Comparison of percolation to batch and sequential leaching tests: Theory and data

Leaching tests are becoming more relevant in assessing solid waste material, particularly with respect to groundwater risks. In the field, water infiltration is the dominant leaching mechanism, which is simulated in the lab with batch and column tests. In this study, we compared percolation, through analytical solutions of the advection–dispersion equation, to laboratory batch and sequential leaching tests. The analytical solutions are supported with comprehensive data from various field and laboratory leaching of different solutes from waste materials and soils collected in long-term joint research projects funded by the German Federal Ministry for Education and Research and the Federal Environment Agency. The comparison of theory and data is facilitated if concentrations and cumulative release are plotted versus the liquid–solid ratios (LS). Both theory and data indicate that leaching behaviour is independent of duration and physical dimensions of the leaching tests. This holds even if field lysimeters are compared to laboratory columns of different size, different flow velocities as well as different contact times. In general, laboratory batch tests over predict effluent concentrations (for LS < K_d). Leaching of solutes from solid samples of certain materials (e.g. chloride from incineration ashes or sulphate from demolition waste) in column and lysimeter tests compares very well and agrees with the analytical solutions. Overall, reproducibility and agreement with theory of column tests are better than batch tests, presumably because the latter are prone to artefacts (e.g. in liquid–solid separation steps). Theory and data fit surprisingly well, despite the fact that the theory is based on the local equilibrium assumption; non-linear sorption and chemical reactions in the solid waste materials are not considered.     

Comparison of the release of constituents from granular materials under batch and column testing

Column leaching testing can be considered a better basis for assessing field impact data than any other available batch test method and thus provides a fundamental basis from which to estimate constituent release under a variety of field conditions. However, column testing is time-intensive compared to the more simplified batch testing, and may not always be a viable option when making decisions for material reuse. Batch tests are used most frequently as a simple tool for compliance or quality control reasons. Therefore, it is important to compare the release that occurs under batch and column testing, and establish conservative interpretation protocols for extrapolation from batch data when column data are not available. Five different materials (concrete, construction debris, aluminum recycling residue, coal fly ash and bottom ash) were evaluated via batch and column testing, including different column flow regimes (continuously saturated and intermittent unsaturated flow). Constituent release data from batch and column tests were compared. Results showed no significant difference between the column flow regimes when constituent release data from batch and column tests were compared. In most cases batch and column testing agreed when presented in the form of cumulative release. For arsenic in carbonated materials, however, batch testing underestimates the column constituent release for most LS ratios and also on a cumulative basis. For cases when As is a constituent of concern, column testing may be required.     

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 11.2 wt% of lead and 3.5 wt% 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.     

Re-cycling of remediated soil - Evaluation of leaching tests as tools for characterization

In Sweden, leaching tests with deionized water (D.W.) are utilized in risk assessment of materials entering landfills, but implementation of these results to evaluate the risk of spreading of pollutants in the environment is difficult. One problem is that most leaching procedures only consider heavy metals release, whereas organic pollutants are left out. The aim of the present study was to assess the possible pollutant mitigation in four remediated soils, three with heavy metals and one with polycyclic aromatic hydrocarbons (PAH) contamination. The mitigation was evaluated by standardized batch and column leaching tests utilizing three different leaching solutions: D.W., a weak ionic solution (0.001 M CaCl₂) and an artificially made soil water (ASW). In general, batch leaching tests implied larger contaminant removal than column leaching test, possibly due to the more rough treatment of the soil particles, and guidelines would at times be exceeded by the batch leaching test but not by column leaching tests. Utilization of CaCl₂ was found to release less heavy metal than D.W., whereas the metals mobilized by ASW were removed from solution by the filtration of soil leachates. Low molecular weight PAH was most efficiently mobilized by CaCl₂, while D.W. worked better for high molecular weight PAH. Despite very low initial PAH-concentrations, tap- and groundwater criteria were exceeded by all leaching solutions.     

