Continuous Leaching Research Articles

Biofilm for leaching precious metals from waste printed circuit boards using biocyanidation technology

Presently, biocyanidation technology is being usually adopted to recover precious metals from an increasing quantity of waste printed circuit boards. The main aim of this work was to investigate the biofilm formation of Pseudomonas and its ability to leach precious metals. Based on batch experiments, strain 113 showed the highest biofilm-forming activity in optimal culture conditions of pH 7.0, 25 °C, and 1/25 NB medium among the Pseudomonas strains isolated. Both low concentrations of Cu2+ (500 ppm) and Ag+ (2.5 ppm) promoted biofilm formation. Under the optimal culture conditions for biofilm formation, the concentration of CN− was up to 5.0 ppm. In the continuous silver leaching experiment, the Ag+ concentration reached 4.0 ppm and the leaching efficiency was 14.7 % at 7 d. The results of this study may contribute to the construction of a bioreactor used for continuous leaching of waste printed circuit boards in an attempt to recover precious metals. Our results may also aid in the industrialization of biocyanidation technology.

Cr(VI) Leached from Lignite Fly Ash—Assessment of Groundwater Contamination Risk

The leaching of Cr(VI) from lignite fly ash was investigated by means of experimental standards and theoretical simulations. Two lignite fly ash samples with different calcium oxide concentration were examined. Acid extraction of fly ash and batch leaching tests were performed at various liquid (water) to solid (fly ash) ratios (L/S) for the estimation of Cr(VI) leaching rate. The mobility of Cr(VI) in soil and soil’s hydraulic conductivity at different temperatures was also examined. The results indicate that chromium in fly ash occurs mainly in its trivalent form and that Cr(VI) can be leached, but at lower degree. Along with Cr(VI), the strong reducing agent Fe(II) is also leached. A portion of the Cr(VI) is readily leached, while several hours are needed for reaching Cr(VI) equilibrium concentration in leachate. The fly ash hydraulic conductivity is greatly affected by its pozzolanic characteristics, decreasing from around 10 mm/h (2.78 × 10−6 m/s) to values lower than 1 mm/h (2.78 × 10−7 m/s) in few hours. In continuous leaching tests, relatively low Cr(VI) concentrations (< 20 μg/L) were detected, while in batch leaching tests with higher water/fly ash contact time, leachate Cr(VI) concentrations up to 700 μg/L were detected depending on the applied L/S ratio. However, for low L/S (0.5–1) ratios, no leachate was obtained. Theoretical calculations were performed in order to estimate the L/S ratios at real conditions (by taking into account the volume and the surface of an ash disposal, the density of ash, the hydraulic conductivity of ash, rainfall rate, and rainfall duration). It was found that in real conditions, high L/S can be obtained only when various specific conditions are fulfilled. Experiments with soil showed that only a small portion (approximately 4%) of the leached Cr(VI) can pass through a relatively thin (12–15 cm) layer of soil, even at low temperatures. Based on the experimental results and the groundwater and surface water quality data of a studied basin, a low contamination risk due to Cr(VI) leaching from lignite fly ash is assessed.

ROLE OF ADDED PHOSPHOGYPSUM AND HUMIC ACIDS IN THE KINETIC RELEASE OF SALT FROM SALT AFFECTED SOIL (SALINE – SODIC)

In order to study the effect of phosphogypsum and humic acids in the kinetic release of salt from salt-affected soil, a laboratory experiment was conducted in which columns made from solid polyethylene were 60.0 cm high and 7.1 cm in diameter. The columns were filled with soil so that the depth of the soil was 30 cm inside the column, the experiment included two factors, the first factor was phosphogypsum and was added at levels 0, 5, 10 and 15 tons ha-1 and the second-factor humic acids were added at levels 0, 50, 100 and 150 kg ha-1 by mixing them with the first 5 cm of column soil and one repeater per treatment. The continuous leaching method was used by using an electrolytic well water 2.72 dS m-1. Collect the leachate daily and continue the leaching process until the arrival of the electrical conductivity of the filtration of leaching up to 3-5 dS m-1. The electrical conductivity and the concentration of positive dissolved ions (Ca, Mg, Na) were estimated in leachate and the sodium adsorption ratio (SAR) was calculated. The results showed that the best equation for describing release kinetics of the salts and sodium adsorption ratio in soil over time is the diffusion equation. Increasing the level of addition of phosphogypsum and humic acids increased the constant release velocity (K) of salts and the sodium adsorption ratio. The interaction between phosphogypsum and humic acids was also affected by the constant release velocity of salts and the sodium adsorption ratio. The constant release velocity (K) of the salts and the sodium adsorption ratio at any level of addition of phosphogypsum increased with the addition of humic acids. The highest salts release rate was 216.57 in PG3HA3, while the lowest rate was 149.48 in PG0HA0. The highest release rate of sodium adsorption ratio was 206.09 in PG3HA3, while the lowest rate was 117.23 in PG0HA0.

