Speciation and Thermodynamic Study of Arsenic(III)-Pharmaceutical Complexes in Aqueous Solutions

Natural water sources are increasingly contaminated with a wide range of pollutants including heavy metals and pharmaceuticals. Arsenic, particularly in its more toxic trivalent form, i.e. As-(III), remains a significant environmental and public health concern due to its widespread presence and carcinogenic effects. In addition to that, pharmaceutical products like metronidazole (MNZ) and nalidixic acid (NAL), persistent in the environment due to their limited biodegradability, also pose significant threats to both ecosystems and human health. Recent research has highlighted the formation of antibiotic-metal complexes (AMCs) where antibiotics interact with heavy metals in aquatic environments, leading to altered physicochemical properties and increased toxicity.

The main objective of the present work is a speciation study on As-(III)-antibiotic complexes and particularly interaction between As-(III) and MNZ or NAL in aqueous solution.

Several temperatures and ionic strengths were probed by potentiometry to determine the formation constants and other thermodynamic parameters of As-(III)-MNZ and As-(III)-NAL complexes. UV spectrophotometric titrations were also employed to confirm formation constants of both systems. An estimation of the sequestering ability of both ligands toward As-(III) under relevant natural water conditions has also been performed. Further, density functional theory calculations have been executed with the purpose of investigating the molecular structure of these complexes and their relative stability.

It turns out that MNZ binds to As-(III) in either a neutral (AsMNZ) or protonated (As-(MNZ)-H) form via As-N and As-O interactions, with the hydroxyl oxygen being the preferred binding site in AsMNZ and both the nitro and hydroxyl groups being equally effective in As-(MNZ)-H, while NAL forms a stable chelated complex through bidentate coordination.

Findings reported in this study contribute to a deeper understanding of the complexes formed by As-(III) with pharmaceuticals and pave the way toward the development of improved technologies for the water treatment and remediation of AMCs.