The effect of crude oil on arsenate adsorption on goethite

This study reports the adsorption of arsenate, As(V), on goethite (α-FeO(OH)) and oil-coated goethite at experimental conditions chosen to mimic settings of wastewater from oil fields being released into marine and freshwater bodies. Similarities are evident between the As(V)-goethite and As(V)-oil-goethite systems: i) Adsorption is fast and saturation is achieved within 180 min, ii) Reaction rates approximate to a pseudo second order rate expression and range between 6.5 and 52.3 × 10(-4)g/μmol/min, iii) Adsorption mechanisms are best described with a Langmuir model, and iv) Adsorption capacity rises with decreasing pH reflecting the increase of positive charges on the goethite surface. A difference is discernable in that the adsorption of As(V) is reduced significantly when the goethite is coated with oil. The similar experimental macroscopic observations for both systems, i.e., Langmuir model fits, reaction rates, and the effect of pH and ionic strength (I), suggest that the oil reduces the effective and/or reactive surface area. The zeta potential (ζ) indicates that the oil coating also changes the surface charge of the goethite, shifting the pH point of zero charge from 9.8 to about 3, thus contributing to the reduced As(V) adsorption. FTIR spectra show that As(V) interacts with the carbonyl functional groups of the oil. Our results suggest that oil-covered goethite significantly reduces the adsorption of As(V) and this points to a potentially significant indirect effect of oil on the cycling of As(V) and other oxyanions in oil polluted waters.