Equilibrium Leaching Tests Research Articles

Stabilization of per- and polyfluoroalkyl substances (PFASs) with colloidal activated carbon (PlumeStop®) as a function of soil clay and organic matter content

The global problem of contamination of drinking water sources and the aquatic environment with per- and polyfluoroalkyl substances (PFASs) originating from highly contaminated soils is addressed in this study. For the first time, a colloidal activated carbon (AC) product (PlumeStop®) was systematically assessed for PFASs stabilization in soil. Colloidal (particle size 0.1–1.1 μm) AC has the advantage that field application is non-intrusive, comprising injection under high pressure in situ at PFAS-contaminated soil hotspots. In the assessment, 10 different soil mixtures with gradually increasing organic carbon and clay fractions were spiked with 18 different PFASs of varying perfluorocarbon chain length and four different functional groups and aged for one year. Equilibrium leaching tests showed that the ability of colloidal AC to increase sorption of PFASs to soil was highly dependent on PFAS perfluorocarbon chain length. The best treatment efficiency was observed for perfluorocarbon chain lengths 6–7 at which colloidal AC resulted in sorption of 81%, 85%, and 86% for perfluorooctanoate (PFOA), 6:2 fluorotelomer sulfonate (6:2 FTSA) and perfluorohexane sulfonate, (PFHxS), respectively. Sorption of individual PFASs decreased significantly (p < 0.05) with increasing organic carbon content in soil treated with colloidal AC indicating stearic hindrance of the ACs pore structure. On the other hand, the sorption of the majority of PFASs increased significantly (p < 0.05) with increasing clay content in colloidal AC-treated soil, which can be explained by increase in surface area that colloidal AC can sorb to. Overall, the results indicate that the colloidal AC product tested can be useful in remediation approaches for certain PFASs under specific field conditions and PFAS contamination.

Influence of leaching conditions on the release kinetics of lead, chromium and nickel from solidified/stabilized cementitious materials

In this work, we investigate, on a laboratory scale, the influence of the leaching conditions on the release of various chemical elements from a cementitious material obtained by solidification of PbO, Ni 2O3 and Cr2O3 with blended cement (CEMII-B32.5, according to European Standards). The pH effects on the pollutants release and the composition of the initial pore solution (target elements: chloride, sulfate, sodium, potassium, calcium, lead, chromium and nickel) were assessed through an experimental procedure consisting of two equilibrium leaching tests [the acid neutralization capacity (ANC) and the pore water (PW) tests] and the maximum mobile fraction (MMF) text. Samples of the same material were submitted in parallel to dynamic leaching tests in order to assess the influence of the boundary conditions (instantaneous liquid/solid (L/S) ratio, solution renewal) on the leaching kinetics of the target elements. The comparison criteria were the leachate saturation state, the released cumulative quantities and the leaching flux. Generally, leachate quantities obtained by the ANC, PW and MMF tests were important. However, the lowest released amount was observed for the monolith leaching test, and leachate saturation slowed down the dynamic release. Finally, experimental results highlighted another important parameter: the influence of the liquid/solid contact type on leaching kinetics.

Mineralogical study and leaching behavior of a stabilized harbor sediment with hydraulic binder

The environmental assessment of potential effects of contaminated harbor sediments stabilized with hydraulic binders and the determination of remediation endpoints require the determination of pollutants leaching potentials. Moreover, little information about the speciation and mobility of inorganic contaminants in these specific solid matrices is available in the literature. The objective of this paper is to investigate the relationship between mineralogy and leachability of contaminants (copper, lead, and zinc) present in a French harbor sediment stabilized with quicklime and Portland cement. Batch equilibrium leaching tests at various pH, chemical analysis of leachates, and mineralogical studies (X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy, and diffuse reflectance infrared Fourier transform) have been combined in the present investigation. The acid neutralization capacity of the stabilized matrix studied is first controlled by the dissolution of portlandite (pH ~12), followed by the dissolution of C-S-H (pH ~11) and the dissolution of ettringite (pH ~10). Finally, a very high buffering capacity of this stabilized sediment is observed for pH values around 6. This equilibrium is mainly controlled by the dissolution of iron sulfides and carbonate minerals. Consequently, the mobilization of inorganic contaminants as a function of pH remains very low (<0.1 wt%) for pH values above 6 and significantly increases for pH below these values. This research confirms the importance of a combined methodology for the intrinsic characterization of potential mobilization of contaminants in a stabilized sediment and for a better understanding of geochemical processes that affect contaminant fate, transformation, and transport in the subsurface environment.

