The chemistry of heavy metals in digested sewage sludge ii. heavy metal complexation with soluble organic matter
Fletcher, P.; Beckett, P.H.T.
Water Research 21(10): 1163-1172
Some characteristics of the binding of the ions Ca(II), Mg(II), Cu(II), Co(II), Ni(II), Zn(II), Mn(II), Pb(II) and Fe(II) on the organic matter in a digested sewage sludge were determined using ion-selective electrodes. Results showed that ion exchange with protons is a principal component in metal binding and that copper and lead can replace more protons from the soluble organic matter than can the other ions. It appeared that the soluble organic matter contained two distinguishable groups of exchange sites. One group of sites was capable of binding all of the above ions plus protons, whereas the second set could bind Cu(II), Pb(II) and protons only. A numerical description of their uptake characteristics, based on two sets of ion exchange constants for each element, was able to predict the measured values of cation activity to within .+-. 10% in the pH range 5.7-7. This thermodynamic data was in reasonable agreement with the general concepts of coordination chemistry. The concentrations of calcium and magnesium in sewage sludge are high enough to compete so strongly with other metal ions that only copper, lead and calcium will be bound significantly. Consequently the model was simplified to predict the ion uptake behaviour of soluble organic matter, in the presence of high concentration of calcium, using a limited set of analytical expressions (analogous to adsorption equations) rather than the complete set of equilibrium constants.