1. Đặc điểm nước thải xi mạ

Nước thải từ ngành xi mạ chứa nhiều kim loại nặng độc hại như Cr⁶⁺, Ni²⁺, Zn²⁺, Cu²⁺, Pb²⁺, cùng với axit, kiềm, cyanide và chất hữu cơ. Những chất này có thể gây ô nhiễm nghiêm trọng nếu không được xử lý đúng cách.

Các đặc trưng chính của nước thải xi mạ:

• pH dao động lớn: Có thể rất axit (pH < 3) hoặc rất kiềm (pH > 10).

• Nồng độ kim loại nặng cao: Độc hại và khó phân hủy sinh học.

• Có thể chứa cyanide (CN⁻): Cực kỳ độc, cần xử lý đặc biệt.

• Có dầu mỡ, chất hữu cơ từ quá trình rửa chi tiết kim loại.

2. Quy trình xử lý nước thải xi mạ bằng phương pháp hóa lý

Phương pháp hóa lý là lựa chọn tối ưu cho xử lý nước thải xi mạ vì nó giúp loại bỏ kim loại nặng và các hóa chất độc hại hiệu quả.

(1) Xử lý riêng cyanide (nếu có)

• Dùng NaOCl (Natri hypoclorit) hoặc Cl₂ (Khí Clo) để oxy hóa cyanide thành CO₂, N₂ và H₂O.

• Phản ứng: $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">l</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">l</span></span>}$$$$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">l</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">l</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}$$

(2) Trung hòa pH

• Điều chỉnh pH về mức 8 - 9 bằng NaOH, Ca(OH)₂ hoặc H₂SO₄ để thuận lợi cho quá trình kết tủa kim loại.

(3) Kết tủa kim loại nặng

• Thêm NaOH hoặc Ca(OH)₂ để kết tủa kim loại dưới dạng hydroxide kim loại: $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">Z</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">n</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">Z</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">n</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">(</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">)</span></span>}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span><span\; style="font-family:\; Arial;">N</span></span>$$$${\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">i</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">i</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">(</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">)</span></span>}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$Ni²⁺ + 2OH⁻ → Ni(OH)₂↓

  Ni2++2OH−→Ni(OH)2​↓

• Thêm Na₂S hoặc FeSO₄ để kết tủa kim loại dưới dạng sunfua (hiệu quả hơn hydroxide): $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">P</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">b</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">P</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">b</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$$$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">u</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">u</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$

(4) Keo tụ - Tạo bông

• Dùng PAC, phèn nhôm (Al₂(SO₄)₃), polymer để liên kết cặn nhỏ thành bông lớn dễ lắng.

(5) Lắng và tách bùn

• Bể lắng giúp tách bùn chứa kim loại nặng ra khỏi nước.

• Bùn thải được thu gom và xử lý theo quy định về chất thải nguy hại.

(6) Lọc & hấp phụ

• Dùng than hoạt tính hoặc cát lọc áp lực để loại bỏ các chất ô nhiễm còn sót lại.

(7) Khử trùng và xả thải

• Sử dụng Clo, Ozone hoặc UV để tiêu diệt vi khuẩn trước khi xả thải ra môi trường.