Advances in preparation of modified activated carbon and its applications in the removal of chromium (VI) from aqueous solutions

Sulfur dioxide removal: An overview of regenerative flue gas desulfurization and factors affecting desulfurization capacity and sorbent regeneration

Numerous mitigation techniques have been incorporated to capture or remove SO₂ with flue gas desulfurization (FGD) being the most common method. Regenerative FGD method is advantageous over other methods due to high desulfurization efficiency, sorbent regenerability, and reduction in waste handling. The capital costs of regenerative methods are higher than those of commonly used once-through methods simply due to the inclusion of sorbent regeneration while operational and management costs depend on the operating hours and fuel composition. Regenerable sorbents like ionic liquids, deep eutectic solvents, ammonium halide solutions, alkyl-aniline solutions, amino acid solutions, activated carbons, mesoporous silica, zeolite, and metal-organic frameworks have been reported to successfully achieve high SO₂ removal. The presence of other gases in flue gas, e.g., O₂, CO₂, NOx, and water vapor, and the reaction temperature critically affect the sorption capacity and sorbent regenerability. To obtain optimal SO₂ removal performance, other parameters such as pH, inlet SO₂ concentration, and additives need to be adequately governed. Due to its high removal capacity, easy preparation, non-toxicity, and low regeneration temperature, the use of deep eutectic solvents is highly feasible for upscale utilization. Metal-organic frameworks demonstrated highest reported SO₂ removal capacity; however, it is not yet applicable at industrial level due to its high price, weak stability, and robust formulation.

Adsorption of hexavalent chromium onto alkali-modified biochar derived from Lepironia articulata: A kinetic, equilibrium, and thermodynamic study

Hydrochar obtained after hydrothermal carbonization (HTC) of Lepironia articulata (LA) was modified into biochar by reacting with a specific amount of KOH in a tubular reactor under slow pyrolysis. The physical and chemical properties of the hydrochar and modified biochar were characterized. The performance of modified biochar (LABC) was investigated through batch sorption experiments. Removal (%) and the maximum adsorption capacity (q_max ) of Cr(VI) onto LABC increased up to 98.9% and 28.75 mg/g relative to 63.44% and 21.90 mg/g in unmodified hydrochar (LAHC) at pH 2.0, 313 K, and 200 mg/L, respectively. The sorption kinetics uptake data were best interpreted with pseudo-second-order model, and sorption isotherm was simulated with the Langmuir adsorption model. The thermodynamic parameters confirm the adsorption process to be an endothermic, spontaneous, and increased disorder. The overall results revealed that LABC can be utilized as an environmentally friendly, inexpensive, and effective adsorbent in Cr(VI) removal. PRACTITIONER POINTS: Hydrochar and modified biochar prepared from a tropical biomass (Lepironia articulata) were successfully used for the removal of Cr(VI) from aqueous solution. Increased specific surface was obtained by applying chemical modification with alkali treatment, contributing to effectiveness as adsorbent. Dimensionless K_c was estimated from the Langmuir fits and then used to estimate thermodynamics of adsorption. The signs of ∆H°, ∆G°, and ∆S° indicate that the adsorption of Cr(VI) onto LABC was an endothermic, spontaneous, and increased disorder.