When discussing water treatment, we learned that high phosphate concentrations are often observed and it is a problem due to its ability to promote eutrophication. One method for removing the phosphate is with a chemical coagulant like alum (Al2(SO4)3¬.) While the reaction series is actually much more complicated, we will assume that the alum dissociates to form aluminum ions and inert sulfate ions. The aluminum cations will react with any available phosphate to form aluminum phosphate and any leftover aluminum ions will reaction with water to precipitate as aluminum hydroxide.
Al2(SO4)3(s) -> 2 Al3+(aq) + 3 SO42-(aq)
Al3+(aq) + PO43-(aq)-> AlPO4(s)
Al3+(aq) + 3 H2O(l) -> Al(OH)3(s) + 3 H+(aq)
A wastewater contains 8.1 mg/L phosphate ions and is treated with an 18 mg/L alum solution. Calculate the mass of inorganic sludge (aluminum precipitates) produced in one day in a plant treating 22,000 m3 of wastewater.