Question

Aluminium sulfate (alum) is the most widely used aluminium-based coagulant in water and wastewater treatment.

a) Briefly describe how alum (Al₂(SO₄)₃) addition assists in water and wastewater treatment.

b) Draw a logC-pH diagram for the aluminium species at equilibrium with Al(OH)₃(s).

c) For the recipe [Al₂(SO₄)₃] = 2 x 10^-3 M, what are the pH and the composition of the system, assuming equilibrium with Al(OH)₃(s)?

d) For what total concentration(s) (minimum and/or maximum) of Al₂(SO₄)₃ is aluminium entirely soluble?

Assume that Al(OH)₃(s) is the only solid that forms and consider only the hydrolysis of Al(III) species. Treat the system as a closed system with respect to CO₂ exchange. Consider the following reaction:

Reaction LogK

Al³⁺ + OH^- = Al(OH)^{2+ 9.0}

Al³⁺ + 2OH^- = Al(OH)²⁺ 18.7

Al³⁺ + 3OH^- = Al(OH)₃⁰ 27.0

Al³⁺ + 4OH^- = Al(OH)₄ 33.0

Al³⁺ + 3OH^- = Al(OH)₃(s) 33.5

Aluminium sulfate (alum) is the most widely used aluminium-based coagulant in water and wastewater treatment. a) Briefly describe how alum (Al2(SO4)3) addition assists in water and wastewater treatment. b) Draw a log C-pH diagram for the aluminium species at equilibrium with Al(OH)3(s). c) For the recipe (Al2(SO4)] = 2 x 10-2M, what are the pH and the composition of the system, assuming equilibrium with Al(OH)3(s)? d) For what total concentration(s) (minimum and/or maximum) of Al2(SO4)3 is aluminium entirely soluble? Assume that Al(OH)3(s) is the only solid that forms and consider only the hydrolysis of Al(III) species. Treat the system as a closed system with respect to CO2 exchange. Consider the following reaction: Reaction LogK Al3+ + OH = Al(OH)2 9.0 Al3+ + 2OH = Al(OH)2 18.7 Al3+ + 3OH = Al(OH)3° 27.0 Al3+ + 4OH = Al(OH)4 33.0 Al3+ + 3OH = Al(OH)3(S) 33.5