Comparison of the efficiency of metal recovery from wet- and dry-discharged municipal solid waste incineration bottom ash by air table sorting and milling.

Separating high-density fractions from municipal solid waste incineration bottom ash (BA) is a promising approach for collecting metal resources and harmful elements. Herein, the efficiency of the recovery of metals (Cu, Zn, Au, Ag, and Pb) in the high-density fractions of two air-dried BAs discharged from wet-based systems (WBA), was compared with that of two BAs discharged from dry-based systems (DBA). WBAs and DBAs (<8 mm) were sorted using sieving, magnetic separation, milling, and air table sorting techniques. Newton's separation efficiency was used to evaluate three scenarios that combined various sorting processes. WBA contained high-density fractions (D1, 2.73-3.63 g/cm3) with a density similar to that of D1 in DBA (2.68-3.67 g/cm3), but the surface of the particles was covered by more minerals. When D1, and the metal particles were recovered after the milling of the low-density fractions, the overall separation efficiency in WBA (0.30-0.40) was lower than that in DBA (0.49) because of the lower separation efficiency for Zn and Pb compared to that for the other target metals. In contrast, no significant difference was recorded in the separation efficiency for Cu, Au, and Ag between WBA and DBA. D1 (18000-132000 mg-Cu/kg) and metal particles (29900-199800 mg-Cu/kg) in both WBA and DBA recorded abundant Cu contents. Moreover, the Au content in some D1 was similar to that found in natural ores. The elemental contents and separation efficiency results indicated that D1 and metal particles should be collected from both WBA and DBA.

Effective immobilization of geogenic As and Pb in excavated marine sedimentary material by magnesia under wet-dry cycle, freeze-thaw cycle, and anaerobic exposure scenarios.

Massive amounts of marine sedimentary materials with geogenic heavy metal(loids) are excavated by the subsurface construction projects and then exposed to weathering conditions, which pose potential threats to the environment. In the present study, 2 % magnesia (MgO) was applied to immobilize geogenic arsenic (As) and lead (Pb) in excavated marine sedimentary material. To better evaluate the immobilization efficiency under different environmental scenarios, the untreated and amended solids were subjected to wet-dry cycles, freeze-thaw cycles, and anaerobic incubation until 49 days. The leaching behaviors of As and Pb were investigated and their size fractionations in the leachates were compared. The results indicate that most Pb exists in particulate and agglomerated colloidal fractions (0.1-5 µm) in the leaching suspensions, while most As is found in dissolved forms (<0.1 µm). It is therefore necessary to consider the element type and exposure scenarios during environmental risk evaluation, particularly using the batch test as a routine compliance testing procedure. In the control test without MgO addition, the wet-dry cycle resulted in the "self-induced" immobilization of As and Pb. The pH decreases to the neutral range and the formation of amorphous Fe-(oxyhydr)oxides following pyrite oxidation largely explained the decreased As and Pb leaching. In comparison, the freeze-thaw cycle and anaerobic incubation tended to enhance As and Pb leaching. Overall, MgO addition significantly reduced the leachability of As and Pb and displayed sustained immobilization performance under all studied scenarios. These findings could be largely attributed to solid particle aggregation induced by MgO addition, including the adsorption of As and Pb onto newly formed Fe-(oxyhydr)oxides and/or MgSi precipitates. This study offers a simple and effective strategy for the sustainable management of excavated marine sedimentary materials contaminated by geogenic As and Pb.

The effects of redox conditions on arsenic re-release from excavated marine sedimentary rock with naturally suppressed arsenic release.

