ABSTRACT:The density functional theory (DFT) at the B3LYP/6-31G (d,p) and B3LYP/6-311G(d,p) basis set levels and ab initio calculations using the HF/6-31G (d,p) and HF/6-311G(d,p) methods were performed on four rhodanine azosulpha drugs (namely 5-sulfadiazineazo-3-phenyl-2-thioxo-4-thiazolidinone, 5-sulfamethazineazo-3-phenyl-2-thioxo-4-thiazolidinone, 5-sulfadimethoxineazo-3-phenyl-2-thioxo-4-thiazolidinone, and 5-sulfamethoxazoleazo-3-phenyl-2-thioxo-4-thiazolidinone) used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between the molecular structure of the rhodanine azosulpha drugs and inhibition efficiency(%IE). The quantum chemical parameters/descriptors, namely, E HOMO (highest occupied molecular orbital energy), E LUMO (lowest unoccupied molecular orbital energy), the energy difference (⌬E) between E HOMO and E LUMO , dipole moment (), electron affinity (A), ionization potential (I), the absolute electronegativity (X), absolute hardness (), softness (), polarizability (␣), the Mulliken charges, and the fraction of electrons (⌬N) transfer from inhibitors to iron, were calculated and correlated with the experimental %IE. Quantitative structure activity relationship (QSAR) approach has been used, and a composite index of some quantum chemical parameters/descriptors was performed to characterize the inhibition performance of the studied molecules. The results showed that the inhibition efficiency (%IE) of the rhodanine azo sulfa drugs studied was closely related to some of the quantum chemical parameters/descriptors but with varying degrees of correlation coefficient (R 2 ). The %IE also increased with the increase in E HOMO and decrease in E HOMO ϪE LUMO ; and the areas containing N atoms are the most possible sites for bonding to the metal iron surface by donating electrons to the metal. The HOMO orbitals consist of 61.73-63.04% double bonded S atom (7(S)), and most of the rest are concentrated on the rhodanine group; so, the rhodanine molecule plays an important role in bonding of sulfa drugs with metal atom in the corrosion process. The calculated/estimated %IE of the compounds studied was found to be close to the experimental %IE.

The prepared chitosan-coated diatomite (CCD) was characterized using Fourier transform infrared spectroscopy and SEM techniques. The effects of pH, contact time, sorbent dosage, metal concentration, and temperature on the adsorption (%) of mercury (Hg­(II)) from aqueous solution using the CCD sorbent were studied systemically. The monolayer sorption capacities of raw diatomite and CCD sorbents at pH 5 were found to be 68.1 and 116.2 mg/g, respectively. The mean adsorption energy (8.2 kJ/mol) calculated from the Dubinin–Radushkevich (D–R) model indicated that the sorption process occurred via chemisorption. The modified sorbent had good adsorption and desorption performance with respect to reusability after 10 repeated cycles. The thermodynamic calculations revealed that the sorption was viable, spontaneous, and exothermic in nature under the conditions studied. The kinetic calculations indicated that the sorption mechanism was well-explained by a pseudo-second-order model. All results demonstrated that the prepared CCD is an effective sorbent for the removal of Hg­(II) from aqueous solution.

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