Sequential Extraction Results Research Articles

Characterization and source identification of organic phosphorus in sediments of a hypereutrophic lake

High phosphorus (P) load and consequent algal bloom are critical issues because of their harmful effects to aquatic ecosystems. The organic phosphorus (Po) cycling and hydrolyzation pathway in the sediments of a hypereutrophic lake area with high algae biomass were investigated using stable isotopes (δ13C and δ15N) along with C/N ratios, a sequential extraction procedure, 31P NMR spectrum, and alkaline phosphatase activity (APA) was measured simultaneously. C/N ratios lower than 10 combined with lighter δ13C (−23.5 to −25.2‰) and δ15N values (3.7–9.5‰) indicated that endogenous algal debris contributed to the predominant proportions of P-containing organic matter in the sediments. Sequential extraction results showed that Po fractions decreased as nonlabile Po > moderately labile Po > biomass-Po. Decreasing humic-associated Po (HA-Po) in sediments downward suggested the degradation of high-molecular-weight Po compounds on the geological time scale to low-molecular-weight Po including fulvic-associated Po (FA-Po), which is an important source of labile Po in the sediment. An analysis of the solution 31P NMR spectrum analysis showed that important Po compound groups decreased in the order of orthophosphate monoesters > DNA-Po > phospholipids. The significant correlation indicated that orthophosphate monoesters were the predominant components of HA-Po. Rapid hydrolysis of labile orthophosphate diesters further facilitated the accumulation of orthophosphate monoesters in the sediments. Additionally, the simultaneously upward increasing trend demonstrated that APA accelerated the mineralization of Po into dissolved reactive phosphorus (DRP), which might feed back to eutrophication in algae-dominant lakes. The significantly low half-life time (T1/2) for important Po compound groups indicated faster metabolism processes, including hydrolysis and mineralization, in hypereutrophic lakes with high algae biomass. These findings provided improved insights for better understanding of the origin and cycling processes as well as management of Po in hypereutrophic lakes.

Availability of copper in mine tailings with humic substance addition and uptake by Atriplex halimus.

The chemical characteristics of mine tailings, organic amendments (doses), and plants are the critical factors that must be evaluated and monitored to ensure the sustainability of phytostabilization. The aim of this study was to evaluate the mobility of copper (Cu) in mine tailings (MT) of the Zone Central of Chile to which commercial humic substances were added, examining their effect on the uptake of Atriplex halimus. Two commercial humic substances (HS1 and HS2) extracted from leonardite (highly oxidized lignite), of different pH and total organic carbon, were evaluated by adsorption curve for Cu. In columns, soluble Cu, pH, and electrical conductivity in leachates were evaluated for MT, MT + HS1, and MT + HS2, and HS1 and HS2 in doses of 120 mg kg-1. In pot assay, seeds were germinated directly in MT and cultivated for 140 days with the addition of HS2 in 120 and 240 mg kg-1. Mine tailing presents high concentration of Cu (2016 ± 223 mg kg-1, pH 6.3 ± 0.1). The results of sequential extraction indicate that Cu is associated with the sulfide fraction of low risk of mobility. The amount of Cu sorbed by HS1 was higher than that sorbed by HS2, and both humic substances showing better fit to the Freundlich than Langmuir model. Lixiviation of Cu was significantly lower in MT + HS1 (0.166 ± 0.043 mg kg-1) and MT + HS2 (0.157 ± 0.018 mg kg-1) than in MT (0.251 ± 0.052 mg kg-1). Copper concentration in plants reached 185.8 ± 37.8 mg kg-1 in the roots and 32.6 ± 7.4 mg kg-1 in the aerial parts cultivated in MT without effect of the humic substance addition in Cu uptake nor growth. Copper concentrations in the aerial parts were adjusted to sufficient or normal levels in plant. A good management of mine tailings through phytostabilization could consider an adequate mixture of humic substances (to avoid leaching of metals) and an organic amendment that provides essential nutrients and increases biomass generation.

