Understanding pH and ionic strength effects on aluminum sulfate-induced microalgae flocculation

Area of Science:

• Environmental Science
• Biotechnology
• Water Treatment

Background:

• Microalgae harvesting is crucial for biofuel and bioproduct applications.
• Aluminum sulfate is a common flocculant, but its efficacy depends on environmental conditions.
• Understanding flocculation mechanisms is key to optimizing microalgae recovery.

Purpose of the Study:

• To investigate the impact of pH and ionic strength of aluminum sulfate on microalgae flocculation.
• To correlate flocculation efficiency with cell surface charge and DLVO interaction energy.
• To compare the responses of different microalgae species to varying conditions.

Main Methods:

• Microalgae species Scenedesmus dimorphus and Nannochloropsis oculata were used.
• Aluminum sulfate concentration, pH, and ionic strength were systematically varied.
• Zeta potential measurements and DLVO theory were applied to analyze cell surface charge and interaction energies.

Main Results:

• Algal flocculation efficiency was directly influenced by pH and ionic strength, altering cell zeta potential.
• Lower total DLVO interaction energy corresponded to higher flocculation efficiency for both species.
• Nannochloropsis oculata showed increased flocculation with higher aluminum sulfate concentrations, favoring extreme pH values (5 and 10).
• Scenedesmus dimorphus exhibited maximal flocculation at low ionic strength (1 μM) or neutral pH (7.5), where surface charges were neutralized.

Conclusions:

• The Derjaguin, Landau, Verwey, and Overbeek (DLVO) model qualitatively predicts microalgae flocculation behavior.
• Species-specific responses to pH and ionic strength necessitate tailored flocculation strategies.
• Optimizing pH and ionic strength is critical for efficient microalgae harvesting using aluminum sulfate.