We present a numerical model to compute heavy metal transport and geochemical reactions during electrokinetic remediation of contaminated marine sediments. The model describes the contaminant transport driven by chemical and electrical gradients, as well as the effect of surface reactions, speciation of chemical species and their interaction. In the model we consider only the main phenomena which most experiment recognised as main contributors to the decontamination of saline sediments. The model accounts for: (1) chemical species transport through the porous matrix by electromigration and electroosmosis, (2) induced pH changes, (3) pH-dependent adsorption of H+ and contaminants onto sediment particle surfaces, (4) aqueous solution speciation (i.e. formation/dissolution of complexes and solid precipitates) and interaction between multiple chemical species. The following simplifications and assumptions were made: constant hydraulic head, saturated porous medium, constant sediment volume and constant voltage gradient. The latter strict condition was verified during laboratory experiments, which showed voltage gradient to be sufficiently stable over the entire duration of the tests. The model was numerically implemented with a finite element software (COMSOL) and coupled with the PHREEQC USGS code for geochemical equilibrium and surface reaction calculations. A two-step non-iterative sequential scheme was used to calculate transport (first step) and reactions (second step) at each time interval. All chemical and geochemical reactions are assumed to be fast enough to reach their chemical equilibrium at each time interval of the numerical integration. Model parameters were either derived from literature (tortuosity, diffusion and ion mobility coefficients) or calibrated through laboratory batch tests and bench-scale electrokinetic tests (surface complexation equilibrium constants, porosity). The outputs of the model are the concentration of each considered chemical species during remediation as a function of time and space. Model predictions are finally compared with experimental results.
Modelling heavy metal trasport and geochemical effects during marine sediment electroremediation
MASI, MATTEO;CECCARINI, ALESSIO;IANNELLI, RENATO
2015-01-01
Abstract
We present a numerical model to compute heavy metal transport and geochemical reactions during electrokinetic remediation of contaminated marine sediments. The model describes the contaminant transport driven by chemical and electrical gradients, as well as the effect of surface reactions, speciation of chemical species and their interaction. In the model we consider only the main phenomena which most experiment recognised as main contributors to the decontamination of saline sediments. The model accounts for: (1) chemical species transport through the porous matrix by electromigration and electroosmosis, (2) induced pH changes, (3) pH-dependent adsorption of H+ and contaminants onto sediment particle surfaces, (4) aqueous solution speciation (i.e. formation/dissolution of complexes and solid precipitates) and interaction between multiple chemical species. The following simplifications and assumptions were made: constant hydraulic head, saturated porous medium, constant sediment volume and constant voltage gradient. The latter strict condition was verified during laboratory experiments, which showed voltage gradient to be sufficiently stable over the entire duration of the tests. The model was numerically implemented with a finite element software (COMSOL) and coupled with the PHREEQC USGS code for geochemical equilibrium and surface reaction calculations. A two-step non-iterative sequential scheme was used to calculate transport (first step) and reactions (second step) at each time interval. All chemical and geochemical reactions are assumed to be fast enough to reach their chemical equilibrium at each time interval of the numerical integration. Model parameters were either derived from literature (tortuosity, diffusion and ion mobility coefficients) or calibrated through laboratory batch tests and bench-scale electrokinetic tests (surface complexation equilibrium constants, porosity). The outputs of the model are the concentration of each considered chemical species during remediation as a function of time and space. Model predictions are finally compared with experimental results.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
