Sequential Leaching Procedure Research Articles

Improvements in recovery of rare earth elements (REEs) from coal via fluidized-bed combustion: Thermal alteration of REE mineralogy and its impact on element extractability

The extraction of rare earth elements (REE) from coal ash has attracted significant attention, but satisfactory solutions have not yet been achieved. Indeed, the REE extractability is largely determined by the transformation behavior of REE during coal combustion. This study found that the controlled combustion conditions of circulated fluidized-bed boiler (CFB) can both utilize the heating value and enhance the recovery of REE from coal. Tests were conducted using a bituminous coal with REE concentration of approximately 149 ppm in an industrial 35 t/h CFB boiler with controlled boiler loads and coal sources. Advanced automated image analysis by scanning electron microscopy (AIA-SEM) and sequential leaching procedure were employed to clarify the speciation transformation behavior of REE in boilers and investigate the effects of these transformations on the leachability of REE. Approximately 75 % of REE partitioned into fly ash and enriched in particles < 45 μm (∼710 ppm) after combustion. Acid leaching tests under mild conditions (1 M HCl at 60 ℃ and S/L = 1:25) performed on coal, bulk ashes, and sieved ashes revealed an improvement in REE leaching ratio from coal (21.34 %) to fly ash (71.07 %). Particle size played a crucial role in acid extractability, particularly for middle and heavy REE in cyclone ash but had no influence on those in fly ash. The dominant forms of REE in coal including Ca-REE fluorocarbonates (36.7 %), aluminophosphates (12.0 %), and organic-bound REE (approximately 20 %) significantly decreased, contributing to the formation of leachable rare earth oxides (∼65 %) in ash. Furthermore, the small size of REE mineral grains in coal (over 50 % below 5 μm) and extensive mineral fragmentation occurring in boiler resulted in the enrichment and enhanced REY leachability in fine ash.

A sequential leaching procedure for efficient recovery of gold and silver from waste mobile phone printed circuit boards

The present study reports a sequential, non-acid process for effective recovery of copper and precious metals from mobile phone printed circuit boards. In this process, gold and silver were first enriched during the synthesis process of cuprous chloride and then leached by thiosulfate method. Results indicated that the distribution of gold and silver in the liquid and solid phases during the synthesis of cuprous chloride process was affected by the [Cu]/[Cu2+] ratio. Enrichment of gold and silver in the residue after the cuprous chloride synthesis could be achieved by the adjusting the [Cu]/[Cu2+] ratio. The silver and gold leaching rates of the residue after cuprous chloride synthesis (93.8 % silver and 99 % gold) were much higher than those of the raw PCB sample (27.0 % silver and 14.2 % gold) under the same conditions. This process has the advantages of high leaching efficiency, high leaching rate and avoiding the use of HNO3 or aqua regia commonly used for copper, gold and silver recovery. Thus, this study offers a promising and environmentally friendly method for recovering valuable metals from e-waste.

Experimental Investigation of Oxide Leaching Methods for Li Isotopes.

To examine the applicability of different leaching methods used to extract secondary oxides from silicate solids for lithium isotope (δ7Li) measurement, this study has conducted leaching experiments on five different types of silicate solids, including a fresh basalt, two weathered basalts, a Yellow River sediment (loess‐dominated) and a shale. Four factors were assessed in the experiments: the concentration of the leaching reagent hydroxylamine hydrochloride (HH), the leaching temperature (20 °C vs 95 °C), the leaching time and the reagent/solid ratio. Based on elemental concentrations and Li isotopes, 0.04 mol l−1 hydroxylamine hydrochloride (HH) in 25% v/v acetic acid at room temperature for 1 h with 40 ml g−1 reagent/solid ratio is recommended. At high temperatures, low δ7Li and high magnesium/iron ratios indicate that minerals other than secondary oxides are dissolved. With increased leaching time, there is no evidence for Li isotopic fractionation at room temperature. However, longer leaching time or increased reagent/solid ratios may increase the risk of leaching from non‐oxide phases. Meanwhile, results suggest that low concentrations of HH are not sufficient to target the secondary oxides evenly, while high concentrations of HH can leach out more non‐oxides. We also examined the optimal oxide leaching method within a full sequential leaching procedure (i.e., exchangeable, carbonate, oxide, clay and residual phases). Elemental concentrations show that no elements exist exclusively in oxides, so it is essential to analyse multi‐elemental concentrations to verify that the leaching has accessed this phase in a given sample. Comparing secondary oxides with their corresponding solutions, we estimate the isotopic fractionation (Δ7Lioxide‐solution) is −16.8‰ to −27.7‰.

