Sustainable treatment of naturally occurring heavy metals in sicilian water via hydrothermal carbonization, secondary biofuel extraction, and activation of Opuntia ficus indica

Ensuring access to clean energy and water to all global citizens is a crucial component of a sustainable, equitable and just society. This work presents an integrated biorefinery concept for water treatment in Sicily, Italy, that transforms prickly pear cactus (Opuntia ficus indica), an abundant and invasive local biomass, into a solid hydrochar and CO2 via hydrothermal carbonization (HTC). The hydrochar is extracted to remove biofuel precursors, then pyrolyzed to recover bio-oils, and finally CO2-activated to create a carbonaceous adsorbent. As HTC temperature increases, the extracted secondary char contains increasing amounts of long-chain fatty acids due to hydrolysis of the Opuntia lipid fraction. Activated hydrochars removed a series of heavy metals present in Sicilian groundwater via chemical adsorption owing to the carboxyl and hydroxyl functional groups present on their surface. Raw CO2-activated prickly pear adsorbents showed a combined metal capacity of 636 mgmetal/gchar, versus 669 mgmetal/gchar for HTC-based adsorbents. While HTC as a pretreatment is not necessary to produce an adsorbent, the hydrochars yield approximately 50 % more of a pyrolysis bio-oil with fewer furfurals than pyrolysis of the raw feedstock, owing to a reduction in sugar content of the prickly pear during HTC. This work demonstrates a series of modular biorefinery processes that could be paired to produce biofuels and a biosorbent from a marginal biomass. Findings suggest that lower-temperature HTC as a pretreatment may result in a partially deoxygenated pyrolysis oil and a more dense, higher adsorbent capacity biosorbent. When HTC is performed at 180 ◦C followed by pyrolysis and activation at 800 ◦C, the approximate product yields are 33 wt% hydrothermal liquids, 22 wt% secondary char, 10 wt% and 18 wt% pyrolysis liquids and gases (respectively) and 15 wt% activated carbon, all on a dry prickly pear basis.