Evolution of As speciation with depth in a soil profile with a geothermal As origin

High contents of arsenic were detected in soils in Guandu plain, northwest Taiwan. To determine the sources and speciation of As in the soils, the depth profiles of soil properties, elemental composition and As speciation were investigated. The As concentrations in the soil profile ranged from 152 to 1222 mg kg−1, with the highest concentration at the depth of 70–80 cm. The As distribution was found to be positively correlated to Fe, Pb, and Ba. The As(V)-adsorbed ferrihydrite and scorodite were the predominant phases in the top layers (<50 cm), while beudantite was the predominant phase below 50 cm along with As(III)- and As(V)-adsorbed ferrihydrite as the minor components. The results of sequential extraction showed that As-associated with noncrystalline and crystalline Fe/Al hydrous oxides and residual phases were predominant at the depths of 0–60, 60–100 and 100–140 cm, respectively, indicating an increasing As recalcitrance with soil depth. Based on the soil properties, and elemental and mineral compositions at different soil depths, the origin of beudantite in the soils was likely allogenic rather than authigenic or anthropogenic. The formation of scorodite in the surface soils was suggested to be transformed from beudantite. As-associated Fe hydrous oxides may be contributed by the progressive dissolution of beudantite and scorodite, and the continuous influxes of As and Fe. While Fe hydrous oxides were able to immobilize As during the dissolution of As-bearing minerals, the increase of As mobility in soils may imply an increase in the environmental risk of As over time.

Reuse of a phosphorus recovery product (struvite/palygorskite) from nutrient wastewater for copper remediation in aqueous solution and soil

In this study, a phosphorus recovery product, struvite palygorskite (S-PAL), obtained from nutrient-rich wastewater by using MgO modified palygorskite was applied for copper remediation in aqueous solution and contaminated soil to achieve waste recycling. The effects of contact time, initial pH, initial Cu(II) concentration and reaction temperature on Cu(II) adsorption in aqueous solution were intensively testified. Pseudo-second-order model was able to properly describe Cu(II) adsorption kinetics by using palygorskite (PAL) and S-PAL, and S-PAL exhibited higher adsorption amount (106.27 mg/g) than PAL (8.46 mg/g) at pH of 4. Cu(II) adsorption on PAL and S-PAL could be well fitted by Freundlich isotherm and Langmuir isotherm, respectively. The calculated thermodynamic parameters indicated that Cu(II) adsorption onto PAL and S-PAL were spontaneous and endothermic. A 28-day soil incubation experiment was conducted to evaluate the effects of PAL and S-PAL with three different rates (1%, 5% and 10% w/w) on Cu immobilization in contaminated soil. In the immobilization test, Cu extracted by 0.01 mol/L CaCl2 after seven days incubation significantly decreased with increasing rate of PAL and S-PAL. BCR sequential extraction results showed the significant decrease of acid soluble Cu and a concomitant increase of the residual fraction of Cu after S-PAL and PAL addition. XRD patterns of soil samples after treatment by PAL and S-PAL showed the formation of Cu0.6Mg1.3Si2O6 and Cu3.04(PO4)2OH0.08·2H2O, which indicated that silanol groups and phosphate exhibited affinity for Cu in the soil.

Oil physical status in lacustrine shale reservoirs – A case study on Eocene Shahejie Formation shales, Dongying Depression, East China

