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Adsorption and heavy metal partitioning in soils and sediments of the Salmon River area, British Columbia Bhoojedhur, Seewant

Abstract

The role of soils in the adsorption and release of heavy metals and their interaction in soil systems has not been extensively investigated. The interface between soils and water systems and the effects of land use practices on heavy metal adsorption, partitioning and release is an area of inquiry that requires study. In this study, both empirical and mechanistic approaches were utilized for evaluating the fate and behaviour of the heavy metals cadmium, lead, copper and zinc in clay minerals, and soils and sediments of the Salmon River watershed. Various extracting agents were chosen to study their effectiveness of heavy metal extraction following sample fortification. The extracting agent 0.05 M E.D.T.A. and a shaking period of two hours were selected as a good compromise for studying the distribution of heavy metals. Lead was retained to a greater extent than Cd, Cu and Zn and correlated with organic matter, and to a lesser extent with cation exchange capacity of the soils and sediments. The physico-chemical approaches utilized for studying adsorption of Cd, Pb, Cu and Zn showed adsorption phenomena for the smectite group of minerals to be different than the 2:1 non-expanded type (illite) and the 1:1 clay mineral (kaolinite), when the data were incorporated into the Langmuir equation. Irrespective of clay mineral type, adsorption conformed to the Freundlich equation at the pH values of 4, 5, 6 and 7 and over a concentration range of 0-3000 ppm. Common among adsorption on clay minerals, soils and sediment was that heavy metal adsorption at low pH values was in excess of the corresponding cation-exchange capacity and adsorption increased with increasing pH. Adsorption mechanisms other than the monolayer type are suggested. Conformity with the Langmuir equation for soil and sediment adsorption was found to vary with metal type, concentration range and pH. No differences in bonding energy of lead were observed among the soils at the same pH. Adsorption of Pb, Cd and Zn on soils and sediments at pH 5.0, were positively correlated with clay content, organic matter and cation exchange capacity; negative correlations by Pb and Cd were observed with oxalate extractable Fe and Al, but Zn was positively correlated. Adsorpti by soils and sediments followed the Freundlich equation over the entire 0-3000 ppm concentration range. Competition studies showed adsorption of an equivalent mixture (1:1:1:1) of Cd, Pb, Cu and Zn on montmorillonite at pH 5 to follow the Langmuir equation. Adsorption from the same equivalent mixture on kaolinite and two soils of differing texture and organic matter content did not fit the Langmuir equation. Comparative data from soils under different land use practices showed Pb and possibly Cd to be associated with traffic proximity. Analysis of the sediments indicated higher metal values in industrial, agricultural and near the mouth of the river than from upstream rural sites. An extension of the physico-chemical approach to soils and sediments provides adsorption maxima that are of predictive value for the heavy metals studied. This approach is useful to evaluate the effect of land use practices on heavy metals.

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