Cadmium contamination of agricultural soils and crops resulting from sphalerite weathering

The biogeochemistry and bioavailability of cadmium, released during sphalerite weathering in soils, were investigated under contrasting agricultural scenarios to assess health risks associated with sphalerite dust transport to productive soils from mining. Laboratory experiments (365 d) on temperate and sub-tropical soils amended with sphalerite (<63 μm, 0.92 wt.% Cd) showed continuous, slow dissolution (0.6-1.2% y(-1)). Wheat grown in spiked temperate soil accumulated ≈38% (29 μmol kg(-1)) of the liberated Cd, exceeding food safety limits. In contrast, rice grown in flooded sub-tropical soil accumulated far less Cd (0.60 μmol kg(-1)) due to neutral soil pH and Cd bioavailability was possibly also controlled by secondary sulfide formation. The results demonstrate long-term release of Cd to soil porewaters during sphalerite weathering. Under oxic conditions, Cd may be sufficiently bioavailable to contaminate crops destined for human consumption; however flooded rice production limits the impact of sphalerite contamination.

The solid-solution partitioning of heavy metals (Cu, Zn, Cd, Pb) in upland soils of England and Wales

Ninety-eight surface soils were sampled from the uplands of England and Wales, and analysed for loss-on-ignition (LOI), and total and dissolved base cations, Al, Fe, and trace heavy metals (Cu, Zn, Cd, Pb). The samples covered wide ranges of pH (3.4-8.3) and LOI (9-98%). Soil metal contents measured by extraction with 0.43 mol l-1 HNO3 and 0.1 mol l-1 EDTA were very similar, and generally lower than values obtained by extraction with a mixture of concentrated nitric and perchloric acids. Total heavy metal concentrations in soil solution depend positively upon soil metal content and [DOC], and negatively upon pH and LOI, values of r2 ranging from 0.39 (Cu) to 0.81 (Pb). Stronger correlations (r2=0.76-0.95) were obtained by multiple regression analysis involving free metal ion (Cu2+, Zn2+, Cd2+, Pb2+) concentrations calculated with the equilibrium speciation model WHAM/Model VI. The free metal ion concentrations depend positively upon MHNO3 and negatively upon pH and LOI. The data were also analysed by using WHAM/Model VI to describe solid-solution interactions as well as solution speciation; this involved calibrating each soil sample by adjusting the content of "active" humic matter to match the observed soil pH. The calibrated model provided fair predictions of total heavy metal concentrations in soil solution, and predicted free metal ion concentrations were in reasonable agreement with the values obtained from solution-only speciation calculations.

Seasonal and annual variations in air concentrations of Pb, Cd and PAHs in Cairo, Egypt

A study is presented concerning the degree and extent of air pollution by cadmium, lead and polycyclic aromatic hydrocarbons (PAHs) in atmospheric particulates of an urban area (El-Abasya) in Cairo, Egypt. The average yearly and seasonal concentrations are presented. The correlation coefficients between metals, and between metals and suspended PAHs are also reported. Statistical analysis showed seasonal and annual variations. Concentrations of PAHs, Pb and Cd were higher in winter than those measured at other times in the study area. The concentration of lead decreased by 40% from 3.36 microg(-3) in 1994 to 2.4 microg m(-3) in 1997 in consequence of the reduction of the Pb concentration in leaded gasoline fuel. The average cadmium concentrations showed significant decrease in 1997. The sum of the concentrations of PAHs was higher during the winter season 1997 (14.79 ng m(-3)) than in summer (7.53 ng m(-3)) and a highly significant difference was observed between the two seasons. In addition, the ratio of benzo(a)pyrene [B(a)P] to Pb was higher in winter than in summer, suggesting the contribution of a non-vehicular source of lead. The levels of lead observed are higher than those recorded in various other parts of the world.