The leaching of major and trace elements from MSWI bottom ash as a function of pH and time

In this paper, the leaching behaviour of major components (Al, Ca, SO₄, Mg, Si, Fe, Na and DOC) and trace elements (Ni, Zn, Cd, Cu, Pb, Mo and Sb) from MSWI bottom ash is studied as a function of time over a wide range of pH, under pH-controlled conditions. Equilibrium geochemical modelling using the modelling framework ORCHESTRA is used to enable a process-based interpretation of the results and to investigate whether ‘equilibrium’ is attained during the time scale of the experiments. Depending on the element and setpoint-pH value, net concentration increases or decreases of up to one order of magnitude were observed. Different concentration–time trends (increase or decrease) are observed in different pH ranges. The direction of the concentration–time trends depends on: (1) the shape of the ‘equilibrium’ solubility curve, and (2) the position of the setpoint-pH in the leaching test relative to the natural pH of the sample. Although the majority of the elements do not reach steady state, leached concentrations over a wide pH range have been shown to closely approach ‘equilibrium’ model curves within an equilibration time of 168 h. The different effects that leaching kinetics may have on the pH dependent leaching patterns have been identified for a wide range of elements, and can generally be explained in a mechanistic way. The results are in support of the currently prescribed equilibration time of 48 h in the European standard for the pH-static leaching test (TS14997). Finally, this study demonstrates that pH-static leaching experiments such as described in the European standards (TS14497 and TS14429), in combination with selective chemical extractions and a mechanistically based modelling approach, constitute a powerful set of tools for the characterization of leaching processes in waste materials over a wide range of conditions.     

Speciation of Cu in MSWI bottom ash and its relation to Cu leaching

In Flanders, recycling of bottom ash is mainly inhibited by the high leaching of Cu. Although it has been proved that dissolved organic C plays a major role in the Cu leaching, the possible role of inorganic Cu mineral speciation has never been experimentally examined. In this study the speciation of Cu is investigated using a combination of optical microscopy and electron microprobe –WDX/EDX. Several Cu species were determined. Metallic Cu (with or without an oxide shell), CuO and Cu₂O were the most abundant. These particles were most likely present in wire-like structures. Copper also occurred as alloy (brass, bronze, zamak), and was found frequently together with typical elements such as Ca, Cl and S. Finally, small metallic Cu particles seemed to be trapped in or precipitated on oxides and silicates. Based on this Cu speciation study, pure Cu minerals were selected and leached as a function of time. The solubility after equilibrium of all studied Cu minerals never exceeded 20 μg/L (which equals 10% of the total Cu leaching).The effect of heating (2 h at 400 °C) on the speciation of Cu was investigated using the same combination of techniques. Results show that metallic Cu seemed to be converted to Cu oxide (mostly CuO) and that the particles were more porous after heating. These conclusions were verified by XRD analysis of the heated pure Cu minerals. After heating, the Cu minerals were also leached as a function of time, to study the impact on Cu leaching. Results indicate that their leaching had slightly increased in comparison with the non-heated Cu minerals. However, the major decrease in Cu leaching in heated bottom ash, more than neutralizes this effect and thus can be attributed to the destruction of organic matter and not to the (small) change in Cu speciation.     

Speciation of trace metals in leachate from a MSWI bottom ash landfill

Trace metal concentrations and speciation were determined in leachate from a municipal solid waste incinerator bottom ash landfill both experimentally and by thermodynamic model calculations. Total dissolved Cr, Sb and W concentrations determined directly by ICP-MS were up to two orders of magnitude higher than that determined upon preconcentration by anin-situ solid phase extraction technique based on 8-HQ cation exchanger which indicates oxyanion complex formation of these metals in the leachates. Speciation modeling suggests that a similar difference for Cu is caused by organic complexation. Lead and Zn concentrations determined by both methods were fairly comparable but very low, in the range 4–60 nmol l⁻¹. The low mobility of both metals can be modeled by assuming adsorption onto Fe-oxyhydroxides oxycoprecipitation with Ca-silicate hydrate phases. The resulting high retardation coefficients between 500 and 800 indicate that scavenging by these secondary weathering products in the MSWI bottom ash deposit can cause an efficient immobilization of both Pb and Zn.     

