Acid-extractable Pb Research Articles

Neighborhood-scale lead (Pb) speciation in Akron, Ohio (USA) soils: primary sources, post-deposition diagenesis, and high concentrations of labile Pb

Lead (Pb) poses a significant risk to infants and children through exposure to contaminated soil and dust. However, there is a lack of information on Pb speciation and distribution at the neighborhood-scale. This work aimed to determine: (1) the distribution of acid-extractable (labile) Pb and other metals ([M]AE) in two neighborhoods in Akron, Ohio (USA) (Summit Lake and West Akron; n = 82 samples); and (2) Pb speciation and potential sources. Total metal concentration ([M]T) and [M]AE was strongly correlated for Pb and Zn (R2 of 0.66 and 0.55, respectively), corresponding to 35% and 33% acid-extractability. Lead and Zn exhibited a strong positive correlation with each other (R2 = 0.56 for MT and 0.68 for MAE). Three types of Pb-bearing phases were observed by electron microscopy: (1) galena (PbS)-like (5–10 μm); (2) paint chip residuals (10–20 μm); and (3) Pb-bearing Fe-oxides (20 μm). Isotope ratio values for PbAE were 1.159 to 1.245 for 206Pb/207Pb, and 1.999 to 2.098 for 208Pb/206Pb, and there was a statistically significant difference between the two neighborhoods (p = 0.010 for 206Pb/207Pb and p = 0.009 for 208Pb/206Pb). Paint and petrol are the dominant sources of Pb, with some from coal and fly ash. Lead speciation and distribution is variable and reflects a complex relationship between the input of primary sources and post-deposition transformations. This work highlights the importance of community science collaborations to expand the reach of soil sampling and establish areas most at risk based on neighborhood-dependent Pb speciation and distribution for targeted remediation.

Mn-modified bamboo biochar improves soil quality and immobilizes heavy metals in contaminated soils

Soil contamination by heavy metals is becoming increasingly serious and needs to be solved urgently. Modified biochar is an effective strategy to solve this problem, and previous studies have only focused on the remediation of single heavy metal contaminated soils through biochar. However, the effects of metal-modified bamboo biochar on soil improvement and the passivation of heavy metals are still unknown. In this study, the Mn-modified bamboo biochar was prepared and the effects of Mn-modified bamboo biochar on soil improvement and the passivation of heavy metals were investigated. The results showed that Mn-modified bamboo biochar can improve the physical and chemical properties of soil, and immobilize heavy metals (i.e., Pb, As, Cd, and Cu). After adding Mn-modified bamboo biochar, soil pH decreased by 0.08–0.70, organic matter content increased by 8.00–45.13%, and alkaline hydrolysis nitrogen content increased by 1.84–5.78%. The diethylene triamine penta-acetic acid (DTPA)-extractable Pb, As, Cd, and Cu decreased by 21.25–58.65%, 7.76–34.02%, 36.44–46.50%, and 3.13–35.21%, respectively, while the acid-extractable Pb, As, Cd, and Cu decreased by 27.81–43.31%, 23.91–26.01%, 17.56–50.60%, and 7.94–18.09% respectively. These results indicated that Mn-modified bamboo biochar can improve the physicochemical properties of the soil and effectively inactivate multi-heavy metals in contaminated soil, Mn-modified bamboo biochar is a potential alternative agent for the management of multi-heavy metal contaminated soils.

Effect of Controlling Heavy Metals in Soil of Rare Earth Mining Area by Biochar Supported Graphene Oxide

Using navel orange peels and natural graphite as raw materials, biochar-supported graphene oxide (BGO) material was prepared using an improved hummer and co-pyrolysis method. The effects of BGO on the forms of heavy metals in the soil of a rare earth mining area were investigated via a soil passivation experiment. The soil column leaching experiment was carried out to explore the change characteristics of heavy metal content in leaching filtrate and the vertical migration law of heavy metals in soil, and the accumulation and release model of heavy metals under leaching conditions was determined. The results showed that pH value and organic matter content of soil with BGO composite increased, and acid-extractable Pb of raw ore and tailings soil decreased by 17% and 8.6%, respectively. The content of Mn form in the raw ore soil did not change significantly, whereas the content of acid-extractable, reducible, and oxidizable state in tailings soil decreased. The accumulation and release characteristics of heavy metals in soil could be divided into two stages:rapid release stage and slow stage. The release rate of heavy metals in soil with BGO composite was lower than that without addition, and the Pb and Mn removed from the tailings soil decreased by 2.5% and 28.4%, respectively, compared with that of the control group, whereas the raw ore soil decreased by 5.7% and 1.1%, respectively. The release of heavy metals in soil is a complex reaction process controlled by a variety of diffusion mechanisms. BGO composites can effectively inhibit the migration of heavy metals by increasing soil pH, surface complexation, and precipitation.

