Gerhard W. Brümmer

The relative affinities of Cd, Ni and Zn for different soil clay fractions and goethite

Cadmium, Ni and Zn ions in aqueous solution were allowed to react with clay fractions (< 2 μm) separated from soils with a wide range of mineralogical composition and properties. Sorbed metals were separated into two components, termed specifically and non-specifically bound, by a controlled washing procedure using 10−2M Ca(NO3)2. Sorption reactions were characterized by Δ pH50 values, by shapes of adsorption curves, and by measuring separation factors and distribution coefficients under prescribed conditions. Three reaction types were identified, viz., (i) those associated with soil adsorbing surfaces dominated by iron oxides; these appear to be controlled by mechanisms which involve metal-ion hydrolysis and result accordingly in relative sorption affinities of Zn > Ni > Cd; (ii) those associated with organic surfaces for which metal-ion hydrolysis was of little significance and little difference in metal-ion affinity was evident; at lower pH-values, Cd and Ni were somewhat preferred over Zn, with the converse at higher pH-values; (iii) those associated with 2:1 layer lattice silicates which exhibit greater preference for Zn, i.e., Zn >> Ni, Cd and higher affinities for each metal at lower pH-values (< 5) than is shown by clays dominated by iron oxides. There was also evidence of greater relative affinity for Ni shown by clay fractions dominated by fine kaolinites when compared with other clays. This investigation has shown that a range of sorption processes are involved in reactions of heavy metals with soils. We caution against undue emphasis on any particular sorption process in developing theoretical sorption models as a basis of understanding and solving problems connected with pollution and plant nutrition; we also stress the need for studies with colloids separated from soils in conjunction with those using synthetic adsorbents as models for soil constituents.

Adsorption-desorption and/or precipitation: dissolution processes of zinc in soils

Abstract The concentrations of trace and toxic metals in soil solutions are explained by several authors either in terms of adsorption—desorption or precipitation—dissolution reactions in soils. Data have been given for zinc to test the applicability of both concepts. The results show that the concentrations of zinc in equilibrium solutions with soil clay fractions and whole soil samples at pH values below 7 are determined exclusively by adsorption—desorption reactions for various pHs, contents of bound zinc and compositions of soils. At neutral to alkaline pH values precipitation—dissolution reactions of zinc may take place. There is some evidence that formation of zinc silicates may control the zinc concentration in solution provided natural complexing agents are absent, the affinity of the soil for zinc is low and the content of reaching zinc is high (> ∼ 100 ppm). Even at pH values above 7, the formation of other zinc compounds is unlikely in most soils because additions of large amounts of zinc are required to ensure saturation of the adsorption sites of different soil components before the zinc concentration in the soil solution can increase sufficiently to bring about the precipitation of definite compounds. Model experiments in CaCO3-buffered systems showed that the adsorption capacity for specifically adsorbed zinc (in μmole/g) by the following components increased in the order CaCO3 (0.44), bentonite (44), humic acid (842), amorphous Fe- and Al-oxides (1190, 1310) and δ -MnO2 (1540) and demonstrated the importance of Mn-, Fe-, and Al-oxides and humic substances for the binding of zinc in soils containing carbonates, and thus indicate the special role of these components in limiting precipitation reactions.

The sorption of Cd, Zn and Ni by soil clay fractions: Procedures for partition of bound forms and their interpretation

Abstract This work is the first of several projects concerned with the study of higher-affinity reactions of Cd, Zn and Ni ions with soil clay fractions. Procedures for the separation of sorbed metals into fractions of lower and higher affinity for soil surfaces are described and evaluated. Various concentrations of Cd, Zn and Ni were allowed to react in the presence of 0.01 M Ca(NO3)2 with soil clays for 1 week after stabilization of suspension pH. The adsorbed metals were partitioned by a brief extraction with 0.01 M Ca(NO3)2 and the resultant fractions, called specifically and non-specifically sorbed metals, were measured by radioisotopic procedures. Measured separation factors showed that the fraction of sorbed metals that was desorbed by a rapid Ca(NO3)2 extraction still had a preference, sometimes marked, over Ca on the soil clay fraction. Separation of fractions of sorbed metals on the basis of affinity was reproducible, but the boundary conditions defined by separation factors vary appreciably between adsorbents, with values in the range 3–20 for amounts sorbed equivalent to ≦ 0.05% of cation exchange capacity and for pH values The proportions of Cd, Zn and Ni bound at high-affinity sites were strongly dependent on experimental conditions of pH, equilibrium time and surface saturation in relation to each soil clay. Hence, comparisons of affinities of trace metals for soils by reliance on measures of total sorption only, without assessing the contribution of lower-affinity forms, may prejudice conclusions and predictions arising from studies of the possible retention of metal pollutants in soils and fixation of micronutrients from fertilizers.

