Desorption process and morphological analysis of real polycyclic aromatic hydrocarbons contaminated soil by the heterogemini surfactant and its mixed systems
Graphical abstract
Introduction
The soil environmental pollution incidents caused by polycyclic aromatic hydrocarbons (PAHs) often have occurred over the years, resulting in serious harm to the ecosystem and human health (Pies et al., 2007; Vogt and Richnow, 2014; Xue et al., 2016). As a group of hydrophobic organic compounds (HOCs), PAHs are almost pervasive even in the Polar Regions (Abakumov et al., 2015). Because of frequent anthropogenic activities, incomplete combustion of fossil fuels releases large amounts of PAHs into the environment and then soil, a reservoir for pollutants burdens most of them. In China, the emitted PAHs for this reason in 2006 reached about 32,720 tons. Among them, the coking plant is a contribution source which cannot be ignored (Wild and Jones, 1995; Trellu et al., 2016; Han et al., 2019). Today, many coking plants had to be relocated, catering to urbanization, left over brownfield sites to be reconstructed (Cao et al., 2019). However, the residual PAHs in sites have “three effect” (mutagenesis, carcinogenesis, teratogenesis) on human beings and are classified as priority pollutants by the US Environmental Protection Agency (EPA) and the European Community (Acimovic et al., 2017).
In natural condition, indigenous microorganisms in the soil can gradually break the benzene ring structure of PAHs through metabolism and enzymes, eventually degrade into small molecules with chain structure. Meanwhile for H-PAHs (High molecular weight PAHs, 5–6 rings), which is difficult to degrade, fused ring is broken firstly, then is converted into compounds with few benzene rings or even chain (Mandal and Das, 2018; Guntupalli et al., 2019; Torres-Farrada et al., 2019; Wang et al., 2019). Actually, due to high hydrophobicity of PAHs, biotic degradation is hard to occur in natural condition, especially M-PAHs (Medium molecular weight PAHs, 4 rings) and H-PAHs (Tunega et al., 2009; Zhao et al., 2019). Therefore, the treatment of abandoned coking sites contaminated by PAHs has caught a great number of attentions and become an urgent matter. The hydrophobicity will greatly limit the effect of traditional technology including chemical oxidation and bioremediation (Ranc et al., 2017; Subashchandrabose et al., 2017; Kottuparambil and Agusti, 2018). Although thermal desorption is a common technology of removing PAHs in contaminated site soil, it incurs high cost (Harmon et al., 2001). On the contrast, surfactant-enhanced remediation (SER) is cost-effective due to inexpensive surfactants and simple operation (Zhou et al., 2013; Liang et al., 2017). Surfactant whose special structure (a kind of amphipathicity compound with hydrophilic head and hydrophobic tail) competes strongly with soil organic matter (SOM) to incorporate PAHs by forming micelles when its concentration is greater than critical micelle concentration (CMC) and reducing interfacial tension (Paria, 2008; Laha et al., 2009; Pei et al., 2017). The formed micelle can bind to PAHs through hydrophobic interior, simultaneously hydrophilic exterior contacts aqueous phase, which increase their mobilization into solution (Chang et al., 2009; Elgh-Dalgren et al., 2009).
In the recent, Gemini surfactant, a new generation surfactant, is favored in HOCs desorption. Its typical structure can be thought of as the formation of two conventional surfactant molecules linked together by a chemical bond at or near the hydrophilic head group, of which there are at least two hydrophobic chains and two hydrophilic groups in the molecular structure. It has many unique properties, such as lower CMC (Tikariha et al., 2011; Kumar et al., 2013; Yadav et al., 2015). Furthermore, a novel generation Gemini surfactant possessing asymmetric topology (named as Heterogemini surfactant) was first synthesized in 1996 (Jaeger et al., 1996; Alami and Holmberg, 2003; Xu et al., 2019). This distinct spatial structure endows Heterogemini surfactants with more powerful self-assembly behavior owing to small micelle ionization, forming strings of micelles easily. Correspondingly, the morphology of micelles is more diverse and the micelles are more dispersed. (Xu et al., 2016, 2019; Yoshimura and Akiba, 2016; Zhou et al., 2016; Yoshimura and Nyuta, 2017; Gao et al., 2018; Mao et al., 2018). There are few related reports due to the difficulty in the synthesis of Heterogemini surfactants, let alone their application. Almost no research has used Heterogemini-traditional mixed surfactants to remediate real PAHs-contaminated soil collected from an abandoned coking plant which was planned for construction land.
As a consequence, we evaluated the feasibility of Heterogemini surfactant (Dodecyldimethylammonium bromide/tetradecyldimethylammonium bromide, DBTB) and two traditional surfactants (Hexadecyl trimethyl ammonium bromide, CTAB; Sorbitan monolaurate, Span 20) as well as their mixed surfactants for remediating real PAHs-contaminated soil. The interaction of mixed surfactants, the desorption of PAHs from contaminated soil and the distribution of PAHs in micelle-aqueous phase have been investigated, respectively. In addition, we also explained the difference in desorption of each surfactant from the perspective of surfactant micelle morphology.
Section snippets
Materials
The Heterogemini surfactant (Dodecyldimethylammonium bromide/tetradecyldimethylammonium bromide, DBTB), with a purity>99%, has obtained authorization use from the Dao Chun Chemical Technology Co. Ltd (Zhengzhou, China). Non-ionic surfactant, Sorbitan monolaurate (Span 20), was purchased from BASF Biotechnology Co. Ltd (Hefei, China). The cationic surfactant, Hexadecyl trimethyl ammonium bromide (CTAB), was purchased from Jinshan Chemical Reagent Co. Ltd (Chengdu, China). They are all synthetic
Critical micelle concentration of single/mixed surfactants
CMC is a crucial parameter to characterize surfactants and sudden changes in the properties of surfactant can be observed, around the CMC (Fluksman and Benny, 2019). With a more accurate surface tension method adopted in our study, the surface tension decreased with the concentration increasing, as shown in Fig. A2. The plot of surface tension (γ) showed that each surface tension curve has only one turning point, and the corresponding surfactant concentration is the CMC of the surfactant. The
Conclusion
Surfactants could promote the desorption capability of PAHs from SOM in contaminated soil. The desorption effect of DBTB, CTAB, Span 20 and their mixed systems at different ratios on real PAHs contaminated soil were studied. Among the single surfactants, the novel Heterogemini surfactant DBTB performed the best, the highest desorption rate for TPAHs 38.7%. The desorption rate for TPAHs of Span 20/DBTB at 3:7 could reach 68.83%, higher than CTAB/DBTB’s highest value of 57.18%. However, both
CRediT authorship contribution statement
Wei Wei: Writing - original draft. Zongxin Ran: Investigation. Huan He: Software. Kuan Zhou: Resources. Zhuoxi Huangfu: Validation. Jiang Yu: Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by National Key Research and Development Program (No. 2018YFC1802605), Sichuan Provincial Major Science and Technology Project (No. 19ZDZX011), Nature Science Foundation of Sichuan Province (No. 2017SZ0181), International Cooperation Project of Sichuan Province (No. 2019YFH1027). Also the authors thank to Xiangwei Wang, Jialu Yao and Sichuan metallurgical geological exploration bureau 605 brigade as well as Dao Chun Chemical Technology Co. Ltd for experiment support.
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