Mn Oxide Fractions Research Articles

The role of Mn and Fe oxides in risk elements retention in soils under different forest types

Retention of four selected risk elements (SRE) – arsen (As), cadmium (Cd), lead (Pb), zinc (Zn) – by manganese (Mn) and iron (Fe) oxides in different forest soils was investigated. The abundance of oxides and the association between oxides and SRE were observed in 91 soil samples of different forests divided into 5 categories covering the variability of forests in the Czech Republic. The sequential extraction procedure was used for dissolution of Mn and Fe oxide fractions (MnOF and FeOF) separately and together with the associated elements. It was found that Mn associated with MnOF is significantly higher in deciduous forests. These differences were ascribed mainly to the higher pH and lower DOC (dissolved organic carbon) in deciduous forests. The association of SRE with MnOF declines in the order Cd » Pb › Zn › As and the association with FeOF in the order As ≈ Pb » Cd › Zn. Despite their relatively lesser occurrence, MnOFs were found to be very significant for retention of Cd. FeOF is strongly associated with As and Pb. The category of forest has been identified to influence the amount of SRE retained by oxides especially in two cases: (1) the category of forests on Fluvisols differs from the other categories; (2) the retention of Cd by MnOF or FeOF is larger in deciduous forests compared with coniferous forests.

Metal fractionation in topsoils and bed sediments in the Mero River rural basin: Bioavailability and relationship with soil and sediment properties

Comprehensive studies that examine the transfer of metals across soil-river bed sediment systems at the basin scale are scarce. An understanding of the distribution of metals among various fractions of soils and sediments within a basin is fundamental to understand their environmental behavior and, thus, to define watershed management plans. This study aims to determine the distribution of metals (Fe, Mn, Cu, and Zn) in topsoils under different land uses (cultivation, pastures, and forests) and river surface sediments in the upper Mero River basin (NW Spain) and to assess these metals' mobility and bioavailability in both environmental compartments to detect potential threats to crops and the water quality. Sequential extraction is used to fractionate the four metals into six fractions: soluble/exchangeable/specifically adsorbed, metals that are bound to Mn oxides, amorphous compounds, metals that are bound to organic matter, crystalline Al and Fe oxides, and residual metals (metals within the crystal lattices of minerals). The fractionation results for both soils and sediment indicate an abundance of metals in the residual fraction (except for Mn in soils) followed by relatively high percentages of oxide-bound fractions (mostly crystalline Fe oxides for both Fe and Zn, amorphous for Cu, and Mn oxides for Mn). On average, the Mn residual fraction in soils (30.2%) is only slightly exceeded by the Mn oxide fraction (34.7%). The land uses do not affect the sequence of metal-binding phases in soils, except for Mn, which is only slightly altered. The results also suggest that Fe, Mn, Cu and Zn in both soils and sediments are not readily mobile under current environmental condition and hence pose no threat to crops or the water quality. In overall terms, the mobility and bioavailability of the four metals (considering the sum of the three least mobile fractions, namely, organic matter, crystalline Al and Fe oxides, and residual) in the soils decrease in the following order: Mn>Cu>Fe>Zn. The same sequence occurs in the sediments, but the order between Fe and Zn is reversed. The soils and sediments that are studied cannot be considered polluted in terms of these metals because of their low levels and prevalence in the residual fraction, which is typical of contributions from natural sources and indicates the pedogenic/lithologic origin of these metals.

