In situ fixation of metals in soils using bauxite residue: chemical assessment

Contamination of soils with heavy metals and metalloids is a widespread problem all over the world. Low cost, non-invasive, in situ technologies are required for remediation processes. We investigated the efficiency of a bauxite residue (red mud) to fix heavy metals in two soils, one contaminated by industrial activities (French soil), and one by sewage sludge applications (UK soil). This Fe-oxide rich material was compared with lime, or beringite, a modified aluminosilicate that has been used for in situ fixation processes. Four different crop species were successively grown in pots. Metal concentrations in the soil pore waters were analyzed during the growing cycles. At the end of the experiment fluxes of heavy metals were measured using a diffusive gradient in thin film technique (DGT). Furthermore, a sequential extraction procedure (SEP) and an acidification test were performed to investigate the mechanisms of metal fixation by different soil amendments. In both soils, the concentrations of metals in the soil pore water and metal fluxes were greatly decreased by the amendments. An application of 2% red mud performed as well as beringite applied at 5%. Increasing soil pH was a common mechanism of action for all the amendments. However, the red mud amendment shifted metals from the exchangeable to the Fe-oxide fraction, and decreased acid extractability of metals. The results suggest that specific chemisorption, and possibly metal diffusion into oxide particles could also be the mechanisms responsible for the fixation of metals by red mud.     

In situ stabilization of trace metals in a copper-contaminated soil using P-spiked Linz-Donawitz slag

Purpose  

A former wood exploitation revealing high Cu and As concentration of the soils served as a case study for assisted phytoextraction.     

In situ remediation of soils contaminated with toxic metal ions using microwave energy

Following onto our work on the in situ remediation of soils contaminated with PAH's, PCB's and other polychlorinated organic compounds using microwave energy, we now report a preliminary investigation on the in situ remediation of soils contaminated with toxic metal ions: Cd(II), Mn(II), Th(IV), Cr(III) and mainly Cr(VI). The soil is partially vitrified in the process, and extraction with hot (70 degrees C) 35% nitric acid for 4.5 h leads to the recovery of very small amounts of the metals which had been spiked into the clean soil: Cd, Mn, and Cr(III) are completely immobilized (unextractable), Th is mostly unextractable, and Cr(VI) partially extractable at very high levels of spiking, but almost completely unextractable using the US EPA Toxicity Characteristic Leaching Procedure. This suggests that contaminated soils which are not going to be used for agricultural purposes can be remediated safely to preset depths without fear of the toxic metal ions leaching out for a long time.     

In situ transformations of fine lead oxide particles in different soils

We present a field application of a new in situ technique to analyze phase transformations of fine lead oxide particles (50-100mum) in different soils directly in the field over 18 months. After the first month of exposure to a calcareous sand we found newly precipitated secondary mineral phases on the lead oxide. The samples exposed to two loamy soils (Dystric Cambisol and Luvisol) showed only very few traces of new phases. We identified the new phases as mainly lead-hydroxy carbonates (hydrocerussite). Whereas the results confirm the transformation of lead phases to lead carbonates in calcareous soil, they also show that an easily soluble phase such as PbO may persist in soils virtually unweathered for more than one year. The formation of a weathering crust is therefore not necessarily needed for preservation of easily soluble phases. The use of experimental in situ methods is thus giving new information on contaminant mineral behavior under field conditions.     

Fractionation and mobility of metals in bauxite red mud

Red mud (RM) is a strongly alkaline residue generated in enormous amounts worldwide from bauxite refining using the Bayer chemical process. RM is composed mainly of Fe, Ti and Al oxides and hydroxides, but it also contains an array of trace metals and metalloids at different concentrations. The purpose of this paper is to assess the potential mobility of metals in RM, with special emphasis on pH effect. The ‘operational’ distribution and leachability of metals within/from RM was studied by applying a sequential extraction procedure (SEP) and several leaching tests (rapid titration, equilibration acidification, batch leaching with acetic acid and also the toxicity characteristics leaching procedure (TCLP) and the DIN 38414-S4 procedures, used as reference methods) carried out at different pH, solid/liquid ratio, extraction period and type of acid (HCl or acetic acid). Chemical analysis showed that, in addition to the major metals Fe, Al and Ti, RM contains several trace metals, some of them (Cr, Cu and Ni) in concentrations exceeding the regulatory limits. SEP showed that a majority of the metals in the RM (between the 32.2?±?8.5 for Cd and 95.3?±?0.4 % for Ni) were found in the residual fraction, suggesting that they are not readily mobile under normal environmental conditions. Leaching tests performed at different pH showed that a significant fraction of the metals is mobilised from RM only under very strong acid conditions (pH?<?2), whereas Al is released in considerable amounts at pH?<?5.3. Among the trace metals, Cr requires special attention because of its relative high concentration in RM and the higher concentrations of this metal mobilised at low pH. The leaching tests using acetic acid showed that the standard TCLP largely underestimates the release of trace metals from RM, and therefore it is not advisable to evaluate the actual potential leaching of trace metals from this residue.     

