A wastewater contains a high soluble copper II concentration. Alkaline precipita

Question

A wastewater contains a high soluble copper II concentration. Alkaline precipitation with caustic is planned to meet the NPDES PSES discharge standard for copper. Determine the pH where the minimum solubility of Cu(OH)2 occurs, assuming a constant equilibrium total nitrogen concentration (NH4+ plus NH3 ) of 300 mg/l as N. At this pH, evaluate the total soluble copper concentration. Neglect ionic strength corrections and assume the solution temperature is 25 oC. Provide a logarithm concentration diagram of [Cu+2] versus pH.

Explanation / Answer

Ka = [Cu(OH)2]/[Cu2+][OH-]

pKa = log[OH-] + log ([Cu2+]/[Cu(OH)2]

= pH - 14 +  log ([Cu2+]/[Cu(OH)2]

log ([Cu2+]/[Cu(OH)2] = 14 + (pKa - pH)

Increasing pH decreases the [Cu2+]

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