Amendment application in a multi-contaminated mine soil: effects on soil enzymatic activities and ecotoxicological characteristics

Synergistic effect of pyrene and heavy metals (Zn, Pb, and Cd) on phytoremediation potential of Medicago sativa L. (alfalfa) in multi-contaminated soil

The environment in India is contaminated with polycyclic aromatic hydrocarbons (PAHs) due to the occurrence of large anthropogenic activities, i.e., fuel combustion, mineral roasting, and biomass burning. Hence, 13 toxic PAHs were detected: phenanthrene, anthracene, fluoranthene, pyrene, and benz(a) anthracene, ben-zo; (b) fluoranthene, benzo(k) fluoranthene, benzo(a) pyrene, benzo(ghi)perylene, dibenz (ah) anthracene, indeno1,2,3-(cd) pyrene, coronene and coronene in the environment (i.e., ambient particulate matter, road dust, sludge, and sewage) of the most industrialized area. Pollutants such as heavy metals and polycyclic aromatic hydrocarbons co-contaminate the soil and pose a significant hazard to the ecosystem because these pollutants are harmful to both humans and the environment. Phytoremediation is an economical plant-based natural approach for soil clean-up that has no negative impact on ecosystems. The aim of this study was to investigate the effects of pyrene (500 mg kg-1), Zn (150 mg kg-1), Pb (150 mg kg-1), and Cd (150 mg kg-1) alone and in combination on the phytoextraction efficiency of Medicago sativa growing in contaminated soil. Plant biomass, biochemical activities, translocation factors, accumulation of heavy metals, and pyrene removal were determined. After 60 days of planting, compared with those of the control plants, the growth parameters, biomass, and chlorophyll content of the M. sativa plants were significantly lower, and the reactive oxygen species activity, such as proline and polyphenol content and metallothionein protein content, was markedly greater in the pyrene and heavy metal-polluted soils. Furthermore, the combined toxicity of pyrene and all three metals on M. sativa growth and biochemical parameters was significantly greater than that of pyrene, Zn, Pb, or Cd alone, indicating the synergistic effect of pyrene and heavy metals on cytotoxicity. Pyrene stress increased Cd accumulation in M. sativa. After pyrene exposure alone or in combination with Zn-pyrene, a greater pyrene removal rate (85.5-81.44%) was observed than that in Pb-pyrene, Cd-pyrene, and Zn-Pb-Cd-pyrene polluted soils (62.78-71.27%), indicating that zinc can enhance the removal of pyrene from contaminated soil. The resulting hypotheses demonstrated that Medicago sativa can be used as a promising phytoremediation agent for co-contaminated soil.

Aided phytostabilisation over two years using iron sulphate and organic amendments: Effects on soil quality and rye production

An outdoor macrocosm experiment using Fe-based and organic amendments over 2 years was set up to evaluate the effectiveness of aided-phytostabilisation. For that, a soil contaminated with As- and Cu-rich waste material (∼13000 mg As kg^-1 and ∼500 mg Cu kg^-1) was treated with combinations of iron sulphate (Fe) with lime, paper mill sludge (PS), holm-oak biochar (BC), olive mill waste compost (OMWC) or green waste compost (GWC). Rye (Secale cereale L.) was grown in the treated and non-treated soils 16 months after addition of the amendments. Arsenic and Cu dynamics in soil were assessed throughout the experiment and soil quality parameters (soil nutrients, organic matter and soil biology) were measured almost two years after addition of the amendments. All treatments resulted in a reduction of soluble and extractable Cu during the experiment and, despite the increase in soil pH (from 5 to 68) and DOC (from 10 up to 50 mg DOC L^-1) provoked by the amendments, As was not significantly mobilised in the treated soils. Treatments combining Fe sulphate with the organic materials, especially biochar and both composts, resulted in an increase in soil available nutrients and enhanced rye growth. In this semi-field scale experiment, the combination of Fe sulphate with holm-oak biochar showed the most promising results in terms of soil fertility (nutrient availability), plant As and Cu uptake and soil C sequestration. Further research should focus on monitoring long-term effects of the soil amendments on crops, following repeated applications.

