Modeling the effects of Ca2+ and clay-associated organic carbon on the stability of colloids from topsoils

The goals of the study were to investigate the effects of the soil-water phase ionic strength, mainly monitored by the calcium ion (Ca(2+)) concentration, on the stability behavior of easily dispersed topsoil colloidal clay-sized particles (<2 microm). The aggregation kinetics as a function of the Ca(2+) concentration was monitored by measuring the increase of the particle size over time with photon correlation spectroscopy. The critical coagulation concentrations (CCC) of Ca(2+) were measured. The Hamaker constants (A) characterizing the attractive chemical properties of the topsoil colloid surface were thus scaled according to the Derjaguin, Landau, Verwey, Overbeek (DLVO) theory by taking into account the electrokinetic behavior of the particles, measured by the zeta-potential. Effective values for the Hamaker constants of topsoil clay-sized colloids, clay minerals, and metal oxides were calculated by referring to reported values for crystalline silica or sand (quartz) particles. Potential-energy diagrams of interacting topsoil clay-sized colloids were calculated. The primary energy maximum and secondary energy minimum were used for modeling the aggregation kinetics along the Ca(2+) concentration by employing Marmur's model. Coagulation in the secondary energy minimum can only explain the aggregation efficiency of topsoil colloids at low Ca(2+) concentrations (<2 mM Ca(2+)) under unfavorable electrostatic conditions. The effect of surface-associated organic matter on the colloidal electrosteric stability was also investigated by comparing the topsoil colloid stability after the removal of organic matter

Physicochemical characterization of potential colloids from agricultural topsoils

A method based on sedimentation and centrifugation steps has been developed to fractionate agricultural topsoils after suspension in water. A high release of potential colloids has been observed with low Ca2+ concentration under soil solid-to-water weight ratio conditions higher than 1/3. The colloidal stability is characterized in terms of particle size and electrokinetic properties. There is no systematic effect of the colloid phase organic carbon content varying from 2 to 5% on the critical coagulation concentration (CCC) and zeta potential results. Consequently, the observed CCC was tested by the Derjaguin-Landau-Verwey-Overbeeck theory by taking into account physicochemical properties of the colloid phase mineral content. (C) 2002 Elsevier Science B.V. All rights reserved

Sorption properties of hydrophobically modified poly(acrylic acids) as natural organic matter model substances to pyrene

Amphiphilic natural organic matter (NOM) in soil, water and sediments plays an important role in the fate of low-soluble hydrophobic organic compounds (HOC) in the environment. Scaling NOM structural factors, which affect the transport and immobilization of HOC, is rather involved due to the complexity of the NOM materials. Sorption properties of hydrophobically modified polyacrylic (HMPA) polymers as NOM model substances for pyrene were investigated and discussed with regard to reported NOM results. Emission fluorescence spectroscopic properties of pyrene and potentiometric titrations were used to characterize the hydrophobic and acidic properties of HMPA samples. Excitation fluorescence spectroscopy allowed a speciation of pyrene molecules. Overall complexation constants, beta(1) and beta(2) of pyrene with hydrophobic moieties (-C8H17) and (-C16H33) were calculated depending on the pH and electrolyte cation nature at an ionic strength of 10 mM, typical of soil solution conditions. Aggregation of HMPA molecules was detected by photon correlation spectroscopy. Normalized sorption constant K-oc values for pyrene with HMPA samples similar to reported values with NOM support recent observations on the contribution of aliphatic moieties to the binding of polyaromatic hydrocarbons (PAH). The HMPA samples also model the partitioning behavior of hydrophobic micelle-like structures of NOM. (c) 2005 Elsevier B.V. All rights reserved