Chemical speciation of cadmium and lead and their bioavailability to cole (Brassica campestris L.) from multi-metals contaminated soil in northwestern China

A pot experiment was conducted to study the relationship between speciation distribution of cadmium (Cd) and lead (Pb) and their availability to cole (Brassica campestris L.) grown on the Cd–Pb polluted soil in northwest of China. The results showed that Cd in the unpolluted soil was mainly bound to carbonate fraction (F2) and Fe–Mn oxide fraction (F3), and Pb was mainly bound to carbonate fraction (F2) and residual fraction (F5). However, marked change of Cd and Pb fractionation was observed with increasing soil Cd and Pb concentrations, where the concentrations of Cd in F1 (exchangeable fraction), F2 and F3 increased significantly (p < 0.001 for F1, F2 and F3), and Pb in F1, F2, F3 and F4 increased significantly (p < 0.001 for F1, F2, F3 and F4).The correlation analysis between the fraction distribution coefficient of Cd and Pb in the soil and Cd and Pb concentration accumulated in cole showed that both Cd and Pb in F1 fraction in the soil made the greatest contribution on the accumulation of Cd and Pb in cole. Higher bio-concentration factors (BCFs) and translocation factors (TFs) for Cd and lower BCFs and TFs for Pb were observed in the cole, respectively. Cd had higher accumulation in the edible parts of the cole, but Pb had lower accumulation in that. Therefore, Cd has higher risk to human health than Pb when people eat the coles grown in Cd–Pb polluted soil in northwestern China.

Leaching and fractionation of heavy metals in mining soils amended with biochar

Contaminated mining soils might pose risk to the environment due to leaching of heavy metals into ground water, especially under acid conditions. Biochar might be an option for remediation of contaminated mining soils. The aim of this study was to evaluate the effect of soil acidification and biochar (BC) application on Cd, Pb and Zn mobility and chemical fractionation in two mining soils (A and B). Fifteen leaching columns per soil were packed, applying the following treatments: soil+3% BC; acidified soil; acidified soil+3% BC and control soil+CaCO3. The control treatment was constituted by the original polluted soil (soil A or soil B). BC was produced by slow pyrolysis at 700°C from sugarcane straw. Ten leaching events were performed and Cd, Pb, Zn and pH were determined for each leaching. A sequential extraction procedure was performed after leaching was finished. The following fractions were evaluated: mobile, exchangeable, bound to Mn oxide, bound to OM, bound to Fe oxide and residual. In the polluted control soil metal mobility was very low. In acidified soils metal mobility increased drastically and BC application reduced the Cd (57–73%), Pb (45–55%) and Zn (46%) concentrations in the leachate. With the sequential extraction it was observed that Cd was mostly bound to the mobile fraction (15–35%) and exchangeable fraction (7–38%), Pb was mainly bound to the Mn oxide fraction (32–70%) and OM (31–43%) and Zn was principally bound to the residual fraction (31–68%). Cadmium presented the highest mobility index (33–53%) and Pb the lowest (1–3%), which is related to the fractions they are bound in the soil. Field studies would be necessary to confirm the effectiveness of BC application on heavy metals’ retention, prior to large scale application.

Chromium(VI) transport and fate in unsaturated zone and aquifer: 3D Sandbox results

The simulation of Cr(VI) behavior in an unsaturated zone and aquifer, using a 3D experimental set-up were performed to illustrate the distribution, transport and transformation of Cr(VI), and further to reveal the potential harm of Cr(VI) after entering the groundwater. The result indicated that chromium(VI) was transported in the vertical direction, meanwhile, was transported in the horizontal direction under the influence of groundwater flow. The direction and distance away from the pollution source zone had great effect on the chromium(VI) concentration. At the sampling sites near the pollution source zone, there was a sudden increase of chromium(VI) concentration. The concentration of chromium(III) concentration in some random effluent samples was not detected. Chromium had not only transported but also had fraction and specie transformation in the unsaturated zone and aquifer. The relative concentration of residue fraction chromium was decreased with time. The content of Fe–Mn oxide fraction chromium was increased with time. The relative content of exchangeable and carbonate-bound fraction chromium was lower and the content variations were not obvious. Chromium(VI) (91–98%) was first reduced to chromium(III) rapidly. The oxidation reaction occurred later and the relative content of chromium(VI) was increased again. The presence of manganese oxides under favorable soil conditions can promote the reoxidation of Cr(III) to Cr(VI).

