pH-dependent Experiments Research Articles

Kinetics of Ganglioside-Rich Supported Lipid Bilayer Formation with Tracer Vesicle Fluorescence Imaging.

Gangliosides, forming a class of lipids complemented by sugar chains, influence the lateral distribution of membrane proteins or membrane-binding proteins, act as receptors for viruses and bacterial toxins, and mediate several types of cellular signaling. Gangliosides incorporated into supported lipid bilayers (SLBs) have been widely applied as a model system to examine these biological processes. In this work, we explored how ganglioside composition affects the kinetics of SLB formation using the vesicle rupturing method on a solid surface. We imaged the attachment of vesicles and the subsequent SLB formation using the time-lapse total internal reflection fluorescence microscopy technique. In the early phase, the ganglioside type and concentration influence the adsorption kinetics of vesicles and their residence/lifetime on the surface before rupturing. Our data confirm that a simultaneous rupturing of neighboring surface-adsorbed vesicles forms microscopic lipid patches on the surface and it is triggered by a critical coverage of the vesicles independent of their composition. In the SLB growth phase, lipid patches merge, forming a continuous SLB. The propagation of patch edges catalyzes the process and depends on the ganglioside type. Our pH-dependent experiments confirm that the polar/charged head groups of the gangliosides have a critical role in these steps and phases of SLB formation kinetics.

One-pot synthesis of superparamagnetically modified zeolite chabazite for removal of Cs+ from radioactively contaminated water

Composites of sodium-exchanged zeolite chabazite particles with attached superparamagnetic nanoparticles of Fe3O4 and CoFe2O4 ferrite spinels have been produced using a novel solvothermal route. These have been characterized by a combination of techniques, including powder x-ray diffraction, x-ray fluorescence spectroscopy, vibrating sample magnetometry, and both scanning and transmission electron microscopy. These confirm the nature of the attached nanoparticles, and the microscope images show a good dispersion of nanoparticles with a narrow size range deposited uniformly on the surface of the zeolite particles. The magnetization is of sufficient strength to allow magnetic separation from a solution. Both time and pH dependent Cs uptake experiments show that the magnetized systems are still excellent for the rapid uptake of Cs via an ion exchange process with no reduction in property due to the attached nanoparticles.

Unraveling the reaction mechanisms for furfural electroreduction on copper.

Electrochemical routes for the valorization of biomass-derived feedstock molecules offer sustainable pathways to produce chemicals and fuels. However, the underlying reaction mechanisms for their electrochemical conversion remain elusive. In particular, the exact role of proton-electron coupled transfer and electrocatalytic hydrogenation in the reaction mechanisms for biomass electroreduction are disputed. In this work, we study the reaction mechanism underlying the electroreduction of furfural, an important biomass-derived platform chemical, combining grand-canonical (constant-potential) density functional theory-based microkinetic simulations and pH dependent experiments on Cu under acidic conditions. Our simulations indicate the second PCET step in the reaction pathway to be the rate- and selectivity-determining step for the production of the two main products of furfural electroreduction on Cu, i.e., furfuryl alcohol and 2-methyl furan, at moderate overpotentials. We further identify the source of Cu's ability to produce both products with comparable activity in their nearly equal activation energies. Furthermore, our microkinetic simulations suggest that surface hydrogenation steps play a minor role in determining the overall activity of furfural electroreduction compared to PCET steps due to the low steady-state hydrogen coverage predicted under reaction conditions, the high activation barriers for surface hydrogenation and the observed pH dependence of the reaction. As a theoretical guideline, low pH (<1.5) and moderate potential (ca. -0.5 V vs. SHE) conditions are suggested for selective 2-MF production.

