Study of the antimony species distribution in industrially contaminated soils

Purpose

The purpose of the present study was to investigate the distribution of antimony (Sb) and its species in soil fractions in order to understand better the real risk associated with Sb in the environment.

Materials and methods

Nine surface soil samples contaminated from lead/zinc and iron smelting operations and coal fired power plants were examined using: (1) four-step sequential extraction procedure (BCR); (2) two-step sequential extraction including ethylenediaminetetraacetic acid (EDTA), sodium hydroxide (NaOH) and NH₄F; and (3) single extraction with EDTA and NaOH. Liquid phase extraction was used for redox speciation of Sb. The distribution of Sb between soil fulvic and humic acids was determined after their chemical separation. The concentrations of Sb were measured by electrothermal atomic absorption spectrometry.

Results and discussion

The main part of total Sb (2.5–105 mg?kg^?1) was associated with the residual fraction in all soils. The exchangeable/carbonate-bound concentrations were 0.83–4.7 % of total Sb. Up to 6.8 % was in the reducible and up to 1.4 % was in the oxidizable fraction. EDTA removed 7.2–11.4 % of total content. Sb(V) was the predominant form in acetic acid and EDTA extracts. Single extraction with 0.1 mol?l^?1 NaOH released up to 13.7 % of soil antimony. The main part of Sb was complexed to the higher molecular weight fraction of soil-derived humic substances.

Conclusions

For highly contaminated soils, 4 % solubility in acetic acid could represent risk of contamination of ground water under specific conditions. Also, the relatively high phytoavailable Sb (7–11 %) can represent a significant proportion in highly polluted soils. Pentavalent antimony was the main antimony species extracted from soils. The main part of the organically antimony was found to be present as complexes with higher molecular weight humic acids fraction.     

Evaluation of toxicity risk of polycyclic aromatic hydrocarbons (PAHs) in crops rhizosphere of contaminated field with sequential extraction

Purpose

The assessing bias of rhizosphere effect on polycyclic aromatic hydrocarbons (PAHs) degradation in soils would come out from formation of nonextractable PAHs and extractability difference of various solvents. The aim of this study was to evaluate the role of rhizosphere effect in long-term PAHs polluted soils by using sequential extraction approach.

Material and methods

The scheme of sequential extraction included methanol/water extractable PAHs, butanol extractable PAHs, DCM extractable PAHs, humic acid-bound PAHs, crude humin-bound PAHs, and organic-C enriched humin-bound PAHs. PAHs in plant tissues were extracted by dichloromethane after saponifying. The correlations between PAHs in plant tissues and sequentially extracted fractions were generated by partial least squares regression.

Results and discussion

The profiles of sequentially extracted PAHs varied with plant species. The discrepancy of toxicity equivalency concentrations between rhizosphere and bulk soils was much more significant than that of total PAHs concentrations. In partial least squares regression models, the concentration of PAHs in plant tissues was correlated with fractions strongly associated with soil.

Conclusions

The novelty of this study is the evaluation of concentration and toxicity equivalency concentration of PAHs in rhizosphere of crops sampled in a field polluted with PAHs for long term. This study has highlighted more significant role of rhizosphere in cleanup of cancerogenic toxicity of soil than amount of PAHs in polluted soils.     

Humic substances and aggregate stability in rhizospheric and non-rhizospheric soil

Purpose

The paper describes rhizospheric (Rs) and non-rhizospheric (nRs) soil to demonstrate the zone of the plant root impact on physical and chemical properties of the soil. The effects of the process accompanying the transformations of organic matter into humic substances in the rhizosphere of “common dandelion” Taraxacum officinale have been determined, and the properties of humic acids (HAs) were described. The importance of iron and clay minerals for the formation of a stable and water-resistant soil structure has been emphasized.

Materials and methods

The laboratory analysis involved determination of basic physical and chemical soil properties: texture, pH, cation exchange capacity (CEC), electrical conductivity, and content of total organic carbon (TOC) and dissolved organic carbon (DOC) and quality of humic substances: optical properties of HAs and its separation into hydrophilic (HIL) and hydrophobic (HOB) fractions, speciation of iron, glomalin operationally described as an easily extractable glomalin-related soil protein (EE-GRSP), and soil aggregate stability (SAS) of six size classes of soil aggregates.