Boron recovery from montmorillonite clay waste using sequential leaching followed by cooling crystallization techniques

A four-stage sequential leaching (SL) and subsequently cooling crystallization (CC) techniques have been implemented for the recovery of boron from the clay waste (CW) induced by the borax derivatives production facility located in Kirka district of Eskisehir province, NW of Turkey. Optimum experimental solid-liquid ratio and agitation time for SL tests were determined by preliminary experiments. In the first leaching stage, approximately 85% of boron in the CW was transited to the leachate. A logarithmic decrease in the boron transition efficiency was obtained as a function of stages. After four-stage SL procedure, CC procedure was carried out with the last leachate at 4.0 °C ±0.3 stabilized refrigerator. Approximately 77% of dissolved boron in the last leachate was crystallized by CC as high purity borax crystals. Mineralogical, chemical, and physical definitions of the crystals were determined by X-ray diffraction (XRD), instrumental element analysis following microwave digestion, and scanning electron microscopy (SEM) procedures, respectively. As a result, sequential leaching followed by cooling crystallization system was determined as an effective methodology for the recovery of boron from the CW as high purity borax form.

The heavy metal partitioning in the particle flux of the subarctic White Sea (Northwestern Russia)

This research addresses the environmental factors that affect the heavy metal accumulation and partition in the particle fluxes of the White Sea. The particulate material was collected between 2006 and 2014 with sediment traps at 9 stations from depths of 25–220 m to determine its geochemical composition. Contribution of terrigenous and hydrogenous processes was estimated based on the sequential leaching procedure which allowed distinguishing the geochemical fractions: exchangeable, Fe–Mn authigenic oxyhydroxides; organic matter/sulfides; residual (fixed in clastic minerals). Terrigenous processes were dominated in the accumulation of Al, Fe, Cr, Ni, Co, and Mo (60–90% of the total content), while the accumulation of Mn, Cu, Cd, and Pb was primarily associated with hydrogenic processes (authigenic oxyhydroxides' formation, sorption processes, and organic compounds). These two groups of chemical elements demonstrated a different distribution pattern in particle fluxes with water depth. Growth of the total particle fluxes with the water depth took place as a rule against the background of authigenic mineral formation (Fe–Mn oxyhydroxides), a slight increase in the lithogenic components, and the slow destruction of organic matter. In areas, where the vertical particle flux dominates, increased content of elements related to the Fe–Mn oxyhydroxides and bound to organic matter, were detected. The influence of anthropogenic sources based on the criteria of contamination level (EF x and I geo) was found only for Cd in some samples. An insignificant anthropogenic contribution is also possible for Cu and Ni, comparing with the background concentrations.

Urban Soils and Road Dust—Civilization Effects and Metal Pollution—A Review

Urban soils have been changed much by human impacts in terms of structure, composition and use. This review paper gives a general introduction into changes from compaction, mixing, water retention, nutrient inputs, sealing, gardening, and pollution. Because pollutions in particular have caused concerns in the past, metal pollutions and platinum group metal inputs have been treated in more detail. Though it is not possible to cover the entire literature done on this field, it has been tried to give examples from all continents, regarding geochemical background levels. Urban metal soil pollution depends on the age of the settlement, current emissions from traffic and industry, and washout. It seems that in regions of high precipitation, pollutants are swept away to the watershed, leaving the soils less polluted than in Europe. Health hazards, however, are caused by ingestion and inhalation, which are higher in 3rd world countries, and not by concentrations met in urban soils as such; these are not treated within this paper in detail. With respect to pollutants, this paper is focused on metals. Contrary to many reviews of the past, which mix all data into one column, like sampling depth, sieved grain sizes, digestion and determination methods, these have been considered, because this might lead to considerable interpretation changes. Because many datasets are not Gaussian distributed, medians and concentration ranges are given, wherever possible. Urban dust contains about two to three fold the hazardous metal concentrations met in urban soils. Some data about metal mobilities obtained from selective and sequential leaching procedures, are also added. Soil compaction, pollution, sealings and run-offs cause stress situations for green plants growing at roadside locations, which is discussed in the Section 5. Environmental protection measures have led to decrease metal pollutions within the last decade in many places.

Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling in Cryogenian Seawater

The behaviour of bioavailable trace metals and their stable isotopes in the modern oceans is controlled by uptake into phototrophic organisms and adsorption on and incorporation into marine authigenic minerals. Among other bioessential metals, Cd and its stable isotopes have recently been used in carbonate lithologies as novel tracer for changes in the paleo primary productivity and (bio)geochemical cycling. However, many marine sediments that were deposited during geologically highly relevant episodes and which, thus, urgently require study for a better understanding of the paleo environment are rather composed of a mixture of organic matter (OM), and detrital and authigenic minerals. In this study, we present Cd concentrations and their isotopic compositions as well as trace metal concentrations from sequential leachates of OM-rich shales of the Cryogenian basal Datangpo Formation, Yangtze Platform (South China). Our study shows variable distribution of conservative and bioavailable trace metals as well as Cd isotope compositions between sequential leachates of carbonate, OM, sulphide, and silicate phases. We show that the Cd isotope compositions obtained from OM leachates can be used to calculate the ambient Cryogenian surface seawater of the restricted Nanhua Basin by applying mass balance calculations. By contrast, early diagenetic Mn carbonates and sulphides incorporated the residual Cd from dissolved organic matter that was in isotopic equilibrium with deep/pore waters of the Nanhua Basin. Our model suggests that the Cd isotopic composition of surface seawater at that time reached values of modern oxygenated surface oceans. However, the deep water Cd isotope composition was substantially heavier than that of modern fully oxygenated oceans and rather resembles deep waters with abundant sulphide precipitation typical for modern oxygen minimum zones. This argues for incomplete recycling of Cd and other bioavailable metals shortly after the Sturtian glaciation in the redox stratified Cryogenian Nanhua Basin. Our study highlights the importance of sequential leaching procedures when dealing with impure authigenic sediments such as OM-rich carbonates, mudstones, or shales to achieve reliable trace metal concentrations and Cd isotope compositions as proxies for (bio)geochemical metal cycling in past aquatic systems.

Strontium isotope stratigraphy and paleomagnetic age constraints on the evolution history of coral reef islands, northern South China Sea

Abstract Understanding the history of the response of coral reefs to past climate changes can provide valuable information for predicting the future response of modern reefs. However, dating such ancient biotic carbonate is still challenging because of its sensitivity to diagenetic alteration processes, scarcity of well-preserved fossils, and low magnetic mineral content. There have been a long debates about the origin and evolutionary history of coral reefs in the northern South China Sea, mainly due to the lack of direct and reliable age constraints. This provides us with a good opportunity to verify the practicability of different dating approaches, especially the strontium (Sr) isotope analysis of bulk carbonate. Here, we retrieved a 972.55-m-long core from the Xisha Islands to provide a credible chronologic constraint on the carbonate platform evolution. The lithostratigraphy, strontium isotope stratigraphy, and magnetostratigraphy were analyzed throughout the whole reef sequence. The lithostratigraphic results show that the 873.55 m reef sequence developed on an ancient volcaniclastic basement and experienced multiple evolutionary phases. The 87Sr/86Sr results of all 100 bulk carbonate samples vary from 0.708506 to 0.709168 and show a monotonic increase with decreasing depth, except for a few outliers. Trace-element criteria and stable isotope (δ18O and δ13C) methods were applied to these bulk carbonate samples, and results imply that the primary or near-primary seawater 87Sr/86Sr values were likely preserved, although different degrees of diagenetic alteration occurred. In addition, the paleomagnetic results indicate 10 normal polarity and eight reversed polarity magnetozones. Based on the 87Sr/86Sr ratios of the selected 58 samples and paleomagnetic reconstruction of polarity reversals, the bottom of the reef sequence is dated to 19.6 Ma, and the observed polarity chronozones extend from chron C6 (19.722–18.748 Ma) at 866.60 m to present at the top. Based on the new data, we propose a new chronologic framework for the evolutionary history of the reef islands, where: (1) the reefs initiated in the early Miocene (19.6 Ma) and were drowned until 16.26 Ma; (2) during 16.26–10.66 Ma, lagoon to lagoon slope environments prevailed; (3) the lagoon environment progressively transformed into a reef crest environment from 10.66 to 4.36 Ma and 4.36 to 1.59 Ma; and (4) the reef started to be drowned again during 1.59–0 Ma. Our study provides a new and reliable chronologic constraint on the general evolutionary history of the reef islands in the northern South China Sea. Furthermore, the 87Sr/86Sr results from bulk carbonate indicate that strontium isotope stratigraphy is a powerful dating tool only when rigid sample selection, sequential leaching procedures, and strict trace-element and isotopic criteria are applied.

