Hydrothermal and Thermochemical Processing for Resource Recovery in Wetland Engineering: Synthesis and Characterization of Willow-Based Chars, Activated Carbons, and Platform Chemicals

TY - BOOK

T1 - Hydrothermal and Thermochemical Processing for Resource Recovery in Wetland Engineering

T2 - Synthesis and Characterization of Willow-Based Chars, Activated Carbons, and Platform Chemicals

AU - Acosta, Andrés

PY - 2024/7

Y1 - 2024/7

N2 - The immediate challenges we face in treating wastewater, recovering resources from waste streams,capturing CO2 from the atmosphere, and the transition from fossil-based chemicals to biomass-derivedbiorefineries are topics of rising global interest. Engineered wetland systems (EWS), at the forefront of theexpanding field of nature-based solutions (NBS), offer a sustainable approach to wastewater treatment andbiomass production. Notably, willows grown in EWS, particularly those in zero-discharge systems, exhibit rapidbiomass growth (17–31 t DM⋅ha−1⋅a−1) and superior nutrient absorption compared to those in bioenergy-focusedsystems like short-rotation willow coppices (SRWC). Our study explores the potential of EWS-willow biomassas a novel feedstock to produce novel materials such as hydrochars, pyrochars, and activated carbons. Weinvestigated the as-yet unexplored potential of EWS-willows for resource recovery and carbon sequestration usingslow pyrolysis (600 °C), hydrothermal carbonization (HTC at varied reaction severities), as well as chemical(KOH) and physical (steam) activation processes.In our first manuscript, we explored the innovative densification of phosphorus and carbon in hydrocharsby leveraging the precursory chemical composition of willows using an accelerated HTC approach, free ofcatalysts, involving fast heating rates, moderate water loading to the reactor, sudden cooling times, and shortreaction times. We experimented with custom-engineered reactors immersed within a preheated fluidized sandbath and a two-factor experimental design that varied the HTC temperatures (225, 250, 275, and 300 °C) andreaction times (10, 20, 30, and 60 min), maintaining a 1:10 dry biomass-to-water mass ratio and rapid heating rateat ~59 °C⋅min–1. Hydrochars showed improved fuel properties and energy content than the EWS-woodchips—Higher heating value (HHV) of 22–28 MJ⋅kg–1, aligning closely with mineral coals. These hydrochars alsoshowcased enhanced thermal stability, heightened aromaticity, and increased fixed carbon content (14 to 45 %).We found out that periodically fertilized natural willows—not previously exposed to wastewater—could achievenear-complete P-retention (~100 %) with our accelerated HTC approach after accumulating sufficient multivalentelements—namely Ca and Mg. Interestingly, we observed that substantial P-recovery might occur relativelyquickly, and compared to previous literature using HTC reaction times between 1 to 13-hours, the reaction timewith our approach could be reduced to 30-minutes at 275 and 300 °C to obtain full phosphorus densification.However, we also documented a lower P-retention of 12–63 % in EWS-hydrochars across temperatures of 225–300 °C with the same approach and rapid heating rates.In our second manuscript, we compared EWS-based woodchips, pyrochars, and hydrochars. Here, weoptimized our HTC method to overcome the previously limited retention of phosphorus in EWS-hydrochars dueto a lower multivalent element content in the EWS-willows compared to their natural counterparts. We conductedHTC at 250 °C without catalysts, applying a slow heating rate of ~3.75 °C⋅min−1 and incorporating process waterrecirculation through the EWS-hydrochars, resulting in a near-complete P-retention. Specifically, we usedsequential P-extractions with a modified Hedley's method and observed a high P-bioavailability in the willowwoodchipsand a