Results and discussion

The Rs was reported with a higher TOC and DOC content (measured in the CaCl₂ extracts), however not significantly. The HAs isolated from Rs revealed a significantly higher content of humic substances at its initial decomposition stage, as compared with nRs. A significantly higher concentration of EE-GRSP was noted in the aggregates of the rhizospheric zone (mean 1.14 g kg^?1) than in the aggregates collected from root-free soil (mean 0.94 g kg^?1). There was noted the highest mean share of 1–3 mm soil aggregates in Rs as well as in nRs, respectively 44.4 and 38.3%. The soil material both in Rs and in nRs contained high amounts of exchangeable Ca²⁺, and smectite is the predominant clay mineral. It was favorable for the accumulation of organic carbon and for the formation of good soil physical condition (tilth). Higher but insignificant SAS values were observed for Rs (mean SAS?=?95.6%) than for nRs (mean SAS?=?93.9%).

Conclusions

The studies confirm the role of common dandelion roots in the process of organic carbon accumulation in rhizospheric zone and a favorable effect on the mechanism of the formation of water-resistant aggregates. Higher values of SAS for the Rs were affected by the content of TOC, DOC, exchangeable Ca²⁺ and the concentration of EE-GRSP, and, less considerably, the content of Fe and clay minerals.

     

Polycyclic aromatic hydrocarbons (PAHs) in rural soils of Dongjiang River Basin: occurrence,source apportionment,and potential human health risk

Purpose

A comprehensive study was conducted to investigate the presence of polycyclic aromatic hydrocarbons (PAHs) in Dongjiang River Basin (DRB) soils and to evaluate their sources and ecological and health risk. In addition, factors affecting the distribution and fate of PAHs in the soils such as emission density, soil organic matter, degradation, etc. were studied.

Materials and methods

Surface soil (0–20 cm) samples from 30 sampling sites in the rural areas of DRB were collected and analyzed for 17 polycyclic aromatic hydrocarbons (16 EPA priority PAHs and perylene). Positive matrix factorization model was used to investigate the source apportionment of these PAHs, and an incremental lifetime cancer risk (ILCR) was used to estimate the integrated lifetime risks of exposure to soil-borne PAHs through direct ingestion, dermal contact, and inhalation collectively.

Results and discussion

The total PAH concentrations in the rural soils in DRB range from 23.5 to 231 μg/kg with a mean concentration of 116 μg/kg. The predominant PAHs in the rural soils were naphthalene, fluoranthene, phenanthrene, and benzo(b)fluoranthene. Cluster analysis was performed to classify the soil PAHs into three clusters, which could be indicative of the soil PAHs with different origins and different properties. Source apportionment results showed that coal, biomass, oil, commercial creosotes, and vehicle contributed 24 %, 24 %, 17 %, 17 %, and 18 % of the total soil PAH burden, respectively. The ILCR results indicated that exposure to these soil-borne PAHs through direct ingestion, dermal contact, and inhalation collectively produces some risk.

Conclusions

PAHs in the soils of the DRB will produce long-term influences on rivers and oceans via soil erosion and river transport. Therefore, PAHs in rural soils of DRB have potential impacts on the water supply and human health risk.     

Distribution of PAH residues in humic and mineral fractions of sediments from stormwater infiltration basins

Purpose

The management of sediments from stormwater infiltration basins is nowadays a key issue for local authorities to ensure long-term performance. Speciation of pollutants is particularly required in view of reuse of these materials. If fractionation of trace metals in sediments is relatively well described, polycyclic aromatic hydrocarbons (PAHs) speciation was only studied using particle size distribution. Therefore, this study aims at the characterization of the PAHs-bearing fractions in sediments.

Materials and methods

Three sediments with various physicochemical properties were sampled in the west and north of France to characterize the distribution of PAHs among organic and inorganic components. Respective organic and inorganic matrixes were obtained by alkaline extraction and methyl isobutyl ketone (MIBK) fractionation procedure. The nature of the solid fractions was assessed through microanalyses: infrared spectroscopy (Fourier transform infrared spectroscopy), X-ray diffraction (XRD), and scanning electron microscopy with X-ray spectroscopy. Bulk sediments and extracted fractions were analyzed for organic matter parameters: elemental analysis (C, N, and H), total organic matter, total organic carbon, hydrocarbons (C10–C40), and PAHs.

Results and discussion

The characterization of bulk sediments highlighted that they were mainly composed of single particles, originating from the geological background, and aggregates (10 to 300 μm) composed of minerals and large organic matter content. The C/N ratio and PAH ratios were shown to be helpful for the determination of the natural or anthropogenic origin of organic matter or to evaluate additional contribution of industrial activities. The fractionation results underlined the role of the aggregates on the distribution of PAHs. Humin fraction and bound-humic acids were mainly composed of aggregates (10 to 150 μm) and accounted for 60 to 70 % of sample mass. The PAHs are mainly sequestrated in these fractions. Only up to 1 % of PAHs are adsorbed on the mineral fraction.

