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The binding of metal to humic substances is problematical. The approaches for studying metal binding to organic matter are briefly reviewed. Ion-selective electrodes (Cu 2+ and Pb 2+ ) were used to measure metal complexation by a whole peat and an extracted humic acid (HA) fraction. Scatchard plots and calculation of incremental formation constants were used to obtain values for the binding constants for the metals onto both peat and HA. Both the peat and the humic acid had a larger maximum binding capacity for Pb 2+ than for Cu 2+ (e.g. at pH = 5 HA gave 0.188 mmol Cu 2+ g -1 and 0.564 mmol Pb 2+ g -1 : peat gave 0.111 mmol Cu 2+ g -1 and 0.391 mmol Pb 2+ g -1 ). Overall, the humic acid had a larger metal binding capacity, suggesting that extraction caused conformational or chemical changes. The binding constants (K 1 ) for Cu 2+ increased with increasing pH in both peat and humic acid, and were larger in the peat at any given pH (e.g. at pH = 5 HA gave log K 1 = 2.63, and peat gave log K 1 = 4.47 for Cu 2+ ). The values for Pb 2+ showed little change with pH or between peat and humic acid (e.g. at pH = 5 HA gave log K 1 = 3.03 and peat gave log K 1 = 3.00 for Pb 2+ ). In the peat, Cu 2+ may be more able to bind in a 2:1 stoichiometric arrangement, resulting in greater stability but smaller binding capacity, whereas Pb 2+ binds predominantly in a 1:1 arrangement, with more metal being bound less strongly. Whole peat is considered to be more appropriate than an extracted humic acid fraction for the study of heavy metal binding in organic soils, as this is the material with which metals introduced into an organic soil would interact under natural conditions. |
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