The effect of manipulating sediment pH on the porewater chemistry of copper- and zinc-spiked sediments

Griffith University, Southport, Queensland, Australia
Chemosphere (Impact Factor: 3.34). 11/2007; 69(7):1089-99. DOI: 10.1016/j.chemosphere.2007.04.029
Source: PubMed


Spiking of sediment with metal cations that readily hydrolyse causes the sediment pH to decrease. Displaced iron and manganese also oxidise and hydrolyse, further lowering sediment pH. The lower pH of metal-spiked sediments requires a subsequent sediment neutralisation. This research compared the pH adjustment of Cu- and Zn-spiked sediments using single and multiple additions of 1M NaOH. Sediment pH, redox potential, and porewater metal concentrations were monitored over 40 days. Depth profiles were also measured to investigate stratification. A single pH adjustment to pH 7 and 8 initially counteracted the pH change caused by metal additions, however, pH continued to decrease slowly thereafter. Multiple pH adjustments diminished porewater Cu, Zn and Fe concentrations to a greater extent than a single pH adjustment, but the ongoing oxidative precipitation of porewater metals continued to consume OH(-) ions and impede pH maintenance. Displacement of high iron(II) concentrations and the opposing rates of iron(II) oxidative precipitation and bacterially-mediated iron(II) production, affected the partitioning of the added metals between the sediment and pore water. Despite similar pH over the spiked-metal concentration gradient following pH adjustment, sediments spiked with higher metal concentrations produced lower porewater Fe concentrations, possibly due to toxicity to iron(III) oxyhydroxide reducing bacteria. Distinct stratification of redox potential and dissolved Fe and Cu developed over a depth of 6cm during the 40-day equilibration period. Recommendations are provided on methods for preparing metal-spiked sediments in which the partitioning of metals between dissolved and particulate phases better resembles that of in situ (field) metal-contaminated sediments.

