Adsorptive removal of micropollutants from wastewater with floating-fixed-bed gasification char

Abstract

The global detection of organic micropollutants (OMPs) in waterbodies and the search for low-cost alternatives to fossil activated carbon have fostered research on the application of modified biochars in complex wastewater treatment plant (WWTP) effluents. Gasification char – a promising prospect for adsorptive OMP removal – was obtained as by-product in a novel, multi-staged gasification process. The gasification system was designed for medium-scale energy production (530 kW combined heat and electrical power output) from spruce woodchips and relies on a floating-fixed-bed gasifier with a preceding pyrolysis step. The resulting floating-fixed-bed gasification char (FFBC) ‘as-is’ – i.e. without further modification – and a commercial powdered activated carbon (PAC) were used in adsorption, desorption and kinetic assays with pharmaceuticals, X-ray contrast media, and industrial chemicals, in order to test the suitability of FFBC as OMP adsorbent in different water matrices (spiked water and WWTP effluent). OMP levels were quantified via HPLC or HPLC-MS/MS and Langmuir and Freundlich isotherm fitting was performed. The characterization of FFBC showed a high C-content (91.43%), low ash-content, porous texture (ABET = 308.15 m2 g−1), and aromatic, heterogeneous structure (confirmed via FTIR) with basic character (pH = 9.50). For both tested adsorbents, high removal rates >90% of OMPs with high logD were found at an equivalent dosing of 60 mg l−1 FFBC ≈ 10 mg l−1 PAC, whereas X-ray contrast agents exhibited lower affinity. The governing removal mechanisms are hydrophobic and π-interactions, pore-size effects and – to a limited extent – hydrogen bond formation. Dissolved organic matter in WWTP effluent inhibits OMP adsorption, as observed in decreasing isotherm fitting parameters. The investigated bio-adsorbent – derived from renewable energy, regional supplies, and a process lean in the use of chemicals – could be an inexpensive alternative in adsorptive wastewater treatment.