The article focuses on adsorption of heavy metal ions on soils and soils constituents such as clay minerals, metal (hydr)oxides, and soil organic matter. Empirical and mechanistic model approaches for heavy metal adsorption and parameter determination in such models have been reviewed. Sorption mechanisms in soils, the influence of surface functional groups and surface complexation as well as parameters influencing adsorption are discussed. The individual adsorption behavior of Cd, Cr, Pb, Cu, Mn, Zn and Co on soils and soil constituents is reviewed.

Adsorption of heavy metal ions on soils and soils constituents

Preface.- Contributors.- List of Abbreviations.- Section 1: Basic Principles: Introduction.-Sources of Heavy Metals and Metalloids in Soils.- Chemistry of Heavy Metals and Metalloids in Soils.- Methods for the Determination of Heavy Metals and Metalloids in Soils.- Effects of Heavy Metals and Metalloids on Soil Organisms.- Soil-Plant Relationships of Heavy Metals and Metalloids.- Heavy Metals and Metalloids as Micronutrients for Plants and Animals.-Critical Loads of Heavy Metals for Soils.- Section 2: Key Heavy Metals And Metalloids: Arsenic.- Cadmium.- Chromium and Nickel.- Cobalt and Manganese.- Copper.-Lead.- Mercury.- Selenium.- Zinc.- Section 3: Other Heavy Metals And Metalloids Of Potential Environmental Significance: Antimony.- Barium.- Gold.- Molybdenum.- Silver.- Thallium.- Tin.- Tungsten.- Uranium.- Vanadium.- Glossary of Specialized Terms.- Index.

Heavy metals in soils : trace metals and metalloids in soils and their bioavailability

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Sorption isotherms: A review on physical bases, modeling and measurement

VII. Synthesis 2569 VIII. Adsorption and Solid Solution 2570 A. General Observations 2570 B. Adsorption of Specific Species 2571 1. Various Cations 2571 2. Various Anions 2572 3. Organic Species 2573 C. Environmental Implications 2573 IX. Transformation of Ferrihydrite 2574 A. Dry Thermal Transformation 2574 B. Aqueous Transformation 2575 C. Effects of Various Ions on the Aqueous Transformation of Ferrihydrite 2576

Occurrence and Constitution of Natural and Synthetic Ferrihydrite, a Widespread Iron Oxyhydroxide

Removal of copper, nickel, cobalt and manganese from aqueous solution by kaolinite.

Organic Matter Characterization

Oxides of Fe and Mn in soils are capable of sorbing large amounts of trace metal ions and can therefore be important in controlling trace metal concentrations in soil solution, and hence trace metal bioavailability in soils. There is, however, relatively little information on the rates of desorption of trace metals from oxide materials or on the factors affecting desorption rates. The objective of this study was to examine the kinetics of desorption of Cd and Co from two Fe oxides, goethite and ferrihydrite, and from two Mn oxides, hausmannite and cryptomelane. The concentrations of Cd and Co specifically sorbed by the oxides at pH 6.0 were greater for the Mn oxides than for the Fe oxides. The metals were also much less readily desorbed from the Mn than the Fe oxides and, in general, Cd was more readily desorbed than Co from all four oxides. Increasing the initial sorption period from 1 to 15 wk substantially decreased the proportion of sorbed Cd or Co subsequently desorbed from goethite, with a similar but much smaller effect also observed with the Mn oxides. Desorption kinetics for both Cd and Co were found to be described well by assuming either the occurrence of two simultaneous first-order desorption reactions, or by a continuous distribution of reaction sites, distributed lognormally with respect to desorption first-order rate constant. With increasing initial sorption period, the parameters obtained from fitting either type of kinetic equation to the experimental data could be interpreted as indicating a movement of metal ions to sites with slower desorption reactions.

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Kinetics of cadmium and cobalt desorption from iron and manganese oxides

Soil solution metal concentrations and hence metal bioavailability are controlled by sorption-desorption reactions at the surfaces of soil colloids. Compared with the numerous studies on sorption of metals by soils, there are relatively few that examine desorption, and even fewer that have measured desorption kinetics. The objective of this study was, using techniques and models developed previously for studies with synthetic oxides, to examine the effect of sorption period on the subsequent desorption kinetics of Cd and Co from two natural soil clay fractions. Cobalt was less readily desorbed from the clay fractions than Cd, but with both metals and clay fractions, increases in sorption period were accompanied by substantial decreases in the proportions of sorbed metal desorbed. As with the oxides studied previously, desorption kinetics for Cd and Co sorbed onto clay fractions were found to be described well by both a two-site first-order model and a model involving a lognormal distribution of first-order rate constants. Reaction half-lives derived from the models suggested a movement of Cd and Co to slower desorption reactions with increasing sorption period. However, some caution should be used in interpreting such parameters.

