Hydrothermal carbonization of arecanut husk biomass: fuel properties and sorption of metals

In this study, hydrothermal carbonization of arecanut husk and its potential application as an alternate solid fuel and heavy metal sorption (adsorbent) was attempted. Arecanut husk (AH) treated with citric acid for 9 h resulted in the hydrochar yield of 58.7% with increased fixed carbon from 17 to 39.7% and HHV from 16.98 to 21.14 MJ kg −1 compared to the parent biomass. The arecanut husk hydrochar (AHH) had H/C of 1.27, O/C ratio of 0.59 with an energy yield of 72.81%, and energy densification ratio of 1.24, and the values have the similar fuel characteristics of lignite. The pH, zeta potential, and surface of the hydrochar were 5.56, − 22.60 mV, and 0.98 m 2 g −1 respectively. Hydrochar with a dosage of 0.1% showed a maximum rate of adsorption for Pb 2+ (79.86 mg g −1 ) at the initial concentration of 100 mg L −1 in the aqueous solution compared to Zn 2+ , Cr 6+ , and Ni 2+ . SEM with EDAX and FT-IR spectroscopy results confirmed the presence of Pb 2+ and changes in functional groups in arecanut husk hydrochar after adsorption of heavy metals in the aqueous solution.