Three types of passivators on the stabilization of exogenous lead-contaminated soil with different particle sizes

Study on the form partitioning and content of heavy metals in soil particles with different sizes is crucial for preventing and controlling heavy metals pollution, but few studies regard soil contaminated by heavy metals as a homogeneous body. In this study (Fig. 1), goat manure, lime and phosphate were used to stabilize exogenous lead (Pb). These soil passivators’ differential effects on total Pb and Pb with different chemical forms in soil particles of different sizes as well as Pb immobilization in soil were investigated. By passivation experiment in laboratory for 45 days, the passivation effect of the single and combined application treatments on exogenous Pb and partitioning characteristics were analyzed and compared. The characterization method of fine sand microstructure and mineral composition analysis was used. The results showed that the single application of P5 and combined application of LP5 had optimum passivation efficiency. The content of DTPA-Pb was reduced with P5 by 65.27% and the percentage of available Pb decreased significantly in soil particles of the four sizes. The content of TCLP-Pb and available Pb (weak acid extraction and reducible Pb) significantly decreased by 71.60 and 25.12% respectively after the application of LP5 in the original soil. Furthermore, most of the total Pb was enriched in coarse sand and clay, while its content was lower in fine sand and silt. The combined application treatment of GL5 significantly increased the content of weak acid extractable and reducible Pb in fine sand, silty sand and clay. Through SEM and XRD analysis, it was found that the diffraction peak of P5 treatment groups might be related to the formation of insoluble Pb that contained compounds, which were mainly mineral components, including quartz, feldspar and mica, and LP showed a big potential in the study on passivation of heavy metal Pb-contaminated soil in the natural environment. In conclusion, further studies on the different dosage and metal-contamination levels as well as different combination forms of passivators should be considered under natural conditions, the selection of suitable passivators according to soil texture is of great significance for remediation of Pb-contaminated soil.

Effects of Soil Amendments on Soil Pb Bioavailability and Pb Absorption by a low-Pb Accumulator Kumquat Grown in Two Types of Pb-Contaminated Soils.

A pot experiment was conducted to investigate the effects of 0.5% and 1% alkaline, clay mineral and phosphorus amendments, as well as 2% and 5% organic amendments, on lead (Pb) soil bioavailability and Pb absorption by the low-Pb accumulator kumquat (Citrus japonica Thunb.) 'Cuipi' in two typical Pb-contaminated soils, Jiyuan and Yangshuo, from northern and southern China, respectively. The diethylenetriaminepentaacetic acid-extractable Pb soil concentration and Pb accumulation in kumquat significantly decreased with amendment additions. High amendment doses had greater inhibitory effects than low doses but no significant effects on the kumquat's biomass in the two typical soils. Alkaline, but not clay mineral, amendments greatly increased the soil pH level. Organic amendments effectively reduced Pb accumulation owing to their strong adsorptive capacities. Thus, using organic amendments combined with a low-Pb accumulator kumquat forms a suitable farming practice for producing safe fruit in the two common types of Pb-contaminated soils in China.

