On equilibration of pore water in column leaching tests

Post-measurement compressed calibration for ICP-MS-based metal quantification in mine residues bioleaching

Bioleaching is an actual economical alternative to treat residues, which allows, depending on the chosen strategy, two possible outcomes: (1) a leachate enriched with target metals, or (2) a residue enriched in target metals through the leaching of interfering components (IC). This work aimed to study the metals released by bioprocessing the Panasqueira mine tailings, as a strategy to increase critical metals' relative concentration in residues. Biostimulation of the local microbiota was compared to a bioaugmentation approach using the autochthonous Diaphorobacter polyhydroxybutyrativorans strain B2A2W2. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was selected to study the metals released in the leachate through multi-element external standards. A new data treatment method was developed to use a preliminary sweep of intensities to quantify the non-initial target metals concentration in the leachate, based on preliminary ICP-MS intensity measurements. The results demonstrated that biostimulation was an efficient bioleaching strategy for the IC silicon, aluminium, magnesium, selenium, manganese, zinc, iron, and copper, by decreasing concentration, resulting in a relative increase in the gallium and yttrium (10x) levels in the treated residue. The strategy followed to quantify a large number of elements with ICP-MS using a reduced number of data points for calibration proved valid and speeded up the analytical process.

First order approximation for coupled film and intraparticle pore diffusion to model sorption/desorption batch experiments

A simple first order approximation was derived to model sorption/desorption kinetics of hazardous compounds in batch experiments based on a coupled film and intraparticle diffusion model. The solution is accurate enough to replace infinite series expansions needed in analytical solution for intraparticle diffusion and it accounts for the mass transfer shift from diffusion in the external aqueous boundary layer to the intraparticle pore space. With increasing distribution coefficient (K_d) and intraparticle particle porosity (ε) or decreasing Sherwood number (Sh) this mass transfer shift from film diffusion to intraparticle pore diffusion is delayed. The simple first order approximation equation allows analyses of mass transfer resistances and calculation of characteristic times which is relevant for the planning of batch experiments. The proposed solution is verified by a semi-analytical solution in Laplace space for fractional mass uptakes in the solid phase at equilibrium ranging from 50% to 91%, representing scenarios typically encountered in batch experiments.

Long-Term Leaching Behavior of Organic and Inorganic Pollutants after Wet Processing of Solid Waste Materials

The recycling of mineral materials is a sustainable and economical approach for reducing solid waste and saving primary resources. However, their reuse may pose potential risks of groundwater contamination, which may result from the leaching of organic and inorganic substances into water that percolates the solid waste. In this study, column leaching tests were used to investigate the short- and long-term leaching behavior of “salts”, “metals”, and organic pollutants such as PAHs and herbicides from different grain size fractions of construction & demolition waste (CDW) and railway ballast (RB) after a novel treatment process. Specifically, silt, sand and gravel fractions obtained after a sequential crushing, sieving, and washing process (“wet-processing”) of very heterogeneous input materials are compared with respect to residual contamination, potentially limiting their recycling. Concentrations in solid fractions and aqueous leachate were evaluated according to threshold values for groundwater protection to identify relevant substances and to classify materials obtained for recycling purposes according to limit values. For that, the upcoming German recycling degree was applied for the first time. Very good agreement was observed between short and extensive column tests, demonstrating that concentrations at L/S 2 ratios are suitable for quality control of recycling materials. Different solutes showed a characteristic leaching behavior such as the rapid decrease in “salts”, e.g., SO₄²⁻ and Cl⁻, from all solid fractions, and a slower decrease in metals and PAHs in the sand and silt fractions. Only the gravel fraction, however, showed concentrations of potential pollutants low enough for an unlimited re-use as recycling material in open technical applications. Sand fractions may only be re-used as recycling material in isolated or semi-isolated scenarios. Leaching from heterogeneous input materials proved harder to predict for all compounds. Overall, column leaching tests proved useful for (i) initial characterization of the mineral recycling materials, and (ii) continuous internal (factory control) and external quality control within the upcoming German recycling decree. Results from such studies may be used to optimize the treatment of mixed solid waste since they provide rapid insight in residual pollution of material fractions and their leaching behavior.

Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials

Initial conditions (pre-equilibrium or after the first flooding of the column), mass transfer mechanisms and sample composition (heterogeneity) have a strong impact on leaching of less and strongly sorbing compounds in column percolation tests. Mechanistic models as used in this study provide the necessary insight to understand the complexity of column leaching tests especially when heterogeneous samples are concerned. By means of numerical experiments, we illustrate the initial concentration distribution inside the column after the first flooding and how this impacts leaching concentrations. Steep concentration gradients close to the outlet of the column have to be expected for small distribution coefficients (Kd<1 L kg-1) and longitudinal dispersion leads to smaller initial concentrations than expected under equilibrium conditions. In order to elucidate the impact of different mass transfer mechanisms, film diffusion across an external aqueous boundary layer (first order kinetics, FD) and intraparticle pore diffusion (IPD) are considered. The results show that IPD results in slow desorption kinetics due to retarded transport within the tortuous intragranular pores. Non-linear sorption has not much of an effect if compared to Kd values calculated for the appropriate concentration range (e.g., the initial equilibrium concentration). Sample heterogeneity in terms of grain size and different fractions of sorptive particles in the sample have a strong impact on leaching curves. A small fraction (<1%) of strongly sorbing particles (high Kd) carrying the contaminant may lead to very slow desorption rates (because of less surface area)-especially if mass release is limited by IPD-and thus non-equilibrium. In contrast, mixtures of less sorbing fine material ("labile" contamination with low Kd), with a small fraction of coarse particles carrying the contaminant leads to leaching close to or at equilibrium showing a step-wise concentration decline in the column effluent.

Determining Adsorption Parameters of Potentially Contaminant-Releasing Materials Using Batch Tests with Differing Liquid-Solid Ratios

Adsorption parameters such as the distribution coefficient are required to predict the release behavior of contaminants using advection-dispersion models. However, for potentially contaminant-releasing materials (PCMs) such as dredged sludge and coal ash, these parameters cannot be obtained by conventional adsorption tests. This study developed a method to determine adsorption parameters for PCMs from a set of batch tests conducted in parallel as a function of the liquid-solid ratio (LS-parallel test). This LS-parallel test was performed on sandy soil derived from marine sediment using liquid-solid ratios from 1 to 300 L/kg. The water-contact time was also changed from 10 min to 28 d to elucidate the kinetics or equilibrium of contaminants released from the sample. Adsorption parameters were successfully obtained if the substance was under adsorption control. A column percolation test was performed to confirm the effectiveness of the obtained parameters. Good agreements were observed for SO₄^2- and B, but discrepancies remained for other substances such as F^- and As suggesting that improvements are necessary in both the LS-parallel test procedure and the advection-dispersion model.

Experiment and multicomponent model based analysis on the effect of flow rate and nitrate concentration on denitrification in low-permeability media

To better understand the combined effects of flow rate and NO₃^- concentration on denitrification rate and NO₃^- removal efficiency in the low-permeability media, a set of column experiments with different flow rates and injected NO₃^- concentrations were conducted. Denitrification processes under these different conditions were simulated using the PHREEQC code that couples the biogeochemical reactions and hydrological transport processes. In these reactive transport models, Monod kinetics were applied to describe the denitrification process. It was found that, among the experiments conducted in this study, the low flow rate (0.023 m/d) resulted in the low denitrification rate but high NO₃^- removal efficiency. Meanwhile, NO₃^- removal efficiency was the highest (85%) under moderate NO₃^- concentration of 1.29 mmol/L, although denitrification rate increased in response to the increase of NO₃^- concentration. The model results also indicated that NO₃^- removal efficiency of 97% can be achieved with relatively low flow rate and high influent NO₃^- concentration. The results in this study provide insights into NO₃^- remediation, and the temporal and spatial flow rate, as well as NO₃^- concentration distribution, should be pre-evaluated for the effective removal strategies.

Using Environmental Simulations to Test the Release of Hazardous Substances from Polymer-Based Products: Are Realism and Pragmatism Mutually Exclusive Objectives?

The potential release of hazardous substances from polymer-based products is currently in the focus of environmental policy. Environmental simulations are applied to expose such products to selected aging conditions and to investigate release processes. Commonly applied aging exposure types such as solar and UV radiation in combination with water contact, corrosive gases, and soil contact as well as expected general effects on polymers and additional ingredients of polymer-based products are described. The release of substances is based on mass-transfer processes to the material surfaces. Experimental approaches to investigate transport processes that are caused by water contact are presented. For tailoring the tests, relevant aging exposure types and release quantification methods must be combined appropriately. Several studies on the release of hazardous substances such as metals, polyaromatic hydrocarbons, flame retardants, antioxidants, and carbon nanotubes from polymers are summarized exemplarily. Differences between natural and artificial exposure tests are discussed and demonstrated for the release of flame retardants from several polymers and for biocides from paints. Requirements and limitations to apply results from short-term artificial environmental exposure tests to predict long-term environmental behavior of polymers are presented.