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Cadmium and cobalt desorption kinetics from soil clays: effect of sorption period

Availability of soil Cu to plants is dependent on the desorption into the soil solution of Cu from the surfaces of soil colloidal materials. Although there have been many studies of Cu sorption by soils, however, few have examined the process of Cu desorption. The objective of this study was to examine some of the factors likely to affect the desorption from soils of both native Cu and Cu added to soils. Ten New Zealand soils were used in this study and Cu desorption was determined by repeated equilibrations in 0.01 M Ca(NO,)2 solution. Desorption of native soil Cu varied between soils and the proportion of the total labile Cu in the soil (as determined by EDTA [ethylenediamine tetraacetic acid] extraction) that could be desorbed readily was strongly influenced by pH. Below pH 6.5, desorption increased with decreasing pH, and above pH 6.5 increased with increasing pH. When Cu was added to the soils (7 mg Cu kgsoil) with an initial contact period between added Cu and soil of 24 h, <8.5% of the added Cu could be desorbed readily. The proportion of added Cu desorbed was reduced substantially by increasing the contact period to 12 wk before desorption. Desorption of native soil Cu was increased by increasing the temperature at which desorption was carried out. The effect of temperature on the desorption of added Cu differed between soils. The results of this study provide more evidence for the existence of slow reactions between added Cu and soil that reduce the ability of the Cu to desorb back into the soil solution. I is NOW generally accepted that soil solution concentrations of micronutrients (and hence availability to plants) are probably controlled by sorptiondesorption reactions at the surfaces of soil colloidal materials (Swift and McLaren, 1991). However, although numerous studies have examined the sorption of Cu by soils and soil components (McLaren and Crawford, 1973; Jarvis, 1981; McLaren et al., 1981; Raikhy and Takkar, 1981; Sanders, 1982), very few studies have examined the reverse process, i.e., desorption of Cu into solution. As noted by Swift and McLaren (1991), this is in some ways a paradox, since it is desorption processes that are likely to control the amount and rate of release of Cu (or other micronutrients) into solution for plant uptake. Of the few Cu desorption studies reported in the literature, most have examined desorption from individual soil components, e.g., McLaren et al., 1983; Padmanabham, 1983a. An exception to this is the study reported by Lehmann and Harter (1984), who examined the desorption kinetics of Cu applied to an intact soil. However our understanding of the factors affecting desorption of Cu from soils is far from complete. Such information is required in order to improve our ability to predict the release of both native Cu from soils and the release of Cu added to soils as a fertilizer or as a pollutant. The objective of our study was to examine the deD.S. Hogg, Woodward-Clyde Consultants, 1550 Hotel Circle North, San Diego, CA 92108; R.G. McLaren, Dep. of Soil Science, Lincoln Univ., Canterbury, New Zealand; and R.S. Swift, Dep. of Soil Science, Univ. of Reading, London Road, Reading, England. Received 20 Apr. 1992. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 57:361-366 (1993). sorption of both native and applied Cu from a range of New Zealand soils. In particular, the effects of soil pH and temperature on Cu desorption were studied and, in the case of applied Cu, the effect of contact time between the Cu and the soil. MATERIALS AND METHODS

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Desorption of copper from some New Zealand soils

The dissociation reactions of copper(II)-humic acid complexes were investigated using a competing ligand spectrophotometric technique. The effects of pH, initial Cu:HA ratio, electrolyte concentration, and predissociation equilibration time on the rates of complex dissociation were studied and are discussed in terms of equilibrium speciation and macromolecular conformation. In general, increasing pH or predissociation equilibration time decreased Cu-HA complex dissociation rates; increasing Cu:HA ratio or electrolyte concentration increased Cu-HA dissociation rates. These trends are shown to be consistent with slower dissociation from more thermodynamically stable complexes

Roger S. Swift

Papers

Organic Matter Characterization

Kinetics of cadmium and cobalt desorption from iron and manganese oxides

Cadmium and cobalt desorption kinetics from soil clays: effect of sorption period

Desorption of copper from some New Zealand soils

Response of copper(II)-humic acid dissociation kinetics to factors influencing complex stability and macromolecular conformation