Encapsulating sorptive materials and biodegrading microorganisms in composite alginate bead geomedia to capture and remove stormwater trace organics and nutrients

Urban areas covered with impervious surfaces generate rapid and intense runoff during rain events and/or after snowmelts. Urban stormwater runoff is known to contain complex mixtures of both dissolved phase and particle-bound pollutants that degrade water quality of receiving waterbodies. The mixture of pollutants frequently includes various nutrients, metals, microplastics, and trace organic contaminants. Green stormwater infrastructure such as bioretention cells (also called raingardens) are increasingly being applied in urban areas to remove stormwater contaminants. Even though bioretention cells can effectively remove particle-bound pollutants, most hydrophilic compounds generally pass through conventional bioretention cells without treatment. Thus, there is a growing need to amend bioretention cells to enable the removal of hydrophilic stormwater pollutants. Bioretention amendment with different sorbent materials (e.g., black carbon materials, such as granular activated carbon [GAC], powdered activated carbon [PAC], biochar) can temporarily enable removal of different hydrophilic stormwater contaminants in bioretention cells. Even the amendment of sorptive material in bioretention cells, however, does not provide a complete solution because the cells would become ineffective over extended time periods when the sorption capacities would get consumed. As such, there is a need to develop an improved media amendment for bioretention cells that would not only enable contaminant sorption but also biodegrade the captured contaminants in situ to renew bioretention sorption capacities.In the first study, I developed and characterized a novel biologically active compound alginate bead geomedia (called “BioSorp Beads”) that could be used to bioaugment bioretention cells and enable contaminant capture during infiltration and biodegradation during antecedent dry periods. I thoroughly mixed powdered activated carbon (sorbent), iron water treatment residual (sorbent, increases bead density), white rot fungi (a representative biodegrading microorganism), wood flour (maintenance substrate for the encapsulated microbes), and AQDS (a model electron shuttle) in sodium alginate. The mixture was then added dropwise into a divalent/trivalent cationic crosslinker solution (CaCl2 or FeCl3) using a peristaltic pump to produce wet beads. Finally, the instantaneously formed wet beads were air-dried to produce the BioSorp Beads. I investigated the effects of different bead compositions on various physical properties of the beads, such as mechanical strength, swelling, surface area, pore volume, leaching, pH. Bead properties could be customized to cater for specific application needs. For example, BioSorp Beads could be produced using FeCl3 as crosslinkers instead of CaCl2 if the encapsulated microbes need a more acidic environment (the case for fungi). Additionally, the encapsulated white rot fungi remained viable in the dried beads at room temperature over an extended period (3 months) and could grow from the beads when nutrients were present, demonstrating the bioaugmentation potentials of the BioSorp Beads.