Competition between humic acid and carbonates for rare earth elements complexation

Area of Science:

• Environmental Chemistry
• Geochemistry
• Analytical Chemistry

Background:

• Rare earth elements (REE) are crucial in various industrial applications and environmental processes.
• Understanding REE speciation in natural waters is vital for assessing their mobility and bioavailability.
• Previous models proposed ternary REE-carbonate-humate complexes, requiring experimental validation.

Purpose of the Study:

• To experimentally investigate the competitive binding of REE to humic acid (HA) and carbonates.
• To determine the influence of pH and alkalinity on REE complexation.
• To validate the accuracy of Model VI for predicting REE speciation in natural waters.

Main Methods:

• Ultrafiltration coupled with inductively coupled plasma mass spectrometry (ICP-MS) for REE analysis.
• Experimental study across a range of pH and alkalinity conditions.
• Data modeling using Model VI and established stability constants.

Main Results:

• REE form binary humate or carbonate complexes, not ternary REE-carbonate-humate complexes.
• REE complexation shows strong dependence on pH and alkalinity.
• Systematic fractionation of REE occurs, with heavier REE favoring carbonate complexation at higher pH and alkalinity.

Conclusions:

• Model VI accurately reproduces experimental data, confirming its utility for REE speciation in natural waters.
• Recently published stability constants for REE-humic acid complexation are reliable.
• This study clarifies REE competition with carbonates and HA, enhancing understanding of speciation in alkaline, organic-rich waters.