Optimization of a German short term percolation test to determine the leaching of granular materials

Different percolation tests were developed worldwide to characterize the leaching and to evaluate the environmental compatibility of granular materials. The German standard up-flow percolation test has a short testing time and can be used for both coarse and fine-grain materials. Some very fine-grain materials are difficult to percolate. According to the standard, admixture of 80% quartz sand (20% material) can be used for cohesive materials. It is assumed that equilibrium concentrations are reached and therefore the sand does not cause any interfering processes. However, the 80% sand admixture cannot be used for coarse materials due to dilution. A standardized sand admixture for both coarse and fine-grain materials is beneficial for the routine of laboratories. The sand admixture has the further advantage that it reduces the testing time. The experimental and the analysis procedures of the German standard were checked, specified, and optimized. An admixture of 50% sand is a good compromise for cohesive and coarse materials. The statistical variations of heavy metal and polycyclic aromatic hydrocarbons from the optimized test with and without sand admixture were determined with an 8-fold intralaboratory and an interlaboratory test. Then the sand admixture was validated for 16 materials (soils, demolition wastes, ashes and other industrial wastes).

Recycled concrete aggregate in base course applications: Review of field and laboratory investigations of leachate pH

The prevalence of construction and demolition (C&D) waste and the concurrent demand for construction aggregate presents the opportunity to recycle C&D waste materials as substitutes for virgin aggregate. Commonly, recycled concrete aggregate (RCA) is used as base course in pavement construction. Environmentally responsible applications of RCA must consider the high pH leachate and trace element leaching risks reported in the literature. This review presents the methodology, results, and limitations of existing laboratory and field investigations of RCA leachate chemistry. Long-term highway field studies of RCA leachate illustrate that an initially high leachate pH approaches neutral within approximately one to two years of construction. Conversely, laboratory investigations of RCA leachate pH using batch reactor leaching tests and column leaching tests measure consistently high leachate pH (pH > 10). The discrepancies between field and laboratory measurements of RCA leachate pH suggest that the current laboratory methodology inadequately describes leachate conditions in the field. The authors recommend that future laboratory investigations consider intermittent wetting and drying cycles, eliminate particle abrasion, employ relevant contact times, and consider additional environmental processes that reduce leachate pH such as soil acidity and carbonation.

Shift in Mass Transfer of Wastewater Contaminants from Microplastics in the Presence of Dissolved Substances

In aqueous environments, hydrophobic organic contaminants are often associated with particles. Besides natural particles, microplastics have raised public concern. The release of pollutants from such particles depends on mass transfer, either in an aqueous boundary layer or by intraparticle diffusion. Which of these mechanisms controls the mass-transfer kinetics depends on partition coefficients, particle size, boundary conditions, and time. We have developed a semianalytical model accounting for both processes and performed batch experiments on the desorption kinetics of typical wastewater pollutants (phenanthrene, tonalide, and benzophenone) at different dissolved-organic-matter concentrations, which change the overall partitioning between microplastics and water. Initially, mass transfer is externally dominated, while finally, intraparticle diffusion controls release kinetics. Under boundary conditions typical for batch experiments (finite bath), desorption accelerates with increasing partition coefficients for intraparticle diffusion, while it becomes independent of partition coefficients if film diffusion prevails. On the contrary, under field conditions (infinite bath), the pollutant release controlled by intraparticle diffusion is not affected by partitioning of the compound while external mass transfer slows down with increasing sorption. Our results clearly demonstrate that sorption/desorption time scales observed in batch experiments may not be transferred to field conditions without an appropriate model accounting for both the mass-transfer mechanisms and the specific boundary conditions at hand.