Distribution of Rare Earth Elements plus Yttrium among Major Mineral Phases of Marine Fe–Mn Crusts from the South China Sea and Western Pacific Ocean: A Comparative Study

Marine hydrogenetic Fe–Mn crusts on seamounts are known as potential mineral resources of rare earth elements plus yttrium (REY). In recent years, increasing numbers of deposits of Fe–Mn crusts and nodules were discovered in the South China Sea (SCS), yet the enrichment mechanism of REY is yet to be sufficiently addressed. In this study, hydrogenetic Fe–Mn crusts from the South China Sea (SCS) and the Western Pacific Ocean (WPO) were comparatively studied with mineralogy and geochemistry. In addition, we used an in situ REY distribution mapping method, implementing laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a sequential leaching procedure to investigate the partitioning behavior of REY in the Fe–Mn crusts. The typical Fe–Mn crusts from SCS were mainly composed of quartz, calcite, vernadite (δ-MnO2), and amorphous Fe oxides/hydroxides (FeOOH). The Fe–Mn crusts from the Central SCS Basin and the WPO contained quartz, δ-MnO2, FeOOH, todorokite, and phillipsite. Furthermore, geochemical analysis indicated that the typical SCS crusts had a higher growth rate and lower REY concentrations. The LA-ICP-MS mapping results showed that the δ-MnO2 and FeOOH dominated the occurrence phases of REY in the SCS crusts. Four mineral phases (i.e., easily exchangeable and carbonate, Mn-oxide, amorphous FeOOH, and residual aluminosilicates) in these Fe–Mn crusts were separated by a sequential leaching procedure. In the SCS and WPO crusts, the majority of total REY (ΣREY) was distributed in the Mn-oxide and amorphous FeOOH phases. The post-Archean Australian shale-normalized REY patterns showed that light REY (LREY) and heavy REY (HREY) were preferentially adsorbed onto δ-MnO2 and FeOOH, respectively. It is noteworthy that ~27% of ΣREY was associated with the residual aluminosilicates phase of the WPO crusts. The La/Al ratios in the aluminosilicates phase of the typical SCS crusts were the values of the upper crust. We conclude that large amounts of terrigenous materials dilute the abundance of REY in the SCS crusts. In addition, the growth rates of Fe–Mn crusts have a negative correlation with the FeOOH-bound and aluminosilicate-bound REY. As a result of the fast growth rates, the SCS crusts contain relatively low concentrations of REY.

Seasonal variations of phosphorus species in the Tuohe River, China. Part I. Sediments

Sediment cores were collected at an upstream site (Jintang) and a downstream site (Neijiang) in summer and winter in the Tuohe River, which is one of the five largest tributaries of the Changjiang (Yangtze) River in China. A sequential leaching procedure was applied to determine the concentrations of the different forms of solid phosphorus, including exchangeable phosphorus (Exc-P) as well as phosphorus associated with iron oxides (Fe-P), with aluminum oxides (Al-P), with apatite (Ca-P) and with the residual fraction (Res-P), as correlated with water content and total organic carbon. The seasonal variations and the vertical distributions of phosphorus species in sediments at both sampling sites demonstrated that: 1) total phosphorus (TP) in summer and winter were ranged between 1 313–2 330, 1 491–2 228 mg/kg in Jintang and 543–2 128, 603–1 175 mg/kg in Neijiang, respectively. It can predicated the pollution of TP in Jintang is serious than that in Neijiang both in summer and winter; 2) total inorganic phosphorus (TIP) was the dominant form of TP; 3) Ca-P was the main chemical forms of TIP in the sediments. Based on the profiles in sediments and bio-available phosphorus data, it revealed that bio-available phosphorus (BAP) represented only a minor portion (0.61%–3.59%) of TP, and the vertical distribution of soluble reactive phosphorus (SRP) in corresponding porewaters was more abundant in the upper layer of the sediment, which suggests that BAP may be converted to non-bioavailable phosphorus in deeper layer of the sediment of this dynamic system.

