A model was tested which predicts the pH and solution metal concentration in the solution phase of soil amended with (waste-) incinerator fly ash (FA). Graded quantities of calcareous metal-rich FA were equilibrated with an acid clay soil, in aerated CaCl2 suspensions (0.01 M), to give a pH range of 3.1 (100% soil) to 7.5 (100% FA). As the FA loading was increased, the concentrations of Zn, Cd and Pb in solution passed through a maximum and then declined until the pH of the soil/ash mixtures approximated that of the pure FA (pHFA). This apparently complex pattern was accurately described by a simple pH- dependent adsorption equation relating adsorbed metal (Mads) to divalent metal concentration in solution (M2+) and pH through 3 constants designated n, Kads and m:
However, at greater ash loadings the solution metal concentration and pH remained constant with FA addition and a solubility product (Ks) could be applied:
Metal concentrations in solution [Msoln] were greatest at very low FA loadings (around 2%); at lower FA additions [Msoln] was limited by total metal concentration while at higher additions of ash the solubility of metals was suppressed by the liming effect of the fly ash. It was therefore concluded that low levels of dust transfer from disposal sites to surrounding acidic soils may be the greatest source of metal pollution to biological and aquatic systems.