Leaching Of Cr Research Articles

Green conversion of hazardous red mud into diagnostic X-ray shielding tiles

Red mud is a solid hazardous alumina industrial waste, which is rich in iron, titanium, aluminum, silicon, calcium, etc. The red mud contains 30–60% of hematite, which is suitable for shielding high energy X- and gamma rays. So, the iron rich red mud was converted into diagnostic X-ray shielding tiles through ceramic route by adding a certain weight percentage of BaSO4 and binders (kaolin clay or sodium hexametaphosphate) with it. The kaolin clay tile possess sufficient impact strength (failure point is 852 mm for 19 mm steel ball) and flexural strength of ~25 N/mm2, which is suitable for wall applications. The 10.3 mm and 14.7 mm thick red mud:BaSO4:kaolin clay tile possess the attenuation equivalent to 2 mm and 2.3 mm lead at 125 kVp and 140 kVp, respectively. No heavy elements were found to leach out except chromium and arsenic from the sintered tiles. However, the leaching of Cr (0.6 ppm) and As (0.015 ppm) was found to be well below the permissible limit. These tiles can be used in the X-ray diagnosis, CT scanner, bone densitometry, and cath labs instead of toxic lead sheet and thereby to protect the operating personnel, public, and environment from radiation hazards.

On leaching mechanisms of major and trace elements from concrete - Carbonation, exposure to deicing salt and external sulphates

Model simulations and tank leach tests are compared to investigate the surface release of major and trace elements from ordinary Portland cement concrete and how this is affected by carbonation, NaCl or Na2SO4 in the eluent. It is distinguished between surface dissolution processes and diffusion of species from inside the concrete. Whereas alkalis, Cr(VI) and Sr are mainly released by diffusion, Ca, SO42−, Al, V, Ba and Zn are primarily released by surface dissolution. Changes in the leaching of Cr(VI) and V from concrete exposed to carbonation, NaCl or Na2SO4 are explained by the solubility equilibria of ettringite in which SO42− is substituted by CrO42− or VO43−. Barite and strontianite control the release of Ba and Sr. It is likely that C–S–H with incorporated Zn controls the release of Zn.

Beneficial utilization of sewage sludge ash residues for the stabilization/solidification of As-, Cr-, and Cu-contaminated marine sediments

The management of incinerated sewage sludge ash (ISSA) residue after phosphorus recovery and dredged marine sediment contaminated with heavy metal(loid)s such as As(V), Cr(VI), and Cu(II) in coastal cities is a major concern due to the limited land space and stringent environmental regulations for solid wastes. This study proposes a novel recycling routine for the generated ISSA residue based on its high iron content. Briefly, the high iron ISSA residue was co-pyrolyzed with a biomass waste (peanut shell) at 1050 °C under N2 atmosphere, yielding a composite material containing zero valent iron (ZVI). Due to the versatile applications of (ZVI) in environmental remediation, including the adsorption removal of heavy metal(loid)s, it could be an alternative low-cost candidate for the solidification/stabilization (S/S) treatment of contaminated marine sediments. The composite material was reused as a partial substitute of cement (at substitution rates of 20%) for the S/S treatment of contaminated marine sediments. The results showed that all the treatment groups could effectively reduce the leaching of As and Cu from contaminated sediments to a comparable level according to the toxicity characteristic leaching procedure (TCLP). CK (cement alone), SBC and BC groups showed a poor immobilization effect toward Cr(VI). Nevertheless, FBC, FSBC, and RBC significantly reduced the leaching of Cr(VI). Replacement of cement by these composite materials significantly reduced the mechanical strength of the S/S products compared to CK. Particularly, BC group demonstrated the lowest mechanical strength, which was 49% lower than that of CK. Even though the lowest mechanical strength was greater than 3 MPa, S/S products met the requirement for recycling as filling materials under local law. The results demonstrated the feasibility of producing ZVI containing biochar composites from ISSA residue and peanut shell, which could be further reused to substitute cement and enhance immobilization efficacy, particularly toward Cr(VI) for contaminated sediment.