ROLE OF ADDED PHOSPHOGYPSUM AND HUMIC ACIDS IN THE KINETIC RELEASE OF SALT FROM SALT AFFECTED SOIL (SALINE – SODIC)

In order to study the effect of phosphogypsum and humic acids in the kinetic release of salt from salt affected soil, a laboratory experiment was conducted in which columns made from solid polyethylene were 60.0 cm high and 7.1 cm in diameter. The columns were filled with soil so that the depth of the soil was 30 cm inside the column, the experiment included two factors, the first factor was phosphogypsum and was added at levels 0, 5, 10 and 15 tons ha-1 and the second factor humic acids were added at levels 0, 50, 100 and 150 kg ha-1 by mixing them with the first 5 cm of column soil and one repeater per treatment. The continuous leaching method was used by using an electrolytic well water 2.72 dS m-1. Collect the leachate daily and continue the leaching process until the arrival of the electrical conductivity of the filtration of leaching up to 3-5 dS m-1. The electrical conductivity and the concentration of positive dissolved ions (Ca, Mg, Na) were estimated in leachate and the sodium adsorption ratio (SAR) was calculated. The results showed that the best equation for describing release kinetics of the salts and sodium adsorption ratio in soil over time is the diffusion equation. Increasing the level of addition of phosphogypsum and humic acids increased the constant release velocity (K) of salts and the sodium adsorption ratio. The interaction between phosphogypsum and humic acids was also affected by the constant release velocity of salts and the sodium adsorption ratio. The constant release velocity (K) of the salts and the sodium adsorption ratio at any level of addition of phosphogypsum increased with the addition of humic acids. The highest salts release rate was 216.57 in PG3HA3, while the lowest rate was 149.48 in PG0HA0. The highest release rate of sodium adsorption ratio was 206.09 in PG3HA3, while the lowest rate was 117.23 in PG0HA0.

Leachability of endocrine disrupting chemicals (EDCs) in municipal sewage sludge: Effects of EDCs interaction with dissolved organic matter

In this study, experiments were performed to assess the significance of dissolved organic matter (DOM) on the leachability of four common EDCs, i.e., bisphenol A (BPA), 17α-ethinylestradiol (EE2), progesterone (PGT) and testosterone (TST), in municipal sewage sludge (MSS) under landfill conditions. The DOM was derived from two sources: MSS (MDOM), and natural soil represented by organic matter obtained from the Suwannee River (NDOM). Fluorescence excitation-emission matrix quenching combined with parallel factor analysis was adopted to characterize the interaction properties between the EDCs and DOM. The accumulative leachability of the target EDCs ranged from 0.09% (PGT) to 3.8% (TST). In particular, the leaching of BPA, EE2 and TST followed S-shaped curves, while PGT exhibited continuous leaching potential in untreated MSS. With the introduction of DOM, (i) the leachability of BPA and EE2 increased to 13.4% and 61.6%, respectively, whereas those of PGT and TST declined by 61.3% and 45.8%, respectively, and (ii) BPA, EE2 and PGT no longer reached leaching equilibrium but the S-shaped leaching property of TST persisted. The differential effects of MDOM and NDOM at identical concentrations on the EDCs leachability increased with curing time. BPA, EE2 and PGT quenched the MDOM fluorophores attributed to aromatic protein-like components. The fluorescence quenching of NDOM by BPA, EE2 and PGT was centered on soluble microbial by-product-like and humic-like substances. Compared with PGT, EE2 and BPA had greater capability for binding with DOM components largely via hydrophobic interactions, whereas PGT preferentially interacted with the DOM hydrophilic functionalities through specific interactions. TST had no binding capability but displayed potentials competing for sorption sites with DOM moieties. Our findings suggested that the management of MSS increased the risk of environmental contamination by EDCs for a long duration and that DOM was a useful indicator to predict the migration and transport properties of EDCs.

