Silver in Welsh soils: Physical and chemical distribution studies

Abstract

Profile distributions of Ag and other metals (Cd, Cu and Pb) in contaminated and uncontaminated soils from west Wales, U.K. are presented. Background Ag levels were <0.1 μg Ag g −1, whilst profiles contaminated by past mining activities contained up to 9 μg Ag g −1. Silver showed a marked surface enrichment in all cases. Adsorption of AgNO 3 by various soil samples was satisfactorily modelled by the Freundlich isotherm equation. Values for k (a constant related to the binding capacity) ranged between 50 and 1450. The Langmuir isotherm equation gave a less satisfactory summary of the adsorption behaviour. Radioactive 110mAg added to intact cores of an alluvial and a peat soil and subjected to leaching over one year (at the annual precipitation rate) remained in the surface layers; after this time 110mAg was not leached appreciably below 40 mm. A chemical fractionation procedure for soil Ag was applied to the contaminated soils. In soils contaminated between 80 and 150 years ago there were negligible amounts of readily exchangeable Ag; nearly half the total Ag was “residual” (i.e., solubilised only in concentrated nitric acid). The two intact soil columns still contained a considerable proportion of the 110mAg in the readily exchangeable form after one year, but most was bound in ‘acid-reducible’ or ‘oxidisable-organic’ forms. The evidence for a strong association between soil organic matter and Ag indicates that humus may control the availability of Ag in the short term and that mineral prospecting for Ag by the chemical analysis of soil humus may be a useful reconnaissance technique. Fixation in a residual fraction may be a mechanism which reduces the bioavailability of the element in the long term.