Urban legacy pollution: Novel techniques for phytoremediation of marginal brownfield sites

Urban soils often retain heavy metal contamination from historical industrial activity, leading to long-term degradation of soil and water quality. Conventional remediation approaches, including physical removal and chelate-assisted phytoextraction, are often expensive, disruptive, and limited in their capacity to restore ecological function. This research investigates the potential of co-planted systems to enhance both contaminant removal via phytoextraction and ecological recovery. Greenhouse and in-situ trials employing four hyperaccumulator species (Helianthus annus, Zea mays, Trifolium pratense, Lolium perenne) compare co-planted assemblages with monocultures to assess effects on plant growth, heavy metal uptake, and soil chemistry. Salix viminalis was also included as a companion plant due to previous evidence of woody species facilitating growth and heavy metal uptake. By examining differences in growth rates, plant tissue heavy metal concentration and root exudate composition between monocultures and co-planting treatments the project seeks to identify synergistic mechanisms that promote effective remediation while supporting ecosystem resilience.Early greenhouse trials have demonstrated that heavy metal uptake (figure 1), soil acidity and total organic carbon (TOC) of root exudates can be increased in co-planted treatments. For example, non-purgeable organic carbon (NPOC) increased in Helianthus annus root exudates from 2ppm to 7ppm when planted with Salix viminalis.   However, the differences are not universal; introducing Lolium perenne increased Ni uptake for plots with Zea mays (0.011mg/kg/day to 0.057mg/kg/day), whereas it inhibited Ni uptake for Helianthus annus (0.81mg/kg/day to 0.21mg/kg/day). As root exudates are often composed of organic and amino acids, they have potential to significantly affect both the soil microbiome and heavy metal mobility. Therefore, ongoing investigations will observe the plant physiological mechanisms affecting heavy metal mobility in soils, by identifying key metabolites in root exudate samples and their release in co-planted treatments versus monocultures.Ultimately, this work demonstrates how diverse planting strategies can improve heavy metal uptake of remediator species whilst increasing soil nutrient content and enhancing ecosystem multifunctionality. The findings thus far have indicated a complex network of plant-plant and plant-soil interactions with practical implications for scalable, low-cost land management practices in urban areas – For instance, pairing Zea mays with Lolium perenne and woody species in marginal industrial areas, to increase Ni uptake and soil carbon.  Figure 1 – Results from preliminary ICP-OES detection for Nickel concentration in leaf material of Zea maize when cultivated in monocultures, compared to co-planted with Helianthus annus or Lolium perenne. Units are mg/kg/day to show rate of nickel accumulation over time.