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

Abstract

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