Cytotoxicity of stabilized/solidified municipal solid waste incineration fly ash

Low-carbon stabilization/solidification (S/S) is of increasing importance as an option for the treatment of municipal solid waste incineration fly ash (MIFA). This study tailored four binders (e.g., ordinary Portland cement (OPC), calcium aluminate cement (CAC), phosphate-modified OPC, and phosphate-modified CAC) for S/S of MIFA and evaluated the cytotoxicity of treated MIFA by using A549 cell-based in-vitro assay. After S/S treatment, the leachability of Cr, Cu, Zn and Pb from MIFA decreased by 76.1%, 93.4%, 69.6%, and 85.5%, respectively. Spectroscopic analysis indicated that the hydration products determined the immobilization efficiencies of various binders, and strong bonding between metallic cations and phosphate enhanced the immobilization efficiency. The treated MIFA showed significantly lower cellular reactive oxygen species (ROS)-inducing abilities than original MIFA, in which with phosphate-modified OPC treated MIFA showed the lowest ROS levels. Intracellular ROS and multicytotoxicity results also revealed that the treated MIFA not only decreased the cytotoxicity-inducing capability but also enhanced the tolerant dosage of cytotoxicity, in which phosphate-modified S/S treatments showed more effective mitigation (25% less cytotoxicity) than plain cement treatments due to the high-efficiency immobilization of potentially toxic elements. This study develops a pioneering assessment protocol to measure the success of sustainable treatment of MIFA in human health perspective.

Chromium speciation, mobility, and Cr(VI) retention-release processes in ultramafic rocks and Fe-Ni lateritic deposits of Greece.

Water contamination by geogenic hexavalent chromium is an emerging issue in areas developed on ultramafic rocks and their weathering products. In this study, samples of serpentinites, soil, and laterites were collected and analyzed for the levels of Cr species, distribution into phases of different mobility, mineralogy, Cr oxidation capacity, and leaching of Cr(VI). Total chromium (2176-21,929mgkg-1) was mainly found in Cr spinels (~ 50% wt as Cr2O3) and Fe (hydr)oxides (2.5% wt). Total Cr(VI) contents (0.49-11.5mgkg-1) increased from the serpentinites to the soil and lateritic deposit, i.e., during the advanced stages of weathering, which were accompanied by increased Cr mobility. Batch experiments of 500-h duration showed that Cr(VI) released in water at rates of 0.25-1.20nMh-1. Rates were higher in water of pH 5.7 rather than pH 8.5, because more Cr(VI) was formed during the experimental period. Asbolane-type Mn oxides and Mn-bearing crystalline Fe oxides were responsible for Cr(III) oxidation. Most of the generated Cr(VI) (52-79% of total Cr(VI)) remained solid-bound by adsorption and/or precipitation processes. Because all samples had a self-capacity to oxidize Cr(III), it seems that retention processes will ultimately define the extent of geogenic Cr(VI) contamination of aquatic systems.

EAF slag as alternative filler for epoxy screeds, an example of green reuse

In this study, electric arc furnace (EAF) slag as a filler in an epoxy matrix is investigated to validate its use instead of sand in construction applications, such as epoxy screeds. The slag chemical composition was determined and morphological and elemental analysis of its phases was analysed. The leaching of Cr, Mo, and V was evaluated both on EAF slag and on the composites materials, demonstrating that polymeric matrix inhibits the release of heavy metals. Epoxy composites with different percentages of slag were mechanically characterized by flexural test, compression test, and Vickers microhardness. The samples were compared to those of epoxy resin filled with sand in the same volume percentage. As a reference, a commercial product specifically formulated for laying epoxy screeds was also tested. The results demonstrated that EAF slag could substitute the sand in epoxy screeds, obtaining a composite material with equal and even superior mechanical properties.

