Crude oil and its constituents can have adverse effects on ecological and human health when released into the environment. The Canadian Council of Ministers of the Environment (CCME) has developed remedial guidelines and a risk assessment framework for both ecological and human exposure to PHC. One of the assumptions used in the derivation of these guidelines is that plants are unable to take up PHC from contaminated soil and therefore subsequent exposure at higher trophic levels is not a concern. However, various studies suggest that plants are indeed able to take up PHC into their tissues. Consumption of plants is a potential exposure pathway in both ecological (e.g., herbivorous and omnivorous birds, and mammals) and human health risk assessments. If plants can uptake PHC, then the current approach for risk assessment of PHC may underestimate exposures to ecological and human receptors. The present review aims to assess whether or not plants are capable of PHC uptake and accumulation. Twenty-one articles were deemed relevant to the study objective and form the basis of this review. Of the 21 primary research articles, 19 reported detectable PHC and/or its constituents in plant tissues. All but five of the 21 articles were published after the publication of the CCME Canada-Wide Standards. Overall, the present literature review provides some evidence of uptake of PHC and its constituents into plant tissues. Various plant species, including some edible plants, were shown to take up PHC from contaminated soil and aqueous media in both laboratory and field studies. Based on the findings of this review, it is recommended that the soil-plant-wildlife/human pathway should be considered in risk assessments to avoid underestimating exposure and subsequent toxicological risks to humans and wildlife.

Land degradation by salinization and sodification changes soil function, destroys soil health, and promotes bioaccumulation of heavy metals in plants, but little is known about their fundamental mechanisms in shaping microbial communities and regulating microbial interactions. In this study, we explored the impact of saline-alkaline (SA) stress on soil bacterial and fungal community structures in different Cd-contaminated soils of Dezhou, Baoding, Xinxiang, Beijing and Shenyang cities from the North China Plain, China. Increased soil salinity and alkalinity enhanced Cd availability, indicated by significant increases in available Cd2+ in soil solution of 34.1%–49.7%, soil extractable Cd of 32.0–51.6% and wheat root Cd concentration of 24.5%–40.2%, as well as decreased activities of antioxidative enzymes of wheat root when compared with CK (no extra neutral or alkaline salts added). Soil bacteria were more active in response to the SA stress than fungi, as the significant structural reorganization of soil bacterial microbiota rather than fungal microbiota between SA and CK treatments was illustrated by principal component analysis. Adding neutral and alkaline salts enriched oligotrophic and haloalkaliphilic taxa in the Sphingobacteriaceae, Cellvibrionaceae, and Caulobacteraceae bacterial families, but decreased some Acidobacteria such as subgroup 6_norank, which was a sensitive biomarker that responded only to Cd contamination in CK-treated soils. Conversely, fungi were more sensitive to soil differences than bacteria: the composition of the fungal community was significantly different among different soil types. Phylogenetic molecular ecological network (pMEN) analysis further indicated that the microbial community structure and network interactions were altered to strengthen the adaptability of microorganisms to SA stress; the changes in structure and network interactions were proposed to contribute to competitive interactions. Most of the keystone genera identified in SA-treated soils, such as Blastococcus, Gemmatimonas, RB41, or Candida, had relatively low abundances (<1%), indicating their disproportionate ecological roles in triggering resistance or tolerance to SA stress and Cd toxicity.

This study aims to summarize results on potential phytomanagement of two metal(loid)-polluted military soils using Miscanthus x giganteus. Such an option was tested during 2-year pot experiments with soils taken from former military sites in Sliač, Slovakia and Kamenetz-Podilsky, Ukraine. The following elements were considered: As, Cu, Fe, Mn, Pb, Sr, Ti, Zn and Zr. M. x giganteus showed good growth at both military soils with slightly higher maximum shoot lengths in the second year of vegetation. Based on Principal Component Analysis similarities of metal(loid) uptake by roots, stems and leaves were summarized. Major part of the elements remained in M. x giganteus roots and rather limited amounts moved to the aerial parts. Levels taken up decreased in the second vegetation year. Dynamics of foliar metal(loid) concentrations divided the elements in two groups: essential elements required for metabolism (Fe, Mn, Cu, and Zn) and non-essential elements without any known metabolic need (As, Sr, Ti, and Zr). Fe, Mn, Ti and Sr showed similar S-shaped uptake curve in terms of foliar concentrations (likely due to dilution in growing biomass), while Cu exhibited a clear peak mid-season. Behavior of Zn was in between. Foliar Zr and As concentrations were below detection limit. The results illustrated a good potential of M. x giganteus for safely growing on metal-polluted soils taken from both military localities.