The leaching of trace elements from municipal solid waste incinerator bottom ash at different stages of weathering

For a proper assessment of the environmental impact of the utilisation and disposal of Municipal Solid Waste Incinerator (MSWI) bottom ash it is necessary to understand weathering processes and their effects on (trace) element leaching. The authors have investigated the processes that control the leaching of Cd, Pb, Zn, Cu, and Mo from 3 categories of bottom ash: (A) unweathered bottom ash (grate siftings and unquenched samples), (B) quenched/non-carbonated bottom ash (freshly quenched and 6-week-old samples), and (C) weathered bottom ash (1.5- and 12-year-old samples). Leaching experiments were performed in a pH-stat at a large range of pH values. The speciation code MINTEQA2 was used for subsequent modelling of precipitation/dissolution processes. The speciation of trace elements in weathered bottom ash was also investigated by microanalytical techniques. In A- and B-type bottom ash the general controlling processes are thought to be precipitation/dissolution of relatively soluble minerals or, in the case of Cu in particular, extensive complexation with dissolved organic C. At the “natural” pH of the samples, the leaching of Cd, Pb, Cu, Zn and Mo is generally significantly lower from C-type bottom ash than from less weathered types of bottom ash. This reduction in leaching is due to the neutralisation of bottom ash pH and the formation of less soluble species of these elements as weathering continues. In the more weathered (C-type) bottom ash trace element leaching does not seem to be solubility-controlled; although slow precipitation reactions cannot be totally excluded, it is hypothesised that the controlling mechanism in those samples is sorption to neoformed minerals.     

高铝粉煤灰中部分主微量元素的分布规律研究

本文旨在研究部分主微量元素在高铝粉煤灰中的分布规律,从而为其资源化利用提供科学依据。首先根据高铝粉煤灰的物相组成特点采用新的分离方法将之分离为三个相(或相组合):铁质微珠、莫来石-刚玉相以及玻璃相;然后分别用化学分析和ICP-MS测试主微量元素在高铝粉煤灰、铁质微珠以及莫来石-刚玉相中的含量,并结合相关数据计算出这些元素在玻璃相中的含量。结果表明,除Al、Mn外,其他元素均在玻璃相中有不同程度的富集;除Al外,其他元素则在莫来石-刚玉相中有不同程度的贫化;除Ti以外的铁族元素以及除Ga、Pb以外的金属成矿元素均在铁质微珠中富集,其他元素则在其中贫化。     

Mineralogical study of secondary mineral phases from weathered MSWI bottom ash: implications for the modelling and trapping of heavy metals

A mineralogical study of 3 samples of municipal solid waste incineration bottom ash collected from different storage sites and with storage times varying from 3 weeks to 2 years, has enabled identification of the main secondary mineral species formed during weathering. The frequencies of the secondary phases were determined and a diagram is proposed for the relative distribution of the newly formed mineral phases: calcite ≫ Fe oxides quartz ⩾ sulphates and/or ettringite (depending on the amount of reactive Al present in the bottom ash). This approach, involving careful sampling, sample preparation and the combined use of various analytical techniques, also showed the high frequency of Al hydroxides and amorphous phases and helped to identify more than 30 sulphates s.l. (sulphates, chromates, vanadates, etc.). Most of the secondary minerals (carbonates and sulphates s.l.) have broad metal trapping capacities for heavy element uptake (Pb, Zn, Cd, As, V, Cr, etc.) due to their crystal-chemistry characteristics. Ca-oxalates were also identified. Mineralogical data from the study provide new input for thermochemical models. The relative stability of metal uptake and the extent of associated neogenesis occurring during bottom-ash decomposition is discussed. Sulphate minerals (and certain heavy metal oxides (zincite)), which are extremely sensitive to environmental conditions, can trap metals only temporarily, as opposed to Fe oxyhydroxides (As, etc.) and carbonates (Pb, Zn, Cd), which are more stable under atmospheric conditions and constitute more sustainable trapping media with higher liquid/solid (L/S) ratios. Finally, a composite predictive diagram is proposed for the mineralogical evolution of bottom ash that accounts for variations in L/S ratios.     

Spatial geochemical variation of major and trace elements in the marine sediments of Mumbai Harbor Bay

The resulting concentration data sets of major (Na, K, Mg, Ca and Fe) and trace elements (Cu, Ni, Co and Mn) in bed and suspended sediments were used to evaluate the enrichment factor for anthropogenic influences and principal component analysis for identifying the origin of source contributions in the studied area. Normalization of metals to Fe indicated that high enrichment factors in the bed sediment were in the order of Co > Cu > Na > Ca > Ni except Mg, K and Mn while for suspended sediments, only Co has a high enrichment factor. High enrichment of Co and Cu reflected the contamination of sediments from anthropogenic sources. The high influence of Na and Ca in sediments may be caused for seawater salinization factor. A significant positive correlation among enrichment factors of various elements of interest suggests a common origin/identical behavior during transport in the sediment system.     

