Biotic Ligand Model Research Articles

Coherent toxicity prediction framework for deciphering the joint effects of rare earth metals (La and Ce) under varied levels of calcium and NTA

With the development of modern technologies, the exploitation and application of rare earth metals (REMs) have increased parallelly. Consequently, more REMs are entering into the environment and therefore there is a pressing need to assess their potential environmental hazards. Here, a standard toxicity test with wheat (Triticum aestivum) was conducted to investigate the single and mixture toxicity of La and Ce in solutions with different levels of calcium and nitrilotriacetic acid (NTA) and results were deciphered by different modeling approaches. Both La and Ce caused adverse effect to wheat, but the presence of Ca and NTA alleviated their toxicity. The obtained EC50 for [La] or [Ce] changed by more than 28-fold and by 4-fold, respectively, with the increase of Ca or NTA. The biotic ligand model (BLM) explained approximately 93% variation of single La or Ce toxicity. The binding constants obtained were 4.14, 6.67, and 6.59 for logKCaBL, logKLaBL, and logKCeBL respectively. The electrostatic toxicity model (ETM) was proved as effective as the BLM, with R2 = 0.93 for La and R2 = 0.92 for Ce. For La–Ce mixtures, parameters from single toxicity approaches were applied successfully to predict the mixture toxicity with concentration addition (CA) model based on the BLM or ETM theory (R2 = 0.92 and RMSE = 8.56; R2 = 0.90 and RMSE = 9.6, respectively). Thus, the results obtained in this study prove that both ETM and BLM theories are appropriate to predict single and mixture REMs toxicity, providing coherent and promising tools for the risk assessment of REM pollution.

Time series analysis for determining ecologically acceptable Cu concentration from species sensitivity distribution with biotic ligand models in soil pore water

A site-specific, ecologically acceptable concentration of Cu in soil pore water was determined with four trophic levels of soil-residing organisms. Specifically, soil pore water was periodically collected from a site contaminated with heavy metals using in-situ samplers. Dissolved Cu concentration, Ca2+, Mg2+, Na+, K+, Cl-, SO42-, NO3-, dissolved organic carbon, pH, and temperature were analyzed to derive a half-maximal effective concentration of Cu (EC50[Cu]T) using a biotic ligand model (BLM). The BLM parameters, such as binding constants (logKXBL) and the fraction of biotic ligand sites occupied by Cu ions (f), were adapted from previous studies. The EC50{Cu2+,} values were used to construct a species sensitivity distribution (SSD) curve from which the hazardous concentration, protecting 95% of the soil-residing organisms (HC5), was determined. Using ten BLM-based acceptable concentrations of Cu obtained by combining BLM and SSD, time series analysis was conducted with the fixed monitoring benchmark method to obtain maximum Cu concentration as an endpoint exhibiting no-adverse-effect which was found to be 0.084 mg/L of Cu in soil pore water at the test site. This study provides a systematic tool for determining an ecologically acceptable concentration of Cu in the soil by incorporating soil pore water chemistry and time series analysis.

The vital function of humic acid with different molecular weight in controlling Cd and Pb bioavailability and toxicity to earthworm (Eisenia fetida) in soil

Humic acid (HA) plays vital roles in regulating the environmental behaviors of metals and thus their toxicity to biota. However, the inner relation between metal bioavailability to soil organisms and the presence of HA with different molecular weight (Mw) is not well documented. In this study, we separated HAs into four fractions with Mw range of 5-30k Da, and discussed their ability to alleviating the toxicity of Cd and Pb to earthworm. The bioaccumulation capacities (Cmax) increased in order of: UF1<UF2<UF3<UF4, which is in line with the variations of bioavailable concentrations of Cd and Pb in soil. Variations of Mw and binding capacities of HA determine the accumulation behavior in soil solution. The unsatisfactory of biotic ligand model fitting and the differences in fractions of the total biotic ligand sites (f) in earthworm bound by Cd and Pb suggested that only free species of Cd could be considered as biological available to earthworm, while the Pb–HAs complexes have potential ability to interact with earthworm membrane. Antioxidant enzymes are effective biomarkers, and HA with lower Mw play more important roles in restricting the toxicity of soil Cd and Pb to earthworm. These results reveal the different mechanism for HA controlling metal bioavailability between Cd and Pb in soil environment.

