Zirconium speciation in microgels: kinetics aspects

Recent works demonstrated that the formation of gels under shear flow that leads to size-controlled microgels, is very promising for several applications in oil and gas industry and could be of great interest to develop new environmental processes that require the control of both water mobility and soil permeability. X-ray absorption spectroscopy (XAS) at the Zr K-edge was performed at the European Synchrotron Radiation Facility (ESRF) to determine the speciation of Zr used as crosslinker to form microgels of a terpolymer of acrylamide having 2% of zirconium reactive acrylate side groups (e.g. HSPAM). The fluorescence detection was used to carry out successful in-situ speciation at concentrations as low as 37 ppm. In polymer microgels and in excess of lactate, Zr species appear to be dimers at pH 6 while tetramers are found when gelation occurred at pH 7. The comparison of the XAS spectra before and after gelling indicates that a ligand exchange between lactate and acrylate around Zr occurs. This ligand replacement ensures HSPAM gelation. For the experiment at pH 6 at various aging times but with lower lactate amount, the evolution of the number of zirconium atoms at 3.6 Å from central zirconium ( N Zr), suggests that the Zr oligomerization is slow. The important result from this kinetic experiment is that the lactate-acrylate replacement is not complete for gels already formed. In other words only a small amount of crosslinkers react with HSPAM to form gels. For long aging time the crosslinking species can evolve from zirconium dimers to tetramers.