Trace Elements

    Potential hazards associated with trace elements pertain to their accumulation in soils which may (1) lead to a plant toxicity condition or (2) result in increased uptake of metals into the food chain.  Ten elements are of primary concern are arsenic, boron, cadmium, copper, mercury, molybdenum, nickel, lead, selenium, and zinc.  Of these cadmium has the most dangerous long-term effects on human health.  Many of these are amplified in the food chain.  All logical pathways by which pollutants in biosolids could reach various biological species in the environment were included in USEPA's models that were examined in preparing the 40 CFR Part 503.

 
 
Adverse effects

When an plant reaches the threshold concentration limit for a particular element, the growth and the metabolism of that plant is severely effected.  Even essential elements for plant growth have a threshold limit concentration limit.  Applying sewage sludge may lead to higher yields by supplying essential nutrients and improving soil properties. This practice is also one of the more common methods for concentrating trace elements in the soils.

Cadmium, Copper, Nickel and Zinc are four trace elements in sludge that pose a hazard to plants, animals and humans. Cadmium has caused adverse health effects in humans who have eaten food grown on Cd rich soils. Copper, Nickel and Zinc were shown to accumulate to toxic levels in plants.
Any element can have adverse health effects if the concentrations are high enough. The following table shows the species groups that are at risk from trace and toxic elements. (Hagedorn, 96)
 
 

This next table shows the biological functions and toxicity of important trace elements. (Hagedorn, 96)
 
 

Mobility in Soils
 

Small amounts of Cd, Cu, Ni, and Zn occur in soil solution because they are tightly held by the soil surfaces.  The largest threat to surface waters is from soil erosion rather than leaching of these metals to the ground water.  Soils with large amount of hydrous oxides and phyllosilicates are best to adsorb these metals and reduce the chance of leachate problems.

The metals Cd and Hg are adsorbed less intensely and pose a larger threat to movement and availiblity.  Cd is a slightly soluble metal that behaves similarly to Ca in the soil solution.  Above a pH of 7, Cd will  precipitate which will limit solubility and mobility.  Mercury has a different fate when released into a soil body.  Mercury volitalizes and is released as a gas when in a soil body. This reaction is greatly increased when saturated soil conditions exsist.  The toxic substance dimethyl mercury is quite toxic and can be found in reduced aquatic environments.

Fate of trace elements in soils.

There are four main processes for the solution phase concentration of trace elements:

• Ion exchange on layer silicates
• Precipitation reactions
• Sorption to hydrous oxide surfaces
• Complex formation with soil organic matter

Ionic exchange

Layer silicates in  the soils provide  permanent charges and ph dependent charges which retain trace metal cations by nonspecific electrostatic forces. These elements compete with Ca and Mg for the available exchange sites.  Trace elements are retained to higher concentrations when in lower ph systems.  At a higher ph, metal hydrolysis is more prevalent and is the domimant reaction.  Cd, Cu, Cr,Hg,Pb Ni and Zn are the main trace elements that are of greatest concern on a waste treatment facility.

Precipitation reactions

When an element undergoes a precipitaition reaction, a certain sequence happens in  the process as the concentration of the element in solution increases.
 

• elements are absorbed on the particle surfaces.
• The reactants are supersaturated in the reaction.
• crystal growth

The major classes of precipitates found in soils are silicates oxides ,carbonates, phosphates and sulfates. These precipitates form salts in the system which are not beneficial to the soil system or the waste treatment system. Precipitate reactions are completely reversible and still have some dissolution properties even in stable solid phases.

Sorption to Hydrous Oxide surfaces

 This process involves the altering of the surface charge using adsorbtion or chemisorption. Trace elements in the form of cations and anions will form short directional bonds with oxide surfaces.
The following table shows cations and anions relative strengths in forming bonds with soil particles. (Hagedorn, 96)

Cation's relative bonding strengths
 

Anion's relative strengths
 
 

Complex formation with soil organic matter

Soil organic matter has many functional groups contained in it that can serve as exchange sites.  Most  of the compounds in question are functional groups high in oxygen.  Groups found in soil organic matter and react with trace elements will most likely contain either COOH groups or OH groups.  These functional groups help drive the complex reactions in the organic matter.  Trace metal compounds are tied up by the highly reactive oxygen groups and hold the metals in place.
 

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