1. Đặc điểm nước thải axit/kiềm trong ngành mạ điện

Nước thải từ ngành mạ điện có thể có tính axit mạnh (pH < 3) hoặc kiềm mạnh (pH > 10) do sử dụng các hóa chất như:

• H₂SO₄, HCl, HNO₃, HF (trong quá trình tẩy rửa, đánh bóng kim loại).

• NaOH, KOH (trong quá trình làm sạch, xử lý bề mặt kim loại).

Ngoài ra, nước thải còn chứa kim loại nặng (Cr⁶⁺, Ni²⁺, Zn²⁺, Cu²⁺) và một số hóa chất độc hại như cyanide (CN⁻), cần được xử lý trước khi thải ra môi trường.

2. Phương pháp xử lý nước thải axit/kiềm

(1) Trung hòa pH

• Nếu nước thải có tính axit (pH < 6):

  • Dùng NaOH, Ca(OH)₂, Na₂CO₃ để nâng pH lên mức trung tính (6.5 - 8.5).
  • Phản ứng điển hình: $${\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;">S</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;">4</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;">a</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;">\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;">a</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;">O</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">4</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </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;">2</span></span>}}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}$$$$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">l</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">a</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">a</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">l</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}$$

• Nếu nước thải có tính kiềm (pH > 9):

  • Dùng H₂SO₄, HCl, CO₂ để hạ pH về trung tính.
  • Phản ứng điển hình: $$\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;">a</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;">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;">S</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;">4</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;">a</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;">S</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;">4</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) Xử lý kim loại nặng

Sau khi trung hòa pH, kim loại nặng cần được kết tủa để loại bỏ.

• Dùng NaOH, Ca(OH)₂ để kết tủa hydroxide kim loại:

  • $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">r</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">3</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">3</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">r</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">(</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">)</span></span>}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">3</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$
  • $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">i</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">N</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">i</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">(</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">)</span></span>}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$
  • $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">Z</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">n</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}}^{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">Z</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">n</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">(</span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">O</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">H</span></span>}{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">)</span></span>}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$

• Dùng Na₂S để kết tủa dưới dạng sunfua (tốt hơn hydroxide):

  • $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">P</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">b</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">P</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">b</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$
  • $$\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">u</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">+</span></span>{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}}^{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">2</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">-</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\to </span></span>\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">C</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">u</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">S</span></span>}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Arial;">\downarrow </span></span>$$

(3) Keo tụ - Tạo bông

• Sử dụng PAC, phèn nhôm (Al₂(SO₄)₃), polymer để kết dính các hạt cặn nhỏ thành bông lớn dễ lắng.

(4) Lắng và tách bùn

• Bể lắng giúp loại bỏ bùn chứa kim loại nặng.

• Bùn thải nguy hại cần xử lý theo quy định môi trường.

(5) Lọc & Hấp phụ

• Lọc qua than hoạt tính hoặc cột lọc áp lực để loại bỏ các hóa chất còn sót lại.

(6) Khử trùng và xả thải

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

3. Kết luận

• Trung hòa pH là bước quan trọng đầu tiên trong xử lý nước thải axit/kiềm ngành mạ điện.

• Kết tủa kim loại nặng bằng NaOH, Ca(OH)₂ hoặc Na₂S giúp loại bỏ độc tố.

• Keo tụ - lắng - lọc giúp nước thải đạt chuẩn xả thải.

• Bùn thải chứa kim loại nặng cần được thu gom và xử lý theo quy định.

 

Việc áp dụng đúng quy trình sẽ giúp doanh nghiệp đáp ứng tiêu chuẩn môi trường, giảm ô nhiễm và tối ưu chi phí xử lý.