Interaction between humic substances and arsenic species simulating environmental conditions.

The importance of evaluating how natural organic matter influences the mobility of arsenic species in an ecosystem is an environmental concern. This work aimed to evaluate the interaction between humic substances (HS) and four arsenic species of high toxicity [As(III), As(V), MMA(V), and DMA(V)] (HS-As) under the influence of HS concentration and pH. Next, the complexing capacity (CC) of HS by As(III) was determined in function of pH, ionic strength and co-existing ions. The free arsenic (Asfree) was determined after a tangential flow ultrafiltration procedure by hydride generation atomic fluorescence spectrometry. The better HS-As interactions for As(III) and As(V) at pH 10.5 reached 52% and 61%, respectively. The increase in pH and ionic strength, as well as co-existing ions increased the CC, which reached 1.57 mg of As(III) g-1 HS. We proposed a HS-As interaction model based on the inner and outer binding sites of HS from these results. The inner sites were occupied through hydrogen bonds, Pearson acid-base, hydrophobic, and van der Waals interactions for trivalent arsenic species, while the interactions through the outer sites for pentavalent arsenic species were mostly by hydrogen bonds and electrostatic forces. According to ecotoxicological studies against Artemia salina, the presence of HS decreased the toxicity of As(III) and As(V) as the lethal concentration increased from 5.81 to 8.82 mg L-1 and from 8.82 to 13.37 mg L-1, respectively. From the results through the proposed model, it was possible to successfully understand the interaction dynamic between soil HS and As(III), As(V), MMA(V) and DMA(V) under simulated environmental conditions.

Chemical characterization and ligand behaviour of Pseudomonas veronii 2E siderophores.

Siderophores are low-molecular weight ligands secreted by bacteria as a survival strategy in Fe(III)-lacking environments. They bind not only Fe(III), but Co(II), Zn(II), Mn(II), Ni(II), Ga(III) as a detoxification alternative. The synthesis, purification and characterization of siderophores produced by Pseudomonas veronii 2E were evaluated to be applied in future environmental technologies. Optimal production was obtained in Fe(III)-free M9-succinate at 25 °C, 40 h and pH 6.9. Siderophores were chemically characterized as hydroxamate and catechol mixed-type. Spectroscopic analysis indicated their belonging to the pyoverdine family, behaving as ligand to Cd(II), Zn(II), Cu(II), Ni(II) and Cr(III), which promoted siderophoregenesis during growth. Siderophore-Cd(II) complexation was studied by electrochemical monitored titration revealing one family of moderate-strength binding sites. Mass spectral analysis evidenced the secretion of a variety of molecules (molecular mass ca.1200 u). Non pathogenic Pseudomonas veronii 2E siderophores represent a safe alternative for the concrete application of environmental technologies and clinical procedures.