Heavy Metals Removal from Wastewater to Meet Discharge Limits

Why is dissolved metal so hard to remove?

Heavy metals in industrial effluent are typically present as dissolved cations (Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Pb²⁺) or oxyanions such as chromate (CrO₄²⁻). Unlike suspended solids, they cannot be screened or settled out while they remain in solution. The core strategy is a phase change: shift the metal into a solid you can then separate by clarification, DAF or filtration.

The dominant route is hydroxide precipitation. Raising pH with lime (Ca(OH)₂) or caustic (NaOH) supplies hydroxide ions that drive the reaction below:

That K_sp relationship is the whole game: for a divalent metal, [M²⁺] = K_sp / [OH⁻]², so residual dissolved metal is a strong function of pH. This is why pH control, not reagent choice, dominates outcomes.

Why does each metal need a different pH?

Every metal hydroxide has its own K_sp, so its pH of minimum solubility is different. Copper reaches its minimum near pH 8–9, nickel and cadmium nearer pH 10–11, and zinc close to pH 9–9.5. Set the pH to favour one metal and you can leave another well above its limit.

The problem is compounded by amphoterism. Metals such as zinc, lead, chromium(III) and aluminium form soluble hydroxo-complexes at high pH — for zinc, Zn(OH)₂ redissolves as zincate (Zn(OH)₄²⁻) above about pH 10.5. The solubility curve is therefore U-shaped: too low and the hydroxide never forms, too high and it dissolves again.

For a mixed-metal stream there is no single pH that minimises every metal at once. Designers pick a compromise pH — commonly 9.0–9.5 — that keeps the worst-case metal within limits, and accept that others sit above their individual optimum. Where limits are very tight, the stream is treated in stages at two different pH set-points, or polished after precipitation.

What residuals can hydroxide precipitation actually reach?

The table below gives indicative minimum-solubility pH values and the dissolved metal residuals typically achievable by conventional hydroxide precipitation followed by clarification and filtration. Real numbers depend on temperature, ionic strength, co-precipitation and the presence of complexing agents.

Hydroxide precipitation reliably reaches the low-mg/L to few-hundred-µg/L range. It struggles below roughly 0.1–0.5 mg/L, which is where sulphide precipitation and polishing steps take over.

When should you use sulphide instead of hydroxide?

Metal sulphides are far less soluble than the corresponding hydroxides — solubility products for CuS, CdS and PbS are of the order 10⁻³⁶ to 10⁻²⁸, against roughly 10⁻²⁰ to 10⁻¹⁵ for the hydroxides. Sulphide precipitation therefore reaches much lower residuals, often into the µg/L range, and is less sensitive to pH and to amphoteric redissolution.

The trade-offs are real: excess sulphide is toxic and can release H₂S gas, so dosing is controlled by ORP/sulphide-selective electrode; the fine precipitate can be colloidal and needs coagulation; and residual sulphide must itself be removed. In practice many plants run a hybrid scheme — bulk removal by hydroxide, then sulphide polishing of the last fraction. For streams needing consistent sub-100 µg/L performance, a downstream membrane or ultrafiltration polishing stage captures the fine metal-sulphide and metal-hydroxide particulates that gravity clarification lets through.

How do you handle chelated metals and hexavalent chromium?

Two chemistries defeat plain precipitation and must be dealt with first.

Chelated metals. Complexing agents — EDTA, NTA, citrate, ammonia, phosphonates from plating and cleaning baths — hold the metal in a soluble complex so it never sees the free hydroxide. Simply raising pH does little. The chelate must be broken: by acidifying and oxidising (e.g. with ferrous sulphate/Fenton chemistry), by adding a competing metal such as ferric or calcium that displaces the target, or by using a metal-selective precipitant (e.g. dithiocarbamate). Only then will the liberated metal precipitate.