Continuous leaching and separation of Pb in CH3COOH-CH3COONa buffered electrolyte

Metal lead and zinc survive with each other in the lead-zinc sulfide ore. Green recycling of high quality lead products is an urgent challenge. The electro-oxidative hydrometallurgical process provides another new idea of green recycling for the treatment of metals in other polymetallic sulfide minerals. In the CH3COOH-CH3COONa buffered electrolyte with pH of 5.7, the optimal electrolysis system was formed: S/L ratio = 0.02 g/mL, stirring speed = 700 rpm, anodic current density = 659.4 A/m2, temperature = 20 °C，reaction time = 60 min, NaCl concentration = 3 mol/L, applied potential = 3.8 V – 4.4 V. The recovery process of metal lead and zinc was systematically investigated. The addition of galena-sphalerite to the electrolyte solution proved that more than 71.55% of the lead was recovered with less than 2.20% zinc was dissolved. The ClO− has been proven to be a mineral leaching catalyst. In addition, the reaction mechanism was studied by linear sweep voltammetry (LSV). The basic principle of lead selective recovery has been studied, and the electrolyte solution in the system can be recycled many times.

A population balance approach to predict the performance of continuous leaching reactors: Model validation in a pilot plant using a roasted zinc concentrate

A Population Balance model for the continuous leaching of a roasted zinc concentrate was developed. The Maximum Mixedness and the Segregated Flow approaches were chosen to model the flow patterns and mixing conditions in the reactors. In order to compare these approaches and validate the proposed model, leaching experiments were carried out in a pilot plant. For that, it was also determined the residence time distributions in this plant reactors. As a result, it was observed that the Segregated Flow did not represent the actual pilot plant operation, since the conditions in which the Segregated Flow is recommended (i.e. segregation in the micromixing of the slurry phase, low diffusivity, and high slurry viscosity) were not present. On the other hand, using the Maximum Mixedness resulted in an excellent agreement between the experimental and modeled data (mean absolute error of 0.5% for the zincite conversion). This indicates that the complex particulate phenomena that occur during the leaching, as well as the flow patterns in the continuous reactors were adequately accounted in the model. Therefore, a robust tool for the design and control of leaching plants is delivered, since it is able to predict the effect of operational parameters (e.g. particle size distribution, solid/liquid ratio, feed flow rate) on the reactors performance. This can lead to better decision-making and costs reduction, contributing also to the prediction of problems faced in the operation of these plants. In a direct application of the developed model, it was predicted that the pilot plant could operate with the doubled capacity.

Correction to: Engineering behaviour of lime- and waste ceramic dust-stabilized expansive soil under continuous leaching

The published version of this article unfortunately contained a mistake in Table 2. Incorrect values of maximum dry density (γdmax) were mistakenly entered in Table 2.

Simulating Continuous Counter-Current Leaching Process for Indirect Mineral Carbonation Under Microwave Irradiation

Mineral carbonation is a promising avenue to realize a deep reduction in carbon dioxide emissions. Though many methods were studied to improve the leaching ratio of mineral leached by ammonium salt, little attention has been received to the problem that the calcium leaching ratio increases while its concentration drops rapidly with the liquid-solid ratio increasing. The continuous counter-current leaching for mineral carbonation process under microwave irradiation is proposed in this study, and the results show that the simulating continuous counter-current leaching process in this article not only is beneficial to improve the leaching ratio and concentration of calcium ions in solution at the same time, but also increases the relative purity of calcium in leached solution. And the produced calcium carbonate products meet the requirements of industrial precipitation of calcium carbonate.

