Effect of Water on the Structure and Stability of Hydrogen-Bonded Oxalic Acid Dimer

Vibrational Stark fields in carboxylic acid dimers

Carboxylic acids form exceptionally stable dimers and have been used to model proton and double proton transfer processes. The stabilization energies of the carboxylic acid dimers are very weakly dependent on the nature of substitution. However, the electric field experienced by the OH group of a particular carboxylic acid is dependent more on the nature of the substitution on the dimer partner. In general, the electric field was higher when the partner was substituted with an electron-donating group and lower with an electron-withdrawing substituent on the partner. The Stark tuning rate (Δ) of the O-H stretching vibrations calculated at the MP2/aug-cc-pVDZ level was found to be weakly dependent on the nature of substitution on the carboxylic acid. The average Stark tuning rate of O-H stretching vibrations of a particular carboxylic acid when paired with other acids was 5.7 cm^-1 (MV cm^-1)^-1, while the corresponding average Stark tuning rate of the partner acids due to a particular carboxylic acid was 21.9 cm^-1 (MV cm^-1)^-1. The difference in the Stark tuning rate is attributed to the primary and secondary effects of substitution on the carboxylic acid. The average Stark tuning rate for the anharmonic O-D frequency shifts is about 40-50% higher than the corresponding harmonic O-D frequency shifts calculated at the B3LYP/aug-cc-pVDZ level, much greater than the typical scaling factors used, indicating the strong anharmonicity of O-H/O-D oscillators in carboxylic acid dimers. Finally, the linear correlation observed between pK_a and the electric field was used to estimate the pK_a of fluoroformic acid to be around 0.9.

Molecular-Scale Mechanism of Sequential Reaction of Oxalic Acid with SO3: Potential Participator in Atmospheric Aerosol Nucleation

Recent research has shown the almost barrierless cycloaddition reaction of the carboxylic acid with one SO₃ to form products with group of -OSO₃H, which can form stable clusters with the nucleation precursors through hydrogen bonds (Mackenzie et al., Science 2015, 349, 58). Oxalic acid (OA), the simplest and prevalent dicarboxylic acid, was selected as an example to clarify the possibility to react with two SO₃ sequentially and the nucleation potential of products. The results indicate that OA can sequentially react with two SO₃ through low reaction barriers to form the primary product (oxalic sulfuric anhydride (OSA)) and the secondary product (oxalic disulfuric anhydride (ODSA)). Interactions between atmospheric nucleation precursors and OSA, ODSA, or OA are in the order of ODSA > OSA > OA through evaluating the stability of generated clusters by the topological, thermodynamics, and kinetic analysis, which implies generated products could be nucleation stabilizers with nucleation potential positively correlating with the number of -OSO₃H. This reaction mechanism contributes to a comprehensive understanding of the reactivity of dicarboxylic acid in the polluted environment as well as the role of products in organosulfur chemistry and, to some extent, help to explain the missing sources of new particle formation.

A one-step acidification strategy for sewage sludge dewatering with oxalic acid

Sewage sludge dewatering is an efficient approach to reduce the volume of sludge for the subsequent disposal. In this study, a novel one-step acidification sludge dewatering method was developed with using oxalic acid as a conditioner. In laboratory-scale experiments with the dosage of 200 mg/g dry solid (DS), the normalized capillary suction time and the specific resistance to filtration were respectively decreased by 78.7% and 60.0% after 30 min of oxalic acid conditioning, much more efficient than those conditioned with sulfuric acid and hydrochloric acid at the same pH value. This superior dewatering performance was attributed to two factors. One was that oxalic acid could more efficiently promote the hydrolysis of polysaccharide, especially pectins, to release bound water. The other was that OA could dissolve more Fe³⁺ and Al³⁺, as well as form precipitate with Ca²⁺ in sludge, which may act as flocculants or co-precipitator for the subsequent sludge particles coagulation. In pilot-scale experiments, the water content of oxalic acid conditioned sludge cake was reduced to 60% under the optimum conditions, while the reagent cost was as low as 110.0 USD/t DS. This work provides a cost-effective and easy-operated sewage sludge disposal technique, and also sheds light on the potential of oxalic acid in environmental waste treatment.