Release Of Some Trace Elements From Sluiced Fly Ash On Acidic Soils With Particular Reference To Boron

Abstract

The purpose of this study was to examine the relationship between the concentration of boron (B) and some other selected trace elements in soil solution as effected by hydrogen ion activity within the normal pH range for acidic soils commonly amended with agricultural limestone and, alternatively, alkaline fly ash. Sluiced alkaline fly ash was applied to an acidic, clay textured soil at rates equivalent to 0, 42, 84, 125 and 167 tonne ha-1 based on the soil lime requirement. After wheat was grown and harvested the soil-ash mixtures were maintained at field capacity moisture content for an additional 4 months before pore water samples were extracted by immiscible displacement. The total concentrations of Co, Cr, Fe, V and Zn in the ash treated soils increased by < 10% at the highest application rate of ash, the content of Cu was increased by 13% and B by 38%. Only the concentration of boron increased appreciably in the pore water extracts. Release of B from the ash was correlated with the solubility behaviour of Ca and Mg, and not with the dissolution of glass phases in the ash. Speciation and adsorption calculations for the extracts were carried out using the program MINTEQ. Common Ca, Mg and Na borate minerals were undersaturated with respect to the equilibrated solutions. Application of the constant capacitance model to the adsorption of B on mineral surfaces suggested that adsorption had little effect on total dissolved B at pH values below 6.0. Predicted concentrations of B in solutions, equilibrated with calcite in a subsurface horizon, were up to 10.6 mg dm-3; more than double the recommended maximum concentration for B (5mg dm-3) in potable water in Ontario.