Identification of Sulfonated and Hydroxy-Sulfonated Polychlorinated Biphenyl (PCB) Metabolites in Soil: New Classes of Intermediate Products of PCB Degradation?

Environmental fate of sulfonated-PCBs: Soil partitioning properties, bioaccumulation, persistence, and mobility

Two new classes of PCB metabolites were recently discovered: sulfonated-polychlorinated biphenyls (sulfonated-PCBs) and hydroxy-sulfonated-polychlorinated biphenyls (OH-sulfonated-PCBs). These metabolites, originating from PCB degradation, seem to possess more polar characteristics than their parent compounds. However, no other information, such as their chemical identity (CAS number) or their ecotoxicity or toxicity, is available so far, although more than about one hundred different chemicals were observed in soil samples. In addition, their physico-chemical properties are still uncertain since only estimations are available. Here we show the first evidence on the fate of these new classes of contaminants in the environment, producing results from several experiments, to evaluate sulfonated-PCBs and OH-sulfonated-PCBs soil partition coefficients, degradation in soil after 18 months of rhizoremediation, uptake into plant roots and earthworms, as well as a preliminary analytical method to extract and concentrate these chemicals from water. The results give an overview of the expected environmental fate of these chemicals and open questions for further studies.

Synthesis of a new sulfonated-hexachlorobiphenyl standard for environmental analysis, ecotoxicological, and toxicological studies

Sulfonated-polychlorinated biphenyls (sulfonated-PCBs) are a newly discovered class of PCB metabolites. They were observed for the first time in polar bear serum and lately, in soil, together with hydroxy-sulfonated-PCBs. Their presence is ubiquitous in soils, and their estimated physical chemical properties show high mobility in water, compared to the parent compounds. However, no single pure standards exist so far and therefore their quantification in the environmental matrices is not accurate. Additionally, pure standards are needed to experimentally determine their physical chemical properties, as well as the ecotoxicological and toxicological characteristics. In the present work, the challenging goal of preparing a polychlorinated biphenyl monosulfonic acid was achieved exploring different synthetic approaches, along which the selection of the starting material resulted in a crucial point. Using PCB-153 (2,2'-4,4'-5,5'-hexachloro-1,1'-biphenyl) the synthesis afforded, as the major species, a side compound. On the contrary, the use of PCB-155 (2,2'-4,4'-6,6'-hexachloro-1,1'-biphenyl), a symmetric hexachlorobiphenyl derivative showing chlorine atoms at all the ortho positions, gave the target sulfonated-PCB compound. In this case, sulfonation was successfully carried out through a two-step procedure, involving chlorosulfonylation and the subsequent hydrolysis of the chlorosulfonyl intermediate.

Enhanced rhizoremediation of polychlorinated biphenyls by resuscitation-promoting factor stimulation linked to plant growth promotion and response of functional microbial populations

Rhizoremediation is acknowledged as a green technology for removing polychlorinated biphenyls (PCBs) in soil. However, rhizoremediation is limited because most soil microorganisms enter into a viable but non-culturable (VBNC) state under PCBs stress. This work was to study the effect of resuscitation-promoting factor (Rpf) on rhizoremediation efficiency of PCBs in alfalfa and rhizosphere microbiological communities. Results suggested that Rpf promoted alfalfa growth in PCB-contaminated soil by improving antioxidant enzymes and detoxification metabolites in alfalfa. After 40 d Rpf treatment, removal rate for five selected PCBs significantly increased by 0.5-2.2 times. Rpf enhanced relative abundances of bphA and bphC responsible for degrading PCBs, and enzymatic activities of metabolizing exogenous compounds in rhizosphere soil. High-throughput sequencing showed that Rpf did not change the dominant microbial population at phyla and genera levels, but caused variation of the bacterial community structures. The promoting function of Rpf was linked to the shift of various key populations having different functions depending on Rpf concentrations. Pseudomonas and Rhizobium spp. enrichment might stimulate PCB degradation and Streptomyces and Bacillus spp. primarily contributed to alfalfa growth. Predicted functions in rhizosphere soil bacterial community indicated Rpf facilitated soil nutrient cycling and environmental adaptation. This study indicated that Rpf was an active additive for strengthening rhizoremediation efficiency of PCB-contaminated soil and enhancing their in-situ remediation.

