Sequential Ammonia and Carbon Dioxide Adsorption on Pyrolyzed Biomass to Recover Waste Stream Nutrients.

The amine-rich surfaces of pyrolyzed human solid waste (py-HSW) can be "primed" or "regenerated" with carbon dioxide (CO) to enhance their adsorption of ammonia (NH) for use as a soil amendment. To better understand the mechanism by which CO exposure facilitates NH adsorption to py-HSW, we artificially enriched a model sorbent, pyrolyzed, oxidized wood (py-ox wood) with amine functional groups through exposure to NH. We then exposed these N-enriched materials to CO and then resorbed NH. The high heat of CO adsorption ( ) on py-HSW, 49 kJ mol, at low surface coverage, 0.4 mmol CO g, showed that the naturally occurring N compounds in py-HSW have a high affinity for CO. The of CO on py-ox wood also increased after exposure to NH, reaching 50 kJ mol at 0.7 mmol CO g, demonstrating that the incorporation of N-rich functional groups by NH adsorption is favorable for CO uptake. Adsorption kinetics of py-ox wood revealed continued, albeit diminishing NH uptake after each CO treatment, averaging 5.9 mmol NH g for the first NH exposure event and 3.5 and 2.9 mmol NH g for the second and third; the electrophilic character of CO serves as a Lewis acid, enhancing surface affinity for NH uptake. Furthermore, penetration of NH and CO measured by NanoSIMS reached over 7 μm deep into both materials, explaining the large NH capture. We expected similar NH uptake in py-HSW sorbed with CO and py-ox wood because both materials, py-HSW and py-ox wood sorbed with NH, had similar N contents and similarly high CO uptake. Yet NH sorption in py-HSW was unexpectedly low, apparently from potassium (K) bicarbonate precipitation, reducing interactions between NH and sorbed CO; 2-fold greater surface K in py-HSW was detected after exposure to CO and NH than before gas exposure. We show that amine-rich pyrolyzed waste materials have high CO affinity, which facilitates NH uptake. However, high ash contents as found in py-HSW hinder this mechanism.