Soft X-ray spectromicroscopy investigation of the interaction of aquatic humic acid and clay colloids

Soft X-ray spectromicroscopy investigations of the interaction of aquatic h umic acid (HA) and montmorillonite colloids have been performed in situ at the NSLS X1-A STXM endstation. Images have been recorded of montmorillonite particles, HA aggregates, and mixed suspensions of both montmorillonite HA and montmorillonite + carboxyl polystyrene microspheres, as reference or ganic colloids. Special emphasis has been placed on the sample preparation technique in order to keep the colloid particles hydrated during all measur ements. C Is near edge absorption fine structure extracted from STXM image stacks reveals electronic transitions corresponding to functional -COOH and -C6H5 groups present in HA and polystyrene. XANES peak intensities reflect the relative amounts of these two carbon functional groups in the organic particles. For example, the greater amount of carboxyl groups in HA compare d to the latex reference particles affects a larger 1s --> pi* transition i ntensity. A specific form of montmorillonite-HA particle agglomeration at n ear-neutral pH has been observed. Under these conditions, we found no separ ate clustering of HA. Instead, STXM images show the HA to coat the clay min eral surface, leading to nearly a fractal manner of aggregation. (C) 2000 A cademic Press.