Kraft lignin-derived micro- and mesoporous nitrogen-doped carbon adsorbent for air and water purification

The study presents a streamlined one-step process for producing highly porous, metal-free, N-doped activated carbon (N-AC) for CO2 capture and herbicide removal from simulated industrially polluted and real environmental systems. N-AC was prepared from kraft lignin - a carbon-rich and abundant by-product of the pulp industry, using nitric acid as the activator and urea as the N-dopant. The reported carbonization process under a nitrogen atmosphere renders a product with a high yield of 30% even at high temperatures up to 800 °C. The N-AC exhibited a substantial high N content (4–5 %), the presence of aliphatic and phenolic OH groups, and a notable absence of carboxylic groups, as confirmed by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and Boehm’s titration. Scanning electron microscopy revealed the presence of mesopores with an average diameter of 3 nm. Porosity analysis indicated that micropores constituted the majority of the pore structure, with 86% of pores having diameters less than 0.6 nm. According to BET adsorption analysis, the developed porous structure of N-AC boasted a substantial specific surface area of 1000 m2 g−1. N-AC proved to be a promising adsorbent for air and water purification. Specifically, N-AC exhibited a strong affinity for CO2, with an adsorption capacity of 1.4 mmol g−1 at 0.15 bar, and 20 °C and it demonstrated the highest selectivity over N2 from the simulated flue gas system (27.3 mmol g−1 for 15:85 v:v CO2:N2 at 20 °C) among all previously reported nitrogen-dopped AC materials from kraft lignin. Moreover, N-AC displayed excellent reusability and efficient CO2 release, maintaining an adsorption capacity of 3.1 mmol g−1 (at 1 bar and 25 °C) over ten consecutive adsorption-desorption cycles, confirming N-AC as a useful material for CO2 storage and utilization. The unique cationic nature of N-AC enhanced the adsorption of herbicides in neutral and weakly basic environments, which is relevant for real waters. It exhibited an impressive adsorption capacity for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) at 96 ± 6 mg g−1 under pH 6 and 25 °C according to the Langmuir-Freundlich model. Notably, N-AC preserves its high adsorption capacity towards 2,4-D from simulated groundwater and runoff from tomato greenhouse, while performance in real samples from Fyris river in Uppsala, Sweden, causes a decrease of only 4-5%. Owing to the one-step process, high yield, the annual abundance of kraft lignin, and the use of environmentally friendly activating agents, N-AC has substantial potential for large-scale industrial applications.