Influence of pH on the nature of aluminum-selenium precipitates

James J. Dynes^a, A. G. Hardie^b, T.Z. Regier^a and D. Peak^c

^aCanadian Light Source, University of Saskatchewan, Canada

^bDepartment of Soil Science, Stellenbosch University, South Africa

^cDepartment of Soil Science, University of Saskatchewan, Canada

james.dynes@lightsource.ca

Selenium is an essential element for humans and animals at low concentrations, while at slightly higher concentrations it is toxic. Whether Se is toxic or essential for plants has yet to be established. Plants are the main nutritional source of Se for humans and animals, and their Se content depends on the bioavailability of Se within the soil environment. An understanding of Se transformations and transport processes, is, thus, essential for assessing the bioavailability of Se in the environment.

Adsorption/desorption of Se by soils has been identified as an important mechanism controlling Se levels in soil solution and natural waters. Whether or not precipitation/dissolution reactions are a mechanism that controls Se bioavailability is still not clear. Aluminum is ubiquitous in mineral soils, and through chemical weathering it is undergoes hydrolysis, forming Al precipitates. The conditions under which Al-Se minerals and noncrystalline Al-Se precipitates form is incomplete.

Al-Se precipitates were prepared by adjusting the pH of Al(ClO₄)₃-SeO₃ and Al(ClO₄)₃-SeO₄ solutions to 3.5, 4.5, 5.5, 6.5, 7.5 and 8.5. The pH was periodically maintained for 1 year, after which the precipitates were filtered, washed with water and freeze-dried. The precipitates were characterized by X-ray diffraction, FTIR spectroscopy, solid state ²⁷Al NMR, X-ray absorption spectroscopy (Al and O K-edges, Se L-edge) and chemical methods. The study showed that both noncrystalline and crystalline Al-Se materials can form under ambient conditions and that the pH of the system was the overlying controlling factor. The results indicate that precipitation/dissolution reactions under the pH range prevailing in soil environments can control the Se concentration in soil solutions.