Chapter 1 The Biosphere Chapter 2 The Anthroposphere Introduction Air Pollution Water Pollution Soil Plants Chapter 3 Soils and Soil Processes Introduction Weathering Processes Pedogenic Processes Chapter 4 Soil Constituents Introduction Trace Elements Minerals Organic Matter Organisms in Soils Chapter 5 Trace Elements in Plants Introduction Absorption Translocation Availability Essentiality and Deficiency Toxicity and Tolerance Speciation Interaction Chapter 6 Elements of Group 1 (Previously Group Ia) Introduction Lithium Rubidium Cesium Chapter 7 Elements of Group 2 (Previously Group IIa) Beryllium Strontium Barium Radium Chapter 8 Elements of Group 3 (Previously Group IIIb) Scandium Yttrium Lanthanides Actinides Chapter 9 Elements of Group 4 (Previously Group IVb) Titanium Zirconium Hafnium Chapter 10 Elements of Group 5 (Previously Group Vb) Vanadium Niobium Tantalum Chapter 11 Elements of Group 6 (Previously Group VIb) Chromium Molybdenum Tungsten Chapter 12 Elements of Group 7 (Previously Group VIIb) Manganese Technetium Rhenium Chapter 13 Elements of Group 8 (Previously Part of Group VIII) Iron Ruthenium Osmium Chapter 14 Elements of Group 9 (Previously Part of Group VIII) Cobalt Rhodium Iridium Chapter 15 Elements of Group 10 (Previously Part of Group VIII) Nickel Palladium Platinum Chapter 16 Elements of Group 11 (Previously Group Ib) Copper Silver Gold Chapter 17 Trace Elements of Group 12 (Previously of Group IIb) Zinc Cadmium Mercury Chapter 18 Elements of Group 13 (Previously Group IIIa) Boron Aluminum Gallium Indium Thallium Chapter 19 Elements of Group I4 (Previously Group IVa) Silicon Germanium Tin Lead Chapter 20 Elements of Group 15 (Previously Group Va) Arsenic Antimony Bismuth Chapter 21 Elements of Group 16 (Previously Group VIa) Selenium Tellurium Polonium Chapter 22 Elements of Group 17 (Previously Group VIIa) Fluorine Chlorine Bromine Iodine

Trace elements in soils and plants

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

The essential micronutrients for field crops are B, Cu, Fe, Mn, Mo, and Zn. Other mineral nutrients at low concentrations considered essential to growth of some plants are Ni and Co. The incidence of micronutrient deficiencies in crops has increased markedly in recent years due to intensive cropping, loss of top soil by erosion, losses of micronutrients through leaching, liming of acid soils, decreased proportions of farmyard manure compared to chemical fertilizers, increased purity of chemical fertilizers, and use of marginal lands for crop production. Micronutrient deficiency problems are also aggravated by the high demand of modern crop cultivars. Increases in crop yields from application of micronutrients have been reported in many parts of the world. Factors such as pH, redox potential, biological activity, SOM, cation-exchange capacity, and clay contents are important in determining the availability of micronutrients in soils. Plant factors such as root and root hair morphology (length, density, surface area), root-induced changes (secretion of H + , OH − , HCO 3 − ), root exudation of organic acids (citric, malic, tartaric, oxalic, phenolic), sugars, and nonproteinogenic amino acids (phytosiderophores), secretion of enzymes (phosphatases), plant demand, plant species/cultivars, and microbial associations (enhanced CO 2 production, rhizobia, mycorrhizae, rhizobacteria) have profound influences on plant ability to absorb and utilize micronutrients from soil. The objectives of this article are to report advances in research on the micronutrient availability and requirements for crops, in correcting deficiencies and toxicities in soils and plants, and in increasing the ability of plants to acquire needed amounts of micronutrient elements.

Micronutrients in Crop Production

Soil chemists have long-recognized that knowledge of the elemental composition of soils is generally of little use in assessing the availability of these elements to plants. An obvious illustration of this principle is the common occurrence of Fe and Mn deficiency in plants despite the relatively high levels of Fe and Mn in many soils. For this reason, chemical soil tests have relied on measurement of extractable or “labile” fractions of elements. Such tests are empirical and provide little basis to relate metal extractability to the chemical forms of the metal in the soil. As soils are increasingly used in our society for purposes other than agriculture, the frequency and extent of soil contamination by toxic metals will increase. Empirical relationships may have to be replaced by a more fundamental understanding of the soil processes controlling metal solubility to prevent practices that could have deleterious effects on soil productivity and environmental quality.