Natural and assisted dissipation of polycyclic aromatic hydrocarbons in a long-term co-contaminated soil with creosote and potentially toxic elements

An enhanced bioremediation strategy was applied to an industrial soil co-contaminated with Polycyclic Aromatic Hydrocarbons (PAHs) and Potentially Toxic Elements (PTEs). Hydroxypropyl-β-cyclodextrin (HPBCD) and a natural mixture of two rhamnolipids (RL) were added to increase PAHs bioavailability, and combined with a microbial consortium (MC) to biodegrade soil PAHs. Bioavailability of only six PAHs (3-, 4-ring PAHs) increased when using HPBCD, with a maximum increase about 2.8-fold higher. The highest dose of HPBCD (5%) enhanced PAH degradation, with the best results for 4-ring PAHs with treatments of HPBCD + MC (up to 48% degradation for pyrene and 43% for fluoranthene), whereas dissipation for 5-ring PAHs was very low and for 6-ring was negligible. The use of RL increased the bioavailability of 13 of the 16 PAHs studied, reaching up to 60-fold higher values for phenanthrene or 18-fold higher for acenaphtene. RL addition did not show degradation improvement in any situation, and even inhibited the scarce degradation observed in the control treatment. The high increase in availability of both PAHs and mainly PTEs when using RL as amendment could make them toxic for microorganisms. In fact, Microtox Acute Toxicity test using Aliivibrio fischeri and the absence of colony forming units (CFUs) of indigenous bacteria demonstrated the extremely high levels of toxicity in RL treated soil.

Biochar, wood ash and humic substances mitigating trace elements stress in contaminated sandy loam soil: Evidence from an integrative approach

We conducted a pot experiment with biochar (BC), wood ash (WA), and humic substances (HS) to investigate their effect on As, Zn, Cu, Cd and Pb mobility in soil, as well as enzyme activities involved in C-, N-, and P-cycles, and Eisenia foetida toxicity in multi-contaminated soils. Amendments were dosed to increase еру soil pH from initial 6.0 to ∼6.5 and ∼7.0. Applying amendments has revealed, that WA significantly immobilized Cu, Zn and Pb, BC - Cu and Zn, and HS decreased solely Cu mobility in soil. The partition indices of Zn, Cu, and Pb, quantitatively describing the bioavailable species of elements in soil, were the lowest for WA. Changes in the water-soluble species of metals were more pronounced than in the exchangeable ones for all amendments. An opposite effect was observed on enzyme activity and earthworm toxicity for the WA and carbonaceous amendments. The BC and HS provided favourable soil conditions to dehydrogenase, β-glucosidase, urease activity and fluorescein diacetate hydrolysis, while WA significantly decreased the activity of all the mentioned enzymes in soil. The results are supported by an enzymes-based weighted mean index, being the highest for BC and HS and the lowest for WA (lower than in the control sample). At the same time, WA was suitable to eliminate the trace elements' stress to earthworms (biomass endpoints and cocoons production). Our data revealed that each amendment has its own advantages and disadvantages. The choice of the most suitable amendment therefore should always be made within an integral approach and based on the purpose of remediation.

Synergic use of chemical and ecotoxicological tools for evaluating multi-contaminated soils amended with iron oxides-rich materials

Abandoned waste piles from ancient mining activities are potential hot spots for the pollution of the surrounding areas. A pot experiment was carried out to check the potential toxicity of the dumping material present in one of these scenarios, and several amendments were tested to attenuate the spread of the contamination events. The waste material had an acid pH and a large total concentration of As and Cu. A dose-response experiment was performed with this material following OCDE 208 test. A proportion 90:10 uncontaminated soil: dumping material (% w/w) was selected for the following experiment, in order to surpass the amount of dumping material that caused 50% reduction in plant growth. Pots were filled with the 90:10 mixture, planted with seeds of Brassica napus and amended with the following materials: three iron oxides of Bayoxide® E33 series, iron (II) sulphate in combination with de-inking paper sludge (Fe+PS), iron oxide-rich rolling mill scale (ROL) and iron oxide-rich cement waste (CEM). Amendment effectiveness evaluation was based on chemical and biological assays: extractable trace element concentration, soil enzymatic activities, inhibition of light emission of V. fischeri and Anabaena sp., B. napus L. fresh weight and screening test for emergence of B. napus L. seedlings. Amendments E33HCF and Fe+PS were the most effective in reducing extractable As and Zn concentration. B. napus weight and dehydrogenase and β-glucosidase activities were positively increased with the two above mentioned treatments but they triggered more toxic effects for V. fischeri luminescence. E33P treatment was the only in which the EC₅₀ was higher than in the control. Anabaena sp. was less sensitive than V. fischeri as its luminescence was not hampered by any treatment. Trace element concentration did not significantly affect the failure in seed emergence. E33HCF and Fe+PS could act as proper amendments as they decreased extractable As and Zn. Further, plant fresh weight, enzymatic activities and some of the bioassays identified the latter treatments as the best ones among those tested here to this type of multi-contaminated soil.

