Facile Synthesis of Activated Carbon from Cellulose in the Presence of Air for Ammonia Recovery in Resource-Constrained Settings

Abstract

As efforts to provide sanitation services in low-resource rural settings of the world proliferate, nutrient recovery via source separation of urine becomes important for economic and environmental implications in these regions of challenging socio-economic constraints. Substantial N (in some cases > 50%) is lost from urine via off-gassed ammonia which can be captured and utilized more efficiently by the use of sorbents such as activated carbon synthesized locally in resource-constrained settings.

My work is focused on developing a simple method to synthesize functionalized-activated carbon from locally available biomass at moderate temperatures (400-450)°C in the presence of air. Using cellulose as a model biomass and diammonium hydrogenphosphate (DAP) as the activating agent, the production of activated carbon is explored in a simple semi-batch Partial Oxidation (POX) reactor setup. DAP helps to facilitate low temperature pyrolysis of cellulose by the action of phosphoric acid which helps in depolymerization and the breakdown of glycosidic linkages present in lignocellulosic biomass. Further, the film of condensed phosphates prevents carbon oxidation at high temperature in the presence of air. This study provides insight into the interaction between DAP and biomass, as well as the char forming mechanism. Various characterization techniques such as N2 physisorption, XPS, DRIFTS, SEM, TEM, surface charge measurements and Raman spectroscopy are utilized to compare the properties between activated carbon formed under nitrogen and partial oxidative conditions. The interaction of DAP with cellulose is investigated and the nature of bonding of the heteroatoms to the carbonaceous matrix is elucidated. Our results indicate that the quality of activated carbon prepared under partial oxidation condition is comparable to carbon prepared under nitrogen, leading to the possibility of an activated biochar production scheme on a small scale.

The prepared activated carbon is utilized to recover ammonia off-gassed from urine providing multiple benefits including odor removal and nutrient recovery since ammonia augmented biochar can serve as a soil amendment. The hygroscopic nature of the prepared activated biochar is useful for holding soil moisture during the dry season and is envisioned to provide better soil health for round the year crop production. The efficacy of the synthesized activated biochar as an ammonia adsorbent is evaluated from adsorption isotherms by studying equilibrium capacity and elucidating the nature of the interaction by Temperature Programmed Desorption (TPD) and DRIFTS measurements. Our results show promising capacities for ammonia adsorption from the gas phase. The total dry ammonia adsorption capacity of the synthesized activated carbons was found to be ~ (24-28) mg/g at 50°C, comparable in magnitude to zeolites. It was observed that > 90% of the adsorbed ammonia could be easily recovered by simple water washing at room temperature, facilitating facile regeneration.