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Division S-2—Soil Chemistry

Properties of Water-Dispersible Colloids from Macropore Deposits and Bulk Horizons of an Agrudalf

^a Chemistry Dep., The Royal Veterinary and Agricultural Univ., Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
^b DHI Water & Environment, Agern Allé 11, DK-2970 Hørsholm, Denmark
^c Currently at: Dep. of Agroecology, Danish Institute of Agricultural Sciences, PO Box 50, DK-8830 Tjele, Denmark

^* Corresponding author (C.Kjaergaard{at}agrsci.dk)

Mobility of colloids depends in part on the release from aggregates and the stability in suspension. This study determined the soil dispersibility of the different horizons in a Typic Agrudalf. Water-dispersible colloids (WDC) from bulk horizons and macropore deposits were characterized for mineralogy and physical chemical properties. The effect of solution pH and ionic strength on -potential and flocculation behavior was evaluated using dynamic light scattering, and the role of colloid associated organic C (OC) and sesquioxides were elucidated. The soil dispersibility did not reflect the contrasting physicochemical conditions, but was directly correlated with total clay content. Generally, WDC were enriched in OC and sesquioxides. Water-dispersible colloids from the Ap-horizon and from macropore deposits differed markedly from the remaining WDC-fractions due to a significantly higher content of OC (14–35 g kg^–1), Al_CBD (9.3–10.6 g kg^–1) and a much smaller N₂–adsorption surface area (14–25 m² g^–1). Treatment with Na₂S₂O₈ for removal of OC increased the surface area by 171–225%, indicating surface coatings of OC. The contribution of OC to the colloidal stability was inferred from: (i) a more negative -potential and larger suspension stability of WDC with larger content of OC, and (ii) reduced negative -potential as well as suspension stability after OC removal. Large variations were observed in the flocculation behavior for WDC with rather similar mineralogical composition. A two-fold increase of the initial particle diameter occurred at an electric conductivity of 91 µS cm^–1 for the least stable colloids and at 1023 µS cm^–1 for the most stable and OC-rich colloids. The effect of solution pH on flocculation was significant only at pH below 4.5.

Abbreviations: CBD, citrate–bicarbonate–dithionite • EC, electric conductivity • EM, electrophoretic mobility • MM, macropore matrix • MP, macropore deposits • NOM, natural organic matter • OC, organic carbon • OX, treated with sodium peroxodisulphate • PSD, particle-size distribution • WDC, water-dispersible colloids

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