Effective Modeling Framework for Quantifying the Potential Impacts of Coexisting Anions on the Toxicity of Arsenate, Selenite, and Vanadate.

Hardly any study has focused on the quantitative modeling of the toxicity of anionic metal(loid)s and their mixtures in the presence of potentially competing anions. Here, we designed a univariate experiment (420 treatments) to investigate the influence of various anions (phosphate, sulfate, carbonate, and OH-) on the toxicity of single anionic metal(loid)s (arsenate, selenite, and vanadate) and a full factorial mixture experiment (196 treatments) to examine the interactions and toxicity of As-Se mixtures at 4 phosphate levels. Standard root elongation tests with wheat (Triticum aestivum) were performed. A modeling framework, resembling the biotic ligand model (BLM) for cationic metals, was developed, extended, and applied to explain anion competitions and mixture effects. Carbonate significantly alleviated the toxicity of all three metal(loid)s. The toxicity of As was significantly mitigated by phosphate, while V toxicity was significantly relieved by OH-. The BLM-like model successfully explained more than 93% of the observed variance in toxicity. With the parameters derived from single-metal(loid) exposures, the developed BLM-toxic unit model reached an overall prediction performance of 78% in modeling the toxicity of As-Se mixtures at varying phosphate levels, validating the effectiveness of the model framework. It is concluded that by taking possible anion competitions and interactions into account, the BLM-type approaches can serve as promising tools for the risk assessment of single and mixed metal(loid)s contamination.

A review of chemical speciation techniques used for predicting dissolved copper bioavailability in seawater

Environmental contextCopper (Cu) is a metal of ecotoxicological concern in contaminated coastal areas. Cu present as the free ion is considered the most bioavailable fraction of the metal, and there is a need for the correct measurement or estimation of free Cu-ion concentrations in seawater. I review studies that have combined a biological measure of Cu bioavailability with chemical speciation measurements, and critically assess the ability of current methods to predict Cu bioavailability in contaminated coastal ecosystems. AbstractCopper (Cu) is an essential metal, but it is also toxic at concentrations reached in polluted coastal areas. In seawater, the speciation of this metal is largely controlled by the presence of dissolved organic matter (DOM), which binds Cu ions decreasing the concentration of inorganic and free forms of the metal. This is important to aquatic life, given that the bioavailability of dissolved metals is generally expected to be determined by the free ion concentration according to bioavailability models such as the free ion activity model and biotic ligand model (FIAM/BLM). The analytical determination of free metal concentrations in seawater is a challenging task that is needed (in combination with toxicity tests or other means of testing bioavailability) in order to test the applicability of the FIAM/BLM in particular systems and also for its application in monitoring and risk assessment of metals. This review summarises the studies that combine a biological measure of Cu bioavailability with the use of a chemical speciation technique for the measurement of Cu speciation in seawater, and it presents a critical examination of the results of those studies in order to determine which techniques are more suitable for the prediction of Cu bioavailability in seawater and to highlight research needs in the field. The technique showing the highest level of agreement with bioavailability data is anodic stripping voltammetry (ASV). Cathodic stripping voltammetry (CSV), aluminium hydroxide coated exchange resin (ALSA), and diffusion gradients in thin films (DGT) are also promising in this regard, although DGT slightly overestimates bioavailable Cu. More research is needed comparing the performance of different chemical speciation techniques with Cu bioavailability in seawater, especially at environmentally relevant concentrations of Cu.

