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Q: Comparison of bioremediation effects among sodium percarbonate，calcium peroxide，magnesium as oxygen release compounds

A: Oxygen-release compounds increase the oxygen content of contaminated areas, enhancing biological activity and thus promoting natural attenuation. The specific compound used will depend on soil chemistry, concentration of target organics, type of target organics and cleanup levels. Parameters of interest are release rate of oxygen at different effective partial pressures and ratio of oxygen released to amount of oxygen applied. Researchers studied the solid oxidants below with respect to dissolution rate and ease of movement through other media:

• Na2CO3-1.5H2O2 encapsulated sodium percarbonate

• Free sodium percarbonate crystals

• CaO2, calcium peroxide

• MgO2, magnesium peroxide

 

Oxygen movement

Oxygen movement in the subsurface is influenced by：

• Soil heterogeneity

• Moisture content, which can hinder O2 movement

• Pore size——a function of sediment age and history

• Tortuosity, caused by small pore sizes, which increases O2 path distance

 

Soil morphology directly influences O2 diffusion through the soil and soil redox potential, and the biological degradation that will occur at interfacial areas. In the interstitial pores, microbes are protected from toxic compounds. "Interstitial pore space attachment also makes predation more difficult. Solid oxidants can exhibit slow dissolution and fall into a reaction-limited domain. Conversely, these compounds can release oxygen from their surfaces rapidly, exhibiting transport limitations. Researchers predicted that the encapsulated Na2CO3 +1.5H2O2's release of O2 was by diffusion-limited transport while the other studied oxidants were controlled by chemical reaction kinetics of dissolution. The kinetics of dissolution have both chemical and thermodynamic limitations. Reactions are as follows：

2H2O + MgO2 ↔ Mg(OH)2(s) + H2O2 

2H2O + CaO2(s) + ↔ Ca(OH)2(s) + H2O2 

4Na2CO3•1.5H2O2 ↔ 8Na+ + 4CO3- + 6H2O2 

H2O2 + H2O2 ↔ O2 + 2H2O 

 

Some of the reaction products produced Mg(OH)2 and Ca(OH)2 have solubility values lower than the ions added. Such precipitates may coat reactant particles and block pores in both the soil and reactant particles, limiting transport of reacting ions and particles. Sodium percarbonate would release O2 by diffusion-limited transport whereas chemical kinetic reactions would control dissolution rate of other oxidants. Release rates of MgO2 and CaO2 could be limited because of self-encapsulation. 

 

Experiments and results：The unencapsulated Na2CO3• 1.5H2O2 had the most rapid release rate, followed by CaCO2, and encapsulated Na2CO3•1.5H2O2. MgO2 had the slowest O2 release by several orders of magnitude. However, the large size of both forms of Na2CO3•1.5H2O2 slows transport of bulk particles. CaO2 and MgO2 both have fractions small enough to permit migration where soil particles, and thus pore spaces, are larger than the particles of soil oxidant. In some cases, lack of movement of oxidant particles may be desirable in establishing stationary oxidative zones. Adding oxidants to water also changes the water's pH, usually in the range of 10 to 12. Shifts to high pH conditions generally have a negative effect on indigenous bacteria, but soils can have a buffering capacity to counteract or neutralize the pH shifts. 

 

Other conclusions：Release rates that are too rapid for biological uptake rates will prevent the utilization of all O2. Oxygen release rates below optimum may result in reduced aerobic metabolism or failure to maintain aerobic respiration. Of the oxidants tested, MgO2 has the widest application based on 

• O2 release rate, which was the longest

• pH shift, which was lowest

• O2 release per mass, which was highest

A: Oxygen-release compounds increase the oxygen content of contaminated areas, enhancing biological activity and thus promoting natural attenuation. The specific compound used will depend on soil chemistry, concentration of target organics, type of target organics and cleanup levels. Parameters of interest are release rate of oxygen at different effective partial pressures and ratio of oxygen released to amount of oxygen applied. Researchers studied the solid oxidants below with respect to dissolution rate and ease of movement through other media:

• Na2CO3-1.5H2O2 encapsulated sodium percarbonate

• Free sodium percarbonate crystals

• CaO2, calcium peroxide

• MgO2, magnesium peroxide

 

Oxygen movement

Oxygen movement in the subsurface is influenced by：

• Soil heterogeneity

• Moisture content, which can hinder O2 movement

• Pore size——a function of sediment age and history

• Tortuosity, caused by small pore sizes, which increases O2 path distance

 

Soil morphology directly influences O2 diffusion through the soil and soil redox potential, and the biological degradation that will occur at interfacial areas. In the interstitial pores, microbes are protected from toxic compounds. "Interstitial pore space attachment also makes predation more difficult. Solid oxidants can exhibit slow dissolution and fall into a reaction-limited domain. Conversely, these compounds can release oxygen from their surfaces rapidly, exhibiting transport limitations. Researchers predicted that the encapsulated Na2CO3 +1.5H2O2's release of O2 was by diffusion-limited transport while the other studied oxidants were controlled by chemical reaction kinetics of dissolution. The kinetics of dissolution have both chemical and thermodynamic limitations. Reactions are as follows：

2H2O + MgO2 ↔ Mg(OH)2(s) + H2O2 

2H2O + CaO2(s) + ↔ Ca(OH)2(s) + H2O2 

4Na2CO3•1.5H2O2 ↔ 8Na+ + 4CO3- + 6H2O2 

H2O2 + H2O2 ↔ O2 + 2H2O 

 

Some of the reaction products produced Mg(OH)2 and Ca(OH)2 have solubility values lower than the ions added. Such precipitates may coat reactant particles and block pores in both the soil and reactant particles, limiting transport of reacting ions and particles. Sodium percarbonate would release O2 by diffusion-limited transport whereas chemical kinetic reactions would control dissolution rate of other oxidants. Release rates of MgO2 and CaO2 could be limited because of self-encapsulation. 

 

Experiments and results：The unencapsulated Na2CO3• 1.5H2O2 had the most rapid release rate, followed by CaCO2, and encapsulated Na2CO3•1.5H2O2. MgO2 had the slowest O2 release by several orders of magnitude. However, the large size of both forms of Na2CO3•1.5H2O2 slows transport of bulk particles. CaO2 and MgO2 both have fractions small enough to permit migration where soil particles, and thus pore spaces, are larger than the particles of soil oxidant. In some cases, lack of movement of oxidant particles may be desirable in establishing stationary oxidative zones. Adding oxidants to water also changes the water's pH, usually in the range of 10 to 12. Shifts to high pH conditions generally have a negative effect on indigenous bacteria, but soils can have a buffering capacity to counteract or neutralize the pH shifts. 

 

Other conclusions：Release rates that are too rapid for biological uptake rates will prevent the utilization of all O2. Oxygen release rates below optimum may result in reduced aerobic metabolism or failure to maintain aerobic respiration. Of the oxidants tested, MgO2 has the widest application based on 

• O2 release rate, which was the longest

• pH shift, which was lowest

• O2 release per mass, which was highest