Metal loading effect on rare earth element binding to humic acid: Experimental and modelling evidence

Abstract

The effect of metal loading on the binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE–HA complexation experiments were performed at pH 3 for REE/C molar ratios ranging from ca 4 × 10 −4 to 2.7 × 10 −2. Results show that the relative amount of REE bound to HA strongly increases with decreasing REE/C. A middle-REE (MREE) downward concavity is shown by patterns at high metal loading, whereas patterns at low metal loading display a regular increase from La to Lu. Humic Ion Model VI modelling are close to the experimental data variations, provided that (i) the ΔLK 2 parameter (i.e. the Model VI parameter taken into account the presence of strong but low density binding sites) is allowed to increase regularly from La to Lu (from 1.1 to 2.1) and (ii) the published log K MA values (i.e. the REE–HA binding constants specific to Model VI) are slightly modified, in particular with respect to heavy REE. Modelling approach provided evidence that log K d REE patterns with varying REE/C likely arises because REE binding to HA occurs through two types of binding sites in different density: (i) a few strong sites that preferentially complex the heavy REE and thus control the log K d REE atterns at low REE/C; (ii) a larger amount of weaker binding sites that preferentially complex the middle-REE and thus control the log K d REE pattern at high REE/C. Hence, metal loading exerts a major effect on HA-mediated REE binding, which could explain the diversity of published conditional constants for REE binding with HA. A literature survey suggests that the few strong sites activated at low REE/C could be multidentate carboxylic sites, or perhaps N-, or P-functional groups. Finally, an examination of the literature field data proposed that the described loading effect could account for much of the variation in REE patterns observed in natural organic-rich waters (DOC > 5 mg L −1 and 4 ⩽ pH ⩽ 7).