Assessing the Ability of Green Adsorbents to Improve Water Quality and Green Stormwater Infrastructure (GSI) Performance
Stormwater runoff from urban areas is primary cause of flooding and erosion of urban streams, as well as a major source of pollution of receiving water bodies. Green stormwater infrastructure (GSI) is increasingly being employed as a stormwater management tool in urban areas, with the intent of using infiltration to address both water quantity and quality concerns. GSI helps maintain or restore the natural hydrologic balance through infiltration-bases approaches. However, GSI media has limited removal capacity for dissolved contaminants; hence, amendments might be required to increase pollutant removal. Adsorption could be an effective method for removing dissolved organic and inorganic pollutants thus minimizing their mobility into the adjacent ecosystems, and limiting their negative effects on GSI plants.
The potential of five low-cost and globally available materials including three raw waste products – waste tire crumb rubber (WTCR), coconut coir fiber (CCF) and blast furnace slag (BFS) – and two modified materials – biochar (BC) and iron coated biochar (FeBC) – as adsorbents for removing several classes of pollutants – dissolved metals, nutrients, and polycyclic aromatic hydrocarbons – from stormwater were investigated in this study. Adsorption capacities, kinetics and environmental parameters effects (pH, ionic strength and dissolved organic carbon) were investigated through batch tests using simulated stormwater spiked with a mixture of pollutants including; metals (Cr, Cd, Cu, Pb, Ni and Zn), nutrients (NH4+, NO3- and PO43-) and four PAHs: pyrene (PYR), phenanthrene (PHE), acenaphthylene (ACY) and naphthalene (NAP). The physicochemical characteristics of the adsorbents were assessed by BET-N2 surface area, CHN elemental analysis, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) equipped with energy dispersive x-ray (EDS). Column study are undertaken to investigate sorbent performance under more realistic dynamic conditions and to assess pollutant transport and breakthrough.
Batch test results revealed that adsorbents with higher organic contents (e.g., BC, FeBC and WTCR) were more efficient for organic contaminants (e.g., PAHs) sorption, while adsorbents with higher polarity surface (e.g., CCF and BFS) were more efficient for inorganic contaminants such as metals. All selected sorbents (except BFS) could remove all four PAHs very well (80-100%). BFS and CCF had very high removal efficiencies (90-100%) for all 6 metals, while WTCR and BC had > 90% removal for three metals; Cr, Pb and Cu. For nutrients, only phosphate could be removed well by BFS and FeBC (60-80 %). In terms of environmental parameters, pH and ionic strength of solution did not have significant effect on the adsorption of PAHs by all sorbents, while pH substantially affected metals sorption capacities as sorption increased by increasing pH. In contrast, dissolved organic matter negatively affected both organic (e.g., PAHs) and inorganic (e.g., metals) pollutant sorption. In column experiments all PAHs and metals were 100% removed by all media over the tested period. Nutrients behaved differently; ammonium was not removed by all media, but phosphate and nitrate were removed by some of media. This research will improve our understanding of GSI amendments that can improve water quality while simultaneously enhancing GSI hydrologic function.