A suite of 24 lacustrine shale core samples from the Eocene Shahejie Formation of the Dongying Depression, East China have been geochemically and petrologically characterized to investigate shale oil production potential and its constraints. Rock-Eval analysis indicates that majority samples are organic-rich with average TOC content of 3.69 wt%, dominated by Type II kerogen at early to peak oil generation stage. Shale oil physical status was defined as free, adsorbed and residual oils based on sequential extraction from different particle sizes under cold and Soxhlet extraction conditions. Quantitative data illustrate that free oil dominates the total extractable organic matter (EOM) yields (with an average of 66%). EOM bulk compositions are dominated by the saturated hydrocarbons, which decreases sequentially from free to residual status, corresponding to the increase of polar fractions. Several major factors, including TOC content, brittleness index, clay mineral content and thermal maturity, which may control relative proportions of different status oils in shale reservoirs are discussed herein. TOC and clay mineral content exert critical impact on adsorption ability and retention proportion, while brittleness index shows a positive correlation to EOM amounts. Thermal maturity plays the principle role in controlling hydrocarbon generation and then the expulsion behavior which in turn have major impacts on the total extract yields. Relatively low maturity coupled with high clay mineral contents were considered as main restrictions on shale oil producibility in the study area. Our sequential extraction results also demonstrated marked compositional fractionation during primary migration in shale reservoirs.

Residence Time Effects on Molybdenum Adsorption on Soils: Elucidation by Multi-Reaction Modeling and XANES Analysis

To investigate the influence of residence time on molybdenum [Mo(VI)] adsorption behavior in soil environments, kinetic batch experiments coupled with X-ray near-edge structure (XANES) spectroscopy were performed for a neutral-pH soil (Webster loam) and two acidic soils (Mahan sand and Windsor sand) at different time scales (1 day–1 year). Batch-type experiments indicated that retention of Mo(VI) was rate limited and typical biphasic for soils. Initial rapid retention was followed by a continued slow retention with increasing aging time for Mahan and Windsor soils. In contrast, the reaction for Webster soil was nearly complete after 8 h, reflecting difference in soil properties. XANES analysis for Webster soil confirmed that most of Mo was bound to montmorillonite during long-term reaction time, whereas kaolinite constitutes a very important host phase for Mahan and Windsor soils. Sequential extraction results indicated that the percentages of Fe/Al oxide and residual fractions increased at the advanced time periods for Mahan and Windsor soils. The goodness-of-fit of numerical modeling results indicated that a simple version of multi-reaction model (MRM) with equilibrium and kinetic sorption sites was capable of describing Mo(VI) retention data for Webster loam. However, for Windsor and Mahan soils, an additional irreversible sorption site was required to simulate Mo(VI) retention over time. Although each site from MRM model cannot be unequivocally clarified from each other by either XANES analysis or sequential extraction results, their finding provided evidence of surface irreversible reactions at long residence times.

The importance of drying and grinding samples for determining mobile chromium fractions in polluted river sediments

A possible impact of sample preparation on the chemical fractionation results is generally underestimated in studies of forms of occurrence of heavy metals in river sediments. Our analysis of the recently published results of sequential extraction of chromium has revealed the effect of sample grinding on the result of determination of mobile chromium fractions in river sediments. This observation has been experimentally verified along with the analysation of potential effect of river sediment drying conditions on chromium distribution pattern. The studies were carried out on river sediments polluted with tannery effluents (Cr, 29.2–233 mg/kg). The determined content of chromium bound to carbonates in powdered samples was 2 to 7 times higher than those in raw river sediment samples. It was shown that the main reason was the different kinetic characteristics of chromium leaching in these sediments. Using the shrinking core model, it was found that diffusion through the “ash layer” was the rate-controlling step during the extraction of the carbonate fraction of chromium. It has been additionally confirmed that common air drying of sediment samples does not affect the results of chemical fractionation of chromium.The results of our studies are also vital for the assessment of environmental risk posed by river sediments polluted with heavy metals. In the case of sediment samples used in this study, powdering changed the risk category (RAC) from low risk to high risk. Hence, in order to achieve a realistic assessment of chromium mobility and environmental risk, it is advisable to use raw samples, despite their poorer homogeneity, and thus, lower precision of chemical fractionation results.

