Adsorption and desorption of copper and zinc in the surface layer of acid soils

Abstract

The environmental and health effects of the contamination of soils by heavy metals depend on the ability of the soils to immobilize these contaminants. In this work, the adsorption and desorption of Cu and Zn in the surface layers of 27 acid soils were studied. Adsorption of Cu^II from 157–3148 μmol L⁻¹ solutions was much greater than adsorption of Zn^II from solutions at the same concentration. For both Cu and Zn, the adsorption data were fitted better by the Freundlich equation than by the Langmuir equation. Multiple regression analyses suggest that Cu and Zn adsorption depends to a significant extent on pH and CEC: for both metals these variables accounted for more than 80% of the variance in the Freundlich pre-exponential parameter $K_{\mathrm{F}}$, and pH also accounted for 57% of the variance in $1/n$ for Zn and, together with carbon content, for 41% of the variance in $1/n$ for Cu. The percentage of adsorbed metal susceptible to desorption into 0.01 M NaNO₃ was greater for Zn than for Cu, but in both cases depended significantly on pH, decreasing as pH increased. In turn, both pH_H2O and pH_KCl are significantly correlated with cation exchange capacity. Desorption of metal adsorbed from solutions at relatively low concentration (787 μmol L⁻¹) exhibited power-law dependence on $K_{\mathrm{d}}$, the quotient expressing distribution between soil and soil solution in the corresponding adsorption experiment, decreasing as increasing $K_{\mathrm{d}}$ reflected increasing affinity of the soil for the metal. The absence of a similarly clear relationship when metal had been adsorbed from solutions at relatively high concentration (2361 μmol L⁻¹) is attributed to the scant between-soil variability of $K_{\mathrm{d}}$ at these higher concentrations. In general, adsorption was greater and subsequent desorption less in cultivated soils than in woodland soils.

Introduction

Heavy metals can be introduced into agricultural soils by industrial emissions and by a variety of agricultural treatments, including fertilizers, liming materials, manure, sewage sludges, and composts [1]. Metal accumulation in arable soils causes degradation of soil and soil productivity losses [2]. The hazard posed by these contaminants through uptake by plants and passage into groundwater depends on their nature and concentration in the soil solution, and their concentration depends in turn on their adsorption by and desorption from soil colloids. These species may be adsorbed either nonspecifically, acting as counterions in the diffuse layer, or by surface complexation processes in which they form more specific bonds with colloid components. The adsorbent colloidal materials can be clay minerals [3], [4], [5], organic matter [6], iron and aluminum oxides [7], organo-mineral associations [8], [9], [10], or natural zeolites [11], and the behavior of heavy metals in soil therefore depends on the type of soil.

In the cases of copper and zinc, there is considerable literature on their adsorption onto soil components, but very little available information on the effects of soil properties on their desorption, which may not be the same as the effects on adsorption. Here we report the results of a study of how the adsorption and desorption of copper and zinc in the surface layers of a series of acid soils, in which these metals are present mainly as divalent cations, varied with the properties of these media.