Sodium Percarbonate for Eco-Efficient Cyanide Detoxification in Gold Mining Tailings

Cyanide-containing effluents from hydrometallurgical gold extraction pose significant environmental risks due to their high toxicity. This study investigates the detoxification of cyanide-laden tailings from the Altyntau Kokshetau gold extraction facility (Kazakhstan) using sodium percarbonate in alkaline conditions. Employing response surface methodology (RSM) and central composite design (CCD), we optimized key parameters—pH (10–12), sodium percarbonate dosage (1.5–4.0 g), reaction time (10–40 min) and temperature (20–25 °C)—achieving 83.33% detoxification efficiency within 40 min and 99.99% after 8 h, reducing cyanide from 443.2 mg/L to 0.05 mg/L. The process follows biphasic pseudo-first-order kinetics ((k₁ = 0.0517) min^–1 initially, (k₂ = 0.01665) min^–1 subsequently), driven by HO^• radical-mediated oxidation of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>C</mi><mi>N</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></semantics></math></inline-formula> to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>C</mi><mi>N</mi><mi>O</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></semantics></math></inline-formula>, as described by <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>(</mo><msup><mrow><mi>C</mi><mi>N</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>+</mo><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub><msub><mrow><mi>O</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>→</mo><msup><mrow><mi>C</mi><mi>N</mi><mi>O</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>+</mo><mo> </mo><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub><mi>O</mi></mrow></semantics></math></inline-formula>). pH emerged as the dominant factor, optimizing radical stability and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>C</mi><mi>N</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></semantics></math></inline-formula> protonation (pK_a ≈ 9.21) at pH 10. Infrared spectroscopy confirmed the presence of cyanide complexes (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>[</mo><mi>A</mi><mi>u</mi><msub><mrow><mo>(</mo><mi>C</mi><mi>N</mi><mo>)</mo></mrow><mrow><mn>2</mn></mrow></msub><msup><mrow><mo>]</mo></mrow><mrow><mo>−</mo></mrow></msup></mrow></semantics></math></inline-formula>, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>[</mo><mi>F</mi><mi>e</mi><msub><mrow><mo>(</mo><mi>C</mi><mi>N</mi><mo>)</mo></mrow><mrow><mn>6</mn></mrow></msub><msup><mrow><mo>]</mo></mrow><mrow><mn>4</mn><mo>−</mo></mrow></msup></mrow></semantics></math></inline-formula>) in tailings, underscoring the need for effective treatment. The method ensures compliance with stringent environmental standards (e.g., ICMI limit of 0.2 mg/L), offering a scalable, eco-efficient solution for mitigating the environmental footprint of gold mining operations.