Immobilization of Cd and Pb in a contaminated acidic soil amended with hydroxyapatite, bentonite, and biochar

This study aims to investigate the dynamic changes over time in the immobilization efficiency of hydroxyapatite, bentonite, and biochar for Cd and Pb co-contaminated soil given the potential remobilization of immobilized heavy metals for long-term stabilized soils. Hydroxyapatite, bentonite, and biochar were applied only once at 2.5% (w/w) of soil during a 5-year pot experiment with pepper-cabbage (Capsicum annuum L.-Brassica pekinensis) rotation. The dynamic changes of soil pH, soil organic carbon (SOC), TCLP (toxicity characteristic leaching procedure)-, DTPA-, and PBET (simulated human gastric solution extraction)-Cd and Pb in soil, fractionation of Cd and Pb in soil, as well as the dynamic variations of bioaccessible Cd and Pb in vegetables were determined. The results indicated the application of hydroxyapatite, bentonite, and biochar significantly increased the proportion of residual fraction of Cd and Pb, and reduced the mobility, bioavailability, and bioaccessibility of Cd and Pb analyzed by TCLP, DTPA, and PBET, respectively. Moreover, the concentrations of TCLP-, DTPA-, and PBET-Cd and Pb gradually increased over time. The proportions of acid-extractable Cd and Pb in amended treatments in 2019 were higher than those in 2016. The increment for acid-extractable Cd fraction was 4.69%, 5.53%, and 4.19% in hydroxyapatite, bentonite, and biochar, respectively. While the increment for acid-extractable Pb fraction was 1.48%, 2.08%, and 1.91%, respectively. The bioaccessible Cd and Pb in pepper and cabbage significantly decreased after remediation while no remarkable changes were observed for Pb from 2017 to 2019. The three amendments were effective ameliorants for Cd and Pb co-contaminated acidic soil, but the immobilization efficiency decreased over time. Moreover, hydroxyapatite showed the highest efficiency on metal stabilization and vegetable safety.

Applying modified biochar with nZVI/nFe3O4 to immobilize Pb in contaminated soil.

Lead (Pb) pollution in soil has become one of the most serious environmental problems, and it is more urgent in areas where acid rain is prevalent. Curing agents to solidify heavy metals in soil are efficiently applied to remediate Pb-contaminated soil. In this study, we prepared biochar, biochar loaded with nano-zero-valent iron (BC-nZVI), and biochar loaded with nano-ferroferric oxide (BC-nFe3O4), and investigated the Pb-immobilizing efficiency in contaminated soil in the condition of acid rain by them. The results showed that 8g/kg is the best added dosage of curing agents for immobilizing Pb, which of the immobilizing efficiency of Pb were 19% (biochar), 42% (BC-nZVI), and 23% (BC-nFe3O4), respectively. Besides, the curing agents had positive effects on immobilizing Pb under acid rain condition, which could significantly reduce the content of acid extractable Pb, especially BC-nZVI (1.5%). And the immobilization efficiency of modified biochar was better than biochar, especially BC-nZVI (66%). BC-nZVI showed a more ideal effect on decreasing the leaching amount of Pb in the condition of acid rain. The results highlighted that biochar-loaded nano-iron-based materials, especially BC-nZVI, was promising and environmentally friendly materials for remediating Pb-contaminated soils, which provided scientific reference and theoretical basis for the treatment of Pb-contaminated soils around industrial sites particularly in acid rain area.

The Effects of Inoculation with Three Glomus Species on Growth and Pb Uptake by Hemp (Cannabis Sativa) in a Pb-Contaminated Soil

Increasing levels of hazardous heavy metals, which enter soil and water ecosystems have had a detrimental effect on global living organisms and needs special attention. The effect of inoculation with three Glomus mycorrhizal fungi (G. mosseae, G. etunicatum and G. constrictum) on hemp growth and some biological properties was evaluated in a soil contaminated with different levels of lead (0, 600, 1200 and 1800mg·kg-1 Pb soil) in the form of Pb(NO3)2. The results show that shoot and root dry weights decreased with increasing Pb levels in soil. But Pb concentration in roots and shoots of hemp increased significantly with increasing Pb levels in soil. However the growth of mycorrhizal inoculated hemps was significantly higher than that of non-inoculated hemp. Mycorrhizal inoculation increased the concentration of Pb in the shoot and root of hemp. In this study, both translocation factor (TF) and enrichment factor (EF) were < 1 and decreased with increasing Pb concentration in soil. The TF was higher in non-inoculated hemp than inoculated hemps. It shows the inhibition effect of mycorrhiza in translocation of Pb from root to shoot. The EF of all inoculated hemps (0.18) was significantly greater than non-inoculated hemp (0.17), showing increased uptake of Pb due to inoculation with mycorrhiza. Simple linear regressions show that the diethylenetriaminepentaacetic acid (DTPA) extractable Pb is more suitable index for evaluating the toxicity effect of soil Pb than the total Pb content.