Rapid and selective leaching of actinides and rare earth elements from rare earth-bearing minerals and ores

Proper handling of the radioactive fraction associated with the production of rare earth elements (REEs) in the mining industry is essential for a sustainable environment and human health. Actinides, such as uranium and thorium, have similar chemical properties to REEs, and are often co-dissolved in current hydrometallurgical processes. This causes radioactive contamination of the infrastructures through the extraction cycle. The present study optimizes, through experimental design, a sequential leaching procedure with Na2CO3/NaHCO3 and HCl for the rapid and selective dissolution of REEs and actinides. Optimization focuses on the main operating parameters (time, temperature, and concentration); different types of minerals, REE composites, and ores; on achieving maximal leaching yield; and on minimizing dissolution of the gangue metals (Fe, Si, and Ca). The final sequential procedure is rapid (1 h), effective, and lead to the segregation of Th, U, and REEs with limited dissolution of the gangue metals.

Zinc and strontium isotope evidence for climate cooling and constraints on the Frasnian-Famennian (~372 Ma) mass extinction

The Frasnian-Famennian (F-F) mass extinction is one of the “Big Five” biotic crises during the Phanerozoic, whereas the cause of this catastrophic event still remains debated. Here we present for the first time high-resolution Zn and Sr isotope data obtained by a sequential leaching procedure for carbonate rocks across the F-F boundary from the Fuhe section in South China. Both δ66Zn and 87Sr/86Sr values rapidly increase in the Lower and Upper Kellwasser Horizons and Palmatolepis delicatula platys to P. minuta minuta conodont zones, and the positive isotope shifts are exactly located in the same horizons characterized by low ∆13C values (equivalent to low atmospheric PCO2 level, reflecting climate cooling) and sea-level fall stages. The strong temporal coupling of δ66Zn values raise with elevated 87Sr/86Sr ratios is most likely to have been caused by input of isotopically heavy Zn and radiogenic Sr from carbonate-dominated weathering, and/or uptake of isotopically light Zn into primary producers during climate cooling. Climate cooling could cause relative sea-level falls resulting in widespread exposure of carbonate platform, enhance carbonate influx proportion through differential weathering of carbonate and silicate rocks, and cause greater productivity by promoting oceanic thermohaline circulation and enhancing upwelling of bio-limiting nutrients. All of these processes would have contributed to the positive shifts of δ66Zn as observed. This proposed scenario is in accordance with the fact that the F-F mass extinction was almost limited to the shallow-water tropical stenothermal faunas, whereas the high latitude and deep-water faunas were less affected. Our data support and represent the composite effect of a series of events linked to climate cooling which may have induced this catastrophic event.

Comparative Analysis of Sequential Leaching Procedures for Dissociation of Rare Earth Elements in Gold-Bearing Material

Sequential leaching of gold-bearing ore of the residuum is described. The interconnection between rare earth elements and their mineral matrix is defined. The mobility of REE in the initial ore and during hypergenesis is assessed. Two kinds of REE are revealed: fine monazite cerium–neodymium crusts in iron-bearing minerals and dense monazite pockets in titanium-bearing minerals. The five-stage sequential leaching procedure allows more detailed distribution of elements in the rebellious matrix while the threestage procedure focuses on mobile forms.

Occurrence and Distribution of Gallium, Scandium, and Rare Earth Elements in Coal Gangue Collected from Junggar Basin, China

ABSTRACTIn order to extract precious metals from coal gangue (CG) collected from the Junggar coal mining fields, Inner Mongolia of China, the occurrence and distribution of elements including gallium (Ga), scandium (Sc), and rare earth elements (REE) were studied by means of sequential leaching procedures. Kaolinite and boehmite were identified as the major crystalline phases in the CG. No observable crystalline minerals were found for bearing Ga, Sc, or REE. The percentage of REE (~88.6%) were able to be leached out by acid (25%, HCl) treatment at room temperature, which indicates the REE are mainly present in the amorphous phases. The structure of boehmite was destroyed when the temperature of acid treatment was improved to 160°C, and about 85% of Ga together with 36.8% of Sc was leached out in this leaching step. The majority of Ga was indicated as isomorphic substitutions in kaolinite and boehmite. The 36.8% scandium was confirmed as Al-substitutions in the Al–O octahedrons of kaolinite, and others (55.6%) are present in boehmite. Only a little portion (1.9%) of scandium was found in the Si–O tetrahedrons. Therefore, this research provided basic data for occurrence and distribution of precious metals in view of the future industrialization.

