Weight-of-Evidence Approach for Assessing Removal of Metals from the Water Column for Chronic Environmental Hazard Classification

The United Nations and the European Union have developed guidelines for the assessment of long-term (chronic) chemical environmental hazards. This approach recognizes that these hazards are often related to spillage of chemicals into freshwater environments. The goal of the present study was to examine the concept of metal ion removal from the water column in the context of hazard assessment and classification. We propose a weight-of-evidence approach that assesses several aspects of metals including the intrinsic properties of metals, the rate at which metals bind to particles in the water column and settle, the transformation of metals to nonavailable and nontoxic forms, and the potential for remobilization of metals from sediment. We developed a test method to quantify metal removal in aqueous systems: the extended transformation/dissolution protocol (T/DP-E). The method is based on that of the Organisation for Economic Co-operation and Development (OECD). The key element of the protocol extension is the addition of substrate particles (as found in nature), allowing the removal processes to occur. The present study focused on extending this test to support the assessment of metal removal from aqueous systems, equivalent to the concept of "degradability" for organic chemicals. Although the technical aspects of our proposed method are different from the OECD method for organics, its use for hazard classification is equivalent. Models were developed providing mechanistic insight into processes occurring during the T/DP-E method. Some metals, such as copper, rapidly decreased (within 96 h) under the 70% threshold criterion, whereas others, such as strontium, did not. A variety of method variables were evaluated and optimized to allow for a reproducible, realistic hazard classification method that mimics reasonable worst-case scenarios. We propose that this method be standardized for OECD hazard classification via round robin (ring) testing to ascertain its intra- and interlaboratory variability. Environ Toxicol Chem 2019;38:1839-1849. © 2019 SETAC.

Method Development for Determining the Removal of Metals from the Water Column under Transformation/Dissolution Conditions for Chronic Hazard Classification

An extension of the transformation/dissolution protocol (T/DP) was developed and evaluated as a tool to measure the removal of metals from the water column for chronic aquatic hazard classification. The T/DP extension (T/DP-E) consists of 2 parts: T/DP-E part 1, to measure metal removal from the water column via binding of metals to a substrate and subsequent settling, and T/DP-E part 2, to assess the potential for remobilization of metals following resuspension. The T/DP-E methodology (672-h [28-d] removal period, 1-h resuspension event, and 96-h resettling period) was tested using Cu, Co, and Sr solutions in the presence of a substrate. The metal removal rates varied from rapid removal for Cu to slower rates of removal for Co and Sr. The resuspension event did not trigger any increase in dissolved Cu, Co, or Sr. Additional 96-h experiments were conducted using dissolved Ni, Pb, Zn, and Ag and supported the conclusion that the T/DP-E is sufficiently robust to distinguish removal rates between metals with a wide range of reactivities. The proposed method provides a means to quantify the rate of metal removal from the water column and evaluate remobilization potential in a standardized and reliable way. Environ Toxicol Chem 2019;38:2032-2042. © 2019 SETAC.

The Fate of Copper Added to Surface Water: Field, Laboratory, and Modeling Studies

The fate and effects of copper in the environment are governed by a complex set of environmental processes that include binding to inorganic and organic ligands in water, soil, and sediments. In natural waters, these interactions can limit copper bioavailability and result in copper transport from the water column to the sediment. In the present study, data on the fate of copper added to lakes, microcosms, and mesocosms were compiled and analyzed to determine copper removal rates from the water column. Studies on copper behavior in sediment were also reviewed to assess the potential for remobilization. A previously developed, screening-level fate and transport model (tableau input coupled kinetic equilibrium transport-unit world model [TICKET-UWM]) was parameterized and applied to quantify copper removal rates and remobilization in a standardized lake setting. Field and modeling results were reconciled within a framework that links copper removal rates to lake depths and solids fluxes. The results of these analyses provide converging evidence that, on a large scale, copper is removed relatively quickly from natural waters. For the majority of studies examined, more than 70% of the added copper was removed from the water column within 16 d of dosing. This information may be useful in the context of environmental hazard and risk assessment of copper. Environ Toxicol Chem 2019;38:1386-1399. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Modeling the Fate of Metal Concentrates in Surface Water

