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Organo-metalic Complexes
Soil organic matter consists of two major types of compounds namely: (1) non-humic substances and (2) humic substances formed by secondary synthesis reactions (Stevenson, 1982). Humic substances in the soil environment are polydisperse polyelectrolytes and the charges are pH dependent (Clapp et al., 1993). Predominant sources of charge are carboxyl groups and under acid conditions these functional groups behave like neutral polar molecules. Under normal agricultural conditions these molecules are negatively charged and these charges are balanced by cations (Clapp et al., 1993). Due to the relatively large surface areas associated with small colloidal particles, chemical and physical reactions tend to be greatly enhanced (Stevenson, 1982). The significance of complexation is emphasized by Logan (1992) in that he states that: “Complexation is a more significant mechanism than ion exchange for immobilization of trace metals because of the much higher complexation constants for trace metals relative to macro metals, as opposed to ion exchange in which selectivity for trace metals does not compensate for the higher concentrations of macro metals. Even though complexation is an important mechanism in the immobilisation of heavy metals humic substances can also form water-soluble complexes with metal ions (such as Cu2+, Zn2+, Mn2+, Co2+, and others) and hydrous oxides (Schnitzer, 1978; Stevenson, 1982). These metal ions insolubilize humic substances by forming inter- and intra-molecular bridges between charges on humic macromolecules or between these and negatively charged inorganic colloids in soils. When these insolubilizing cations are removed the anions become soluble in water with the most highly charged macromolecules (higher oxygen and lower carbon content) solubilizing first (Clapp et al., 1993). Water-soluble complexes of fulvic acids with toxic metals can increase their concentrations in soil solutions in excess of their normal solubilities and are a major cause for concern (Davies, 1980; Tate, 1987; Bourg, 1995). Stability constants that have been determined by a number of workers vary widely though. These stability constants are usually lower than those formed with synthetic complexing agents such as EDTA (Schnitzer, 1978). Other organic complexing agents may also influence the availability of trace elements to higher plants (Stevenson, 1982). Before application to soil the heavy metals that are present in the biosolids are probably complexed with the organic material or biotic surfaces (Nederlof and Van Riemsdijk, 1995) in different forms or incorporated into the cell structures of dead and living microorganisms. During organic matter mineralization the unstable sludge component is degraded in soil and the organic component of the organo-metalic complexes is either mineralised or altered into humic compounds by microbial and biochemical processes, thereby releasing the metals into the soil solution. Upon release into the soil solution organic material and mineral surfaces of Fe, Mn, Al, and Si as well as other soil conditions determine the sorption rate onto soil particle surfaces. (The influence of Fe and Mn is discussed in more detail later in the chapter).
Clay Content and Type
The clay content and clay type of a soil can often be positively correlated with the amount of metals taken up by plants (Hooda et al., 1997; Kabata-Pendias, 2001). In many cases the horizon in which an increased content of metals is found, the metal is associated with certain clay minerals (illuvial horizon) or organic material in the Ahorizon (Aubert and Pinta, 1977). Structural aspects of soils also play a role in that lithogenic metals such as Al, Fe, or Cr show lower total concentrations on aggregate surfaces than in aggregate cores, whereas ubiquitously deposited metals such as Cd, Pb, or Zn show higher total concentrations on aggregate surfaces (Wilcke and Amelung, 1996).
Different clay minerals in the soil have different affinities for metals. This effect is mainly attributed to the effects of pH on variable-charged sorption sites, which also leads to the increased mobility of Cr6+ at higher pH mainly due to the form of the oxyanions (CrO42-) in solution (McLaughlin et al., 2000). Jinadasa et al. (1995) found that the metal ion adsorption on synthetically prepared goethite was strongly pHdependent and that Cr was more strongly adsorbed than Cd and Pb. Fendorf et al. (1996) found that Cr3+ was more stable when precipitated on goethite than on silica. When a soil system buffers the addition of acid, heavy metals (e.g. Cr) bound in silicates are released into the soil solution due to the silicate’s destruction during the buffering process (Kaupenjohann and Wilcke, 1995).
CHAPTER 1: General Introduction
1.1 Introduction
1.2 Critical Research Questions
1.3 Broad Aim of the Study
1.4 Communication of Results of the Study
CHAPTER 2: Literature Review on the Effect of Liming on Heavy Metal Mobility in Acid Soils After Long-term Biosolids Disposal – A South African Perspective
2.1. Introduction
2.2 Background
2.3 Soil Characteristics Determining Metal Mobility
2.4 The Effect of Liming and Increased pH on Organic Material Solubility and Metal Mobility
2.5 EDTA Extractability of Metals and Organic Matter After Liming
2.6 Concluding Remarks
2.7 Aim of the Study
CHAPTER 3: Soil Description
3.1 Introduction
3.2 Materials and Methods
3.3 Results and Discussion
3.4 Conclusions and Recommendations
CHAPTER 4: Increase in Metal Extractability After Liming of Sacrificial Sewage Sludge Disposal Soils
4.1 Abstract
4.2 Introduction
4.3 Materials and Methods
4.4 Results and Discussion
4.5 Conclusions and Recommendations
CHAPTER 5: The Influence of Increasing Lime Rates on Ammonium EDTA (NH4-EDTA) Extractable Metals and Organic Matter from Two Acid Long-term Biosolids Disposal Soils
5.1 Abstract
5.2 Introduction
5.3 Materials and Methods
5.4 Results and Discussion
5.5 Conclusions and Recommendations
CHAPTER 6: Changes in Ammonium Nitrate (NH4NO3) and Ammonium EDTA (NH4EDTA) Extractable Metals from Two Long-Term Biosolids Disposal Soils due to Intensive Liming and Incubation
6.1 Abstract
6.2 Introduction
6.3 Materials and Methods
6.4 Results and Discussion
6.5 Conclusions and Recommendations
CHAPTER 7: Heavy Metal Uptake by Wheat from Two Sacrificial Biosolids Disposal Soils at Differential Liming Rates
7.1 Abstract
7.2 Introduction
7.3 Materials and Methods
7.4 Results and Discussion
7.5 Conclusions and Recommendations
CHAPTER 8: Concluding Remarks and Recommendations
References
