Zirconium speciation in lactate solutions and polyacrylate gels

Controlling gelation kinetics is an important objective for several applica tions (ceramic and thin film syntheses, reduction in permeability of porous rock, etc). There is a growing interest in studying the gelation of polyme rs by zirconium, a crosslinker of lower toxicity than the chromium which is still commonly used. XAS at the Zr K-edge was performed at the European Sy nchrotron Radiation Facility (ESRF) on the BM32 beamline. The fluorescence detection was used to carry out successful in situ speciation at concentrat ions as low as 36 ppm. The Zr speciation was determined both in ZrLa (where La stands for lactate) aqueous solutions and in gels of a terpolymer of ac rylamide having 2% of zirconium reactive acrylate side groups and 2% of sul fonate groups introduced to prevent syneresis. XANES results show that Zr i s always in a dodecahedral geometry. In ZrLa solutions, EXAFS results indic ate that Zr species grow from a dimer Zr-2(La)(6) to a tetramer (Zr-4(La)(x )) and then to larger polymers resulting from tetramer associations, as the Zr concentration decreases from 51840 ppm to 36ppm. In polymer gels, Zr sp ecies appear to be dimers at pH 6 while tetramers are found when gelation o ccurred at pH 7. Calculations taking into account multiple scattering effec ts as well as dynamic molecular calculation confirmed conclusions derived f rom conventional EXAFS analysis.