Massive quantities of marine sedimentary rock are excavated from urban coastal areas. The excavated rock often releases arsenic with concurrent oxidation of framboidal pyrite, but the arsenic release is naturally suppressed with subsequent atmospheric exposure. The present study evaluated the re-release of arsenic from excavated rock in which arsenic release has been naturally suppressed by the atmospheric exposure in the presence of sulfate ions under various redox conditions using the biological reduction method. The atmospheric exposure and subsequent batch leaching test revealed that the amount of arsenic release that was naturally suppressed corresponded to 1.2% of the total arsenic content. The sequential extraction analysis also showed that the arsenic in the exposed rock was altered to insoluble phases. We observed a re-release of 6.0-18.2% of the total arsenic content under reductive conditions (< + 70 mV of Eh), exceeding the amount of arsenic that was naturally suppressed, even in the presence of sulfate ions. The correlation in the amount of arsenic and iron re-released demonstrates that arsenic re-release under reductive conditions is mainly regulated by the iron dissolution up to 10 mg kg-1 even in the presence of sulfate ion. Further reduction and dissolution of iron did not cause further increase in the arsenic re-release. Therefore, excavated marine sedimentary rock should be reused under redox conditions in which iron is not reduced. Otherwise, treatments such as chemical immobilization should be performed.

Impact of diatomite addition on lead immobilization in air pollution control residues from a municipal solid waste incinerator.

Air pollution control (APC) residues, which are known to be the byproducts of incineration treatment, exhibit a high leaching potential of toxic metals. Calcium silicate hydrate (C-S-H), which is a major hydration product of hardened cement and immobilizes toxic metal, can be formed by the reaction of Ca with pozzolanic Si in a highly alkaline environment. Toxic metals might be immobilized by the addition of pozzolanic material to APC residues (instead of using cement), which is a Ca source and provides an alkaline condition. In this study, diatomite, which mainly comprises amorphous silica (SiO2·nH2O), was investigated as a pozzolanic material for Pb immobilization in APC residues obtained from a municipal solid waste incinerator. APC residues were cured with and without the addition of diatomite at different temperatures. When diatomite was added to APC residues, pozzolanic phases such as C-S-H gel were formed via the consumption of Ca(OH)2 and CaClOH. Compared to APC residues cured without diatomite, the leaching of Pb decreased by 99% for APC residues cured for 14 days with 10% diatomite at 70 °C. The results of sequential chemical extraction showed that water-soluble Pb in APC residues was reduced from 10.3% to nearly zero by the pozzolanic reaction. Consequently, the leaching amount of Pb dropped below 0.3 mg/L (Japanese criteria for landfill disposal). Overall, these experiments provide promising results regarding the possibility of using diatomite for pretreating APC residues.

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

Adsorption parameters such as the distribution coefficient are required to predict the release behavior of contaminants using advection-dispersion models. However, for potentially contaminant-releasing materials (PCMs) such as dredged sludge and coal ash, these parameters cannot be obtained by conventional adsorption tests. This study developed a method to determine adsorption parameters for PCMs from a set of batch tests conducted in parallel as a function of the liquid-solid ratio (LS-parallel test). This LS-parallel test was performed on sandy soil derived from marine sediment using liquid-solid ratios from 1 to 300 L/kg. The water-contact time was also changed from 10 min to 28 d to elucidate the kinetics or equilibrium of contaminants released from the sample. Adsorption parameters were successfully obtained if the substance was under adsorption control. A column percolation test was performed to confirm the effectiveness of the obtained parameters. Good agreements were observed for SO42- and B, but discrepancies remained for other substances such as F- and As suggesting that improvements are necessary in both the LS-parallel test procedure and the advection-dispersion model.

Distribution of recoverable metal resources and harmful elements depending on particle size and density in municipal solid waste incineration bottom ash from dry discharge system.

Although municipal solid waste incineration bottom ash (BA) has the potential to be used as a metal resource, it raises concerns about the potential release of harmful elements into the environment. Element distribution in terms of particle size and density should be assessed to determine the fractions for the metal resources' recovery and to remove harmful elements. For this purpose, this study proposed a series of sorting processes based on the distribution of 25 elements in the sorted fractions by sieving, magnetic separation, air table sorting, and milling from dry BA < 8 mm. The Ca, Na, Mg, P, S, Cl, and Ti contents exhibited a decreasing tendency with increasing particle density and could affect the formation of low-density particles. The highest density fraction of non-magnetic components of 0.5-8 mm had abundant metal particles and recorded high Cu, Zn, Cr, Ni, Mo, Fe, Pb, Sb, and Au contents. In particular, the Cu (132000 mg-Cu/kg) and Zn (43000 mg-Zn/kg) contents demonstrated potential as metal resources. The fraction contained considerable proportions of Mo (77%), Cd (46%), Cu (39%), Zn (34%), Pb (26%), Au (40%), and Ag (18%) of the total amount. After milling and sieving of the highest density fraction, a substantial amount of Cd (44%), Cu (18%), Zn (12%), Pb (13%), and Ag (11%) were found in residual minerals; they could become harmful elements when recycled for construction purposes. The results show that air table sorting can separate metal resources and harmful elements before milling of BA.

