Characterization of Phosphate Sequestration by a Lanthanum Modified Bentonite Clay: A Solid-State NMR, EXAFS, and PXRD Study

Phosphate (P-i) sequestration by a lanthanum (La) exchanged clay mineral (La-Bentonite), which is extensively used in chemical lake restoration, was investigated on the molecular level using a combination of P-31 and La-139 solid state NMR spectroscopy (SSNMR), extended X-ray absorption spectroscopy (EXAFS), powder X-ray diffraction (PXRD) and sorption studies. P-31 SSNMR show that all P-i was immobilized as rhabdophane (LaPO(4.)n (HO)-O-2, n <= 3), which was further supported by La-139 SSNMR and EXAFS. However, PXRD results were ambiguous with respect to rhabdophane and monazite (LaPO4). Adsorption studies showed that at dissolved organic carbon (DOC) concentration above ca. 250 mu M the binding capacity was only 50% of the theoretical value or even less. No other La or Pi phases were detected by SSNMR and EXAFS indicating the effect of DOC is kinetic. Moreover, P-31 SSNMR showed that rhabdophane formed upon P-i sequestration is in close proximity to the clay matrix.