PHREEQC Modeling of molybdenum in the Botlek area

Abstract

The Botlek in the Port of Rotterdam contains molybdenum contaminations exceeding the intervention value. Because little is known about the geochemical behaviour of molybdenum in the subsurface, a study consisting of a part literature review and a part geochemical modeling with the program PHREEQC was done. Based on measurements from the field, and the information available from literature, PHEEQC was used to determine the speciation, degree of adsorption, and precipitation reactions that control molybdenum concentrations and transport in the subsurface. In addition, the possibility of making time scenarios for future contamination development was investigated. The conclusion of this study is that the fate of molybdenum in the subsurface is determined by three processes: the precipitation of molybdenite, the precipitation of iron molybdate, and the adsorption of molybdate on iron oxyhydroxides in the subsurface. However, as essential information about the kinetics of the precipitation reactions is missing, no further conclusions regarding time and extent of these precipitation reactions can be done. Adsorption does occur, but values determining the degree of adsorption are taken from literature and not from data from the Botlek itself. Based on this study, a table of recommended measurements is made to do further research on the kinetics of these precipitation reactions, and measure the subsurface of the Botlek for a better insight in the groundwater composition and to obtain more information about the subsurface, its constituents and its relationship with adsorption.