State of the Science on Metal Bioavailability Modeling: Introduction to the Outcome of a Society of Environmental Toxicology and Chemistry Technical Workshop

A Society of Environmental Toxicology and Chemistry technical workshop was held in December 2017 to critically evaluate the current state of the science of metal bioavailability modeling. The availability of mechanistic models such as the biotic ligand model and the rapid development of empirical models such as multiple linear regressions means that choices are available in terms of bioavailability normalization approaches that can be used in metal risk assessments and the development of risk-based protective values for aquatic life. A key goal of the workshop was to provide potential users of metal bioavailability models with the information required to make appropriate decisions when choosing among mechanistic and empirical models. Workshop participants focused on the state of the science of metal bioavailability modeling, mechanistic and empirical model frameworks, validation of bioavailability models, and application of bioavailability models in risk-based decision-making approaches. The output of this workshop provides the necessary scientific information to incorporate bioavailability normalization in regulations pertaining to metals in freshwater systems. Environ Toxicol Chem 2019;39:42-47. © 2019 SETAC.

Metal Bioavailability Models: Current Status, Lessons Learned, Considerations for Regulatory Use, and the Path Forward.

Since the early 2000s, biotic ligand models and related constructs have been a dominant paradigm for risk assessment of aqueous metals in the environment. We critically review 1) the evidence for the mechanistic approach underlying metal bioavailability models; 2) considerations for the use and refinement of bioavailability-based toxicity models; 3) considerations for the incorporation of metal bioavailability models into environmental quality standards; and 4) some consensus recommendations for developing or applying metal bioavailability models. We note that models developed to date have been particularly challenged to accurately incorporate pH effects because they are unique with multiple possible mechanisms. As such, we doubt it is ever appropriate to lump algae/plant and animal bioavailability models; however, it is often reasonable to lump bioavailability models for animals, although aquatic insects may be an exception. Other recommendations include that data generated for model development should consider equilibrium conditions in exposure designs, including food items in combined waterborne-dietary matched chronic exposures. Some potentially important toxicity-modifying factors are currently not represented in bioavailability models and have received insufficient attention in toxicity testing. Temperature is probably of foremost importance; phosphate is likely important in plant and algae models. Acclimation may result in predictions that err on the side of protection. Striking a balance between comprehensive, mechanistically sound models and simplified approaches is a challenge. If empirical bioavailability tools such as multiple-linear regression models and look-up tables are employed in criteria, they should always be informed qualitatively and quantitatively by mechanistic models. If bioavailability models are to be used in environmental regulation, ongoing support and availability for use of the models in the public domain are essential. Environ Toxicol Chem 2019;39:60-84. © 2019 SETAC.

Bioavailability Assessment of Metals in Freshwater Environments: A Historical Review.

Many metals (aluminum, cadmium, cobalt, copper, nickel, lead, zinc) are widely studied environmental contaminants because of their ubiquity, potential toxicity to aquatic life, and tendency for toxicity to vary widely as a function of water chemistry. The interactions between metal and water chemistry influence metal "bioavailability," an index of the rate and extent to which the metal reaches the site of toxic action. The implications of metal bioavailability for ecological risk assessment are large, with as much as a 100-fold variability across a range of water chemistries in surface waters. Beginning as early as the 1930s, considerable research effort was expended toward documenting and understanding metal bioavailability as a function of total and dissolved metal, water hardness, natural organic matter, pH, and other water characteristics. The understanding of these factors and improvements in both analytical and computational chemistry led to the development of modeling approaches intended to describe and predict the relationship between water chemistry and metal toxicity, including the free ion activity model, the gill surface interaction model, the biotic ligand model, and additional derivatives and regression models that arose from similar knowledge. The arc of these scientific advances can also be traced through the evolution of the US Environmental Protection Agency's ambient water quality criteria over the last 50 yr, from guidance in the "Green Book" (1968) to metal-specific criteria produced in the last decade. Through time, water quality criteria in many jurisdictions have incorporated increasingly sophisticated means of addressing metal bioavailability. The present review discusses the history of scientific understanding of metal bioavailability and the development and application of models to incorporate this knowledge into regulatory practice. Environ Toxicol Chem 2019;39:48-59. © 2019 SETAC.

