Analysis of metal speciation dynamics in clay minerals dispersion by stripping chronopotentiometry techniques

Investigating the Binding Heterogeneity of Trace Metal Cations With SiO2 Nanoparticles Using Full Wave Analysis of Stripping Chronopotentiometry at Scanned Deposition Potential

Silica oxides nano- and microparticles, as well as silica-based materials, are very abundant in nature and industrial processes. Trace metal cation binding with these bulk materials is generally not considered significant in speciation studies in environmental systems. Nonetheless, this might change for nanoparticulate systems as observed in a previous study of Pb(II) with a very small SiO₂ particle (7.5 nm diameter). Besides, metal binding by those nanoparticles is surprisingly characterized by a heterogeneity that increases with the decrease of metal-to-particle ratio. Therefore, it is interesting to extend this study to investigate different trace metals and the influence of the nanoparticle size on the cation binding heterogeneity. Consequently, the Cd(II), Pb(II), and Zn(II) binding by two different sized SiO₂ nanoparticles (Ludox LS30 and TM40) in aqueous dispersion was studied for a range of pH and ionic strength conditions, using the combination of the electroanalytical techniques Scanned Stripping ChronoPotentiometry and Absence of Gradients and Nernstian Equilibrium Stripping. The coupling of these techniques provides the free metal concentration in the bulk (AGNES) and information of the free and complex concentration at the electrode surface for each Stripping Chronopotentiometry at Scanned deposition Potential (SSCP). A recent mathematical treatment allows the reconstruction of a portion of the metal to ligand binding isotherm with the included heterogeneity information using the full SSCP wave analysis. In this work, we observed that the Zn(II) binding is homogeneous, Cd(II) is slightly heterogeneous, and Pb(II) is moderately heterogeneous, whereas the results obtained with the 7.5 nm diameter nanoparticle are slightly more heterogeneous than those obtained with the one of 17 nm. These findings suggest that the Zn(II) binding is electrostatic in nature, and for both Cd(II) and Pb(II), there should be a significant chemical binding contribution.

Towards improving the electroanalytical speciation analysis of indium

The geochemical fate of indium in natural waters is still poorly understood, while recent studies have pointed out a growing input of this trivalent element in the environment as a result of its utilisation in the manufacturing of high-technology products. Reliable and easy-handling analytical tools for indium speciation analysis are, then, required. In this work, we report the possibility of measuring the total and free indium concentrations in solution using two complementary electroanalytical techniques, SCP (Stripping chronopotentiometry) and AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) implemented with the TMF/RDE (Thin Mercury Film/Rotating Disk Electrode). Nanomolar limits of detection, i.e. 0.5 nM for SCP and 0.1 nM for AGNES, were obtained for both techniques in the experimental conditions used in this work and can be further improved enduring longer experiment times. We also verified that AGNES was able (i) to provide robust speciation data with the known In-oxalate systems and (ii) to elaborate indium binding isotherms in presence of humic acids extending over 4 decades of free indium concentrations. The development of electroanalytical techniques for indium speciation opens up new routes for using indium as a potential tracer for biogeochemical processes of trivalent elements in aquifers, e.g. metal binding to colloidal phases, adsorption onto (bio)surfaces, etc.

Free indium concentration determined with AGNES

Indium is increasingly used in electronic devices, from which it can be mobilized towards environmental compartments. Speciation of In in waters is important for its direct ecotoxicological effects, as well as for the fate of this element in the environment (e.g. fluxes from or towards sediments). Free indium concentrations in the environment can be extremely low due to hydrolysis, especially important in trivalent cations, to precipitation and to complexation with different ligands. In this work, the free indium concentration (which is a toxicologically and geochemically relevant fraction) in aqueous solutions at pH3 has been measured with an adapted version of the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). Speciation measurements in mixtures of indium with the ligands NTA (nitrilotriacetic acid) and oxalate indicate that the values of their stability constants in the NIST46.6 database are less adequate than those published in some more recent literature. The extraordinary lability and mobility of In-oxalate complexes allow the measuring of free indium concentrations below nmol/L in just 25s of deposition time.

Relationship of Biodiversity with Heavy Metal Tolerance and Sorption Capacity: A Meta-Analysis Approach

Microbial remediation of metals can alleviate the concerns of metal pollution in the environment. The microbial remediation, however, can be a complex process since microbial metal resistance and biodiversity can play a direct role in the bioremediation process. This study aims to understand the relationships among microbial metal resistance, biodiversity, and metal sorption capacity. Meta-analyses based on 735 literature data points of minimum inhibitory concentrations (MIC) of Plantae, Bacteria, and Fungi exposed to As, Cd, Cr Cu, Ni, Pb, and Zn showed that metal resistance depends on the microbial Kingdom and the type of heavy metal and that consortia are significantly more resistant to heavy metals than pure cultures. A similar meta-analysis comparing 517 MIC values from different bacterial genera (Bacillus, Cupriavidus, Klebsiella, Ochrobactrum, Paenibacillus, Pseudomonas, and Ralstonia) confirmed that metal tolerance depends on the type of genus. Another meta-analysis with 195 studies showed that the maximum sorption capacity is influenced by microbial Kingdoms, the type of biosorbent (whether consortia or pure cultures), and the type of metal. This study also suggests that bioremediation using microbial consortia is a valid option to reduce environmental metal contaminations.