In situ immobilization of Cu(II) in soils using a new class of iron phosphate nanoparticles

This study tested the feasibility of using a new class of iron phosphate (vivianite) nanoparticles synthesized using sodium carboxymethyl cellulose (NaCMC) as a stabilizer for in situ immobilization of Cu(II) in soils. Transmission electron microscopy measurements demonstrated that the particle size was about 8.4+/-2.9 nm. Batch tests showed that nano-sized vivianite particles can effectively reduce the leachability and in vitro bioaccessibility of Cu(II) in three representative soils (calcareous, neutral, and acidic) at the low doses of 0.61 and 3.01 mg PO(4) g(-1) soil. The Cu leachability was evaluated by the toxicity characteristic leaching procedure and in vitro bioaccessibility was evaluated by the physiological based extraction test. In the case of soil amendment with nanoparticles in 3.01 mg PO(4) g(-1) soil, Cu leachability reduced 63-87% and Cu concentrations in TCLP extract decreased from 1.74-13.33 mg l(-1) to 0.23-2.55 mg l(-1) after those soils were amended for 56 d. Meanwhile, the bioaccessibility of Cu was reduced by 54-69%. Sequential extraction procedures showed the significant decrease of water soluble/exchangeable Cu(II) and carbonate bound fractions and concomitant increase of Cu residual fraction after the soils were amended with the nanoparticles, suggesting that the formation of copper phosphate minerals through precipitation and adsorption was probably responsible for the decrease of Cu availability in soils. Visual MINTEQ modeling further revealed that Cu(3)(PO(4))(2) and Cu(5)(PO(4))(3)OH were formed in the vivianite-solid Cu(II) system, resulting in the decreased solubility of the Cu(II) in the acidic pH range.     

In situ anion diffusion experiments using radiotracers

Diffusion experiments in compacted bentonite have been carried out in situ using the borehole laboratory CHEMLAB. The "ordinary" anion iodide and the redox-sensitive pertechnetate ion have been investigated. In spite of strongly reducing groundwater conditions, technetium was found to diffuse mostly unreduced as TcO4-, although in some spots in the compacted clay, the activity was significantly higher, which may be explained by reduction of some TcO4- by iron-containing minerals in the bentonite. The measured concentration profiles in the clay cannot be accommodated by assuming one single diffusion process. The experimental data are modeled assuming two diffusion paths, intralamellar diffusion and diffusion in external water. The apparent diffusivity for the intralamellar diffusion was found to be 8.6 x 10(-11) m2 s(-1) for iodide with a capacity factor of 0.1, while the apparent diffusivity for the diffusion in external water was found to be 5 x 10(-14) m2 s(-1) with alpha=2.26. The corresponding values for Tc were found to be Da= 6 x 10(-11) m2 s(-1), alpha=0.1 and Da= 1 x 10(-13) m2 s(-1), alpha=0.46, respectively. The diffusion constants and capacity factors obtained in this study are in accordance with data from laboratory experiments.     

Bioavailability and cellular effects of metals on Lumbricus terrestris inhabiting volcanic soils

Whether the radial thickness (RT) of the chloragogenous tissue and intestinal epithelium of earthworms (Lumbricus terrestris) reflects the bioavailability of metals in soils was investigated in two areas, one with active volcanism (Furnas) and another with no volcanic activity since 3 million years ago (Santa Maria), in the Azores. Metal contents in soil samples and earthworms from the two areas were analyzed. Autometallography and measurements of the RT were performed in the chloragogenous tissue and intestinal epithelium. Earthworms from the active volcanic area demonstrated lower RT of chloragogenous tissue and intestinal epithelium as well as higher levels of bioavailable metals, especially Zn and Cd. Comparison of bioavailable metal contents between both areas suggests a higher risk for uptake of potentially toxic metals in the active volcanic area than in the non-active volcanic area, which is reflected by the lower RT of the chloragogenous tissue and intestinal epithelium in the former.     