Contaminant leaching from gold mining tailings dams in the Witwatersrand Basin,South Africa: A new geochemical modelling approach

In this study a new geochemical reaction modelling methodology is used to shed light on the geochemical processes within Witwatersrand tailings impoundments and the evaluation of the geochemical impacts for future mining projects is evaluated. Information from international and local studies on tailings sulphidic tailings impoundments is used to develop a conceptual understanding on a typical Witwatersrand gold tailings impoundment. The tailings impoundments consist of 3 distinct geochemical zones: Oxidation Zone (OZ), a Transition Zone (TZ) and a Reduction Zone (RZ). Individual reaction models are developed for each of the 3 zones. The output of one model is used as an input to the next in spatial order. The results of the final model represent the tailings basal seepage characteristics, which indicate the most likely impacts on groundwater resources. The model results agree with existing information on AMD in the Witwatersrand. The results indicate that the tailings basal seepage is likely to be acidic (pH of ∼3.5), containing elevated concentrations of SO₄ and trace metals (Al, Co, Cr, Cu, Mn, Ni, Zn and U). Predicted Fe concentrations are low, due to the low $f_{{\text{O}}_2}$ in the TZ and RZ and the buffering effect of the precipitation of K-jarosite in the OZ and TZ. This study therefore indicates that the methodology employed produces results that can be correlated to existing information and can thus be used as a methodology in the assessments of impacts from sulphidic tailings material for future mining projects.     

Temporal transport of major and trace elements in the upper reaches of the Xijiang River,SW China

This study investigated the temporal variations of major and trace element contents and controlling factors in the upper reaches of the Xijiang River, SW China. The results showed the major ions principally derived from the weathering of carbonate and silicate rocks, with a seasonal variation impacted by the monsoonal climate. The contents of dissolved trace elements also varied seasonally in the two main tributaries: the Nanpan and Beipan rivers. Most dissolved trace elements such as Mn, Zn and Pb being affected by anthropogenic contributions displayed a large seasonal variability, from onefold to hundred-fold, while V, Rb and U show a relative stable trend uncorrelated with the different seasons, suggesting a natural origin by rock weathering. Suspended particle matter in the Nanpan and Beipan rivers was depleted in Ca, Mg, Na, K and Sr, but enriched in Al, Fe and Sb compared to the upper crust (UC) values. A binary mixture of silicate-rich and carbonate-rich sources dominates the behavior of elements such as Ca and Sr in riverine sediments, while a third end-member with an anthropogenic signature influenced the content of trace elements like Mn, Sb, Pb, Zn, Cd, Cr and Cu. The enrichments of Sb relative to the UC would be related to the mining activity in the studied area. The study highlights the importance of investigating temporal variations of major and trace element contents of rivers controlled by the monsoon climate and impacted by human activity.     

Stable isotope evidence for the atmospheric origin of CO2 involved in carbonation of MSWI bottom ash

Stable isotopes were used to constrain the origin of CO₂ involved in the ageing process of municipal solid waste incineration (MSWI) bottom ash under open-air conditions. The δ¹³C and δ¹⁸O values of CaCO₃ occurring in MSWI bottom ash samples of variable age and the δ¹³C of the residual organic matter content were measured, and laboratory assessments made of the isotopic fractionation accompanying CaCO₃ neo-formation during accelerated carbonation experiments of bottom ash or pure lime with atmospheric or industrial CO₂. The results indicate that stable isotopic compositions exhibited by fresh and aged bottom ash samples reflect non-equilibrium processes resembling those described in the carbonation of concrete and mortar. They also lead to conclusions on the prevalent involvement of atmospheric CO₂ in the open-air carbonation of MSWI bottom ash.     

Spatial distribution and geochemical baselines of major/trace elements in soils of Medak district,Andhra Pradesh,India