Effects of membrane filter material and pore size on turbidity and hazardous element concentrations in soil batch leaching tests.

Soil batch leaching tests are conducted worldwide to quantify the leaching of hazardous substances from contaminated soil. In the extracts of soil batch leaching tests, some inorganic substances such as arsenic and lead are released both in colloidal and dissolved form. Recent studies have found that soil colloidal particles with small diameters persist in the filtrate even after the extracts are filtered through a membrane filter (MF) with a pore size of 0.45 µm, and they might affect the concentration of arsenic or lead. This study evaluated the effects of 0.45- or 0.4-µm MF materials on filtrate turbidity and leaching concentrations of inorganic hazardous elements during batch leaching tests. Turbidity and arsenic and lead concentrations in the filtrates of the tested soil samples varied greatly depending on the MF material. These findings indicate that the MF material affects the removal rate of colloidal arsenic or lead and therefore affects the results of leaching tests.

Determination of metal-abundant high-density particles in municipal solid waste incineration bottom ash by a series of processes: Sieving, magnetic separation, air table sorting, and milling.

Valuable non-ferrous (NFe) metals are contained in municipal solid waste incineration (MSWI) bottom ash (BA). The applicability of an air table for separating NFe metal-abundant particles in MSWI BA (<8 mm) was studied. A stepwise separation procedure was developed based on the performance tests of the air table conducted by changing three variables (air injection, vibration, and end slope). As a result of the stepwise separation, six bulk density ranges (<0.7 to >1.1 g/cm3 at intervals of 0.1 g/cm3) were prepared from non-magnetic fractions with four size ranges (4-8, 2-4, 1-2, and 0.5-1 mm). The effectiveness of air table sorting was evaluated based on dry particle density and proportion of metals. NFe metals were obtained by sieving (>0.5 mm) after a ball mill process, confirming that NFe metals accounted for 3.2% of the < 8 mm MSWI BA. The highest particle density of each size fraction and the proportion of NFe metals in the fractions were 3.19 g/cm3 and 59.9% (4-8 mm), 2.97 g/cm3 and 28.8% (2-4 mm), 2.78 g/cm3 and 10.4% (1-2 mm), and 2.87 g/cm3 and 4.4% (0.5-1 mm). In the 0.5-2 mm particles, the highest density fraction contained 68.7% of the NFe metals, which were expected to be recovered by applying an air table separator to MSWI BA 0.5-2 mm.

Speciation and Fractionation of Soil Arsenic from Natural and Anthropogenic Sources: Chemical Extraction, Scanning Electron Microscopy, and Micro-XRF/XAFS Investigation.

A large amount of excavated soils with low-level As contamination caused by civil construction projects is of great concern in Japan. This study investigated the chemical speciation and extractability of As in 24 soil samples from the sites affected and unaffected (naturally contaminated) by anthropogenic pollution. The results of As K-edge XANES demonstrated that naturally contaminated soils were grouped into two types: (i) soils containing FeAsS-like and As2S3-like species (ave. 53%, hereafter As-S species) and (ii) soils with no or minor As-S species (ave. 3%). Clear differences were found in As, Fe, and S fractionations by sequential extraction. From naturally contaminated soils enriched with As-S species, more than 50% of As was extracted in the oxidizable fraction. Arsenic was mainly recovered in the reducible fraction for naturally contaminated soils with no or minor As-S species and anthropogenically contaminated soils. The µ-XRF and µ-XAFS revealed that the naturally contaminated soils containing As-S species were abundant in pyrite framboids (∼20 µm in diameter) in which As occurred as multiple oxidation states. The results suggest that framboidal pyrite becomes a source of As in naturally contaminated soils after being excavated and exposed to the surface environment.

