Adsorption and toxicity of metal contaminants in soils and sediments: Predicting toxicity of cadmium to microorganisms with the free ion activity model

The influence of adsorption on cadmium toxicity to soil microorganisms in smectite-rich soils and sediments was quantified as a function of solution and sorbent characteristics. One of the central tenets of ecotoxicology and geochemistry is that toxic effects of metals to organisms is correlated with the activity of the uncomplexed metal (Mn+)_aq. This concept is the major assumption in the free ion activity model (FIAM). The FIAM has successfully explained the influence of solution-phase complexation and precipitation on metal toxicity to microorganisms. However, the ability of the FIAM to explain the effect of adsorption on metal toxicity to the microorganisms has not been tested.; Cadmium adsorption and surface characterization experiments were conducted to infer Cd sorption mechanisms to a reference smectite and three fractions of a Veritsol soil. Cadmium adsorption isotherms conformed to the Langmuir adsorption model. The reference smectite had the highest total charge, the highest percentage of permanently charged (85%) sites, and formed the highest proportion of inner-sphere complexes with the cadmium.; Equilibrium geochemical modeling (MINTEQA2) was used to predict the speciation of Cd in the soil suspensions using Langmuir and Triple Layer surface complexation models. Model predictions matched observed Cd adsorption over a range of pHs. Model calculations predicted that edge-site aluminols are the dominant surface functional group responsible for Cd adsorption.; The influence of adsorption on cadmium toxicity to soil microorganisms was assessed indirectly through the relative change in microbial hydrolysis of fluorescein diacetate (FDA) as a function of total Cd concentration and sorbent characteristics. The sequence of relative percentage of FDA hydrolysis was reference smectite > untreated Vertisol > DCB-treated Vertisol > H ₂O₂-treated Vertisol in suspensions containing the same total Cd concentrations. Adsorption decreased the toxicity of Cd to soil microorganisms. Inner-sphere complexation is more effective than outer-sphere complexation in reducing the bioavailability and toxicity of heavy metals in soils. The experimental results support the hypothesis of this study that Cd adsorption influences microbial activity and Cd toxicity in soil.