Effect of time and pH on the lability of copper and zinc sorbed on humic acid particles
1992
Waller, Pamela A. | Pickering, William F.
The relative lability of copper and zinc present as soluble humate complexes or sorbed on humic acid particles has been evaluated by measuring the degree of metal transfer to cation exchangers, after mixing for time periods ranging from 2 to 114 h, with system pH values ranging from 2.7 to 8. Three major forms of metal humate species were postulated, namely, metal chemisorbed on solid; a soluble non-labile complex (possibly MA₂); and a soluble labile complex (MA). The amount present in each form varied with time, pH, temperature, amount of exchanger resin present, and amount of metal ion initially loaded onto humic acid particles (40 to 480 μmol g⁻¹). The percentage of total metal content which remained ‘fixed’ to the humic acid particles peaked in the pH 4 to 5.5 region. Below pH 4 an increasing percentage of the metal was mobilised, presumably as a result of protons competing for the functional groups involved in metal bonding while at pH 5.5 the solid particles dissolved and most metal ion was present as soluble complexes. These humates partially dissociated in the presence of the cation exchangers (Zn more than Cu). At all pH values transfer of metal ion to the exchanger was slow, with equilibrium generally not being fully established after 5 days of mixing. The amount of non-labile soluble humate complex detected was generally <6% of total metal content, except in the pH 6.5 to 7.5 region, where it was the major species present during the first 24 hours of mixing. With longer mixing times the pH increased, and most of the zinc (and about a third of the copper) was transferred to the exchanger resin. The apparent lability of the sorbed Cu and Zn was thus relatively high at low and high pH, and minimal in the 4.5 to 5.5 region.
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