Availability and leaching of polycyclic aromatic hydrocarbons: Controlling processes and comparison of testing methods

We have studied the availability and leaching of polycyclic aromatic hydrocarbons (PAHs) from two contaminated materials, a tar-containing asphalt granulate (Sigma16 US-EPA PAHs 3412mg/kg) and gasworks soil (SigmaPAHs 900mg/kg), by comparing results from three typical types of leaching tests: a column, sequential batch, and two different availability tests. The sequential batch test was found to largely resemble the column test. However, the leaching of particularly the larger PAHs (>5 aromatic rings) was found to be enhanced in the batch test by up to an order of magnitude, probably due to their association with large DOC (dissolved organic carbon) molecules generated by the vigorous mixing. The release of PAHs in the two availability tests, in which the leaching is facilitated by either a high concentration of DOC or Tenax resin, was similar, although the latter test was easier to perform and yielded more repeatable results. The availability was much higher than the amount leached in the column and sequential batch tests. However, biodegradation had apparently occurred in the column test and the total amount of PAHs released by either leaching or biodegradation, 9% and 26% for asphalt granulate and gasworks soil, respectively, did equal the amount leached in the availability tests. Therefore, the availability was found to provide a relevant measure of the PAH fraction that can be released from the solid phase. These results stress the importance of using the available instead of the total amount of contaminant in the risk analysis of solid materials in utilization or disposal.     

Evaluation of leaching and extraction procedures for soil and waste

Laboratory leaching tests may be used for source term determination as a basis for risk assessment for soil-groundwater pathways on contaminated sites. In order to evaluate different leaching procedures, batch extraction tests and percolation tests were performed using three reference materials produced from contaminated soil, demolition waste and municipal solid waste incinerator bottom ash. Emphasis was placed on the investigation of the leachability of the heavy metals copper and chromium, polycyclic aromatic hydrocarbons (PAHs) and the anions chloride and sulfate. Significant discrepancies between column experiments and batch/extraction tests were found for the release of PAHs and to a lesser extent for the heavy metals Cu and Cr. Additionally interlaboratory comparisons were conducted based on different leaching tests with the reference materials and evaluated using the criteria of comparability and reproducibility. The best reproducibility was achieved for all investigated substances in column tests. The reproducibility of batch tests was acceptable except for PAHs. The results from the experimental work will help establish standardized and feasible laboratory procedures as fundamental for substance specific risk assessment of contaminated sites.     

Percolation and batch leaching tests to assess release of inorganic pollutants from municipal solid waste incinerator residues

In this study, percolation and batch leaching tests were considered in order to characterize the behaviour of air pollution control (APC) residues produced in a municipal solid waste incinerator (MSWI) as a function of the liquid to solid ratio (L/S). This waste is hazardous, and taking into account their physical and chemical properties, leaching of contaminants into the environment is the main concern. In our work the leaching behaviour of toxic heavy metals (Pb, Zn, Cr, Ni and Cu) and inorganics associated with soluble salts (Na, K, Ca and Cl) was addressed. Although pH of the leaching solution is the most important variable, L/S may also play an important role in leaching processes. In our work, results from column and batch tests were compared in terms of concentration (mg/L) and releasing (mg/kg). The APC residues revealed to be hazardous according to both tests, and both Pb and Cl⁻ far exceeded the regulatory thresholds. The material exhibits high solubility, and when the liquid to solid ratio was high, more than 50% can be solubilised. The patterns of release may be in some cases availability or solubility controlled, and the former was easier to identify. When the results from column and batch experiments were compared by representing the cumulative released amounts (in mg/kg) as a function of L/S, both curves match for Zn, Ni, Cu, K, Na, Cl and Ca, but for Cr and Pb a significant difference was observed. In fact, the column experiments revealed that under percolation conditions it should be expected slow releasing of Pb along time. From this study, it can be concluded that the released amounts obtained in batch experiments for a certain L/S should be considered as the worst case for medium term. Some simple models proposed on the literature and based on local equilibrium assumption showed good fitting to experimental data for soluble species (non-reactive solutes).     

Assessment of long-term leaching from waste incineration air-pollution-control residues

Assessment of long-term leaching from MSWI air-pollution-control (APC) residues is discussed with respect to use in environmental impact assessment, such as life-cycle assessment (LCA). A method was proposed for estimating leaching as a function of the liquid-to-solid (L/S) ratio in a long-term perspective (L/S 5000l/kg). Data for changes in residue pH as a function of L/S was used in combination with pH dependent leaching data to predict leachate concentrations of Al, Ca, Cd, Ba, Mg, Ni, Pb, S, Pb, V and Zn as a function of L/S. Mass balance calculations were used to determine the element fractions leached with respect to L/S. The estimated long-term leaching from a semi-dry residue and a fly ash was compared with short-term leaching determined by batch tests at L/S 10l/kg, both carbonated and non-carbonated versions of the residues were investigated. Generally, very high L/S ratios above 2000l/kg were required to leach 20-30% of the solid contents. However, Ca and S were depleted at L/S 200-900l/kg. The long-term leachate concentrations were found to either remain at the same level as the initial leaching determined by the L/S 10 batch test, or to significantly decrease compared with the initial leaching. Only Al and Zn were found to show higher leachate concentrations at L/S ratios above 3000-5000l/kg. Carbonation generally prolonged the time needed for depletion from the solid residues; however, Ca and S were depleted faster than in the case of non-carbonated residues. This study shows that uncritical use of batch leaching data for assessing the potential leaching is highly problematic, and evaluations of residue disposal should include scenario specific quantification of the long-term leaching.     

