Sustainability of an in situ aided phytostabilisation on highly contaminated soils using fly ashes: Effects on the vertical distribution of physicochemical parameters and trace elements.

Aided phytostabilisation using trees and fly ashes is a promising technique which has shown its effectiveness in the management of highly metal-contaminated soils. However, this success is generally established based on topsoil physicochemical analysis and short-term experiments. This paper focuses on the long-term effects of the afforestation and two fly ashes (silico-aluminous and sulfo-calcic called FA1 and FA2, respectively) by assessing the integrity of fly ashes 10 years after their incorporation into the soil as well as the vertical distribution of the physicochemical parameters and trace elements (TEs) in the amended soils (F1 and F2) in comparison with a non-amended soil (R). Ten years after the soil treatment, the particle size distribution analysis between fly ashes and their corresponding masses (fly ash + soil particles) showed a loss or an agglomeration of finer particles. This evolution matches with the appearance of gypsum (CaSO4 2H2O) in FA2m instead of anhydrite (CaSO4), which is the major compound of FA2. This finding corresponds well with the dissolution and the lixiviation of Ca, S and P included in FA2 along the F2 soil profile, generating an accumulation of these elements at 30 cm depth. However, no variation of TE contamination was found between 0 and 25 cm depth in F2 soil except for Cd. Conversely, Cd, Pb, Zn and Hg enrichment was observed at 25 cm depth in the F1 soil, whereas no enrichment was observed for As. The fly ashes studied, and notably FA2, were able to reduce Cd, Pb and Zn availability in soil and this capacity persists over the time despite their structural and chemical changes.

Use of an in vitro digestion method to estimate human bioaccessibility of Cd in vegetables grown in smelter-impacted soils: the influence of cooking.

Metal contamination of urban soils and homegrown vegetables has caused major concern. Some studies showed that cadmium (Cd) was among the most significant hazards in kitchen garden soils and prolonged exposure to this metal could cause deleterious health effects in humans. In general, most risk assessment procedures are based on total concentrations of metals in vegetables. The present study assesses human bioaccessibility of Cd in vegetables cultivated in smelter-impacted kitchen garden soils. Seven vegetables (radish, lettuce, French bean, carrot, leek, tomato, and potato) were considered. Using the UBM protocol (unified BARGE bioaccessibility method), the bioaccessibility of Cd was measured in raw/cooked vegetables. A considerable amount of Cd was mobilized from raw vegetables during the digestion process (on average 85% in the gastric phase and 69% in the gastrointestinal phase), which could be attributed to a high uptake of Cd during the growth of the vegetables. Most Cd is accumulated in the vacuoles of plant cells, except what is absorbed by the cell wall, allowing Cd to be released from plant tissues under moderate conditions. Cooking by the steaming process generally increased the bioaccessibility of Cd in French bean, carrot, and leek. For potato, few or no significant differences of Cd bioaccessibility were observed after the steaming process, while the frying process strongly decreased bioaccessibility in both phases. The estimation of metal bioaccessibility in vegetables is helpful for human health risk assessment.

Radial metal concentration profiles in trees growing on highly contaminated soils.

The soil around Metaleurop, a big smelter, is heavily contaminated by Zn, Pb, Cd and Cu. In order to compare the impact of different soil amendments on the metal availability to trees, the polluted soil section was divided in a reference parcel and two others with either sulfo-calcic or silico-aluminous ash amendments. Five different tree species were planted on the parcels and the uptake of heavy metals in these trees was studied. Total and labile metal fractions were assessed in each of the 3 parcels. The mobility and assimilation of the metals was highest in the non-amended, reference soil parcel which had the lowest pH, organic matter and carbonate content. In all soils, pH decreased while organic matter content and mobility of the metals increased over time. Highest bulk concentrations of trace metals were found in white willow trees (Salix alba L.). Laser ablation-ICPMS was used to study changes in metal accumulation over a period of 10 years after planting the trees. The radial metal profiles in the trunk core samples varied between elements and tree species, however, in all willow trees the radial Cd and Zn profiles were significantly correlated. Radial pollutant concentration patterns are discussed in terms of seasonal effects, health status, tree species and metal mobility in the soil. For Cd and Zn, the profiles were influenced by their mobility in the soils. In general, periodical patterns were observed for Pb. Cu concentration profiles were decreasing over time, with the strongest decrease in the initial growth period.