Highlights (3 to 5 bullet points, maximum 85 characters, including spaces) Trace element time-dependent release based on first order series reactions. The model fits initial delay of Zn, Pb, Cd and Cu and adsorption of As and Cr. The model is valid when applied to sediments with different levels of oxidation. Prediction of time-dependent leaching behaviour in scenarios of acidification.
The contaminant release from estuarine sediment due to pH changes was investigated using a modified CEN/TS 14429 pH-dependence leaching test. The test is performed in the range of pH values of 0-14 using deionised water and seawater as leaching solutions. The experimental conditions mimic different circumstances of the marine environment due to the global acidification, carbon dioxide (CO2) leakages from carbon capture and sequestration technologies, and accidental chemical spills in seawater. Leaching test results using seawater as leaching solution show a better neutralisation capacity giving slightly lower metal leaching concentrations than when using deionised water. The contaminated sediment shows a low base-neutralisation capacity (BNCpH 12 = -0.44 eq/kg for deionised water and BNCpH 12 = -1.38 eq/kg for seawater) but a high acid-neutralisation capacity when using deionised water (ANCpH 4 = 3.58 eq/kg) and seawater (ANCpH 4 = 3.97 eq/kg). Experimental results are modelled with the Visual MINTEQ geochemical software to predict metal release from sediment using both leaching liquids. Surface adsorption to iron- and aluminium-(hydr)oxides was applied for all studied elements. The consideration of the metal-organic matter binding through the NICA-Donnan model and Stockholm Humic Model for lead and copper, respectively, improves the former metal release prediction. Modelled curves can be useful for the environmental impact assessment of seawater acidification due to its match with the experimental values.
Carbon dioxide (CO) Capture and Storage (CCS) is a technology to reduce the emissions of this gas to the atmosphere by sequestering it in geological formations. In the case of offshore storage, unexpected CO leakages will acidify the marine environment. Reductions of the pH might be also caused by anthropogenic activities or natural events such as acid spills and dredging operations or storms and floods. Changes in the pH of the marine environment will trigger the mobilisation of elements trapped in contaminated shallow sediments with unclear redox boundary. Trace element (As, Cd, Cr, Cu, Ni, Pb and Zn) release from anoxic and oxic estuarine sediment is analysed and modelled under different laboratory acidification conditions using HNO (l) and CO (g): acidification at pH = 6.5 as worst-case scenario in events of CO leakages and acid spills, and acidification at pH = 7.0 as a seawater scenario under CO leakages, acid spills, as well as sediment resuspension. The prediction of metal leaching behaviour appear to require sediment specific and site specific tools. In the present work it is demonstrated that the proposed three in-series reactions model predicts the process kinetics of the studied elements under different simulated environmental conditions (oxidation levels and acid sources). Differences between HNO and CO acidification are analysed through the influence of the CO gas on the ionic competition of the medium. The acidification with CO provokes higher released concentrations from the oxic sediment than from the anoxic sediment, except in the case of Zn, which influences the release of the other studied elements. Slight acidification can endanger the aquatic environment through an important mobilisation of contaminants. The obtained prediction of the contaminant release from sediment (kinetic parameters and maximum concentrations) can contribute to the exposure assessment stage for risk management and preincidental planning in accidental CO leakages and chemical spills scenarios.
CO 2 seawater acidification by CCS-simulated leakage: Kinetic modelling of Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediment using pH-static leaching tests a b s t r a c t A modified pH-dependent leaching test with continuous pH control that employed CO 2 to acidify a seawater-sediment mixture is used to address Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediments under the influence of acidification processes. Long-term (480 h) leaching experiments at pH values of 7.0, 6.5 and 6.0 are performed. The different evolutionary patterns of the redox potential and Fe release at pH = 6 with respect to the other pH values shows the need to assess the influence of the initial Fe content in seawater upon elemental release. Hence, assays at pH = 6.0 are conducted using natural seawater with Fe concentrations between 9.02 and 153 g/L. A set of in-series reactions for trace elements, Fe and other ions associated with Fe is proposed to model a Fe/multi-ion-dependent mechanism for trace metal release. The maximum concentration of each contaminant that can be released from the sediment and the kinetic parameters of the proposed model are completed for the studied pH values, for good consistency between the experimental and simulated mobilisation of each studied element.
This study provides a better knowledge of key parameters controlling the mobility of Dissolved Organic Carbon (DOC), As, Cd, Cr, Cu, Ni, Pb and Zn from contaminated marine sediment in contact with acidified seawater using static and dynamic standard leaching tests. These procedures have been modified in order to use different leaching agents, L/S ratios, contact times and pH values that simulate seawater acidification under CO 2 leakages scenarios. Studied sediment from a potential area of CO 2 storage, shows a high acid neutralisation capacity (ANC pH=4 =3.58 eq/kg) for deionised water as well as for seawater (ANC pH=4 =3.97 eq/kg). The availability control mechanism is shown by releasing Cd with seawater at pH values 6, 7 and 8 and by the releasing of Zn with seawater at pH 6; the solubility control mechanism appears for Ni release using natural seawater. Experimental results of metal release from the pH dependence leaching test are modelled with Visual MINTEQ geochemical software to predict metal release from sediment, obtaining minor differences with experimental values. An improvement in the metal release results has been obtained considering in the model the influence of the DOC, Fe-and Al-(hydr)oxides, humic acids and fulvic acids. The obtained results would be useful as a line of evidence input for the risk assessment of a Carbon Capture and Storage site where acidified seawater at different concentrations of CO 2 is in contact with sediment.
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