Metal speciation in soil at a contaminated glassworks site in southeastern Sweden

Risk assessments of metal contaminated land are often based on total metal concentrations of the soil material, although it is well known that the potential for leaching is determined by the fractionation between labile metal forms and forms that are not to be assessed as geochemically active. In the present study, a 4-step sequential leaching procedure was applied to solid material from five subareas of a glass waste site in southeastern Sweden (including, for example, landfill areas and natural till material). The aim was to assess whether the major fraction of the total Cd and Pb content should be considered labile (easily mobilizable) or relatively stably sorbed, which is a question that is motivated by the fact that previous site-specific investigations have shown strongly elevated metal concentrations in soil, but very little impact on local groundwater. Although the total metal concentrations as well as the amount of visable glass particles differed between the subareas, the fractions extracted in the different steps of the sequential leach were similar for all areas. Only a minor part of the total content of both Cd and Pb was released in the final strong acid residual leach (9-19 %), which indicates that there has been a significant release of metal ions from the glass waste material in the past, followed by a transition to secondary phases. Of the two metals, Cd was assessed to be the most labile one. The first extraction, targeting the exchangeable and carbonate bound fraction, released 43-59 % of the total Cd content. So, although the relatively small effect seen on groundwater quality indicates an effective retention of these metal cations by the soil solid matrix, the relatively high fraction of potentially mobile forms highlights the need of further assessments of the stability in major retention mechanisms.

Study on the species of heavy metals in MSW incineration fly ash and their leaching behavior

This study aims to identify the chemical species and leaching behavior of Cd, Zn, Pb, Cu and Cr in a fly ash with high content of calcium collected from the incineration of municipal solid waste (MWS), based on the sequential leaching procedure. A thermodynamic pseudo-equilibrium model was also developed to evaluate the possible chemical compounds of these heavy metals in the fly ash. The results indicate that, approximate 30% of Cu was distributed in the organic bound fraction and very likely combined with some organic ligands. Pb exhibited the highest fraction among these five metals in water soluble fraction, accounting for about 7.5%. It thus potentially causes a menace to the surroundings. In terms of model calculation, the metallic chlorides in the fly ash were responsible for the leaching of Pb, Zn, Cd and Cu even under a rigorous environmental condition (pH=2) where the oxides and/or metallic ferrites were rarely mobile. The leaching of Cr and Cd in the fly ash was controlled by a dissolution mechanism whereas the fate of Pb, Zn and Cu was controlled by the precipitation/sorption. Cu and Zn in fly ash have been proven to associate with Ca-bearing compounds through precipitation/sorption during leaching test while Pb mainly exists as sulfate and phosphate.

Alkali activation of recovered fuel–biofuel fly ash from fluidised-bed combustion: Stabilisation/solidification of heavy metals

Recovered fuel–biofuel fly ash from a fluidized bed boiler was alkali-activated and granulated with a sodium-silicate solution in order to immobilise the heavy metals it contains. The effect of blast-furnace slag and metakaolin as co-binders were studied. Leaching standard EN 12457-3 was applied to evaluate the immobilisation potential. The results showed that Ba, Pb and Zn were effectively immobilised. However, there was increased leaching after alkali activation for As, Cu, Mo, Sb and V. The co-binders had minimal or even negative effect on the immobilisation. One exception was found for Cr, in which the slag decreased leaching, and one was found for Cu, in which the slag increased leaching. A sequential leaching procedure was utilized to gain a deeper understanding of the immobilisation mechanism. By using a sequential leaching procedure it is possible fractionate elements into watersoluble, acid-soluble, easily-reduced and oxidisable fractions, yielding a total ‘bioavailable’ amount that is potentially hazardous for the environment. It was found that the total bioavailable amount was lower following alkali activation for all heavy metals, although the water-soluble fraction was higher for some metals. Evidence from leaching tests suggests the immobilisation mechanism was chemical retention, or trapping inside the alkali activation reaction products, rather than physical retention, adsorption or precipitation as hydroxides.

