Process identification and model development of contaminant transport in MSWI bottom ash

In this work we investigate to what extent we are able to predict experimental data on column leaching of heavy metals from municipal solid waste incinerator (MSWI) bottom ash, using the current knowledge on processes controlling aqueous heavy metal concentrations in combination with a multicomponent reactive transport computer model. Heavy metal concentrations were modelled with a surface complexation model for metal sorption to (hydr)oxide minerals in the bottom ash matrix. For transport modelling it was necessary to simplify the sorption modelling approach. Therefore, we determined a minimal set of components and species that still provided an adequate description of the pH dependent heavy metal behaviour. The concentration levels of the heavy metals are generally predicted to within one order of magnitude. Discrepancies between the model and the data are caused by uncertainty in modelling parameters and a still insufficient description of the dynamics of macroelement leaching and pH. In general, the simulated leaching curves show much more abrupt changes than the measurements. This observation might be an indication of non-equilibrium. Processes that have to be taken into account for further model development are the influence of non-equilibrium effects and the facilitated transport of heavy metals by dissolved organic matte

Process identification and model development of contaminant transport in MSWI bottom ash

In this work we investigate to what extent we are able to predict experimental data on column leaching of heavy metals from municipal solid waste incinerator (MSWI) bottom ash, using the current knowledge on processes controlling aqueous heavy metal concentrations in combination with a multicomponent reactive transport computer model. Heavy metal concentrations were modelled with a surface complexation model for metal sorption to (hydr)oxide minerals in the bottom ash matrix. For transport modelling it was necessary to simplify the sorption modelling approach. Therefore, we determined a minimal set of components and species that still provided an adequate description of the pH dependent heavy metal behaviour. The concentration levels of the heavy metals are generally predicted to within one order of magnitude. Discrepancies between the model and the data are caused by uncertainty in modelling parameters and a still insufficient description of the dynamics of macroelement leaching and pH. In general, the simulated leaching curves show much more abrupt changes than the measurements. This observation might be an indication of non-equilibrium. Processes that have to be taken into account for further model development are the influence of non-equilibrium effects and the facilitated transport of heavy metals by dissolved organic matte

The leaching of major and trace elements from MSWI bottom ash as a function of pH and time

In this paper, the leaching behaviour of major components (Al, Ca, SO4, Mg, Si, Fe, Na and DOC) and trace elements (Ni, Zn, Cd, Cu, Pb, Mo and Sb) from MSWI bottom ash is studied as a function of time over a wide range of pH, under pH-controlled conditions. Equilibrium geochemical modelling using the modelling framework ORCHESTRA is used to enable a process-based interpretation of the results and to investigate whether 'equilibrium' is attained during the time scale of the experiments. Depending on the element and setpoint-pH value, net concentration increases or decreases of up to one order of magnitude were observed. Different concentration-time trends (increase or decrease) are observed in different pH ranges. The direction of the concentration-time trends depends on: (1) the shape of the 'equilibrium' solubility curve, and (2) the position of the setpoint-pH in the leaching test relative to the natural pH of the sample. Although the majority of the elements do not reach steady state, leached concentrations over a wide pH range have been shown to closely approach 'equilibrium' model curves within an equilibration time of 168 h. The different effects that leaching kinetics may have on the pH dependent leaching patterns have been identified for a wide range of elements, and can generally be explained in a mechanistic way. The results are in support of the currently prescribed equilibration time of 48 h in the European standard for the pH-static leaching test (TS14997). Finally, this study demonstrates that pH-static leaching experiments such as described in the European standards (TS14497 and TS14429), in combination with selective chemical extractions and a mechanistically based modelling approach, constitute a powerful set of tools for the characterization of leaching processes in waste materials over a wide range of conditions

The leaching of major and trace elements from MSWI bottom ash as a function of pH and time

In this paper, the leaching behaviour of major components (Al, Ca, SO4, Mg, Si, Fe, Na and DOC) and trace elements (Ni, Zn, Cd, Cu, Pb, Mo and Sb) from MSWI bottom ash is studied as a function of time over a wide range of pH, under pH-controlled conditions. Equilibrium geochemical modelling using the modelling framework ORCHESTRA is used to enable a process-based interpretation of the results and to investigate whether 'equilibrium' is attained during the time scale of the experiments. Depending on the element and setpoint-pH value, net concentration increases or decreases of up to one order of magnitude were observed. Different concentration-time trends (increase or decrease) are observed in different pH ranges. The direction of the concentration-time trends depends on: (1) the shape of the 'equilibrium' solubility curve, and (2) the position of the setpoint-pH in the leaching test relative to the natural pH of the sample. Although the majority of the elements do not reach steady state, leached concentrations over a wide pH range have been shown to closely approach 'equilibrium' model curves within an equilibration time of 168 h. The different effects that leaching kinetics may have on the pH dependent leaching patterns have been identified for a wide range of elements, and can generally be explained in a mechanistic way. The results are in support of the currently prescribed equilibration time of 48 h in the European standard for the pH-static leaching test (TS14997). Finally, this study demonstrates that pH-static leaching experiments such as described in the European standards (TS14497 and TS14429), in combination with selective chemical extractions and a mechanistically based modelling approach, constitute a powerful set of tools for the characterization of leaching processes in waste materials over a wide range of conditions

