Rare earth elements complexation with humic acid

Abstract

The binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE­HA complexation experiments were performed at various pH conditions (ranging from 2 to 10.5) using a standard batch equilibration method. Results show that the amount of REE bound to HA strongly increases with increasing pH. Moreover, a Middle-REE (MREE) downward concavity is evidenced by REE distribution patterns at acidic pH. Modelling of the experimental data using Humic Ion Binding Model VI provided a set of log KMA values (i.e., the REE­HA complexation constants specific to Model VI) for the entire REE series. The log KMA pattern obtained displays a MREE downward concavity. Log KMA values range from 2.42 to 2.79. These binding constants are in good agreement with the few existing datasets quantifying the binding of REE with humic substances but quite different from a recently published study which evidence a lanthanide contraction effect (i.e., continuous increase of the constant from La to Lu). The MREE downward concavity displayed by REE­HA complexation pattern determined in this study compares well with results from REE­fulvic acid (FA) and REE­acetic acid complexation studies. This similarity in the REE complexation pattern suggests that carboxylic groups are the main binding sites of REE in HA. This conclusion is further illustrated by a detailed review of published studies for natural, organic-rich, river- and ground-waters which show no evidence of a lanthanide contraction effect in REE pattern. Finally, application of Model VI using the new, experimentally determined log KMA values to World Average River Water confirms earlier suggestions that REE occur predominantly as organic complexes (= 60%) in the pH range between 5­5.5 and 7­8.5 (i.e., in circumneutral pH waters). The only significant difference as compared to earlier model predictions made using estimated log KMA values is that the experimentally determined log KMA values predict a significantly higher amount of Light-REE bound to organic matter under alkaline pH conditions

Characterization of the Coal Humic Acids From the Candiota Coalfield, Brazil

“…The purification process performed after the extraction causes a significant loss of cations, thereby decreasing the elemental concentration. Very few of the metallic elements detected can be incorporated into the CHA structure because of the capacity of the CHA to bind many heavy metallic ions through complexation [31,32]. Other elements such as Si can originate from impurities.…”

Distribution and fractionation of the rare earth elements in Brazilian soils

“…This result corroborates Palumbo et al (2005) studies on the affinity of REEs to oxides, where it was reported that Fe, Al and Mn (hydr) oxides can co-precipitate, adsorb and incorporate various REEs in their structure, in amounts varying with type (Wang et al, 2001), depth (Yan et al, 1999) and soil pH (Hu et al, 2006), and sometimes without correlation with Fe and Mn contents (Yan et al, 1999). Organic matter can also retain significant amounts of REEs, because it can complex, adsorb or chelate positively charged REEs in amounts varying with the type, composition and content of OM, soil pH and redox conditions (Pourret et al, 2007). The adsorption of REEs increases with increasing soil pH and CEC values because the surface of soil particles charged with OH -increases as the soil pH increases, thus increasing CEC values.…”

Rare earth element sorption onto hydrous manganese oxide: A modeling study

“…The linear free energy relationship (LFER) or the correlation between the first hydrolysis constant for aqueous species and the corresponding surface complexation constant was used to estimate the sorption stability constant as has been previously done in numerous studies [16,28,36,37]. The LFER can be used to extend results from a limited dataset to other metals.…”