Influence of Dissolved Organic Carbon on the Acute Toxicity of Copper and Zinc to White Sturgeon (Acipenser transmontanus) and a Cladoceran (Ceriodaphnia dubia).

We conducted acute lethality tests with white sturgeon (Acipenser transmontanus) and Ceriodaphnia dubia exposed to copper and zinc at dissolved organic carbon concentrations ranging from 0.5 to 5.5 mg/L. Dissolved organic carbon had minimal effects on zinc toxicity but did have a protective effect on acute copper toxicity, which was equal to that predicted by the copper biotic ligand model (BLM). The BLM-adjusted copper median effect concentrations for A.transmontanus ranged from 2.4 to 8.2 mg/L. Environ Toxicol Chem 2019;38:2682-2687. Published 2019 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US government work, and as such, is in the public domain in the United States of America.

Extension of biotic ligand model to account for the effects of pH and phosphate in accurate prediction of arsenate toxicity

Biotic ligand model (BLM) was extended to predict the toxicity of inorganic arsenate (iAs(V)) to the luminescent bacteria, Aliivibrio fischeri. As the pH increased from 5 to 9, the HAsO42- form predominated more than the H2AsO4- form did, and the EC50[As]T (50% effective iAs(V) concentration) decreased drastically from 3554 ± 393 to 39 ± 6 μM; thus, the HAsO42- form was more toxic to A. fischeri than H2AsO4-. As the HPO42- activity increased from 0 to 0.44 mM, the EC50{HAsO42-} values (50% effective HAsO42- activity) increased from 31 ± 6 to 859 ± 128 μM, indicating that the toxicity of iAs(V) decreased, owing to the competition caused by the structural similarity between iAs(V) and phosphate ions. However, activities of Ca2+, Mg2+, K+, SO42-, NO3-, and HCO3- did not significantly affect the EC50{HAsO42-} values. The BLM was reconstructed to take into account the effects of pH and phosphate, and the conditional binding constants for H2PO4-, HPO42-, H2AsO4-, and HAsO42- to the active binding sites of A. fischeri were obtained; 3.424 for logKXH2PO4, 4.588 for logKXHPO4, 3.067 for logKXH2AsO4, and 4.802 for logKXHAsO4. The fraction of active binding sites occupied by iAs(V) to induce 50% toxicity (fmix50%) was found to be 0.616.

Predicting metal toxicity to aquatic life:a first step towards integrating a QICAR approach with the BLM framework

Abstract The biotic ligand model (BLM) is a tool that quantifies the bioavailability of metals, accounting for both metal speciation and hydrogeochemical properties. The BLM assumes that metal ions bind to biologically active sites, the so-called biotic ligands (BLs), and that metal toxicity is caused by the binding of metal ions to BLs. The binding affinity of a given metal ion for the BLs is represented by a conditional binding constant (K). This parameter is typically obtained via metal toxicity testing. However, metal toxicity testing is time consuming, labor intensive, and expensive. Thus, there is an urgent need for a system to predict BLMs for data-poor metals. Herein, relationships between the log K values of known metals (derived from the literature) and metal ionic characters were investigated to construct quantitative ion character-activity relationship (QICAR) equations. The results showed that log K was significantly correlated with the softness index (σp) in rainbow trout (adjusted R2 = 0.901; F = 65.0; p

An improved biotic ligand model (BLM) for predicting Co(II)-toxicity to wheat root elongation: The influences of toxic metal speciation and accompanying ions