Hexavalent chromium. Cr(VI) exists as soluble, toxic chromate/dichromate that does not precipitate as a hydroxide. It must first be reduced to Cr(III) at low pH, then precipitated as Cr(OH)₃:

Because Cr(III) is amphoteric, keep the precipitation pH near 8.5 and avoid overshoot, or it redissolves as chromite.

Worked example: caustic dose and residual copper at target pH

Take a rinse-water stream of 100 mg/L dissolved Cu²⁺ (molar mass 63.5 g/mol) that must meet a 0.5 mg/L copper limit. We size the caustic dose to precipitate the copper and check the residual predicted by the solubility product.

• Molar copper load: 100 mg/L ÷ 63.5 g/mol = 1.57 mmol/L Cu²⁺.
• Stoichiometric hydroxide: Cu²⁺ + 2·OH⁻ → Cu(OH)₂, so 2 × 1.57 = 3.15 mmol/L OH⁻. As NaOH (40 g/mol): 3.15 × 40 = 126 mg/L NaOH, plus a small excess to raise and hold the target pH.
• Residual at pH 9: at pH 9, [OH⁻] = 10⁽⁹⁻¹⁴⁾ = 10⁻⁵ mol/L. With K_sp(Cu(OH)₂) ≈ 2.2 × 10⁻²⁰, dissolved [Cu²⁺] = K_sp / [OH⁻]² = 2.2 × 10⁻²⁰ / (10⁻⁵)² = 2.2 × 10⁻¹⁰ mol/L.
• Convert to mass: 2.2 × 10⁻¹⁰ mol/L × 63.5 g/mol = 1.4 × 10⁻⁸ g/L = 0.000014 mg/L equilibrium copper.

Equilibrium says copper could reach far below the 0.5 mg/L limit. In practice the achieved residual is set by solid–liquid separation, not thermodynamics: fine Cu(OH)₂ floc carried over the clarifier weir keeps real effluent nearer 0.1–0.3 mg/L. The lesson — precipitation chemistry is rarely the bottleneck; capturing the last of the precipitate is. That is why polishing filtration or membranes are added when limits fall into the µg/L range.

How do you polish down to µg/L limits?

When permit limits are set in µg/L — as they increasingly are for priority substances — precipitation alone is not enough and a polishing stage removes the residual dissolved and colloidal metal:

• Ion exchange. Chelating resins (iminodiacetic or aminophosphonic) selectively capture trace metal cations to low µg/L, even against a high calcium/sodium background. See our guide to ion exchange and water softening for the underlying resin chemistry.
• Adsorption. Granular ferric hydroxide, activated alumina and functionalised media adsorb specific metals (notably arsenic, lead) and suit low, steady loads.
• Membranes. Ultrafiltration removes precipitated metal particulates; reverse osmosis and nanofiltration reject dissolved ions for the tightest limits or for reuse.

Selection follows the flow, the residual concentration and whether the water is being recycled. Trace polishing is inherently a low-capacity duty, so it belongs downstream of bulk precipitation, never as the primary removal step.

What do UK and EU compliance limits require?

Discharges to sewer are controlled by a trade effluent consent from the water company; discharges to a watercourse need an Environment Agency permit with Emission Limit Values (ELVs) for named metals. These are backed by the Water Framework Directive (WFD), which lists several metals — cadmium, lead, nickel and mercury — as priority (or priority hazardous) substances with Environmental Quality Standards (EQS) expressed as dissolved concentrations in the receiving water. Zinc and copper are controlled as specific pollutants under UK-specific EQS.

Two practical consequences follow. First, permit limits are increasingly bioavailability-based and set in µg/L, which is why polishing is often unavoidable. Second, the metal-bearing sludge from precipitation is usually classified as hazardous waste (mirror-hazardous entries in the List of Wastes), so it must be characterised, stored and consigned accordingly — a cost that belongs in the treatment business case. For the wider framework of numeric limits and monitoring, see our overview of industrial discharge compliance requirements and the detailed guide to UK effluent discharge standards.