Effects of associations between humic acids and iron or aluminium on the flocculation and aggregation of kaolin and quartz

The aggregative effect of organic matter on soil particles is conditioned by its composition and concentration and the degree to which it is associated with metal ions and oxides in the soil. We treated suspensions 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 substrate) 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 kaolinitic 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 removed from solution increased and was always >91%. Untreated kaolin flocculated between pH 2.0 and 4.5. This flocculation interval narrowed as the dose of HA increased, widened as the dose of HA-A1 increased, but was barely affected by HA-Fe. Similarly, the aggregative effect of HA-Fe associations on kaolin was smaller than that of HA-A1 associations, which induced formation of aggregates >100 μm that were essentially absent in the untreated substrate. There was a similar but less marked difference between the aggregative effects of HA-Fe and HA-A1 on quartz.