Reactions of Copper and Cadmium in Soils

[摘要] This project examines some of the factors influencing the interactions of the metals copper and cadmium with a range of soils from the United Kingdom, at levels which may be said to mimic low level contamination. Applying the traditional batch technique, in alliance with AAS (Atomic Absorption Spectrophotometry) and ISE's (Ion Selective Electrodes), the physical and chemical parameters of the soils which influence the removal of these metals from incident solutions, as well as the effect the solution chemistry has upon the process, was examined. No clear relationship could be found between the 'classical' soil properties and the retention of either metal, although pH did seem to play a role in solution Cu chemistry, and the presence of carbonates in soils greatly increased their ability to remove Cd from solution. The relationship between pH and organic matter in solution can be crucial in considerations of solution chemistry. The presence of other polyvalent cations had a minimal influence upon the uptake characteristics for either metal, leading to the conclusion that groups of discrete, high affinity, sites exist, over and above the CEC (Cation Exchange Capacity), for Cu and Cd at up to 2.5muM Cu or Cd/g of soil. Anions, in terms of the type and their concentration had a variable influence upon Cu and Cd soil-solution interactions. An organic ligand such as Citrate acts much as expected, complexing and maintaining metal levels (total) in solution. Inorganic complexation, by NO3- Cl- and SO42- has a smaller influence, although chloride is expected to, and does sequester Cd to the solution phase - sulphate -Cd complexation is anomalous in this respect, implying that the (CdSO4) species may be being sorbed. Cu uptake follows a pattern which is inversely related to the influence each anion has on the pH of the system. Changing the ionic strength (I) of the systems has a predictable influence on Cd uptake; as I increases, [Cd]sorbec decreases ie. not all of the differences in the results described above can be due solely to the type of anion. In this instance, AI has little influence on Cu uptake, suggesting that the formation of Nitrate complexes is not a limiting factor with regard to soil uptake, and that more stable species control its destiny. Sorption kinetics suggest that the free ion levels stabilize within minutes of the solution coming into contact with the soil, although longer term studies suggest that there is a prolonged change in the activity, but on a far less dramatic scale than in the initial minutes of the reaction. In the short term, uptake may be followed by a release of the free metal ion - possibly due to a pH feedback mechanism. Data summation could be achieved for Cd via the logarithmic van Bemmelen-Freundlich relationship, whilst for Cu no mathematical relationship could be found which resulted in a single, easily rationalized, straight line - a two-constant version of the above equation is the simplest to use.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry, Soil sciences