PNNL

Abstract

Untreated fecal waste in unsewered communities and reactive nitrogen (N) loading from wastewater and urine disposal are important environmental challenges. Through pyrolysis, untreated fecal waste can be converted into amine-rich carbon dioxide (CO2) sorbents, the chemisorption of which tunes the surface chemistry toward ammonia (NH3) uptake. We compared the N-loading potential within amine-rich pyrolyzed human solid waste through exposure to CO2 followed by NH3 with the N-loading potential on carboxyl-rich, pyrolyzed oxidized wood using sequential adsorption of isotopically labeled CO2 and NH3. The total N content of pyrolyzed oxidized wood exposed once to NH3 was similar to native, pyrolyzed human solid waste (3.7% compared to 4.6%). However, sorption kinetics and spectroscopic evidence revealed distinct N forms between the two materials affecting subsequent CO2 and NH3 uptake. Tertiary amines comprised 70% of N in pyrolyzed waste, compared to only 53% in pyrolyzed oxidized wood. The high heat of adsorption of CO2 on pyrolyzed human solid waste (50 kJ mol-1) at low concentrations (0.05 mmol CO2 g-1) was indicative of CO2 reactivity with tertiary amines, consistent with the formation of carbamic acid (Figure S1). Conversely, after exposure to NH3, pyrolyzed oxidized wood chemisorbed CO2 at much higher concentrations (0.7 mmol CO2 g-1), pointing to primary amines as the active functional moieties. Adsorption kinetics of pyrolyzed oxidized wood revealed continued, albeit diminishing NH3 uptake with each consecutive CO2 treatment, averaging 5.9 mmol NH3 g-1 for the first NH3 exposure event, and still 3.5 and 2.9 mmol NH3 g-1 for the second and third. Penetration of 15NH3 and 13CO2 measured by NanoSIMS reached over 7 µm deep into both materials, explaining the large NH3 capture. The significantly lower NH3 adsorption compared to pyrolyzed oxidized wood may be a result of potassium (K) carbonate precipitation precluding a Lewis base interaction between NH3 and chemisorbed CO2, suggested by a 2-fold increase in surface K concentrations after exposure of pyrolyzed human solid waste to CO2+NH3 detected by XPS.