Toxicity of nanoparticles of CuO, ZnO and TiO2 to microalgae Pseudokirchneriella subcapitata

Estimation of Cumulative Aquatic Exposure and Risk Due to Silver: Contribution of Nano-Functionalized Plastics and Textiles

The biotic ligand model (BLM) was developed to explain and predict the effects of water chemistry on the acute toxicity of metals to aquatic organisms. The biotic ligand is defined as a specific receptor within an organism where metal complexation leads to acute toxicity. The BLM is designed to predict metal interactions at the biotic ligand within the context of aqueous metal speciation and competitive binding of protective cations such as calcium. Toxicity is defined as accumulation of metal at the biotic ligand at or above a critical threshold concentration. This modeling framework provides mechanistic explanations for the observed effects of aqueous ligands, such as natural organic matter, and water hardness on metal toxicity. In this paper, the development of a copper version of the BLM is described. The calibrated model is then used to calculate LC50 (the lethal concentration for 50% of test organisms) and is evaluated by comparison with published toxicity data sets for freshwater fish (fathead minnow, Pimephales promelas) and Daphnia.

Biotic ligand model of the acute toxicity of metals. 2. Application to acute copper toxicity in freshwater fish and Daphnia.

During recent years, the biotic ligand model (BLM) has been proposed as a tool to evaluate quantitatively the manner in which water chemistry affects the speciation and biological availability of metals in aquatic systems. This is an important consideration because it is the bioavailability and bioreactivity of metals that control their potential to cause adverse effects. The BLM approach has gained widespread interest amongst the scientific, regulated and regulatory communities because of its potential for use in developing water quality criteria (WQC) and in performing aquatic risk assessments for metals. Specifically, the BLM does this in a way that considers the important influences of site-specific water quality. This journal issue includes papers that describe recent advances with regard to the development of the BLM approach. Here, the current status of the BLM development effort is described in the context of the longer-term history of advances in the understanding of metal interactions in the environment upon which the BLM is based. Early developments in the aquatic chemistry of metals, the physiology of aquatic organisms and aquatic toxicology are reviewed first, and the degree to which each of these disciplines influenced the development of water quality regulations is discussed. The early scientific advances that took place in each of these fields were not well coordinated, making it difficult for regulatory authorities to take full advantage of the potential utility of what had been learned. However, this has now changed, with the BLM serving as a useful interface amongst these scientific disciplines, and within the regulatory arena as well. The more recent events that have led to the present situation are reviewed, and consideration is given to some of the future needs and developments related to the BLM that are envisioned. The research results that are described in the papers found in this journal issue represent a distinct milestone in the ongoing evolution of the BLM approach and, more generally, of approaches to performing ecological assessments for metals in aquatic systems. These papers also establish a benchmark to which future scientific and regulatory developments can be compared. Finally, they demonstrate the importance and usefulness of the concept of bioavailability and of evaluative tools such as the BLM.

The biotic ligand model : a historical overview

The rational for the study was to review the literature on the toxicity and corresponding mechanisms associated with lead (Pb), mercury (Hg), cadmium (Cd), and arsenic (As), individually and as mixtures, in the environment. Heavy metals are ubiquitous and generally persist in the environment, enabling them to biomagnify in the food chain. Living systems most often interact with a cocktail of heavy metals in the environment. Heavy metal exposure to biological systems may lead to oxidation stress which may induce DNA damage, protein modification, lipid peroxidation, and others. In this review, the major mechanism associated with toxicities of individual metals was the generation of reactive oxygen species (ROS). Additionally, toxicities were expressed through depletion of glutathione and bonding to sulfhydryl groups of proteins. Interestingly, a metal like Pb becomes toxic to organisms through the depletion of antioxidants while Cd indirectly generates ROS by its ability to replace iron and copper. ROS generated through exposure to arsenic were associated with many modes of action, and heavy metal mixtures were found to have varied effects on organisms. Many models based on concentration addition (CA) and independent action (IA) have been introduced to help predict toxicities and mechanisms associated with metal mixtures. An integrated model which combines CA and IA was further proposed for evaluating toxicities of non-interactive mixtures. In cases where there are molecular interactions, the toxicogenomic approach was used to predict toxicities. The high-throughput toxicogenomics combines studies in genetics, genome-scale expression, cell and tissue expression, metabolite profiling, and bioinformatics.

A review of toxicity and mechanisms of individual and mixtures of heavy metals in the environment.