Distribution and mode of occurrence of uranium in bottom ash derived from high-germanium coals

The radioactivity of uranium in radioactive coal bottom ash (CBA) may be a potential danger to the ambient environment and human health. Concerning the limited research on the distribution and mode of occurrence of uranium in CBA, we herein report our investigations into this topic using a number of techniques including a five-step Tessier sequential extraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magnetic separation, and electron probe microanalysis (EPMA). The Tessier sequential extraction showed that the uranium in the residual and Fe–Mn oxide fractions was dominant (59.1% and 34.9%, respectively). The former was mainly incorporated into aluminosilicates, retained with glass and cristobalite, whereas the latter was especially enriched in the magnetic fraction, of which about 50% was present with magnetite (Fe3O4) and the rest in other iron oxides. In addition, the uranium in the magnetic fraction was 2.6 times that in the non-magnetic fraction. The experimental findings in this work may be important for establishing an effective strategy to reduce radioactivity from CBA for the protection of our local environment.

Fractionation and risk assessment of Fe and Mn in surface sediments from coastal sites of Sonora, Mexico (Gulf of California).

The aim of this study was to evaluate Fe and Mn distribution in geochemical fractions of the surface sediment of four oyster culture sites in the Sonora coast, Mexico. A selective fractionation scheme to obtain five fractions was adapted for the microwave system. Surface sediments were analyzed for carbonates, organic matter contents, and Fe and Mn in geochemical fractions. The bulk concentrations of Fe ranged from 10,506 to 21,918mg/kg (dry weight, dry wt), and the bulk concentrations of Mn ranged from 185.1 to 315.9mg/kg (dry wt) in sediments, which was low and considered as non-polluted in all of the sites. The fractionation study indicated that the major geochemical phases for the metals were the residual, as well as the Fe and Mn oxide fractions. The concentrations of metals in the geochemical fractions had the following order: residual >> Fe and Mn oxides > organic matter > carbonates > interchangeable. Most of the Fe and Mn were linked to the residual fraction. Among non-residual fractions, high percentages of Fe and Mn were linked to Fe and Mn oxides. The enrichment factors (EFs) for the two metals were similar in the four studied coasts, and the levels of Fe and Mn are interpreted as non-enrichment (EF < 1) because the metals concentrations were within the baseline concentrations. According to the environmental risk assessment codes, Fe and Mn posed no risk and low risk, respectively. Although the concentrations of Fe and Mn were linked to the residual fraction, the levels in non-residual fractions may significantly result in the transference of other metals, depending on several physico-chemical and biological factors.

Evaluation of soil washing process with carboxymethyl-β-cyclodextrin and carboxymethyl chitosan for recovery of PAHs/heavy metals/fluorine from metallurgic plant site

Polycyclic aromatic hydrocarbons (PAHs)/heavy metals/fluorine (F) mixed-contaminated sites caused by abandoned metallurgic plants are receiving wide attention. To address the associated environmental problems, this study was initiated to investigate the feasibility of using carboxymethyl-β-cyclodextrin (CMCD) and carboxymethyl chitosan (CMC) solution to enhance ex situ soil washing for extracting mixed contaminants. Further, Tenax extraction method was combined with a first-three-compartment model to evaluate the environmental risk of residual PAHs in washed soil. In addition, the redistribution of heavy metals/F after decontamination was also estimated using a sequential extraction procedure. Three successive washing cycles using 50g/L CMCD and 5g/L CMC solution were effective to remove 94.3% of total PAHs, 93.2% of Pb, 85.8% of Cd, 93.4% of Cr, 83.2% of Ni and 97.3% of F simultaneously. After the 3rd washing, the residual PAHs mainly existed as very slowly desorbing fractions, which were in the form of well-aged, well-sequestered compounds; while the remaining Pb, Cd, Cr, Ni and F mainly existed as Fe–Mn oxide and residual fractions, which were always present in stable mineral forms or bound to non-labile soil fractions. Therefore, this combined cleanup strategy proved to be effective and environmentally friendly.

