The valorization of lignin into advanced materials for water and soil remediation is experiencing a surge in demand. However, it is imperative that material research and manufacturing be sustainable to prevent exacerbating environmental issues. Meeting these requirements necessitates a deeper understanding of the role of lignin's functional groups in attracting targeted species. This research delves into the interaction mechanisms between lignin and organic molecules, using the adsorption of the cationic dye Methylene Blue (MB⁺) as a case study. Herein, we aim to quantitatively estimate the contribution of different interaction types to the overall adsorption process. While carbonyl groups were found to have no significant role in attraction, carboxylic groups (-COOH) exhibited significantly lower adsorption compared with hydroxyl groups (-OH). Through alternately blocking aliphatic and phenolic -OH groups, we determined that 61% of the adsorption occurred through hydrogen bonding and 38% via electrostatic interactions. Performance studies of modified lignin along with spectroscopic methods (XPS, FTIR) confirmed the negligible role of π-π interactions in adsorption. This study offers fundamental insights into the mechanistic aspects of MB adsorption on lignin, laying the groundwork for potential modifications to enhance the performance of lignin-based adsorbents. The findings underscore the importance of hydroxyl groups and provide a roadmap for future studies examining the influence of steric factors and interactions with other organic molecules.

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 byproduct 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. 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. 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-doped 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 10 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 toward 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, annual abundance of kraft lignin, and use of environmentally friendly activating agents, N-AC has substantial potential for large-scale industrial applications.

Because of its high surface area and combination of various functional groups, graphene oxide (GO) is currently one of the most actively studied materials for electroanalytical applications. It is not practical to utilize self-supported GO on its own and thus it is commonly integrated with different supporting carriers. Having a large lateral size, GO can only wrap the particles of the support and thus can significantly reduce the surface area of porous materials. To achieve synergy from the high surface area and polyfunctional nature of GO, and the rigid structure of a porous support, the lateral size of GO must essentially be decreased. Recently reported graphene oxide quantum dots (GOQDs) can fulfil this task. Here we report the successful preparation of an SiO₂-GOQDs hybrid, where GOQDs have been incorporated into the mesoporous network of silica. The SiO₂-GOQDs emit a strong luminescence with a band maximum at 404 nm. The Raman spectrum of SiO₂-GOQDs shows two distinct peaks at 1585 cm⁻¹ (G-peak) and 1372 cm⁻¹ (D-peak), indicating the presence of a graphene ordered basal plane with aromatic sp²-domains and a disordered oxygen-containing structure. Covalent immobilization of GOQDs onto aminosilica via such randomly structured oxygen fragments was proven with the help of Fourier transform infrared spectroscopy, solid-state cross-polarization magic angle spinning ¹³C nuclear magnetic resonance, and X-ray photoelectron spectroscopy. SiO₂-GOQDs were used as a modifier of a carbon paste electrode for differential pulse voltammetry determination of two antibiotics (sulfamethoxazole and trimethoprim) and two endocrine disruptors (diethylstilbestrol (DES) and estriol (EST)). The modified electrodes demonstrated a significant signal enhancement for EST (370%) and DES (760%), which was explained by a π–π stacking interaction between GOQDs and the aromatic system of the analytes.