Conclusions

Both alkaline extraction and MIBK procedure demonstrated that PAH residues were readily sequestrated in humin and bound-humic acids fractions and that fulvic acids, humic acids, and mineral fractions contained poor concentrations of PAHs. Microanalyses underlined the high level of aggregation of particles in sediments and their mixed inorganic and organic nature. In case of stormwater sediments, the location of PAHs in highly organic aggregates is consistent with their sources, being both oil products and debris from vehicles and road.     

Mutual relations between PAHs derived from atmospheric deposition,enzymatic activity,and humic substances in soils of differently urbanized areas

Purpose

The aim of this study was to determine the mutual relations between polycyclic aromatic hydrocarbons (PAHs) originated from atmospheric emissions and enzymatic activity and humic substances in soils at differently urbanized area, on an example of the Lublin city, east Poland.

Materials and methods

The chosen areas represented three differently urbanized environments: old tenement houses and modern residential blocks, mixture of different building and rural landscape, and typical rural environment with smallholding farms, respectively. On each of the urban, suburban, and rural areas, one representative plot was chosen on fallow lands classified as luvisol derived from loess. The soil samples were collected from the top 25 cm layer. The following properties were determined: pH, organic carbon, total nitrogen, humic and fulvic acids, PAHs content (14 PAHs from US EPA list), and the activities of the following enzymes: dehydrogenases, acid phosphatase, alkane phosphatase, protease, and urease.

Results and discussion

Higher contents of organic C and total N were found in the rural soil samples. The share of humic acid was similar in all soils investigated, ranging from 19.38 to 25.27%, while fulvic acid values differ significantly between urban and rural areas. The urban soils indicated much lower share of fulvic acids (9.78–10.99%) than those of rural (29.02–29.32%). Consequently, the values of the C_HA:C_FA ratio of the urban soil were approximately two times higher than those of the rural soil. The results showed that both the rate of humification and the activity of dehydrogenases, acid phosphatase, alkaline phosphatase, and proteases in the soils increased in the following sequence: urban < suburban < rural.

Conclusions

The results showed that an increase of PAHs in the urbanized areas affect other soil properties. The phenanthrene/anthracene and fluoranthene/pyrene ratios pointed to coal combustion as the principal source of PAHs in the investigated soils. The PAH content in the urbanized area inhibit humification processes in the soil and the activity of dehydrogenases, acid phosphatase, alkaline phosphatase, and proteases.

     

Influence of mixtures of acenaphthylene and benzo[a]anthracene on their degradation by Pleurotus ostreatus in sandy soil

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds commonly found as soil contaminants. Fungal degradation is considered as an environmentally friendly and cost-effective approach to remove PAHs from soil. Acenaphthylene (Ace) and Benzo[a]anthracene (BaA) are two PAHs that can coexist in soils; however, the influence of the presence of each other on their biodegradation has not been studied. The biodegradation of Ace and BaA, alone and in mixtures, by the white rot fungus Pleurotus ostreatus was studied in a sandy soil.

Materials and methods

Experimental microcosms containing soil spiked with different concentrations of Ace and BaA were inoculated with P. ostreatus. Initial (t ₀) and final (after 15 days of incubation) soil concentrations of Ace and BaA were determined after extraction of the PAHs.

Results and discussion

P. ostreatus was able to degrade 57.7% of the Ace in soil spiked at 30 mg kg^?1 dry soil and 65.8% of Ace in soil spiked at 60 mg kg^?1 dry soil. The degradation efficiency of BaA by P. ostreatus was 86.7 and 77.4% in soil spiked with Ace at 30 and 60 mg kg^?1 dry soil, respectively. After 15 days of incubation, there were no significant differences in Ace concentration between soil spiked with Ace and soil spiked with Ace + BaA, irrespective of the initial soil concentration of both PAHs. There were also no differences in BaA concentration between soil spiked with BaA and soil spiked with BaA + Ace.

Conclusions

The results indicate that the fungal degradation of Ace and BaA was not influenced by the presence of each other’s PAH in sandy soil. Bioremediation of soils contaminated with Ace and BaA using P. ostreatus is a promising approach to eliminate these PAHs from the environment.     

Sorption of humic acid on Fe oxides,bacteria, and Fe oxide-bacteria composites

Purpose

Sorption of humic substances on other soil components plays an important role in controlling their function and fate in soil. Sorption of humic substances by individual soil components has been studied extensively. However, few studies reported the sorption characteristic of humic substances on composites of soil components. This study aimed to investigate the sorption characteristics of humic acid on Fe oxide-bacteria composites and improve the understanding on the interaction among humic substance Fe oxide bacteria in soil.