Heavy metals in urban soils of East St. Louis, IL, Part I: Total concentration of heavy metals in soils

The city of East St. Louis, IL, has a history of abundant industrial activities including smelters of ferrous and non-ferrous metals, a coal-fired power plant, companies that produce organic and inorganic chemicals, and petroleum refineries. A protocol for soil analysis was developed to produce sufficient information on the extent of heavy metal contamination in East St. Louis soils. Soil cores representing every borough of East St. Louis were analyzed for heavy metals--As, Cd, Cu, Cr, Hg, Ni, Pb, Sb, Sn, and Zn. The topsoil contained heavy metal concentrations as high as 12.5 ppm Cd, 14,400 ppm Cu, ppm quantities of Hg, 1860 ppm Pb, 40 ppm Sb, 1130 ppm Sn, and 10,360 ppm Zn. Concentrations of Sb, Cu, and Cd were well correlated with Zn concentrations, suggesting a similar primary industrial source. In a sandy loam soil from a vacated rail depot near the bank of the Mississippi River, the metals were evenly distributed down to a 38-cm depth. The clay soils within a half-mile downwind of the Zn smelter and Cu products company contained elevated Cd (81 ppm), Cu (340 ppm), Pb (700 ppm), and Zn (6000 ppm) and displayed a systematic drop in concentration of these metals with depth. This study demonstrates the often high concentration of heavy metals heterogeneously distributed in the soil and provides baseline data for continuing studies of heavy metal soil leachability.     

Influence of organic waste and residue mud additions on chemical,physical and microbial properties of bauxite residue sand

Background, aim and scope  

In an alumina refinery, bauxite ore is treated with sodium hydroxide at high temperatures and pressures and for every tonne of alumina produced, about 2 tonnes of alkaline, saline bauxite processing waste is also produced. At Alcoa, a dry stacking system of disposal is used, and it is the sand fraction of the processing waste that is rehabilitated. There is little information available regarding the most appropriate amendments to add to the processing sand to aid in revegetation. The purpose of this study was to investigate how the addition of organic wastes (biosolids and poultry manure), in the presence or absence of added residue mud, would affect the properties of the residue sand and its suitability for revegetation.     

Long-term residue of DDT compounds in forest soils in Maine

Soils in forests sprayed aerially with DDT in 1958-1967 have been sampled for persistence of residues at intervals since then and most recently in 1993. Results of all the samples are presented and show persistence through 30 years, with evidence of decline of residue only in the third decade. The metabolites of DDT--DDE and DDD--increased over time, each comprising about a third of total residue in 1993. Residue continues to be held in the organic mat with little evidence of movement downward to the inorganic soil horizons.     

Salinity increases mobility of heavy metals in soils

The effect of salinity induced by CaCl₂, MgCl₂, NaCl and Na₂SO₄ on the mobility of Cu, Cd, Pb and Zn was studied. An increase of ionic strength by any salts promoted a higher release of Cd than the others metals. When CaCl₂ and NaCl were applied, Cd and Pb showed the highest degree of mobilization. When MgCl₂ was applied, Cd and Cu were mobilized the most. Finally, an increase of Na₂SO₄ also promoted the strongest mobilization of Cd and Cu.As the total heavy metal content was higher, the percentage of Pb and Cu released upon salinization decreased, indicating that these metals are strongly bound to soil constituents. An increase of carbonates in the soil promoted a higher release of Pb for all used salts and for Zn when MgCl₂ and NaCl were used. This indicates that Pb and Zn are adsorbed on the surface of carbonate crystals. An increase of fine particles promoted a decrease of percentage of released Cd for all salts, indicating that Cd is strongly retained in the fine fractions.The main mechanism regulating Pb and Cd mobility was competition with Ca²⁺ for sorption sites followed for metal chloro-complexation, association between the Cd/Pb-sulfates and competition with Mg²⁺. The main mechanism regulating Cu mobility was the formation of Cu-sulfate, followed by competition with cations (Mg > Ca) and chloride. For Zn, competition with Ca²⁺ for sorption sites was the most important process for its mobility; followed by Zn-sulfate association and, finally, chloride and competition with Mg with the same effect.     

Cation and anion leaching and growth of Acacia saligna in bauxite residue sand amended with residue mud, poultry manure and phosphogypsum

Purpose  

To examine (1) the effect of organic (poultry manure) and inorganic (residue mud and phosphogypsum) amendments on nutrient leaching losses from residue sand and (2) whether amendments improve the growth of plants in residue sand.     

