Heavy metal ions (Pb, Cr, Hg) in industrial wastewater are typically removed by:
Choose the correct answer
Biological nitrification
Chemical precipitation as metal hydroxides by pH adjustment with lime
Filtration through sand bed only (no chemical treatment)
UV disinfection
Correct Answer
B. Chemical precipitation as metal hydroxides by pH adjustment with lime
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Heavy metals: removed by chemical precipitation — pH adjustment (lime addition) precipitates metal hydroxides [M(OH)n]. Example: Cr⁶⁺ → reduced to Cr³⁺ by SO2 or FeSO4 → then precipitated as Cr(OH)3 at pH 8–9. Pb²⁺ precipitates at pH > 9. For trace concentrations: ion exchange or adsorption (activated carbon). Coagulation + sedimentation after precipitation.
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