Efficient Capture of Sr2+ Ions by a Layered Potassium Neodymium Phosphate

90Sr has a long half-life, strong radioactivity, and high mobility. The removal of radioactive strontium from the water environment is of great significance to human safety and the sustainable development of nuclear energy. In this study, a two-dimensional rare earth phosphate K3Nd(PO4)2 efficiently captured Sr2+ ions in aqueous solutions. At room temperature, the adsorption isotherm, kinetics, and pH dependence experiments of K3Nd(PO4)2 for Sr2+ ions were examined (V/m = 1000 mL/g, 12 h contact time). The experimental results show that the maximum adsorption capacity of K3Nd(PO4)2 for Sr2+ (qmSr) was 42.35 mg/g. The removal efficiency for Sr2+ (RSr) was 87.47% within 24 h. It had a good affinity with Sr2+ ions in neutral or even high alkaline environments (distribution coefficient KdSr = 1.46 × 106 mL/g, RSr = 99.93%). The adsorption mechanism was attributed to the ion exchange between Sr2+ and K+ ions by batch adsorption experiments combined with multiple characterizations, including XPS, EDS, and PXRD. This is the first report of Sr2+ removal by ion exchange via rare earth phosphate materials with a two-dimensional structure. This work provides insight into the future development of rare earth phosphates as ion exchange materials for radionuclide remediation.

Insight into the role of adsorbed formate in the oxidation of formic acid from pH-dependent experiments with Pt single-crystal electrodes

The concentration dependence of the activity for the oxidation of formic acid at pH 1.2, 2.4, and 3.9 has been studied on Pt(1 1 1), Pt(1 0 0), and stepped Pt[n(1 1 1) × (1 1 0)] electrodes. The results clearly demonstrate that, in this pH range, formate, and not formic acid, is the active species. To analyse the data, a kinetic model in which adsorbed monodentate formate is the active intermediate has been proposed. On Pt(1 1 1), a steady increase in the reaction rate with increasing pH and a reaction order of 1 for the bulk formate concentration was obtained, in agreement with the proposed model. On the other hand, the competition between adsorbed hydrogen and formate for the adsorption sites on Pt(1 0 0) cancels both the concentration and the pH dependence of the reaction rate. On this electrode, the potential dependence of the reaction rate for the dehydrogenation of formic acid to COad was also studied and found to go through a maximum both at pH 1.2 and at pH 3.9, although the rate of dehydrogenation is slower and the maximum is broader at pH 3.9. The slower rate at the higher pH is consistent with the well-known fact that dehydrogenation of HCOOH is an acid-catalysed reaction. On Pt[n(1 1 1) × (1 1 0)] electrodes the behaviour observed is similar to that of Pt(1 1 1), with a reaction order of 1 for formate. The only significant difference with respect to the Pt(1 1 1) surface is the formation of COad due to the presence of (1 1 0) steps.

A fluorescence “turn-on” probe for Cu (Ⅱ) based on flavonoid intermediates generated by copper-induced oxidative cyclization and its fluorescence imaging in living cells

A fluorescence “turn-on” probe for Cu (Ⅱ) ions was prepared based on the condensation reaction of coumaraldehyde and 1-hydroxy-2-acetylnaphthalene. A strong fluorescent flavonoid intermediate was formed and verified by the NMR and ESI-MS experiments. The water-soluble and pH dependence experiments were performed to confirm the optimal solvent condition (CH3CN: HEPES = 1:1, v/v, pH = 7.2–7.4). The dynamic experiments indicated that the formation process of the intermediate catalyzed by Cu(Ⅱ) ions was probably pseudo-first-order reaction process. The probe showed good selectivity toward copper ions and almost no interference except Ag+ ions by the selectivity and competitive experiments. The HeLa cells were used in the cell fluorescence imaging tests and it was demonstrated that the probe could be used in the phycological condition and showed weak cytotoxicity by the MTT experiments.

Sorption of selected pharmaceutical compounds on polyethylene microplastics: Roles of pH, aging, and competitive sorption

