Characteristics of fulvic and humic acids isolated from various sources and their role in iron and aluminum mobilization.

摘要

Fulvic and humic acids from four water sources and an alkaline-extraction of forest floor litter were fractionated using the XAD-8 resin method. Molecular characteristics of these samples, and one natural organic matter (NOM) sample collected by reverse osmosis (RO), were determined by major and minor elemental analysis; {dollar}sp{lcub}13{rcub}{dollar}C-NMR, infrared, UV, and visible spectroscopy, and titration of acidic groups. The fulvic and humic acids displayed trends characteristic for these acids. RO-isolated NOM recovered approximately 40% more carboxylic carbon than organics fractionated by the XAD-8 technique.; The fulvic and humic acid samples were used in experiments investigating their role in mobilizing Al- and Fe-hydroxides (as soluble and colloidal complexes), and models of their interactions were developed. Both fulvic and humic acid led to lower mobilization of Al-hydroxide by inhibiting proton-promoted dissolution and by preventing Al-hydroxide flocs from dispersing. At the same NOM concentration, Fe-hydroxide mobilization was enhanced by formation of colloidal size particles, ligand-promoted dissolution, and reductive dissolution. Colloids formed due to disaggregation of the Fe-hydroxide floc. Ligand-promoted dissolution occurred only at low levels, while reductive dissolution was typically the major pathway for mobilization at pH 4. Reductive dissolution was most rapid initially, and then slowed to progressively slower rates until a linear or zero rate was attained. Concurrent with reductive dissolution, adsorbed NOM was released, presumably by oxidation of organics coupled to reductive dissolution. Reductive dissolution was catalyzed by exposure to visible light, but even without light catalysis, Fe-II typically became the dominant Fe species within time periods of days to weeks. Fulvic and humic acids isolated from the same source displayed remarkably similar abilities to mobilize Fe-hydroxide after 12 hours of exposure.; The feasibility of the Fe mobilization model was considered by analyzing lysimeter-collected soil solutions collected from various soil horizons (Spodosol, Findley Lake, WA). In soil solutions from the upper horizons, approximately one-third to one-half of the total Fe occurred as Fe-II, and conditions were conducive for reductive dissolution. It was proposed that removal of Fe occurred by oxidation and adsorption of Fe-NOM complexes. An increase in pH as the soil solutions contact the lower horizons was suggested to be primarily due to release of hydroxyls as DOC was adsorbed in the lower soil horizons.