Eggshell membrane derived nitrogen rich porous carbon for selective electrosorption of nitrate from water

Nitrate (NO₃^-) is a ubiquitous contaminant in water and wastewater. Conventional treatment processes such as adsorption and membrane separation suffer from low selectivity for NO₃^- removal, causing high energy consumption and adsorbents usage. In this study, we demonstrate selective removal of NO₃^- in an electrosorption process by a thin, porous carbonized eggshell membrane (CESM) derived from eggshell bio-waste. The CESM possesses an interconnected hierarchical pore structure with pore size ranging from a few nanometers to tens of micrometers. When utilized as the anode in an electrosorption process, the CESM exhibited strong selectivity for NO₃^- over Cl^-, SO₄^2-, and H₂PO₄^-. Adsorption of NO₃^- by the CESM reached 2.4 × 10^-3 mmol/m², almost two orders of magnitude higher than that by activated carbon (AC). More importantly, the CESM achieved NO₃^-/Cl^- selectivity of 7.79 at an applied voltage of 1.2 V, the highest NO₃^-/Cl^- selectivity reported to date. The high selectivity led to a five-fold reduction in energy consumption for NO₃^- removal compared to electrosorption using conventional AC electrodes. Density function theory calculation suggests that the high NO₃^- selectivity of CESM is attributed to its rich nitrogen-containing functional groups, which possess higher binding energy with NO₃^- compared to Cl^-, SO₄^2-, and H₂PO₄^-. These results suggest that nitrogen-rich biomaterials are good precursors for NO₃^- selective electrodes; similar chemistry can also be used in other materials to achieve NO₃^- selectivity.

Synthesis and NOx removal performance of anatase S-TiO2/g-CN heterojunction formed from dye wastewater sludge

In this study, sludges generated from Ti-based flocculation of dye wastewater were used to retrieve photoactive titania (S-TiO₂). It was heterojunctioned with graphitic carbon nitride (g-CN) to augment photoactivity under UV/visible light irradiance. Later the as-prepared samples were utilized to remove nitrogen oxides (NO_x) in the atmospheric condition through photocatalysis. Heterojunction between S-TiO₂ and g-CN was prepared through facile calcination (@550 °C) of S-TiO₂ and melamine mix. Advanced sample characterization was carried out and documented extensively. Successful heterojunction was confirmed from the assessment of morphological and optical attributes of the samples. Finally, the prepared samples' level of photoactivity was assessed through photooxidation of NO_x under both UV and visible light irradiance. Enhanced photoactivity was observed in the prepared samples irrespective of the light types. After 1 h of UV/visible light-based photooxidation, the best sample STC4 was found to remove 15.18% and 9.16% of atmospheric NO, respectively. In STC4, the mixing ratio of S-TiO₂, to melamine was maintained as 1:3. Moreover, the optical bandgap of STC4 was found as 2.65 eV, where for S-TiO₂, it was 2.83 eV. Hence, the restrained rate of photogenerated charge recombination and tailored energy bandgap of the as-prepared samples were the primary factors for enhancing photoactivity.

Facile synthesis and characterization of anatase TiO2/g-CN composites for enhanced photoactivity under UV-visible spectrum

For the purpose of atmospheric NO removal, anatase TiO₂/g-CN photocatalytic composites were prepared by using a facile template-free calcination route in atmospheric conditions. Considerably fiscal NP400 and laboratory-grade melamine were used as the precursor of the composites. Additionally, samples were prepared with different wt. ratios of TiO₂ and melamine by using two distinct calcination temperatures (550 °C/600 °C). The morphological attributes of the composites were assessed with X-ray diffraction, scanning and transmission electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. Additionally, the optical traits were evaluated and compared using UV-visible diffuse reflectance spectroscopy and photoluminescence analysis. Finally, the photodegradation potentials for atmospheric NO by using the as-prepared composites were assessed under both UV and visible light irradiation. All the composites showed superior NO oxidation compared to NP400 and bulk g-CN. For the composites prepared by using the calcination temperature of 550 °C, the maximum NO removal was observed when the NP400 to melamine ratio was 1:2, irrespective of the utilized light irradiation type. Whereas for increased calcination temperature (600 °C), the maximum NO removal was observed at the precursor mix ratio of 1:3 (NP400:melamine). Successfully narrowed energy bandgaps were perceived in the as-prepared composites. Moreover, a subsequent drop in NO₂ generation during NO oxidation was observed under both UV and visible light irradiation. Interestingly, higher calcination temperature during the synthesis of the catalysts has shown a significant drop in NO₂ generation during the photodegradation of NO.

Selective adsorption of nitrate over chloride in microporous carbons

Activated carbon is the most common electrode material used in electrosorption processes such as water desalination with capacitive deionization (CDI). CDI is a cyclic process to remove ions from aqueous solutions by transferring charge from one electrode to another. When multiple salts are present in a solution, the removal of each ionic species can be different, resulting in selective ion separations. This ion selectivity is the result of combined effects, such as differences in the hydrated size and valence of the ions. In the present work, we study ion selectivity from salt mixtures with two different monovalent ions, chloride and nitrate. We run adsorption experiment in microporous carbons (i.e., without applying a voltage), as well as electrosorption experiments (i.e., based on applying a voltage between two carbon electrodes). Our results show that i) during adsorption and electrosorption, activated carbon removes much more nitrate than chloride; ii) at equilibrium, ion selectivity does not depend strongly on the composition of the water, but does depend on charging voltage in CDI; and iii) during electrosorption, ion selectivity is time-dependent. We modify the amphoteric Donnan model by including an additional affinity of nitrate to carbon. We find good agreement between our experimental results and the theory. Both show very high selectivity towards nitrate over chloride, [Formula: see text] ∼10, when no voltage is applied, or when the voltage is low. The selectivity gradually decreases with increasing charging voltage to [Formula: see text] ∼6 at V_ch = 1.2 V. Despite this decrease, the affinity-effect for nitrate continues to play an important role also at a high voltage. In general, we can conclude that our work provides new insights in the importance of carbon-ion interactions for electrochemical water desalination.