Effects of soil chemical properties and fractions of Pb, Cd, and Zn on bacterial and fungal communities.

Heavy metal contamination in soils poses a serious threat to microorganisms, which play important roles in soil biogeochemical process. However, the key fractions of heavy metals affecting soil microorganisms are still unclear. In this study, DNA sequencing, redundancy and variance partition analysis were performed to investigate the combined effects of heavy metal fractions and soil chemical properties on microbial communities in Pb, Cd, and Zn co-contaminated soils. The results showed that long-term exposure of microorganisms to these metals changed the richness, diversity, and structure of their communities. The bacterial and fungal Chao richness indexes decreased, but only the bacterial Shannon index improved with increasing metal concentrations. Moreover, soil available potassium and acid-extractable Pb made the greatest contributions to variations in the bacterial community structure, while soil pH, water-extractable Pb and Zn were the dominant factors influencing the fungal community structure. In addition, Marmoricola, Nocardioides, and Gibberella were sensitive to these metals. Overall, the effects of different heavy metal fractions on microorganisms varied significantly, and these metal fractions together with soil chemical properties determined the soil microbial communities.

Study on Pb release by several new lixiviants in weathered crust elution-deposited rare earth ore leaching process: Behavior and mechanism.

New leaching agents could lead to a reduction in ammonia nitrogen pollution and the supplementation of soil nutrients during in-situ mining. They could also result in the release of even more toxic heavy metals, which has an impact on the environment as well as human health. In this study, column leaching experiments were used to simulate in-situ leaching, and the leaching behavior and fractional changes of lead in weathered crust elution-deposited rare earth ore by different leaching agents were studied. The experimental results showed that the amount of lead that was leached followed the order of CLA (60% CaCl2+25% NH4Cl+15% MgSO4)>(NH4)2SO4>MgSO4. The lead leaching process was comprised of both an acceleration and deceleration stage that followed the first order kinetic model. The amount of Pb when using compound agent was the greatest most likely because of the presence of Cl-. The soil heavy metal morphology test showed that the three leaching agents primarily leached acid extractable lead, and the compound leaching agent leached the greatest amount of acid extractable Pb, which mainly due to the presence of NH4Cl. The reducible fraction was enriched in the direction of migration of the leachate, which was due to the presence of SO42-. These results indicate that the introduction of leaching agents during the mining process pose a greater risk for the release of heavy metals and provide a theoretical basis for the prevention and remediation of heavy metal pollution in mining areas where new leaching agents were used.

Isotope tracers for lead and strontium sources in the Tieguanyin tea garden soils and tea leaves.

The concentrations of Pb & Sr in Tieguanyin tea leaves and soils from 15 tea gardens of Anxi, China, were determined and the sources of Pb & Sr in soil and leaf samples were analysed using isotope tracing technology. The results showed pH in soils had significant correlations to both acid-extractable Pb & Sr in soils and new leaves. The Pb concentration in leaves was significantly lower than that in soils, especially the acid-extractable Pb in soils. The low Bio-concentration Factor (BCF) indicated the bioavailable Pb in soils could not easily be transferred to leaves. The contribution rates of parent material were 61%-100% and 45%-100% for total Pb isotope and acid-extractable Pb isotope in soils, respectively, indicating a low impact of human activity. A sizeable influence of parent material for leaves was also observed, suggesting that Pb may be present in the dust-fall. Although Sr concentrations in leaves were not high, they exceeded that in soils. The high BCF also indicated that tea has a high capacity to accumulate Sr, with the coincidence that Sr87/Sr86 in the acid-extractable isotope in soils were similar to new leaves. A Pb-Sr joint tracer indicated that Sr in old and new leaves may be influenced by parent material and anthropogenic sources, respectively.

