Effect of Redox Processes on Soil and Water Quality
Investigators (most current known information)
The oxidation state of soils and soil minerals has great influence on the chemistry of the soil system. We found in laboratory studies that the water content or swellability of various clay minerals in their natural oxidized state decreased if the exchangeable cations was changed from Na to any of the organic cations tetramethyl ammonium (TMA+), trimethylphenyl ammonium (TMPA+), or hexadecyltrimethyl ammonium (HDTMA+). If the oxidation state of iron (Fe) in the Na-exchanged clay minerals was reduced from Fe(III) to Fe(II) the clay swelling also decreased; but, if the organo-clay was reduced from Fe(III) to Fe(II), the swelling increased dramatically, to levels even exceeding that of Na-exchanged oxidized clay. This behavior is explained by the effect of Fe reduction on two counter-acting processes governing clay swelling. Iron reduction increases the attractive force between superimposed clay layers, limiting their swellability. Iron reduction also increases the attractive force between the clay surface and adsorbed water, thereby increasing clay swelling. When an inorganic cation such as Na is the exchanged cation, layer collapse becomes the dominant process; whereas, if the organic cation is the exchanged cation, the larger cation precludes layer collapse and surface hydration dominates. An enhanced interaction between clay surfaces and interlayer water was also observed by infrared spectroscopy. We further observed that reduction of Fe in the clay increases the surface basicity of the clay interlayer, causing dehydrochlorination of chlorinated aliphatic organic molecules. Oxidation-reduction reactions also occur at reduced clay surfaces, as evidenced by redox mediated degradation of nitromethanes. Iron reduction also brought about a decrease in specific surface area and an increase in cation exchange capacity.
We also took these studies to the field, measuring the redox potential continuously for one year at depths of 7.5, 15, 30, 60 and 90 cm, using a field station installed in a commercial sod-production operation at Kibbutz Givat-Brenner, Israel. pH was measured at 40 cm, and temperature and water inputs (rain and irrigation) were also recorded. A permanently reduced layer (pe=0.3±3.0) was found at 15 cm depth. Higher and lower horizons were, on average, more oxidized. The pe variability at a given depth was caused by seasonal, soil, and agrotechnical variations. It appears that the low redox potential at the 15 cm horizon was related to a combination of the irrigation regime, input of effluents, root activity and cultivation practices. Higher weighted average redox potential at deeper horizons (to 90 cm) was due to lower local concentrations of organic matter. At the shallow depth (7.5 cm), higher average redox potential was due to faster gas exchange with the atmosphere.
The study is continuing at other sites, having different soil conditions and cultivational histories. The effect of prolonged soil drying on the redox potential in situ is studied as well as the effects of water quality parameters (fresh vs. reclaimed wastewater).
The results of these studies have established the feasibility of long-term, non-disturbing in situ measurement of the redox potential variations in field soils. Further study will contribute to the ability to manage soil redox under conditions of continued heavy loading of wastewater, as is anticipated in Israeli agriculture in view of the scarcity of freshwater.
Continued experimentation at the field site is planned: (a) to impose prolonged drying periods on the plot in order to dry the soil and permit better gas-exchange and soil aeration; (b) to initiate an experiment at an adjacent plot where fresh water will be used for the lawn irrigation instead of treated effluents, and the redox regime will be measured; and (c) sample reduced soils and analyze the changes in the soil solution chemical composition and in Fe and Mn mineralogy of the solid phase, and their correlation with the measured redox regime.
No outcomes reported