Microplastics (MPs) as the carrier of pharmaceuticals in aquatic environments have been concerned in recent years. However, the influences of environmental factors on the sorption of pharmaceuticals onto MPs, particularly the effect of the simultaneous sorption by MPs of different pharmaceuticals in multi-solute systems are still unclear. This study investigated the influences of pH, aging of MPs, and competition of pharmaceuticals on the sorptions of sulfamethoxazole (SMX), propranolol (PRP), and sertraline (SER) onto polyethylene MPs. In the 96 h pH-dependent experiments, the sorptions of the three pharmaceuticals were mainly driven by hydrophobic interaction. Besides, the ionization states of the three pharmaceuticals varied with the pH ranging from 2.00 to 12.00, and electrostatic interaction would affect the sorption affinities of the pharmaceuticals in different ionization states. In the aged MPs experiments, the MPs aged by UV irradiation showed a stronger sorption capacity than the pristine ones. Across the MPs under different UV irradiation durations, the 6 d aged MPs showed the highest sorption percentages of 23.0% and 17.6% for SER and PRP, respectively; for SMX, the highest sorption percentage of 5.4% was recorded with the 10 d aged MPs. In the multi-solute systems, the sorption kinetics of the three pharmaceuticals fit well with the pseudo-second-order model. The sorption quantities of the three pharmaceuticals onto MPs followed the order of SER cations (18.70 μg g−1) > SMX anions (7.83 μg g−1) > PRP cations (3.80 μg g−1) at pH 7.00. The good fitting of the Freundlich model suggested a multilayer sorption of the three pharmaceuticals onto MPs. The SER with higher hydrophobicity would preferentially be adsorbed onto MPs and influenced the subsequently sorption processes of the other pharmaceuticals via electrostatic interactions. This may change the environmental fate of the contaminants, which should be carefully considered in future work.

Synthesis and characterization of Sb2O3 nanoparticles by liquid phase method under acidic condition

Antimony trioxide (Sb2O3) nanoparticles were successfully synthesized via a facile and practical liquid phase method under acidic condition. The structure, composition and surface morphology of the as-prepared products have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), scanning electronic microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The as-obtained products revealed a cubic phase of Sb2O3 (JCPDS No. 05–0534) with a preferred plane of (222) and lattice spacing of a = 11.180 Å. Besides, Sb2O3 octahedra, spherical and polyhedral nanoparticles, coral-like nanorod and paddy-like nanosheets have also been synthesized by the modulating reaction time and solution pH value under acidic condition. According to the results of the time-dependent and pH-dependent experiments, the possible formation mechanisms for Sb2O3 nanocrystals with diverse morphologies have been proposed, which could be a favorable suggestion for the synthesis of other metal-oxide nanomaterials with morphology control.

Experimental and computational study on the enantioseparation of four chiral fluoroquinolones by capillary electrophoresis with sulfated-β-cyclodextrin as chiral selector.

In this work, enantioseparation of four chiral fluoroquinolones (FQs), namely, ofloxacin, gemifloxacin, lomefloxacin, and gatifloxacin, was achieved by capillary electrophoresis with sulfated-β-cyclodextrin (S-β-CD) as chiral selector. Factors affecting the enantiomeric resolution, such as the concentrations of S-β-CD, BGE pH conditions, and the buffer types and concentrations, were optimized and discussed. A BGE consisting of 30 g/L S-β-CD and 30-mM phosphate at pH4.0 was found fit for enantiomeric resolution of ofloxacin and gemifloxacin, while the same BGE at pH3.0 was suitable for enantioseparation of lomefloxacin and gatifloxacin. The pH-dependent experiments showed that separation resolutions of four FQs enantiomers were significantly affected by BGE pH, which was thought to be related with the varying electrostatic attraction between the enantiomers and chiral selector. To verify this speculation, molecular docking studies were used for further investigation of the enantiomeric recognition mechanism of S-β-CD. Molecular model indicated that hydrophobic effect and hydrogen bond were involved in host-guest inclusion, but the electrostatic attraction enhanced the chiral discrimination by increasing the difference in binding energy between individual enantiomers and S-β-CD. This work provided a further insight into the chiral recognition mechanisms of CD derivatives.

Fabrication of three-component hydrogen-bonded covalent-organic polymers for ciprofloxacin decontamination from water: adsorption mechanism and modeling