Effect of drying on leaching testing of treated municipal solid waste incineration APC-residues

Air-pollution-control (APC) residues from waste incinerators are hazardous waste according to European legislation and must be treated prior to landfilling. Batch and column leaching data determine which type of landfill can receive the treated APC-residues. CEN standards are prescribed for the batch and column leaching test; however, these standards do not specify whether or not the residue samples should be dried prior to the leaching testing. Laboratory tests were performed in parallel (dried/non-dried) on treated APC-residue samples and evaluated with respect to Cr, Cd, Cu, Pb and Zn leaching. The effect of drying of the wet APC-residue samples was particularly dramatic regarding the leaching of Cr. Drying resulted in 10-100 times more Cr leaching in both batch and columns test. Drying also affected the leaching of Cd, Cu and Pb. Initial Cd leaching was up to 100 times higher in column tests with dried APC-residue than in tests with wet residues. The effect of drying appeared to be a combination of decreasing the reduction capacity of the sample (Cr), decreasing pH (Cd, Cu) and in column tests also a wash-out of salts (probably affecting Cd and Pb). If the leaching tests are intended to mimic landfill conditions, the results of this paper suggest that the tests should be done on wet, non-dried residue samples, although this may be less practical than testing dried samples.     

Leaching from MSWI bottom ash: evaluation of non-equilibrium in column percolation experiments

Impacts of non-equilibrium on results of percolation experiments on municipal solid waste incineration (MSWI) bottom ash were investigated. Three parallel column experiments were performed: two columns with undisturbed percolation and one column with two sets of 1-month-long flow interruptions applied at liquid-to-solid (L/S) ratios of L/S 2L/kg and 12L/kg, respectively. Concentrations of Na, K, Cl(-), Ca, Si, SO(4)(2-), Al, Cu, Ni, Mo, Ba, Pb, Zn, and dissolved organic carbon (DOC) were monitored throughout the entire leaching period; geochemical modeling was used to identify non-equilibrium-induced changes in the solubility control. Despite both physical and chemical non-equilibrium, the columns were found to provide adequate information for readily soluble compounds (i.e., Na, Cl(-), and K) and solubility-controlled elements (i.e., Ca, SO(4)(2-), Ba, Si, Al, Zn, and Pb). The leaching of Cu and Ni was shown to depend strongly on DOC leaching, which was likely affected by physical non-equilibrium during flow interruptions. Consequently, the leaching of Cu and Ni in the undisturbed columns was shown to be by about one order of magnitude lower compared with the interrupted column. The results indicate that the leaching of DOC-related metals in laboratory column experiments may be considerably underestimated compared with full-scale scenarios in which the impacts from non-equilibrium may be significantly lower. The leaching of Mo (or MoO(4)(2-)) may be controlled solely by its availability in the mobile zone, which in turn appeared to be controlled by diffusion from the stagnant zone; no Mo controlling minerals were predicted by the geochemical modeling.     

Kinetics of steel slag leaching: Batch tests and modeling

Reusing steel slag as an aggregate for road construction requires to characterize the leaching kinetics and metal releases. In this study, basic oxygen furnace (BOF) steel slag were subjected to batch leaching tests at liquid to solid ratios (L/S) of 10 and 100 over 30 days; the leachate chemistry being regularly sampled in time. A geochemical model of the steel slag is developed and validated from experimental data, particularly the evolution with leaching of mineralogical composition of the slag and trace element speciation. Kinetics is necessary for modeling the primary phase leaching, whereas a simple thermodynamic equilibrium approach can be used for secondary phase precipitation. The proposed model simulates the kinetically-controlled dissolution (hydrolysis) of primary phases, the precipitation of secondary phases (C-S-H, hydroxide and spinel), the pH and redox conditions, and the progressive release of major elements as well as the metals Cr and V. Modeling indicates that the dilution effect of the L/S ratio is often coupled to solubility-controlled processes, which are sensitive to both the pH and the redox potential. A sensitivity analysis of kinetic uncertainties on the modeling of element releases is performed.     

