University of Saskatchewan
Department of Soil Science
College of Agriculture and Bioresources

Environmental Soil Chemistry

Oxyanion reaction mechanisms with soil components

Many different oxyanions are found in the soil environment, and the chemistry of these oxyanions is quite varied. Oxyanions are studied for both agronomic and environmental reasons by soil chemists. Some oxyanions such as phosphate and sulfate are essential nutrients for plant growth and are found in relatively high concentrations in soils. Other oxyanions such as borate are micronutrients. They are essential for plant growth at low concentrations but become toxic at higher concentrations. The range between deficiency symptoms and toxicity is usually quite narrow. A third group of oxyanions, such as arsenate, arsenite, selenate, selenite, and chromate are frequently studied because they have little agronomic use and are instead detrimental to human health. Understanding the mechanisms by which these oxyanions react with mineral surfaces is vital for predicting their bioavailability, toxicity, and transport in natural systems.

The environmental soil chemistry program at University of Saskatchewan uses an array of advanced spectroscopic techniques to determine the nature of interactions between oxyanions and soil components. Several types of X-Ray Absorption Spectroscopy (XAS) have been utilized as well as Attenuated Total Reflectance (ATR) FTIR spectroscopy. While the mechanisms of adsorption are unique to both the sorbate and sorbent being studied, there is a recurring theme in all studies that is significant.

In many cases, more than one adsorption mechanism can be seen to occur simultaneously. In systems where only adsorption is occurring, then both outer-sphere and inner-sphere complexation are often observed at the same time. This distribution of outer-sphere and inner-sphere surface complexes has been seen with sulfate, selenate, and borate on iron oxides. Reaction conditions such as pH, ionic strength, and reactant concentration have all been shown to influence the relative distribution of the simultaneously occurring sorption complexes. Therefore it is necessary to conduct spectroscopic studies at a wide range of reaction conditions to truly understand the reactivity of oxyanions. It is also important to account for outer-sphere complexation of oxyanions when attempting to describe their adsorption with surface complexation models.