Degradation of metolachlor in surface soil is extremely important to its potential mobility and overall persistence. In this study, the effects of earthworms (Eisenia fetida) on the degradation of metolachlor at two concentration levels (5 and 20 mg kg⁻¹) in soil were investigated via the column experiment. The degradation kinetics of metolachlor indicate that addition of earthworms enhances metolachlor degradation significantly (P < 0.05), with the enhanced degradation rate of 30% and 63% in the low and high concentration treatments at the 15th day, respectively. Fungi rather than bacteria are primarily responsible for metolachlor degradation in soil, and earthworms stimulate metolachlor degradation mainly by stimulating the metolachlor-degrading functional microorganisms and improving fungal community structure. Earthworms prefer to promote the possible fungal degraders like order Sordariales, Microascales, Hypocreales and Mortierellales and the possible bacteria genus Rubritalea and strengthen the relationships between these primary fungi. Two metabolites metolachlor oxanilic (MOXA) and moetolachlor ethanesulfonic acid (MESA) are detected in soil and earthworms in the high concentration treatments. Earthworms stimulate the formation of MOXA and yet inhibit the formation of MESA in soil. Another metabolite metolachlor-2-hydroxy (M2H) is also detected in earthworms, which is reported firstly. The study provides an important information for the remediation of metolachlor-polluted soil.

Enhancing metals phytoextraction using gentile mobilizing agents might be an appropriate approach to increase the phytoextraction efficiency and to shorten the phytoremediation duration. The effect of sulfur-impregnated organoclay (SIOC) on the redistribution of potentially toxic elements (PTEs) among their geochemical fractions in soils and their plant uptake has not yet been studied. Therefore, our aim is to investigate the role of different SIOC application doses (1%, 3% and 5%) on operationally defined geochemical fractions (soluble + exchangeable; bound to carbonate; manganese oxide; organic matter; sulfide; poorly- and well-crystalline Fe oxide; and residual fraction) of Cd, Cr, Cu, Ni, Pb, and Zn, and their accumulation by pea (Pisum sativum) and corn (Zea mays) in a greenhouse pot experiment using a polluted floodplain soil. The SIOC caused a significant decrease in soil pH, and an increase in organic carbon and total sulfur content in the soil. The addition of SIOC increased significantly the soluble + exchangeable fraction and bioavailability of the metals. The SIOC leads to a transformation of the residual, organic, and Fe-Mn oxide fractions of Cd, Cu, Ni, and Zn to the soluble + exchangeable fraction. The SIOC addition increased the potential mobile (non-residual) fraction of Cr and Pb. The SIOC increased the sulfide fraction of Cr, Ni, and Zn, while it decreased the same fraction for Cd, Cu, and Pb. The effect of SIOC on the redistribution of metal fractions increased with enhancing application dosages. Pea accumulated more metals than corn with greater accumulation in the roots than shoots. Application of the higher dose of SIOC promoted the metals accumulation by roots and their translocation to shoots of pea and corn. Our results suggest the potential suitability of SIOC for enhancing the phytomanagement of PTEs polluted soils and reducing the environmental risk of these pollutants.

A bacterial community was enriched with polycyclic aromatic hydrocarbons (PAHs) polluted soil to better study PAH degradation by indigenous soil bacteria. The consortium degraded more than 52% of low molecular weight and 35% of high molecular weight (HMW) PAHs during 16 days in a soil leachate medium. 16S rRNA gene high-throughput sequencing and quantitative polymerase chain reaction analyses for alpha subunit genes of ring-hydroxylating-dioxygenase (RHDα) suggested that Proteobacteria and Actinobacteria at the phylum level, Pseudomonas, Methylobacillus, Nocardioides, Methylophilaceae, Achromobacter, Pseudoxanthomonas, and Caulobacter at the generic level were involved in PAH degradation and might have the ability to carry RHDα genes (nidA and nahAc). The community was selected and collected according to biomass and RHDα gene contents, and added back to the PAH-polluted soil. The 16 EPA priority PAHs decreased from 95.23 to 23.41 mg kg⁻¹ over 35 days. Compared with soil without the introduction of this bacterial community, adding the community with RHDα genes significantly decreased soil PAH contents, particularly HMW PAHs. The metabolic rate of PAHs in soil was positively correlated with nidA and nahAc gene contents. These results indicate that adding an indigenous bacterial consortium containing RHDα genes to contaminated soil may be a feasible and environmentally friendly method to clean up PAHs in agricultural soil.

