Colloidal and solid phase partitioning between ferrihydrite, humic acid and copper coprecipitates

Area of Science:

• Environmental Chemistry
• Geochemistry
• Biogeochemistry

Background:

• Understanding environmental interactions between iron (Fe) oxides, humic acids (C), and copper (Cu) is crucial for predicting Cu fate.
• Current limitations exist in measuring specific forms and chemical interactions of these elements in environmental matrices.
• Dissolved and colloidal Cu concentrations are key indicators of Cu bioavailability and mobility.

Purpose of the Study:

• To investigate the effects of pH, Fe:C molar ratio, and Cu concentration on dissolved and colloidal Cu.
• To characterize the colloidal phases formed during sorption (SOR) and coprecipitation (CPT) reactions.
• To evaluate the performance of modeling approaches in predicting Cu partitioning.

Main Methods:

• Utilized asymmetrical flow field-flow fractionation (AF4) coupled with total organic carbon (TOC) analysis and inductively coupled plasma mass spectrometry (ICP-MS).
• Examined reactions across a range of pH (5-7) and Fe:C molar ratios (1:0 to 1:3).
• Applied an additivity approach with NICA-Donnan and ferrihydrite surface complexation models.

Main Results:

• Fe:C ratio and pH significantly influenced Cu partitioning between dissolved, colloidal, and solid phases.
• At pH 7, Fe-C-Cu ternary phases comprised ~10% of dissolved Cu; at pH 6, only Fe-Cu colloids were observed.
• Modeling approaches showed discrepancies, often overpredicting dissolved Cu without C and underpredicting with C present.

Conclusions:

• Environmental factors like pH and humic acid content critically alter Cu speciation and phase distribution.
• The formation of distinct colloidal phases (Fe-C-Cu, Cu-C, Fe-Cu) impacts Cu behavior.
• Current models require refinement to accurately predict dissolved and colloidal Cu concentrations in complex environmental systems.