Oil spills are a hazard during exploration activities in aqueous environments.
The spilled oil can coat sediments and changes the adsorption capacities of the
minerals. To constrain this process, arsenate (AsV) adsorption on natural and
oil-coated mineral surface was investigated using batch experiments. Arsenate
was chosen because of its high concentration in oil and its high toxicity to marine
organisms. Adsorption isotherms, kinetics, and mechanisms were assessed.
Solutions with low and high pH and ionic strengths (I) were used to mimic
relevant environmental conditions and goethite, illite, montmorillonite, kaolinite,
and chlorite were chosen as they are typically found in sediments.
Isotherms show that AsV adsorption on natural and oil-coated clay minerals
follows the Langmuir model. Thermodynamic equilibrium is reached between
2 and 24 h and is delayed on oil-coated minerals. The adsorption capacity is
reduced on oil-coated minerals because of surface area reduction except for
montmorillonite where the surface area is increased due to dispersion of the
mineral in water. Adsorption increases at lower pH on both natural and oil-coated
minerals because the affinity of the negatively charged AsV oxyanion is higher for
positively charged surfaces. At the same pH, maximum adsorption capacity is
slightly higher at higher I. Spectroscopic investigations suggest that AsV
adsorption occurs via an inner-sphere complex on the mineral surface. For clay
minerals, AsV adsorption on basal surface via Na+ bridging is also important.
Oil coating changes the adsorption enthalpy and entropy suggesting important
effects of the oil on bonding structures and complexes. Simple compensation plots
show that the adsorption of AsV on oil-coated minerals is entropy-controlled.
In conclusion, oil coating not only reduces the adsorption capacity of minerals but
also affects bonding structures and energies. These effects must to be taken into
account in oil exploration and future risk assessment.