Beyond cadmium accumulation: Distribution of other trace elements in soils and cacao beans in Ecuador

Since cacao beans accumulate Cd in high levels and restrictions have been imposed on safe levels of chocolate consumption, concern about whether or not cacao trees store other toxic elements seems to be inevitable. Following a previous study in Ecuador examining Cd content in five cacao varieties collected in pristine areas and in places impacted by oil activities, we present here the concentrations of 11 trace elements (TEs) (As, Ba, Co, Cu, Cr, Mo, Mn, Ni, Pb, V and Zn) in soils, cacao tissues (leaves, pod husks, beans) and cocoa liquor (CL). Several TEs showed concentrations in topsoils above the Ecuadorian limits, and may have a mixed natural and anthropogenic origin. Ba and Mo concentrations in cacao tissues are slightly higher than those reported in other surveys, but this was not the case for toxic elements (As and Pb). TE contents are lower in CL, than in beans, except for Pb and Co, but no risk was identified for human health. Compared with control areas, Enrichment Factors were below 2 in impacted areas, except for Ba. Transfer factors (from soils to cacao) indicated that cacao does not accumulate TEs. A positive correlation was found between Cd and Zn in topsoils and cacao tissues for the CCN-51 variety, and between Cd and Ni for the Nacional variety. Identifying patterns of TE distribution and potential interactions in order to explain plant internal mechanisms, which is also dependent on the cacao variety, is a difficult task and needs further research.

Influence of earthworm bioturbation on metals phytoavailability and human gastric bioaccessibility

At the global scale, urban agriculture is increasingly developing in cities due to demographic growth and sustainable food concerns. But, urban soils are frequently polluted with metals. In urban gardens, organic matter is also commonly added both to valorize organic household waste and to promote biophysicochemical fertility. As earthworms promote the decomposition and the recycling of soil organic matter, they can also influence the biogeochemical cycle of metals in urban polluted soils. In order to produce safe vegetables in urban areas, it is crucial to highlight the mechanisms involved in complex soil-earthworm-plant ecosystems. An experiment was set up to examine these relationships using lettuce cultivated in controlled conditions with RHIZOtest® devices. Thanks to the RHIZOtest® devices, metal transfer and bioaccessibility were for the first time compared for urban polluted soil without (1-urban soil polluted with Pb, Cd, Cu, and Zn: essential or toxic metals currently found in environment, SNB) and with bioturbation (2-this metal-polluted soil subjected to earthworm bioturbation, SB) and earthworm casts (3-earthworm casts produced in this polluted soil and naturally enriched in organic matter and microorganisms, T). Metal concentration, phytoavailability, and human gastric bioaccessibility were determined in the different samples. Results showed that earthworm bioturbation increased the phytoavailability of all the metals. For the experimental condition SB, the phytoavailability of metals was increased up to 75% compared to SNB. In addition, surprisingly, metal phytoavailability was always superior in SB compared to earthworm casts (T). Moreover, earthworms led to an increase in Zn gastric bioaccessibility up to 10% in the soils in the same way as for phytoavailability, meaning Zn bioaccessibility in SB > T > SNB, whereas it remained unchanged in the lettuces. These data are important to promote sustainable agriculture activities in urban areas; actually, databases concerning different experimental conditions are needed to develop decision support tools.

Cadmium bioaccumulation and gastric bioaccessibility in cacao: A field study in areas impacted by oil activities in Ecuador

Cacao from South America is especially used to produce premium quality chocolate. Although the European Food Safety Authority has not established a limit for cadmium (Cd) in chocolate raw material, recent studies demonstrate that Cd concentrations in cacao beans can reach levels higher than the legal limits for dark chocolate (0.8 mg kg^-1, effective January 1st, 2019). Despite the fact that the presence of Cd in agricultural soils is related to contamination by fertilizers, other potential sources must be considered in Ecuador. This field study was conducted to investigate Cd content in soils and cacao cultivated on Ecuadorian farms in areas impacted by oil activities. Soils, cacao leaves, and pod husks were collected from 31 farms in the northern Amazon and Pacific coastal regions exposed to oil production and refining and compared to two control areas. Human gastric bioaccessibility was determined in raw cacao beans and cacao liquor samples in order to assess potential health risks involved. Our results show that topsoils (0-20 cm) have higher Cd concentrations than deeper layers, exceeding the Ecuadorian legislation limit in 39% of the sampling sites. Cacao leaves accumulate more Cd than pod husks or beans but, nevertheless, 50% of the sampled beans have Cd contents above 0.8 mg kg^-1. Root-to-cacao transfer seems to be the main pathway of Cd uptake, which is not only regulated by physico-chemical soil properties but also agricultural practices. Additionally, natural Cd enrichment by volcanic inputs must not be neglected. Finally, Cd in cacao trees cannot be considered as a tracer of oil activities. Assuming that total Cd content and its bioaccessible fraction (up to 90%) in cacao beans and liquor is directly linked to those in chocolate, the health risk associated with Cd exposure varies from low to moderate.

