Numerical approach to modelling pulse-mode soil flushing on a Pb-contaminated soil

Purpose Soil flushing can represent a suitable technology inremediation of soils, sediments and sludge contaminated bypersistent species (e.g. toxic metal). This paper presents a modelspecifically developed to evaluate the feasibility of chelatingagent-enhanced flushing. The model, here applied to the remediationof real Pb-contaminated soils, was conceived also tosimulate an innovative pulse-mode soil flushing technique.Materials and methods The soil flushing application wasfirstly carried out through columns laboratory experiments.Columns were filled with a real Pb-contaminated soil(3,000 mg kg−1 of dry soil) and flushing was operated in apulse mode with different chelating agent dosages (3 and4.3 mmol kg−1soil). Experimental results were used to calibrateand validate the developed reactive transport model thataccounts for transport of ethylenediamine tetraacetic acid(EDTA) and EDTA–Pb chelate complexes, Pb residual concentrationon soil and the reduction in permeability by soildissolution. Determination of hydrodynamic and hydrodispersiveparameters was carried out through a numericalapproach incorporating the use of neural network as interpolatingfunction of breakthrough data obtained by a tracer test.Results and discussion The EDTA dosage strongly influencedthe efficiency in Pb extraction and soil permeability.Cumulative extractions of Pb were found to be 20 and 29 %for the EDTA concentrations of 3 and 4.3 mmol/kg of dry soil,respectively. The soil dissolution caused a significant flow ratedecrease, as a consequence of the increase in chelating agentconcentration. Therefore the recovery phase duration increasedfrom 738 to 2,080 h. The ability of the model insimulating all the examined phenomena is confirmed by agood fit with experimental results in terms of (a) soil permeabilityreduction, (b) eluted Pb and (c) residual Pb in the soil.Conclusions Results highlighted as the model, supported bya preliminary and careful characterization of the soil, can beuseful to assess the feasibility of the flushing treatment(avoiding soil clogging) and to address the choice of theoperating parameters (flow rate, chelating agent dosage andapplication method). On the basis of the present researchresults, a protocol is suggested for in situ soil pulse–flushingapplication.