A three-component hydrogen-bonded covalent organic polymer, namely JLUE-HCOP-66, was fabricated via a facile multiple-linking-site solvothermal approach to overcome the weakness of poor function complexity and limited structure diversity of the pure covalent skeletons. The as-prepared JLUE-HCOP-66 polymers were employed to decontaminate ciprofloxacin (CIP), a popular F-quinolones (FQNs) antibiotic, from water and exhibited satisfactory adsorption performance. Specifically, JLUE-HCOP-66 polymers have high adsorption capacity with the maximum contribution of 111.1 mg/g according to the Langmuir model, good antiinterference to NaCl salts, and excellent regeneration property. The pH-dependent experiment results signified the probably dominated mechanism of electrostatic interaction. In addition, adsorption studies and structural characterization in combination illustrated that the pore-filling effect, hydrogen bonding formation might also govern the whole process, accompanied by electrostatic interaction, dipole-dipole complexation, π-π EDA interaction, and hydrophobic-hydrophobic interaction besides. Moreover, electrostatic potentials, as well as the frontier molecular orbital distributions (HOMO and LUMO) of CIP and JLUE-HCOP-66 fragment, were calculated using density functional theory to theoretically support the research. Furthermore, the response surface methodology (RSM) according to the CCD matrix was used to not only study the interactive and cooperative effects of initial CIP concentration, initial pH, ionic strength along with JLUE-HCOP-66 dosage on CIP removal using JLUE-HCOP-66 but also optimize the operation conditions. Given the peculiar structure and functional feature, this work could hopefully bring HCOPs into the practical applications of such challenging and persistent ciprofloxacin potent removal with further large-scale efficiency.

Potassium isotopic fractionation during clay adsorption

Clay adsorption is a critical process responsible for the mobilization and cycling of potassium (K) on Earth’s surface. Recent studies emphasized the potential of using stable K isotopes (δ41K) to understand chemical weathering. However, the direction, degree, and mechanism of K isotopic fractionation linked to clay K uptake during chemical weathering remain poorly constrained.This work investigated the mechanism of K adsorption on clays (kaolinite and smectite) and the isotopic fractionation in three experimental sets with K-containing solutions. The time-series experiments revealed that the adsorption and isotope equilibria were attainedafter less than 12-hour reaction. Potassium adsorption rate slowed down and its isotopic fractionation approached the steady-state during 15-day reaction. The pH-dependent experiments demonstrated that the percentage of clay K adsorption and the isotopic composition of adsorbed K (and aqueous K) display negative linear correlations. Net isotopic fractionation between adsorbed and aqueous phases (Δ41Kad-aq) remained near-constant (0.6–0.8‰), regardless of variations in pH ranging from 4 to 10. The concentration-control experiments demonstrated that the percentage of K adsorption decreased with increasing KCl concentrations from 0.005 to 20 mM. The δ41K values of aqueous K reached the minimum of −0.53‰ after 92.7% K adsorbed (initial KCl of 0.005 mM). Potassium adsorption was substantially suppressed as ionic strength (fixed by Na+) increased from 0.001 to 0.5 M without apparent Δ41Kad-aq variations. The K K-edge XANES demonstrated that primary K incorporated in clay lattice and surface KCl derived from sorbed K+ and Cl− synchro-dehydration can be identified after drying of clays. These features indicate that adsorbed K+ was bounded onto clays as outer-sphere complexes, which can be replaced with excess Na+ at high ionic strength. Based on experimental results, we cannot distinguish specific mineralogy regulation on K isotopic fractionation. In sum, isotopically heavy K is preferentially sorbed on clay minerals. The results confirm an equilibrium fractionation path independent of reaction time, pH, ionic strength, and initial KCl concentration. Observed K isotopic fractionation is best fitted by an equilibrium isotopic fractionation law with a fractionation factor αad-aq of 1.00075. We highlight the opposite direction of K isotopic fractionation in clay adsorption and structural incorporation during chemical weathering, and their comparative contributions should be considered for future field investigations.

The Curious Case of Aqueous Warfarin: Structural Isomers or Distinct Excited States?

Warfarin is a potent anti-coagulant drug and is on the World Health Organization's List of Essential Medicines. Additionally, it displays fluorescence enhancement upon binding to human serum albumin, making warfarin a prototype fluorescent probe in biology. Despite its biological significance, the current structural assignment of warfarin in aqueous solution is based on indirect evidence in organic solvents. Warfarin is known to exist in different isomeric forms-open-chain, hemiketal, and anionic forms-based on the solvent and pH. Moreover, warfarin displays a dual absorption feature in several solvents, which has been employed to study the ring-chain isomerism between its open-chain and hemiketal isomers. In this study, our pH-dependent experiments on warfarin and structurally constrained warfarin derivatives in aqueous solution demonstrate that the structural assignment of warfarin solely on the basis of its absorption spectrum is erroneous. Using a combination of steady-state and time-resolved spectroscopic experiments, along with quantum chemical calculations, we assign the observed dual absorption to two distinct π → π* transitions in the 4-hydroxycoumarin moiety of warfarin. Furthermore, we unambiguously identify the isomeric form of warfarin that binds to human serum albumin in aqueous buffer.