In order to explore the effects of pH and accompanying ions on divalent cobalt (Co(II)) toxicity to the wheat root elongation, an improved biotic ligand model (BLM) to predict Co(II) toxicity was developed in solution culture. The results showed that the Co(II)-toxicity decreased with the increases of K+, Ca2+ and Mg2+ activities, and the activity of Na+ had no impact on the Co(II)-toxicity. High H+ activity reduced the Co(II)-toxicity by the competitive effect of H+, while low H+ activity affected the toxicity by the change in the type of Co(II) in culture medium. Co2+ and CoOH+ were toxic to wheat root elongation, and Co(OH)2 was not. On the basis of BLM theory, the conditional equilibrium constants for Co2+, CoOH+, H+, Mg2+, Ca2+, K+ were obtained: logKCoBL = 4.65, logKCoOHBL = 6.62, logKHBL = 4.53, logKMgBL = 3.65, logKCaBL = 2.36 and logKKBL = 2.17. Free Co2+ and CoOH+, and the competitions of K+, Mg2+ and Ca2+ were suggested to be considered when developing the Co(II)-BLM.

Combined effects of waterborne copper exposure and salinity on enzymes related to osmoregulation and ammonia excretion by blue crab Callinectes sapidus.

Copper is essential, but can be toxic to aquatic organisms when present in high concentrations. In freshwater crustaceans, copper inhibits enzymes related to ionic and osmoregulation and to the ammonia efflux, that leads to Na+ imbalance and inhibition of ammonia excretion. In the animals inhabiting estuarine or seawater, mechanisms of copper toxicity is not clear, but had been described as disruption of ionregulation and metabolism. To clarify the mechanism of copper toxicity in crustaceans inhabiting variable salinity, this work investigated whether copper affects ammonia excretion and enzymes used for ammonia balance and osmoregulation in the blue crab Callintectes sapidus acclimated to salinity 2 and 30 ppt. To achieve this, juveniles of the blue crab were exposed to 63.5 µg/L of copper at both salinities for 96 h. This is an environmentally realistic copper concentration. Results of ammonia efflux, free amino acids and Na+ concentrations in hemolymph, Na+/K+-ATPase, H+-ATPase and, carbonic anhydrase (CA) activities in gills were consistent with the osmoregulatory pattern adopted by the blue crab, which hyperosmoregulates at salinity 2 ppt and osmoconforms at 30 ppt. At 30 ppt copper reduced free amino acid in hemolymph of crabs, suggesting an effect of the metal on osmotic performance. At 2 ppt, copper significantly increased the H+-ATPase activity involved in ammonia excretion. This may be a compensatory response of crabs to maintain low levels of ammonia in their hemolymph; which can be increased by copper exposure. Results presented here are useful for the improvement of the Biotic Ligand Model (BLM) to predict copper toxicity for saltwater environments.

Model construction for estimating potential vulnerability of Japanese soils to cadmium pollution based on intact soil properties.

Prediction of heavy metal bioavailability in intact soil is important to manage soil pollution risks. We developed a regression model for representative Japanese soils to judge their potential vulnerability to cadmium (Cd) pollution. We added four rates of Cd to 17 sample soils to mimic artificial contamination. After aging the contaminated soils, we measured Cd’s bioavailability using the diffusive gradients in thin-films (DGT) technique. We then evaluated the relationships between bioavailability of Cd ([CdDGT]) and intact soil properties by statistical analyses. Cation exchange capacity (CEC) and pH emerged as significant factors to explain the cadmium bioavailability in Japanese soils. Specifically, lower CEC and lower pH were associated with higher [CdDGT], which poses a higher risk for soil ecosystems. The correlation between pH and [CdDGT] had a high dependence on [CdAdd], whereas that for CEC did not. Regression analysis also showed that the interaction between intact soil pH and spiked concentration ([CdAdd]) had a significant contribution to [CdDGT]. The regression model developed was rationally supported by a biotic ligand model. This simplified but realistic model would be useful in estimating the vulnerability of representative Japanese soils and determining the risk for Japanese soils in relation to Cd contamination.

Unravelling the molecular mechanisms of nickel in woodlice.