Spatial distribution and ecological risks of polychlorinated biphenyls in a river basin affected by traditional and emerging electronic waste recycling in South China

With development of e-waste related legislation in China, formal recycling activities are designated in some areas while informal ones are illegally transferred to emerging areas to avoid supervision. However, the resulting environmental impact and ecological risks are not clear. Here, we investigated the discharge of polychlorinated biphenyls (PCBs) to soil and aquatic environments by e-waste recycling activities in the Lian River Basin, China. The study area included a designated industrial park in the traditional e-waste recycling area (Guiyu, known as the world's largest e-waste center), several emerging informal recycling zones, and their surrounding areas and coastal area. A total of 27 PCBs were analyzed, and the highest concentration was found in an emerging site for soil (354 ng g^-1) and in a traditional site for sediment (1350 ng g^--1) respectively. The pollution levels were significantly higher in both the traditional and emerging recycling areas than in their respective upstream countryside areas (p = 0.0356 and 0.0179, respectively). Source analysis revealed that the traditional and emerging areas had similar PCB sources mainly associated with three PCB technical mixtures manufactured in Japan (KC600) and the USA (Aroclor 1260 and Aroclor 1262). The PCB pollution in their downstream areas including the coastal area was evidently affected by the formal and informal recycling activities through river runoff. The ecological risk assessments showed that PCBs in soils and sediments in the Lian River Basin could cause adverse ecotoxicological consequences to humans and aquatic organisms.

Bioaccumulation of PCBs and their hydroxy and sulfonated metabolites in earthworms: Comparing lab and field results

Sulfonated and hydroxy-sulfonated PCBs were recently discovered by our group as new PCB soil contaminants, constituting about 1% of their parent compounds in soil. Here we investigate for the first time the bioaccumulation of these metabolites as well as hydroxy-PCBs and native PCBs in earthworms. A sequence of three experiments, at increasing complexity and ecological realism, were performed with four different earthworm species (Eisenia foetida Savigny, Lumbricus terrestris L, Allolobophora chlorotica Savigny and Aporrectodea caliginosa Savigny) exposed to contaminated soils. The first experiment confirmed that when exposing earthworms to soil contaminated with a single hexa-chlorinated congener (PCB 155), no formation of polar metabolites in earthworms could be detected. This allowed to plan the following two experiments, using a soil from a PCB contaminated site and rich in relatively high levels (10-130 μg kg^-1) of hydroxy-, sulfonated-, and hydroxy-sulfonated-PCBs. Bioaccumulation factors (BAFs) and bioconcentration factors (BCFs) were then obtained in the second and third experiments, to compare the accumulation behavior of these chemicals in laboratory and natural conditions. Regressions between BAF/BCF and Log Kow/Log D, produced a variety of results, being generally significant between BCF and PCBs and not significant in the other cases. In general, the metabolites accumulated in earthworms with detectable concentrations in their tissues (8-115 μg kg^-1), although sulfonated and hydroxy-sulfonated PCBs showed BAF and BCF values lower (up to two orders of magnitude) than those calculated for the parent PCBs, given their lower lipophilicity.

Mercury vertical and horizontal concentrations in agricultural soils of a historically contaminated site: Role of soil properties, chemical loading, and cultivated plant species in driving its mobility

The long term vertical and horizontal mobility of mercury (Hg) in soils of agricultural areas of a historically contaminated Italian National Relevance Site (SIN Brescia-Caffaro) was investigated. The contamination resulted from the continuous discharge of Hg in irrigation waters by an industrial plant (Caffaro S.p.A), equipped with a mercury-cell chlor-alkali process. The contamination levels with depth ranged from about 20 mg/kg dry weight (d.w.) of soil in the top (plow) layer to less than 0.1 mg/kg d.w. at 1 m depth. The concentrations varied also spatially, up to one order of magnitude within the same field and showing a decreasing trend from the Hg source (i.e., irrigation ditches). The concentration profiles and gradients measured were explained considering Hg loading, soil properties, such as the texture, organic carbon content, pH and cation exchange capacity. A Selective Sequential Extraction (SSE) was also applied on soil samples from an ad hoc greenhouse experiment to investigate the role of different plant species in influencing Hg speciation in soils. Although most of the extracted Hg was included in scarcely mobile or immobile forms, some plant species (i.e., alfalfa) showed to importantly increase the soluble and exchangeable fractions with respect to the unplanted control soils, thus affecting mobility and potential bioavailability of Hg.

Interconversion between methoxylated, hydroxylated and sulfated metabolites of PCB 3 in whole poplar plants

