Acidic surface chemical modification of biomass combustion ash-derived activated carbon for CO2 adsorption

Abstract

To maximise the output and optimise a given adsorbent-adsorbate system, tailoring the material to the specific task at hand is a prominent approach. In the context of CO2 adsorption, terminal amine groups are most common due to their exceptional affinity towards carbon dioxide. However, other methods involving introduction of alternative functionalities are often overlooked despite providing major benefits: e.g. cost-effectiveness of sorbent production and regeneration, thermal stability and etc. On these grounds, a physically activated carbon has been chemically modified with a mixture of nitric and sulphuric acid to introduce novel acidic functional groups to facilitate CO2 adsorption. The experimental campaign was conducted by employing a randomised Box-Behnken design, evaluating the modification time and temperature as well as the ratio at which the acids (of different concentrations) have been mixed. CO2 uptake was maximised (0.96 mmol/g at 50 °C) when treating the sorbent for 3 hours at 90 °C with 1 molar acids at a volumetric HNO3/H2SO4 ratio of 1:2. Successful grafting of the nitro group was confirmed via spectroscopic studies, increased nitrogen content as well as other indicators. The sample was shown to possess a high working capacity, losing less than 5 % of its original uptake even after 40 adsorption-desorption cycles. As such, nitration of the surface may be viewed as a novel yet cost-effective surface modification method in the context of CO2 adsorption.