Optimal design and operation of HMs removal from soil by EDDS enhanced washing
The subject of the present research work is the optimization of soil-washing processes applied to heavy metal contaminated soils.
The work focuses on the whole cycle of the soil washing technology, including the possible recovery and the proper disposal of the used washing solution.
Both the design and the exploitation of a soil washing treatment are investigated, in order to maximize their efficacy, in terms of cost and process efficiency. At this aim process parameters and reactor configurations are studied in details through lab-scale tests, and the observed kinetics are simulated through mathematical modeling.
Soil samples used for the experimental activity were collected from an agricultural field located in Southern Italy, mainly contaminated by copper.
Among several Aminopolycarboxylate (APC) chelating agents, Ethylenediamine-N,N’-disuccinic acid (EDDS) was selected, for its recognized biodegradability, widely reported in literature, and its efficiency as extracting agent towards several heavy metals, including Cu that was selected as main model metal in this thesis.
Literature review allowed determining the two most important process parameters to be investigated for washing optimization. The two parameters were identified as EDDS:Cu molar ratio and liquid to soil ratio (L/S). In order to investigate the effect of these parameters on process kinetics and Cu extraction yield, batch washing tests in completely stirred tank reactor configuration (CSTR) were carried out.
EDDS:Cu molar ratio increase was found to be able to enhance process efficiency more than L/S increase. Batch tests clearly displayed a first fast kinetic step at the beginning of the treatment, followed by a second slower kinetic extraction step, which lasted until the end of the treatment. According to this observation, an empirical mathematical model based on two-kinetic terms was formulated. Model parameters were firstly calibrated and then validated using two different sets of experimental data. The derived mathematical model was useful to assess the validity of the twokinetic steps process hypothesis, and to provide a tool for process efficiency prediction depending on EDDS:Cu molar ratio and treatment time.
Exploitation costs of the process were minimized studying different treatment configurations. In details two Plug-Flow configurations were analyzed and compared to the CSTR one. The two PlugFlow configurations were simulated using several reactors in series, varying the hydraulic retention time of the reactors, and fractionating the injection of the washing solution. Achieved results displayed improvements in terms of Cu extraction yield and process kinetics for the tested PlugFlow conditions compared to the CSTR one, and showed that the use of a Plug-Flow reactor allows to reduce the amount of required washing solution.
Finally, an electrochemical process was tested for the treatment and the recovery of the spent EDDS solution. Batch tests were carried out to optimize electrochemical process parameters (e.g. current density, washing solution pH and conductivity). The recovered solution was also used for a multiwashing test. Results proved the effectiveness of the electrochemical treatment for EDDS solution recovery and its potential application as technique for EDDS-enhanced soil washing costs reduction.