Soil Washing Optimization, Recycling of the Solution, and Ecotoxicity Assessment for the Remediation of Pb-Contaminated Sites Using EDDS
This paper presents the results of an experimental study aimed at investigating the applicability of ethylenediamine-N,N -disuccinic acid (EDDS) as a washing solution for the remediation of Pb-contaminated soil. All aspects of the treatment are analyzed and optimized, including the reuse and the final disposal of the EDDS spent solution. Different molar concentrations of the washing solutions and the efficiencies of varying solid/liquid ratio are tested at different pH values. More than 90% of
... . More than 90% of the mobile Pb fraction is removed in about 24 h at pH 6. Accordingly, soil toxicity strongly decreases as a consequence of the treatment. The regenerated solution exhibits a reduced, but not negligible, extractive capacity. The total extraction of Pb is approximately 50% of the initial value after one regeneration cycle, and almost 20% after a second regeneration cycle. Respirometric tests, conducted using an activated sludge sampled in a municipal wastewater treatment plant, indicate that the spent solutions are not biodegradable, but they do not exert any toxic effect on the biomass. On the contrary, tests on regenerated solutions displayed the same biodegradability as the fresh ones. also includes the more famous and less expensive ethylenediamine-tetracetic acid (EDTA). As is well known, these acids have the capacity to form stable complexes with metal ions, because of the presence in their structure of carboxylate groups linked to nitrogen atoms, and therefore, they are excellent chelating agents. Unfortunately, most of these acids are characterized by an elevated toxicity    , as well as low biodegradability. As a result, APCs' applicability for soil washing treatment is somehow limited, because of the mentioned concern related to the management of SWSs. However, EDDS, or in particular, one of its stereoisomers [S,S]-EDDS, displayed complete degradation in natural environmental conditions, in less than 10 days [16, 17] . EDDS has been, therefore, widely used for in-site interventions, exploiting its ability to increase the PTMs' mobility, so that the latter are successively removed by soil flushing or phytoextraction [18, 19] . Less numerous, instead, are the off-site and on-site applications  . This is probably not ascribable to the extractive efficiencies, which have always been very high, but to the uncertainties related to the management of the SWSs. While EDDS is highly biodegradable, its metal complexes may not be to the same extent [21, 22] . Moreover, the cost of EDDS solution is quite significant, and therefore, the regeneration and reuse of SWSs could play an important role in decreasing the costs of the process. Both aspects are investigated, in the present paper, referring to a Pb-contaminated site, in order to make the use of EDDS solutions for soil washing treatment in similar studies more appealing and feasible. At this aim, along with the optimization of the washing process operative parameters involving fresh EDDS solutions, the results are presented below related to (i) soil washing conducted with regenerated solution; (ii) respirometric tests simulating the disposal of the SWSs in a municipal wastewater treatment plant; (iii) ecotoxicity tests, carried out on treated and untreated soil. These aspects are rarely considered all together in a single research work, reflecting the uniqueness of this study. The choice of Pb-contaminated soil is due to the large number of sites contaminated by this metal and to the high toxicity of Pb    . Lead, in fact, is used for many industrial processes, including the production of paints, glasses, batteries, explosives, fertilizers, and pesticides. Moreover, Pb has been used, and is still used in many countries, as a fuel additive. It follows that Pb contamination is common, not only in industrial areas, but can also occur in rural areas, and sediments of urban creeks [26, 27] . However, it is also reported that Pb displays limited solubility in soil, as well as low availability for plant uptake, due to the formation of insoluble precipitates [28, 29] . Nonetheless, this Pb is mainly bioavailable for human beings, due to its transfer through the food chain , representing a serious threat, because of its documented toxicity  .