Contributions of artifactual materials to the toxicity of anthropogenic soils and street dusts in a highly urbanized terrain

A study was undertaken to test the hypothesis that the presence of fly ash and other artifactual materials (AMs) significantly increases the toxicity of urban soil and street dust. AMs were distinguished as artifacts (artificial particles > 2 mm in size), and particulate artifacts (≤2 mm in size); street dust was the <63 μm fraction of street sediments. Reference artifacts, street dusts, and topsoils representing different land use types in Detroit, Michigan were analyzed for miscellaneous radionuclides, trace elements, magnetic susceptibility (MS), and acetic acid-extractable (leachable) Pb. Background levels were established using native glacial sediments. Street sediments were found to have a roadside provenance, hence street dusts inherited their contamination primarily from local soils. All soils and dusts had radionuclide concentrations similar to background levels, and radiological hazard indices within the safe range. Artifacts, fly ash-impacted soils and street dusts contained elevated concentrations of toxic trace elements, which varied with land use type, but none produced a significant amount of leachable Pb. It is inferred that toxic elements in AMs are not bioavailable because they are occluded within highly insoluble materials. Hence, these results do not support our hypothesis. Rather, AMs contribute to artificially-elevated total concentrations leading to an overestimation of toxicity. MS increased with increasing total concentration, hence proximal sensing can be used to map contamination level, but the weak correlation between total and leachable Pb suggests that such maps do not necessarily indicate the associated biohazard. Home site soils with total Pb concentrations >500 mg kg−1 were sporadically toxic. Thus, these results argue against street dust as the local cause of seasonally elevated blood-Pb levels in children. Lead-bearing home site soil tracked directly indoors to form house dust is an alternative exposure pathway.

Changes in Metal Availability and Improvements in Microbial Properties After Phytoextraction of a Cd, Zn and Pb Contaminated Soil.

Assessing the effects of phytoextraction on soil properties is important for successful implementation of this method. This study was conducted to evaluate the effects of phytoextraction by Sedum alfredii Hance on the availability of metals and improvement of the microbial community (biomass and structure) of a Cd, Zn and Pb contaminated soil. Phytoextraction significantly decreased the acid extractable, Mn/Fe oxide and organic matter bound fractions of Cd and Zn as well as the acid extractable Pb in the rhizosphere soil. Soil microbial biomass, total, bacterial, actinomycete, fungal, AM fungal, and protozoa phospholipid fatty acids (PLFAs) were significantly enhanced. The ratio of fungal to bacterial and gram-positive to gram-negative bacterial PLFAs were significantly changed. Redundancy analysis showed that microbial biomass and specific groups of PLFAs were negatively correlated with available metals while positively correlated with dissolved organic carbon/organic acids. In conclusion, phytoextraction by S. alfredii reduced available metal concentrations and improved soil microbial properties.

EFFECT OF ROCK PHOSPHATE AND SUPERPHOSPHATE APPLICATION ON MOBILITY OF ELEMENTS (Cd, Zn, Pb, As, Fe, Mn) IN CONTAMINATED SOILS

Weakly acidic Litavka and alkaline Mal n soils are some of the most contaminated soils by multiple risk elements in the Czech Republic. The aim of this paper was to determine the effect of P sources (rock phosphate, superphosphate), each applied at three rates on the mobility of risk elements (Cd, Zn, Pb, As) and micronutrients (Fe, Mn) in soils and to compare their effectiveness with lime and dolomite tested in our previous study. In 7, 14, 28, and 42 days, we determined CaCl2-extractable and acid-extractable concentrations of elements by ICP-OES. In alkaline soil, there was an increase in Cd, Zn, As, and Mn CaCl2-and acid-extractable concentrations after superphosphate application but no effect on concentrations of these elements after rock phosphate application. In acidic soil, there was a decrease in CaCl2-extractable concentrations of Cd and Zn and no effect on CaCl2-extractable Pb, As, Fe, and Mn after rock phosphate application. With the exception of a decrease in Pb and Mn, the same trends were recorded for acid-extractable concentrations. Superphosphate was less effective than rock phosphate for immobilisation of CaCl2-extractable Zn, As, and Mn. Phosphate additives were ineffective for immobilisation of all tested elements in alkaline soils. In acidic soils, phosphate additives were ineffective in immobilising As and Fe compared to lime application and were substantially less effective in Cd and Zn immobilisation than lime. Superphosphate application was a suitable measure to decrease acid-extractable Pb concentrations only in acidic soils at higher rates and was comparable with lime application at lower rates.

