Anionic Effects on Flocculation and Consolidation of Sediments Contaminated by Heavy Metals

The remediation of heavy metal-contaminated sediments is a significant environmental challenge. While cation effects are well studied, the influence of common co-existing anions on treatment efficiency remains poorly quantified. This study systematically investigates the effects of nitrate (NO₃⁻), chloride (Cl⁻), and sulfate (SO₄²⁻) ions on the flocculation and consolidation of copper (Cu)- and zinc (Zn)-contaminated sediments through settling column tests, turbidity measurements, and oedometer consolidation tests. Results demonstrated that NO₃⁻ achieved the highest flocculation efficiency, with a final settling height of 3.52 cm and a supernatant turbidity of 4.6 NTU, and the best consolidation performance, with a coefficient of 1.27 × 10⁻³ cm²/s. In contrast, SO₄²⁻ yielded the poorest outcomes. The superior performance of NO₃⁻ is attributed to its low charge density, which promotes the formation of denser flocs. These findings underscore that anion selection is a critical factor for optimizing sediment dewatering processes, suggesting that strategies favoring nitrate conditions can enhance the efficiency of techniques like pressure filtration and vacuum pre-compression.