Clay mineral dependent desorption of pyrene from soils by single and mixed anionic–nonionic surfactants
Graphical abstract
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are considered as priority pollutants that pose serious health and environmental threats [1], [2], based on the carcinogenic, mutagenic and teratogenic properties of certain PAHs [3], [4], [5], [6], [7]. Moreover, such compounds are likely to deposit on the soils because of their poor water solubility [8]. The contamination of soils by PAHs have been recognized as a major environmental problem [9], [10], [11].
Various physical, chemical, biological, and their combined technologies have been attempted to remediate PAHs-contaminated soils [12], [13]. Surfactant enhanced remediation (SER) has emerged as a promising option for the removal of sorbed PAHs because of its improved desorption effectiveness [14], [15], [16], [17], [18]. The hydrophobic PAHs can be desorbed by being partitioned into the hydrophobic cores of surfactant micelles above their critical desorption concentrations [17], [19]. The aqueous solubility of the compounds increases with increasing numbers of micelles formed by the surfactants in the soil–water–surfactant system [20].
In some cases, a single surfactant can sorb on the surface of the soil, effectively lowering its ability to desorb PAHs [17]. To address this limitation, a mixture of anionic–nonionic surfactants has been investigated in a number of scientific and industrial applications [15], [21]. Compared with single surfactants, the mixtures of anionic and nonionic surfactants form mixed micelles that enhance desorption of PAHs [17], [22], [23], [24] because addition of nonionic surfactants to anionic surfactant solutions can decrease the precipitations that can occur between anionic surfactants and divalent electrolytes (e.g. Ca2+, Mg2+). Also, sorption of nonionic surfactants in soil can be reduced by the presence of anionic surfactants [17], [25], leading to formation of more micelles and, hence, more pollutant removal [26], [27]. Consequently, in the SER of contaminated soils, anionic-nonionic mixed surfactants may be more effective than single surfactants alone [16], [17], [28], [29], [30], [31].
Clay minerals (e.g. montmorillonite and kaolin) are important components of soils and sediments. Many have strong sorption affinities, large specific surface areas and high cation exchange capacities (CECs). The mineralogical composition of the soil clay fraction influences the sorption of surfactants. For instance, in recent work [32] researchers have found that sorption of SDBS to montmorillonite is similar to that observed with kaolin. However, the sorption of TX-100 to montmorillonite is much greater than it is to kaolin [33]. Therefore, the relative sorption of TX-100 and SDBS on clay minerals may play a significant role in controlling the desorption of PAHs from clay-containing soils. Although the use of mixed surfactants has received considerable attention in the literature, to our knowledge the impact of using mixed surfactants in desorption of PAHs from soils containing various clay mineral compositions has not been reported.
Pyrene was chosen as a model PAHs in this study because of its abundance and hydrophobicity in the polluted soils [34], [35]. Montmorillonite and kaolin were used to represent the mineral clays that are found in soils of Guangdong, China [36]. The surfactants SDBS and TX-100 were selected because it has reported that they effectively remove hydrophobic pollutants from soils [17]. The aims of this study were to investigate (1) the performance of single and mixed anionic–nonionic surfactants in SER of artificially pyrene-contaminated soils with varying montmorillonite and kaolin compositions in Guangdong, China and (2) the distribution of TX-100 and SDBS during desorption of pyrene from these soils.
Section snippets
Chemicals and clays
Pyrene with a purity greater than 98% was obtained from Aldrich Chemical Company. The nonionic surfactant t-octylphenoxypolyethoxyethanol (TX-100) and sodium dodecyl benzene sulfonate (SDBS) with purity of 99.9% were obtained from Sigma Chemical Company. Both surfactants were used without further purification. The properties and molecular structures of pyrene, TX-100 and SDBS are given in Table S1 and Fig. S1 (Supplementary Material). Two standard clay minerals, Ca-montmorillonite (purchased
Physicochemical characteristics of the soils and the clays
As shown in Table 1, the clay minerals in soil A and soil B were dominated by kaolin, and the clay minerals in soil C were dominated by montmorillonite. As the content of the montmorillonite in soil increased, the BET and CEC values increased (Table 2). Wang’s study [42] shows that the sorption capacity for TX-100 was highly related to CEC with a correlation coefficient of 0.9. With different CEC values montmorillonite-rich soil (soil C) and kaolin-rich soil (soil A and soil B) may have a
Conclusions
Understanding the effect of clay minerals on SER within soil–water–surfactant environment is a key to improving soil washing effectiveness. This study investigated pyrene desorption from soils and pure clay minerals using single or mixed anionic–nonionic surfactants. The main conclusions are as follows:
- (1)
Desorption of pyrene from kaolin and kaolin-rich soil was well described by a first-order two-compartment model and a simple first-order model, respectively.
- (2)
Increasing temperature and shaking
Acknowledgments
This study was financially supported by the National High Technology Research and Development Program of China (2012AA101403), Guangzhou Science and Information Project (No. 2011J2200064), the National Natural Science Foundation of China (41101291), and Ministry of Science and Technology of China (2012AA06A203). We thank our visiting professor Dr. Donald Barnes for providing valuable comments on the manuscript.
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