Mobility of heavy metals after spiking in relation to sediment and metal properties: leaching column study

Abstract

Sorption, desorption, and leaching of metals in the sediments of the aquatic environment are paramount important in their bioavailability. Also, behavior of these metals in such sediments as the riverbed sediments is affected by the characteristics of the sediments such as grain size, pore size distribution, and pH of the medium. Four riverbed sediments with varying properties were artificially contaminated with cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn). Then, using leaching columns, they all were subjected to drying and wetting phases to study the competitive binding and release of metals. Sediment samples were collected from 0 to 10 cm depth of Abshineh and Ghareh Chai River during the dry season. Samples were spiked with 400 mg Zn kg−1, 200 mg Cu kg−1, 200 mg Ni kg−1, and 50 mg Cd kg−1 (dry weight equivalent) in plastic container and then after drying seated at a height of 10 cm in leaching column. Sediment columns were incubated at room temperature (22–24 °C) and leached with 10–20 pore volumes (PVs) with distilled water. The distribution of metals over different extractable fractions was evaluated before leaching. The four metals were separated into two classes according to their leachability. In the sediment columns that contained more clay, commonly mobile metals like Zn, Ni, and Cd that appeared to be rather immobile and commonly immobile metals like Cu were unexpectedly quite mobile. The observation that copper leachability was higher for the sediments with finer grain size and higher organic matter content indicates that pore size distribution, the presence of organically complexes, and colloidal particles, may have had major role in this mobility behavior of copper. The mobility of Zn and Cd are strongly governed by pH, where the mobility of Cd is typically much greater than Zn under acidic conditions. The metal addition caused the displacement of Ca, Mg, K, and Na their subsequent leachate from the spiked sediment columns. Despite the spiking of inorganic metals into the sediments and drying before leaching, the mobilization of metals with distilled water (to mimic rainwater) was small in the different sediments relative to the total amounts of each metal added. This would indicate that the major part of the added metals were likely to be bound in forms that exhibit a low potential for release to the environment and low bioavailability to organisms.