Identification of Pb sources using Pb isotopic compositions in the core sediments from Western Xiamen Bay, China

Pb concentrations and Pb isotopic compositions (total and acid-extractable) in the core sediments collected from Western Xiamen Bay, China, were determined to investigate the Pb pollution history and trace the Pb sources. Pb concentration showed an increasing trend (from 40.3 to 64.2mgkg−1) with obvious fluctuation from 1967 to 2013, reflected by the core sediments. The enrichment factors of Pb were between 2 and 5, indicating a moderate enrichment. The relative contributions of Pb-Zn deposit and parent material to total Pb in the core sediments were 51%–62% and 38%–49%, respectively. Acid-extractable Pb isotopic signature was more sensitive than total Pb isotopic signature in identifying anthropogenic Pb sources due to the substantial variability of acid-extractable Pb isotopic ratios exhibited in the core sediments. Based on the acid-extractable Pb isotopic ratios, Pb-Zn deposit and industrial emission were regarded as the main anthropogenic Pb contributors.

Effects of Amendments and Water Conditions on the Chemical Speciation of Cd and Pb in Contaminated Paddy Soil in a Mining Area

ABSTRACTLime, sepiolite and peat were used as amendments to study their effects on the chemical speciation of Cd and Pb in a polluted paddy soil under moist and water-logged conditions. The results showed that application of amendments reduced acid-extractable Cd (up to 12.56% and 5.07%) and Pb (up to 6.69% and 2.68%) in moist and water-logged soils, respectively. The reducible Cd and residual Cd was increased while the oxidizable Cd was decreased after adding amendments. However, the oxidizable Pb was increased while the residual Pb was decreased in amended soil, and the reducible Pb presented a different changing trend under moist and water-logged conditions. The soil pH was significantly correlated with different fractions of Cd and Pb in soil, which indicated a distinct influence of soil pH on the redistribution process of Cd and Pb fractions in amended soil.

Tracing Sources of Heavy Metals in the Soil Profiles of Drylands by Multivariate Statistical Analysis and Lead Isotope

Two profile soils in drylands of Xiamen City were collected and 26 kinds of heavy metals were determined by ICP-MS. The distribution characteristics, environmental risk and pollution sources of heavy metals in soil profiles were studied. The results of Nemero comprehensive pollution index method, the geological accumulation index method and potential ecological hazard index method indicated that Cd was the main pollution factor in the drylands of Xiamen City; the soil pollution was serious in 50-60 cm depth. Multivariate statistical analysis showed that the main sources of heavy metals were the compound pollution of agricultural production activities and natural resources, and the main pollution factor Cd was from agricultural production. Composition of acid-extractable Pb isotopic ratios in soil profiles and potential pollution sources were analyzed. The results showed that Pb in soil profiles was from sludge and parent material. The contribution rates of the two sources in the soil profiles by ²⁰⁶Pb/²⁰⁷Pb-²⁰⁶Pb/²⁰⁴Pb were calculated by using the two mixed models. In the two profiles, parent material contribution rates were 54.36% and 42.88%, and sludge contribution rates were 45.64% and 57.12%, respectively. The main sources of lead of drylands in Xiamen were parent material and sludge, which was consistent with the results of the multivariate statistical analysis.

Bioavailability of zinc and lead from a contaminated soil amended with pine bark-goat manure compost

ABSTRACTThe distribution in soil and plant uptake of zinc (Zn) and lead (Pb) as influenced by pine bark-goat manure (PBG) compost additions were investigated from the soils artificially contaminated with Zn or Pb ions using maize (Zea mays L.) as a test crop. Soils were amended with four rates of pine bark-goat manure compost (0, 50, 100, and 200 tons ha−1) and four rates (0, 300, 600 and 1200 mg kg−1) of Zn or Pb. Maize was planted and grown for 42 days. At harvest, plants samples were analyzed for Zn and Pb concentration. Soils samples were analyzed for pH, extractable and diethylene triamine pentaacetic acid (DTPA) extractable Zn and Pb. Extractable Zn and Pb was lower in PBG compost amended soils than in unamended soils and steadily declined with increasing amount of compost applied. The extractable fraction for Zn dropped by 62.2, 65.0 and 44.6% for 300, 600 and 1200 mg Zn kg−1, respectively when 200 t ha−1 of PBG compost was applied. Metal uptake by maize plants were directly related to the rate of applied heavy metal ions with greater concentrations of metals ions found where metal ions were added to non-amended soils.