Heavy metals of the Tibetan top soils

Objective

Due to its high elevation, rare human activities and proximity to south Asia where industries are highly developed, it is required to investigate the fragile environment of the Tibetan Plateau. We are aiming to obtain the concentration level, source, spatial distribution, temporal variation and potential environmental risk of Tibetan soils.

Methods

A total of 128 surf ace soil samples were collected and analyzed f or V, Cr, Mn, Co, Ni, Cu, Zn, As, Cd and Pb, and an additional 111 samples were analyzed f or Hg and total organic carbon. Concentration comparisons coupled with multivariate statistics were used to analysis the sources of elements of soils. We also carried out Risk assessment on the soils.

Results

Concentrations of Hg, Cr, Ni, Cd and Pb are slightly higher than those of the late 1970s. Concentrations of Cr and Ni are higher than averaged world background values. Tibetan soils present a high natural As concentration level.

Discussion

Anthropogenic sources may partly contribute to the elevated Hg, Cd and Pb concentrations. Cr and Ni are mainly originated from soil parent materials. Soil elements in Anduo and Qamdo regions may threaten the health of local people.

Conclusion

Heavy metal elements of Tibetan Plateau are mainly from the natural source. Arsenic present a high background level. Soil elements in Anduo and Qamdo regions may threaten the health of local people, which should be of concern to scientists and the government.     

Removal of heavy metals and arsenic from contaminated soils using bioremediation and chelant extraction techniques

A combined chemical and biological treatment scheme was evaluated in this study aiming at obtaining the simultaneous removal of metalloid arsenic and cationic heavy metals from contaminated soils. The treatment involved the use of the iron reducing microorganism Desulfuromonas palmitatis, whose activity was combined with the chelating strength of EDTA. Taking into consideration that soil iron oxides are the main scavengers of As, treatment with iron reducing microorganisms aimed at inducing the reductive dissolution of soil oxides and thus obtaining the release of the retained As. The main objective of using EDTA was the removal of metal contaminants, such as Pb and Zn, through the formation of soluble metal chelates. Experimental results however indicated that EDTA was also indispensable for the biological reduction of Fe(III) oxides. The bacterial activity was found to have a pronounced positive effect on the removal of arsenic, which increased from the value of 35% obtained during the pure chemical treatment up to 90% in the presence of D. palmitatis. In the case of Pb, the major part, i.e. approximately 85%, was removed from soil with purely chemical mechanisms, whereas the biological activity slightly improved the extraction, increasing the final removal up to 90%. Co-treatment had negative effect only for Zn, whose removal was reduced from 80% under abiotic condition to approximately 50% in the presence of bacteria.     

The effect of heavy metals on dinitrogen fixation by Rhizobium-white clover in a range of long-term sewage sludge amended and metal-contaminated soils

An investigation was conducted to determine whether effective strains of Rhizobium leguminosarum biovar. trifolii capable of symbiotic N2 fixation with white clover (Trifolium repens) were present in a range of metal-contaminated soils. A number of historically sewage-amended sites (including experimental, pasture grassland and arable sites) were selected and compared with highly contaminated samples from abandoned heavy metal mines. Many sites had metal concentrations above the limits established by the UK Government, based on those developed by the European Commission (EC) for sludge-amended soils. Acetylene reduction activity (ARA) was used to screen the samples for effective N2 fixation. When the host plant was indigenous to the sward, rhizobia were found in the nodules and in the soil rhizosphere at all the sites tested. They were shown to be capable of effective symbiosis and N2 fixation, even though metal concentrations greatly exceeded the soil metal limits in some cases. However, nodulation failed to occur in some cases where T. repens was not indigenous to metal-contaminated soils. This indicated either that an ineffective rhizobial population was present, or that effective cells were absent from the soil. The influence of individual metals on ARA could not be determined conclusively because of the confounding effects of soil physicochemical variability and the presence of different metals at high concentrations together in the soil. However, Cd concentrations appeared to be particularly important in determining the presence of effective ARA in soils with no indigenous clover. In contrast to previous studies, the results presented here suggest that heavy metals may have had a quantitative effect on the free-living population of rhizobia, rather than a genetic effect.     

