EFFECTS OF ASSOCIATIONS BETWEEN HUMIC ACIDS AND IRON OR ALUMINUM ON THE FLOCCULATION AND AGGREGATION OF KAOLIN AND QUARTZ

The aggregative effect of organic matter on soil particles is conditio ned by its composition and concentration and the degree to which it is associated with metal ions and oxides in the soil. We treated suspens ions of kaolin or quartz substrates at pH 5 with metal (Fe or Al; c. 3 to 23 mg g(-1) of substrate) or humic acid (HA; 7 to 53 mg g(-1) of s ubstrate) or both. In the absence of added Fe or Al, the proportion of HA retained by the substrate decreased from 98.5 to 50.8% for kaolini tic samples, and from 35.8 to 23.4 for quartz-based samples, as the HA dose increased. In the presence of Fe or Al, the proportion of HA rem oved from solution increased and was always >91%. Untreated kaolin flo cculated between pH 2.0 and 4.5. This flocculation interval narrowed a s the dose of HA increased, widened as the dose of HA-Al increased, bu t was barely affected by HA-Fe. Similarly, the aggregative effect of H A-Fe associations on kaolin was smaller than that of HA-Al association s, which induced formation of aggregates >100 mu m that were essential ly absent in the untreated substrate. There was a similar but less mar ked difference between the aggregative effects of HA-Fe and HA-Al on q uartz.