Adsorption of fulvic and humic like acids on surfaces of clays: Relation with SUVA index and acidity

In the context of sustainable development, economic and environmental issues encourage the valorization of local materials such as clay and agro-industrial waste. One of the major problems nowadays is the management of municipal waste. The degradation of this waste produces leachate rich in humic-like substances which can be valued for the consolidation of earth bricks. The characterization of the interaction between humic-like substances with clay minerals could provide a better understanding of the consolidation mechanism. In this respect, the adsorption behavior of fulvic and humic like adds on surfaces of distinctive raw clays has been studied. Five types of products were tested: green waste extract, municipal solid waste extract, two commercial products and one product synthetised from xylose. The quantities of fulvic acids adsorbed by the surfaces of raw clays (expressed in mg per g of clay) are greater than the quantities of humic acids. A raw day with a large specific surface area and a large amount of iron compounds adsorbs a higher quantity (expressed in mg per g of clay) of fulvic acid than a raw clay, rich in kaolinite, containing less iron compounds and with a lower specific surface area. In alkaline conditions, the adsorbed quantities are lower than in alkaline conditions, and the surface area or the presence or iron compounds influence less the adsorption capacity than in acid conditions. It is interesting to note that if the adsorbed quantities are expressed in mg divided by the specific surface area of the clay, it appears that a kaolinite rich clay adsorbs more fulvic or humic like substances than a lateritic clay which is rich in iron oxides. Lastly, in acid (resp. alkaline) conditions, the parameter which governs the adsorption is the acidity (resp. the Specific Ultra-Violet Absorbance, SUVA index): the lower (resp. the higher) the parameter, the greater is the adsorption capacity. The interest in these adsorption reactions is the possibility to use these acids in the building sector for the manufacture of clay bricks at low temperature (compared to fired clay bricks).