The use of a four-stage sequential leaching procedure and the corresponding one-phase extractions for risk assessment of potential harmful substances in waste rock utilized in railway ballast

The purpose of environmental legislation in the EU is prevention, minimization, and utilization of waste, respectively. When utilization is not possible, the purpose becomes the safe landfill disposal of waste, or disposal by other ecologically beneficial methods. In addition, material efficiency is an essential topic nowadays to promote the sustainable use of natural resources, waste materials, and industrial by-products, in agreement with the principle of sustainable development and LCA. To promote these goals, a four-stage sequential leaching procedure and determination of total concentrations was used in this research to determine the distribution of Cu, Pb, Zn, and V samples taken from waste rock material, originating from a Finnish zinc mine, and used as railway ballast in Northern Finland. The leaching procedure consists of the following five sequential fractions: (i) an acidic water-soluble fraction (H2O, pH = 4), (ii) an exchangeable fraction (CH3COOH), (iii) an easily reduced fraction (NH2OH-HCl), and (iv) oxidizable fraction (H2O2 + CH3COONH4). The results show that conditions and the size of ballast have a significant effect on the solubility of all heavy metals, and therefore on their mobility, bioavailability, and environmental risk. In addition, the total concentration of every element is much larger than its solubility in each four fractions (i)–(iv) or the sum of these concentrations – this sum can be called by the potential bioavailability – because the highest concentration is in the residual fraction. The leachability results determined here for waste rocks utilized as railway ballast show in good agreement with all earlier investigations determined for other waste or industrial by-products. Sequential leaching studies provide valuable information about the effect of conditions on the leachability/solubility, mobility, and bioavailability for risk assessment of harmful heavy metals. This information is necessary if we want to know the real environmental risk of metals in different conditions, possible in natural conditions now and in the future, i.e. not only in terms of the conditions pertaining to permission applications.

Leaching of Heavy Metals from Personal Computer Components: Comparison of TCLP with a European Leaching Test

AbstractThe objective of this work was to quantify the leaching of nine heavy metals from four desktop personal computer components (PCCs), namely, integrated drive electronics (IDE) cables, video cards, random-access memory (RAM), and CPUs, using two different leaching tests. One test was the toxicity characteristic leaching procedure (TCLP), which is widely used in the United States. The TCLP was augmented using a five-step sequential leaching procedure to further investigate the influence of increasing liquid-to-solid (L/S) ratios on the amount of metals leached. The other leaching test was based on European standards following the requirements of a European decision regarding acceptance of waste at landfills. The European leaching test was performed at the natural pH of the materials. Results showed that the masses of some metals leached during the TCLP were up to two orders of magnitude greater than the corresponding masses measured by the European leaching test. Lead and zinc were the dominant metal...

The Use of a Five-stage Sequential Leaching Procedure for Risk Assessment of Heavy Metals in Waste Rock Utilized in Railway Ballast

A five-stage sequential leaching procedure was used to determine the distribution of 10 metals in three combination samples taken from waste rock material, originating from a Finnish zinc mine, and used as railway ballast in Northern Finland. The leaching procedure consists of the following five sequential fractions: a water-soluble fraction (H2O), an exchangeable fraction (CH3COOH), an easily reduced fraction (NH2OH-HCl), an oxidizable fraction (H2O2 + CH3COONH4), and a residual fraction (HF + HNO3 + HCl). The results show, in accordance with all earlier corresponding studies, that a sequential extraction procedure is very suitable for evaluating the effects of external conditions on the solubility of harmful heavy metals, and that external conditions have a large effect on their leachability/solubility, and therefore on their mobility, bioavailability, and environmental risk. In addition, the total concentration of each element is much larger than its solubility in each of the first four fractions (1)–(4) – the potential bioavailability fractions – because the highest concentrations of all metals occurred in the residual fraction (5). The conditions of this phase – three strong acids and strong, long-lasting shaking – are never possible in nature, and therefore the residual fraction is called the inert phase. Thus, the total concentrations of the heavy metals are poor measures of real environmental risk and give no information about the effect of external conditions on their solubility. In addition, the dependency of different elements on the external conditions differs.