Facile, Electrochemical Chlorination of Graphene from an Aqueous NaCl Solution.

We report a facile approach to directly chlorinate graphene from an aqueous sodium chloride solution under ambient conditions. By applying a moderate anodic voltage to substrate-supported monolayer graphene, the resultant chlorine radicals generated at the graphene surface enable efficient chlorination: X-ray photoelectron spectrum confirms the formation of C-Cl bonds, and reaction voltage-tunable Cl:C atomic ratios of up to 17% are achieved. In comparison, we find the corresponding electrochemical graphene bromination and iodination reactions much less viable. Electrical and Raman characterizations show substantial p-doping for the chlorinated graphene, yet good basal-plane integrity and electrical properties are maintained. Interference reflection microscopy and pH-dependent experiments next help elucidate the competition between the radical-mediated electrochemical chlorination and oxidation in the process, and rationalize acidic conditions for optimal chlorination. Reaction in a mixed NaCl-NaN3 solution shows the electrochemical chlorination to be fully suppressed by azidation, yet a sequential, two-step chlorination-azidation approach permits facile bifunctionalization.

γ-FeOOH and γ-FeOOH decorated multi-layer graphene: Potential materials for selenium(VI) removal from water

The development of robust, cost-effective remediation technologies of selenium uptake from ground and surface water is challenging and at the same time urgently needed. The present study provides a new, facile synthesis protocol for lepidocrocite (γ-FeOOH) nanoflakes and lepidocrocite decorated on multi-layered graphene (γ-FeOOH-MLG) and their possible application for Selenate [Se(VI)] ions removal from drinking water. The morphological and structural properties of γ-FeOOH were studied by using different techniques. There is a limited data on the adsorption of Se(VI) ions onto the 2D γ-FeOOH nanosheets. This work delineates adsorption mechanism and merits of incorporating graphene for enhancing adsorbent’s potential for selenium removal from contaminated water. The extraordinary electronic and textural properties of graphene sheets make it a physical support to stabilize γ-FeOOH nanoflakes, substantially improving adsorption capacities and kinetics besides strengthening particle stability. Maximum adsorption capacity of γ-FeOOH and γ-FeOOH-MLG samples estimated by Langmuir adsorption isotherms observed to be ∼40.09 and ∼83.44 mg/g, respectively. The adsorption mechanism of Se(VI) to the surfaces of adsorbents was found to be monolayer and of chemisorption in nature. The pH dependence experiments showed that γ-FeOOH exhibited maximum adsorption capacity at a pH close to the PZC 5.97 (±1), which are in suitable for real ground water conditions.

Radium and strontium binding by a modified bauxite refinery residue – isotope exchange studies of pH-dependence, reversibility and ageing

The pH-dependence and reversibility of radium (Ra) and strontium (Sr) binding to a modified bauxite refinery residue (MBRR), and the effects of sample ageing, were studied in laboratory uptake/leaching experiments. Natural 226 Ra and stable strontium (Sr nat ) were placed in contact with the MBRR for an 8-day loading period (equilibrium pH c . 8.5). Following the addition of exchange isotopes 228 Ra and 85 Sr, the samples were then leached in pH-dependent experiments, where the pH was decreased incrementally from 9 to 3 over 7 h. A further suite of samples was aged at 4°, 23° and 65°C for 6 months after the initial addition of 226 Ra and Sr nat and then studied in a similar set of pH-dependent exchange experiments. The relative concentrations of the Ra and Sr isotope pairs ( 226 Ra/ 228 Ra and Sr nat / 85 Sr) provided insights into the adsorption strength, incorporation, reversibility and ageing effects. The Sr nat data showed that the amount of bound Sr released from aged MBRR samples as the pH decreases is substantially lower than the unaged sample, showing that Sr has been incorporated in less accessible phases during the ageing period. It appears that the uptake of 228 Ra by the higher-temperature aged samples is somewhat stronger than the unaged samples; however, Ra isotope exchange plots for aged and unaged samples are similar. Consequently, the stronger binding of 228 Ra by the higher-temperature aged samples is probably driven by translocation to kinetically less-favourable surface sites rather than a significant incorporation of Ra within the mineral solids during ageing.

