Advancing prediction of polycyclic aromatic hydrocarbon bioaccumulation in plants for historically contaminated soils using Lolium multiflorum and simple chemical in-vitro methodologies

This study compared chemical extraction methods for the prediction of PAH bioaccumulation in ryegrass (Lolium multiflorum) roots in four Manufactured Gas Plant (MGP) historically (>50 years) contaminated soils. The in-vitro methods compared were butanol (BuOH), non-buffered and buffered 2-hydroxypropyl-β-cyclodextrin extractions (HPCD, Buf-HPCD), potassium persulfate oxidation (KPS), solid phase extraction using Tenax resin (Tenax), and polyoxymethylene solid-phase extraction (POM). Extractions were directly compared with bioaccumulation and modelled using equilibrium partitioning theory (EqPT) with a combination of different partitioning parameters (K_OC and K_OW values) that aimed to improve predictions. The PAH accumulation in plant roots showed good correlation with concentrations in soils, and higher concentrations of the 4-6 ring PAHs compared with 2-3 ring PAHs. Plant accumulation of 16 PAHs in L. multiflorum was estimated within a factor of 5 using direct comparison for all bioaccessibility extraction methods. Accumulation values predicted using the calculation approach depended on the combination of K_OC, K_OW parameters and root components (total lipid vs total dry weight) used in calculations. Using K_OC values derived from historically contaminated soils improved accuracy of predicted total root accumulation although precision was low. The combined contribution of PAH in lipid and carbohydrate root components (total dry weight) overestimated accumulation and a lipid only approach using generic partitioning parameters provided more accurate and precise approximation of bioaccumulation in roots of L. multiflorum in the soils. Overall, Tenax, POM and HPCD-based extractions showed promising results for predicting L. multiflorum root accumulation using the different approaches. This work significantly extends current knowledge for integrating simple chemical extractions into ecological risk assessment frameworks for the prediction of plant PAH bioavailability in historically contaminated soils.

A field assessment of bauxite residue rehabilitation strategies

Bauxite residue, the by-product of the alumina industry, is mainly stored in land-based bauxite residue disposal areas (BRDAs). Environmental concern has been raised due to the large volumes in stockpile, the high alkalinity of the material, as well as the presence of elevated concentrations of trace elements. If not adequately managed, BRDAs can act as a source of pollution. In order to minimize the environmental risk, revegetation is implemented to stabilize the residue against water and wind erosion. Currently, two main approaches are used: the use of amendments or the installation of a capping layer. However, few studies evaluating the long-term success and self-sustainability of the rehabilitation programs have been published. A series of field-established rehabilitation strategies reflecting both direct revegetation and revegetation on capping layer were assessed in terms of both soil and plant quality. Soil physico-chemical properties, including pseudo-total and plant-available fractions of nutrients and trace elements, were determined over a summer and winter seasons and aerial portions of vegetation were analysed for nutrients and trace elements. Failure to adequately lower alkalinity remains the major constraint to long-term rehabilitation success of bauxite residue. This is evidenced from poor soil properties in unamended residue and in residue capped with a shallow soil layer, as well from vegetation displaying excessive concentrations of certain elements. Certain elements exceeded typical ranges for non-contaminated soils (i.e. Cr, Fe, Na, Ni and V), with some showing excessive plant-available fractions (i.e. of Al, As, Cr, Hg and V). Vegetation analysis found excessive uptake of some elements (i.e. of Al, Na, Fe, Cr and V). Future attempts for bauxite residue rehabilitation should include both gypsum and organic amendments, while a capping layer may only be effective if either a deep layer (>1 m) is installed or if the underlying residue is sufficiently treated prior to capping.

Concentration of cadmium in cacao beans and its relationship with soil cadmium in southern Ecuador

Cadmium (Cd) content in cacao beans above a critical level (0.6 mg kg(-1)) has raised concerns in the consumption of cacao-based chocolate. Little is available regarding Cd concentration in soil and cacao in Ecuador. The aim of this study was to determine the status of Cd in both, soils and cacao plants, in southern Ecuador. Soil samples were collected from 19 farms at 0-5, 5-15, 15-30, and 30-50 cm depths, whereas plant samples were taken from four nearby trees. Total recoverable and extractable Cd were measured at the different soil depths. Total recoverable Cd ranged from 0.88 to 2.45 and 0.06 to 2.59, averaged 1.54 and 0.85 mg kg(-1), respectively in the surface and subsurface soils whereas the corresponding values for M3-extractable Cd were 0.08 to 1.27 and 0.02 to 0.33 with mean values of 0.40 and 0.10 mg kg(-1). Surface soil in all sampling sites had total recoverable Cd above the USEPA critical level for agricultural soils (0.43 mg kg(-1)), indicating that Cd pollution occurs. Since both total recoverable and M3-extractable Cd significantly decreased depth wise, anthropogenic activities are more likely the source of contamination. Cadmium in cacao tissues decreased in the order of beans>shell>>leaves. Cadmium content in cacao beans ranged from 0.02 to 3.00, averaged 0.94 mg kg(-1), and 12 out of 19 sites had bean Cd content above the critical level. Bean Cd concentration was highly correlated with M3- or HCl-extractable Cd at both the 0-5 and 5-15 cm depths (r=0.80 and 0.82 for M3, and r=0.78 and 0.82 for HCl; P<0.01). These results indicate that accumulation of Cd in surface layers results in excessive Cd in cacao beans and M3- or HCl-extractable Cd are suitable methods for predicting available Cd in the studied soils.