Green and sustainable zero-waste conversion of water hyacinth ( Eichhornia crassipes ) into superior magnetic carbon composite adsorbents and supercapacitor electrodes.

Troublesome aquatic weed, water hyacinth ( Eichhornia crassipes ) was converted into solid and liquid fractions via green and energy-saving hydrothermal carbonization (HTC). The solid product, hydrochar, was employed as a precursor to prepare magnetic carbon materials by simple activation and magnetization using KOH and Fe 3+ ions, respectively. The obtained magnetic adsorbent possessed good magnetic properties and presented outstanding capacities to adsorb methylene blue (524.20 mg g -1 ), methyl orange (425.15 mg g -1 ) and tetracycline (294.24 mg g -1 ) with rapid adsorption kinetics even at high concentrations (up to 500 mg L -1 ), attributed to high specific surface area and mesopore porosity. Besides the solid hydrochar, the water-soluble liquid product was used to fabricate carbon-based supercapacitors through facile KOH activation with a considerably lower KOH amount in comparison to conventional activation. The supercapacitor electrode made from activated liquid product possessed an extremely high specific surface area of 2545 cm 2 g -1 and showed excellent specific capacitance (100 F g -1 or 50 F cm -3 at 1 A g -1 ) and good retention of capacitance (92% even after 10 000 cycles). This work demonstrated that both solid and liquid HTC fractions from this bio-waste can serve as effective sources to prepare functional carbon materials, making this approach a sustainable zero-waste biomass conversion process.