Adsorption and solubility of ten metals in soil samples of different composition

We conducted batch experiments for ten metals [Mg, Cr(III), Fe(III), Co, Ni, Cu, Zn, Sr, Cd, Pb] and four soil samples of different composition to determine the relation of the soluble fraction (’intensity’) to an adsorbed or precipitated metal pool (’quantity’) and, thus, to investigate the buffer function of soils. The soil samples were spiked with 6 to 12 exponentially increasing metal doses added as metal nitrates. The native metal pool involved in sorption processes was characterized by an extraction with 0.025 M (NH4)2EDTA (pH 4.6). The quantity-intensity (Q/I) relations of eight metals [except Cr(III) and Fe(III)] were governed by sorption and complexation processes and can be fitted by Freundlich isotherms. Q/I relations for Cr(III) and two soils indicate a sorption maximum, which can be approximated with the Langmuir isotherm. In a calcareous soil high Cr doses induced the precipitation of a Cr oxide. The solution concentrations of Fe are primarily a function of the pH-dependent solubility of ferrihydrite. For all metals pH was the predominant factor controlling the partitioning between the solid and the liquid phase. Drastic losses in the buffer function of soils primarily occurred in the slightly acidic range. Furthermore, adsorption was also metal specific. On the basis of median Freundlich K values, adsorption increased in the order [median KF values and KF range (mg kg—1) in brackets]: Mg (2.9: 0.9—19) < Sr (4.7: 0.6—21) << Co (17.7: 1.1—143) < Zn (26.7: 1.8—301) = Ni (27.6: 2.4—120) < Cd (71: 2.5—405) << Cr(III) (329: 45—746) < Cu (352: 30—1200) < Pb (1730: 76—4110). Adsorption und Loslichkeit von zehn Metallen in Boden variierenden Stoffbestandes Schuttelversuche mit zehn Metallen [Mg, Cr(III), Fe(III), Co, Ni, Cu, Zn, Sr, Cd, Pb] und vier Bodenproben wurden durchgefuhrt, um die Beziehungen zwischen den gelosten Anteilen (“Intensitat„) und den sorbierten bzw. gefullten Anteilen (“Quantitat„) zu ermitteln und damit das Puffervermogen der Boden zu bestimmen. Die Metalle wurden als Nitratsalzlosungen mit exponentiell steigenden Konzentrationen in 6—12 Dosen zugegeben. Die bodenburtige, an Sorptionsprozessen beteiligte Metallfraktion wurde als der mit 0,025 M (NH4)2EDTA (pH 4,6) extrahierbare Anteil berucksichtigt. Die Q/I-Beziehungen von acht Metallen [ohne Cr(III) und Fe(III)] wurden durch Sorptionsprozesse und Komplexbildungen gesteuert, die sich mit Freundlich-Isothermen beschreiben liesen. Q/I-Beziehungen fur Cr und zwei von vier Bodenproben zeigten ein Adsorptionsmaximum und folgten einer Langmuir-Anpassung. In einer kalkhaltigen Bodenprobe bewirkten hohe Cr-Gaben die Ausfullung eines Cr-Oxids. Die Losungsgehalte von Fe wurden uberwiegend durch die pH-abhangige Loslichkeit des Ferrihydrits bestimmt. Bei allen Metallen hatte der pH-Wert den grosten Einflus auf Sorption und Loslichkeit. Insbesondere im schwach sauren Bereich war eine drastische Abnahme des Puffervermogens der Boden festzustellen. Daneben war das Adsorptionsverhalten stark metallspezifisch. Auf der Grundlage medianer Freundlich-K-Werte stieg die Adsorbierbarkeit in folgender Reihe an [mediane KF-Werte und KF-Bereiche (mg kg—1) in Klammern]: Mg (2.9: 0.9—19) < Sr (4.7: 0.6—21) << Co (17.7: 1.1—143) < Zn (26.7: 1.8—301) = Ni (27.6: 2.4—120) < Cd (71: 2.5—405) << Cr(III) (329: 45—746) < Cu (352: 30—1200) < Pb (1730: 76—4110).

Bioformation of polycyclic aromatic hydrocarbons in soil under oxygen deficient conditions

Abstract Possible formation of polycyclic aromatic hydrocarbons (PAHs) through biological processes in soil was investigated. Soil, plant material, and a mixture of both were waterlogged and incubated for 2 years, resulting in long lasting oxygen deficient conditions. Sample extracts were analysed by HPLC for 15 PAHs. In soil with or without added plant material, the contents of 4-, 5-, and 6-ring PAHs increased significantly to 139–238% of the contents before incubation. Concentrations decreased for several 3-ring PAHs, probably due to anaerobic biodegradation. When only plant material was incubated, the increase was significant for the 4-ring PAHs benzo[ a ]anthracene and chrysene and all 5- and 6-ring PAHs (250–674%). It was assumed that PAHs are formed from aromatic compounds of plant material and humus precursors. Under oxygen deficient conditions this may lead to natural background levels of PAHs in soils.

