Addition of calcareous metal wastes to acid soils: Consequences for metal solubility

A model was tested which predicts the pH and solution metal concentration i n the solution phase of soil amended with (waste-) incinerator fly ash (FA) . Graded quantities of calcareous metal-rich FA were equilibrated with an a cid 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 con centrations 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 pur e FA (pH(FA)) This apparently complex pattern was accurately described by a simple pH-dependent adsorption equation relating adsorbed metal (M-ads) to divalent metal concentration in solution (M2+) and pH through 3 constants designated n, K-ads and m: For pH < pH(FA). log(M-ads/(M2+)(n)) = K-ads + m pH However, at greater ash loadings the solution metal concentration and pH re mained constant with FA addition and a solubility product (K-s) could be ap plied: For pH greater than or equal to pH(FA), log(M2+) = log K-s - 2 pH Metal concentrations in solution [M-soln] were greatest at very low FA load ings (around 2%); at lower FA additions [M-soln] was limited by total metal concentration while at higher additions of ash the solubility of metals wa s suppressed by the liming effect of the By 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 aqua tic systems.