Enhanced electrokinetic remediation of polluted soils by anolyte pH conditioning

Abstract

In the treatment of a polluted soil, the pH has a strong impact on the development of different physicochemical processes as precipitation/dissolution, adsorption/desorption or ionic exchange. In addition, the pH determines the chemical speciation of the compounds present in the system and, consequently, it conditions the transport processes by which those compounds will move. This question has aroused great interest in the development of pH control technologies coupled to soil remediation processes. In electrokinetic remediation processes, pH has usually been controlled by catholyte pH conditioning with acid solutions, applied to cases of heavy metals pollution. However, this method is not effective with pollutants that can be dissociated in anionic species. In this context, this paper presents a study of the electrokinetic remediation of soils polluted with 2,4-Dichlorophenoxyacetic acid, a common polar pesticide, enhanced with an anolyte pH conditioning strategy. A numerical study is proposed to evaluate the effectiveness of the strategy. Several numerical tests have been carried out for NaOH solutions with different concentrations as pH conditioning fluid. The results show that the anolyte pH conditioning strategy makes it possible to control the pH of the soil and, consequently, the chemical speciation of pollutant species. Thus, it is possible to achieve an important flux of pesticide into the anolyte compartment (electro-migration of anionic species and diffusive transport of acid species). This way, it possible to maximise the pesticide accumulation in this compartment, allowing a much more effective removal of pollutants from the soil than without the anolyte pH conditioning strategy.

Calcite buffer effects in electrokinetic remediation of clopyralid-polluted soils

“…The pH of the soil pore water affects the chemical speciation of the substances contained in the soil [22,57], so acidic species are present in the low pH soil zone, generally between CA and MpHGP in EKR processes. In the case studied, Clopyralidacid species are concentrated in this zone, favouring the transport of these species to the anodic compartment mainly by diffusion [36]. Furthermore, Clopyralidanion species, in equilibrium with acidic species, also accumulate in this zone and is susceptible to being moved towards the anodic compartment mainly by electromigration [36].…”

“…In the case studied, Clopyralidacid species are concentrated in this zone, favouring the transport of these species to the anodic compartment mainly by diffusion [36]. Furthermore, Clopyralidanion species, in equilibrium with acidic species, also accumulate in this zone and is susceptible to being moved towards the anodic compartment mainly by electromigration [36]. For these reasons, the performance of the EKR treatment of soils with this type of pollutant (acid pollutant with anionic dissociation) improves when the volume of the acid soil zone is reduced as has been proven in previous works [36].…”

“…The level of electrolyte in compartments is constant during the tests by adding anolyte through gravity and extracting the catholyte by overflow of the CC. This modelled experimental setup has been outlined in detail in preceding studies [26,36,37]. Fig.…”

“…The M4EKR model has been implemented in COMSOL Multiphysics and is capable of simulating the general behaviour trends of an EKR process by solving a coupled problem of reactive transport and electric field. This makes the M4EKR module a powerful computational tool that has been used to carry out different studies [36][37][38].…”

Transient reactive transport model for physico-chemical transformation by electrochemical reactive barriers

“…This is mainly explained by effects of pH on chemical speciation as well as inducing physicochemical processes such as precipitation/dissolution, adsorption and ionic exchange between solid matrix and electrolyte ( e.g. [17–19]). For groundwater treatment, pH and redox shifts can be manipulated to support various transformation/degradation mechanisms ( e.g.…”