The biotic ligand model (BLM) of acute metal toxicity to aquatic organisms is based on the idea that mortality occurs when the metal-biotic ligand complex reaches a critical concentration. For fish, the biotic ligand is either known or suspected to be the sodium or calcium channel proteins in the gill surface that regulate the ionic composition of the blood. For other organisms, it is hypothesized that a biotic ligand exists and that mortality can be modeled in a similar way. The biotic ligand interacts with the metal cations in solution. The amount of metal that binds is determined by a competition for metal ions between the biotic ligand and the other aqueous ligands, particularly dissolved organic matter (DOM), and the competition for the biotic ligand between the toxic metal ion and the other metal cations in solution, for example, calcium. The model is a generalization of the free ion activity model that relates toxicity to the concentration of the divalent metal cation. The difference is the presence of competitive binding at the biotic ligand, which models the protective effects of other metal cations, and the direct influence of pH. The model is implemented using the Windermere humic aqueous model (WHAM) model of metal-DOM complexation. It is applied to copper and silver using gill complexation constants reported by R. Playle and coworkers. Initial application is made to the fathead minnow data set reported by R. Erickson and a water effects ratio data set by J. Diamond. The use of the BLM for determining total maximum daily loadings (TMDLs) and for regional risk assessments is discussed within a probabilistic framework. At first glance, it appears that a large amount of data are required for a successful application. However, the use of lognormal probability distributions reduces the required data to a manageable amount. Keywords—Bioavailability Metal toxicity Metal complexation Risk assessment

Hazard/Risk Assessment BIOTIC LIGAND MODEL OF THE ACUTE TOXICITY OF METALS. 1. TECHNICAL BASIS

It is critically important to consider bioavailability when developing water quality criteria (WQC) and sediment quality guidelines (SQGs). This is one of the most important lessons learned as WQC and SQGs have evolved over the previous 30 years. Bioavailability is particularly important for metals, as they are present in a variety of forms (metal species) in aquatic settings, and these species may differ widely in their degree of availability to aquatic organisms. The need to consider bioavailability of metals in sediments was motivated by the common observation that similar dry weight metal concentrations (e.g. µg metal/g dry sediment) exhibit a wide range of effects on sediment organisms when the sediments are from different locations and have varying properties. This finding is demonstrated in Figure 1, where mortality may vary widely at a fixed sediment concentration. The challenge in deriving meaningful sediment quality guidelines is to learn why these differences in effects occur and to devise a method that accounts for these differ

Use of the sem and avs approach in predicting metal toxicity in sediments

A time variable model has been used to analyze a field scale experiment in which a flooded limestone quarry was dosed with equal quantities of Lindane and DDE. The water column and sediment chemical concentrations were monitored for one year after the initial dosing. The markedly different physical-chemical characteristics of the test chemicals provided an interesting contrast for model application.

A simplified time variable model of partitioning chemicals is cast in the form of analytical solutions for the total, dissolved and particulate concentrations of chemical in completely mixed, interactive, water column and sediment compartments. The model formulation incorporates chemical decay and transport mechanisms of particulate and diffusive exchange between water column and sediment. The Lindane and DDE model calibrations based on data from the first year after dosing are presented. A preliminary verification of the model, obtained by projecting DDE levels in the water and sediment five years after initial dosing, when the quarry was revisited, is also shown. The model results underscore the significance of chemical partitioning on chemical fate and highlight the importance and utility of a modeling framework which incorporates realistic mechanisms of water column and sediment interaction.

Time variable model of the fate of DDE and lindane in a quarry

Developing site-specific water quality criteria for metals using the biotic ligand model

To understand the cause-effect relationships controlling water quality in dynamically narrow estuaries, a time dependent branching, laterally averaged (longitudinal and vertical) hydrodynamic model was joined together with a similarly formulated water quality model. The governing equations consist of equations for continuity, temperature, conductivity (or salinity) and momentum conservation for channels of variable width and depth. A turbulence closure model is embedded within the circulation model to provide a parameterization of the turbulence mixing processes. The water quality component of the unified model consists of conservation equations for dissolved oxygen and biochemical oxygen demand, both of which are formulated to resolve the vertical and longitudinal directions. A simulation conducted with the model for an observationally well documented and interesting rainfall event in the Milwaukee Harbor estuary is described.

Paul R. Paquin

Papers

Hazard/Risk Assessment BIOTIC LIGAND MODEL OF THE ACUTE TOXICITY OF METALS. 1. TECHNICAL BASIS

Use of the sem and avs approach in predicting metal toxicity in sediments

Time variable model of the fate of DDE and lindane in a quarry

Developing site-specific water quality criteria for metals using the biotic ligand model

A Unified Hydrodynamic-Water Quality Model