Groundwater risk assessment investigations for oil shale exploitation in El-Lajjun area of central Jordan

Oil shales are sedimentary rocks that contain organic material in its inorganic matrix. The oil shale deposits of Jordan have been investigated since the 1960s. An extensive investigation studies have been commenced to evaluate the quality and quantity of these resources. In 2006, the government of Jordan invited pioneer companies in oil shale field to submit their proposal to utilize and develop the oil shale of Jordan using different extraction technologies. Jordan is considered one of the driest countries in the world, which poses an additional challenge for oil shale development, because a vast quantity of water is needed for this process. El-Lajjun study area is located in central Jordan; the oil shale in the study area was deposited in marine environment; it comprises massive beds of brown-black, kerogen-rich, bituminous chalky marl. Ten representative oil shale rock samples were collected for risk assessment investigations from different localities of oil shale exposures. A standardized laboratory Fischer assay test delivered oil shale characteristics. Sequential extraction was used to evaluate mobility and distribution of the trace elements Ti, V, Cr, Co, Zn, As, Zr, Cd, Pb, and U. Column leaching experiments were performed to simulate the leaching behavior of the above elements from oil shale and spent shale to evaluate the possible negative impact on groundwater in the study area. Results of sequential extraction indicate that concentration of trace elements and trace elements are richer in oil shale and in the spent shale compared to most rocks and soils. The concentrations in the leachate were below the maximum contaminant levels of the Environmental Protection Agency (EPA) and the Jordanian standards for drinking water. The significance of this study lies in that its utilizes for the first time the sequential extraction and column leaching techniques to investigate the Jordanian oil shale rocks.

Temporal trends of &amp;lt;sup&amp;gt;137&amp;lt;/sup&amp;gt;Cs activity concentration in pond waters in the vicinity of Fukushima Dai-ichi nuclear power plant

Abstract. Closed and semi-closed water bodies, such as lakes and ponds, are important water resources in Fukushima area and they are the most sensitive environments to radioactive contamination after the Fukushima Dai-ichi nuclear power plant accident. Wakiyama et al. (2017) investigated 137Cs activity in water and bottom sediment in four ponds; Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), within 10 km zone from the FDNPP during 2015–2016. This study follows up their observation to address longer time trends of 137Cs activity concentration in pond waters and to show speciation of 137Cs in soil and bottom sediment. Mean total 137Cs activity concentration in water ranged from 2.5 to 29 Bq L−1. There was not found steady trend in the activity concentration of total and particulate 137Cs for four ponds. The concentration of dissolved 137Cs was usually low in winter during the entire observation period in all four ponds. A tendency to a decrease in the 137Cs activity concentration in suspended sediments was found for four ponds and the decreasing rate constants, including radiological decay, on SU, FS, IZ and KM were 0.33, 0.53, 0.29 and 0.25 yr−1, respectively. The results of sequential extractions of soil and bottom sediment samples showed higher proportion of bioavailable 137Cs, i.e., exchangeable and organic bound 137Cs, in bottom sediment than in the soil.

Geochemical solid characterization of drill cuttings, core and drilling mud from Marcellus Shale Energy development

Million tons of drill cuttings generated from shale gas development are currently disposed of in landfills, buried in-situ, or reused as road fill. Cuttings, core samples and operating drilling mud from the Marcellus Shale Energy and Environmental Laboratory in WV were studied to better characterize drill cuttings and to evaluate trace metal mobility in various disposal environments. Results showed that physical and chemical properties of drill cuttings are impacted by the host-rock formation, calcium sources and the residual drilling-mud. Barite in residual drilling mud forms a coating around rock cuttings and influences the mobility of Sr in drill cuttings. Trace metals in drill cuttings are primarily associated with pyrite and total organic carbon (TOC) phases in the shale. Based on sequential extraction results, various trace metal mobilities are controlled by additional solid fractions in drill cuttings, such as pyrite, TOC, calcite, barite and exchangeable clays in the shale.

