Chemical Treatment Processes
Core Concepts
Chemical treatment transforms contaminants into removable forms. Key processes include neutralization, precipitation, coagulation/flocculation, oxidation/reduction, ion exchange, and disinfection. pH adjustment is fundamental; precipitation of heavy metals as hydroxides (e.g., Cd(OH)2, Pb(OH)2) or sulfides is common. Coagulation destabilizes colloids (e.g., with alum, ferric chloride); flocculation agglomerates them into larger flocs. Oxidation (e.g., with chlorine, ozone, hydrogen peroxide, Fenton's reagent) destroys organics; reduction (e.g., with sulfur dioxide, sodium bisulfite) converts hexavalent chromium (Cr6+) to trivalent chromium (Cr3+), which precipitates. Ion exchange removes dissolved ions. Disinfection (chlorination, UV, ozonation) inactivates pathogens. Traps include confusing coagulation vs. flocculation, misapplying solubility product (Ksp) concepts (e.g., minimum solubility of metal hydroxides occurs at specific pH), and overlooking competing ions in precipitation. Also, ORP (oxidation-reduction potential) is a key control parameter for redox reactions. Another trap: chlorine demand vs. residual chlorine; breakpoint chlorination. Hexavalent chromium reduction requires acidic pH (2-3) and then precipitation at higher pH (8-10).
Worked Example
To precipitate Cr3+, the pH must be raised to about 8-9. If Cr6+ is present, it must first be reduced to Cr3+ at pH 2-3 using a reducing agent (e.g., sodium metabisulfite). Rule of thumb: Cr6+ reduction: pH 2-3; precipitation: pH 8-9. This shows the trap of trying to precipitate Cr6+ directly-it is soluble at high pH.