Paper No. 20
Presentation Time: 9:00 AM-6:00 PM
THE ROLE OF CALCIUM IN HUMIC-CLAY COMPLEX FORMATION IN SOILS
OUFQIR, Sofia, Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, Saint Paul, MN 55108, TONER, Brandy M., Soil, Water, and Climate, University of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, Saint Paul, MN 55104, BLOOM, Paul R., Saint Paul, MN 55108, HATCHER, Patrick, Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529 and EL AZZOUZI, Mohammed, Rabat, 10000, oufqi001@umn.edu
Humic-clay complexes are recognized to be important geocolloids because of their influence on soil aggregate stability and stabilization of carbon in the global C cycle. The presence of Ca ions largely enhances humic sorption on clay particles through cation bridges that represent one of the most important sorption mechanisms. However, this mechanism has received little scientific scrutiny. Our goal was to study the chemistry of Ca in the humic acid (HA), clay mineral and humic-clay matrixes by means of solid state
13C CP-MAS NMR and FTIR spectroscopy, and compare it to those of strontium and barium to assess the effect of alkaline earth in the interaction between HAs and clay minerals.Clay-humic composites were obtained by dispersion-sedimentation-siphoning processes, while HAs and clay minerals were separated from the soil using standard methods. Samples have been treated with chloride salts of Ca, Sr or Ba prior to spectroscopic measurements.
FTIR results for HA set of samples revealed structural changes stemming from their interaction with alkaline earth cations. Drastic decrease in O-H/C-O carboxylic and phenolic bands combined with strong increase in the intensity of ionized carboxylic and alcoholic groups peaks indicate the formation of metal complexes with HA. Furthermore, calcium was abundantly fixed to HA as compared to barium and strontium. On the other hand, clay mineral spectra showed particularly barium and strontium complexation with Al-Al-OH groups, whereas calcium was fixed through water bridging. The NMR results confirmed these findings, and further showed that only the aliphatic fraction of organic matter was bonded to clay minerals by means of carboxylic and hydroxyl groups. In other respects, the association of Ca, Ba, and Sr ions with carboxylic groups of HAs caused a conformation change substantiating the liability of forming intermolecular interactions. However, only calcium produced significant structural transition when bonded to humic-clay composites owing likely to the formation of highly efficient bridging links.Therefore, the conclusions drawn from our investigation help to explain the effective role of calcium in the sorption of humic substance by clay minerals, and yet understand its effect in stabilizing the carbon in soils and hence in the global C cycle.