Metals present in concentrates are in a solid form and are not bioavailable, but they can dissolve or potentially transform to more soluble forms. Transformation/dissolution laboratory protocols have been developed to assess the importance of dissolution of sparingly soluble metal substances in the context of hazard classification; however, these tests represent worst-case scenarios for metal bioavailability because attenuation mechanisms such as complexation, sorption, and transport to the sediment are not considered. A unit world model (UWM) for metals in lakes, tableau input coupled kinetics equilibrium transport (TICKET)-UWM, has been developed that considers key processes affecting metal transport, fate, and toxicity including complexation by aqueous inorganic and ligands, partitioning to dissolved organic carbon (DOC) and particulate organic carbon (POC), precipitation, and transport of dissolved metals and solids between the water column and sediment. The TICKET-UWM model was used to assess the fate of a metal concentrate and dissolved metal ions released from the concentrate following an instantaneous input to a generalized lake. Concentrate dissolution rates in the water column were parameterized using results from batch transformation/dissolution tests for 2 specific concentrates containing lead (Pb), copper (Cu), and cobalt (Co). The TICKET-UWM results for a generalized lake environment showed that water column concentrations of metals in the lake environment after 28 d were several orders of magnitude lower than the 28-d concentration from the transformation/dissolution tests because Pb, Cu, and Co partitioned to POC in the water column and were subsequently removed due to settling. Resuspension of sediment served to increase total metal in the water column, but the resulting concentrations were still much lower than the 28-d concentrations from the transformation/dissolution tests. Information from TICKET-UWM could be used to refine the environmental hazard profiles of metals. Environ Toxicol Chem 2019;38:1256-1272. © 2019 SETAC.

Bioretention storm water control measures decrease the toxicity of copper roof runoff

The present study evaluated the ability of 2 different bioretention storm water control measures (SCMs), planter boxes and swales, to decrease the toxicity of sheet copper (Cu) roofing runoff to Daphnia magna. The present study quantified changes in storm water chemistry as it passed through the bioretention systems and utilized the biotic ligand model (BLM) to assess whether the observed D. magna toxicity could be predicted by variations found in water chemistry. Laboratory toxicity tests were performed using select storm samples with D. magna cultured under low ionic strength conditions that were appropriate for the low ionic strength of the storm water samples being tested. The SCMs decreased toxicity of Cu roof runoff in both the BLM results and the storm water bioassays. Water exiting the SCMs was substantially higher than influent runoff in pH, ions, alkalinity, and dissolved organic carbon and substantially lower in total and dissolved Cu. Daphnids experienced complete mortality in untreated runoff from the Cu roof (the SCM influent); however, for planter and swale effluents, survival averaged 86% and 95%, respectively. The present study demonstrated that conventional bioretention practices, including planter boxes and swales, are capable of decreasing the risk of adverse effects from sheet Cu roof runoff to receiving systems, even before considering dilution of effluents in those receiving systems and associated further reductions in copper bioavailability. Environ Toxicol Chem 2017;36:1680-1688. © 2016 SETAC.

Attenuation of copper in runoff from copper roofing materials by two stormwater control measures

Concerns have been raised over diffuse and non-point sources of metals including releases from copper (Cu) roofs during storm events. A picnic shelter with a partitioned Cu roof was constructed with two types of stormwater control measures (SCMs), bioretention planter boxes and biofiltration swales, to evaluate the ability of the SCMs to attenuate Cu in stormwater runoff from the roof. Cu was measured as it entered the SCMs from the roof as influent as well as after it left the SCMs as effluent. Samples from twenty-six storms were collected with flow-weighted composite sampling. Samples from seven storms were collected with discrete sampling. Total Cu in composite samples of the influent waters ranged from 306 to 2863 μg L(-1) and had a median concentration of 1087 μg L(-1). Total Cu in the effluent from the planter boxes ranged from 28 to 141 μg L(-1), with a median of 66 μg L(-1). Total Cu in effluent from the swales ranged from 7 to 51 μg L(-1) with a median of 28 μg L(-1). Attenuation in the planter boxes ranged from 85 to 99% with a median of 94% by concentration and in the swales ranged from 93 to 99% with a median of 99%. As the roof aged, discrete storm events showed a pronounced first-flush effect of Cu in SCM influent but this was less pronounced in the planter outlets. Stormwater retention time in the media varied with antecedent conditions, stormwater intensity and volume with median values from 6.6 to 73.5 min. Based on local conditions, a previously-published Cu weathering model gave a predicted Cu runoff rate of 2.02 g m(-2) yr(-1). The measured rate based on stormwater sampling was 2.16 g m(-2) yr(-1). Overall, both SCMs were highly successful at retaining and preventing offsite transport of Cu from Cu roof runoff.

TICKET-UWM: a coupled kinetic, equilibrium, and transport screening model for metals in lakes

The tableau input coupled kinetic equilibrium transport-unit world model (TICKET-UWM) has been developed as a screening model for assessing potential environmental risks associated with the release of metals into lakes. The model is based on a fully implicit, one-step solution algorithm that allows for simultaneous consideration of dissolved and particulate phase transport; metal complexation to organic matter and inorganic ligands; precipitation of metal hydroxides, carbonates, and sulfides; competitive interactions of metals and major cations with biotic ligands; a simplified description of biogeochemical cycling of organic carbon and sulfur; and dissolution kinetics for metal powders, massives, and other solid forms. Application of TICKET-UWM to a generalized lake in the Sudbury area of the Canadian Shield is presented to demonstrate the overall cycling of metals in lakes and the nonlinear effects of chemical speciation on metal responses. In addition, the model is used to calculate critical loads for metals, with acute toxicity of Daphnia magna as the final endpoint. Model results show that the critical loads for Cu, Ni, Pb, and Zn varied from 2.5 to 39.0 g metal/m(2) -year and were found to be one or more orders of magnitude higher than comparable loads for pesticides (lindane, 4,4'-DDT) and several polyaromatic hydrocarbon (PAH) compounds. In sensitivity calculations, critical metal-loading rates were found to vary significantly as a function of the hydraulic detention time, water hardness, and metal dissolution kinetic rates.

