Research Article: Metal removal and associated binding fraction transformation in contaminated river sediment washed by different types of agents

Date Published: March 28, 2017

Publisher: Public Library of Science

Author(s): Hong Wang, Tongzhou Liu, Shuai Feng, Weihua Zhang, Manuel Joaquín Reigosa.

http://doi.org/10.1371/journal.pone.0174571

Abstract

In ex-situ washing, HCl, EDTA and H2O2 solutions can effectively extract heavy metals in river sediment. Nevertheless they often target different sediment components, possibly transforming metal species into more bioavailable and hence toxic ones. This study, in batch settings, investigated the influences of different types of washing agents (i.e. HCl, EDTA and H2O2) on metal (i.e. Cu and Zn) removal from contaminated river sediment, destroy or dissolution of sediment components, and transformation of metal fractions during chemical washing treatment. Additionally, bioavailability of these metals left in the washed sediment was assessed. Results showed that HCl obtained the highest Cu and Zn removal through destroying the reducible, oxidizable and residual sediment components. Meanwhile, it transformed metal fractions to acid extractable one, resulting in an increase in metal bioavailability. Thus, the feasibility of washing with HCl for sediment remediation shall be reconsidered due to the caused high metal bioavailability. EDTA was capable of removing metals via direct complexation of labile metal species and indirect dissolution of reducible and oxidizable sediment components, where the transformation of corresponding metal binding fraction may occur. H2O2 obtained the lowest total Cu and Zn removal, but it preferentially removed the oxidizable metal species by oxidizing sulfides in the sediment. The bioavailable levels of Cu and Zn in the sediment washed by EDTA or H2O2 seemed not increase. To maintain a good balance between labile metal species removal and avoiding increase of metal bioavailability, EDTA and H2O2 are promising additives for metal removal by sediment washing.

Partial Text

Heavy metal contaminated sediment in rivers those are subject to anthropogenic discharges is a major problem from environmental perspective. Considerable heavy metal pollutants initially generated from manufacturing activities, have been discharged or leaked into the surface waters in the Pearl River Delta of South China. Majority of these heavy metals are first retained onto suspended solids, and then deposited on the river bed, leading to heavy metal enrichment in river sediment. When river environment is disturbed by water currents change (e.g. flooding or tidal intrusion) and anthropogenic disturbance (e.g. river channel maintenance dredging) sediment re-suspension would occur and heavy metals in sediment can release back into the overlying water [1], threatening the aquatic biota. Therefore, it’s necessary to develop cost-effective techniques to clean up the heavy metal contaminated sediment.

This study investigated Cu and Zn removal performance in river sediment by ex situ washing with three types of agents, i.e. HCl, EDTA, and H2O2. The associated potential metal fraction transformation was examined. The bioavailability of these metals left in the washed sediment was also assessed. HCl obtained the highest Cu and Zn removal through destroying the reducible, oxidizable and residual sediment components. Meanwhile it apparently transformed metal fractions to acid extractable one, resulting in an increase of metal bioavailability. Therefore, the feasibility of washing with HCl for sediment remediation should be reconsidered due to the caused high metal bioavailability. EDTA was capable of removing metals via direct complexation of labile metal species and indirect dissolution of reducible and oxidizable sediment components, where the transformation of corresponding metal binding fraction may occur. The bioavailable Cu and Zn levels in the sediment washed by EDTA seemed not increase. H2O2 obtained the lowest total Cu and Zn removal, but it preferentially removed the oxidizable metal species by oxidizing sulfides in the sediment. To maintain a good balance between labile metal species removal and avoiding increase of metal bioavailability, EDTA and H2O2 are promising additives for metal removal by sediment washing. Besides the types of washing agents employed, the influences of washing duration as well as the aging effect after washing also necessitate further investigations.

 

Source:

http://doi.org/10.1371/journal.pone.0174571

 

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