Migration of leather tannins and chromium in soils under the effect of simulated rain

Chromium (Cr) and tannin are two major pollutants in leather industry. However, little is known about the co-migration of leather tannins and Cr in soils. In this study, column experiments were conducted to estimate Cr leaching behavior from topsoil and the environmental risk of the leachate at various tannin dosages and different ways (tannin either directly adding to the Cr-contaminated soil or adding stepwise through simulated rain) into the soil. The total Cr concentration in leachate was positively related with tannin content in soil, while Cr (Ⅵ) concentration was negatively correlated. The maximum cumulative leaching efficiency of total Cr from soil after six leaching events was 44.65% with 3 mg/g tannin adding into soil directly, and the maximum cumulative leaching efficiency of Cr (Ⅵ) was 38.75% with simulated rain leaching Cr-contaminated soil. With 3 mg/g tannin adding into soil, tannin concentration in the top layer (0–7 cm) lost by 32.67% after leaching, the amount of decomposed tannin was 0.25 mg/g, excluding the amount of tannin in leachate (3.63 mg/L) and the original amount in the soil (0.34 mg/g), indicating a slow degradation under natural condition. Both of the total Cr and Cr (Ⅵ) concentration in each layer of the soil columns decreased under tannin treatments compared with control. Compared with tannin adding stepwise into simulated rain, adding tannin into soil significantly (p < 0.05) affected the migration of Cr. Tannin increased the residual fraction while decreased the exchangeable fraction of Cr in the soils. Overall, this research can provide reference information for environmental risk assessment of contaminants in tanning sites.

Microwave-enhanced reductive immobilization of high concentrations of chromium in a field soil using iron polysulfide

High concentrations of Cr(VI) are often detected in contaminated soil. Yet, cost-effective remediation technologies have been lacking. In this study, we prepared a type of FeSx based on commercial FeSO4.7H2O and CaSx and tested a microwave-assisted technology based on FeSx for reductive immobilization of high concentrations of Cr(VI) in a field contaminated soil. The as-prepared FeSx particles appeared as a honeycomb-like and highly porous structure. The microwave-assisted FeSx reduction process was able to rapidly reduce the TCLP-based reachability of Cr(VI) from 391.8 to 2.6 mg·L-1. The dosage of FeSx, S/Fe molar ratio, initial moisture content, microwave power, and irradiation time can all affect the treatment effectiveness. After 500 days curing under atmospheric conditions, the TCLP-leached concentration of Cr remained below the regulatory limit of 5 mg·L-1, while other treatments failed to meet the goal. Sx2- or S2- served as the primary electron donors, whereas Fe facilitated the microwave absorption and the formation of the stable final product of FeCr2O4. S and Fe are mostly precipitated in soil. The microwave-assisted FeSx reduction was shown to be an effective approach to rapidly reduce the leachability of Cr(VI) in contaminated soil, especially in heavily contaminated soil.

Synthesis of a magnetically separable LDH-based S-scheme nano-heterojunction for the activation of peroxymonosulfate towards the efficient visible-light photodegradation of diethyl phthalate

Herein, a stable and S-scheme Fe3O4@CuCr-LDH nanocomposite was synthesized and used for the photocatalytic degradation of diethyl phthalate (DEP) through the activation of peroxymonosulfate (PMS). In comparison with pure Fe3O4, and LDH, the Fe3O4@CuCr-LDH nanocomposite demonstrated significantly enhanced photocatalytic activity due to its S-scheme charge-carrier migration mechanism. The prepared composite was also magnetically recoverable from the treated water which is favorable to avoid the production of secondary pollution. The integration of the composite and visible light showed the presence of a synergy factor of 14 for the degradation of diethyl phthalate. However, the photocatalytic performance of Fe3O4@CuCr-LDH was dependent on the different physicochemical parameters; wherein, the higher degradation efficiency was achieved using the solution pH of 8, the catalyst, DEP, and PMS concentrations of 1 g L−1, 20 mg L−1, and 8 mM, respectively. The high photocatalytic activity of the catalyst was maintained after 5 consecutive reaction runs and the stability of the material was proved by the XPS. Moreover, the ICP-AES analysis proved that the leaching of Fe, Cr, and Cu is lower than the standard concentration in the drinking water. Finally, the mineralization ability and the decreased toxicity of the treated solution of DEP were assessed.

