Sorption of pentachlorophenol on surficial sediments: The roles of metal oxides and organic materials with co-existed copper present

Abstract

The sorption characteristics of pentachlorophenol (PCP) in the surficial sediments were investigated using a selective extraction procedure. The results show that the Γ_max of PCP sorption decreased from 1.60 μmol g⁻¹ to 0.69 μmol g⁻¹ by approximately 60% after selective removal of organic materials from the sediments. The sorption of PCP in the sediments after selective removal of Mn oxides increased nearly up to 600% (from 1.60 μmol g⁻¹ to 11.11 μmol g⁻¹) and, to a less degree, the PCP sorption in the sediments after simultaneous removal of Fe/Mn oxides (Γ_max = 3.53 μmol g⁻¹). The analysis of the data using an additional model indicates that the contribution of Mn oxides to PCP sorption was negative, and Fe oxides and organic materials both have greater potential for sorption of PCP with less contribution from residues including Mn and Fe oxides in the residual fractions determined by a sequential extraction procedure and clay and silicate minerals. The differences in the decreased degrees of PCP sorption with increasing of Cu suggest that competition between Cu and PCP for sorption sites mainly takes place on Fe oxides.

Introduction

Pentachlorophenol (PCP) was once used worldwide as wood preservative, herbicide, insecticide and fungicide. It is a probable human carcinogen and has been designated as a priority pollutant by USEPA. In the 1970s, PCP was heavily used in China for fighting against schistosomiasis and as a herbicide (He et al., 2006). PCP has a slow biodegradation rate, and furthermore has a tendency to accumulate in soil and sediment, and may be a toxicity risk in contaminated aquatic or edaphic systems for a long period of time (Estevinho et al., 2006, He et al., 2006).

The surficial sediments are considered as main carriers and potential sources of both organic and inorganic pollutants in aquatic environments. The environmental fate of organic substances in aquatic system, especially in sediment, depends primarily on their solubility and physical–chemical factors affecting the partition between the solid and soluble phases (Esteves da Silva and Marques, 2007). The sorption properties of chlorophenol are controlled largely by its degree of substitution and hydrophobicity (Divincenzo and Sparks, 1997). In normal water and sediment in the northeastern river in China (pH 6–9), PCP can be totally or partially ionized as PCP is a hydrophobic ionizable organic compound with a pK_a of 4.75, affecting its solubility, sorption, transport and bioavailability (Divincenzo and Sparks, 1997, Divincenzo and Sparks, 2001). For example, when the pH of river water is 6.75, the PCP existed as ionized form reaches 99% of the total PCP in the water, and PCP sorption can be well described by Langmuir isotherm when PCP is mainly existed as ionized form or linear isotherm when PCP is mainly existed as nonionic form (Divincenzo and Sparks, 2001).

Meanwhile, the physical, chemical, and biological characteristics of the sediment also influence the fate and transport of PCP (Cea et al., 2007). Sediment particles contain minerals (iron oxides, manganese oxides, and clay minerals) and organic materials, which consist of different domains that play different roles in the sorption of organic contaminants (Li and Werth, 2001). To date, there are many studies on the roles of sediment components, but they mainly focus on the organic materials because most studies indicated that organic materials have been identified as predominant sorbents unless the content of organic carbon is below 0.1% by mass (Kile et al., 1995, Wang et al., 2005, He et al., 2006). However, the other components except for organic materials may also indirectly contribute to the sorption of organic pollutants although there is no direct contribution existed, because the interactions between the various sediment components generally take place. For instance, both organic matter and metal oxyhydroxides may coat surfaces of silicate minerals changing their affinity to organic pollutants (Laird et al., 1994). Researches on the sorption behaviors of PCP in the sediment components, including not only organic materials, but also iron oxides, manganese oxides, and clay minerals, in other words, the studies on the relative contributions of sediment components to the sorption of PCP, especially in the presence of co-existed pollutants are rarely reported. The purpose of the present paper is to investigate the sorption characteristics of PCP in the surficial bulk sediments and sediments after selective removal of organic materials, Mn oxides, and Fe/Mn oxides in a unitary solution of PCP or in a binary solution containing PCP and Cu, using a selective extraction procedure, and to understand the relative contributions of the various sediment components to PCP sorption and the interactions between sediment components that influence PCP sorption.