Nickel speciation and ecotoxicity in European natural surface waters: development, refinement and validation of bioavailability models

The accurate prediction of Ni ecotoxicity in natural surface water with bioavailability models such as the biotic ligand model (BLM) depends on how well these models can predict both the speciation of Ni (i.e. Ni2+ concentration), the toxicity of Ni2+ ions to an organism, and the effects of water chemistry parameters thereupon, such as dissolved organic carbon (DOC), pH, and water hardness. The overall aim of the study was to calibrate existing speciation models to Ni speciation in natural surface waters and to use these data to validate and/or refine bioavailability models for aquatic organisms from three trophic levels, i.e. algae, invertebrates (daphnids), and fish. The developed chronic Ni toxicity models for daphnids, fish and algae exhibit sufficiently high predictive capacities to yield a marked reduction of uncertainty associated with differences in chronic Ni bioavailability among different test waters. This is due to the fact that they can predict both Ni2+ concentrations as a function of dissolved Ni and water chemistry (mainly DOC, pH, Ca, Mg), as well as the toxicity of the Ni2+ ion as a function of water chemistry (mainly pH, Ca, Mg). The use of the models presented in the present study for normalizing Ni toxicity data will therefore decrease the overall uncertainty of the risk assessment, provided that the variability of bioavailability modifying parameters across different EU regions and water bodies is acknowledged

Migratie van zink in metaalverontreinigde oeverbodems

In the Noorderkempen (northwest Belgium), about 280 km2 land is contaminated with cadmium (Cd) and zinc (Zn) due to a historical pollution by the Zn smelters. Current atmospheric and aquatic emissions of metals are strongly reduced from historic ones, however the environmental contamination remains. Surface water Zn concentrations peak during winter periods, thereby largely exceeding water quality limits. This work was initiated to identify if such peaks originate from Zn that is leached from riparian soils, i.e. soils periodically waterlogged adjacent to rivers. Soil chemical and physical studies and a three-year monitoring program were set-up to address the issue. A batch experiment with sixteen acid sandy soil samples from the region was performed to identify processes explaining dynamics of Zn upon waterlogging. The experiment confirmed contrasting trends in trace metal solubility in response to waterlogging due to counteracting effects of increased pH and increased dissolved Fe(II) concentrations. After two months of waterlogging, pore-water Zn concentrations increased by, on average, a factor of 18 (range 0.6 80, immobilization in only 1 soil) despite an increase in pH. The mobilization was most pronounced in soils with low (background) Zn concentration. It is hypothesized that Zn mobilization results from Fe2+ desorbing Zn2+ from particulate organic matter and from reductive dissolution of Zn bearing Fe/Mn oxyhydroxides. This hypothesis is based on Zn speciation modelling and on a significant positive correlation between the Zn mobilization factor and the soil Fe/organic carbon ration (Fe/OC; r = 0.87, n = 13). The Fe2+ is a significant competing ion determining fate of toxic trace metals in anoxic soils.A column experiment with three contaminated soils was designed to mimic soil profiles of metal contaminated riparian soils. The surface layers were either waterlogged or not to test its effects on subsequent vertical migration of Zn. Pore-water Zn concentrations in the saturated columns were 2.5 to 40-fold lower than in corresponding unsaturated ones in which nitrification and associated acidification mobilized Zn. Zinc in pore-water was explained by the pore-water pH across treatments and time. In the leachates of the saturated treatments, Fe breakthrough was observed, while weighted average Zn concentrations were unaffected (P>0.1) by the topsoil treatments. The Zn immobilization upon waterlogging in the column experiment contrasts the general Zn mobilization in the batch experiment because of different properties of soil samples used, i.e. lower Fe/OC ratio and higher soil Zn concentration in the soil samples, both factors likely leading to a lower fraction of Fe bound Zn in the aerobic soil. Long-term (15 y) monitoring data of river water quality and new data of a three-year field monitoring study show that Zn and Cd concentrations exceed quality standards 4-200-fold. A statistical analysis revealed a significant effect of month of sampling with lower Zn concentrations in winter and higher in summer months. Surface water Zn and Cd concentrations were positively related to Fe and Mn but not to Ca, K or Na concentrations, suggesting that redox reactions and not dilution processes are involved. In ground- and pore-water of the associated riparian soils, correlations between Zn and Fe or Mn, as proxies for reduced conditions, are lacking. Overall, the field study suggested that river sediments and not riparian soils may be the cause of the winter peaks of Zn and Cd in these rivers.A 1-D simulation study was conducted to predict current vertical Zn distributions in riparian soils using the atmospheric emission history of the Zn smelters. Periodic waterlogging in the riparian soils could explain larger dispersion of Zn in riparian soils compared to acid, unsaturated soils but was not able to explain the enrichments of Zn in the deeper soil layers as observed. The vertical Zn distributions were better predicted if horizontal Zn transport was included, this transport occurs via the aquifer from the dry podsols towards the lower horizon of riparian soils. In addition, the area based Zn stock in the riparian soils is 2-8 fold larger than in dry podsols of the same area which corroborates the hypothesis that the riparian soils are a net sink of Zn in the contaminated area by sequestering metals transported via the groundwater.Summarizing, Zn can be immobilized or mobilized during waterlogging because of counteracting effect of pH increase and reductive dissolution of Zn bearing Fe(III)/Mn oxyhydroxides. The net effect depends on soil properties and extent of reduction. The redox reactions are unlikely promoting migration of Zn to the subsurface. Subsurface Zn concentrations in such soils probably originate from horizontal transport and seasonal Zn and Cd release in the surface waters is likely related to in-stream chemical or physical processes.Dankwoord i Summary iii Samenvatting v List of abbreviations vii List of symbols viii Table of contents ix CHAPTER 1 Mobility of zinc and cadmium in riparian soils in response to periodic waterlogging: background, mechanisms and research objectives 1 CHAPTER 2 Mobilization of zinc upon waterlogging riparian podsols is related to reductive dissolution of Fe-minerals 19 CHAPTER 3 Lower mobility of zinc in flooded than in unsaturated soils is related to soil pH in repacked soil columns 45 CHAPTER 4 Seasonal trends of dissolved zinc and cadmium concentrations in surface waters and in associated riparian soils of the Noorderkempen, Belgium 63 CHAPTER 5 The effect of periodic waterlogging on vertical migration of zinc in riparian soil: a simulation study 95 CHAPTER 6 General conclusions and future perspectives 115 Annex 121 Bibliography 127 List of publications 135nrpages: 135status: publishe