significant P-retention in EWS-chars—up to 92 % in pyrochars and 100 % retention inhydrochars, along with a higher labile-P fraction of 21 % in hydrochars than 5 % in pyrochars. Our findings gobeyond the limited previous studies on HTC in willows and represent a fundamental advance in improving the Pretentioncapabilities of EWS-hydrochars. Utilizing X-ray-based techniques, Raman spectroscopy, scanningelectron microscopy, and gas physisorption, we characterized the EWS-chars' structures. We revealed innovative 3D-visualization, which transcends previous literature by providing insights into the chars' internal porosity, waterretention potential and quantifying—for the first time—their carbonaceous structural thickness via a meshingalgorithm and the calculation of their porosity and mean Feret diameter. EWS-pyrochars exhibited remarkablearomaticity with a higher concentration of overall sp2 C-atoms at 63 % vs. 43 % in hydrocars. Moreover, unlikehydrochar, which depicts occluded porosity, EWS-pyrochars contain 92 % water storage-like pores. Althoughhydrochars indicated lower carbonization and thermal stability than pyrochars, their higher carbon retention (54vs. 41 % in pyrochar) suggest superior annual benefits—on a 10 ha EWS scale—of 79-tons of carbonsequestration and 334 kg of phosphorus recovery versus 60-tons of carbon and 298 kg of phosphorus withpyrochars.Our third manuscript delves into the continuous catalytic hydrothermal synthesis (HTS) of 5-hydroxymethylfurfural (HMF) from fructose. We report HMF yields up to 56.5 mol% with a selectivity of 63 %,marking an opportunity to further research the HTS process for platform chemical production. A pivotal part ofour research is the innovative production of EWS-activated carbons, which has a novel application for the selectiveadsorption of HMF from the HTS product mixtures. Here, we demonstrate that activated carbons derived fromEWS-hydrochars exhibit superior HMF adsorption capacities compared to commercial activated carbon, withselective adsorption of HMF even in the presence of fructose and levulinic acid. Our findings reveal that physicallyEWS-activated carbons achieved a remarkable HMF capture of 0.33 g HMF⋅g–1 AC, significantly higher than0.22 g HMF⋅g–1 AC of commercial activated carbon Norit® GCN-830. We used as precursors the EWS-pyrocharsand hydrochars obtained from our second manuscript and experimented with chemical (KOH ratio 1:1 at 650 °C)and physical (using steam at 800 and 900 °C) activation processes with a heating rate ~10 °C⋅min−1. Employingstate-of-the-art analytical methods—including high-resolution X-ray photoelectron spectroscopy, X-raydiffraction, Raman spectroscopy, X-ray Fluorescence, high-resolution transmission electron microscopy (HRTEM),scanning electron microscopy, derivative thermogravimetry, vacuum FT-IR spectrometry, and gasphysisorption with N2 and CO2—we meticulously characterize these innovative materials to explore the futuredevelopment of innovative and sustainable adsorbents for the chemical industry. Preliminary findings highlightthe unique crystallinity and structural qualities of EWS-derived activated carbons, including the presence ofcrystalline phases CaCO3 and Ca3(PO4)2. Despite a lower BET surface area of 531–882 m2⋅g–1 compared toNorit® 1150 m2⋅g–1, the EWS-activated carbons' microporous structure and functional group richness, includingoxygen-containing, pyrrolyc nitrogen, and phosphate-like functional groups position these materials asexceptional candidates for efficient and selective HMF adsorption.The Ph.D. thesis's findings suggest innovative materials for high-value resource recovery and carbonsequestration, moving the engineered wetland systems field forward, shifting their traditional use, and openingthe opportunity for the future integration of EWS-willow biomass in biorefineries.