EoW criteria for waste-derived aggregates

Waste-derived aggregates are being considered as possible candidates for development of End-of-Waste (EoW) criteria at European Union (EU) level in accordance with Article 6 (1) of the EU Waste Framework Directive (2008/98/EC) as a means of increasing the recovery of resources from waste. If a waste-derived aggregate achieves EoW status, it will become a (construction) product and hence be regulated by the Construction Products Regulation (CPR) which means that in most EU Member States there will be no applicable environmental protection regulation. It is therefore important that the criteria a waste-derived aggregate must fulfil to achieve and maintain EoW status ensure sufficient protection of the environment and human health. It is shown that EoW criteria that do not include restrictions on the conditions of the use of waste-derived aggregates for specific construction purposes will result in leaching limit values that are so stringent that very few, if any, waste-derived aggregates can meet them. It is therefore proposed to impose restrictions and conditions on the use as part of possible future EoW criteria for waste-derived aggregates, and a step-wise methodology for development of more realistic leaching limit values for EoW is outlined. The methodology incorporates the mitigating effects of various measures that reduce the potential environmental impact of construction applications with waste-derived aggregates. Recommendations are also made of the practical testing and documentation procedures for aggregates with EoW status within the framework of the CPR

A consistent geochemical modelling approach for the leaching and reactive transport of major and trace elements in MSWI bottom ash

To improve the long-term environmental risk assessment of waste applications, a predictive "multi-surface" modelling approach has been developed to simultaneously predict the leaching and reactive transport of a broad range of major and trace elements (i.e., pH, Na, Al, Fe, Ca, SO4, Mg, Si, PO4, CO3, Cl, Ni, Cu, Zn, Cd, Pb, Mo) and fulvic acids from MSWI bottom ash. The geochemical part of the model approach incorporates surface complexation/precipitation on Fe/Al (hydr)oxides, complexation with humic and fulvic acids (HA and FA, respectively) and mineral dissolution/precipitation. In addition, a novel approach is used to describe the dynamic leaching of FA, based on the surface complexation of FA on Fe/Al (hydr)oxides. To enable reactive transport calculations, the geochemical part of the model is combined with advective/dispersive transport of water and first-order mass transfer between mobile and stagnant zones. Using a single, independently determined set of input parameters, adequate model predictions are obtained for the leaching of a broad range of elements under widely different conditions, as verified with data from the European standardised pH-static and percolation leaching tests (TS 14997 and TS 14405, respectively). The percolation tests were operated at different flow velocities and with flow interruptions to enable verification of the local equilibrium assumption. Although the combination of experimental and modelling results indicates that the leaching of major solubility-controlled elements occurs largely under local equilibrium conditions, this study has led to the identification of physical non-equilibrium processes for non-reactive soluble salts, as well as possible sorption-related non-equilibrium processes for the leaching of Mo, FA and associated trace metals. Further improvement of the reactive transport model can be achieved by a more mechanistic description of the (dynamic) leaching behaviour of humic substances. As the modelling approach outlined in this study is based on the fundamental processes that underlie leaching, the approach is expected to be also applicable to other granular contaminated materials application scenarios and conditions. Therefore, the combination of standardized leaching test methods, selective chemical extractions and mechanistic modelling, constitutes a promising generic approach to assess the long-term environmental impact of the application of granular contaminated materials in the environment

Arsenic, antimony and vanadium in the North Atlantic Ocean

New measurements of arsenic, antimony and vanadium in samples from vertical profiles at nine stations between 50° and 70°N in the Atlantic Ocean are presented. Antimonate concentrations exhibit little variation with depth and location, the average concentration being 1.16 nmol 1-1. The vertical distribution of arsenate is similar to that reported for the South Atlantic and Pacific Ocean, with somewhat lower surface concentrations (14 to 18 nmol 1-1) and uniform deep water concentrations (mean, 19 nmol 1-1). Uniform vertical profiles for vanadium with a mean concentration of 32.6 nmol 1-1 are in agreement with data for the Central Atlantic, but differ from profiles in the Pacific which exhibit surface water depletion