The bioavailability of the metalloid arsenic (As) in paddy soil is controlled by microbial cycling of As and other elements such as iron (Fe) and sulfur (S), which are strongly influenced by water management in paddy fields. In this study, we evaluated how water management affects As bioavailability by growing rice plants in a geogenic As-contaminated soil. We determined As speciation in soil porewater and the diversity of the associated microbial community. Continuous flooding enhanced the release of Fe and As and increased arsenite (As(III)) and methylated As species concentrations in the rice grain compared with aerobic treatment. Total inorganic and organic As in the grain was 84% and 81% lower, respectively, in the aerobic treatment compared with the continuous flooding treatment. The amounts of Fe(III)-reducing bacteria (FeRB) increased in the flooded rhizosphere soil. The abundance of FeRB in the soil correlated with the dissolution of Fe and As. Among the As-transformation genes quantified, the aioA gene for As(III) oxidation and arsM gene for As(III) methylation were most abundant. The arsM copy number correlated positively with the levels of dsrB (dissimilatory (bi) sulfite reductase β-subunit), suggesting that dissimilatory sulfate-reducing bacteria (SRB) may play an important role in dimethylarsenate (DMAs(V)) production in soil. Our results show that decreased populations of rhizosphere FeRB and SRB contributed to a lower bioavailability of As, and decreased production of methylated arsenicals under oxic conditions.

Dissolved organic carbon (DOC) has a major influence upon sorption/desorption and transport of hydrophobic organic contaminants (HOCs) in soil environments. However, the molecular mechanisms of DOC sorption and its effects on aged HOC desorption in contaminated soils still remain largely unclear. Here, effects of three different DOC (one from commercial peat and two from biochars produced at 300 °C and 500 °C pyrolysis temperatures, respectively) and oxalate (as a reference) on abiotic desorption behavior of aged phenanthrene from three agricultural soils were investigated. Results showed that desorption of aged phenanthrene from soils was predominantly dependent on soil organic carbon content. The presence of DOC and oxalate resulted in higher desorption of phenanthrene compared to water alone, and the effects were positively related to soil organic carbon content and DOC/oxalate concentration. The facilitating effects of DOC were further increased during the second consecutive desorption, whereas oxalate had no such effect. Ultra-high-resolution Fourier transform-ion cyclotron resonance-mass spectrometry confirmed the molecular fractionation of DOC at the soil-water interface during DOC sorption. Specifically, the DOC molecules with O-rich moieties were preferentially adsorbed, whereas the molecules with phenolic and aromatic structures were selectively retained in the soil solutions through competitive displacement and co-sorption reactions during sorption. The enriched phenyl structures in the retained DOC facilitated its association with phenanthrene in the solutions and thus the release of phenanthrene from the soils. In contrast, oxalate replaced some organic carbon from the soils and thus released the associated phenanthrene into the solutions. Our findings highlight the importance of the molecular composition and structure of DOC for the desorption of phenanthrene in soil-water environments, which may help improve our understanding of the release and transport of organic compounds in the environments.

Remediation of soil chromium (Cr) pollution is becoming more and more urgent. In this study, a multi-loaded nano-zero-valent iron (nZVI) material (CNH) was prepared by carboxymethyl cellulose (CMC) and humic acid (HA) as dispersant and support agent, respectively, and the remediation effect of CNH, HA and CN (CNH without HA) for Cr contaminated soil was investigated within 90 d cycle. After 7 d treatment of CNH, the HOAc-extractable Cr decreased significantly. After the 90 d remediation, the HOAc-extractable Cr decreased most in the treatment of 3% CNH, about 74.48% lower than control. All treatments eventually caused different decline of soil pH, with a range of 0.12–0.54, in which the CNH treatment group had the least depression. HA loading significantly weakened the toxicity of nZVI, resulting in the higher soil microbial quantity and enzyme activities compared with CN. Additionally, the improvement of soil microecology by CNH and HA was positively correlated with the ratio of application, while CN was negatively correlated (except FDA enzyme activity) with these indexes. These results emphasized the potential of the synthesized CNH as a promising material to remediate Cr contaminated soil. Furthermore, details of possible mechanistic insight into the Cr remediation were carefully discussed.

Remediation of hexavalent chromium [Cr(VI)] has been widely studied for its high mobility and toxicity. As Cr(VI) migrates in natural environment, both soils and groundwater are contaminated simultaneously. In the present study, a novel reactor combining adsorption and microbial fuel cell (A-MFC) using Platanus acerifolia leaves was developed for removing Cr(VI) from groundwater and soils. When initial Cr(VI) concentration was 50 mg/L, the adsorption efficiency of A-MFC achieved 98% after 16 h. Afterwards, the leaves were used for fabricating an MFC-integrated leaching reactor. The A-MFC significantly improved the overall Cr(VI) removal efficiency through leaching and 40% of Cr(VI) in the soil column was removed. The electrical voltage and current of A-MFC reactor achieved averagely 343 mV and 141 μA to maintain the system operation without extra energy supply. This novel A-MFC reactor is an environmentally friendly technology which achieved efficient Cr(VI) removal from groundwater and soils using natural materials, proving the concept that integrated self-remediation of Cr(VI) in contaminated soil and groundwater with natural material and energy.