Effect of acid rain on the fractionation of heavy metals and major elements in contaminated soils

Acid rain is a serious environmental problem worldwide. In the present study, we investigated the effect of acid rain (1:1 equivalent basis H2SO4:HNO3) at pH values of 2.0, 4.0 and 7.0 on the fractionation of heavy metals (Cd, Cu, Fe, Mn, Ni, Pb and Zn) and major elements (K, Na, Ca, and Mg) in contaminated calcareous soils over a 2084 h period. Heavy metals and major elements in soil samples were fractionated before and after 2084 h kinetic release using a sequential extraction procedure. Before kinetic studies the predominant fractions of K, Na, Ca, Mg, Cd and Ni were mainly associated with carbonate fraction (CARB), whereas Fe, Mn and Zn were associated with the Fe–Mn oxide fraction (Fe–Mn oxide). The highest percentage of Pb and Cu were found in the exchangeable (EXC) and organic matter (OM) fractions, respectively. After kinetic study using different simulated acid rain solutions, the major fractions of heavy metals (expect of Cu) and Na was the same as before release. Upon the application of different acid rain solutions, K and Mg were found dominantly in Fe–Mn oxide fraction, whereas Ca was in the EXC fraction. The results provide valuable information regarding metal mobility and indicated that speciation of metals (Cu and Zn) and major elements in contaminated calcareous soils can be affected by acid rain.

Hazardous elements speciation in sandy, alkaline coal mine overburden by using different sequential extraction procedures

This paper deals with the problem of hazardous elements (HE) speciation in sandy, alkaline coal mine overburden destined for restoration. Two sequential extraction procedures [five stages Tesier’s (T) and seven stages Zeien and Brümmner’s method (ZB)] were employed that focused on potentially bioavailable HE fractions previously found in plants growing during this soil restoration process. The results showed that the selection of the extraction procedure in HE speciation is essential, due to the different potential threats of HE fractions obtained for plants. The biggest residual fraction obtained by both extractions suggest geogenic origin of Cu, Zn, Ni, Cr and Pb. Tesier’s extraction procedure works reasonably well for low concentration of water soluble and exchangeable HEs, whilst giving higher values for their organically bound forms. Zeien and Brümmner’s method, however, was found to be more appropriate for evaluation of the mobility of HE as Mn and Fe oxide fractions. The principal component analysis test estimated three main groups where the relationship between soil properties (pH, clay and silt), total content of element and available fractions (Zn, Ni, Cr, Pb-T and Ni-ZB) was defined as a PC 1 group. Other PC 2 and PC 3 from the PCA analysis include Corg and CEC, and available Cu-T and Cr, Pb-ZB, respectively. The PC 3 factor linked fine sand and available Zn-ZB. Pearson’s correlation coefficients showed a high correlation between available HE and high pH(KCl) obtained by T-extraction, and Ni by Zn-extraction. Also, clay, silt and fine sand were highly correlated with the Ni-ZB available forms.

Biomobilization of Heavy Metals from the Sediments Affect the Bacterial Population of Al-Ghadir River (Lebanon)

Although there is no doubt about the importance of the bacterial activity on the solubilisation and the distribution of heavy metals in aquatic sediments, hydromorphic soils and ground waters; little is known about the involvement of bacterial dissolution in periodically anaerobic environments like that found in dredged sediments and little is known about the processes and the environmental factors controlling this process.The aim of this paper was to study the effect of the autochthonous bacterial activity on the biodegradation of organic matter and the mobilization of heavy metals in the sediments of Al-Ghadir river (Mount Lebanon). Sediments were incubated under standard anaerobic conditions and enriched with glucose to stimulate and accelerate microbial metabolism. The evolution of carbon metabolism (Organic matter evolved, carbon consumed and organic acids produced) and metals released in batch reactors were followed over time. Under the adopted conditions, analysis of the chemical parameters indicated that the incubated sediments showed a significant release of organic carbon corresponding to bacterial development. Mineral analysis showed an important solubilisation of Fe2+ and Mn2+ indicating the presence of Fe- and Mn-reducing bacteria in sediments. Pb, Cd and Cr solubilisation profiles were observed and appeared concomitant to the solubilisation profiles of Fe and Mn indicating that the redox cycle has been well installed and that Pb, Cd and Cr were associated to Fe and Mn oxides. The production of Cu appeared in parallel to the mineralization of the organic matter in the sediment indicating that Cu was associated to this fraction. Zn appeared associated to the sulphide fraction than to the Fe and Mn oxides fraction. Microbiological and genetic analysis showed a decrease and the disappearance of some bacterial strains due to the shift in the culture conditions and the toxicity of the released heavy metals but at the same time the development and the growth of many other populations which showed to be tolerant to the same conditions