Engineering behaviour of lime- and waste ceramic dust-stabilized expansive soil under continuous leaching

Numerous distresses in pavement structures on chemically stabilized expansive soils have raised considerable debate regarding the long-term performance of lime-stabilized subgrades. Therefore, this paper examines the effects of continuous leaching on the durability of lime- and waste ceramic dust (WCD)-stabilized expansive subgrades. The tests were performed with different percentages of quicklime (3.5, 4.5, 5 and 10%), while the combined effects of lime and WCD were evaluated by adding 1.5% of WCD to the mixtures containing 3.5 and 4.5% lime. Geotechnical tests such as Atterberg limits, compaction and California bearing ratio (CBR), in addition to microstructural and leachate analyses, were used to evaluate the engineering properties of the stabilized soils. The CBR result shows that early strength development occurred in the soils mixed with lime and WCD. However, at longer curing periods, the beneficial effect of lime on the engineering properties of the soil increased with an increase in lime content. Results of the leaching tests show considerable changes in the physicochemical properties of the soil mixtures, while the estimated service life of the stabilized soils increased from 3.4 to 9.2 years, as lime content increased from 3.5 to 10%. Microstructural analysis results reveal that the flocculated nature of the soil fabric and the density of the fibrous cementitious compounds increased with an increase in lime content. This study clearly indicates that electrical conductivity, pH and pore-water cation concentrations can be used in combination with geotechnical tests, to correctly evaluate the durability of lime- and lime-WCD-stabilized expansive soils.

Application of Principal Component Analysis and Correlation for Assessing Groundwater Contamination in and around Municipal Solid Waste Landfill of Ghazipur, Delhi

The present study has been executed to assess the groundwater quality and to investigate the possible sources of contamination in the adjoining areas of Ghazipur landfill site, Delhi. The groundwater samples were collected during pre and post monsoon seasons and various physico-chemical and heavy metals concentration were also analyzed. The results show that the samples which were taken adjacent to landfill site had a higher concentration of total dissolved solids (2383 mg/l), chloride (991mg/l), total hardness (659 mg/l), ammonical nitrogen (2.3mg/l), sodium (350 mg/l), calcium (534 mg/l) and sulphate (631 mg/l). Most of the heavy metals concentration in groundwater samples was found within the prescribed limit except Cu and Pb. The principal component analysis (PCA) extracted 3 factors which collectively explained 80.1% of the variation in data during post-monsoon whereas 75.4% of the variation in pre-monsoon season. The obtained results clearly show that K+, Mg2+, Na+, SO42−, and NO3− ions significantly contribute towards the high temporal pollution load to groundwater quality. Factor analysis also suggested that the continuous percolation of these contaminants from the nearby landfill could be the possible sources of contamination into groundwater aquifer system. The concentration of heavy metals in leachate have followed the order as- Fe>Zn>Cu>Pb>Ni>Cr>Cd. The overall result indicates that the landfill leachate is significantly affecting the nearby groundwater quality through continuous leaching over a long period of time. As the groundwater is used by some local residents for domestic purposes, immediate action and alternative effort should be made to supply clean water.

Saline-sodic soil treated with gypsum, organic sources and leaching for successive cultivation of sunflower and rice

ABSTRACT Correction of saline and sodic soils aims to reduce salts dissolved in the solution and exchangeable sodium, respectively, to allow the growth and production of crops. In this context, an experiment was carried out between August/2011 and September/2012, in saline-sodic soil of the Irrigated Perimeter of São Gonçalo, in the municipality of Sousa, PB, Brazil. Agricultural gypsum, organic sources and continuous leaching for reducing salinity, sodicity and alkalinity in the saline-sodic soil and their effects on the production of the sunflower cultivar Embrapa 122/V-2000 and the rice variety Diamante were evaluated. The treatments were distributed in four randomized blocks and the soil was subjected to continuous leaching for 50 days and evaluated for salinity, sodicity and alkalinity before and after leaching, as well after sunflower and rice cultivation, in the 0-0.20 and 0.20-0.40 m layers. Leaching and the application of gypsum and organic sources reduced the initial salinity in both soil layers, to a greater extent in the surface layer. Exchangeable sodium decreased in 0-0.20 m and increased in 0.20-0.40 m. After rice cultivation, the soil in the 0-0.20 m layer changed from saline-sodic to non-saline in the treatments gypsum + bovine manure and gypsum + rice husk. The reduction of salinity, sodicity and alkalinity in the soil was higher during rice cultivation than during sunflower cultivation.

