Montserrat Filella

Antimony in the environment: a review focused on natural waters: II. Relevant solution chemistry

This paper discusses the main characteristics of the solution chemistry of antimony in relation to its behaviour and fate in natural waters. It is based on a careful and systematic examination of a comprehensive collection of solution equilibrium and environmentally oriented studies, some published more than 100 years ago. General principles are outlined and main unknowns highlighted. Existing information on antimony reactivity in solution is scarce and often relatively old. Studies performed under conditions relevant to natural water conditions are particularly scant. D 2002 Elsevier Science B.V. All rights reserved.

Antimony in the environment : knowns and unknowns

Environmental context. Antimony first attracted public attention in the mid-1990s amid claims that it was involved in Sudden Infant Death Syndrome. A substantial number of papers have now been published on the element and its behaviour in the natural environment. However, many key aspects of the environmental chemistry of antimony remain poorly understood. These include critical areas such as its ecotoxicology, its global cycling through different environmental compartments, and what chemical form it takes in different environments. More focussed research would help the situation. The present review highlights several areas of environmental antimony chemistry that urgently need to be addressed. Abstract. The objective of the present article is to present a critical overview of issues related to the current state of knowledge on the behaviour of antimony in the environment. It makes no attempt to systematically review all published data. However, it does provide a list of the main published reviews on antimony and identifies subjects where systematic reviews are needed. Areas where our knowledge is strong – and the corresponding gaps – in subjects ranging from total concentrations and speciation in the various environmental compartments, to ecotoxicity, to cycling between compartments, are discussed, along with the underlying research. Determining total antimony no longer poses a problem for most environmental samples but speciation measurements remain challenging throughout the process, from sampling to analysis. This means that the analytical tools still need to be improved but experience shows that, to be useful in practice, this should be directly driven by the requirements of laboratory and field measurements. Many different issues can be identified where further research is required, both in the laboratory and in the field, the most urgently needed studies probably being: (i) long-term spatial and temporal studies in the different environmental compartments in order to collect the data needed to establish a global biogeochemical cycle; (ii) laboratory studies of antimony interactions with potential natural binders; (iii) reliable ecotoxicological studies.

Complexation by natural heterogeneous compounds: Site occupation distribution functions, a normalized description of metal complexation

Abstract This paper presents a new conceptual approach to interpreting titration curves of metal complexation by physically and chemically heterogeneous natural complexants such as humic acids, clays, complete soils, or sediments. The physico-chemical and analytical difficulties encountered with such systems are reviewed by comparison with a system containing only a few simple ligands, followed by discussion of the new approach on the same basis. It is shown that interpretation of heterogeneous complexant properties necessitates a preliminary transformation of experimental raw data into a function sufficiently normalized so as to allow comparison of results obtained under different conditions. A normalized function called a Site Occupation Distribution Function (SODF) and its potential usefulness is described here. The SODF is a readily computable function which relates the complexation buffer intensity of the system to the differential free energy of the complexation sites present. Its major interest is that it enables one to obtain both a rigorous mathematical description of the complexant properties (even when highly heterogeneous) at the macroscopic level and, in certain cases, an estimation of the molecular-scale behavior of particular site types. The relationship of the SODF to other distribution functions proposed in the literature is discussed and applications are exemplified using simulated and real natural systems. In particular, its utility is discussed in detail for 1. (1) discriminating between different site types (major, minor, dominant, background), 2. (2) evaluating the degree of heterogeneity of an unknown complexant system, 3. (3) estimating the nature and true thermodynamic constants of complexes, and 4. (4) yielding a rigorous definition of “complexation capacity.”

Effect of physico-chemical heterogeneity of natural complexants. Part I. Voltammetry of labile metal-fulvic complexes.

A theoretical study of the effect of the physico-chemical heterogeneity of complexants such as fulvic compounds on d.c. polarographic curves is described. These compounds have been strongly implicated in playing a decisive role in trace cation complexation in natural systems. Their main feature is their wide range of complexation energy, which stems from their intrinsic properties: polyfunctionality, polyelectrolytic character and conformational properties. The whole of these complexing properties may be represented by complexation distribution functions. It is shown that, in the formation of labile metal—fulvic complexes and in the absence of adsorption and kinetic effects, it is possible to obtain from a single voltammetric curve important parameters related to metal speciation, namely the average diffusion coefficient of the complexes and the parameters describing the equilibrium complexation distribution functions.

Computer simulation of the low-molecular-weight inorganic species distribution of antimony(III) and antimony(V) in natural waters

Computer speciation models of antimony in aqueous, multicomponent solutions of environmental and biological interest have been developed. Antimony is an element of increasing environmental significance but one whose chemical speciation has not previously been modelled in any comprehensive way. The available thermodynamic data have been critically evaluated and entered into a thermodynamic database. The JESS suite of computer programs has been used to develop the thermodynamically-consistent mass balance equations used for modelling purposes. The calculated speciation distributions successfully account for some but not all key known properties of antimony in natural waters. It is believed that the main difference is most likely due to kinetic factors.

