Adding Value to Food Waste Hydrothermal Liquefaction:\nUse of the Aqueous Phase for Compost and the Char for Adsorption of\nLead, Copper, and Nitrate

Composting and adsorption water treatment\napplications were evaluated\nfor their potential to add value to the aqueous phase and char products\nfrom hydrothermal liquefaction (HTL) of food waste. The aqueous phase\nfrom uncatalyzed HTL was successfully utilized in a hemp and cattle\nmanure compost system, evaluated over 60 days based on compost temperature,\nwater recovery, nutrient recovery, and fungal growth. Germination\nindex measurements showed that the HTL aqueous phase is compostable\nbut may require longer composting times to avoid inhibition of plant\ngrowth. Red mud, a byproduct of alumina production, was investigated\nas an inexpensive, homogeneous catalyst for increasing the HTL biocrude\noil yield and for modifying the adsorption properties of the HTL-char.\nRed mud HTL-char (RM-HTL-char) showed a higher adsorption capacity\nof 35.3 mg/g for Pb(II) and 4.0 mg/g for Cu(II) from water compared\nto commercial activated carbon. The adsorption capacity of RM-HTL-char\nfor nitrate (∼4 mg/g) was not as high as that for commercial\nactivated carbon but was substantially higher than that for char from\nuncatalyzed HTL, which usually has negligible adsorption for anions.\nThe Sips isotherm model best fit the Pb and nitrate adsorption data;\nthe Freundlich isotherm model best fit the Cu adsorption data. A pseudo-second-order\nkinetic model described the Pb and Cu adsorption onto RM-HTL-char,\nwhile a pseudo-first-order model described the nitrate adsorption.\nThese results suggest a value-added use for both the red mud and HTL-char\nand the potential for nutrient-relevant anion adsorption on HTL-char.