Reconstruction of the Nd isotope composition of seawater on epicontinental seas: Testing the potential of Fe–Mn oxyhydroxide coatings on foraminifera tests for deep-time investigations

The Fe–Mn oxide fraction leached from deep-sea sediments has been increasingly used to reconstruct the Nd isotope composition of deep water masses, that can be used to track changes in oceanic circulation with a high temporal resolution. Application of this archive to reconstruct the Nd isotope composition of bottom seawater in shallow shelf environments remained however to be tested. Yet as the Nd isotope composition of seawater on continental margins is particularly sensitive to changes in erosional inputs, establishment of neritic seawater Nd isotope evolution around areas of deep water formation would be useful to discriminate the influence of changes in oceanic circulation and in isotopic composition of erosional inputs on the Nd isotope record of deep waters. The purpose of this study is to test the potential of Fe–Mn coatings leached from foraminifera tests to reconstruct the Nd isotope composition of seawater in shelf environments for deep-time intervals. Albian to Turonian samples from two different outcrops have been recovered, from the Paris Basin (Wissant section, northern France) and from the Western Interior Seaway (Hot Spring, South Dakota, USA), that were deposited in epicontinental seas. Rare Earth Element (REE) spectra enriched in middle REEs in the foraminifera leach at Wissant highlight the presence of Fe–Mn oxides. The similarity of the Nd isotopic signal of the Fe–Mn oxide fraction leached from foraminifera tests with that of fish teeth suggests that Fe–Mn oxides coating foraminifera can be good archives of shelf bottom seawater Nd isotopic composition. Inferred bottom shelf water Nd isotope compositions at Wissant range from −8.5 to −9.7 ε-units, about 1.5–2 ε-units higher than that of the contemporaneous local detrital fraction. At Hot Spring, linear REE spectra characterizing foraminifera leach may point to an absence of authigenic marine Fe–Mn oxide formation in this area during the Late Cenomanian–Early Turonian, consistent with dysoxic to anoxic conditions at Hot Spring, contemporaneous to an Oceanic Anoxic Event. The similarity of the Nd isotopic signal of the carbonate matrix of foraminifera with that of fish teeth suggests that it records the Nd isotope composition of bottom shelf seawater as well. Inferred bottom shelf water Nd isotope compositions at Hot Spring are quite radiogenic, between −7 and −6 ε-units, about 2.5–4 ε-units higher than that of the contemporaneous local detrital fraction. In contrast, in both sections Fe–Mn oxides leached directly from the decarbonated sediment tend to yield a less radiogenic Nd isotopic composition, typically between 0.2 and 0.8 ε-units lower, that is intermediate between that of fish teeth and of the detrital fraction. This suggests the contribution of pre-formed continental Fe–Mn oxides to the Nd isotopic signal, along with authigenic marine oxides, or a detrital contamination during leaching.