Effect of physico-chemical heterogeneity of natural complexants: Part II. Buffering action and role of their background sites

The peculiar buffering behaviour of the “background sites” of natural heterogeneous complexants is described. Background sites include most of the minor sites, i.e., a group of sites typically comprising not more than ca. 10% of the total sites. Despite this low proportion, they play an important role in controlling free metal ion concentrations in natural systems, because their complexation energy is higher than that of the major sites and their log K values span a broad range of order of magnitudes, owing to the cooperative effect of the large number of site types that they include, together with such continuous secondary effects as those due to a polyelectrolytic interactions and conformational changes. The main characteristics of these sites are summarized and a mathematical and chemical explanation for their cooperative action in controlling the overall free energy of metal ion complexation and the overall buffering intensity of the system is presented. Comparison is made with a classical system made up of a few simple ligands and devoid of secondary effects. The important parameters useful for describing the properties of background sites as a whole are deduced for complexation data obeying the Freundlich isotherm, which has been shown to fit most of the properties of background sites of natural systems.

A comprehensive systematic compilation of complexation parameters reported for trace metals in natural waters

Abstract: All the data published for complexation (conditional stability constants and effective ligand concentrations) of trace metals (Cu(II), Zn(II), Pb(II), and Cd(II)) in whole seawater, estuarine and freshwater samples over the past 25 years is compiled and presented in a systematic format. This work is intended as a resource for workers in the field of metal ion speciation in natural waters. The experimental techniques and data interpretation protocols used are briefly discussed, and main problems highlighted.

Questions of size and numbers in environmental research on microplastics: methodological and conceptual aspects

Environmental context Microplastics, either purposefully manufactured or formed by fragmentation of discarded ‘end-of-life’ macroplastic items, are accumulating in environmental compartments. As more and more data are collected on microplastics in the environment, discussion of two issues has become indispensable: (i) how reliable are the results in terms of the inherent capabilities and limitations of current methods used for sampling, counting and measuring microplastic particles; and (ii) how can the fate of microplastics be understood in the context of natural particles and colloids? Abstract A first important step in evaluating the impact of microplastic pollution in natural systems is assessing the reliability of the results obtained according to the inherent capabilities and limitations of the methods used for sampling, counting and measuring microplastic particles. This study, based on the critical reading of 55 studies containing quantitative microplastic data in waters and sediments, is an attempt to analyse these issues in the light of existing knowledge in the field of natural colloid studies. Existing results are highly dependent on the sampling and methodological procedure chosen and are essentially descriptive. Moreover, often they lack standardisation and adequate reporting of basic information such as the meaning of the size parameter measured. Colloid theory may provide the theoretical background needed to explain microplastic behaviour or, at least, to identify the parameters (e.g. density, surface characteristics, shape) that need to be known in order to gain a predictive knowledge of the subject. They are introduced and discussed. Finally, microplastics are not alone in environmental compartments. For this reason, when possible, published microplastic particle size distributions in natural waters have been quantitatively situated in the context of natural particles.

Implications of natural organic matter binding heterogeneity on understanding lead(II) complexation in aquatic systems

A critical analysis of data published for complexation (conditional stability constants and complexation capacities) of lead(II) by natural organic matter (NOM) is presented. The same patterns are observed for the different types of NOM, i.e. isolated humic-type substances, isolated algae- and bacteria derived ligands and cell walls, and filtered whole water systems: apparent stronger binding sites are utilised at lower metal ion loadings, progressively weaker sites contribute to metal complexation at higher loadings. Continuous binding functions should thus be used to adequately describe lead-NOM complexation in natural waters. Two important observations arising from our analysis are that: (i) the binding curves (binding affinity as a function of metal ion loading) for whole natural water samples lie between those for compounds representative of the two main NOM types, viz., isolated humic compounds and biota, and (ii) binding constants commonly used in speciation codes probably underestimate the actual extent of lead(II) binding by NOM at natural concentration levels.

Dispersion of antimony from oxidizing ore deposits

The solubilities of brandholzite, [Mg(H2O)6][Sb(OH)6]2, and bottinoite, [Ni(H2O)6][Sb(OH)6]2, at 25 °C in water have been measured. Solubilities are 1.95(4) × 10-3 and 3.42(11) × 10-4 mol dm-3, respectively. The incongruent dissolution of romeite, Ca2Sb2O7, and bindheimite, Pb2Sb2O7, at 25 °C in 0.100 mol dm-3 aqueous HNO3 was also investigated. Equilibrium dissolved Sb concentrations were 3.3 ± 1.0 × 10-7 and 7.7 ± 2.1 × 10-8 mol dm-3, respectively. These values have been used to re-evaluate the geochemical mobility of Sb in the supergene environment. It is concluded that the element is geochemically immobile in solution and in soils. This was in part validated by an orientation soil geochemical survey over the Bayley Park prospect near Armidale, New South Wales, Australia. Anomalous soil Sb levels are confined to within 100 m of known stibnite mineralization.