Dynamic conductivity measurements in humic and fulvic acid solutions

Conductivity changes of dilute aqueous humic and fulvic acids solutions were monitored after the addition of small quantities of Cu, Cd, Pb, and Zn. The solutions were stirred at a constant and reproducible rate, and measurements proceeded until stable conductivities were attained. The values were compared to KCl reference solutions treated in an identical manner, and the results showed that the humic materials significantly reduced metal ion mobilities. Pb and Cu were affected more than Cd and Zn, and especially Zn regained a significant portion of its mobility upon dispersal. Cu, in contrast, was strongly and irreversibly retarded in all cases. It was inferred that Zn underwent transient electrostatic interactions with humics at high local concentration, while associations between Cu and humics were rapid and permanent. Different humic materials showed these effects to different extents. Raising the temperature from 20 to 60 °C did not, as expected, lead to higher relative conductivities in metal–humate solutions, but showed enhanced retardation of the cations. It was noted that temperature induced aggregation (clouding) of humic solutions increased the effective sizes of the metal–humate complexes, reducing their mobilities in aqueous solution.