Field isotopic study of lead fate and compartmentalization in earthworm-soil-metal particle systems for highly polluted soil near Pb recycling factory

Earthworms are important organisms in soil macrofauna and play a key role in soil functionality, and consequently in terrestrial ecotoxicological risk assessments. Because they are frequently observed in soils strongly polluted by metals, the influence of earthworm bioturbation on Pb fate could therefore be studied through the use of Pb isotopes. Total Pb concentrations and isotopic composition ((206)Pb, (207)Pb and (208)Pb) were then measured in earthworms, casts and bulk soils sampled at different distance from a lead recycling factory. Results showed decreasing Pb concentrations with the distance from the factory whatever the considered matrix (bulk soils, earthworm bodies or cast samples) with higher concentrations in bulk soils than in cast samples. The bivariate plot (208)Pb/(206)Pb ratios versus (206)Pb/(207)Pb ratios showed that all samples can be considered as a linear mixing between metallic process particulate matter (PM) and geochemical Pb background. Calculated anthropogenic fraction of Pb varied between approximately 84% and 100%. Based on Pb isotopic signatures, the comparison between casts, earthworms and bulk soils allowed to conclude that earthworms preferentially ingest the anthropogenic lead fraction associated with coarse soil organic matter. Actually, soil organic matter was better correlated with Pb isotopic ratios than with Pb content in soils. The proposed hypothesis is therefore a decrease of soil organic matter turnover due to Pb pollution with consequences on Pb distribution in soils and earthworm exposure. Finally, Pb isotopes analysis constitutes an efficient tool to study the influence of earthworm bioturbation on Pb cycle in polluted soils.

Effects of historic metal(loid) pollution on earthworm communities

The effects of metal(loid)s (Pb, Cd, Cu, Zn, As and Sb) from atmospheric fallout on earthworm communities were investigated in a fallow meadow located close to a 60-year-old lead recycling factory. We examined abundance and species diversity as well as the ratio of adult-to-juvenile earthworms, along five 140 m parallel transects. The influence of soil pollution on the earthworm community at the plot scale was put in context by measuring some physico-chemical soil characteristics (OM content, N content, pH), as well as total and bioavailable metal(loid) concentrations. Earthworms were absent in the highly polluted area (concentration from 30,000 to 5000 mg Pb·kg(-1) of dried soil), just near the factory (0-30 m area). A clear and almost linear relationship was observed between the proportion of juvenile versus mature earthworms and the pollution gradient, with a greater proportion of adults in the most polluted zones (only adult earthworms were observed from 30 to 50 m). Apporectodea longa was the main species present just near the smelter (80% of the earthworms were A. longa from 30 to 50 m). The earthworm density was found to increase progressively from five individuals·m(-2) at 30 m to 135 individuals·m(-2) at 140 m from the factory. On average, metal(loid) accumulation in earthworm tissues decreased linearly with distance from the factory. The concentration of exchangeable metal(loid)s in earthworm surface casts was higher than that of the overall soil. Finally, our field study clearly demonstrated that metal(loid) pollution has a direct impact on earthworm communities (abundance, diversity and proportion of juveniles) especially when Pb concentrations in soil were higher than 2050 mg·kg(-1).

Environmental and health impacts of fine and ultrafine metallic particles: assessment of threat scores

This study proposes global threat scores to prioritize the harmfulness of anthropogenic fine and ultrafine metallic particles (FMP) emitted into the atmosphere at the global scale. (Eco)toxicity of physicochemically characterized FMP oxides for metals currently observed in the atmosphere (CdO, CuO, PbO, PbSO(4), Sb(2)O(3), and ZnO) was assessed by performing complementary in vitro tests: ecotoxicity, human bioaccessibility, cytotoxicity, and oxidative potential. Using an innovative methodology based on the combination of (eco)toxicity and physicochemical results, the following hazard classification of the particles is proposed: CdCl2~CdO>CuO>PbO>ZnO>PbSO(4)>Sb(2)O(3). Both cadmium compounds exhibited the highest threat score due to their high cytotoxicity and bioaccessible dose, whatever their solubility and speciation, suggesting that cadmium toxicity is due to its chemical form rather than its physical form. In contrast, the Sb(2)O(3) threat score was the lowest due to particles with low specific area and solubility, with no effects except a slight oxidative stress. As FMP physicochemical properties reveal differences in specific area, crystallization systems, dissolution process, and speciation, various mechanisms may influence their biological impact. Finally, this newly developed and global approach could be widely used in various contexts of pollution by complex metal particles and may improve risk management.

Green manure plants for remediation of soils polluted by metals and metalloids: ecotoxicity and human bioavailability assessment

Borage, white mustard and phacelia, green manure plants currently used in agriculture to improve soil properties were cultivated for 10 wk on various polluted soils with metal(loid) concentrations representative of urban brownfields or polluted kitchen gardens. Metal(loid) bioavailability and ecotoxicity were measured in relation to soil characteristics before and after treatment. All the plants efficiently grow on the various polluted soils. But borage and mustard only are able to modify the soil characteristics and metal(loid) impact: soil respiration increased while ecotoxicity, bioaccessible lead and total metal(loid) quantities in soils can be decreased respectively by phytostabilization and phytoextraction mechanisms. These two plants could therefore be used for urban polluted soil refunctionalization. However, plant efficiency to improve soil quality strongly depends on soil characteristics.