Materials and methods

Humic acid was purchased from Sigma-Aldrich and was purified. Hematite and ferrihydrite were synthesized in the lab. Bacillus subtilis and Pseudomonas putida were cultivated in Luria-Broth medium and harvested at stationary growth phase. Batch sorption experiments were carried out at pH 5.0. Various amounts of humic acid were mixed with 20 mg of Fe oxide, bacteria, or Fe oxide-bacteria composite (oxide to bacteria of 1:1) in 10 mL of KCl (0.02 mol L^?1) to construct sorption isotherms. The effects of phosphate concentration and addition order among humic acid, Fe oxide, bacteria on the sorption of humic acid were also studied. The sorption of humic acid was calculated by the difference between the amount of humic acid added initially and that remained in the supernatant.

Results and discussion

The maximum sorption of humic acid on hematite, ferrihydrite, B. subtilis and P. putida was 73.2, 153.5, 69.1, and 56.7 mg C g^?1, respectively. The maximum sorption of humic acid on examined Fe oxide-bacteria composite was 28.2–57.2 % less than the predicted values, implying that the sorption of humic acid was reduced by the interaction between Fe oxides and bacteria. The presence of phosphate exerted negligible influence on the sorption of humic acid on bacteria while it inhibited the sorption of humic acid on Fe oxides. On Fe oxide-bacteria composites, inhibiting influences followed by promoting or weak inhibiting effects of phosphate with increasing concentration on the sorption of humic acid were found.

Conclusions

The interaction between Fe oxides and bacteria reduced the sorption of humic acid; moreover, the reduction was greater by the interaction of bacteria with ferrihydrite than that with hematite. Phosphate exerted negligible and inhibiting influence on the sorption of humic acid by bacteria and Fe oxides, respectively. On Fe oxide-bacteria composites, humic acid sorption was initially inhibited and then promoted or weakly inhibited by phosphate with increasing concentration.     

Hydrophobicity of electrophoretic fractions of different soil humic acids

Purpose

The fractionation of soil humic acids (HAs) according to their hydrophobicity is a common procedure in the study of this polydispersed complex natural mixture, so that reversed-phase high-performance liquid chromatography (RP-HPLC) is used resulting in humic components of differing hydrophobic/hydrophilic properties. However, a comparative study of the hydrophobicity of fractions isolated from different soil HAs have not been addressed so far.

Materials and methods

The RP-HPLC with online absorbance detection was used for analysis of International Humic Substances Society soil standard HAs, chernozem soil HAs, and their electrophoretic fractions A, B, and C?+?D, obtained by tandem size exclusion chromatography–polyacrylamide gel electrophoresis. The strong relationship between hydrophobicity, electrophoretic mobility (EM), molecular size (MS), specific absorbance at 280 nm and aromaticity of HAs fractions was found.

Results and discussion

Independently of soil HAs genesis fraction A with lowest EM and highest MS is essentially more hydrophobic (60–73 % of the fraction amount remained adsorbed on the RP column) than medium EM and MS fraction B (33–47 % of the fraction amount remained adsorbed on the RP column). The lowest hydrophobicity belongs to fraction C?+?D with highest EM and lowest MS.

Conclusions

The most hydrophilic aromatic fraction C?+?D seems to have been bound with other mostly aliphatic hydrophobic fractions A and B through non covalent (possibly hydrogen) bonds. These data could be relevant to better understanding the overall makeup of soil HAs and their structural organization.     

The effect of concentrations and properties of phenanthrene, pyrene, and benzo(a)pyrene on desorption in contaminated soil aged for 1 year

Purpose

The choice and timing of microorganisms added to soils for bioremediation is affected by the dominant bioavailable contaminants in the soil. However, changes to the concentration of bioavailable PAHs in soil are not clear, especially when several PAHs coexist. This study investigated the effects of PAH concentration and chemical properties on desorption in meadow brown soil after a 1-year aging period, which could reflect changes of PAH bioavailability during bioremediation.

Materials and methods

Based on the percentage of different molecular weights in a field investigation, high-level contaminated soil (HCS) and low-level contaminated soil (LCS) were prepared by adding phenanthrene (PHE), pyrene (PYR) and benzo(a)pyrene (BaP) to uncontaminated meadow brown soil. The concentrations of HCS and LCS were 250 mg?kg^?1 (PHE, PYR, and BaP: 100, 100, and 50 mg?kg^?1) and 50 mg?kg^?1 (PHE, PYR, and BaP: 20, 20, and 10 mg?kg^?1) respectively. The soils were aged for 1 year, after which desorption was induced by means of a XAD-2 adsorption technique over a 96-h period.