Tableau input coupled kinetic equilibrium transport (TICKET) model

TICKET is a general-purpose, multispecies reactive transport model that is based on the tableau structure in MINEQL. The model can be used in solving problems from simple chemical equilibrium calculations for batch systems to complex one-dimensional, reactive transport computations for surface water, groundwater, and water treatment systems. To streamline model input and model formulation, specifications of equilibrium speciation (including homogeneous precipitation, solid solutions, adsorption, and ion exchange) and linear and nonlinear kinetic reactions are defined directly in the tableau. In addition, the burden of accounting for appearing and disappearing solid phases is circumvented by approximating homogeneous precipitation as a solid solution (with an insoluble seed). The solution technique for the model is based on a one-step algorithm and can be applied to both steady-state and fully implicit, time-variable problems. This approach is particularly well-suited in handling chemical speciation-transport problems with fast, nonlinear reaction kinetics and transient chemical intermediates. TICKET model simulations are presented for several test cases to verify the computational scheme. A model application, which examines the effects of sorption and overlying dissolved oxygen concentration on Fe(II) and As(III) oxidation in a sediment column, is also presented to demonstrate the utility of TICKET in examining complex chemical speciation-transport behavior.

Iron(II)-catalyzed oxidation of arsenic(III) in a sediment column

Arsenic contamination in aquatic systems is a worldwide concern. Understanding the redox cycling of arsenic in sediments is critical in evaluating the fate of arsenic in aquatic environments and in developing sediment quality guidelines. The direct oxidation of inorganic trivalent arsenic, As(III), by dissolved molecular oxygen has been studied and found to be quite slow. A chemical pathway for As(III) oxidation has been proposed recently in which a radical species, Fe(IV), produced during the oxidation of divalent iron, Fe(II), facilitates the oxidation of As(III). Rapid oxidation of As(III) was observed (on a time scale of hours) in batch systems at pH 7 and 7.5, but the extent of As(III) oxidation was limited. The Fe(II)-catalyzed oxidation of As(III) is examined in a sediment column using both computational and experimental studies. A reactive-transport model is constructed that incorporates the complex kinetics of radical species generation and Fe(II) and As(III) oxidation that have been developed previously. The model is applied to experimental column data. Results indicate that the proposed chemical pathway can explain As(III) oxidation in sediments and that transport in sediments plays a vital role in increasing the extent of As(III) oxidation and efficiency of the Fe(II) catalysis.

Measuring the partitioning of silver to organic carbon using solubility enhancement

Ionic silver is toxic to aquatic organisms at low concentrations. However, complexation to binding sites on natural organic matter has been shown to reduce silver toxicity. Research indicates that there is a need to develop reliable methods for characterizing silver binding at very low silver ion concentrations where strong binding sites have a significant influence on silver speciation. This study provides an analytical method for measuring silver binding using a solubility-enhancement procedure. Preliminary experimental results are provided that demonstrate strong silver binding in the presence of natural organic matter at very low silver ion concentrations.

Effect of thioarsenite formation on arsenic(III) toxicity

Soluble arsenic(III)-sulfide complexes (thioarsenites) play a significant role in the chemistry of arsenic in reducing, sulfidic environments at circumneutral pH. Chemical equilibrium calculations using thioarsenite thermodynamic data from the literature indicate that the formation of a dithioarsenite complex, AsS(OH)(SH)(-1), reduces the concentration of the uncomplexed inorganic As(III) species present (defined sigma H3AsO3, where sigma H3AsO3 = AsO3(-3) + HAsO3(-2) + H2AsO3(-1) + H3AsO3). With enough sulfide present, soluble As(III) is dominated by this complex. Therefore, it is of interest to examine the effect of dithioarsenite formation on As(III) toxicity. The Microtox acute toxicity test was used for this purpose. Tests performed on solutions with varying S:As ratios indicate that As(III) toxicity is a function of the uncomplexed As(III) concentration rather than the total As(III) concentration. This suggests that the dithioarsenite species is not bioavailable and that its formation reduces As(III) toxicity. Chemical equilibrium calculations and sediment pore-water field data from various sources indicate that, in many sediments, dithioarsenite formation can reduce toxicity.