Methoxylated polychlorinated biphenyls (MeO-PCBs) are overlooked metabolites of PCBs. In general, they are more toxic to plants than their parent congeners. However, information on the fate of MeO-PCBs and the relationship between methoxylated, hydroxylated and sulfated metabolites of PCBs in plants is scarce. In this work, poplar plants (Populus deltoides × nigra, DN34) were hydroponically and separately exposed to 4'-methoxy-4-monochlorobiphenyl (4'-MeO-PCB 3) and 4'-PCB 3 sulfate for 10 days to investigate the uptake, translocation and metabolism of MeO-PCBs and the relationship between methoxy-PCBs, hydroxyl-PCBs and PCB sulfates within plants. Results showed that 4'-MeO-PCB 3 and 4'-PCB 3 sulfate were taken up by the roots of poplar plants and translocated from roots to shoots and leaves. 4'-OH-PCB 3 and 4'-PCB 3 sulfate were identified as the hydroxylated metabolite and sulfate metabolite of 4'-MeO-PCB 3 in poplar, respectively. In the backward reaction, 4'-OH-PCB 3 and 4'-MeO-PCB 3 were found as metabolites of 4'-PCB 3 sulfate. For exposure groups, the yields of 4'-OH-PCB 3 produced from 4'-MeO-PCB 3 and 4'-PCB 3 sulfate were 1.29% and 0.13% respectively. The yield of 4'-PCB 3 sulfate which originated from 4'-MeO-PCB 3 in wood and root samples of exposure groups was only 0.02%. Only 0.04% of the initial mass of 4'-PCB 3 sulfate was transformed to 4'-MeO-PCB 3 in the exposure groups. The sulfation yield of 4'-OH-PCB 3 was higher than hydrolysis yield of 4'-PCB 3 sulfate, indicating that formation of PCB sulfates was predominant over the reverse reaction, the formation of hydroxy-PCBs. These results provide new perspective on the transport, metabolism, and fate of MeO-PCBs, and also help to better understand sources of OH-PCBs and PCB sulfates in the environment. This study provides the first evidence of interconversion of sulfate metabolites from methoxy-PCBs and methoxy-PCBs from PCB sulfates.

'Cry-for-help' in contaminated soil: a dialogue among plants and soil microbiome to survive in hostile conditions

An open question in environmental ecology regards the mechanisms triggered by root chemistry to drive the assembly and functionality of a beneficial microbiome to rapidly adapt to stress conditions. This phenomenon, originally described in plant defence against pathogens and predators, is encompassed in the ‘cry‐for‐help’ hypothesis. Evidence suggests that this mechanism may be part of the adaptation strategy to ensure the holobiont fitness in polluted environments. Polychlorinated biphenyls (PCBs) were considered as model pollutants due to their toxicity, recalcitrance and poor phyto‐extraction potential, which lead to a plethora of phytotoxic effects and rise environmental safety concerns. Plants have inefficient detoxification processes to catabolize PCBs, even leading to by‐products with a higher toxicity. We propose that the ‘cry‐for‐help’ mechanism could drive the exudation‐mediated recruitment and sustainment of the microbial services for PCBs removal, exerted by an array of anaerobic and aerobic microbial degrading populations working in a complex metabolic network. Through this synergistic interaction, the holobiont copes with the soil contamination, releasing the plant from the pollutant stress by the ecological services provided by the boosted metabolism of PCBs microbial degraders. Improving knowledge of root chemistry under PCBs stress is, therefore, advocated to design rhizoremediation strategies based on plant microbiome engineering.

A new dataset of PCB half-lives in soil: Effect of plant species and organic carbon addition on biodegradation rates in a weathered contaminated soil

This paper presents a new dataset of Polychlorinated Biphenyls (PCBs) half-lives in soil. Data were obtained from a greenhouse experiment performed with an aged contaminated soil under semi-field conditions, collected from a National Relevance Site (SIN) located in Northern Italy (SIN Brescia-Caffaro). Ten different treatments (combination of seven plant species and different soil conditions) were considered together with the respective controls (soil without plants). PCB concentration reduction in soil was measured over a period of 18 months to evaluate the ability of plants to stimulate the biodegradation of these compounds. Tall fescue, tall fescue cultivated together with pumpkin and tall fescue amended with compost reduced more than the 50% of the 79 measured PCB congeners, including the most chlorinated ones (octa to deca-PCBs). However, the data obtained showed that no plant species was uniquely responsible for the effective degradation of all isomeric classes and congeners. The obtained half-lives ranged from 1.3 to 5.6 years and were up to a factor of 8 lower compared to generic HL values reported in literature. This highlighted the importance of cultivation and plant-microbe interactions in speeding up the PCB biodegradation. This new dataset could contribute to substantially improve the predictions of soil remediation time, multimedia fate and the long-range transport of PCBs. Additionally, the half-lives obtained here can also be used in the evaluation of the food chain transfer of these chemicals, and finally the exposure and potential for effects on ecosystems.