Speciation of nickel in surface waters measured with the Donnan membrane technique

The evaluation of the ecotoxicological risk of nickel (Ni) in surface water is hampered by a lack of speciation data. Six surface waters were sampled and speciation of Ni(II) was measured by the Donnan membrane technique (DMT) combined with radiochemical determination of 63Ni. The free Ni2+ ion fraction in the dissolved (two-fold), even when assuming that all dissolved organic matter (DOM) was present as fulvic acid (FA). The impact of several model parameters affecting the prediction of Ni speciation were evaluated, including the solubility product of Fe(OH)3, which affects the Fe competition for complexation by DOM. The best fit (R2=0.88) was obtained by increasing only the distribution term DeltaLK2, which modifies the binding strength of multi-dentate sites, to accommodate the observed dependence of free ion fraction on Ni concentration.status: publishe

Speciation of nickel in surface waters measured with the Donnan membrane technique

The evaluation of the ecotoxicological risk of nickel (Ni) in surface water is hampered by a lack of speciation data. Six surface waters were sampled and speciation of Ni(II) was measured by the Donnan membrane technique (DMT) combined with radiochemical determination of 63Ni. The free Ni2+ ion fraction in the dissolved (two-fold), even when assuming that all dissolved organic matter (DOM) was present as fulvic acid (FA). The impact of several model parameters affecting the prediction of Ni speciation were evaluated, including the solubility product of Fe(OH)3, which affects the Fe competition for complexation by DOM. The best fit (R2=0.88) was obtained by increasing only the distribution term DeltaLK2, which modifies the binding strength of multi-dentate sites, to accommodate the observed dependence of free ion fraction on Ni concentration.Liesbeth Van Laer, Erik Smolders, Fien Degryse, Colin Janssen, Karel A.C. De Schamphelaer

Sn(IV) Sorption onto Illite and Boom Clay: Effect of Carbonate and Dissolved Organic Matter

126Sn is a long-lived fission product and it is important to assess its sorption onto the host rocks surrounding a possible nuclear waste repository. Boom Clay (BC) is under investigation in Belgium as a potential host rock. To better understand Sn(IV) sorption onto the clay minerals constituting BC, sorption of Sn(IV) was here investigated on Illite du Puy (IdP), from pH 3 to 12. Sorption isotherms at pH ~8.4 were acquired in the presence and absence of carbonate, and in the presence and absence of BC dissolved organic matter (DOM). Sn(IV) strongly sorbed on IdP over the full range of the pHs and concentrations investigated. In the presence of carbonates, Sn(IV) sorption was slightly decreased, highlighting the Sn(IV)–carbonate complexation. DOM reduced the Sn(IV) sorption, confirming the strong complexation of Sn(IV) with DOM. The results were modelled with the 2-site protolysis non-electrostatic surface complexation model. The surface complexation constants and aqueous complexation constants with carbonate and DOM were optimized to describe the experimental data. The applicability of the component additivity approach (CAA) was also tested to describe the experimental Sn(IV) sorption isotherm acquired on BC in BC pore water. The CAA did not allow accurate prediction of Sn(IV) sorption on BC, highlighting the high sensitivity of the model to the Sn(IV)-DOM complexation

Sn(IV) Sorption onto Illite and Boom Clay: Effect of Carbonate and Dissolved Organic Matter

126Sn is a long-lived fission product and it is important to assess its sorption onto the host rocks surrounding a possible nuclear waste repository. Boom Clay (BC) is under investigation in Belgium as a potential host rock. To better understand Sn(IV) sorption onto the clay minerals constituting BC, sorption of Sn(IV) was here investigated on Illite du Puy (IdP), from pH 3 to 12. Sorption isotherms at pH ~8.4 were acquired in the presence and absence of carbonate, and in the presence and absence of BC dissolved organic matter (DOM). Sn(IV) strongly sorbed on IdP over the full range of the pHs and concentrations investigated. In the presence of carbonates, Sn(IV) sorption was slightly decreased, highlighting the Sn(IV)–carbonate complexation. DOM reduced the Sn(IV) sorption, confirming the strong complexation of Sn(IV) with DOM. The results were modelled with the 2-site protolysis non-electrostatic surface complexation model. The surface complexation constants and aqueous complexation constants with carbonate and DOM were optimized to describe the experimental data. The applicability of the component additivity approach (CAA) was also tested to describe the experimental Sn(IV) sorption isotherm acquired on BC in BC pore water. The CAA did not allow accurate prediction of Sn(IV) sorption on BC, highlighting the high sensitivity of the model to the Sn(IV)-DOM complexation