Prediction of heavy metal behavior in soil by means of simple field tests

Binding and retention against uptake by plants, and groundwater pollution of the metal ions Cd, Mn, Ni, Co, Zn, Cu, Cr(III), Pb, Hg, Fe(III), and Al by soils in relation to pH, redox potential, texture, organic matter, and iron oxide contents can be diagnosed in the form of rough relative values with simple field methods. A comparison with the results of some pot and field trials showed the practicability of this method.

Gehalte an organischer substanz, schwermetallen und phosphor in dichtefraktionen von fluvialen Unterwasserböden

Abstract Three samples of Unterwasserboden from the River Elbe were separated into different density fractions using heavy liquids. The contents of organic matter, which range between 4.8 and 19.4% in the non-fractionated samples, decrease progressively with increasing density from 80% in the fraction −3 to 2.6 g cm −3 . More than 96% of the organic matter of the non-fractionated sediment is bound as organo-mineral complexes which exist in the greatest proportion in the fraction 1.6–2.2 g cm −3 . According to the distribution of organic matter among the different density fractions, that −3 is named the organic fraction, whereas those between 1.6 and 2.2 g cm −3 are called organo-mineral and those > 2.4 g cm −3 are called mineral fractions. Distribution of the elements Cr, Co, Ni, Cu, Zn, As, Hg, Pb and P, as well as Mn, Fe and Sr, is related to the density fractions of the Unterwasserboden according to the physicochemical properties of the elements and the extent of pollution. The lowest contents of the elements occur in the quartz fractions with a density 2.6–2.7 g cm −3 and 2.4–2.6 g cm −3 in the fluvial and marine sediments, respectively. The contents of the elements generally increase in the fractions with density higher than quartz. Extremely high values for Cu, Pb, Sr, Fe, Cr, As and Co occur in some of the heavy mineral fractions > 2.8 g cm −3 , especially those of highly polluted fluvial samples. In the highly polluted fluvial Unterwasserboden from the mid-part of the Elbe River, the major proportions of some elements occur in the carbon-containing fractions −3 , especially in the organic and organo-mineral fractions −3 . Thus, the percentages of the elements in the fraction −3 of a fluvial sample decrease from 92% to 72% in the order Hg > Zn > As > Cu > Ni > Cr > Pb. Within the fractions −3 the contents of Hg, As and Ni are highly correlated with carbon content and therefore are believed to be especially associated with organic matter. On the other hand, Pb, Sr, Co, Fe, P and Mn in the fractions −3 seem to be mainly bound by minerals, whereas Zn, Cu and Cr seem to be bound in both organic and mineral forms. For most of the elements lower proportions were found in the fraction −3 of a weakly polluted Unterwasserboden from the marine environment of the mouth of the Elbe than of the fluvial samples from upstream. At the same time, the positive correlation of the contents of Cu, Zn, Pb and Ni with contents of organic matter in the individual fractions −3 was better for the marine sample than for fluvial samples. Relatively high contents of Hg, Cu, Zn and Ni in the organic-matter-rich fraction −3 and of Pb in all of the organo-mineral fractions indicate that these metals are transported from the fluvial to the marine region in association with organic matter.

Soil Organic Matter

In most topsoils, the mass of the soil organic matter only amounts to a few percent, but has an important influence on all soil functions and plays a central role in the global carbon cycle. For this reason, the carbon content, or the dark color value, is a differentiating criterion for soil descriptions in German and international classifications.

Soil-Plant Relations

Soils are the natural sites for all terrestrial plants, developing roots in the soil space that anchor them in the soil, and absorbing water, oxygen and nutrients from the soil through their root system. This requires good rootability and distance to bedrock (Sect. 9.1). Furthermore, soils must be able to store sufficient plant available water (Sect. 9.2), provide enough gas exchange (Sects. 6.5 and 9.3) and thermal fluxes (Sects. 6.6 and 9.4), and also contain large enough quantities of available plant nutrients (Sect. 9.5). These properties are significantly determined by the thickness of the rootable soil zone. Because fertile soils are the basis for supplying a growing human population with food and because arable soils are a limited good, they must be protected from damage and destruction, in order to preservetheir fertility and to prevent famine among the population. Their capacity to produce any kind of yield is called soil fertility or productivity (Chap. 11).

Chemical Properties and Processes

Many regulating functions of soils (Sect. 1.2) are based on biogeochemical processes, and are therefore affected by soil chemical properties. Examples are the storage and supply of nutrients, the sorption and degradation of contaminants, as well as the buffering of acid deposition. The chemical processes taking place at biogeochemical interfaces are of outstanding importance. About 40–60 % of the soil volume consists of pores, which can be filled with water (soil solution) or gases (soil air), depending on the actual soil moisture. The soil solids mainly consist of minerals and smaller fractions of organic matter. This porous system of mineral and organic soil particles, gases, aqueous solutions and organisms leads to the formation of very large and chemically reactive interfaces. These interfaces can adsorb, complex, precipitate or chemically transform various ions and molecules. This chapter provides an introduction to the chemical properties and processes regulating the behavior of nutrients and contaminants in soils.