Results and discussion

The range of the rapidly desorbing fraction (F _rap) for PHE, PYR, and BaP in HCS and LCS was from 1.9 to 27.8 %. In HCS, desorption of PYR was most difficult, and the rate constant of very slow desorption (K _vs) of PYR was 8 orders of magnitude lower than that of BaP, which had similar very slow desorbing fractions (49.8 and 50.5 %, respectively). However, in LCS, desorption of PYR was the easiest; the K_vs of PYR was 8–10 orders of magnitude higher than those of PHE and BaP. In HCS, the time scale for release of 50 % of the PAHs was ranked as BaP?>?PYR?>?PHE, while in LCS this was BaP?>?PHE?>?PYR.

Conclusions

The combined effect of PAH concentrations and properties should be taken into account during desorption. The desorption of PAH did not always decrease with increasing molecular weight, and the desorption of four-ring PAHs might be special. These results are useful for screening biodegrading microbes and determining when they should be added to soils based on the dominant contaminants present during different periods, thus improving the efficiency of soil bioremediation.     

Emerging resistant microbiota from an acidic soil exposed to toxicity of Cr, Cd and Pb is mainly influenced by the bioavailability of these metals

Purpose

The effect of pollutants in soil microorganisms is an important issue in order to understand their toxic effects in the environment, as well as for developing adequate bioremediation strategies. In this sense, the main objective of this study was to assess the involvement of the indigenous microbiota of an acidic forest Mediterranean soil by artificial pollution with heavy metals, and to detect and isolate resistant microorganisms that could be useful for bioremediation.

Materials and methods

Samples from a previously unpolluted acidic forest soil were amended with Cr(VI), Cd(II) or Pb(II) at total amounts ranging from 0.1 to 5,000 mg?kg^?1. These soil microcosms were incubated under controlled laboratory conditions for 28 days. Soluble fractions of metals were determined from aqueous extracts. Both activity and composition of the microbial community were assessed, respectively, by respirometric assays and molecular analysis (polymerase chain reaction denaturing gradient gel electrophoresis). The isolation of metal-resistant microorganisms was attempted by culture plating from microcosms incubated with high concentrations of metals. Isolated strains were tested in cultures with minimal medium to check for their metal resistance and their capacity to reduce the presence of toxic Cr(VI).

Results and discussion

A decrease in the soil respirometric activity and changes in the microbial community composition were detected from 10/100 mg?kg^?1 Cr and 1,000 mg?kg^?1 Cd and Pb. Presumably resistant bacterial and fungal populations developed in most of these polluted microcosms; however, the microbiota was severely impaired at the highest additions of Cr. Even though Cr was the most damaging metal in soil microcosms, if the soluble fractions of metals are considered instead of their total added amounts, the comparison among their toxic effects suggests a similar potential toxicity of Cr and Pb. Isolated multiresistant microorganisms were related mainly to Actinobacteria, Firmicutes and Ascomycota. Some of them showed the capacity to reduce Cr(VI) concentrations between 54 % and 70 % of the initial value. These strains were affiliated to several species of Streptomyces and Bacillus.

Conclusions

The combination of respirometric assays with molecular methods has been useful to assess the effect of metals on the soil microbial community, which can greatly be explained by their differential bioavailability. Cultivation-dependent and -independent approaches have proved the presence and development of multiresistant microorganisms in a previously unpolluted soil. Due to their properties, some of the isolated strains are potentially useful for soil bioremediation.     

Impact of flue gas desulfurization gypsum and lignite humic acid application on soil organic matter and physical properties of a saline-sodic farmland soil in Eastern China

Purpose

Appropriate land management is important for improving the soil quality and productivity of the saline-sodic farmland. A recent study has revealed that flue gas desulfurization (FGD) gypsum and lignite humic acid application enhanced the salt leaching and crop production. The purpose of this study was to investigate the effects of applied FGD gypsum and lignite humic acid (powder) on the soil organic matter (SOM) content and physical properties.

Materials and methods

This study was based on a field experiment of five consecutive rapeseed-maize rotations in a saline-sodic farmland soil (Aquic Halaquepts) at coastal area of North Jiangsu Province, China. The soil is sandy clay loam texture with pH of 8.43 and clay content of 185 g kg^?1. Six treatments included three FGD gypsum rates (0, 1.6, and 3.2 Mg ha^?1) and two lignite humic acid rates (0 and 1.5 Mg ha^?1). The amendments were incorporated into 0–20 cm soil depth manually every year. Soil samples were collected from each treatment and analyzed for soil organic matter, water-stable aggregates (wet sieving method), bulk density (clod method), water retention capacity (pressure plate apparatus), total porosity (calculated from bulk density and particle density), and microporosity (calculated from water content at 0.01 MPa).

