Adsorption-desorption of trace elements in sediments of the Maipo river basin

Abstract

We studied the adsorption-desorption of Cu, Mn, Pb and Zn in Maipo river basin, one of the most perturbed rivers in Chile, and thus determined its tolerance to the addition of these metals into the sediments and the possibility of transferring the metals from the sediments to the water column. Sediments were sampled from six sites from the river basin and characterized by their pH, electrical conductivity, organic carbon, total organic carbon, water soluble phosphorus and texture. Batch sorption and kinetics experiments were conducted to obtain the retained amounts onto the sediment samples. The initial concentration of trace elements in the sediment was quantified by AAS. The metals were present in most of the sites. Sorption parameters such as Kd and Koc were determined. Metals were adsorbed in the sediment, with Mn showing the lowest affinity, Kd and Koc values and Freundlich constant (nf) support these results. Desorption less than 1% were observed for Cu, while Pb and Zn ranged from 0.6% to 6.8% and those for Mn ranged from 0% to 17.6% across sites. The tolerance of the Maipo River to metal adsorption in sediments was as follows: Pb> Cu> Zn >Mn. Desorption analysis showed that both Zn and Mn could be transferred from the sediment to the water column. The strongest correlation occurs between the pH and Kd-Pb, indicating that at higher pH values, the Pb adsorption was unfavorable, while the correlation was moderate for Cu, Mn and Zn. On the other hand, there are positive correlations between pairs of metals such as Mn-Cu, Zn-Cu, and Zn-Mn, indicating the competition of these metals for the adsorbent sites. The high concentration of Mn found in the sediment and the possibility of being transferred to the water column, due to its easy of desorption, suggest concern due to its potential risk to aquatic biota.