Regional-scale variations in soil geochemistry were investigated with special reference to differences among soil groups and lithology in an area of 9,699 km² in Medak district, Andhra Pradesh, India. The concentrations of 29 elements (major: Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti, P and trace: As, Ba, Cd, Co, Cr, Cu, F, Mo, Ni, Pb, Rb, Se, Sr, Th, U, V, Y, Zn, Zr) in 878 soil samples collected (557-topsoil, 321-subsoil) at a sampling density of 1 site/17 km² from 557 sites representative of all the soil types present in studied area were determined, and their elemental composition are discussed. The baseline levels of these elements in soils are determined over different lithological units for the identification of anomalous values relative to these. For the first time, geochemical maps for Medak district are prepared on 1:50,000 scale and the lithogeochemical database generated provides information on the lateral and vertical distribution of elements in soil. The spatial variations in the distribution of elements reflect underlying geologic characteristics. Box-plots reveal that the concentration of most of the elements in soils were not strongly dependent on the soil group but the soil-geochemistry abruptly changes with the change in the soil parent materials indicating that the distribution of elements is mostly influenced by the bedrock lithology and other natural processes acting on them. For instance, the concentrations of Co, Cu, Fe, Mn, Ti, V and Zn are high in soils developed on basaltic terrain while the soils developed on granitic and gneissic terrain exhibit high elemental concentrations of K, Pb, Rb, Si, Th and Y. Alfisols had relatively high contents of elements while entisols had lower concentrations of most of the elements. The database can be used in the chemical characterisation of different geological units as well as applications in various environmental and agricultural fields. The results indicate that regional geology is an important determinant of soil geochemical baselines for soil pollution assessment and further emphasizes the importance of determining background levels locally. The defined baselines can be used to establish background values for future soil surveys.     

Acid leaching of ash and coal: Time dependence and trace element occurrences

The leaching of coal and coal/asphaltite/wood-ashes in sulfuric acid (pH 1.0, 25 °C, S/L, 1:10) was studied as a function of time; acid consumption and extracted metal concentrations are presented. Whole coals consumed acid rapidly during the first few minutes, followed by slow acid consumption. Wood-, lignite-, and asphaltite-ashes consumed acid in two stages, the rapid phase extending < 30 min and the slow phase extended up to 10 days. The rapid phase was dominated by the dissolution of Ca, K and Mg ions for wood-ash, by Ca, Al and Mg ions for lignite-ash and Ca and Mg ions for asphaltite-ash. The sulfur concentration in solution and the concentrations of Ca, Fe, K, Mg, Na, P, Al and Mn in the aqueous phase verified the neutralizing capacity of the untreated ashes as well as the formation of insoluble sulfates in the residues. The slow phase kinetics differed for different fuels and exhibited leaching of several abundant elements—Fe, Al, K, Na and Mn. Trace elements (Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Th, U, V, Zn) sometimes required up to 32 h for maximal extraction from ashes. Suggestions are presented regarding the chemical nature of trace elements in the untreated coals and ashes and suitable residence times for economical industrial processes. We think it possible to combine bacteriological oxidation of sulfidic concentrates of acid leaching from ash of various qualities or even whole coals.     

Scenario of particulate trace metal and metalloid transport during a major flood event inferred from transient geochemical signals

High-resolution sampling (every 3 h) of SPM was performed during a major flood event in a heterogeneous, medium scale watershed of the Garonne-Gironde fluvial-estuarine system (the Lot River; A = 10,700 km²; Q = 151 m³/s). Particulate metal and metalloid (Cd, Zn, Pb, Co, Cr, Ni, Mo, V, U, As, Sb, Th) concentrations were compared with monthly data of the same site (Temple site) obtained during 1999–2002. During the flood event, suspended particulate matter (SPM) concentrations closely followed river discharge with a maximum value (1530 mg/L) coinciding with the discharge peak (2970 m³/s). Trace metal/metalloid concentrations showed significant temporal variations and very contrasted responses. Particulate concentrations were similar to baseline values at the beginning of the flood and mostly increased during the event, showing anticlockwise and complex shape hystereses. Comparison of SPM yield (440,000 t) and particulate metal/metalloid fluxes during the flood with annual fluxes (1999–2002) highlights the great importance of major flood events in fluvial transport. Adequate sampling frequency during floods is necessary for reliable annual flux estimates and provides geochemical signals that may greatly improve our understanding of fluvial transport processes. The scenario of SPM and metal and metalloid transport during the flood are reconstructed by combining variations of Zn, Cd and Sb concentrations, concentration ratios (e.g. Zn/Cd, As/Th, Cd/Th) and hysteresis loops. Changes in SPM and metal/metalloid transport during distinct key stages of the flood were attributed to successive dominance of different water masses transporting material from different sources (e.g. industrial point source, bed sediment from reservoirs, plain erosion). Flood management (dam flushing) clearly enhanced the remobilization of up to 30-a old polluted sediment from reservoir lakes. Sediment remobilization accounted for ∼185,000 t of SPM (i.e. 42% of the total SPM fluxes during the flood) and strongly contributed to particulate metal/metalloid transport for Cd (90%), Zn (83%) and Pb (61%). Therefore, flood management needs to be taken into consideration in future models for erosion and pollutant transport.     