PCB vertical and horizontal movement in agricultural soils of a highly contaminated site: Role of soil properties, cultivation history and PCB physico-chemical parameters

The agricultural areas of a historically contaminated National Relevance Site (SIN Brescia Caffaro) in Italy are an ideal case for studying the long term vertical and horizontal movement of polychlorinated biphenyls (PCBs) in soil. Here, a former large producer of PCBs (Caffaro S.p.A.) discharged its wastewaters, contaminated by PCBs and other chemicals, to a ditch used for about 80 years as source of irrigation waters for the adjacent agricultural areas. This caused a spread of contamination along both a vertical and a horizontal soil gradient. PCB concentrations of about 80 congeners, including PCB 209, peculiar of Caffaro production, were measured in three areas, selected for their different soil properties and cultivation history. The contamination levels with depth ranged from about 30 mg/kg dry weight (d.w.) of soil in the top (plow) layer to less than 0.1 mg/kg d.w. at the depth of 1 m. The concentrations varied also horizontally, since each field was surface irrigated from the short edge of each field, showing that PCBs could spread with length halving the initial concentrations in the topsoil only after about 30-35 m. The concentration gradients detected were explained considering the historic soil use and its change with time, the pedological properties as well as PCB physico-chemical parameters and halflives, developing equations which could be employed as guidance tools for evaluating PCBs (and similar chemicals) movement and direct further studies.

New Data Set of Polychlorinated Dibenzo-p-dioxin and Dibenzofuran Half-Lives: Natural Attenuation and Rhizoremediation Using Several Common Plant Species in a Weathered Contaminated Soil

In this paper, a new data set of polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/Fs) half-lives (HLs) in soil is presented. Data are derived from a greenhouse experiment performed with an aged contaminated soil under semi-field conditions, obtained from a National Relevance Site (SIN) located in Northern Italy (SIN Brescia-Caffaro). Ten different treatments (combination of seven plant species with different soil conditions) were considered together with the respective controls (soil without plants). The ability of the plants to stimulate the biodegradation of these compounds was evaluated by measuring the PCDD/F concentration reduction in soil over a period of 18 months. The formation of new bound residues was excluded by using roots as a passive sampler of bioaccessible concentrations. The best treatment which significantly reduced PCDD/F concentrations in soil was the one with Festuca arundinacea (about 11-24% reduction, depending on the congener). These decreases reflected in HLs ranging from 2.5 to 5.8 years. Simulations performed with a dynamic air-vegetation-soil model (SoilPlusVeg) confirmed that these HLs were substantially due to biodegradation rather than other loss processes. Because no coherent PCDD/F degradation HL data sets are currently available for soil, they could substantially improve the predictions of soil remediation time, long-range transport, and food chain transfer of these chemicals using multimedia fate models.

Plants radically change the mobility of PCBs in soil: Role of different species and soil conditions

The mobility of Polychlorinated Biphenyls (PCBs) in soil cultivated with different plant species was evaluated by means of a column experiment to investigate the specific plant influence on PCB environmental fate and the potential for leaching. The soil was collected at a National Relevance Site for remediation located in Northern Italy (SIN Brescia-Caffaro) and underwent a rhizoremediation treatment for 18 months with different plant species (Festuca arundinacea, Cucurbita pepo ssp pepo and Medicago sativa). The same but unplanted soil was also considered as control for comparison. The columns were leached with tap water and PCB concentrations were measured in the leachate after 7 days of soil/water contact. Soil previously cultivated with different plant species exhibited statistically different behavior in terms of chemical leaching among the different fractions. Total PCB bulk concentrations ranged from 24 to 219 ng/L. Leachate samples were enriched in tetra- to hepta-PCBs. While PCB concentrations in the dissolved phases varied within a factor of 2 between controls and treatments, PCB associated to particulate organic carbon (POC) differed by more than one order of magnitude. More specifically, Medicago sativa enriched the soil with POC doubling PCB leaching with respect to the other plant species and the unplanted controls.

Modelling peak exposure of pesticides in terrestrial and aquatic ecosystems: importance of dissolved organic carbon and vertical particle movement in soil

In the present work, an existing vegetation/air/litter/soil model (SoilPlusVeg) was modified to improve organic chemical fate description in terrestrial/aquatic ecosystems accounting for horizontal and vertical particulate organic carbon (POC) transport in soil. The model was applied to simulate the fate of three pesticides (terbuthylazine, chlorpyrifos and etofenprox), characterized by increasing hydrophobicity (log K_OW from about 3 to 7), in the soil compartment and more specifically, their movement towards surface and groundwater through infiltration and runoff processes. The aim was to evaluate the role of dissolved organic carbon (DOC) and POC in the soil in influencing the peak exposure of pesticides in terrestrial/aquatic ecosystems. Simulation results showed that while terbuthylazine and chlorpyrifos dominated the free water phase (C_W-FREE) of soil, etofenprox was mainly present in soil porewater as POC associated chemical. This resulted in an increase of this highly hydrophobic chemical movement towards groundwater and surface water, up to a factor of 40. The present work highlighted the importance of DOC and POC as an enhancer of mobility in the water of poor or very little mobile chemicals. Further studies are necessary to evaluate the bioavailability change with time and parameterize this process in multimedia fate models.