Enhanced Anaerobic Performances of Kitchen Wastes in a Semi-Continuous Reactor by EDTA Improving the Water-Soluble Fraction of Fe

The addition of Fe2+ is considered an effective method for increasing methane production, but the added Fe2+ may not be absorbed by anaerobic microorganisms due to complex chemical reactions. In this study, ethylenediaminetetraacetic acid (EDTA) was used as a ligand of Fe2+ (EDTA-Fe) to promote the dissolution of Fe, and the anaerobic performances of kitchen wastes (KWs) in a semi-continuous reactor were studied. The results indicated that the biogas yields and methane contents were enhanced to 594–613 mL·g−1VSadd·d−1 and 63.6–64.4% at an organic loading rate (OLR) of 2.5 gVSadd·L−1·d−1 due to EDTA-Fe addition. Simultaneously, the EDTA-Fe was more effective than Fe2+ in preventing the acidification of KWs with a high OLR (5.0 gVSadd·L−1·d−1). In addition, the sequential extraction results showed that the water-soluble fraction of Fe in the R3 (EDTA-Fe addition) was 1.49-fold of that in the R2 with Fe2+ addition. The contents of coenzymes F420 and F430 were also improved 1.09 and 1.11 times, respectively. Mechanism analysis confirmed that the EDTA enhanced methane production and operational stability by promoting the dissolution of Fe and maintaining a high content of water-soluble Fe.

Sorption mechanisms of diphenylarsinic acid on ferrihydrite, goethite and hematite using sequential extraction, FTIR measurement and XAFS spectroscopy

Diphenylarsinic acid (DPAA) is an organoarsenic compound derived from abandoned chemical weapons. DPAA sorption by iron (hydr)oxides is of considerable importance but remains largely unexplored. The current study aimed at investigating the sorption mechanisms of DPAA on ferrihydrite, goethite and hematite using both macroscopic sorption kinetics and sequential extraction procedure (SEP) as well as microscopic Fourier transformed infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques. Sorption kinetics studies show that >93% of added DPAA (4–100 mg L−1) was sorbed on ferrihydrite and hematite within 5 min, while only 84% of added DPAA (100 mg L−1) was sorbed on goethite after 24 h. The sequential extraction results and FTIR measurements reveal that DPAA formed simultaneously inner- and outer-sphere complexes on goethite and hematite, but predominantly inner-sphere complexes on ferrihydrite with limited formation of outer-sphere complexes (<15%). A combination of SEP, FTIR and EXAFS techniques further enables identification of the interfacial reactions between DPAA and solid surfaces of iron (hydr)oxides and the mechanisms involved. Results indicate that DPAA interacted with these iron (hydr)oxides via (1) electrostatic attraction or hydrogen bonding, (2) surface complexation and (3) complexation embedded inside the mineral particles. EXAFS studies further demonstrate that DPAA formed mainly bidentate binuclear corner-sharing (2C) complexes regardless of the iron substrate, with As-Fe distances at 3.19–3.32 Å. Comparison of these results with available data in the literature on inorganic, methyl and phenyl arsenics (As) suggests that it is the phenyl group substitution that finally determines the predominance of 2C complexes. Results from the present study will improve our knowledge of DPAA interaction with solid surfaces and may help in the prediction of the environmental fate and environmental risk management of DPAA in the soil-water system.