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Available from: Stuart L Simpson, Dec 14, 2013
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    • "Decreases in pH in samples spiked with all four metals could also have affected uptake and accumulation by embryos exposed to these sediments. The observed lesser pH was likely a consequence of hydrolysis of the added metals, including the displacement of Fe(II) from particulate material by the applied metals followed by oxidative hydrolysis, as well as the competitive displacement of protons from organic matter and metalbinding sites (Simpson et al. 2004; Hutchins et al. 2007). "
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    ABSTRACT: Predicting the bioavailability and effects of metals in sediments is of major concern in context with sediment risk assessment. This study aimed to investigate the bioavailability and molecular effects of metals spiked into riverine sediments to zebrafish (Danio rerio) embryos. Embryos were exposed to a natural and an artificial sediment spiked with cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn) individually or as a mixture at concentrations ranging from 150 to 3000 mg/kg dry weight (dw) over 48 h, and uptake of metals was determined. Furthermore, transcript abundances of the metallothioneins MT1 and MT2, the metal-responsive element-binding transcription factor (MTF) and the genes sod1, hsp70 and hsp90α1 were measured as indicators of metal-induced or general cellular stress. D. rerio embryos accumulated metals from sediments at concentrations up to 100 times greater than those spiked to the sediment with the greatest bioaccumulation factor (BAF) for Cu from artificial sediment (275.4 ± 41.9 (SD)). Embryos accumulated greater concentrations of all metals from artificial than from natural sediment, and accumulation was greater when embryos were exposed to individual metals than when they were exposed to the mixture. Exposure of embryos to Zn or the mixture exhibited up to 30-fold greater transcript abundances of MT1, MT2 and hsp70 compared to controls which is related to significant uptake of Zn from the sediment. Further changes in transcript abundances could not be related to a significant uptake of metals from sediments. These studies reveal that metals from spiked sediments are bioavailable to D. rerio embryos directly exposed to sediments and that the induction of specific genes can be used as biomarkers for the exposure of early life stages of zebrafish to metal-contaminated sediments.
    Full-text · Article · Sep 2015 · Environmental Science and Pollution Research
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    • "In principle, the toxicity of sediments spiked with known concentrations of a metal or metal mixture (added as highly-dissociable salts that allow the metal cations to interact with sediment binding sites) can provide a basis for estimating the contribution of metal(s) to toxicity of sediments collected in the field (e.g., Milani et al., 2003) or for establishing sediment quality guidelines for individual metals (Besser et al., 2013; Vangheluwe et al., 2013). The environmental realism of toxicity tests with metal-spiked sediments depends on many factors, including the metal-binding characteristics of the sediment(s), the methods used to avoid artifacts of metal spiking (e.g., pH control), and the duration of the post-spiking equilibration period (Brumbaugh et al., 2013; Hutchins et al., 2007, 2008). "
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    ABSTRACT: This paper reviews methods for testing the toxicity of metals associated with freshwater sediments, linking toxic effects with metal exposure and bioavailability, and developing sediment quality guidelines. The most broadly applicable approach for characterizing metal toxicity is whole-sediment toxicity testing, which attempts to simulate natural exposure conditions in the laboratory. Standard methods for whole-sediment testing can be adapted to test a wide variety of taxa. Chronic sediment tests that characterize effects on multiple endpoints (e.g., survival, growth, and reproduction) can be highly sensitive indicators of adverse effects on resident invertebrate taxa. Methods for testing of aqueous phases (pore water, overlying water, or elutriates) are used less frequently. Analysis of sediment toxicity data focuses on statistical comparisons between responses in sediments from the study area and responses in one or more uncontaminated reference sediments. For large or complex study areas, a greater number of reference sediments is recommended to reliably define the normal range of responses in uncontaminated sediments -- the ‘reference envelope’. Data on metal concentrations and effects on test organisms across a gradient of contamination may allow development of concentration-response models, which estimate metal concentrations associated with specified levels of toxic effects (e.g. 20% effect concentration or EC20). Comparisons of toxic effects in laboratory tests with measures of impacts on resident benthic invertebrate communities can help document causal relationships between metal contamination and biological effects. Total or total-recoverable metal concentrations in sediments are the most common measure of metal contamination in sediments, but metal concentrations in labile sediment fractions (e.g., determined as part of selective sediment extraction protocols) may better represent metal bioavailability. Metals released by the weak-acid extraction of acid-volatile sulfide (AVS), termed simultaneously-extracted metals (SEM), are widely used to estimate the ‘potentially-bioavailable’ fraction of metals that is not bound to sulfides (i.e., SEM-AVS). Metal concentrations in pore water are widely considered to be direct measures of metal bioavailability, and predictions of toxicity based on pore-water metal concentrations may be further improved by modeling interactions of metals with other pore-water constituents using Biotic Ligand Models. Data from sediment toxicity tests and metal analyses has provided the basis for development of sediment quality guidelines, which estimate thresholds for toxicity of metals in sediments. Empirical guidelines such as Probable Effects Concentrations or (PECs) are based on associations between sediment metal concentrations and occurrence of toxic effects in large datasets. PECs do not model bioavailable metals, but they can be used to estimate the toxicity of metal mixtures using by calculation of probable effect quotients (PEQ=sediment metal concentration/PEC). In contrast, mechanistic guidelines, such as Equilibrium Partitioning Sediment Benchmarks (ESBs) attempt to predict both bioavailability and mixture toxicity. Application of these simple bioavailability models requires more extensive chemical characterization of sediments or pore water, compared to empirical guidelines, but may provide more reliable estimates of metal toxicity across a wide range of sediment types.
    Full-text · Article · Jun 2014 · Applied Geochemistry
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    • "Often, high dissolved concentrations remain in the pore waters, burrow waters, or overlying waters due to decreases in water pH, changes in sediment redox potential, slow reaction kinetics, or slow replenishment rate of overlying water volume (Simpson et al. 2004; Brumbaugh et al. 2013). Along with abnormally high pore water contaminant concentrations, other cations that are relatively weakly bound within the particulate phase, like calcium and magnesium in freshwater sediments, may also be displaced, leading to dissolved concentrations that are much higher than typical field-contaminated sediments (Lee et al. 2000; Simpson et al. 2004; Burton et al. 2006; Hutchins et al. 2007, 2008). An increase in calcium and magnesium ions in the dissolved phase is expected to decrease the bioavailability of other metals due to competitive displacement and physiological effects on membrane permeability (Calamari et al. 1980; Bradley and Sprague 1985; Welsh et al. 2000). "
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    ABSTRACT: Understanding the effects of sediment contaminants is pivotal to reducing their impact in aquatic environments. Outdoor mesocosms enable us to decipher the effects of these contaminants in environmentally realistic scenarios, providing a valuable link between laboratory and field experiments. However, because of their scale, mesocosm experiments are often complex to set up and manage. The creation of environmentally realistic conditions, particularly when using artificially contaminated sediment, is one issue. Here, we describe changes in geochemistry over 1.5 years of a sediment spiked with four different concentrations of copper, within a large freshwater mesocosm facility. The spiking procedure included proportional amendments with garden lime to counteract the decreases in pH caused by the copper additions. The majority of copper within the spiked mesocosm sediments partitioned to the particulate phase with low microgram per liter concentrations measured in the pore waters and overlying waters. The minimum partition coefficient following equilibration between pore waters and sediments was 1.5 × 10(4) L/kg, which is well within the range observed for field-contaminated sediments (1 × 10(4) to 1 × 10(6) L/kg). Recommendations are made for the in situ spiking of sediments with metals in large outdoor mesocosms. These include selecting an appropriate sediment type, adjusting the pH, allowing sufficient equilibration time, and regular mixing and monitoring of metal partitioning throughout the experimental period.
    Full-text · Article · Feb 2014 · Environmental Science and Pollution Research
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