Soft X-ray spectro-microscopy investigation of the formation and ageing of Eu(III)-induced humic acid aggregates

Abstract
Aggregation of Aldrich humic acid at pH 4.3 and after loading with Eu(III)-cations at pH 5.3 is investigated by scanning transmission X-ray microscopy (STXM). C 1s-NEXAFS spectra extracted from STXM image stacks for the low pH aggregates show no significant variation, whereas Eu(III)-humate formation is observed to be accompanied by segregation into zones with different optical densities. These zones exhibit distinctly different NEXAFS features, indicative of the presence of a separation of organic species or components. Increasing humate aggregate densification over a period of several months is observed. The spectrum of the unloaded HA cannot be reproduced by a superposition of spectra extracted from the segregated humate zones; there is a distinct metal bonding effect on the molecular states of the functional groups involved in complexation.