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Preface
Methods for removing heavy metals from water are mainly divided into chemical, physical, and biological methods. The actual choice should consider water quality and treatment goals. The classifications are explained below:
Chemical Methods
01 Chemical Precipitation:
Add agents like lime, sodium hydroxide, and sodium sulfide to react with heavy metal ions to form insoluble hydroxides, sulfides, or salt precipitates, then separate the solids from the liquid. This method is suitable for high-concentration wastewater but produces a lot of sludge that needs further treatment.
02 Redox Method:
By adding an oxidant or reductant, you can change the valence of heavy metals, turning them into forms that are easy to precipitate or less toxic. Common methods include chlorination and potassium permanganate oxidation, which remove dissolved iron, manganese, and other ions. Manganese is harder to treat than iron, and removing iron alone isn't very effective.
03 Chelation Method:
Using metal-capturing agents with specific ions to form stable chelate precipitates, you can selectively remove target heavy metals, but the chemicals can be pretty pricey.
Physical Methods
01 Ion Exchange Method:
This method uses ion exchange resins to react with heavy metal ions in water, absorbing the metal ions onto the resin, which are then recovered using a regenerating solution. It's suitable for low-concentration heavy metal wastewater and allows for metal recovery, but the resin can get contaminated easily and needs to be replaced regularly.
02 Membrane Separation Method:
This includes reverse osmosis, nanofiltration, electrodialysis, etc. It uses the selective permeability of semi-permeable membranes to separate heavy metal ions from water molecules. It has high separation efficiency and can remove multiple pollutants at once, but the equipment is expensive, membranes can get clogged, and pretreatment is needed.
03 Adsorption Method:
Using adsorbents like activated carbon, zeolite, or biomass materials, heavy metal ions are removed through physical or chemical adsorption. The equipment is simple and easy to operate, but the adsorbent has limited capacity and is hard to regenerate.
Biological Methods
01 Biosorption:
Uses microorganisms (like bacteria, algae, fungi) or functional groups on plant cell walls to adsorb heavy metal ions, removing them through solid-liquid separation. It’s low-cost and eco-friendly, but a bit slow and can be sensitive to environmental conditions.
02 Phytoremediation:
Uses hyperaccumulator plants to absorb and concentrate heavy metals, which are then removed by harvesting the plants. Good for low-concentration polluted water, but takes a long time and usually needs to be combined with ecological restoration.
03 Bio-flocculation:
Uses microbial metabolites or flocculants to make heavy metal ions form flocs, removing them through sedimentation or filtration. It’s simple to operate and doesn’t cause secondary pollution, but the flocculation effect can be influenced by water quality.