Evaluation of carbonization as a thermal pretreatment method for landfilling by column leaching tests

To evaluate carbonization as a thermal pretreatment method for landfilling, the releasing characteristics of organic and inorganic constituents from carbonization residue derived from shredded residue of bulky waste was investigated by means of batch and column leaching tests. Shredded residue of bulky waste itself and its incineration ash were tested together to compare pretreatment methods. In batch leaching tests at a liquid/solid ratio of 10, the release of organic carbon from carbonization residue was at a remarkably low level. Besides, carbonization contributed to immobilize heavy metals such as chromium, cadmium, and lead within its residue. In column tests, the discharges of organic constituents were lowest from carbonization residue under aerobic conditions due to microbial activity. The leaching of Cd, Cr, Pb, and Cu from carbonization residue was suppressed under anaerobic conditions; however, this suppression effect tended to be weaker under aerobic conditions. From the results showing that the total releasing amounts of organic and inorganic constituents from carbonization residue are so low as to be comparable to that of incineration ash, carbonization can be considered as one of the thermal pretreatment methods of organic wastes.     

Assessment of long-term pH developments in leachate from waste incineration residues.

Environmental assessment of residue disposal needs to account for long-term changes in leaching conditions. Leaching of heavy metals from incineration residues are highly affected by the leachate pH; the overall environmental consequences of disposing of these residues are therefore greatly influenced by changes in pH over time. The paper presents an approach for assessing pH changes in leachate from municipal solid waste incineration (MSWI) air-pollution-control (APC) residues. Residue samples were subjected to a stepwise batch extraction method in order to obtain residue samples at a range of pH values (similar to common pH-dependence tests), and then on these samples to determine leaching of alkalinity as well as remaining solid phase alkalinity. On a range of APC residues covering various pretreatment and disposal options, this procedure was used to determine leachable and residual alkalinity as a function of pH. Mass balance calculations for typical disposal scenarios were used to provide data on pH as a function of the liquid-to-solid (L/S) ratio in the leaching system. Regardless of residue type and pretreatment, pH was found to stay above 7 for L/S ratios up to about 2000 L kg(-1) corresponding to about 100,000 years in typical landfill scenarios. It was found that pH changes were mainly governed by alkalinity decreases from leaching processes rather than neutralization reactions. The results suggest that leaching testing for assessment purposes should be carried out in the alkaline range, for example, at pH 9. The paper offers a thorough basis for further modelling of incineration residue leaching and for modelling the environmental consequences of landfilling and utilization of these residues.     

The fixation and leaching of cement stabilized arsenic

The solidification/stabilization of sodium arsenite, sodium arsenate, arsenic trioxide and arsenic pentoxide at dosages of approximately 10% has been investigated using sequential batch leaching tests. The leaching of arsenic, which was found to be diffusion based, was clearly least effective for those formulations containing additional iron(II). Calcium was found to influence the leaching of cement immobilized arsenic: those formulations containing the greatest Ca:As mole ratios were generally the most successful. Analysis using both FTIR and SEM revealed substantial changes to the cement matrices of those formulations to which the ferrous sulfate had been added. Ettringite was identified in the cement+ferrous sulfate formulations.     

Quantifying the availability and the stability of trace cationic elements in fly ash

For adsorption-desorption controlled leaching processes, the total leachable mass and the adsorption constant are parameters representing the availability and the stability of trace elements in solid media. With these parameters, one can predict the leaching behavior of trace elements from solids under various pH and solid-to-liquid ratio conditions. An approach was developed in this paper to determine these parameters for model elements Cu(II), Cd(II), and Ni(II) in fly ash. This approach consists of a batch equilibrium titration, a batch equilibrium leaching with and without target element addition, and mathematical modeling. Results indicated that the adsorption constant of a trace element can be determined by modeling the adsorption ratio of the added element to the system as a function of pH. Results also indicated that the trace element originally present in fly ash had similar adsorption-desorption behavior as that added externally. By modeling the batch leaching data with and without external element addition, the total leachable mass and adsorption constant of the target element can be determined simultaneously. The total leachable mass is in agreement with experimental data from 50mM EDTA extraction.     

Leaching tests to evaluate pollution potential of combustion residues from an iron recycling industry

Three leaching tests were applied to an industrial fly ash (cupola dust) for the evaluation of its groundwater pollution potential. The tests comprised two batch shaking tests (the SLT-test and a titration test) and a column leaching test (laboratory scale). All three tests gave valuable and consistent results with respect to the leaching of lead, zinc, cadmium and organic matter. The pH of the leaching environment was the most important factor in governing the leaching process. The results of the leaching seemed to be in accordance with observations of the groundwater quality below an actual industrial fly ash disposal site. The combination of the SLT-test and the titration test appears to be a cost-effective means of evaluating the groundwater pollution potential from industrial fly ashes.     

