Al(III) speciation distribution and transformation in high concentration PACl solutions

Area of Science:

• Inorganic Chemistry
• Water Treatment Chemistry

Background:

• Polyaluminum chloride (PACl) is a widely used coagulant in water treatment.
• Understanding the speciation of aluminum in PACl is crucial for optimizing its performance.
• The Al13 fraction is a key species influencing PACl's effectiveness.

Purpose of the Study:

• To investigate the effects of aluminum(III) concentration and pH on PACl speciation.
• To determine the factors controlling the formation and stability of the Al13 fraction in PACl solutions.

Main Methods:

• Preparation of PACl samples with varying Al(III) concentrations (0.01–2.0 mol/L) and B ratios (1.0–2.5).
• Characterization using ferron assay, pH measurements, and 27Al Nuclear Magnetic Resonance (NMR) spectroscopy.
• Analysis of speciation changes during dilution, aging, and concentration (freeze-drying, heating).

Main Results:

• Al(III) concentration significantly impacts hydrolysis and species distribution, correlating with decreased pH.
• Higher Al(III) concentrations and lower pH led to a decrease in the Al13 fraction.
• Al13 was undetectable at 2.0 mol/L Al(III) and pH 2.73.
• Dilution and aging allowed for the reappearance and increase of Al13 over time.
• Concentration via freeze-drying or heating reduced both pH and Al13 content.

Conclusions:

• pH is the critical factor governing Al13 formation in concentrated PACl.
• Aluminum speciation in PACl is a dynamic process influenced by concentration, pH, time, and temperature.
• Controlling pH is essential for managing the Al13 fraction and optimizing PACl performance.