ADSORPTION OF SOIL-DERIVED HUMIC ACID BY SEVEN CLAY MINERALS: A SYSTEMATIC STUDY R EBECCA A. C HOTZEN,T AMARA P OLUBESOVA*, B ENNY C HEFETZ , AND Y AEL G. MISHAEL * Department of Soil and Water Sciences, Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 7610001, Israel Abstract—Humic acid (HA)-clay complexes are well known for their contribution to soil structure and environmental processes. Despite extensive research, the mechanisms governing HA adsorption are yet to be resolved. A systematic study was conducted to characterize the adsorption of a soil-derived HA to seven clay minerals. Clay surfaces affected HA adsorption directly due to structural differences and indirectly by altering solution pH. The following order of HA removal was obtained for the clay minerals at their natural pH: illite >> palygorskite > kaolinite > sepiolite > montmorillonite = hectorite >> talc. Removal of HA (precipitation and adsorption) by kaolinite and illite was attributed to the low pH they induce, resulting in protonation of the clay and HA surfaces. In spite of the low pH, the zeta potential for HA remained negative, which promoted HA adsorption to the protonated clay surfaces by ligand exchange. Ionic strength did not affect HA adsorption to clay minerals with low zeta potentials, indicating that charge screening is not a major mechanism of HA adsorption for these minerals, and supporting the suggestion that ligand exchange is the main adsorption mechanism to pH-dependent sites. The increase in ionic strength did, however, promote HA adsorption to clay minerals with high zeta potentials. At pH 8 9 the order of HA affinity for clay minerals was: palygorskite > sepiolite > montmorillonite = hectorite > kaolinite > illite > talc, emphasizing strong HA interactions with the fibrous clays. This strong affinity was attributed to their large surface areas and to strong interactions with OH groups on these clay surfaces. Results indicated that HA did not enter the intracrystalline channels of the fibrous clays but suggested that their macro-fiber structure facilitates HA adsorption. The sorption of HA to kaolinite further increased in the presence of Cu 2+ , and the sorption of Cu 2+ increased in the presence of HA, due to a number of synergistic effects. This study emphasizes the diverse effects of clay structure and solution chemistry on HA adsorption. Key Words—Adsorption, Copper, Hectorite, Humic Acid, Illite, Kaolinite, Montmorillonite, Palygorskite, Sepiolite, Talc. INTRODUCTION Soil clay minerals have very large surface areas available for sorption (Grim, 1968; Sposito, 1984; Yapar et al., 2015). In soil, these surfaces are rarely bare but instead coated with organic matter, thus altering the clay-mineral physicochemical properties. Even a small amount of soil organic matter (~1%) can influence significantly the surface chemistry of minerals (Bertsch and Seaman, 1999). Many studies (Kerndorff and Schnitzer, 1980; Sparks, 2003) have examined sorption of contaminants to clays and/or to soil organic matter; to understand these processes better, exploration of the interaction between clay minerals and soil organic matter is required. One of the most prevalent components of humic substances are humic acids (HAs), which are polyelec- trolytes formed by secondary synthesis reactions during the decay process of soil organic matter that undergoes microbial transformation (Stevenson, 1994). The HAs are heterogeneous agglomerations containing varying functionalities ranging from non-polar polymethylene chains to highly polar carboxylic acid fractions (Ghosh et al., 2009). The HAs derived from soils tend to be larger and more aromatic in nature than aquatic HAs (Stevenson, 1994). Due to greater aromaticity, poly- functionality, and hydrophobicity, HAs are more likely to form coatings on soil minerals than other humic substances (Zhou et al., 1994). Adsorption of HA by mineral surfaces is influenced mostly by solution pH, ionic strength, and the type of exchangeable cations. These factors also affect HA conformational changes (Vermeer et al., 1998; Essington, 2015). Complexes of HA and clay contribute to soil structure and water-holding capacity, and are involved in reduc- tive and oxidative reactions, playing a major role in the cycling of various nutrients and pollutants (Sparks, 2003). Interactions between HA and clay have been studied widely, and the reported mechanisms include * E-mail address of corresponding author: [email protected]; [email protected]DOI: 10.1346/CCMN.2016.064027 Clays and Clay Minerals, Vol. 64, No. 5, 628–638, 2016. This paper is published as part of a special section on the subject of ‘Clays in the Critical Zone,’ arising out of presentations made during the 2015 Clay Minerals Society-Euroclay Conference held in Edinburgh, UK.
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ADSORPTION OF SOIL-DERIVED HUMIC ACID BY SEVEN CLAY MINERALS:
A SYSTEMATIC STUDY
REBECCA A. CHOTZEN, TAMARA POLUBESOVA*, BENNY CHEFETZ, AND YAEL G. MISHAEL*
Department of Soil and Water Sciences, Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem,P.O. Box 12, Rehovot 7610001, Israel
Abstract—Humic acid (HA)-clay complexes are well known for their contribution to soil structure andenvironmental processes. Despite extensive research, the mechanisms governing HA adsorption are yet tobe resolved. A systematic study was conducted to characterize the adsorption of a soil-derived HA to sevenclay minerals. Clay surfaces affected HA adsorption directly due to structural differences and indirectly byaltering solution pH. The following order of HA removal was obtained for the clay minerals at their naturalpH: illite >> palygorskite > kaolinite > sepiolite > montmorillonite = hectorite >> talc. Removal of HA(precipitation and adsorption) by kaolinite and illite was attributed to the low pH they induce, resulting inprotonation of the clay and HA surfaces. In spite of the low pH, the zeta potential for HA remainednegative, which promoted HA adsorption to the protonated clay surfaces by ligand exchange. Ionicstrength did not affect HA adsorption to clay minerals with low zeta potentials, indicating that chargescreening is not a major mechanism of HA adsorption for these minerals, and supporting the suggestionthat ligand exchange is the main adsorption mechanism to pH-dependent sites. The increase in ionicstrength did, however, promote HA adsorption to clay minerals with high zeta potentials. At pH 8�9 theorder of HA affinity for clay minerals was: palygorskite > sepiolite > montmorillonite = hectorite >kaolinite > illite > talc, emphasizing strong HA interactions with the fibrous clays. This strong affinity wasattributed to their large surface areas and to strong interactions with OH groups on these clay surfaces.Results indicated that HA did not enter the intracrystalline channels of the fibrous clays but suggested thattheir macro-fiber structure facilitates HA adsorption. The sorption of HA to kaolinite further increased inthe presence of Cu2+, and the sorption of Cu2+ increased in the presence of HA, due to a number ofsynergistic effects. This study emphasizes the diverse effects of clay structure and solution chemistry onHA adsorption.
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Figure 8. SEM images of HA morphology: (a) HA pH 7; (b) HA
pH 4; and (c) Cu2+-HA complexes at pH 5.
Vol. 64, No. 5, 2016 Adsorption of humic acid by clay minerals 637
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