Simplified Sample Embedding and Polishing Methods for Preparing Hydrophilic, Fragile, or Solvent-Susceptible Materials for Thin Sections for Microscopic Analyses.

In the preparation of thin sections for microscopy, embedding and polishing processes in particular can change the composition and morphologies of samples. Soils and ashes are very fragile and solvent-susceptible, and appropriate sample preparation procedures have not been well-established. To improve the existing preparation methods and make them easier and faster, we embedded freeze-dried blocks, polished, and then examined these thin-section samples using polarization microscopy, laser microscopy, and field emission scanning electron microscopy with energy-dispersive X-ray spectrometry, and electron backscattered diffraction (EBSD). Appropriate thin-section samples can be prepared by: (1) rinsing with acetone and then embedding with Spurr resin along with repeated evacuation and ventilation, rather than conventional dehydration/replacement; (2) polishing using silicon carbide paper and diamond slurries, and then wiping with a cloth and a synthetic oil; and (3) slightly rinsing with 100% ethanol to remove the oil. The preparation method minimized contamination and pores, and showed flat surfaces and sometimes EBSD patterns. Freeze-drying has been claimed to cause the development of cracks due to ice crystal formation upon freezing, however, our method not only overcomes such problems for microscopic observation but saves substantial time, taking only 2 days in total to process a specimen, and requiring less than 1 g of resin and ~1 g of sample.

Reproducibility of up-flow column percolation tests for contaminated soils.

Up-flow column percolation tests are used at laboratory scale to assess the leaching behavior of hazardous substance from contaminated soils in a specific condition as a function of time. Monitoring the quality of these test results inter or within laboratory is crucial, especially if used for Environment-related legal policy or for routine testing purposes. We tested three different sandy loam type soils (Soils I, II and III) to determine the reproducibility (variability inter laboratory) of test results and to evaluate the difference in the test results within laboratory. Up-flow column percolation tests were performed following the procedure described in the ISO/TS 21268-3. This procedure consists of percolating solution (calcium chloride 1 mM) from bottom to top at a flow rate of 12 mL/h through softly compacted soil contained in a column of 5 cm diameter and 30 ± 5 cm height. Eluate samples were collected at liquid-to-solid ratio of 0.1, 0.2, 0.5, 1, 2, 5 and 10 L/kg and analyzed for quantification of the target elements (Cu, As, Se, Cl, Ca, F, Mg, DOC and B in this research). For Soil I, 17 institutions in Japan joined this validation test. The up-flow column experiments were conducted in duplicate, after 48 h of equilibration time and at a flow rate of 12 mL/h. Column percolation test results from Soils II and III were used to evaluate the difference in test results from the experiments conducted in duplicate in a single laboratory, after 16 h of equilibration time and at a flow rate of 36 mL/h. Overall results showed good reproducibility (expressed in terms of the coefficient of variation, CV, calculated by dividing the standard deviation by the mean), as the CV was lower than 30% in more than 90% of the test results associated with Soil I. Moreover, low variability (expressed in terms of difference between the two test results divided by the mean) was observed in the test results related to Soils II and III, with a variability lower than 30% in more than 88% of the cases for Soil II and in more than 96% of the cases for Soil III. We also discussed the possible factors that affect the reproducibility and variability in the test results from the up-flow column percolation tests. The low variability inter and within laboratory obtained in this research indicates that the ISO/TS 21268-3 can be successfully upgraded to a fully validated ISO standard.

Column percolation test for contaminated soils: Key factors for standardization.

Column percolation tests may be suitable for prediction of chemical leaching from soil and soil materials. However, compared with batch leaching tests, they are time-consuming. It is therefore important to investigate ways to shorten the tests without affecting the quality of results. In this study, we evaluate the feasibility of decreasing testing time by increasing flow rate and decreasing equilibration time compared to the conditions specified in ISO/TS 21268-3, with equilibration periods of 48h and flow rate of 12mL/h. We tested three equilibration periods (0, 12-16, and 48h) and two flow rates (12 and 36mL/h) on four different soils and compared the inorganic constituent releases. For soils A and D, we observed similar values for all conditions except for the 0h-36mL/h case. For soil B, we observed no appreciable differences between the tested conditions, while for soil C there were no consistent trends probably due to the difference in ongoing oxidation reactions between soil samples. These results suggest that column percolation tests can be shortened from 20 to 30days to 7-9days by decreasing the equilibration time to 12-16h and increasing the flow rate to 36mL/h for inorganic substances.