Selective removal of radiocesium from micaceous clay for post-accident soil decontamination by temperature-controlled Mg-leaching in a column

The effective and efficient removal of radioactive Cs from contaminated soil is highly urgent for the nuclear post-accident remediation. In present study, we achieved rapid Cs desorption from both a typical micaceous clay (i.e., vermiculitized biotite, VB) and actually contaminated soil by high-speed ion exchange through temperature-controlled continuous leaching with Mg-solutions in a column reactor. Cs-sorbed VB was firstly employed as a soil surrogate to explore the macro-Cs desorption process and micro-mechanism in detail. Results showed that VB sandwiched the adsorbed Cs to its interlayers within collapsed structure (10.7 Å) and prevent Cs release even by abundant extraction with H2O at 250 °C or Mg2+ at 25 °C. However, Mg2+-extracted Cs desorption boosted significantly with elevating temperatures and 100 % of sorbed-Cs was removed from Cs-VB leached above 150 °C. Further structural and composition analysis of the leached specimen ensured that solvated Mg2+ preferentially entered into Cs+-collapsed interlayers at 150 °C than K+-interlayers above 200 °C, leading to prior complete Cs removal over K from VB at lower temperatures. By contrast, the Cs-contaminated soil reduced by ∼39 % but ∼82 % of its initial radioactivity after equally leaching with same volumes of Mg2+-solution at 150 and 200 °C, respectively. These temperature-controlled Cs desorption validated that radioactive Cs in actual soil indeed be tightly trapped by micaceous clays nearly in the Cs-K co-collapsed interlayers, to which its extraction by other cations can conditionally occur above enough high leaching temperatures. These superior features would inspire new insights for the design of novel practical technologies for treatment and decontamination of the nuclear post-accident soils.

Effects of continuous leaching on engineering properties of lime-stabilized lateritic soils

This research investigates the performance of lime-stabilized lateritic subgrades under continuous leaching by focusing on changes in the engineering properties of the natural soil together with those treated with varying proportions of lime. Geotechnical tests conducted include California bearing ratio (CBR), Atterberg limits, particle size analysis and Proctor compaction tests. The leaching test was conducted in a constant-head cylindrical Perspex tank for a period of 28 days. X-ray diffraction analysis was conducted on the extracted leachates coupled with other physicochemical tests such as pH, electrical conductivity (EC), and calcium and potassium concentrations. The experimental results indicate that the highest CBR value of 47.1% was obtained from the mixture containing 5% lime after a curing period of 7 days. Further increase in the amount of lime resulted in a decrease in CBR, except for the 10% soil-lime mixture which had a CBR value of 31.4% after a curing period of 28 days. Physicochemical analyses showed a significant decrease in EC, calcium and potassium concentrations from the pore fluid of the soil-lime mixtures. The results suggest that the potential for strength loss was higher in the mixtures stabilized below their optimum lime contents than in those stabilized above their optimum lime contents.

Reactive iron isotope signatures of the East Asian dust particles: Implications for iron cycling in the deep North Pacific