A Comparison in Metal Leachability Between Phosphoric Acid and Ascorbic Acid Stabilised Ferrochrome Slag

Ferrochrome (FeCr) slag was milled and stabilised with either ascorbic acid or phosphoric acid. The stabilised FeCr was then geopolymerised with 1 M KOH in order to obtain a monolith with at least an unconfined compressive strength of 1 MPa. The leachability of metals of the stabilised geopolymerised monoliths were then compared with the unstabilised geopolymerised monolith. Ascorbic acid stabilisation was only effective in Cr leaching reduction by 99.45% but was not effective on immobilisation of Fe, Zn, Ni and Mn. Ascorbic acid stabilisation was thought to proceed via the reduction of Cr(VI) species to insoluble Cr (III) species. Phosphate stabilisation reduced the leachability of Cr, Ni, Zn, Mn, Fe by 99.5%, 67.1%, 71.1%, 96.8% and 85.4% respectively. Phosphate stabilisation was thought to proceed via the formation of phosphate compounds of the metal ions in question. The phosphate stabilised FeCr slag leachability was within the allowable Toxicity Characteristic Leaching Procedure (TCLP) limits and its use is not detrimental to the environment.

Removal of chromate ions from leachate-contaminated groundwater samples of Khan Chandpur, India, using chitin modified iron-enriched hydroxyapatite nanocomposite.

Chromite ore processing residues (COPR) are real environmental threats, leading to CrO42-, i.e., Cr (VI) leaching into groundwater. It is of serious concern as Cr (VI) is proven to be carcinogenic. Here we emphasize the application of novel and eco-friendly chitin functionalized iron-enriched hydroxyapatite nanocomposite (HAP-Fe0-Ct) in the remediation of Cr (VI)-contaminated groundwater samples collected from Khan Chandpur, India, where the level of Cr (VI) is found to be 11.7 mg/L in a complex aqueous matrix having 793 mg/L of total dissolved solids. Chitin functionality in the composite has resulted in positive zeta potential at circum-neutral pH, favoring electrostatic attraction of chromate ions and resulting in its bulk surface transport. The HAP-Fe0-Ct showed faster kinetics of removal with efficiency (qm = 13.9 ± 0.46 mg/g) for Cr (VI). The composite has shown sorption equilibrium and 100% removal of Cr (VI) within 3 h of interaction time in groundwater samples. No Cr (VI) leaching in the acid wash process at pH 3.5 also suggests chromium's strong chemisorption onto nanocomposite. During the interaction in aqueous solutions, thereduced iron (Fe0) on the nanocomposite becomes oxidized, suggesting the probable simultaneous reduction of Cr (VI) and its co-precipitation. Continuous column extraction of chromate ions was also efficient in both spiked solutions (39.7 ± 0.04 mg/g) and COPR contaminated water (13.2 ± 0.09 mg/g). Reusability up to three cycles with almost complete Cr (VI) removal may be attributed to surface protonation, new binding sites generation, and electron transfer from Fe0 core through defects. The study concludes that HAP-Fe0-Ct could be utilized for continuous Cr (VI) removal from COPR contaminated complex groundwater matrices.

Stabilization of chromium(VI) by hydroxysulfate green rust in chromium(VI)-contaminated soils

