How to Solve the Problem of Heavy Metal Ion Residues in Polyaluminium Chloride (PAC)?

Polyaluminum chloride (PAC) as a core coagulant in water treatment directly impacts the safety of effluent quality. However, in practical applications, heavy metal ion residues occasionally occur, posing environmental pollution risks and even threatening human health. These issues typically stem from impurities in PAC raw materials,  production process defects, or water quality complexity, requiring systematic scientific control from source to endpoint. This article will analyze these factors and provide practical solutions. 

Causes of Heavy Metal Residues in PAC

Raw Material Impurities:

Polyaluminum chloride production raw materials (e.g., bauxite, calcium aluminate powder) naturally contain trace heavy metals (e.g., arsenic, lead, cadmium). Inadequate raw material screening or unqualified suppliers may lead to excessive heavy metals in the final product.

Production Process Contamination:

During acid dissolution reactions, equipment made from materials containing heavy metals (e.g., low-quality stainless steel reactors) may corrode and release ions such as iron or chromium.

Complexity of Water Quality:

When treating industrial wastewater containing heavy metals, Polyaluminum chloride may adsorb and carry some heavy metals into water bodies, causing secondary pollution.

Solutions for Heavy Metal Residues in PAC

Raw Material Screening and Supplier Management:

Use high-purity bauxite and require suppliers to provide heavy metal test reports (e.g., complying with GB 15892-2020 standards).

Equipment Upgrades:

Replace ordinary steel with corrosion-resistant materials (e.g., enamel reactors, titanium alloy pipelines) to prevent heavy metal leaching during production.

Process Optimization:

Add sodium sulfide (Na₂S) or calcium sulfide (CaS) during the acid dissolution stage to precipitate heavy metal ions as sulfides for filtration and removal.

Chemical Precipitation Method:

Add sodium sulfide or sodium hydroxide to treated water, adjusting the pH to 8-9 to form hydroxide or sulfide precipitates of heavy metal ions.

Activated Carbon Adsorption:

Install activated carbon adsorption tanks after coagulation to remove residual heavy metals (e.g., 90%+ removal rates for lead and cadmium) using its porous structure.

Ion Exchange Resins:

For high-concentration scenarios (e.g., electroplating wastewater), use chelating ion exchange resins (e.g., DTPA-modified resins) to selectively adsorb heavy metal ions.

Regular Testing:

Perform heavy metal testing on polyaluminum chloride raw materials, finished products, and effluent using methods like atomic absorption spectroscopy or inductively coupled plasma mass spectrometry (ICP-MS).

Heavy metal residues in polyaluminum chloride are a significant but highly controllable risk. By implementing source control (raw material screening), process upgrades, and endpoint treatments (e.g., activated carbon or resin systems), combined with routine monitoring, risks can be effectively mitigated. For sensitive applications (e.g., drinking water treatment), prioritize food-grade PAC supplemented with activated carbon adsorption. For industrial wastewater, chemical precipitation paired with ion exchange resins offers cost-effective efficiency. The ultimate goal is to ensure water safety while maximizing the economic and technical benefits of PAC applications.