Potential influence of CO 2 release from a carbon capture storage site on release of trace metals from marine sediment

One of the main risks of CCS (Carbon Capture and Storage) is CO 2 leakage from a storage site. The influence of CO 2 leakage on trace metals leaching from contaminated marine sediment in a potential storage area (Northern Spain) is addressed using standardized leaching tests. The influence of the pH of the leaching solution on the leachates is evaluated using deionized water, natural seawater and acidified seawater at pH = 5, 6 and 7, obtained by CO 2 bubbling. Equilibrium leaching tests ( EN 12457) were performed at different liquid–solid ratios and the results of ANC/BNC leaching test ( CEN/TS 15364) were modeled using Visual Minteq. Equilibrium tests gave values of the final pH for all seawater leachates between 7 and 8 due to the high acid neutralization capacity of the sediment. Combining leaching test results and geochemical modeling provided insight in the mechanisms and prediction of trace metals leaching in acidified seawater environment.

Modeling of Amphoteric Heavy Metals Solubility in Stabilized/Solidified Steel Foundry Dust

The influence of pH on the leaching of metals from waste materials can be described by geochemical and empirical models. These equations may be integrated into dynamic leaching models in order to describe the long-term behavior of waste-derived forms or they can be used to predict the concentration of metals in equilibrium leaching tests at a given pH. The aim of this work is to describe the equilibrium concentration of the main metals (Zn, Pb, and Cr) contained in untreated and stabilized/solidified (S/S) electric arc furnace dust (EAFD) using experimental data obtained from a pH-dependence leaching test (acid neutralization capacity, ANC). EAFD is a hazardous waste generated in steel factories. Steel foundry dust coming from an electric arc furnace was characterized by acid digestion, x-ray fluorescence (XRF), and x-ray diffraction (XRD). The waste mainly contains Zn and Fe, which were identified in zincite and zinc ferrite phases. Pb and Cr were also detected at lower concentrations. Cement/EAFD formulations ranging from 7 to 20% dry wt of cement were prepared and the ANC leaching test was performed. The amphoteric behavior of Zn, Pb, and Cr was described by the geochemical model Visual MINTEQ and by an empirical model developed for these metals. Zinc and lead solubilities were well described by both models; however, Visual MINTEQ failed to describe the chromium behavior quantitatively.

Multiple Stirred-Flow Chamber Assembly for Simultaneous Automatic Fractionation of Trace Elements in Fly Ash Samples Using a Multisyringe-Based Flow System

There is a current trend in automation of leaching tests for trace elements in solid matrixes by use of flow injection based column approaches. However, as a result of the downscaled dimensions of the analytical manifold and execution of a single extraction at a time, miniaturized flow-through column approaches have merely found applications for periodic investigations of trace element mobility in highly homogeneous environmental solids. A novel flow-based configuration capitalized on stirred-flow cell extraction is proposed in this work for simultaneous fractionation of trace elements in three solid wastes with no limitation of sample amount up to 1.0 g. A two-step sequential extraction scheme involving water and acetic acid (or acetic acid/acetate buffer) is utilized for accurate assessment of readily mobilizable fractions of trace elements in fly ash samples. The fully automated extraction system features high tolerance to flow rates (< or = 6 mL min(-1)) and, as opposed to operationally defined batchwise methods, the solid to liquid ratio is not a critical parameter for determination of overall readily leachable trace elements provided that exhaustive extraction is ensured. Analytical performance of the dynamic extractor is evaluated for fractionation analysis of a real coal fly ash and BCR-176R fly ash certified reference material. No significant differences were found at the 0.05 significance level between summation of leached concentrations in each fraction plus residue and concentration values of BCR-176R, thus revealing the accuracy of the automated method. Overall extractable pools of trace metals in three samples are separated in less than 115 min, even for highly contaminated ashes, versus 18-24 h per fraction in equilibrium leaching tests. The multiple stirred-flow cell assembly is thus suitable for routine risk assessment studies of industrial solid byproduct.

