Hydrothermal carbonization of fruit peels of varied origin forms hydrochar of distinct capability for adsorption of methylene blue

Fruit peel is an important category of food waste with high moisture content, and hydrothermal carbonization (HTC) is an efficient route to produce hydrochar (HC) for versatile applications. Origin of fruit peel might significantly impact nature of resulting HC, which was investigated herein by HTC of some typical fruit peels (dragon fruit, kiwi, lychee, mangosteen, pomegranate and watermelon) at 200 degrees C for 10 h. The results showed that fragmentation reactions determine the yields of HCs. Abundant inorganics in peels of dragon fruit and watermelon interfered condensation reactions but enhanced fragmentation to form more it-conjugated organics in bio-oil but less HC (ca. 35% versus others from ca. 50% to 65%). All the HCs had contents C of 60-65 %, H of 4-6 % and O of ca. 27 %, which was equivalent to a higher heating value of 24-27 MJ/Kg. Although dehydrogenation and deoxygenation were limited during the HTC, the inorganics could be effectively leached. All the HCs became hydrophobic and exhibited flaky-layered and porous surface from degradation of cell wells, enhancing capability for adsorption of methyl blue (MB). Less aliphatic structures in HCs from peels of pomegranate, mangosteen and lychee were more effective for the chemisorption of MB via it-it interaction by following pseudo second order adsorption. The adsorption isotherms of MB followed Langmuir model with monolayer coverage on surface of all the HCs.