(HA) were precipitated by the acidification of the HLS extract to pH 1.0 with concentrated HCl. Precipitated HAs were separated from fulvic acids (FA) by centrifugation at 4000 rpm for 20 min and redissolved in a sodium hydroxide solution (0.1 M).
Fulvic acids were obtained from the supernatant solution recovered after the precipitation
of the humic acids.
The total organic carbon content of HA and FA solutions was measured by the dichromate oxidation method.
Spectrophotometric measurements were carried out with a Perkin Elmer Lambda 20 spectrometer using quartz cells.