Electrodialytic Arsenic Removal from Bulk and Pre-treated Soil

Arsenic (As) is a well-known highly toxic and carcinogenic element. A combination of electrodialytic remediation (EDR) after soil washing with flocculant addition targeting remediation of a soil polluted with As from wood preservation industry is the focus of this paper. The fine fraction (< 0.063 mm) from the washed soil after dry sieving was also considered. The EDR experiments were carried out in a 2-compartment cell applying 0.01 mA/cm2 during 14, 7 and 3 days. The suspended soil slurry was placed in cathode compartment separated by anion exchange membrane (AEM) of the anolyte where the pH was kept at 10. The soil was highly polluted with As, and the EDR was able to remove between 50 and 80% corresponding to 400–478 mg As/kg of soil. The major part of the As was removed within the first 3 days (63%), and approx. 10% more of As was released doubling the time of the experiment: 72% in 7 days and 80% in 14 days. The pre-treated soil showed higher As initial concentration, but did not show a clear advantage in terms of removal rate as the original soil (not washed or sieved) showed 80% of As removal comparing with 61% and 50% for washed and fine fraction, respectively (although the absolute removed concentration was similar). The sequential extraction results confirmed that As was bound into more mobile fractions in original soil, and the higher removal was mainly due to its larger exchangeable and reducible fractions compared to the oxidizable and residual fraction in pre-treated soil.

Fate and migration of arsenic in large-scale anaerobic landfill

The fate and migration of arsenic in a large-scale anaerobic landfill site was investigated by characterization of the total As content and its speciation using a sequential extraction procedure. The total As content in the anaerobic landfill varied greatly with age of the deposited refuse, ranging from 15.26 ± 4.18 mg kg−1 to 38.41 ± 10.41 mg kg−1. There was an increasing trend from the upper layer to a depth of 18–19 m, followed by a decrease in the lower layer. Sequential extraction results showed that As present in exchangeable and weak-acid soluble forms (F1) varied from 0.08 ± 0.01 mg kg−1 to 12.61 ± 0.92 mg kg−1, but from 1.28 ± 0.11 mg kg−1 to 8.40 ± 0.23 mg kg−1 in reducible forms (F2). Oxidizable (F3) and residual (F4) forms of As, which were much more stable and for which the environmental risk correspondingly decreased, accounted for 24.21%–58.70% and 10.11%–30.90% of the total As content, respectively. Nitrate and carbonate had a strong influence on the distribution of As in F1 species; ferric ion affected As distribution in F2; both ferrous ion and dissolved oxygen content contributed to F3 distribution; As in F4 was associated with crystalline minerals structures and was only weakly affected by environmental factors. The deposition process could be divided into six phases to interpret As migration and distribution within the anaerobic landfill after closure.

Effect of sulfur and sulfur-iron modified biochar on cadmium availability and transfer in the soil–rice system

Cadmium (Cd) contamination in paddy soils has aroused global concern. Sulfur modified biochar (BC) could combine the benefits of BC and S for Cd remediation. However, no information is available on the impact of sulfur modified biochar on Cd phytoavailability in paddy soils. In this study, a pot experiment was conducted to investigate the effect of sulfur modified biochar (S-BC) and sulfur and iron (Fe) modified biochar (S-Fe BC) on Cd mobility and Cd transfer in the soil-rice system. The application of S-BC and S-Fe BC effectively reduced pore water Cd in the rhizosphere and non-rhizosphere pore water throughout the rice growth stages. S-BC and S-Fe BC addition increased the total chlorophyll content, as well as the root, shoot and grain biomasses of rice. Furthermore, S-BC and S-Fe BC amendments greatly increase the formation of Fe plaque on rice root surface, thus decreasing Cd accumulation in different rice tissues. In particular, S-Fe BC supplementation significantly reduced the Cd concentration in rice grains to 0.018 mg kg−1 in Cd-contaminated soil, which was lower than the China National standard for food contamination limit (0.2 mg kg−1 Cd). Sequential extraction results showed that S-BC and S-Fe BC can promote the transfer of exchangeable Cd to Fe-Mn oxide, organic and residual bound forms which reduce Cd in paddy soils. Thus, the amendment of S-Fe BC to Cd-contaminated paddy soil is an effective strategy to decrease Cd accumulation in rice grains and thereby protect public health.

