in theextent to which they are adsorbed and rate at which they are degraded.

Runoff

Properties that favor infiltration limit runoff

A contaminant maymove in runoff as thedissolved chemical species or bound to suspended solids

Bioremediation of OrganicContamination

Increase microbial activity to degradecontaminants

Cheaper and greener than excavation andsolvent extraction / incineration

Feed us! Feed us!

Biostimulation

Add nutrient source to raise numbers andactivity of indigenous degraders

Hydrocarbon degraders stimulated byfertilizer application

Why?

Subsurface biostimulation by injection ofair and nutrients

Why?

Adding N, P, K, etc. adds essential elements needed by thepopulation 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.

Super Bug

Bioaugmentation

Inoculate soil

Must be able to compete

Fortify with selected native strains?

Long-term exposure to contaminant mayincrease numbers of degraders

Early work of this type led toisolation of organisms that hada great capacity to degrade a specific contaminant but thatwhen introduced into soil at thecontaminated site, died off.

Isolation ofsuperior degraders from the site worked. site worked.

Inorganics

Sewage sludgeLandfillsSalinity from irrigationArsenical pesticidesLeaded gasolineRadioactive waste storage

Some sources of inorganic contaminants.

Form Fraction Mobility

Exchangeable Small Low ions

Bound to Medium Lowerorganicmatter

Bound by Large Lowestminerals orprecipitated

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

Managing Inorganic Contamination

Reduce mobility and plant availability

Lime to precipitate

Drain to aerate / oxidize

This is what you want to do and often simple prescriptions are effective,like

See next slide for common effectof soil pH on the solubility of manymetals. Recall discussion on metalmicronutrient solubility.

Perhaps surprising but true, the chemically reduced form is more solubleat any pH than the oxidized form. Thus, wet, reducing conditions in the soilfavor 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 adsorptionedges (the figure doesn’t look very edgy but imagine the pH range was expandedand the spacing shortened –more edgy). You get the point, though, decreasing mass in solution with increasing pH.

Phytoremediation

Use of hyperaccumulating plants

Grow, harvest and recover

Requires high uptake and biomass yield

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