Fractional precipitation of humic acid from colored natural waters

Abstract

Humic acid (HA) is usually defined in the soil and aquatic sciences as that fraction of base-soluble soil organic matter or dissolved organic matter (DOM) which is insoluble in acid. Stepwise titration of bulk DOM from four samples of bog pore water and stream water were undertaken to study the fractional precipitation of HA. Three independent methods: DOC analysis, absorbance measurements at 330 nm, and densitometry of photographic negatives of filters, were used to quantify the precipitation of HA. Half of the HA which would be precipitable at pH 2.0 can be precipitated at pH values from 3.4 to 4.6. These preliminary results have implications for the stability of DOM in waters that experience pH decreases, and for the capacity of DOM to complex metals.