Results and discussion

After 5 years, the SOM and soil physical properties were significantly (P?<?0.05) affected by the application of FGD gypsum and lignite humic acid, especially at the 0–20 cm soil depth. The highest amount of SOM with best soil physical condition was observed in the field which was treated with FGD gypsum at 3.2 Mg ha^?1 with lignite humic acid, and the SOM, total porosity (TP), microporosity (MP), mean weight diameter (MWD), water-stable macroaggregate (WSMA), and available water content (AWC) were increased by 22.8, 6.34, 23.2, 48.1, 55.5, and 15.8 %, respectively, while the bulk density (BD) was decreased by 5.9 % compared to no amendments applied. The generalized linear regression analysis showed that the SOM explained 42.9, 55.0, 48.5, and 54.2 % of the variability for BD, MWD, WSMA, and MP, respectively.

Conclusions

This study illustrates the benefits of applying FGD gypsum and lignite humic acid for increasing the soil organic matter content and improving the soil physical properties and suggests a great potential for ameliorating saline-sodic farmland soil (Aquic Halaquepts) by using combined amendment of FGD gypsum with lignite humic acid.

     

Toxicity of arsenic in relation to soil properties: implications to regulatory purposes

Purpose

The present work evaluates the influence of different soil properties and constituents on As solubility in laboratory-contaminated soils, with the aim of assessing the toxicity of this element from the use of bioassays to evaluate the soil leachate toxicity and thereby propose soil guideline values for studies of environmental risk assessment in soil contamination.

Materials and methods

Seven soils with contrasting properties were artificially contaminated in laboratory with increasing concentrations of As. Samples were incubated for 4 weeks, and afterwards, soil solution (1:1) was obtained after shaking for 24 h. The soil leachate toxicity was assessed with two commonly used bioassays (seed germination test with Lactuca sativa and Microtox ® test with Vibrio fischeri).

Results and discussion

The relationship between soluble As and soil properties indicated that iron oxides and organic matter content were the variables most closely related to the reduction of the As solubility, while pH and CaCO₃ increased As solubility in the soil solutions. Toxicity bioassays showed significant differences between soils depending on their properties, with a reduction of the toxicity in the iron-rich soil (no observed effect concentration (NOEC)?=?150 mg kg^?1) and a significant increase in the highly carbonate samples (NOEC between 15 and 25 mg kg^?1).

Conclusions

Soil guideline values for regulatory purposes usually set a single value for large areas (regions or countries) which can produce over- or underestimation of efforts in soil remediation actions. These values should consider different levels according to the main soil properties controlling arsenic mobility and the soil leachate toxicity.     

Using humic products as amendments to restore Zn and Pb polluted soil: a case study using rapid screening phytotest endpoint

Purpose

Heavy metal contamination is a priority issue affecting millions of hectares of soil throughout the world. One of the most promising, environmentally friendly, and cost-effective approaches to restore polluted soils could be applying organic amendments. We investigated the remediation potential of three types of humic products with regard to their effect on the bioavailability of Pb and Zn, content of nutrients, and the ability to mitigate acute phytotoxicity in contaminated soil.

Materials and methods

Spodosol samples were spiked with Pb (550 mg kg^?1) and Zn (880 mg kg^?1). Then, two different commercial humic products (from peat and lignosulfonate) and natural humic acids (from brown oxidized coal) were added in two doses to reach an equal content of carbon: a 10% increment and a 30% increment of the initial total organic carbon in the soil. After 30 days, the content of metals and nutrients (S, K, Na, Ca, Mn, P) was determined by the sequential extraction (i?H₂O, ii?NH₄COOH pH 4.8, iii–CH₃COOH). The effect of humic products on heavy metals bioavailability was evaluated using the calculated partition indexes. Seed germination and root elongation of Sinapis alba were also determined. Chemical and biochemical variables were aggregated by the principal component analysis.

Results and discussion

Humic products reduced the amount of bioavailable fractions of Pb and Zn in soils. The partition index, which quantitatively describes bioavailable fractions of the Zn and Pb in the soil, was 28–49% lower than in the spiked (Pb+Zn) control. The inhibition of root elongation and seed germination of mustard by Zn and Pb was significantly mitigated by humic products; in the soil test, the root length and seed germination were up to 36–87% higher than those of the Pb+Zn control and did not differ from those in the non-amended treatments. This effect may have been associated with the structural differences (H/C and O/C ratio) and content of nutrients (Na and K) in humic products.

Conclusions

Commercial humic products used in poor multi-contaminated soils can maintain plant growth by improving nutrient status due to heavy metals immobilization and can be a promising approach to remediate the soil contaminated with heavy metals at extremely high concentrations.

