RESUMO
The Hazaribagh industrial area in Dhaka city, the capital of Bangladesh, is considered one of the hotspots of chromium (Cr) pollution, due to excessive discharge of Cr contaminated waste over decades by approximately 150 tanneries. In 2000, elevated Cr concentrations were observed in the underlying Dupi Tila Aquifer (DTA), which is heavily deployed for drinking water supply of Dhaka city's population. In the following years, Cr concentrations in the DTA have dropped and apparently stayed low. In 2010, elevated Cr concentrations were found again in the DTA. This study aims to evaluate the status of the total Cr contamination in the surface waters, groundwater, and soils in the area, to clarify the temporal evolution of the total Cr contamination pattern in the DTA. For this, we collected water and soil samples in 2012, 2013, 2014, and 2019 for (hydro)chemical characterization and analyzed new groundwater level data on the development of the cone of depression below the city. Our study indicates that the temporal evolution of the total Cr contamination in the DTA is closely coupled to the groundwater dynamics. The rapid growth of the cone of depression due to excessive pumping resulted (i) in a disconnection of the groundwater table from the heavily contaminated Hazaribagh soils, and (ii) in an increased gradient between the contaminated surface waters and the groundwater, increasing infiltration into the groundwater. Finally, (iii) the further growth of the cone of depression resulted in an inflow of fresh groundwater from the west of Dhaka city, causing a dilution effect. Although in 2017 tanneries were moved out of the Hazaribagh area, the contaminated soils still pose a threat to groundwater quality when groundwater levels would recover.
RESUMO
The efficacy of copper shavings (Cu(0)) for the removal of Hg2+ from aqueous solution by amalgamation is demonstrated. Two kinds of copper shavings were investigated: (a) chemically processed shavings (Fluka) and (b) recycled shavings from scrap metal. Batch sorption experiments yielded very high retardation coefficients of 28 850-82 830 for the concentration range studied (1-10 000 microg/L Hg2+ dissolved in distilled water or in a 0.01 M CaCl2 matrix solution). Sorption data were well-described bythe Freundlich isotherm equation. Kinetic batch sorption experiments showed that 96-98% of Hg2+ was removed within 2 h. Column experiments were performed with a mercury solution containing 1000 microg/L Hg in a 0.01 M CaCl2 matrix with a flow rate of 0.5 m/d. No mercury breakthrough (c/c(0) = 0.5) could be detected after more than 2300 percolated pore volumes, and the high retardation coefficients determined in the batch studies could be confirmed. Copper was released from the shavings due to the amalgamation process and to copper corrosion by oxygen, resulting in concentrations of mobilized copper of 0.2-0.6 mg/L. Due to their high efficiency in removing Hg2+ from aqueous solution, the use of copper shavings for the removal of mercury from contaminated water is suggested, employing a sequential system of mercury amalgamation followed by the removal of mobilized copper by an ion exchanger such as zeolites. Possible applications could be in environmental technologies such as wastewater treatment or permeable reactive barriers for in situ groundwater remediation.