Iron released from wind-blown dust has been widely suggested to be an important nutrient source for oceanic primary productivity. Once introduced into the seawater, the reactive Fe might get dissolved and impart its isotope signature into the water column. Characterization of the reactive iron isotope signatures of eolian dust will thus help to understand the budget of Fe in the global ocean. Yet, there are few systematic studies on the Fe isotope systematics of the dust sources across large spatial scale. Here, we report Fe isotope data from HCl leaching experiment on surficial samples from deserts and loess deposits of different locations spanning >3000 km spatially in east Asia, one of the most important dust source regions. Our data suggest most of the samples have relatively similar leaching signatures (δ56Fe = –0.09±0.07‰, n=9, 2 SD). To understand the kinetics of Fe release during proton promoted leaching of natural particles, a synthetic model taking into account of different dissolution dynamics of easily dissolvable Fe and silicate Fe was developed. The model was tested by continuous leaching of two representative pelagic sediment samples recovered from core LL44-GPC3 in the North Pacific. When introducing only one tunable parameter (i.e., the initial dissolution rate of Fe from hydro(oxides), with other parameters constrained by the dissolution kinetics), the model successfully reproduces leachate δ56Fe evolution and agrees reasonably well with Fe speciation in the studied marine sediment samples. We thus suggest that the leaching process can be well characterized by our kinetic model, while Fe isotopes in the initial leachates of the samples are dominated by endmember composition of the easily dissolvable Fe species. The small but distinct fractionation of Fe isotopes in the desert/loess samples from the parental upper continental silicate materials are suggested to record signatures of the secondary Fe species produced during weathering in the oxic environment. The significantly lighter Fe isotope compositions in the initial leachates of the North Pacific pelagic sediment samples compared to the east Asian desert/loess deposits indicate apparent isotope exchange of the dust reactive components with seawater dissolved species. The sedimentary light reactive Fe isotope signatures in turn point toward the contribution of sources characterized by low δ56Fe to the deep North Pacific and/or preferential uptake of 56Fe by seawater ligands from the dust reactive components particularly at the seawater-sediment interface.

Compound redistribution due to droplet evaporation on a thin polymeric film: Theory

A thin polymeric film in contact with a fluid body may leach low-molecular-weight compounds into the fluid. If this fluid is a small droplet, the compound concentration within the liquid increases due to continuous leaching in addition to the evaporation of the droplet. This may eventually lead to an inversion of the transport process and a redistribution of the compounds within the thin film. In order to gain an understanding of the compound redistribution, we apply a macroscopic model for the evaporation of a droplet and combine that with a diffusion model for the compound transport. In the model, material deposition and the resulting contact line pinning are associated with the precipitation of a fraction of the dissolved material. We find three power law regimes for the size of the deposit area as a function of the initial droplet size, dictated by the competition between evaporation, diffusion, and the initial compound concentrations in the droplet and the thin film. The strength of the contact line pinning determines the deposition profile of the precipitate, characterized by a pronounced edge and a linearly decaying profile toward the center of the stain. Our predictions for the concentration profile within the solid substrate resemble patterns found experimentally.

Comparative study of the adsorption/immobilization of Cu by turmeric residues after microbial and chemical extraction.

The turmeric industry produces a huge amount of residues annually. After undergoing different extraction process, turmeric residue biomass may be transformed from waste to resource. Turmeric residues exhibit different characteristics suitable for various environmental applications. In this work, the adsorption of Cu(II) onto turmeric residues from microbial (TR-A) and chemical (TR-B) extraction was investigated. The characteristics of the residues were examined via Brunauer-Emmett-Teller analysis, thermogravimetric analysis, scanning electron microscopy, Fourier-transform infrared spectroscopy, and elemental analysis. Then, applications to Cu(II) immobilization were identified. Results suggested that although TR-B had better thermal stability, larger surface area, and more pores than TR-A, the adsorption capacity of Cu(II) onto TR-A was higher (13.12 mg/g) than that onto TR-B (7.37 mg/g) because TR-A had more microbial cell debris, metabolites, and S element than TR-B. In practice, TR-A-added soil achieved 40% more Cu immobilization than TR-B-added soil under continuous leaching of simulated acid rain. Consequently, the residues extracted using the microbial method prevented pollution after the traditional extraction process and transformed waste into a material for environmental remediation.

The Influence of Soil Mechanical Redesigning on Polycyclic Aromatic Hydrocarbon (PAH) Release: a Column Approach