Phytoremediation of diphenylarsinic-acid-contaminated soil by Pteris vittata associated with Phyllobacterium myrsinacearum RC6b

A pot experiment was conducted to explore the phytoremediation of a diphenylarsinic acid (DPAA)-spiked soil using Pteris vittata associated with exogenous Phyllobacterium myrsinacearum RC6b. Removal of DPAA from the soil, soil enzyme activities, and the functional diversity of the soil microbial community were evaluated. DPAA concentrations in soil treated with the fern or the bacterium were 35-47% lower than that in the control and were lowest in soil treated with P. vittata and P. myrsinacearum together. The presence of the bacterium added in the soil significantly increased the plant growth and DPAA accumulation. In addition, the activities of dehydrogenase and fluorescein diacetate hydrolysis and the average well-color development values increased by 41-91%, 37-78%, and 35-73%, respectively, in the treatments with P. vittata and/or P. myrsinacearum compared with the control, with the highest increase in the presence of P. vittata and P. myrsinacearum together. Both fern and bacterium alone greatly enhanced the removal of DPAA and the recovery of soil ecological function and these effects were further enhanced by P. vittata and P. myrsinacearum together. Our findings provide a new strategy for remediation of DPAA-contaminated soil by using a hyperaccumulator/microbial inoculant alternative to traditional physicochemical method or biological degradation.

Differences in the response of soil dehydrogenase activity to Cd contamination are determined by the different substrates used for its determination

Dehydrogenase activity (DHA) is an important indicator of heavy metal toxicity in contaminated soils. Different instances of DHA were determined using various substrates and which could affect the description of heavy metal toxicity. Currently, too few investigations have been done on selecting appropriate substrates. This study employed indoor simulation to determine soil DHA and its response to external cadmium (Cd) using two substrates (TTC and INT). Hormesis for DHA obtained using the TTC method (DHA-TTC) in low Cd concentration was observed which was quickly inhibited in high Cd concentration. While DHA obtained using the INT method (DHA-INT) decreased slowly when Cd concentration increased. The DHA-TTC and DHA-INT in soils at Cd concentration of 500 mg kg^-1 decreased 86% and 53%, respectively, compared to the control. The dose-response relationship of Cd to DHA can be well simulated using the logistic model (p < 0.01), which indicated DHA could be used to indicate soil Cd toxicity. Multiple stepwise regression analysis revealed that total organic matter (TOC) is the major factor influencing the toxicity of Cd to DHA-TTC, while TOC, pH and cation exchange capacity (CEC) are major factors influencing the toxicity of Cd to DHA-INT. The different responses of soil DHA-TTC and DHA-INT to Cd are due to the differences in electron transport chain characteristics between TTC and INT, as well as the influence of soil properties. Although both DHA-TTC and DHA-INT can monitor soil Cd contamination, DHA-INT is recommended as a superior bio-indicator to indicate and assess contamination of Cd in soil.

Bioremediation mechanisms of combined pollution of PAHs and heavy metals by bacteria and fungi: A mini review

In recent years, knowledge in regard to bioremediation of combined pollution of polycyclic aromatic hydrocarbons (PAHs) and heavy metals by bacteria and fungi has been widely developed. This paper reviews the species of bacteria and fungi which can tackle with various types of PAHs and heavy metals entering into environment simultaneously or successively. Microbial activity, pollutants bioavailability and environmental factors (e.g. pH, temperature, low molecular weight organic acids and humic acids) can all affect the bioremediation of PAHs and heavy metals. Moreover, this paper summarizes the remediation mechanisms of PAHs and heavy metals by microbes via elucidating the interaction mechanisms of heavy metals with heavy metals, PAHs/PAHs metabolites with PAHs and PAHs with heavy metals. Based on the above reviews, this paper also discusses the potential research needs for this field.

Assessing the combination of iron sulfate and organic materials as amendment for an arsenic and copper contaminated soil. A chemical and ecotoxicological approach

The efficiency of combining iron sulfate and organic amendments (paper mill sludge, olive mill waste compost and olive tree pruning biochar) for the remediation of an As- and Cu-contaminated soil was evaluated. Changes in As and Cu fractionation and solubility due to the application of the amendments was explored by leachate analysis, single and sequential extractions. Also, the effects on Arrhenatherum elatius growth, germination of Lactuca sativa and toxicity to the bacteria Vibrio fischeri were assessed. The combination of iron sulfate and the organic amendments efficiently reduced As solubility and availability through the formation of amorphous iron oxides, while organic matter did not seem to mobilize As. At the same time, copper fractionation was strongly affected by soil pH and organic matter addition. The soil pH significantly influenced both As and Cu mobility. Within all the amendments tested, FeSO4 in combination with compost showed to be the most suitable treatment for the overall remediation process, as it reduced As and Cu availability andenhanced soil nutrient concentrations and plant growth. In sipte of contradictory trends between chemical analyses and ecotoxicity tests, we can still conclude that the application of organic amendments in combination with reactive iron salts is a suitable approach for the remediation of soils contaminated by Cu and As.