The effect of humic acid on the formation and solubility of secondary solid phases (Nd(OH)CO₃ and Sm(OH)CO₃)

Abstract

The formation of secondary Ln(III) solid phases ( e.g. Nd(OH)CO₃ and Sm(OH)CO₃) has been studied as a function of the humic acid (HA) concentration in 0.1 M NaClO₄ aqueous solution and their solubility has been investigated in the neutral pH range (6.5–8) under normal atmospheric conditions. Nd(III) and Sm(III) were selected as analogues for trivalent lanthanide and actinide ions. The solid phases under investigation have been prepared by alkaline precipitation and characterized by TGA, ATR-FTIR, XRD, TRLFS, DR-UV-Vis and Raman spectroscopy, and solubility measurements. The spectroscopic data obtained indicate that Nd(OH)CO₃ and Sm(OH)CO₃ are stable and remain the solubility limiting solid phases even in the presence of increased HA concentration (0.5 g/L) in solution. Upon base addition in the Ln(III)-HA system decomplexation of the previously formed Ln(III)-humate complexes and precipitation of two distinct phases occurs, the inorganic (Ln(OH)CO₃) and the organic phase (HA), which is adsorbed on the particle surface of the former. Nevertheless, HA affects the particle size of the solid phases. Increasing HA concentration results in decreasing crystallite size of the Nd(OH)CO₃ and increasing crystallite size of the Sm(OH)CO₃ solid phase, and affects inversely the solubility of the solid phases. However, this impact on the solid phase properties is expected to be of minor relevance regarding the chemical behavior and migration of trivalent lanthanides and actinides in the geosphere.