Experimental and Computational Study on the Adsorption Mechanism of 2-Arylpropionic Acids on Graphene: Solvent Effects and Aromatic Features Affecting the Adsorption Performance

Adsorption of five 2-arylpropionic acids on graphene indicated that there was a competition between the solute molecules remaining in water solution and depositing on the graphene surface. The pH-dependent experiments showed that the adsorption of five 2-arylpropionic acids on graphene was significantly restrained by the effects of solvent–solute interactions. To find out the relevance between the adsorption capacity and solvent effects, a distribution coefficient (Log D) was introduced and employed to reflect the variation of solvent effects with different pH. An unexpected accordance was observed between the curves of adsorption capacity and Log D values. Then, we extended our work to probe the interactions between 2-arylpropionic acids and graphene. It was found that the experimental adsorption coefficients (Kd) of five adsorbates were closely related to their aromatic features, such as the numbers and distribution of the aromatic rings in their structures, rather than their hydrophobicity, suggesting ...

K2Ti6O13 hybridized graphene oxide: Effective enhancement in photodegradation of RhB and photoreduction of U(VI)

The environmental pollutions by organic pollutants and radionuclides have aroused great concern. Developing highly efficient elimination methods becomes an imperious demand. In this study, a nanocomposite of K2Ti6O13 (KTO) nanobelts hybridized graphene oxide (GO) nanosheets (GO/KTO) was used to photodegrade RhB (dye) and photoreduce U(VI) (radionuclide), which was synthesized by a facile hydrothermal method. The adsorption capacity and the slope (k) of the curve -ln(C/C) versus time in photodegradation of RhB by GO/KTO were higher than that by GO and KTO. In the presence of different free radical scavengers, superoxide radical (·O2−) was found to play the most significant role in the reaction. The XPS experiment indicates U(VI) was successfully photoreduced to less toxic U(IV). The pH dependent photocatalytic experiments on RhB and U(VI) both showed the best performance at neutral pH value (from pH 6 to pH 8). To investigate the reason for the enhanced photocatalysis of GO/KTO, the morphology/microstructure, optical and photo-electrochemical properties were examined. The enhanced abilities of separation of photo electrons and holes and the adsorption of GO/KTO were ascribed to the structure of KTO nanobelts laying on the surface of GO nanosheets, which may maximize the contacting area between KTO and GO, and thus greatly reduce the surface related oxygen defects to enhance the electron interface transfer between KTO and GO and decrease the recombination efficiency of electrons and holes. These results showed the GO/KTO has great application potential in environmental treatment of organic pollutants and high valent heavy/radionuclide ions at neutral condition.

Electropreconcentration-induced local pH change.

Ion-permselective nanochannel-based sample preconcentration or electropreconcentration has been demonstrated as an effective technique for concentrating charged analytes at the interface between a micro- and nanochannel. The anion-selective electropreconcentration involves extraction of hydroxide in the preconcentrated sample plug, resulting in pH decrease. We investigated the pH change in a microchannel using charged pH indicators with different conditions including running buffer pH, sample channel electric field, and salt concentration. The anion-selective preconcentration showed pH decrease from 11 to under 7 in the preconcentrated sample plug. Therefore, careful design and interpretation are required with pH-dependent experiments such as analyzing enzyme or antibody characteristics. The pH change could be mitigated by reducing the sample channel electric field and/or increasing salt concentration in the buffer.

In situ synthesis of rice-like ZnGa2O4 for the photocatalytic removal of organic and inorganic pollutants

Rice-like HT-ZnGa2O4 were successfully synthesized by using commercially available Ga2O3 as an in situ sacrificial templates. The composition, morphology and microstructure of rice-like HT-ZnGa2O4 were characterized in detail. Rice-like HT-ZnGa2O4 were found to be composed of hundreds of nanoparticles. The pH-dependent experiment results indicated that the dispersive rice-like HT-ZnGa2O4 was obtained at pH 8.0. The photocatalytic degradation results showed that the rice-like HT-ZnGa2O4 exhibited high photocatalytic activity and excellent stability for the photocatalytic removal of organic and inorganic pollutants in solution. Finally, the mechanism involved in the photocatalytic process have been investigated by theory and experiment analyses.

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.