Geochemical characteristics and microbial community composition in toxic metal-rich sediments contaminated with Au–Ag mine tailings

The effects of extreme geochemical conditions on microbial community composition were investigated for two distinct sets of sediment samples collected near weathered mine tailings. One set (SCH) showed extraordinary geochemical characteristics: As (6.7–11.5%), Pb (1.5–2.1%), Zn (0.1–0.2%), and pH (3.1–3.5). The other set (SCL) had As (0.3–1.2%), Pb (0.02–0.22%), and Zn (0.01–0.02%) at pH 2.5–3.1. The bacterial communities in SCL were clearly different from those in SCH, suggesting that extreme geochemical conditions affected microbial community distribution even on a small spatial scale. The clones identified in SCL were closely related to acidophilic bacteria in the taxa Acidobacterium (18%), Acidomicrobineae (14%), and Leptospirillum (10%). Most clones in SCH were closely related to Methylobacterium (79%) and Ralstonia (19%), both well-known metal-resistant bacteria. Although total As was extremely high, over 95% was in the form of scorodite (FeAsO4·2H2O). Acid-extractable As was only ∼118 and ∼14mgkg−1 in SCH and SCL, respectively, below the level known to be toxic to bacteria. Meanwhile, acid-extractable Pb and Zn in SCH were above toxic concentrations. Because As was present in an oxidized, stable form, release of Pb and/or Zn (or a combination of toxic metals in the sediment) from the sediment likely accounts for the differences in microbial community structure. The results also suggest that care should be taken when investigating mine tailings, because large differences in chemical/biological properties can occur over small spatial scales.

Metal immobilization and phosphorus leaching after stabilization of pyrite ash contaminated soil by phosphate amendments

In this study we would like to show the importance of a holistic approach to evaluation of chemical stabilization using phosphate amendments. An extensive evaluation of metal stabilization in contaminated soil and an evaluation of the leaching of phosphorus induced after treatment were performed. The soil was highly contaminated with Cu (2894 mg kg(-1)), Zn (3884 mg kg(-1)), As (247 mg kg(-1)), Cd (12.6 mg kg(-1)) and Pb (3154 mg kg(-1)). To immobilize the metals, mixtures of soil with phosphate (from H(3)PO(4) and hydroxyapatite (HA) with varying ratios) were prepared with a constant Pb : P molar ratio of 1: 10. The acetic acid extractable concentration of Pb in the mixture with the highest amount of added phosphoric acid (n(H(3)PO(4)) : n(HA) = 3 : 1) was reduced to 1.9% (0.62 mg L(-1)) of the extractable Pb concentration in the untreated soil, but the content of water extractable phosphorus in the samples increased from 0.04 mg L(-1) in the untreated soil sample up to 14.3 mg L(-1) in the same n(H(3)PO(4)) : n(HA) = 3 : 1 mixture. The high increase in arsenic mobility was also observed after phosphate addition. The PBET test showed phosphate induced reduction in Pb bioavailability. In attempting to stabilize Pb in the soil with the minimum treatment-induced leaching of phosphorus, it was found that a mixture of soil with phosphate addition in the molar ratio of H(3)PO(4) : HA of 0.75 : 1 showed the most promising results, with an acetic acid extractable Pb concentration of 1.35 mg L(-1) and a water extractable phosphorus concentration of 1.76 mg L(-1). The time-dependent leaching characteristics of metals and phosphorus for this mixture were evaluated by a column experiment, where irrigation of the soil mixture with the average annual amount of precipitation in Slovenia (1000 mm) was simulated. The phosphorus concentration in the leachates decreased from 2.60 mg L(-1) at the beginning of irrigation to 1.00 mg L(-1) at the end.