During the last few years, there has been an alarming increase in the amount of nickel (Ni) being released into the environment, primarily due to its use in the production of stainless steel but also from other sources such as batteries manufacturing and consequent disposal. The established biotic ligand models provide precise estimates for Ni bioavailability, in contrast, studies describing the mechanisms underpinning toxicological effect of Ni are scarce. This study exploits RNA-seq to determine the transcriptomic responses of isopods using Porcellionides pruinosus as an example of a terrestrial metal-resistant woodlouse. Furthermore, the recently proposed model for Ni adverse outcome pathways (Ni-AOP) presents an unprecedented opportunity to fit isopod responses to Ni toxicity and define Porcellionides pruinosus as a metalomic model.Prior to this study, P. pruinosus represented an important environmental sentinel, though lacking genetic/omic data. The reference transcriptome generated here thus represents a major advance and a novel resource. A detailed annotation of the transcripts obtained is presented together with the homology to genes/gene products from Metazoan and Arthropoda phylum, Gene Ontology (GO) classification, clusters of orthologous groups (COG) and assignment to KEGG metabolic pathways.The differential gene expression comparison was determined in response to nickel (Ni) exposure and used to derive the enriched pathways and processes. It revealed a significant impact on ion trafficking and storage, oxidative stress, neurotoxicity, reproduction impairment, genetics and epigenetics. Many of the processes observed support the current Ni-AOP although the data highlights that the current model can be improved by including epigenetic endpoints, which represents key chronic risks under a scenario of Ni toxicity.

The direct effects of a tropical natural humic substance to three aquatic species and its influence on their sensitivity to copper.

Few studies have been conducted so far into the effects of humic substances (HS) on aquatic organisms and their influence on the toxicity of chemical pollutants in the tropics. The aim of the present study was therefore to evaluate the direct effects of locally-derived tropical natural HS on the cladoceran Daphnia similis, the midge Chironomus xanthus and the fish Danio rerio. The influence of a HS concentration series on the acute toxicity of copper to these organisms was also assessed through laboratory toxicity testing. The HS did not exert direct acute effects on the test organisms, but long-term exposure to higher HS concentrations provoked a stress response (increase in feces production) to D. rerio and exerted effects on chironomid adult emergence and sex ratio. The biotic ligand model proved to be a useful tool in converting total copper concentrations to the appropriate bio-available fraction to which tropical aquatic organisms are exposed.

Iron Modulation of Copper Uptake and Toxicity in a Green Alga ( Chlamydomonas reinhardtii).

Little attention has been paid to the role of essential trace elements on the toxicity of another element. In this work, we examined if low concentrations of essential elements (Co, Mn, Zn, and Fe) modified the response of a freshwater green alga ( Chlamydomonas reinhardtii) to copper. To do so, we followed cell growth over 72 h in exposure media where the essential element concentrations were manipulated. Among these elements, iron proved to have a strong impact on the cells' response to copper. The free Cu2+ concentrations required to inhibit cellular growth by 50% (EC50) over 72 h decreased from 2 nM in regular Fe medium (10-17.6 M Fe3+) to 4 pM in low iron medium (10-19.0 M Fe3+); a 500-fold increase in toxicity. Moreover, at low Cu2+ concentrations (10-13.0 to 10-10.5 M), Cu uptake increased under low iron conditions but remain relatively stable under regular iron conditions. These results show clearly that iron plays a protective role against copper uptake and toxicity to C. reinhardtii. In freshwaters, iron is always abundant but the expected free iron concentrations in surface waters can vary between 10-14.0 to 10-20.0 M, depending on pH (e.g., when pH increases from 6 to 8). We conclude that copper toxicity in natural waters can be modulated by iron and that, in some conditions, the Biotic Ligand Model may need to be further developed to account for the influence of iron.

