Heavy Metal Removal from Industrial Wastewater

Three Precipitation Methods

Method	Reagent	Optimal pH	Achievable Level	Best For
Hydroxide	NaOH or Ca(OH)₂	8.5–11 (metal-dependent)	0.5–5 ppm	Most metals, lowest cost
Sulfide	Na₂S or FeS	7–9	0.01–0.1 ppm	Ultra-low limits, mixed metals
Chelation + IX	Chelating resin	2–6	< 0.01 ppm	Trace removal after precipitation

Hydroxide Precipitation pH Guide

Each metal has an optimal pH for minimum solubility:

Metal	Optimal pH	Minimum Solubility
Cr³⁺	7.5–8.5	0.01 mg/L
Cu²⁺	8.5–9.5	0.01 mg/L
Zn²⁺	9.0–10.0	0.01 mg/L
Ni²⁺	10.0–11.0	0.1 mg/L
Pb²⁺	9.0–10.0	0.05 mg/L
Cd²⁺	10.5–11.5	0.05 mg/L

Problem: Mixed metals have different optimal pH values. Compromise pH 9.0–9.5 works for most but may not meet ultra-low limits for Ni.

Cr(VI) Reduction

Hexavalent chromium must first be reduced to Cr(III) before precipitation:

1. Lower pH to 2.5–3.0 with H₂SO₄
2. Add FeSO₄ at Fe:Cr ratio 3:1 (or sodium bisulfite)
3. Mix 30 minutes — verify Cr(VI) < 0.1 ppm
4. Raise pH to 8.5 with NaOH — Cr(OH)₃ precipitates
5. Add flocculant, settle, filter

Process Design

1. Equalization — 6–8 hr HRT to smooth out concentration spikes
2. pH adjustment — NaOH (precise) or lime (cheaper for large volumes)
3. Coagulation — FeCl₃ 50–100 ppm improves settling and co-precipitates metals
4. Flocculation — Anionic PAM 0.5–2 ppm
5. Clarification — Lamella settler or conventional
6. Polishing — Sand filter + chelating resin for ultra-low limits
7. Sludge handling — Filter press, hazardous waste disposal