Performance of Filtralite as a filter medium for nickel removal in urban runoff: effects of granulometry

Urban runoff frequently carries elevated concentrations of heavy metals such as nickel (Ni), posing significant environmental and public-health risks. Sustainable Urban Drainage Systems (SUDS) offer a promising pathway to mitigate these impacts, particularly through the use of filter media that enhance water decontamination. This study evaluates Filtralite, a lightweight expanded clay aggregate, as a filtration medium for Ni removal, with special emphasis on the evolution of pH under prolonged operational conditions and on the influence of particle size on the material’s treatment capacity.

The research was based on an 80-day experiment designed to simulate an accelerated weathering process similar to what occurs under real operating conditions when SUDS interact with rainfall. Four granulometric fractions (2 mm, 1 mm, 0.5 mm, and 0.25 mm) were tested under controlled, repeated washing cycles carried out statically: the Filtralite was kept submerged in beakers, and its water was replaced on an approximately daily basis throughout the 80-day period. The pH values of the effluent were systematically recorded and interpreted as a proxy for the material’s alkalinity-generating capacity—an essential driver of Ni removal from the contaminated solution.

Results demonstrate a consistent granulometry-dependent pattern in pH evolution. Coarser fractions (2 and 1 mm) experienced a more rapid decline in alkalinity than finer ones: although initial effluent pH values exceeded 10, they dropped below the threshold required for efficient Ni precipitation (≈8.5–9) after only a few litres of cumulative washing. The 2 mm fraction dropped to pH 8–8.5 after approximately 8–10 L of equivalent runoff, suggesting a short effective lifespan in real SUDS applications. The 1 mm fraction exhibited a slower decline, maintaining pH > 9 for a longer period, but ultimately converging toward circumneutral values at extended washing volumes. In contrast, finer fractions (0.5 and 0.25 mm) preserved alkaline conditions throughout most of the experiment. The 0.5 mm material sustained pH values in the range 9–10 for the majority of the test, indicating a more stable and gradual release of alkaline species. The finest fraction (0.25 mm) provided the most robust performance: effluent pH consistently remained between 9.5 and 10 even under high cumulative washing volumes, reflecting the strong buffering capacity associated with its larger specific surface area.

Overall, the findings confirm that Filtralite is an effective and sustainable medium for Ni removal in SUDS, although its long-term performance is highly sensitive to granulometry. Fine fractions provide a prolonged alkaline environment that enhances precipitation-driven removal. These results suggest that finer Filtralite may offer favourable characteristics for potential field applications, supporting more stable and efficient metal removal over extended periods. However, the reduced particle size also implies lower hydraulic conductivity compared to coarser fractions, which could limit infiltration performance in practical implementations. Validation under real operating conditions is therefore still required.