Determination of some trace elements in geochemical standards by X-ray fluorescence

The following trace elements Cr, Cu, Nb, Ni, Pb, Rb, Sr, Y, Zn, and Zr were determined by X-ray fluorescence in some geochemical standards from Japan and France. The internal standard method was applied using a Philips PW 1410 X-ray spectrometer. The results were compared with the published data.     

A geochemical survey of trace elements in agricultural and non-agricultural topsoil in Dexing area,China

In China, soil pollution is very serious, which has jeopardized the ecology, food safety, the people's health, and even the sustainable development of agriculture. In order to investigate the soil pollution situation, a total of 874 agricultural and non-agricultural topsoil samples were collected from Dexing area, northeast of Jiangxi Province, China. The total elemental concentrations of 17 elements (As, Hg, Mo, Cd, Cr, Zn, Cu, Mn, Ti, Pb, Fe, Ca, K, Si, Al, Mg, and Na) were determined. The geochemical background and threshold was predicted with the method of the median ± median absolute deviation (MAD). The agricultural soil median concentration of trace elements was similar to that of the non-agricultural soil. In contrast to Jiangxi soil background of trace elements, the geochemical background of the study area was obviously higher. The maps of the pollution indices for As, Cd, Cr, Cu, Hg, Mn, Mo, Pb, Ti and Zn of non-agricultural soil and agricultural soils in the study area, showed that the highest level of pollution is distributed near and along the Lean River, especially in the neighboring and surrounding Dexing and Leping mining area.     

A coupled non-isothermal reactive transport model for long-term geochemical evolution of a HLW repository in clay

A numerical model of coupled saturated/unsaturated water flow, heat transfer and multi-component reactive solute transport is presented to evaluate the long-term geochemical evolution in bentonite, concrete and clay formation for a potential geological radioactive waste repository. Changes in formation porosity caused by mineral dissolution/precipitation reactions are taken into account. Simulations were carried out with a general-purpose multicomponent reactive transport code, CORE^2D V4. Numerical results show that pH in the bentonite porewater can vary from neutral to up to 13 over a time scale of 1 Ma although dissolution of silica minerals and precipitation of secondary calcium silicate hydrate (CSH) minerals in bentonite buffer the effect of the hyperalkaline plume. Mineral precipitation reduces the volume of pore space in bentonite close to the bentonite–concrete interface due to the precipitation of CSH minerals. Model results indicate that bentonite porosity decreases less than 25%. The hyperalkaline plume from the concrete only extends to a distance of 0.7 m in the clay formation over the time range of 1 Ma.     

低熟煤中的孢粉与常量元素和微量元素的相关性初探

运用电离耦合等离子体原子发射光谱(ICP-AES)和X射线荧光光谱分析(XRF),对新疆八道湾煤矿中侏罗统西山窑组煤中的孢粉角质层进行了分析,测定了煤中的常量和微量元素。并对煤中孢粉和共伴生元素进行了回归分析,探讨了微量和常量元素的煤岩学及孢粉学属性,在微量元素与孢粉的煤相学应用方面作了初步尝试。结果表明,成煤的蕨类植物孢子和裸子植物花粉与某些常量元素和微量元素有着很高的相关性;蕨类植物孢子和裸子植物花粉的相对含量在煤中具有互补性,决定了它们与微量元素的关系也具有一定的互补性。     

Carbonation processes in municipal solid waste incinerator bottom ash and their effect on the leaching of copper and molybdenum

The interaction of CO₂ with municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the resulting changes in pH and bottom ash mineralogy and the impact that these changes have on the mobility of Cu and Mo. Carefully controlled carbonation experiments were performed on bottom ash suspensions and on filtered bottom ash leachates. Changes in leachate composition were interpreted with the geochemical model MINTEQA2, and neoformed minerals were investigated by means of chemical and spectroscopic analysis. The leaching of Cu and Mo during artificial carbonation is compared to the leachability of Cu and Mo from a sample of naturally carbonated bottom ash from the same incinerator. During carbonation in the laboratory, a precipitate was formed that consisted mainly of Al-rich amorphous material, calcite, and possibly gibbsite. Carbonation to pH ≈8.3 resulted in a reduction of more than 50% in Cu leaching, and a reduction of less than 3% in Mo leaching. The reduction in Cu leaching is attributed to sorption to the neoformed amorphous Al-minerals. During natural weathering/carbonation of bottom ash, additional sorption sites are formed which further reduce the leaching of Cu and Mo on a time scale of months to years.