Long-term leaching test of incinerator bottom ash: evaluation of Cu partition

Two types of leaching tests were performed on the bottom ash from municipal solid waste incinerators. A short-term batch test specified by the America Nuclear Society (ANS) and long-term column tests with acetic acid (pH 5.2) as leaching solution were used to evaluate copper leachability. The Cu leaching after the 5-d ANS test is about 1% of the original Cu content of 5300 mg/kg. Upon addition of a stabilizing agent, the Cu leaching quantity is reduced; the extent of reduction depends on the type of chemical used (phosphate, carbonate and sulfide). The 1.6% Na(2)S addition showed negligible Cu leaching, and Na(2)S was, therefore, used in subsequent column tests. The 30-d column test indicates a steady increase of Cu leaching amount with time and reaches about 1.5% of the original Cu content after 30 d. A 180-d column test further increased the Cu leaching to about 5.1% of the original Cu content, whereas no appreciable Cu leaching was found with the addition of 1.6% Na(2)S. A sequential extraction was conducted on the raw ash, ash with the addition of Na(2)S and the residue ash after 30 d of operation to characterize Cu affinity for different solid fractions. The data were used to evaluate the fate of Cu through these interactions.     

Effect of contact time on the release of contaminants from granular waste materials during column leaching experiments

When reusing or disposing of contaminated granular waste materials there is a need to evaluate how the contaminants will interact on the pathway soil–groundwater and the effect this interaction will have on the surrounding environment. While column testing can provide a closer approximation to field percolation conditions than batch testing, there is still a need to develop column testing procedures that consider the requirements of practical testing time frames. This study evaluates the effect of different column contact times (2.5, 5, and 16 h) on the release of inorganic constituents from bottom ash and demolition waste, two commonly reused granular materials. Leaching data for representative constituents of concern, such as copper, chromium, sulfate and chloride, as well as pH and electrical conductivity was compared for all different contact times studied.Results for the materials investigated in this study showed that variations in contact time have no significant effect on the release of the selected constituents and leaching parameters at low liquid to solid ratios. However, after a liquid to solid ratio of 1 L/kg, the effect is more noticeable, and higher contact times show lower pH values as well as a reduction in the release of constituents of concern from bottom ash. In the case of demolition waste, the variation of contact time did not have a strong effect on the leaching behavior.     

Experimental and numerical analysis of metal leaching from fly ash-amended highway bases

A study was conducted to evaluate the leaching potential of unpaved road materials (URM) mixed with lime activated high carbon fly ashes and to evaluate groundwater impacts of barium, boron, copper, and zinc leaching. This objective was met by a combination of batch water leach tests, column leach tests, and computer modeling. The laboratory tests were conducted on soil alone, fly ash alone, and URM-fly ash-lime kiln dust mixtures. The results indicated that an increase in fly ash and lime content has significant effects on leaching behavior of heavy metals from URM-fly ash mixture. An increase in fly ash content and a decrease in lime content promoted leaching of Ba, B and Cu whereas Zn leaching was primarily affected by the fly ash content. Numerically predicted field metal concentrations were significantly lower than the peak metal concentrations obtained in laboratory column leach tests, and field concentrations decreased with time and distance due to dispersion in soil vadose zone.     

Evaluation of heavy metal leaching from spent household batteries disposed in municipal solid waste

Batch leaching tests and simulated landfill lysimeter tests were performed to evaluate the contents of heavy metals leached from spent batteries in the municipal solid waste. The toxicity characteristic leaching procedure was utilized to perform the batch leaching tests of 36 spent batteries. Four lysimeters were prepared with battery contents ranging from 0% to 100% by weight for column tests, and the experiments were performed at ambient temperature. The age of all the batteries used in the study ranged from freshly disposed up to approximately 3 years old. The results from the batch tests showed that the type of battery influenced the heavy metal concentrations in the leached solutions. The lysimeter experiment results illustrated that at lower pH levels more metals are leached than at higher pH levels. The increasing amount of batteries disposed in landfills can contribute to the leaching of more metals, especially Mn and Zn, into the environment. These results indicate that the direct disposal of spent household batteries into a MSW landfill can increase the heavy metal contents in the landfill leachate.