Effect of simulated acid rain on soil acidification and rare earth elements leaching loss in soils of rare earth mining area in southern Jiangxi Province of China

Acid rain has long been a great concern because of environmental and ecological problems; however, the effect of acid rain on soil acidification, loss of rare earth elements (REEs) via the leaching process, and transformation are rarely reported in rare earth mining areas. Through a simulated acid rain leaching experience, the effect of acid rain was studied on soil acidification and REEs leaching loss. The results showed that the tested soil had certain buffering capacity against nearly neutral rainwater. However, simulated acid rain of low and very low pH (pH ≤ 3.5) had a greater impact on soil acidification. After eluviating by simulated acid rain of pH 3.5 for 36 h, the pH of tailings, garden soil, paddy soil, and alluvial soil decreased by 20.41, 32.03, 13.60, 16.88, and 15.83 %, respectively, from the original values. For simulated acid rain of pH 2.5, it was 31.89, 44.76, 31.26, 29.87, and 29.15 %, respectively. After simulated acid rain eluviations of low and very low pH (pH ≤ 3.5), the order of the leaching rate of REEs in the tested soil was as follows: garden soil > tailings > paddy soil > alluvial soil. For nearly neutral rainwater (pH 4.5 simulated acid rain and pH 5.6 deionized water), the order was tailings > garden soil > paddy soil > alluvial soil. For simulated acid rain of the same pH, the leaching amounts of REEs in tailings and garden soil were higher than those in paddy soil and alluvial soil. After leaching by low and very low pH-simulated acid rain (pH ≤ 3.5), the peak value of the leaching amount of REEs in all tested soil appeared at 2 h, and then gradually reduced and reached a stable leaching state 20 h after leaching. On leaching by simulated acid rain of pH 2.5, the maximum REEs contents of leachate in tailings, garden soil, paddy soil, and alluvial soil were 156.35, 145.82, 99.88, and 85.97 mg/L, respectively. For pH 3.5 of simulated acid rain, it was 130.49, 110.49, 80.57, and 62.73 mg/L, respectively. On leaching by simulated acid rain of pH 4.5, the maximum contents of REEs in the leachate were 53.46 and 29.82 mg/L, respectively, which were observed after leaching for 6 h in tailings and garden soil that became stable 12 h after leaching. The contents of leached REEs in paddy soil and alluvial soil were always in a lower and stable state. After eluviations with deionized water of pH 5.6, the contents of leached REEs in other soils were lower, except for the slight fluctuations in tailings. The maximum content in the leachate of REEs was in the water-soluble and exchangeable fraction. When bound to carbonate fractions, REEs were not detected in the leachate. REEs bound to iron-manganese (Fe–Mn) oxides fraction and to organic matter fraction in the leachate possibly came from the tested soil or from the REEs transformation during the migration process. The content of residual fraction REEs in the leachate was very low.

Chemical forms of heavy metals in soils around municipal waste dumpsites in Asaba Metropolis, Delta State, Nigeria

The chemical forms of copper, lead, manganese and zinc in soils around five selected municipal waste dumpsites in Asaba metropolis, Delta State, Nigeria, were studied, using the Ma and Rao six-step sequential chemical extraction procedure, with a view to providing information on the bioavailability and mobility of the metals in the soil matrix. The results showed that lead and manganese were speciated into residual fractions with averages of 60.32 and 82.24% respectively, while copper and zinc were associated with organic and Fe–Mn oxide fractions respectively with respective averages of 47.91 and 63.21%. The residual fraction also contained relatively high percentages of copper (33.18%) and zinc (20.81%). Relatively very low levels of the metals were found in the water soluble, exchangeable and carbonate fractions. The mobility factors of all the metals in all the soil profiles were generally low ranging from 1.97 to 39.99 indicating generally low availability as well as mobility. Lead and manganese associated with the residual fraction were strongly bound to soil matrix and were not bioavailable since they were occluded within the crystal lattice layer of silicates and well crystallised oxide minerals. Our results show that the metals do not pose any environmental risk or hazard. However, the soils stand the risk of being contaminated by copper and zinc since metals associated with organic and Fe–Mn oxide fractions could be bioavailable and mobilisable when the soil conditions such as pH and redox potentials change.