Chromium (Cr)-contaminated soils pose a great environmental risk, with high solubility and persistent leaching of Cr(VI). In this study, hydroxysulfate green rust (GRSO4), with the general formula Fe(II)4Fe(III)2(OH)12SO4·8H2O, was evaluated for its efficiency in Cr(VI) stabilization via Cr(VI) reduction to Cr(III) in four representative Cr(VI)-spiked soils. The initial concentrations of phosphate buffer-extractable Cr(VI) (Cr(VI)b) in soils 1, 2, 3, and 4 were 382.4, 575.9, 551.3, and 483.7 mg kg–1, respectively. Reduction of Cr(VI) to Cr(III) by structural Fe(II) (Fe(II)s) in GRSO4 in all studied soils was fast, wherein the application of GRSO4 markedly decreased the amount of Cr(VI)b at the Cr(VI)b/Fe(II)s stoichiometric mole ratio of 0.33. The kinetics of Cr(VI) reduction by GRSO4 could not be determined as this reaction coincided with the release of Cr(VI) from soil during the experiment. The concentration of Cr(VI)b decreased, as the Cr(VI)b/Fe(II)s ratio decreased from 0.46 to 0.20, generally to below 10 mg kg–1. Back-transformation of the generated Cr(III) was examined in the presence of manganese oxide birnessite at the birnessite/initial Cr(III) mole ratio of 4.5. The results of batch tests showed that only 5.2% of the initial Cr(III) was converted to Cr(VI) after two months, while under field capacity moisture conditions, less than 0.05% of the initial Cr(III) was oxidized to Cr(VI) after six months. The results illustrated that remediation of Cr(VI)-contaminated soils would be fast, successful, and irreversible with an appropriate quantity of fresh GRSO4.

Resource recycling sustainability assessment in ready-mixed concrete manufactured on energy consumption and environmental safety in China.

With the rapid development of construction industry, consumption of ready-mixed concrete has increased rapidly in China. As a kind of green building material and waste comprehensive utilization product, industrial solid waste-based ready-mixed concrete have better performance in terms of resource conservation and durability. However, some typical issues are associated with industrial solid waste resource recycling in ready-mixed concrete production process such as energy and material consumption, as well as leaching pollutant emissions. So, a "life cycle assessment" of the particular elements has been carried out, determining the resources consumption of all the processes of the ready-mixed concrete production. Through preparation of several different strength grades of concrete, the embodied energy and resources consumption indicator are quantitative evaluation. In addition, the environmental safety assessment was also proposed. The results show a certain linear relationship of concrete with various strength grade and its resources consumption, the higher strength grade, the more resources consumption potentials in the production of concrete. In this case, the linear correlation coefficient R2=0.98313 between them, and the equivalent coefficient's order of the main resources, is as follows: the first is oil, then natural gas, iron ore, limestone, gypsum, and fly ash, and the last is coal. The more preceding shows more scarcity. Meanwhile, the general leaching of heavy metals Cu, Zn, Ni, Cr, Pb, Cd, and Ba of solid waste-based C20 concrete were also checked out. So, to further ensure the environmental safety, the potential ecological risk method was adopted to assess the heavy metal security and solid waste resource utilization.

Anodic Oxidation Enabled Cation Leaching for Promoting Surface Reconstruction in Water Oxidation.

A rational design for oxygen evolution reaction (OER) catalysts is pivotal to the overall efficiency of water electrolysis. Much work has been devoted to understanding cation leaching and surface reconstruction of very active electrocatalysts, but little on intentionally promoting the surface in a controlled fashion. We now report controllable anodic leaching of Cr in CoCr2 O4 by activating the pristine material at high potential, which enables the transformation of inactive spinel CoCr2 O4 into a highly active catalyst. The depletion of Cr and consumption of lattice oxygen facilitate surface defects and oxygen vacancies, exposing Co species to reconstruct into active Co oxyhydroxides differ from CoOOH. A novel mechanism with the evolution of tetrahedrally coordinated surface cation into octahedral configuration via non-concerted proton-electron transfer is proposed. This work shows the importance of controlled anodic potential in modifying the surface chemistry of electrocatalysts.