     

Changes in selected hydrophobic components during composting of municipal solid wastes

Purpose

One of the most practical ways to utilise municipal solid waste is composting, thereby producing materials that may be productively used to improve soil properties. Wastes, as well as mature composts, contain hydrophobic substances, including fats, which are more resistant to microbiological decomposition than other constituents. The aim of this work was to determine qualitative and quantitative changes of hydrophobic substances, especially fatty acids, during the course of municipal solid waste composting. This provides new information on intensity of hydrophobic versus other substances decomposition undergoing during these processes.

Materials and methods

Raw materials, prepared according to MUT-DANO technology, were composted in a pile, and samples were taken after 1, 14, 28, 42, 56, 90 and 180 days of the composting. Temperature, moisture, total organic carbon, hydrophobic substances carbon (HSC) and fatty acid carbon (FAC) contents were determined in all samples. Hydrophobic substances were extracted with 1:2 (v/v) mixture of ethanol/benzene, while fats were extracted with petroleum ether and determined by GC analysis after transesterification with BF₃ in methanol.

Results and discussion

The HSC decreased from 27.8 to 9.3 g kg^?1 during first 90 days of composting, and thereafter remained constant. Similarly, the highest content of FAC was in raw compost, while the lowest was after 90 days. Octadecenoic acid predominated in the raw compost and decreased from 56 to 23 % FAC after 180 days. During the composting processes, domination of octadecenoic acid was replaced by hexadecanoic and octadecanoic acids, which increased from 18.8 to 36.7 % and 8.3 to 19.4 % FAC, respectively. The share of hexadecanoic, eicosanoic and docosanoic acids increased after the thermophilic phase. The presence of odd-numbered fatty acids (pentadecanoic and heptadecanoic) was noted, which are known to be products of the bacterial transformation-synthesis of lipid substances.

Conclusions

The extent of decomposition of hydrophobic substances, especially fatty acids, is greater than other components in composted municipal solid waste, and intensity of the biotransformation is significantly correlated with composting parameters, mainly temperature and time. During the thermophilic phase of municipal solid waste composting, the decrease in total content of hydrophobic substances is approximately fivefold, while the reduction in fatty acids can be about tenfold. Unsaturated fatty acids are more intensively decomposed during the composting processes, while saturated fatty acids are more resistant. Moreover, transformation of fatty matter may result in the creation of specific isomers with odd numbers of carbon atoms.     

Enhanced bioremediation of PAH-contaminated soil by immobilized bacteria with plant residue and biochar as carriers

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are largely accumulated in soils in China. The immobilized-microorganism technique (IMT) is a potential approach for abating soil contamination with PAHs. However, few studies about the application of IMT to contaminated soil remediation were reported. Due to recalcitrance to decomposition, biochar application to soil may enhance soil carbon sequestration, but few studies on the application of biochars to remediation of contaminated soil were reported. In this study, we illustrated enhanced bioremediation of soil having a long history of PAH contamination by IMT using plant residues and biochars as carriers.

Materials and methods

Two PAH-degrading bacteria, Pseudomonas putida and an unidentified indigenous bacterium, were selected for IMT. The extractability and biodegradation of 15 PAHs in solution and an actual PAH-contaminated soil amended with immobilized-bacteria materials were investigated under different incubation periods. The effects of carriers and the molecular weight of PAHs on bioremediation efficiency were determined to illustrate their different bio-dissipation mechanisms of PAHs in soil.

Results and discussion

The IMT can considerably enhance the removal of PAHs. Carriers impose different effects on PAH bio-dissipation by amended soil with immobilized-bacteria, which can directly degrade the carrier-associated PAHs. The removal of PAHs from soil depended on PAH molecular weight and carrier types. Enhanced bio-dissipation by IMT was much stronger for 4- and 5-ring PAHs than for 3- and 6-ring ones in soil. Only P400 biochar-immobilized bacteria enhanced bio-dissipation of all PAHs in contaminated soil after a 90-day incubation.

Conclusions

Biochar can promote bioremediation of contaminated soil as microbial carriers of IMT. It is vital to select an appropriate biochar as an immobilized carrier to stimulate biodegradation. It is feasible to use adsorption carriers with high sorptive capabilities to concentrate PAHs as well as microorganisms and thereby enhance dissipation of PAHs and mitigate soil pollution.     

Fluorescence spectroscopy of sedimentary pore-water humic substances: a simple tool for retrospective analysis of lake ecosystems

Purpose

The feasibility of applying the method of single-scan fluorescence emission spectra of humic substances (HSs) without chemical pretreatments of sediment pore-water samples was tested to ascertain the past productivity and sources of organic matter of lakes.

Materials and methods

Sediment samples were collected from ten Estonian lakes (located between 57°36′ and 59°25′N and 22°12′ and 26°59′E) covering all levels of the trophic scale. The height (fluorescence intensity), location (fluorescence maximum) and shape (fluorescence index, the ratio of intensities at the emission wavelengths 450 and 500 nm) of the fluorescence emission spectrum at an excitation of 340 nm were under consideration.