Modeling of zinc solubility in stabilized/solidified electric arc furnace dust

Equilibrium models which attempt for the influence of pH on the solubility of metals can improve the dynamic leaching models developed to describe the long-term behavior of waste-derived forms. In addition, such models can be used to predict the concentration of metals in equilibrium leaching tests at a given pH. The aim of this work is to model the equilibrium concentration of Zn from untreated and stabilized/solidified (S/S) electric arc furnace dust (EAFD) using experimental data obtained from a pH-dependence leaching test (acid neutralization capacity, ANC). EAFD is a hazardous waste generated in electric arc furnace steel factories; it contains significant amounts of heavy metals such as Zn, Pb, Cr or Cd. EAFD from a local factory was characterized by X-ray fluorescence (XRF), acid digestion and X-ray diffraction (XRD). Zn and Fe were the main components while the XRD analysis revealed that zincite, zinc ferrite and hematite were the main crystalline phases. Different cement/EAFD formulations ranging from 7 to 20% dry weight of cement were prepared and subjected to the ANC leaching test. An amphoteric behavior of Zn was found from the pH dependence test. To model this behavior, the geochemical model Visual MINTEQ (VMINTEQ) was used. In addition to the geochemical model, an empirical model based on the dissolution of Zn in the acidic zone and the re-dissolution of zinc compounds in the alkaline zone was considered showing a similar prediction than that obtained with VMINTEQ. This empirical model seems to be more appropriate when the metal speciation is unknown, or when if known, the theoretical solid phases included in the database of VMINTEQ do not allow to describe the experimental data.

Leaching behaviour of low level organic pollutants contained in cement-based materials: Experimental methodology and modelling approach

The aim of this paper is the investigation of the leaching behaviour of different porous materials containing organic pollutants (PAH: naphthalene and phenanthrene). The assessment methodology of long term leaching behaviour of inorganic materials was extended to cement solidified organic pollutants. Based on a scenario-approach considering environmental factors, matrix and pollutants specificities, the applied methodology is composed of adapted equilibrium and dynamic leaching tests. The contributions of different physical and chemical mechanisms were identified and the leaching behaviour was modelled. The physical parameters of the analysed reference and polluted materials are similar. A difference in the pore size distribution appears for higher naphthalene content. The solubility of the PAH contained in the material is affected by the ionic strength and by the presence of a co-solvent; the solution pH does not influence PAH solubility. The solubility of the major mineral species is not influenced by the presence of the two PAH nor by the presence of the methanol as co-solvent in the range of the tested material compositions. In the case of the leaching of a monolith material the main transport mechanism is the diffusion in the porous system. For both mineral and organic species we observed at least two dynamic domains. At the beginning of the leaching process the released flux is due to the surface dissolution and to the diffusion of the main quantity dissolved in the initial pore solution. The second period is governed by a stationary regime between dissolution in pore water and diffusion. The model, coupling transport and chemical phenomena in the pore solution, at the monolith surface and in the leachate simulates satisfactory the release for both mineral and organic species.

Release dynamic process identification for a cement based material in various leaching conditions. Part I. Influence of leaching conditions on the release amount