Impact of pre-equilibration and diffusion limited release kinetics on effluent concentration in column leaching tests: Insights from numerical simulations

Column leaching tests have become a standard method for assessing leaching of pollutants from materials used, e.g., for road and railway constructions and in landscaping measures. Column tests showed to be practical in laboratories yielding robust and reproducible results. However, considerable uncertainty still exists related particularly to the degree of equilibration of the pore water with the solids during preparation (pre-equilibration) and percolation of the column. We analyse equilibration time scales and sensitivity of concentrations in column leachate with respect to initial conditions in a series of numerical experiments covering a broad spectrum of material and solute properties. Slow release of pollutants from solid materials is described by a spherical diffusion model of kinetic sorption accounting for multiple grain size fractions and sorption capacities. Results show that the cumulative concentrations are rather independent of the pre-equilibration level for a broad spectrum of parameter settings, e.g. if intra-particle porosity is high, grain size is small, or if the sorption coefficient is large. Sensitivity increases with decreasing liquid solid ratios and contact time during percolation. Significant variations with initial column conditions are to be expected for material and compound properties leading to slow release kinetics. In these cases, sensitivity to initial conditions may have to be considered.

Analysis and interpretation of the leaching behaviour of waste thermal treatment bottom ash by batch and column tests

This paper investigates the leaching behaviour of specific types of waste thermal treatment bottom ash (BA) as a function of both pH and the liquid-to-solid ratio (L/S). Specifically, column percolation tests and different types of batch tests (including pH-dependence) were applied to BA produced by hospital waste incineration (HW-I), Refuse Derived Fuel (RDF) gasification (RDF-G) and RDF incineration (RDF-I). The results of these tests were interpreted applying an integrated graphical and modelling approach aimed at identifying the main mechanisms (solubility, availability or time-controlled dissolution and diffusion) governing the release of specific constituents from each type of BA. The final aim of this work was in fact to gain insight on the information that can be provided by the leaching tests applied, and hence on which ones may be more suitable to apply for assessing the leaching concentrations expected in the field. The results of the leaching tests showed that the three samples of analysed BA presented differences of orders of magnitude in their leaching behaviour, especially as a function of pH, but also in terms of the L/S. These were mainly related to the differences in mineralogy of the samples. In addition, for the same type of bottom ash, the comparison between the results of batch and percolation column tests, expressed in terms of cumulative release, showed that for some constituents (e.g. Mg for HW-I BA and Cu for RDF-G BA) differences of over one order of magnitude were obtained due to variations in pH and DOC release. Similarly, the eluate concentrations observed in the percolation tests, for most of the investigated elements, were not directly comparable with the results of the pH-dependence tests. In particular, in some cases the percolation test results showed eluate concentrations of some constituents (e.g. K and Ca in HW-I BA) of up to one order of magnitude higher than the values obtained from the pH-dependence experiments at the same pH value. This was attributed to a rapid washout from the column of the soluble phases present in the BA. In contrast, for other constituents (e.g. Mg and Ba for the RDF-G BA), especially at high L/S ratios, the concentrations in the column tests were of up to one order of magnitude lower than the solubility value, indicating release under non-equilibrium conditions. In these cases, batch pH-dependence tests should be preferred, since column tests results could underestimate the concentrations expected in the field.

Characterization of phosphorus leaching from phosphate waste rock in the Xiangxi River watershed, Three Gorges Reservoir, China

Phosphate mining waste rocks dumped in the Xiangxi River (XXR) bay, which is the largest backwater zone of the Three Gorges Reservoir (TGR), are treated as Type I industry solid wastes by the Chinese government. To evaluate the potential pollution risk of phosphorus leaching from phosphate waste rocks, the phosphorus leaching behaviors of six phosphate waste rock samples with different weathering degrees under both neutral and acidic conditions were investigated using a series of column leaching experiments, following the Method 1314 standard of the US EPA. The results indicate that the phosphorus release mechanism is solubility-controlled. Phosphorus release from waste rocks increases as pH decreases. The phosphorus leaching concentration and cumulative phosphorus released in acidic leaching conditions were found to be one order of magnitude greater than that in neutral leaching conditions. In addition, the phosphorus was released faster during the period when environmental pH turned from weak alkalinity to slight acidity, with this accelerated release period appearing when L/S was in the range of 0.5-2.0 mL/g. In both neutral and acidic conditions, the average values of Total Phosphorus (TP), including orthophosphates, polyphosphates and organic phosphate, leaching concentration exceed the availability by regulatory (0.5 mg/L) in the whole L/S range, suggesting that the phosphate waste rocks stacked within the XXR watershed should be considered as Type II industry solid wastes. Therefore, the phosphate waste rocks deposited within the study area should be considered as phosphorus point pollution sources, which could threaten the adjacent surface-water environment.