Inter-clonal Variation in Copper Sensitivity in Bosmina longirostris with Different Exposure Histories

An acquisition of metal tolerance in cladocerans related to the historical exposure has been well documented for the genera Daphnia and Ceriodaphnia, which are frequently used in ecotoxicological studies. However, small-sized cladocerans are rarely investigated for the inter-clonal variation in metal sensitivity, whereas they often dominate zooplankton community in many lakes and ponds, and even in eutrophicated rivers. We investigated the influence of historical copper exposure on the copper sensitivity of Bosmina longirostris. Copper sensitivity was compared among three clones originating from a site (Lake Yanaka), which located at downstream of historically contaminated river (Watarase River) and clones from five different reservoirs. For reference, the background copper concentration (as Cu2+ activity) at each site and its toxicity to Daphnia magna were estimated by metal speciation and the biotic ligand model (BLM), respectively. Less copper-sensitive Bosmina clones were obtained only from Lake Yanaka, although the background copper concentrations were far below the lethal levels. The results suggested the variability in copper-sensitivity in B. longirostris and its association with historical copper contamination.

The performance of a free surface and metal-removing constructed wetland: How a young wetland becomes mature

The H-02 wetland was constructed on the Savannah River Site (SC, USA) in 2007 to remove Cu and Zn from process and storm runoff water. Water samples were routinely collected from 2008 to 2017 in the primary discharge pipes, influent, effluent, and receiving stream. Water quality parameters were measured in situ, total and dissolved metal (Cu and Zn) concentrations were determined, and metal speciation was calculated with the Biotic Ligand Model. Generally, the H-02 wetland system functioned well during the reporting period. Alkaline waters in the pipe became neutral after running through the wetland. Total metal concentrations in the effluent were lower than the regulatory discharge limit with a removal efficiency of 63.8% (95% CI: 60.8–66.8%) for Cu and 70.5% (95% CI: 67.0–73.9%) for Zn. Reduced metal toxicity was inferred from the observed lower concentrations of free Cu2+ and Zn2+ in the effluent than that in the influent and pipe. As suggested by the higher percentage of Cu-organic matter complexes than Zn-organic matter complexes, Cu had a higher affinity for natural organic ligands than Zn. A three-stage performance pattern of a metal-removing constructed wetland was hypothesized, which includes the plateau stage, the trough stage, and the recovery stage. Further studies were needed to test this hypothesis, monitor the treatment efficiency, improve operation strategies, and ensure the long-term sustainability of H-02 wetland system.

The cation competition and electrostatic theory are equally valid in quantifying the toxicity of trivalent rare earth ions (Y3+ and Ce3+) to Triticum aestivum

There is a lack of appropriate models to delineate the toxicity of rare earth elements (REEs) while taking into account the factors that affect bioavailability. Here, standardized wheat (Triticum aestivum L.) root elongation tests were conducted to examine the impact of exposure conditions (i.e., varying Ca, Mg, Na, K and pH levels) on Y and Ce toxicity. Cation competition and electrostatic theory were examined for their applicability in explaining the observed variations in toxicity. Only Ca2+ and Mg2+ significantly alleviated the toxicity of Y3+ and Ce3+, while Na+, K+ and H+ showed no significant effects. Based on the cation competition, the derived binding constants for the hypothetical biotic ligands of wheat logKCaBL, logKMgBL, logKYBL, and logKCeBL were 3.87, 3.59, 6.70, and 6.48, respectively. The biotic ligand model (BLM) succeeded in predicting toxicities of Y and Ce, with more than 93% of the variance in toxicity explained. Given the BLM requires large data sets for deriving model parameters, attempts were further made to explore a simpler electrostatic based model to quantify REEs toxicity. The results demonstrated that the predictive capacity of the electrostatic approach, which considers ion activities at the plasma membrane surface, was comparable to that of BLM with at least 87% of the variations in toxicity explained. This suggested that the electrostatic theory can serve as a surrogate to BLM in modeling Y and Ce toxicities. Therefore, we recommend the BLM and electrostatic-based model as effective approaches to incorporate bioavailability in quantifying REEs toxicity in the presence of various levels of other major cations.