Size characterization by laser granulometry of colloids extracted from compost at different temperatures.

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English. The objective of this work was to characterize colloids extracted from composts and their potential retention in soils. Compost made of sludge and green wastes was sampled (i) during the fermentation phase and (ii) after maturation. The same kind of compost was used in a long-term field experiment at Feucherolles (France), near Paris where amended and nonamended soils were sampled. The colloidal fraction was extracted from composts in water at room temperature (20 degrees C) and compared to the colloidal fraction extracted from the soil. Composts were also extracted by pressurized hot liquid water at 50, 125 and 175 degrees C. The total organic carbon of the extracts was measured and the particle size distribution (PSD) of colloidal extracts was analyzed by laser granulometry. The diameters of the colloids extracted from the soil ranged between 0.040 and 0.300 mu m, independently of the temperature. For composts, it varied from 0.040 to 3.200 mu m when extraction was done at 20 degrees C, while at higher temperatures, much more organic matter was extracted, and colloid diameters ranged from 0.040 mu m to 0.200 mu m. The water-soluble C decreased and the size of colloids recovered in water at temperatures below 50 degrees C increased when compost maturity increased. The adsorption on soils of colloidal particles extracted from composts was characterized. The largest adsorption (up to 30% of the initial soluble C) occurred with the extracts recovered at high temperature, in relation to the more hydrophobic properties of the colloids extracted with hot water maintained in subcritical conditions. After adsorption, the particle size distribution in the colloidal fraction extracted at 20 degrees C moved towards finer fractions; by contrast, the colloidal fraction extracted at 175 degrees C moved towards coarser fractions. The coarsest colloids coming from the soil disappeared during the adsorption experiment, probably because of the coprecipitation with the finest colloids coming from compost.