Intrinsic molecular parameters (solubility, chemical structure, etc.) as well as physico-chemical properties of the medium (lithology, permeability, organic matter content, etc.) and hydrological specificities (rainfall regimes) are usually put forward to characterize the mobility of polycyclic aromatic hydrocarbons (PAHs). Here, we study the role of aggregate mechanical redesigning on the ability of anciently impacted soils to release polycyclic hydrocarbons. Soil columns were collected in a heavily impacted wasteland (Villeneuve la Garenne, France) for this purpose and submitted to continuous and sequential leaching regimes. The soil’s organo-mineral properties were characterized with suited techniques (Rock Eval, FT/IR) prior to the leaching experiments. The release of 18 PAHs (naphthalene, 1 methyl naphthalene, 2 methyl naphthalene, acenaphtylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benz(b)fluoranthene, benz(k)fluoranthene; benzo(a)pyrene, indeno[1,2,3-cd]pyrene, dibenz(a,h)anthracene, benzo(g,h,i) perylene) was assayed under non-disturbed configuration of the columns, then under reshuffled (10 mm), and finely sieved (4 mm) states in order to evaluate the impact of grain redistribution on the mobilization of the aromatic molecules. For both sequential and continuous leaching regimes, results showed a substantial increase of released amounts when the systems were disturbed. Significant increases of PAH concentrations were indeed observed when the columns were reshuffled (10 mm). The release was more intense when the mechanical reworking was performed with a smaller threshold (4 mm). Greater accessibility of the molecules drove very likely PAH release during the mechanical reworking operations.

Long-term acid generation and heavy metal leaching from the tailings of Shimokawa mine, Hokkaido, Japan: Column study under natural condition

The leaching from mine tailings results in contamination of nearby groundwaters and rivers by hazardous metals like copper (Cu), zinc (Zn), and iron (Fe). In this study, unweathered tailings samples at depths of 1 to 3 m were collected from a tailings dam of an abandoned mine located in the north of Hokkaido, Japan. The mechanisms of long-term tailings weathering were assessed through leaching of hazardous metals by three column experiments. Measurements of mineralogical and chemical constituents, observation by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), sequential extraction of the tailings, and chemical analyses of the leachates were carried out to determine the processes responsible for the leaching of Cu, Zn, Fe, and sulfate ion (SO42−). The contents of Cu, Zn, and Fe in the tailings were mainly associated with ion exchangeable and sulfide fractions. The pH values of the effluents from the columns were 3.0–3.7 throughout the experiments over 84 weeks, and approximately 15–23% of Cu, 35–45% of Zn, 2.5–4% of Fe and 15–20% of S were leached. Higher concentrations of Cu, Zn, Fe, and SO42− at the beginning of the experiments were observed, which could be attributed to the dissolution of soluble sulfate minerals present in the tailings. This indicates that the formation and dissolution of secondary soluble sulfate minerals contributed to Cu and Zn leaching. The continuous leaching of Cu, Zn, Fe, and SO42− suggests the oxidation of pyrite and other sulfide minerals. During these processes, ferrihydrite, goethite, lepidocrocite, and maghemite were formed and these minerals also acted as a sink for Cu and Zn by adsorption, and/or co-precipitation. These results mean the significance of the long-term behavior of hazardous metals released from mine tailings dams, which could provide helpful information on the management of tailings dams after mine closure.

Influence of wood ash pre-treatment on leaching behaviour, liming and fertilising potential

In Denmark, increasing amounts of woody biomass are being used for the production of renewable energy, resulting in more wood ashes being generated. While these materials have been mainly landfilled, wood ashes may also be utilised for fertilizing and liming purposes on top of soils. Pre-treatments involving hardening or granulation may be carried out prior to soil application. In this study, two Danish wood ash samples were hardened and/or granulated. Lab-hardening induced rapid changes in the shape of the acid neutralisation capacity curve of the ashes. Up-flow column tests, assuming local equilibrium conditions, were employed to investigate the leaching from pre-treated ashes. Granules and loose ashes demonstrated similar leaching behaviours, indicating that similar geochemical processes were governing their leaching. In comparison with untreated fresh ashes, the hardened ashes demonstrated reduced leaching of Ca, Ba, Pb and Zn with concentration levels generally below or close to the analytical limits of quantification; to the contrary, the leaching of As, P, Sb, Si, V and Mg was enhanced in the hardened ashes. The release of alkalinity was reduced by hardening. In general, all granules were barely breakable by finger-pinching and they could withstand one month of continuous leaching, preserving their overall shape. The solubility of phosphorous in neutral ammonium citrate indicated that about 30–51% of the total P content in the ash samples was released, suggesting that the ashes could be potentially valuable as P-fertiliser if applied onto soil.