Reactions controlling heavy metal solubility in soils

Summary
Samples of five soils whose pH in the field had been adjusted to between 5.0 and 7.5 were incubated with water or 0.01 m CaCl2 at 90% field capacity. Additional samples of the most acid soil were limed to various pH values immediately before incubation.

Manganese, zinc and cobalt concentrations in the soil solutions, collected by displacement, decreased as the pH increased; the concentrations in calcium chloride solutions were higher than those in water solutions. The free divalent ions Mn2+, Zn2+ and Co2+ were the major metal species in solution at pH 5 but the proportion of the metals present as the free ion decreased as the pH increased.

Differences in the manganese and zinc concentrations in the solutions were due not only to the pH of these solutions but also to the original pH of the soil in the field.

The effect of pH on the total and free ionic concentrations of mnganese, zinc and cobalt in soil solutions

Summary

Soil samples whose pH had been adjusted to between 4.5 and 7.5 either for long periods in the field or short periods in the laboratory were incubated after wetting with water or 0.01 m CaCl2. Copper concentrations in the soil solutions decreased only slightly as the solution pH increased, but free cupric ion concentrations decreased considerably. The copper concentrations were smaller and the proportion of copper present in solution as cupric ion at a given pH was larger when CaCl2 rather than water was used. Complexed organic species made up most of the copper in all solutions. The duration of pH adjustment did not affect these results.



Copper adsorption isotherms were determined on the soils using low equilibrium solution concentrations. As a given copper concentration the quantity of copper adsorbed increased and the proportion of copper in solution present as cupric ion decreased with pH increase; again the duration of pH adjustment did not affect the results.

The effect of pH upon the copper and cupric ion concentrations in soil solutions

Summary

The effects of varying the pH and ionic strength, and the concentrations of organic matter, copper, calcium and phosphate upon the complexing of copper by extracts of humified organic matter from laboratory preparations, soil and peat were measured. The extent of complexing increased as pH increased and ionic strength decreased, and with increasing organic matter: copper ratios in a way consistent with the complexant being heterogeneous. Calcium competed with copper for organic matter, but the effect of phosphate was negligible. The extent of complexing at pH 7 by equal weights of colloidal and dialysable material from the same extract were usually similar. Weight for weight of organic carbon, alkali-extracted humified organic matter complexed copper much more extensively than the corresponding water-soluble material.

The influence of pH, ionic strength and reactant concentrations on copper complexing by humified organic matter.

Summary

The metal complexing properties of the colloidal and non-colloidal fractions of water extracts of peat and aerobically composted lucerne, and of chelating resin and alkali extracts of peat and soil organic matter were compared.



The contributions of the non-colloidal fractions to the complexing properties of the whole extracts are considerable, and on a weight-for-weight basis are somewhat greater than that of the corresponding colloidal material. No one compound is preponderantly responsible for the complexing action. The complexing activity of the non-colloidal fractions is confined almost entirely to the acidic constituents. On the basis of the enhancement of the visible range of optical absorbance of the various preparations, Cu2+ is more extensively complexed than Mg2+, Mn2+, Cd2+, Zn2+, Co2+, Ni2+, and Pb2+.

The complexing of copper by humified organic matter from laboratory preparations, soil and peat

Summary

Samples of humified organic matter were extracted from several sources by water or alkali, and divided into colloidal and dialysable fractions. The variation of the concentration of complexed copper with that of added copper was determined for these fractions using a cupric ion-sensitive electrode. A computer program fitted five equations often used to describe adsorption of ions on soil surfaces to the results. The data did not fit Langmuir or Temkin equations very well, probably because of the heterogeneity of adsorption sites of the organic matter; they fitted double Langmuir, Gunary and Freundlich equations almost equally well, leaving very small residual sums of squares. The Freundlich isotherm is preferred as it has only two parameters and the values of these are fairly stable to changes in the range of copper concentrations considered, or to doubling of the complexant concentration.

J. R. Sanders

Papers

The effect of pH on the total and free ionic concentrations of mnganese, zinc and cobalt in soil solutions

The effect of pH upon the copper and cupric ion concentrations in soil solutions

The influence of pH, ionic strength and reactant concentrations on copper complexing by humified organic matter.

The complexing of copper by humified organic matter from laboratory preparations, soil and peat

The use of adsorption equations to describe copper complexing by humified organic matter