Dissolved metal adsorption capacities and fractionation in filter materials for use in stormwater bioretention facilities.

The dissolved metal adsorption and association was determined for ten different filter materials recommended and/or implemented in bioretention facilities. Batch adsorption and batch kinetic experiments were performed at lab-scale using both single and multi-metal solutions. Metal strengths and association were determined by sequential extraction analysis. All materials adsorbed metals and 90% of adsorption occurred within 1 h. However, as metal solutions became more complex, adsorption behavior changed. Generally, filter materials classified as sand with a naturally high pH, relatively low organic matter (OM) content and large specific surface area seem to be good choices for removing dissolved metals. Additionally, a chalk additive might improve metal adsorption whereas biochar did not significantly improve metal retention and may be an unwanted (due to degradation over time) extra source of OM. Regardless of filter material, metals primarily adsorbed to the exchangeable form which indicates that metal adsorption might not be permanent, but rather substantially reversible in some cases. More research is needed to assess whether dissolved metals adsorbed in filter materials of bioretention systems pose a delayed threat instead of an immediate threat. Finally, the authors strongly recommend filter materials intended for stormwater bioretention facilities to be tested prior to implementation.

Do salt and low temperature impair metal treatment in stormwater bioretention cells with or without a submerged zone?

Although seasonal temperature changes and (road) salt in winter and/or coastal stormwater runoff might interfere with the metal treatment performance of stormwater bioretention cells, no previous study has evaluated the effect of these factors and their interactions under controlled conditions. In this 18week long study 24 well established pilot-scale bioretention columns were employed to evaluate the individual and combined effect(s) of low/high temperature, salt and presence of a submerged zone with an embedded carbon source on metal removal using a three factor, two-level full factorial experimental design. In most instances, the three factors significantly influenced the metal outflow concentrations and thus the treatment performance; the effect of temperature depended on the metal in question, salt had an overall negative effect and the submerged zone with carbon source had an overall positive effect. Despite these statistically significant effects, the discharge water quality was generally markedly improved. However, leaching of dissolved Cu and Pb did occur, mainly from bioretention cells dosed with salt-containing stormwater. The highest concentrations of metals were captured in the top layer of the filter material and were not significantly affected by the three factors studied. Overall, the results confirmed that bioretention provides a functioning stormwater treatment option in areas experiencing winter conditions (road salt, low temperatures) or coastal regions (salt-laden stormwater). However, validation of these results in the field is recommended, especially focusing on dissolved metal removal, which may be critically affected under certain conditions.

The influence of temperature and salt on metal and sediment removal in stormwater biofilters.

Stormwater biofilters are used to treat stormwater runoff. In countries with cold winter climates, biofilters are subject to low temperatures which, in some cases, are combined with potentially high salt concentrations from road de-icing, potentially affecting the biofilter's performance. Since stormwater biofilters have been developed without consideration of their critical winter use, a laboratory study was carried out to evaluate the performance of stormwater biofilters subjected to low and high temperatures, with and without salt. Both factors and their interaction had a significant effect on outflow concentrations and removal percentages. Salt had a negative impact on outflow concentrations, causing lower removal percentages for (especially dissolved) metals, this impact being most pronounced for Cu and Pb. The unrealistic combination of salt with high temperature seemed to further amplify the negative impacts of salt despite the fact that temperature alone did not cause significant differences in outflow concentrations and removal percentages. Still, biofilters showed the ability to treat stormwater efficiently under the simulated winter conditions; outflow concentrations for total metals as a minimum met the class 4 threshold value defined in the Swedish freshwater quality guidelines, while inflow concentrations clearly exceeded the threshold value for class 5. The relatively coarse filter material (which is recommended to facilitate infiltration during winter) did not seem to exacerbate biofilter performance.