AB - The immediate challenges we face in treating wastewater, recovering resources from waste streams,capturing CO2 from the atmosphere, and the transition from fossil-based chemicals to biomass-derivedbiorefineries are topics of rising global interest. Engineered wetland systems (EWS), at the forefront of theexpanding field of nature-based solutions (NBS), offer a sustainable approach to wastewater treatment andbiomass production. Notably, willows grown in EWS, particularly those in zero-discharge systems, exhibit rapidbiomass growth (17–31 t DM⋅ha−1⋅a−1) and superior nutrient absorption compared to those in bioenergy-focusedsystems like short-rotation willow coppices (SRWC). Our study explores the potential of EWS-willow biomassas a novel feedstock to produce novel materials such as hydrochars, pyrochars, and activated carbons. Weinvestigated the as-yet unexplored potential of EWS-willows for resource recovery and carbon sequestration usingslow pyrolysis (600 °C), hydrothermal carbonization (HTC at varied reaction severities), as well as chemical(KOH) and physical (steam) activation processes.In our first manuscript, we explored the innovative densification of phosphorus and carbon in hydrocharsby leveraging the precursory chemical composition of willows using an accelerated HTC approach, free ofcatalysts, involving fast heating rates, moderate water loading to the reactor, sudden cooling times, and shortreaction times. We experimented with custom-engineered reactors immersed within a preheated fluidized sandbath and a two-factor experimental design that varied the HTC temperatures (225, 250, 275, and 300 °C) andreaction times (10, 20, 30, and 60 min), maintaining a 1:10 dry biomass-to-water mass ratio and rapid heating rateat ~59 °C⋅min–1. Hydrochars showed improved fuel properties and energy content than the EWS-woodchips—Higher heating value (HHV) of 22–28 MJ⋅kg–1, aligning closely with mineral coals. These hydrochars alsoshowcased enhanced thermal stability, heightened aromaticity, and increased fixed carbon content (14 to 45 %).We found out that periodically fertilized natural willows—not previously exposed to wastewater—could achievenear-complete P-retention (~100 %) with our accelerated HTC approach after accumulating sufficient multivalentelements—namely Ca and Mg. Interestingly, we observed that substantial P-recovery might occur relativelyquickly, and compared to previous literature using HTC reaction times between 1 to 13-hours, the reaction timewith our approach could be reduced to 30-minutes at 275 and 300 °C to obtain full phosphorus densification.However, we also documented a lower P-retention of 12–63 % in EWS-hydrochars across temperatures of 225–300 °C with the same approach and rapid heating rates.In our second manuscript, we compared EWS-based woodchips, pyrochars, and hydrochars. Here, weoptimized our HTC method to overcome the previously limited retention of phosphorus in EWS-hydrochars dueto a lower multivalent element content in the EWS-willows compared to their natural counterparts. We conductedHTC at 250 °C without catalysts, applying a slow heating rate of ~3.75 °C⋅min−1 and incorporating process waterrecirculation through the EWS-hydrochars, resulting in a near-complete P-retention. Specifically, we usedsequential P-extractions with a modified Hedley's method and observed a high P-bioavailability in the willowwoodchipsand a significant P-retention in EWS-chars—up to 92 % in pyrochars and 100 % retention inhydrochars, along with a higher labile-P fraction of 21 % in hydrochars than 5 % in pyrochars. Our findings gobeyond the limited previous studies on HTC in willows and represent a fundamental advance in improving the Pretentioncapabilities of EWS-hydrochars. Utilizing X-ray-based techniques, Raman spectroscopy, scanningelectron microscopy, and gas physisorption, we characterized the EWS-chars' structures. We revealed innovative 3D-visualization, which transcends previous literature by providing insights into the chars' internal porosity, waterretention potential and quantifying—for the first time—their carbonaceous structural thickness via a meshingalgorithm and the calculation of their porosity and mean Feret diameter. EWS-pyrochars exhibited remarkablearomaticity with a higher concentration of overall sp2 C-atoms at 63 % vs. 43 % in hydrocars. Moreover, unlikehydrochar, which depicts occluded porosity, EWS-pyrochars contain 92 % water storage-like pores. Althoughhydrochars indicated lower carbonization and thermal stability than pyrochars, their higher carbon retention (54vs. 41 % in pyrochar) suggest superior annual benefits—on a 10 ha EWS scale—of 79-tons of carbonsequestration and 334 kg of phosphorus recovery versus 60-tons of carbon and 298 kg of phosphorus withpyrochars.Our third manuscript delves into the continuous catalytic hydrothermal synthesis (HTS) of 5-hydroxymethylfurfural (HMF) from fructose. We report HMF yields up to 56.5 mol% with a selectivity of 63 %,marking an opportunity to further research the HTS process for platform chemical production. A pivotal part ofour research is the innovative production of EWS-activated carbons, which has a novel application for the selectiveadsorption of HMF from the HTS product mixtures. Here, we demonstrate that activated carbons derived fromEWS-hydrochars exhibit superior HMF adsorption capacities compared to commercial activated carbon, withselective adsorption of HMF even in the presence of fructose and levulinic acid. Our findings reveal that physicallyEWS-activated carbons achieved a remarkable HMF capture of 0.33 g HMF⋅g–1 AC, significantly higher than0.22 g HMF⋅g–1 AC of commercial activated carbon Norit® GCN-830. We used as precursors the EWS-pyrocharsand hydrochars obtained from our second manuscript and experimented with chemical (KOH ratio 1:1 at 650 °C)and physical (using steam at 800 and 900 °C) activation processes with a heating rate ~10 °C⋅min−1. Employingstate-of-the-art analytical methods—including high-resolution X-ray photoelectron spectroscopy, X-raydiffraction, Raman spectroscopy, X-ray Fluorescence, high-resolution transmission electron microscopy (HRTEM),scanning electron microscopy, derivative thermogravimetry, vacuum FT-IR spectrometry, and gasphysisorption with N2 and CO2—we meticulously characterize these innovative materials to explore the futuredevelopment of innovative and sustainable adsorbents for the chemical industry. Preliminary findings highlightthe unique crystallinity and structural qualities of EWS-derived activated carbons, including the presence ofcrystalline phases CaCO3 and Ca3(PO4)2. Despite a lower BET surface area of 531–882 m2⋅g–1 compared toNorit® 1150 m2⋅g–1, the EWS-activated carbons' microporous structure and functional group richness, includingoxygen-containing, pyrrolyc nitrogen, and phosphate-like functional groups position these materials asexceptional candidates for efficient and selective HMF adsorption.The Ph.D. thesis's findings suggest innovative materials for high-value resource recovery and carbonsequestration, moving the engineered wetland systems field forward, shifting their traditional use, and openingthe opportunity for the future integration of EWS-willow biomass in biorefineries.

KW - Hydrothermal carbonization

KW - Pyrolysis

KW - Catalytic Hydrothermal Synthesis

KW - HMF

KW - Furfural

KW - Hydrochar

KW - Pyrochar

KW - Activated Carbon

KW - X-rays

M3 - Ph.D. thesis

BT - Hydrothermal and Thermochemical Processing for Resource Recovery in Wetland Engineering

PB - Aarhus University - Department of Biological and Chemical Engineering

CY - Aarhus

ER -