Page 71 - RoadCem Manual - PCT BV

In soil there are aerobic soil reactions. In these reactions oxygen acts as the terminal electron acceptor. This reaction results in the most energy for the living organisms in soil and is preferred. Respiration uses oxygen as the terminal electron acceptor. If the oxygen is removed from the system then the soil is said to be anaerobic, that is without air. Once the oxygen is removed several anaerobic reactions will occur. The sequence that the reactions occur in is dependent on the amount of energy each reaction will produce. The sequence starts with aerobic reactions (respiration), then denitrification, then Mn or Fe reduction, then sulfate reduction, and finally carbon dioxide reduction.
Denitrification occurs when nitrate is used as the terminal electron acceptor. Nitrate is reduced to nitrogen gas. This reaction can be measured in soil by measuring the production of N2 gas. There is no morphologic signature to this reaction.
Iron (Fe) reduction occurs when Fe3+ is used as the terminal electron acceptor. Fe3+ (ferric iron) is reduced to Fe2+ (ferrous iron). Unlike nitrate reduction there is a morphologic signature of Fe reduction. Consider soil particles to be coated or painted with Fe-oxide paint (rust). This is similar to the way in which M&Ms are coated with a colorful candy shell. If the M&Ms get wet the shell is washed or dissolved off leaving the white candy shell beneath. Similarly the rusty coating on soil particles is dissolved off as the Fe3+ is converted to Fe2+. The Fe2+ is colorless and soluble in water. The gray color that remains is the color of the mineral grains.
Sulfate reduction occurs as sulfate is converted to hydrogen sulfide gas, which smells like rotten egg. Generally this odor is only encountered when the soil is saturated and reduced for sulfate.
Redoximorphic features, formerly known as mottles, are formed by changes in redox conditions in seasonally saturated soil, the reduction and oxidation of C, Fe, Mn, and S compounds, and the subsequent translocation of C, Fe, Mn, and S compounds. The best evidence of this process is to find features caused by reduction and oxidation in the same profile. The oxidized features are evidence of translocation. In order to for features to form the following must occur:
• must have anaerobic conditions (reduced and saturated - stagnant);
• must have Fe and/or Mn (electron acceptor);
• must have microbes (bugs);
• must have carbon (food for the bugs).
Within the soil reducing conditions may occur adjacent to organic matter. For example Fe is reduced near the dead root after all the oxygen and nitrate is removed from the water. The reduced Fe is soluble in water and diffuses away from the root area leaving gray minerals behind. As the reduced Fe interacts with the water that has not had all its oxygen removed it will reoxidize or rust leaving a red rim around the reduced area in the middle. Since the bacteria are not mobile the area of reduction will be near a carbon source.
It is important to know the chemical composition of the sub-base to determine how much RoadCem is needed. The organic content has an effect on the binding of the material. When contaminated substances need to be immobilized then the amount of RoadCem that is needed is higher. When there are strongly contaminated substances present then it is maybe required to use ImmoCem. The procedure to use in order to evaluate chemical composition of the soil is specified in figure 4.20.
71