Wood mulch coated with iron-based water treatment residuals for the abatement of metals and phosphorus in simulated stormwater runoff

Laboratory-scale batch and flow-through studies evaluated the performance of iron-based water treatment residuals (Fe-WTR) coated wood mulch for removing two types of urban stormwater runoff pollutants i.e., toxic heavy metals (Cu, Zn, and Pb) and the nutrient, phosphorus (P). Fe-WTR is an industrial waste byproduct generated from the treatment of drinking water, primarily comprising amorphous iron (hydr)oxides. The Fe-WTR particles had a large surface area and were non-hazardous, as indicated by toxicity characteristic leaching procedure (TCLP) and synthetic precipitation leaching procedure (SPLP) tests. Batch studies of WTR-coated mulch loaded with simulated urban stormwater fixed pollutant concentrations showed fast (<5 min), and nearly complete removal of Cu, Zn, and Pb, contrasting with lower and incomplete removal of P. Temperature and pH affected the removal process only for Zn, with increasing pH improving the removal. Flow-through column studies with 5.1 and 10.2-cm filtration bed depths were performed using simulated stormwater. Coated mulch performed better than uncoated mulch for all pollutants and for both bed depths. In the 10.2 cm bed, 21% to 25% higher reductions were observed for metals and 8% higher reduction for P using coated mulch relative to uncoated mulch. The increases were statistically significant in all cases, with p < 0.001 for the 10.2 cm bed. This study demonstrates that Fe-WTR coated wood mulch can serve as an innovative adsorbent media for urban stormwater treatment. Moreover, this application finds a beneficial way of reusing Fe-WTR as a value-added product, saving the cost of its disposal in landfills.