Viability of spent coffee ground biochar as a filtration media to remove organic contaminants from urban stormwater in the Pacific Northwest

Abstract

Rainfall on urban landscapes in densely populated cities can have severe impacts on the hydrologic cycle. Higher volumes of “urban stormwater” runoff can overwhelm stormwater management systems resulting in overflow events where runoff and/or raw sewage are directly discharged to receiving water bodies. Furthermore, urban stormwater contacts engineered and impervious surfaces (e.g., pavement, buildings, vehicles) during conveyance, transporting elevated concentrations of pollutants which threaten human and environmental health. Decentralized approaches known as best management practices (BMPs) are a potential solution to facilitate local groundwater recharge via stormwater infiltration, and to better manage large volumes of urban runoff to prevent overflow events; however, these systems are unable to adequately remove all contaminants present in urban runoff. This study analyzes biochar, a pyrolyzed biomass feedstock adsorbent, produced from spent coffee grounds (SCG) as a possible BMP amendment to treat stormwater contaminants during infiltration. Caffeine, atrazine, diuron, fipronil, and pentachlorophenol compounds were used as representative trace organics found in urban stormwater and exhibit a range of hydrophobicity. Organic compound adsorption capacities and rates on inactivated biochar (SCG 400) and potassium hydroxide (KOH) activated biochar (SCG KOH) were compared through surface characterization (e.g., fixed carbon content, porosity, and surface area) and batch sorption tests in a complex, simulated urban stormwater matrix. SCG KOH showed significant Brunauer-Emmett-Teller (BET) surface area increase over the SCG 400 precursor (852.11 m2/g versus 2.66 m2/g). Both SCG 400 and SCG KOH exhibited high fixed carbon contents (71.1% and 84.8% respectively) when compared to the SCG feedstock (17%). Batch tests indicated lower removal rates by SCG 400 when multi-pollutant contaminant mixtures or natural organic matter were introduced; yet, higher removal rates were observed in the simulated stormwater matrix. The higher rates are likely due to electron bridging effects. SCG KOH removed nearly all contaminants regardless of batch sorption conditions. Isotherm batch sorption tests for SCG KOH were inconclusive as the activated biochar removed even the highest concentrations (10,000 µg/L). Equilibrium adsorption was achieved within 1-12 hours in multi-pollutant systems with no trend of preferential removal for more hydrophobic compounds. Bench-scale column studies amended with 0 wt% (sand only), 0.5 wt%, and 3 wt% biochar showed long lifetimes and high removal capacities of SCG KOH with increased weight percent. For example, complete breakthrough for sand-only columns occurred within 13 pore volumes (1.2 L), while the 3 wt% columns maintained complete removal of all contaminants until receiving 150 pore volumes (16 L) of heavily contaminated stormwater. Complete breakthrough of the 3 wt% columns has not been observed after 1200 pore volumes (130 L). The results of our study indicate that SCG biochar activated with KOH has a high adsorption rate for a wide variety of trace organic contaminants over a long lifetime. Additionally, while this study is focused on applications of biochar amendments to address urban stormwater pollution in the Puget Sound (Pacific Northwest), our approach is applicable worldwide.