Heavy metal contamination (such as lead, cadmium, mercury, arsenic, and chromium) in industrial wastewater poses a significant threat to the environment and human health. Traditional physical filtration and biological treatment methods often struggle to completely remove heavy metals. Chemical methods, however, have become an important means of industrial wastewater treatment due to their high efficiency, controllability, and wide applicability.
Chemical Precipitation (Most Common, Low-Cost)
Chemical precipitation involves adding chemicals to wastewater to cause heavy metal ions to form insoluble precipitates, which are then removed through solid-liquid separation.
Common Precipitants and Reaction Principles
Hydroxide Precipitation (PH Adjustment)
Adding alkaline substances such as NaOH and Ca(OH)₂(lime) causes heavy metals to form hydroxide precipitates.
Applicable heavy metals: Cu²⁺, Zn²⁺, Ni²⁺, Cd²⁺, etc.
Sulfide Precipitation (More Thorough)
Sulfides such as Na2S and H2S are added to produce a metal sulfide precipitate with an extremely low solubility product.
Advantages: More thorough than hydroxide precipitation and suitable for low-concentration heavy metal wastewater.
Carbonate Precipitation
Suitable for metals such as lead and cadmium that can precipitate with CO3²⁻.
Advantages and Disadvantages of Precipitation:
Simple operation, low cost, suitable for large-scale application, and capable of treating multiple heavy metals simultaneously.
However, it produces a large amount of sludge, requiring further treatment.
Sulfide precipitation may produce toxic H2S gas, requiring strict PH control.
Ion Exchange (High-Precision Removal)
Ion exchange uses ion exchange resins to selectively adsorb heavy metal ions and is suitable for low-concentration wastewater or high-purity reuse needs.
Common Resin Types:
Cation Exchange Resins: Remove cations such as Cu²⁺, Pb²⁺, and Cd²⁺.
Chelating resins: Highly selective for specific metals (such as Hg²⁺and Ni²⁺).
Advantages and Disadvantages
High removal rate (>99%), suitable for trace heavy metal removal.
Resin is reusable, and operating costs are manageable.
However, resins are easily contaminated by organic matter and require pretreatment.
Treatment efficiency is low for high-concentration wastewater, making it suitable for subsequent polishing.
Adsorption (Low Cost, Easy Operation)
Adsorption uses porous materials (such as activated carbon, zeolite, and biochar) to adsorb heavy metals.
Advantages and Disadvantages of Adsorption
Simple operation, no secondary pollution.
Adsorbents are regenerable.
However, their adsorption capacity is limited and requires frequent replacement. Treatment effectiveness for high-concentration wastewater is poor.
Electrochemical Method (High Efficiency, Automated)
Electrochemical methods use electrolysis to deposit or precipitate heavy metals on electrodes and are suitable for high-concentration wastewater.
What Are Commonly Used Flocculants
Inorganic flocculants
Aluminum sulfate, polyaluminum chloride (PAC), ferric chloride
Features: Low cost and wide applicability, but high sludge production, and some agents may corrode equipment.
Applications: High-turbidity wastewater, industrial heavy metal wastewater, and municipal wastewater treatment.
Organic polymer flocculants
Polyacrylamide (PAM), chitosan
Features: Low dosage, large flocs, and rapid settling, but high price, and some synthetic PAMs may have residual problems.
Applications: Sludge dewatering, low-turbidity wastewater, and the food and paper industries.
Composite flocculants
PAC+PAM, polysilicate aluminum ferric (PSAF)
Features: Combining the advantages of inorganic and organic flocculants, improving sedimentation efficiency and reducing chemical dosage.
Applicable applications: Treating difficult-to-treat industrial wastewater (such as printing and dyeing, and electroplating wastewater).
Biological flocculants
Features: Environmentally friendly, biodegradable, and pollution-free, but with a high cost. Currently used primarily in the food and pharmaceutical industries.
Key Factors in Selecting Water Treatment Chemicals
- Water Quality Characteristics
Identify pollutant type (heavy metals/organic matter/suspended solids), concentration, and pH.
Select the appropriate chemical for each pollutant (e.g., precipitants or chelating agents for heavy metals, oxidants for organic matter).
- Treatment Objectives
Distinguish between different requirements, such as meeting discharge standards, reuse, or resource recovery.
Select cost-effective and efficient technology routes based on these objectives.
- Chemical Performance
Evaluate reaction efficiency, sludge production, and residual risk.
Balance treatment effectiveness with the difficulty of subsequent disposal.
- Economic Efficiency
Comprehensively calculate chemical costs, equipment investment, and operating energy consumption.
For high-concentration wastewater, consider low-cost precipitants; for low-concentration wastewater, consider adsorption/ion exchange.
- Operational Requirements
Consider dosing method, system stability, and degree of automation.
For fluctuating water quality, select a broad-spectrum chemical with strong adaptability.
- Safety and Environmental Protection
Avoid the use of highly toxic substances and control the risk of secondary contamination.
For food and drinking water treatment, prioritize biodegradable chemicals.