Toxic metals in WEEE: characterization and substance flow analysis in waste treatment processes.

Waste electrical and electronic equipment (WEEE) has received extensive attention as a secondary source of metals. Because WEEE also contains toxic substances such as heavy metals, appropriate management of these substances is important in the recycling and treatment of WEEE. As a basis for discussion toward better management of WEEE, this study characterizes various types of WEEE in terms of toxic metal contents. The fate of various metals contained in WEEE, including toxic metals, was also investigated in actual waste treatment processes. Cathode-ray tube televisions showed the highest concentration and the largest total amount of toxic metals such as Ba, Pb, and Sb, so appropriate recycling and disposal of these televisions would greatly contribute to better management of toxic metals in WEEE. A future challenge is the management of toxic metals in mid-sized items such as audio/visual and ICT equipment because even though the concentrations were not high in these items, the total amount of toxic metals contained in them is not negligible. In the case of Japan, such mid-sized WEEE items as well as small electronic items are subject to municipal solid waste treatment. A case study showed that a landfill was the main destination of toxic metals contained in those items in the current treatment systems. The case study also showed that changes in the flows of toxic metals will occur when treatment processes are modified to emphasize resource recovery. Because the flow changes might lead to an increase in the amount of toxic metals released to the environment, the flows of toxic metals and the materials targeted for resource recovery should be considered simultaneously.

Fate of metals contained in waste electrical and electronic equipment in a municipal waste treatment process.

In Japan, waste electrical and electronic equipment (WEEE) that is not covered by the recycling laws are treated as municipal solid waste. A part of common metals are recovered during the treatment; however, other metals are rarely recovered and their destinations are not clear. This study investigated the distribution ratios and substance flows of 55 metals contained in WEEE during municipal waste treatment using shredding and separation techniques at a Japanese municipal waste treatment plant. The results revealed that more than half of Cu and most of Al contained in WEEE end up in landfills or dissipate under the current municipal waste treatment system. Among the other metals contained in WEEE, at least 70% of the mass was distributed to the small-grain fraction through the shredding and separation and is to be landfilled. Most kinds of metals were concentrated several fold in the small-grain fraction through the process and therefore the small-grain fraction may be a next target for recovery of metals in terms of both metal content and amount. Separate collection and pre-sorting of small digital products can work as effective way for reducing precious metals and less common metals to be landfilled to some extent; however, much of the total masses of those metals would still end up in landfills and it is also important to consider how to recover and utilize metals contained in other WEEE such as audio/video equipment.

A preliminary categorization of end-of-life electrical and electronic equipment as secondary metal resources.

End-of-life electrical and electronic equipment (EEE) has recently received attention as a secondary source of metals. This study examined characteristics of end-of-life EEE as secondary metal resources to consider efficient collection and metal recovery systems according to the specific metals and types of EEE. We constructed an analogy between natural resource development and metal recovery from end-of-life EEE and found that metal content and total annual amount of metal contained in each type of end-of-life EEE should be considered in secondary resource development, as well as the collectability of the end-of-life products. We then categorized 21 EEE types into five groups and discussed their potential as secondary metal resources. Refrigerators, washing machines, air conditioners, and CRT TVs were evaluated as the most important sources of common metals, and personal computers, mobile phones, and video games were evaluated as the most important sources of precious metals. Several types of small digital equipment were also identified as important sources of precious metals; however, mid-size information and communication technology (ICT) equipment (e.g., printers and fax machines) and audio/video equipment were shown to be more important as a source of a variety of less common metals. The physical collectability of each type of EEE was roughly characterized by unit size and number of end-of-life products generated annually. Current collection systems in Japan were examined and potentially appropriate collection methods were suggested for equipment types that currently have no specific collection systems in Japan, particularly for video games, notebook computers, and mid-size ICT and audio/video equipment.

Leaching behavior of CRT funnel glass.

