The sorption of dissolved heavy metals from stormwater: the effect of biofilm, gravel, and two alternative substrates
Thesis DisciplineWater Resource Management
Degree GrantorUniversity of Canterbury
Degree NameMaster of Water Resource Management
Stormwater acts as a major transport pathway for urban contaminants, with heavy metal contaminants of concern primarily being zinc, copper, and lead. In an effort to reduce the environmental impacts of stormwater, a range of stormwater treatment systems have been developed, collectively called Low Impact Developments (LIDs). Environmentally they aid in reducing contaminant loads entering surfacewater and groundwater systems while enhancing the aesthetic values of urban areas. Most current stormwater treatment systems incorporate both filtration and infiltration processes in contaminant removal restricting their remedial properties to particulate contaminants. This can be limiting if much of the contaminants are in dissolved form.
Research into alternative stormwater treatment systems substrate materials (e.g. recycled glass and mussel shell) for stormwater treatment is necessary to provide sustainable and environmentally friendly options for the potential treatment of dissolved heavy metals. In addition to alternative materials, the effect of biofilm on the adsorption of dissolved heavy metals needs to be studied. Biofilms are assemblages of microbial cells connected with extracellular polymeric substances (EPS) that attach themselves to moist abiotic surfaces. Biofilms are known to sorb heavy metals from aqueous environments, in particular, dissolved heavy metals which have the potential to escape treatment in stormwater treatment systems. Thus, there is a need to understand the role of alternative materials and biofilm in the sorption of dissolved heavy metals from stormwater, in an effort to improve the efficiency of stormwater treatment systems.
This thesis investigated the use of recycled glass, mussel shell, gravel, and biofilm for the treatment of dissolved heavy metals in a stormwater. Freshwater biofilm utilised throughout this research was grown using water collected from Okeover and Haytons Streams, as well as, seed biofilm from Haytons Stream and activated sludge from the Bromley Sewage Ponds in Christchurch, New Zealand. A synthetic stormwater, containing typical concentrations of heavy metal contaminants, was used for all experiments.
The biofilm demonstrated an ability to adsorb dissolved zinc (Zn), copper (Cu), and lead (Pb) to high concentrations from the stormwater. However, the biofilm did not have a significant impact on heavy metal removal relative to the substrate on its own. In fact, the results would suggest that the presence of biofilm inhibits the removal of certain dissolved heavy metals in either or both the recycled glass and mussel shell substrates.
The gravel, recycled glass, and mussel shell substrates demonstrated stormwater treatment potential by effectively reducing the concentration of dissolved heavy metals from the synthetic stormwater. Gravel showed a removal efficiency (%) of Pb (99%) ≥ Zn (99%) > Cu (96%) and mussel shell one of Pb (97%) ≥ Zn (97%) > Cu (89%). Meanwhile, recycled glass achieved an order of removal efficiency of Pb (96%) > Zn (86%) > Cu (73%). Dissolved heavy metal removal mechanisms were investigated using scanning electron microscope and energy-dispersive X-ray spectroscopy analyses and it is likely that both the composition and surface texture of the substrate influence the removal of dissolved heavy metal from aqueous solutions.
The results of this thesis demonstrated benefits in the use of waste products for the protection of the environment. Using recycled glass and mussel shells for stormwater treatment contributes to waste minimisation and increased value associated with the extended lifecycle of these products.