In this paper we investigate at laboratory scale the influence of the liquid/solid leaching conditions on the release of different chemical species from a reference porous material obtained by solidification of PbO and CdO with Portland cement. The pH influence on the dissolution of pollutants and the initial pore solution composition (target elements: Na +, K +, Ca 2+, Pb 2+, Cd 2+, SO 4 2 − ) were assessed by applying a methodology consisting of two equilibrium leaching tests, the Acid Neutralization Capacity (ANC) and the Pore Water (PW) tests and geochemical modelling. Samples of the same material were submitted in parallel to four different dynamic leaching tests in order to determine the influence of the sample shape (monolithic or granular) and eluate hydrodynamics (instantaneous L/S ratio, eluate renewal) on the leaching of the target elements. The comparison criteria were the eluate saturation state, the cumulative release and the released flux. Generally, the eluates obtained in the tests applied on granular material were more concentrated, even saturated for the eluate pH value with respect to Ca 2+, Pb 2+ and SO 4 2 − . The consequence of the eluate saturation is the slowing down effect on the dynamic release. The highest released flux was observed for the Monolith Leaching Test (MLT) involving the highest instantaneous L/S ratio and the lowest solid/liquid exchange surface and for which no saturation was observed, except Pb 2+ and SO 4 2 − in some eluates. The maximum cumulative released-mass was obtained for the Column Leaching Test (CLT) applied on granular material having the highest exchange surface, the lowest instantaneous L/S and a continuous input flow of the leachant. The experimental results demonstrate the significance of the liquid/solid contact type which is also a scenario specific parameter.

Release dynamic process identification for a cement based material in various leaching conditions. Part II. Modelling the release dynamics for different leaching conditions

This paper deals with process identification and model development for the case of a porous reference material leaching under certain hydrodynamic conditions. Four different dynamic leaching tests have been applied in order to take into account different types of solid/liquid contact conditions corresponding to various real leaching scenarios: monolithic and granular material with sequential eluate renewal, and granular material and continuously renewed eluate with different hydrodynamic conditions (dispersion, residence time). A coupled chemical-mass transfer model has been developed to describe the leaching behaviour under all experimental conditions. Diffusion has been considered as the mass transport mechanism inside the saturated porous material and dispersive convection as that in the leachate. Two specific phenomena have been identified and considered in the model: (i) the early surface dissolution of the material which results in high Ca concentration and (ii) the late weak dissolution of Na and K giving rise to a long-term residual release. The intrinsic material parameters such as the initial concentrations in the pore water and solid phases were determined by applying equilibrium leaching tests and geochemical modelling. Diffusion coefficients for different elements and the late solubility of alkalines have been found to reach the same values in the four tests. The estimated values of the surface dissolution kinetic constant have shown a dependence on leachate hydrodynamics when the thickness of the degraded layer is nearly the same in the four tests (intrinsic parameter of the material). The competition between the four main dynamic processes, i.e. diffusion, convection, late dissolution, and surface dissolution, has been emphasized and compared in the four leaching tests: the hydrodynamic dispersion and the residence time had no effect on the leaching behaviour of alkalines, which is controlled by diffusion, whereas the behaviour of calcium (a major element of the material) was strongly influenced. This has significant effects on eluate pH values and on the concentration of Pb (the monitored pollutant). The model was then applied to simulate a landfill scenario in the case of a stabilized/solidified incinerator residue containing heavy metals and chloride. A high rain infiltration level and the use of small blocs are favourable conditions for enhanced pollutant release.

Leaching assessment of road materials containing primary lead and zinc slags

Characterisation of the leaching behaviour of waste-containing materials is a crucial step in the environmental assessment for reuse scenarios. In our research we applied the multi-step European methodology ENV 12-920 to the leaching assessment of road materials containing metallurgical slag. A Zn slag from an imperial smelting furnace (ISF) and a Pb slag from a lead blast furnace (LBF) are investigated. The two slags contain up to 11.2wt% of lead and 3.5wt% of zinc and were introduced as a partial substitute for sand in two road materials, namely sand-cement and sand-bitumen. At the laboratory scale, a leaching assessment was performed first through batch equilibrium leaching tests. Second, the release rate of the contaminants was evaluated using saturated leaching tests on monolithic material. Third, laboratory tests were conducted on monolithic samples under intermittent wetting conditions. Pilot-scale tests were conducted for field testing of intermittent wetting conditions. The results show that the release of Pb and Zn from the materials in a saturated scenario was controlled by the pH of the leachates. For the intermittent wetting conditions, an additional factor, blocking of the pores by precipitation during the drying phase is proposed. Pilot-scale leaching behaviour only partially matched with the laboratory-scale test results: new mass transfer mechanisms and adapted laboratory leaching tests are discussed.