The leaching behavior of cathode ray tube (CRT) funnel glass containing 23 mass percent of Pb in 0.001 N HCl, distilled water, and 0.001 N NaOH at 90°C was investigated using a static method. The weight loss and leached amount of each component was measured and surface changes observed by SEM. The leaching mechanism is discussed. In acid solution, the leached amount of Pb showed t(1/2) dependence, that is, diffusion-controlled dependence, which is common in lead silicate glasses. In water and basic solutions, the leached amount showed saturation after higher initial dissolution than in acid and the deposition of many particles on the surface was observed. The amount leached was less for Pb than other components. The deposited particles formed a protective layer, which suppressed the dissolution of the glass. This dense layer must be formed as a result of a high initial dissolution rate.

Alkaline solution neutralization capacity of soil.

Alkaline eluate from municipal solid waste (MSW) incineration residue deposited in landfill alkalizes waste and soil layers. From the viewpoint of accelerating stability and preventing heavy metal elution, pH of the landfill layer (waste and daily cover soil) should be controlled. On the other hand, pH of leachate from existing MSW landfill sites is usually approximately neutral. One of the reasons is that daily cover soil can neutralize alkaline solution containing Ca(2+) as cation. However, in landfill layer where various types of wastes and reactions should be taken into consideration, the ability to neutralize alkaline solutions other than Ca(OH)(2) by soil should be evaluated. In this study, the neutralization capacities of various types of soils were measured using Ca(OH)(2) and NaOH solutions. Each soil used in this study showed approximately the same capacity to neutralize both alkaline solutions of Ca(OH)(2) and NaOH. The cation exchange capacity was less than 30% of the maximum alkali neutralization capacity obtained by the titration test. The mechanism of neutralization by the pH-dependent charge can explain the same neutralization capacities of the soils. Although further investigation on the neutralization capacity of the soils for alkaline substances other than NaOH is required, daily cover soil could serve as a buffer zone for alkaline leachates containing Ca(OH)(2) or other alkaline substances.

Effect of exposure test conditions on leaching behavior of inorganic contaminants from recycled materials for roadbeds.

Throughout the utilization of recycled materials, weathering factors such as humidity, gas composition and temperature have the potential to change the material properties and enhance the release of inorganic contaminants. In this research, the effects of weathering factors on recycled gravel materials for roadbeds were evaluated by applying three kinds of accelerating exposure tests: freezing-melting cycle test, carbonation test, and dry-humid cycle test. The effects of exposure tests were determined by X-ray diffraction (XRD) analysis and serial batch leaching test, making it possible to identify the change in release mechanisms. Sixteen elements, mainly metals, were investigated. Tested samples were molten slag from municipal solid waste, molten slag from automobile shredded residue, and crushed natural stone. After the exposure tests, the increase of cumulative release in the leaching test was generally less than 2.0 times that of the samples without the exposure test. Among the three test conditions, freezing-melting showed a slightly higher effect of enhancing the release of constituents. XRD analysis showed no change in chemical species. From these results, it was determined that the stony samples were stable enough so that their properties were not significantly changed by the exposure tests.

Formation and durability of dithiocarbamic metals in stabilized air pollution control residue from municipal solid waste incineration and melting processes.

In Japan, chemical stabilization of metals using a chelating agent composed of dithiocarbamic salt is the prevailing technique for the final disposal of air pollution control (APC) residue from municipal solid waste (MSW) incineration and melting processes. The objective of this study is to quantitatively evaluate the formation and durability of dithiocarbamic metals (DCM) in treated APC residue. In this research, sodium N,N--diethyldithiocarbamate trihydrate was used as a chelating agent. An extraction method using n-butyl acetate was developed to separate DCM from residue. This method elucidated the competitive formation of DCM among Cu, Pb, Cd, and Zn. The metal fraction forming DCM was about 70 and 90% of the total content of Pb in APC residue from incineration and melting, respectively. Regarding durability in the environment, 240 day curing of the chelate-treated APC residue showed that air-contact conditions induced degradation of DCM with time. On the other hand, sealed conditions did not bring about a decrease over the course of the experiment. These results imply that DCM degradation proceeds when the treated residue comes into contact with air in actual landfills.