Results and discussion

Pore-water humic substances (pwHSs) from sediments of eutrophic lakes had generally a high fluorescence intensity and fluorescence index and their fluorescence maximum was located at shorter wavelengths. Characteristic features of pwHSs from oligotrophic lakes were low fluorescence intensity, emission of maximum fluorescence at shorter wavelengths and high fluorescence index values. Pore-water humic substances from sediments of dystrophic lakes were characterized by a low fluorescence intensity and fluorescence index and their spectral peak was shifted to longer wavelengths. The study also demonstrated that a shift in the peak location of pwHSs fluorescence was accompanied with a change in the C/N ratios of sedimentary organic matter, and the alteration in the fluorescence index of pwHSs was synchronous with the changes in their molecular weight.

Conclusions

The obtained results suggest that fluorescence spectroscopy of pwHSs without using chemical pretreatments has a great potential in the reconstruction of past lake conditions.     

Mobility of mercury in soil as affected by soil physicochemical properties

Purpose

The interaction of mercury with organic matter was studied on three soils with distinct physical-chemical compositions (Fluvisol, Luvisol, and Chernozem) and three vermicomposts based on various bio-waste materials (digestate, kitchen waste with woodchips, and garden bio-waste).

Materials and methods

Laboratory batch experiments, in which organic matter composition was modeled by adding graded doses of vermicompost to individual soils, were carried out. The composition of organic matter in these vermicomposts was assessed via fractionation of humic acids, fulvic acids, hydrophilic compounds, and possible hydrophobic neutral organic matter. Furthermore, the samples were artificially contaminated with inorganic and organic mercury. Prepared samples were stored under constant temperature of 25 °C. The incubation experiments lasted for 56 days, in which the samples were taken ten times. During the experiments, the changes in mercury mobile phase amount were observed, and the influence of the source of contamination was evaluated.

Results and discussion

The amount of mobile mercury increased and then decreased during the time. In most of the soils and vermicompost combinations, the content of mercury bound to the soil was stable after 21 days. The effects of the mercury source on the exchangeable portion of Hg in the soils were most obvious in samples without added vermicompost. Nevertheless, differences between mobile inorganic and organic forms of Hg were lower in the case of Fluvisol compared to other soils. Moreover, in this soil, the content of available mercury was higher than from others.

Conclusions

In general, the smallest differences between mobile inorganic and organic forms of Hg were observed in the case of soil with the highest content of organic matter. Also higher doses of vermicomposts decreased the amount of mercury mobile phase available. Additionally, the largest positive influence of vermicompost dose on Hg mobility was measured in soils combined with vermicompost with the highest portion of humic acids.

     

Humic and fulvic acids as indicators of soil and water pollution

Humic substances are the major organic components of soil and sediments, but little is known on how they are affected by environmental and industrial pollution. To find out whether such effects could be recognized, a number of analytical characteristics were compared of humic and fulvic acids extracted from unpolluted and polluted soils and sediments. The main differences were that, per unit weight, polluted humic and fulvic acids contained more N, S and H but fewer CO₂H groups, and were more aliphatic than unpolluted samples. Unusually high N and S contents of humic materials appear to be the most valid indicators of pollution. Humic acids are preferred to fulvic acids as indicators of pollution because the former are more readily separated and purified.     

Heavy metal contamination of urban topsoils in a typical region of Loess Plateau,China

Purpose

Heavy metals pollution of city soil has become a serious environmental issue. Attention has been given to the issue of soil contamination in big cities, but little research has been done in the Loess Plateau, which is the largest loess deposition area in the world. The aim of this study was to assess the contamination of topsoil.

Materials and methods

Forty soil samples were collected from different districts and sieved through nylon sieves. The coarse particles (2 mm) were used to determine pH and electrical conductivity using a suspension of 1:5 soil to deionized water. The fine particles (150 μm) were used to determine soil organic matter and selected heavy metals. Metals were measured in digested solutions by a flame atomic absorption spectrophotometer.

Results and discussion

The mean concentrations of heavy metals in urban soils in the study area are significantly lower than the mean concentrations across China. The integrated pollution index was determined to be 1.13, indicating moderate pollution. Weathering of parent material, the use of pesticide and fertilizer, discharge of waste from traffic, wastes from commodities and industry, and coal combustion are considered to be the main sources of heavy metal pollution in the study area.

Conclusions

The results indicate that, at least in the study area, land use greatly influences the soil quality and heavy metal contents in urban topsoils. Soil backfill may change heavy metal contents to some extent. Deep digging and backfill can be effectively used for the remediation of heavy metal contaminated soil and sediments.