Thallium concentrations and sources in the surface sediments of Bohai Bay

The Tl concentrations and chemical speciation were determined in the surface sediments of Bohai Bay to evaluate its biogeochemical characteristics. The total Tl concentrations were in the range of 0.506–0.770 μg/g and correlated significantly with clay and total organic carbon (TOC) contents, suggesting that the grain size and TOC were major factors controlling Tl distribution. The sequential extraction performed to indicate Tl speciation and availability in Bohai Bay sediments suggested that Tl occurred mainly in the residual fraction and Tl came mainly from natural input. In the non-residual fractions, Fe–Mn oxide and organic matter fractions accounted for the main portions, suggesting that the labile Tl adsorption was dominated by Fe–Mn oxides and organic matter. In addition, according to our estimate, about 2.7 t/yr and 0.16 t/yr of Tl reached Bohai Bay via rivers and atmosphere, respectively.

Treatment of zinc-rich acid mine water in low residence time bioreactors incorporating waste shells and methanol dosing

Bioreactors utilising bacterially mediated sulphate reduction (BSR) have been widely tested for treating metal-rich waters, but sustained treatment of mobile metals (e.g. Zn) can be difficult to achieve in short residence time systems. Data are presented providing an assessment of alkalinity generating media (shells or limestone) and modes of metal removal in bioreactors receiving a synthetic acidic metal mine discharge (pH 2.7, Zn 15 mg/L, SO 4 2− 200 mg/L, net acidity 103 mg/L as CaCO 3) subject to methanol dosing. In addition to alkalinity generating media (50%, v.v.), the columns comprised an organic matrix of softwood chippings (30%), manure (10%) and anaerobic digested sludge (10%). The column tests showed sustained alkalinity generation, which was significantly better in shell treatments. The first column in each treatment was effective throughout the 422 days in removing >99% of the dissolved Pb and Cu, and effective for four months in removing 99% of the dissolved Zn (residence time: 12–14 h). Methanol was added to the feedstock after Zn breakthrough and prompted almost complete removal of dissolved Zn alongside improved alkalinity generation and sulphate attenuation. While there was geochemical evidence for BSR, sequential extraction of substrates suggests that the bulk (67–80%) of removed Zn was associated with Fe–Mn oxide fractions.

Fractionation and speciation of arsenic in three tea gardens soil profiles and distribution of As in different parts of tea plant ( Camellia sinensis L.)

The distribution pattern and fractionation of arsenic (As) in three soil profiles from tea ( Camellia sinensis L.) gardens located in Karbi–Anglong (KA), Cachar (CA) and Karimganj (KG) districts in the state of Assam, India, were investigated depth-wise (0–10, 10–30, 30–60 and 60–100 cm). DTPA-extractable As was primarily restricted to surface horizons. Arsenic speciation study showed the presence of higher As(V) concentrations in the upper horizon and its gradual decrease with the increase in soil depths, following a decrease of Eh. As fractionation by sequential extraction in all the soil profiles showed that arsenic concentrations in the three most labile fractions (i.e., water-soluble, exchangeable and carbonate-bound fractions) were generally low. Most arsenic in soils was nominally associated with the organic and Fe–Mn oxide fractions, being extractable in oxidizing or reducing conditions. DTPA-extractable As (assumed to represent plant-available As) was found to be strongly correlated to the labile pool of As (i.e. the sum of the first three fractions). The statistical comparison of means (two-sample t-test) showed the presence of significant differences between the concentrations of As(III) and As(V) for different soil locations, depths and fractions. The risk assessment code (RAC) was found to be below the pollution level for all soils. The measurement of arsenic uptake by different parts of tea plants corroborated the hypothesis that roots act as a buffer and hold back contamination from the aerial parts.

Metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment

A free water surface wetland was built in 2002 to treat wastewater from a tool factory containing metals (Cr, Ni, Zn and Fe), nutrients and organic matter. Until 2006, the last reported period, the wetland retained metals and stored them primarily in the bottom sediment and in the biomass of macrophytes secondarily. The aim of this work was to study metal retention and distribution in the sediment of a constructed wetland for industrial wastewater treatment. Total concentrations and fractions (exchangeable, carbonate-bound, Fe–Mn oxides-bound, organic matter-bound and residual) of metals in sediment were analyzed in this treatment wetland, in order to estimate the fate of metals over time. Metal concentrations were significantly higher in the inlet than in the outlet sediment; concentrations in the latter remained without significant differences throughout the testing period. Metal concentrations and redox potential decreased with depth within the sediment. The lowest metal concentrations and pH and the highest redox values were attained in spring, in agreement with the period of maximum macrophyte growth. Ni and Zn were mainly stored associated with the carbonate fraction; Cr was mainly associated with the Fe–Mn oxides fraction, while Fe was mainly associated with the residual fraction, probably as pyrite. The incoming wastewater composition containing high pH, carbonate, calcium and Fe concentrations favored the observed association in the surface sediment. It would be expected that sediment will continue retaining metals in fractions that will not release them into the water while the chemical and environmental conditions remain unchanged.

Cobalt Distribution and Speciation: Effect of Aging, Intermittent Submergence, In Situ Rice Roots

The speciation and distribution of Co in soils is poorly understood. This study was conducted using x-ray absorption spectroscopy (XAS) techniques to examine the influence of soluble cobalt in the +2 oxidation state (Co[II]) aging, submergence-dried cycling, and the presence of in vivo rice roots on the speciation and distribution of added Co(II) in soils. In the aging and submerged-dried cycling studies, Co was found to be associated with Mn oxide fraction (23 to 100% of total Co) and Fe oxide fractions (0 to 77% of total Co) of the soils as either Co(II) species or a mixed Co(II), and Co in the +3 oxidation state (Co[III]) species. The surface speciation of Co in the Mn oxide fraction suggests an innersphere complex was present and the speciation of Co in the Fe oxide fraction was an innersphere surface complex. The in vivo root box experiments showed similar Co speciation in the Mn oxide fraction (13 to 76% of total Co) as the aging and submerged-dried cycling studies. However, the Fe oxide fraction of the soil was unimportant in Co retention. A significant amount (24 to 87% of total Co) of the Co in root box treatments was identified as a Co precipitate. The importance of this finding is that in the presence of rice roots, the Co is redistributed to a Co precipitate. This work confirmed earlier macroscopic work that Mn oxides are important in the sequestration of Co in soils and the influence of roots needs to be taken into account when addressing Co speciation. The information gained from this study will be used to improve models to predict the lability and hence the availability of Co in terrestrial environments.

Copper contamination of soils and vegetables in the vicinity of Jiuhuashan copper mine, China

Copper contamination in soils and vegetables in the vicinity of an abandoned copper mine in China was investigated. The Cu concentrations of 93 soil samples ranged from 30.4 to 3,191 mg kg−1 soil for a mean of 816.8 mg kg−1 soil. Among 15 samples from a 0 to 20-cm soil layer used for the toxicity characteristic leaching procedure (TCLP) test, the highest value of Cu-TCLP was 133.8 mg kg−1 soil and the TCLP values were positively correlated with the total Cu content of the soils. The sequential extraction of soils in the 0–20-, 20–40-, and 40–60-cm soil layers showed that Cu existed mainly in the Fe–Mn oxide fraction, sulfide/organic fraction, and residual fraction. The copper contamination of 21 species of vegetables from in situ sampling was also examined. Cu concentrations in the edible portions of Brassica chinensis and Solanum melongena were higher than the FAO/WHO standard (40 mg kg−1 DW). The health risk of copper for local inhabitants from consuming these vegetables was assessed on the basis of the target hazard quotient. Enriched concentrations of copper were also found in situ in eight cultivars of B. chinensis planted in the fields, with two levels of Cu concentration. The results showed that there is severe copper contamination in this mine area, and the pollutant in soils show a high risk of leaching into the groundwater and diffusing through the food chain.