Biochar induced Pb and Cu immobilization, phytoavailability attenuation in Chinese cabbage, and improved biochemical properties in naturally co-contaminated soil

Contamination of agricultural soils with potential toxic elements (PTEs) has caused serious health and environmental concern all over the world. Therefore, eco-friendly solutions for Pb and Cu immobilization were required to minimize their mobility through various types of biochar amendments in contaminated soil. A greenhouse pot study was conducted to evaluate the comparative effect of rapeseed residue (RP), and rice straw (RS)-derived biochars, produced at different temperatures on Pb and Cu mobility and their accumulation in a high metal accumulating crop such as Chinese cabbage (Brassica chinensis L.). Sequential extraction, leaching toxicity, and phytoavailability techniques were used to assess Pb and Cu mobility. The impacts of various types of biochars on soil surface electronegative charge and soil enzymatic activity were also investigated in contaminated soil. The sequential extraction results revealed that the application of 5% of high-temperature rice straw biochar (RS550) and rapeseed residue biochar (RP550) significantly reduced the acid-soluble fractions (active fractions) of Pb and Cu by 53.50 and 50.35% respectively. Following the addition of 5% RS550, the residual fraction (stable fractions) of Pb and Cu were increased by 36.14 and 50.94% respectively. A remarkable decrease in Pb and Cu phytoavailability and solubility was observed after the soil was treated with 5% of both RS550 and RP550 biochars. In all biochar treatments, the maximum reduction of Pb concentration in roots and shoots of the cabbage was 37.81 and 26.54%, respectively, while the concentration of Cu decreased by 50.41 and 46.25%, respectively. Incorporation of biochar into contaminated soils also improved soil urease, catalase, and acid phosphatase activities as compared to the control soil. Biochar application greatly enhanced the Pb and Cu immobilization, reduced phytoavailability to Chinese cabbage, and improved soil enzymes activity. The significant increment in soil surface electronegative charges and soil pH after biochar incorporation was considered one of the Pb and Cu immobilization mechanisms. Overall, application of rice straw biochar produced at 550 °C could potentially be a safe, efficient, and sustainable approach to stabilize Pb and Cu and restore soil functions in contaminated soil.

Mobility and Behaviour of Metals in Copper Mine Tailings and Soil at Khetri, India

The mobility and behaviour of metals (Cu, Zn, Ni, Cr, and Pb) in mine tailings and neighbouring soils were studied in the Khetri copper mine region, Rajasthan, India. Single reagent extraction was used to assess the mobility and availability of metals, while sequential chemical fractionation (water soluble, acidic, and reducible) was used to evaluate their behaviour in the environment. Samples were divided into two different physical grain size fractions to assess the impact of grain size on extraction. The mobility trend in both tailings and soil followed the order Ni > Zn > Cu. Metals of lithogenic origin (Cr and Pb) were less extracted in the water soluble fraction or non-residual fractions than those associated with mining. The acid leachable fraction was high in the tailings due to the presence of sulphide minerals. Hence, acidification of tailings would enhance leaching of metals. Results of single and sequential chemical extraction indicate that the Cu, Zn, and Ni are anthropogenic, as they were mostly associated with the non-residual fraction of the soil.

Chromium speciation and its stable isotopic signature in the dolomite–terra rossa weathering system