Anodic Oxidation Enabled Cation Leaching for Promoting Surface Reconstruction in Water Oxidation

AbstractA rational design for oxygen evolution reaction (OER) catalysts is pivotal to the overall efficiency of water electrolysis. Much work has been devoted to understanding cation leaching and surface reconstruction of very active electrocatalysts, but little on intentionally promoting the surface in a controlled fashion. We now report controllable anodic leaching of Cr in CoCr2O4 by activating the pristine material at high potential, which enables the transformation of inactive spinel CoCr2O4 into a highly active catalyst. The depletion of Cr and consumption of lattice oxygen facilitate surface defects and oxygen vacancies, exposing Co species to reconstruct into active Co oxyhydroxides differ from CoOOH. A novel mechanism with the evolution of tetrahedrally coordinated surface cation into octahedral configuration via non‐concerted proton‐electron transfer is proposed. This work shows the importance of controlled anodic potential in modifying the surface chemistry of electrocatalysts.

Process for Enhancing the Valuable Metal Recovery from "Electric Arc Furnace" (EAF) Slags

Given the current depletion of the world's natural resources, the strong demand and the induced future deposit of primary mineral resources carrying metals (such as molybdenum, vanadium and chromium), one of the solution to this problem is the recycling of metals from by-products (such as steel slags) containing them. Steel slags are major by-products produced by the steel industry. Although they are considered as industrial waste that may cause public and environmental concerns, slags represent an important potential economic resource because they often contain significant amounts of valuable "Strategic Metals" (SMs). In order to minimize energy costs, environmental impacts and meet the metals demand for new high technologies, it will be more and more necessary to use cleaner and more economical technologies to recover these metals from slags. In this study, steel slags, generated from electrical arc furnace and provided by the Industeel France ArcelorMittal Company located in Châteauneuf (France), were studied. The chemical and mineralogical characterization results show that the investigated slags contain approximately 20–22% Fe, 3–4% Cr, 1 500 ppm V and are constituted of the following main crystalline phases: larnite (Ca2SiO4), magnetite (Fe3O4), wustite (FeO) and chromite (FeCr2O4). Slag samples underwent different treatment steps before leaching (magnetic separation, grinding at d ≤ 25 μm and roasting). Various alkaline reagents (NaOH, KOH, Na2CO3 and K2CO3) have been studied with a slag/reagent weight ratio of 1, sufficiently high to provide an excess of alkaline reagent and allows the complete modification of the metal species able to be modified with these reagents at different co-grinding residence times. Three temperatures (400, 600 and 800 °C) were tested. The results show that the Cr leaching rate reaches 97.5% for one hour of the slag co-grinding with a NaOH and Na2CO3 mixture followed by roasting at 800 °C, while the V maximum leaching rate of 62.5% of vanadium is obtained after three hours of slag co-grinding with NaOH and Na2CO3 and roasted at 600 °C. The Mo leaching rate, meanwhile, reaches 95.8% using KOH for one hour of co-grinding the slag followed by roasting at 600 °C.

Conversion of chromium-containing solid wastes into value-added products through a plasma-assisted aluminothermic process.

Chromium-containing solid wastes have been generated by chemical and leather/tanning industries, and the management and proper disposal of the same wastes have been challenging tasks. A significant fraction of these wastes contains chromium compounds with chromium present in the hexavalent (Cr+6) form, which is hazardous to human beings, animals, and ecosystems. Since these wastes are discarded largely without proper treatments, soil and groundwater get contaminated and they can cause several health issues to human beings. Conventional methods developed to convert hazardous Cr6+ to Cr3+/Cr metal either generate secondary toxic wastes and unwanted by-products and/or are time-consuming processes. In this work, a plasma-assisted aluminothermic process is developed to convert the toxic waste into non-toxic products. The waste was mixed with aluminium powder and subjected to transferred arc plasma treatment in a controlled air atmosphere. Chemical analysis and Cr leachability studies of the waste material prior to plasma treatment have shown that it is highly toxic. Analysis of the products obtained from the plasma treatment showed that Cr and Fe present in the waste could be recovered as a metallic mixture as well as oxide slag, which were found to be non-toxic. Easy separation of the metallic fraction and the slag from the treated product is one of the merits of this process. Besides converting chromium-containing toxic waste to non-toxic materials, the process is rapid and recovers the metals from the waste completely.