Transient behavior of heavy metals in soils during electrokinetic remediation

This paper presents a systematic bench-scale laboratory study performed to assess the transient behavior of chromium, nickel, and cadmium in different soils during electrokinetic remediation. A series of laboratory electrokinetic experiments was conducted using two different clayey soils, kaolin and glacial till. For each type of soil, four electrokinetic experiments with 1, 2, 4, and 10 d of treatment time were performed. In all tests, the contaminants were Cr(VI), Ni(II), and Cd(II) combined in the soil. A geochemical assessment was performed using the geochemical model MINEQL(+) to determine the partitioning of the heavy metals in soils as precipitated, adsorbed, and aqueous forms. Results showed that in kaolin, the extent of Ni(II) and Cd(II) migration towards the cathode increased as the treatment time increased. Unlike kaolin, in glacial till treatment time had no effect on nickel and cadmium migration because of its high buffering capacity. In both kaolin and glacial till, the extent of Cr(VI) migration towards the anode increased as the treatment time increased. However, Cr(VI) migration was higher in glacial till as compared to kaolin because of the high pH conditions that existed in glacial till. In all tests, some Cr(VI) was reduced to Cr(III), and the Cr(VI) reduction rate to Cr(III) as well as the Cr(III) migration were significantly affected by the treatment time. Overall, this study showed that the electroosmotic flow as well as the direction and extent of contaminant migration and removal depend on the polarity of the contaminant, the type of soil, and the treatment duration.     

Removal of harmful cyanobacterial blooms in Taihu Lake using local soils. III. Factors affecting the removal efficiency and an in situ field experiment using chitosan-modified local soils

Effects of ionic strength, pH, organic content, cell concentration, and growth phase on the removal of MA cells using chitosan-modified sepiolite were studied in the laboratory. The MA removal efficiency increased with the increase of salinity for normal clay flocculation. In contrast, for chitosan-modified clays/soils, MA removal efficiency increased with the decrease of salinity. The removal efficiency of chitosan-modified sepiolite was not significantly affected by pH (6-9), but dropped dramatically beyond pH 10. Humic acid had a small negative effect on the removal of MA cells. Cells were removed more effectively by clays around the early senescence growth phase than other growth stages. The removal efficiency increased as the cell concentration increased. In a field enclosure of Taihu Lake, a loading of 0.025 g/L chitosan-modified local soils removed 99% algal cells and no increase of chlorophyll-a was observed during the following one month's monitoring process.     

In situ diffusion experiment in granite: phase I

A program of in situ experiments, supported by laboratory studies, was initiated to study diffusion in sparsely fractured rock (SFR), with a goal of developing an understanding of diffusion processes within intact crystalline rock. Phase I of the in situ diffusion experiment was started in 1996, with the purpose of developing a methodology for estimating diffusion parameter values. Four in situ diffusion experiments, using a conservative iodide tracer, were performed in highly stressed SFR at a depth of 450 m in the Underground Research Laboratory (URL). The experiments, performed over a 2 year period, yielded rock permeability estimates of 2 x 10(-21) m(2) and effective diffusion coefficients varying from 2.1 x 10(-14) to 1.9 x 10(-13) m(2)/s, which were estimated using the MOTIF code. The in situ diffusion profiles reveal a characteristic "dog leg" pattern, with iodide concentrations decreasing rapidly within a centimeter of the open borehole wall. It is hypothesized that this is an artifact of local stress redistribution and creation of a zone of increased constrictivity close to the borehole wall. A comparison of estimated in situ and laboratory diffusivities and permeabilities provides evidence that the physical properties of rock samples removed from high-stress regimes change. As a result of the lessons learnt during Phase I, a Phase II in situ program has been initiated to improve our general understanding of diffusion in SFR.     

In situ phytoremediation of arsenic- and metal-polluted pyrite waste with field crops: effects of soil management

Sunflower, alfalfa, fodder radish and Italian ryegrass were cultivated in severely As-Cd-Co-Cu-Pb-Zn-contaminated pyrite waste discharged in the past and capped with 0.15 m of unpolluted soil at Torviscosa (Italy). Plant growth and trace element uptake were compared under ploughing and subsoiling tillages (0.3 m depth), the former yielding higher contamination (∼30%) in top soil.Tillage choice was not critical for phytoextraction, but subsoiling enhanced above-ground productivity, whereas ploughing increased trace element concentrations in plants. Fodder radish and sunflower had the greatest aerial biomass, and fodder radish the best trace element uptake, perhaps due to its lower root sensitivity to pollution. Above-ground removals were generally poor (maximum of 33 mg m⁻² of various trace elements), with Zn (62%) and Cu (18%) as main harvested contaminants. The most significant finding was of fine roots proliferation in shallow layers that represented a huge sink for trace element phytostabilisation.It is concluded that phytoextraction is generally far from being an efficient management option in pyrite waste. Sustainable remediation requires significant improvements of the vegetation cover to stabilise the site mechanically and chemically, and provide precise quantification of root turnover.