We present the results of a study of chromium (Cr) content, Cr speciation and its stable isotope composition in a terra rossa weathering profile, which includes ferromanganese concretions (FMCs) and gibbsite spots, developed on dolomite in the Yunnan–Guizhou Plateau in south central China. Sequential extraction results show that the Cr in all samples is present mainly within the residual fraction and oxidizable fraction, and that the Cr content of the exchangeable fraction and reducible fraction is very low. The significantly high Cr content (mean of 21.0 ppm) of the oxidizable fraction is a potential environmental hazard. The Cr in the terra rossa and gibbsite spots exhibits absolute accumulation throughout the profile; correspondingly, the FMCs are significantly enriched in most trace elements, except for Cr. Re-oxidation of Cr(III) to soluble Cr(VI) by MnO2 is likely responsible for the relative depletion of Cr in the FMCs. In the bulk dolomite, the δ53Cr values of the carbonate component and detrital component are 0.447‰ and −0.437‰, respectively. The δ53Cr signal in both fractions is heterogeneous, while both δ53Cr signals are significantly different from that of bulk silicate Earth (BSE; −0.124 ± 0.101‰). Our results provide a proof of concept that the allogenic Cr-isotopic composition of terrigenous minerals in carbonate rocks, can provide new insights into paleo-terrestrial oxidative weathering. Overall, the terra rossa is characterized by the enrichment of heavy Cr isotopes compared to the detrital component of dolomite. Hence, the isotopic signature of Cr does not support the idea that the detrital component in dolomite is the sole parent material of the terra rossa. The absolute accumulation of Cr and heavy Cr isotope enrichment in terra rossa indicate that the extraneous input of dissolved Cr(VI) with a heavy isotopic signature occurred during dolomite weathering and terra rossa formation.

Heavy metals of surface sediments in the Changjiang (Yangtze River) Estuary: Distribution, speciation and environmental risks

Terrigenous sediment in estuarine environments serves as a major sink for heavy metals sourced from land, and thus has been widely used for the investigation of anthropogenic pollution and environmental risk. This study presents the concentrations and chemical speciation of eight heavy metals (Zn, Cu, Ni, Pb, Cr, Hg, As, Cd) in 34 surface sediment samples collected from the Changjiang (Yangtze River) estuarine area, which aims to investigate the potential sources and environmental risks of heavy metals. The concentrations of these metals in inner estuary are overall higher than those in coastal and offshore areas, with the highest values occurring in the South Branch, South Channel, and North Hangzhou Bay. Although the Changjiang river input is the dominant source of these heavy metals, the local contamination and variations in sediment characters especially the grain size and particulate organic matter account for the heterogeneous compositions of these heavy metals in these estuarine sediments. The sequential extraction results suggest that most of these heavy metals, except Cd, are bound in the geochemically inert residual fractions, and primarily determined by the Changjiang input from natural weathering sources. The results of geoaccumulation index (Igeo) and enrichment factor (EF) suggest that these heavy metals do not show obvious enrichments and contamination in this region, while the higher bulk concentrations of Cd in the estuarine and offshore sediments are predominantly inherited from the Changjiang sediments, although the residual Cd in the crystal lattice of minerals reflects the average Cd content in the upper continental crust. The extra Cd adsorbed to the river sediments is strongly bound to exchangeable-, carbonate- and Fe-Mn oxides-associated phases, which may be chemically reactive during estuarine processes and thus has high environmental risks under enhancing human activities.

Removal and immobilization of heavy metals in contaminated soils by chlorination and thermal treatment on an industrial-scale

Heavy metals contaminated soils are a potential exposure hazard to the public. This study demonstrated the application of chlorination and thermal treatment for remediation of heavy metals-contaminated soils on a full-industrial scale. Five soil samples were treated at a soil remediation factory equipped with an internally-heated rotary kiln (length: 17 m; diameter: 4 m; rotating speed: 0.60 rpm). The target soil temperature was set at about 950 °C. During treatment, calcium chloride was added to facilitate chlorination. The chlorination and thermal treatments effectively removed Pb and Cd and led to the slight removal of Cu and Zn but was not effective for Cr and Ni. Chlorination and thermal treatment also significantly reduced the bioaccessibility of heavy metals in soils, particularly Pb and Cd. The results of sequential extraction revealed that thermal treatment generally decreased the carbonate (C2), Fe/Mn oxide (F3) and organic matter (O4) fractions but tended to increase the residual (R5) fractions of heavy metals, indicating that the chlorination and thermal treatment also favored the immobilization of heavy metals. Overall, the results of this study suggest that remediation of heavy metals-contaminated soils by chlorination and thermal treatment is technologically feasible and that there is great potential for practical application of this method.