Synthesis, characterization and stability of Cr(III) and Fe(III) hydroxides

Area of Science:

• Environmental Chemistry
• Water Treatment Technologies
• Materials Science

Background:

• Chromium contamination in soil and aquifers is a significant environmental issue, with hexavalent chromium (Cr(VI)) posing toxicity and carcinogenicity risks.
• Current water treatment methods focus on reducing toxic Cr(VI) to less harmful trivalent chromium (Cr(III)) and precipitating it as stable solid compounds.
• The stability of chromium precipitates is influenced by the presence of iron, forming mixed iron-chromium hydroxide phases.

Purpose of the Study:

• To synthesize, characterize, and evaluate the stability of mixed iron(III)-chromium(III) hydroxides.
• To compare the stability and effectiveness of mixed hydroxides against pure chromium(III) hydroxide precipitates in water treatment.

Main Methods:

• Synthesis of mixed (Fex,Cr1-x)(OH)3 hydroxides.
• Characterization of synthesized hydroxides using structural and solubility analyses.
• Thermodynamic calculations to assess stability and solubility limits.

Main Results:

• Crystalline chromium(III) hydroxide (Cr(OH)3·3H2O) is unstable and soluble, tending to transform into amorphous Cr(OH)3(am).
• Mixed Fe0.75Cr0.25(OH)3 hydroxides exhibit a ferrihydrite structure and their solubility aligns with a solid solution model.
• Mixed Fe(III)-Cr(III) hydroxides demonstrate significantly enhanced stability across a wider pH range (4.8–13.5) compared to pure Cr(III) hydroxides.

Conclusions:

• Mixed iron(III)-chromium(III) hydroxides are more effective than pure chromium(III) hydroxides in improving groundwater quality.
• The amorphous Cr(OH)3(am) phase meets drinking water standards (Cr level < 50μg/l) within a specific pH range (5.7–11).
• Mixed Fe0.75Cr0.25(OH)3 hydroxides offer extended stability, making them a promising material for effective chromium removal in water treatment.