Phosphate (P) sequestration by a lanthanum (La) exchanged bentonite (a clay mineral), which is extensively used in chemical lake restoration, was investigated on the molecular level using a combination of 31P and 139La solid state NMR spectroscopy (SSNMR), extended X-ray absorption spectroscopy (EX-AFS) and powder X-ray diffraction (PXRD) in combination with sorption studies. 31P SSNMR show that all phosphate is immobilized as rhabdophane, LaPO4·xH2O, which is further supported by 139La SSNMR and EXAFS; whereas PXRD results are ambiguous with respect to rhabdophane and monazite (LaPO4). Adsorption studies show that, at humic acids (HA) concentrations above ca. 250 µM the binding capacity is only 50 % of the theoretical value or even less. No other lanthanum or phosphate phases are detected by SSNMR and EXAFS indicating the effect of HA is kinetic. Moreover, 31P SSNMR shows that rhabdophane formed upon P sequestration is in close proximity to the clay matrix.
Revised: February 21, 2020 |
Published: March 6, 2015
Citation
Dithmer L., A.S. Lipton, K. Reitzel, T.E. Warner, D. Lundberg, and U. Nielsen. 2015.Characterization of phosphate sequestration by a lanthanum modified bentonite clay: A solid- state NMR, EXAFS and PXRD study.Environmental Science & Technology 49, no. 7:4559-4566.PNNL-SA-107380.doi:10.1021/es506182s