Modelling Al competition for heavy metal binding by dissolved organic matter in soil and surface waters of acid and neutral pH

Published field data on the activity of Al3+ (aAl3+) in temperate soil and surface waters of pH 3.3-7.1 have been assembled, and empirical equations describing the dependence of log aAl3+ on pH derived. For soils, two equations are necessary, one describing mineral, or high-Al, soils and the other low-Al soils. In each case, the data at higher pH (>5.5 or 6) can be described with a limiting relationship in which aAl3+/aH+3 was constant, while at lower pH the aH+ exponent is smaller, 1.02 for low-Al soils and 1.59 for high-Al soils. The majority of data for surface waters can be represented by a single equation, involving either a limiting aAl3+/aH+3 value, or a quadratic expression. Exceptions are acid bog waters, which have appreciably lower values of aAl3+ than other waters. For each equation, estimates of uncertainty are derived. The equations have been used to estimate competition by Al3+ and AlOH2+ for the binding of heavy metals (Cu, Zn, Pb) by dissolved fulvic acid (FA) under natural conditions, also taking into account competition by Ca and Fe(III) species. The simulations suggest that Al significantly decreases the extents of organic complexation of all three metals, but that the concentrations of Cu2+ and Pb2+ are substantially more sensitive to competition than is the concentration of Zn2+.