Environmental assessment of waste matrices contaminated with arsenic

The use of equilibrium-based and mass transfer-based leaching tests has been proposed to provide an integrated assessment of leaching processes from solid wastes. The objectives of the research presented here are to (i) validate this assessment approach for contaminated soils and cement-based matrices, (ii) evaluate the use of diffusion and coupled dissolution–diffusion models for estimating constituent release, and (iii) evaluate model parameterization using results from batch equilibrium leaching tests and physical characterization. The test matrices consisted of (i) a soil contaminated with arsenic from a pesticide production facility, (ii) the same soil subsequently treated by a Portland cement stabilization/solidification (S/S) process, and (iii) a synthetic cement-based matrix spiked with arsenic(III) oxide. Results indicated that a good assessment of contaminant release from contaminated soils and cement-based S/S treated wastes can be obtained by the integrated use of equilibrium-based and mass transfer-based leaching tests in conjunction with the appropriate release model. During the time scale of laboratory testing, the release of arsenic from the contaminated soil matrix was governed by diffusion and the solubility of arsenic in the pore solution while the release of arsenic from the cement-based matrices was mainly controlled by solubilization at the interface between the matrix and the bulk leaching solution. In addition, results indicated that (i) estimation of the activity coefficient within the matrix pore water is necessary for accurate prediction of constituent release rates and (ii) inaccurate representation of the factors controlling release during laboratory testing can result in significant errors in release estimates.

Leaching of inorganic contaminants from cement-based waste materials as a result of carbonation during intermittent wetting

Characterization of the leaching behavior of wastes is a crucial step in the environmental assessment for reuse or disposal scenarios. The release of inorganic contaminants from waste materials is typically evaluated by tank leaching of continuously water-saturated material. However, materials, in many field or management scenarios, experience cyclic wetting and drying under varied environmental conditions (i.e. variable relative humidity, atmospheric CO 2 or CO 2 from biologic activities). During periods of storage in an unsaturated environment, many processes may occur that can influence the release potential and release rate of inorganic constituents. The research presented here was carried out to examine how the phenomena of carbonation during drying influence the release of inorganic contaminants from Portland cement-based materials during cyclic wetting and storage. Batch equilibrium leaching tests were used to determine constituent solubility as a function of pH. Dynamic leaching tests on monolithic material were carried out to determine the rate of constituent release as a function of leaching time and intermittent storage conditions. This paper presents the results observed for three typical waste constituents, arsenic, cadmium and lead.

Environmental assessment of a cement-based solidified soil contaminated with lead

The use of equilibrium-based and mass transfer rate extraction tests has been proposed to provide an integrated assessment of leaching processes from stabilized/solidified wastes. The objectives of the research presented here are to validate this assessment approach and to evaluate the applicability of (i)using models assuming diffusive controlled release or coupled dissolution and diffusion for estimating rates of constituent release from cement solidified matrices, and (ii)using results from batch equilibrium leaching tests and physical characterization to obtain parameters for release rate models. The test matrix consisted of soil fines contaminated with lead during manufacturing operations that were subsequently treated by two methods of Portland cement solidification/stabilization. Fundamental leaching parameters (i.e., acid neutralization capacity of the waste, constituent solubility as a function of pH, pore water properties and, release rates from monolithic leach tests) were measured. Results were interpreted according to behavioral models based on mechanistic understanding of leaching processes. Results showed that (i)a good assessment of the release of contaminants from cement-based S/S treated wastes can be obtained by integrated use of equilibrium-leaching tests and mass transfer leaching tests, (ii)equilibrium-leaching tests are sensitive to the extent of particle size reduction and contact time and need therefore to be adapted according to waste properties to ensure equilibrium, (iii)mass transfer leaching tests need to be defined according to the specified scenario, (iv)estimation of the activity coefficient within the waste pore water is necessary for accurate prediction of constituent release rates, (v)results need to be interpreted according to the considered scenario and the relevant behavioral model, (vi)the release of lead was governed by solubilization phenomenon at the solid–liquid interface (i.e., at the interface between the matrix and the leaching solution), and (vii)when release of lead is controlled by solubilization at the leachant–monolith interface, the release flux of lead can be directly calculated from the release flux of hydroxide and consequently from pH data measured in the leachate.