Laboratory investigation of transport and treatment of chromium in groundwater at Liwa district, Abu Dhabi

Abstract

Groundwater quality is a critical aspect affecting utilization of this source for certain purposes. In the UAE, utilization of groundwater for domestic purposes is well practiced. While the quality of groundwater at some locations does not permit direct utilization of this source, groundwater, in many cases, is mixed with desalinated water at certain proportion before being disinfected and then distributed to customers. Groundwater of Liwa aquifer contains high level of Cr(VI). It is believed that this contaminant originates from natural sources, but ittle has been done to investigate the fate of this contaminant. Spreading of chromium to nearby aquifers may render the quality of these aquifers un-suitable for certain uses. Furthermore, groundwater in Liwa district may not be suitable for drinking purposes even when mixed with desalinated water. It was, therefore, necessary to understand the impact of different transport mechanisms including advection, dispersion, and soil interaction on the transport of Cr(VI) in groundwater at Liwa district. This is important for the success of future measures to decontaminate groundwater in Liwa area. Another objective of this study was to investigate removal of Cr(VI) using activated carbon and metal iron. Several batch and column studies were conducted to investigate the transport of Cr in Liwa soil. Batch studies revealed that Liwa soil has low sorption capacity to Cr(VI) and higher sorption capacity to Cr(III). Sorption distribution coefficients for Cr(VI) determined from column experiments were more reliable than results obtained using batch tubes since the latter method is not sensitive to chemicals that are slightly sorbed to solids. Miscible displacement experiments conducted using Cr(VI) on columns packed with Liwa soil suggested that differences in the initial concentration used or differences in the applied flow rates employed do not result in a significant impact on the shape of the breakthrough curve. Normalized effluent Cr(VI) concentration along with the corresponding values of the pore volume (PV) were utilized to determine the value of the hydrodynamic dispersion coefficient (D) and the retardation factor and (R) using CXTFIT2.0. The model was further used to simulate Cr transport under different conditions. In conclusion, columns packed with Liwa soil have dispersivity of 0.4 cm and a retardation coefficient for Cr(VI) of about 1.5. Reduction of Cr(VI) using iron was investigated under different pH values. Addition of iron to a solution containing Cr(VI) caused the disappearance of the chemical. It was also found that as the pH is reduced from 7 to 4, higher removal occurred with almost complete disappearance of Cr(VI) from solution at pH 4 after 2 hrs of contact time.The rate of change of Cr(VI) and total Cr concentration with the use of granular activated carbon (GAC) at different pH values was investigated. The results of the experiments showed that the concentration of Cr(VI) decreases with time in the presence of GAC, with more disappearance from solution as the pH drops. In both the carbon and the iron experiment, the actual mechanism of Cr(VI) disappearance from solution is not clear.