Field availability and mobility of metals in Ferralsols developed on ultramafic rock of Niquelândia, Brazil

Ultramafic (UM) rocks are defined as igneous rocks that contain more than 90% of mafic minerals. Soils derived from ultramafic rock are generally nutrient-deficient and have concomitant high concentrations of potentially phytotoxic trace elements (Ni, Cr, Co, Mn). Consequently, to assess the dynamics of nutrients and metals in the ultramafic complex of Niquelândia (Brazil), soil solutions have been sampled in soils characterized by high Cr(VI) availability. The metal contents in surficial water have also been analyzed to investigate the metals’ leaching and mobility. Soil solutions featured low nutrient contents, a large Ca:Mg imbalance, and high Ni and Cr concentrations. Chromium was present in its toxic dissolved form (Cr(VI)) in the soil and surficial solutions. Metals concentrations were often above the toxic limit for biota and were therefore able to affect soil functioning. Ni behavior in the topsoil appeared to be primarily controlled by organic matter, while Cr was more likely to be released from Fe-oxides by anionic exchange. This result agreed with the Cr(VI) lability assessed using isotopic exchange kinetics in a companion study. In these serpentinic tropical soils, the highly leached Fe-oxide horizons appear to play a large role in the sequestration and diffuse leaching of labile Cr(VI) and Ni, respectively, in the deeper part of soil profiles and the topsoil. At the catchment scale, surficial solutions results suggest that metals may be exported to surrounding ecosystems that are not adapted to these metals.

Chromium mobility in ultramafic areas affected by mining activities in Barro Alto massif, Brazil: An isotopic study

This work studies the potential release of Cr from solids to surface water and groundwater, and the related isotopic compositions, in a nickel laterite ore deposit from Barro Alto in Brazil (in the State of Goiás). This ultramafic system is characterized by elevated concentrations of chromium (Cr). Even largely immobile Cr may be naturally leached from weathering profiles such as laterite to the surface and groundwater. This system is exploited for metal production, and both mining and metallurgical activities result in a significant increase in the trivalent (Cr(III)) and hexavalent chromium (Cr(VI)) concentrations released into the environment via runoff. Among all the samples collected, ores contained higher amounts of chemically and isotopically exchangeable Cr(VI) (ECr(VI)) with values as high as 104 (± 8) mg kg−1. δ53Cr increased from −0.28 (± 0.01)‰ to −0.05 (± 0.01)‰ and the ECr(VI) value was up to 30 times higher in deep soils than at the surface (up to 7 (± 1) mg kg−1). Chemically extracted Cr(VI) (ECr(VI)-KH2PO4) displayed positive δ53Cr value (1.69 (±0.03) ‰) with a similar Cr isotopic composition as the ones measured in freshwater sampled in and around the mine area. It appears that Cr is primarily released as Cr(VI), i.e. the toxic species, and becomes increasingly more available as it moves from the soil profile to the ores and mining residues. Based on the differences in the Cr isotopic composition, this study proves that δ53Cr can be used in environmental studies to trace Cr leaching.

Study on migration characteristics of heavy metals during the oil sludge incineration with CaO additive

The volatilization of heavy metals in oil sludge (OS) restricts the development of OS incineration. In order to explore the effect of CaO on the migration characteristics of heavy metals during OS incineration, incineration experiments of OS with and without CaO were carried out respectively in a horizontal tube furnace at the temperature ranging from 800 °C to 1000 °C. The total amount, speciation fractions, and leachability of Cr, Cu and Zn in OS and bottom ash were investigated and presented in this work. The results showed that the total amount of Cr, Cu, and Zn in the bottom ash decreased with the increasing of experimental temperature. CaO could solidify the heavy metals in the bottom ash with an order of Cu, Cr, and Zn from strong to weak. In the presence of CaO additive, the exchangeable and acid-soluble fractions of Cu and Zn in bottom ash were significantly reduced. Moreover, the leaching levels of Cr, Cu and Zn in bottom ash were also obviously lowered. The results will supply essential data and theoretical support for the innocuous treatment of heavy metals in the bottom ash of OS incineration.