Kill two birds with one stone: Solubilizing PAHs and activating PMS by photoresponsive surfactants for the cycle remediation of contaminated groundwater
Graphical abstract
Introduction
Polycyclic aromatic hydrocarbons (PAHs) bring considerable damage to the health of humans and other living organisms. PAHs are toxic, mutating carcinogens, remaining stable in the aquatic environment [1]. Immediate action must therefore be taken to eliminate PAHs from affected areas. Surfactant-enhanced remediation (SER), which can increase the efficiency of desorption and solubilization of PAH contaminants, is seen as a promising solution to this problem [2]. Yet the application of SER is hindered by some obvious drawbacks. On the one hand, the separation process of pollutants after solubilization is slow and costly; on the other hand, flushing agents can cause secondary contamination [3]. Specific methods of removing PAHs from surfactant solutions incorporate pervaporation, organic extraction, air stripping and precipitation, yet most are cumbersome and energy consuming [4], [5].
Reversible surfactants may solve this problem. Reversible surfactants allow the reuse of surfactants by regulating the dissolution and release of insoluble contaminants in water. External stimuli such as pH [6], light [7], temperature [8] and electrical potential [9] have been reported to stimulate the recycling of reversible surfactants. At present, some reversible surfactants have been applied to treat soil and groundwater contaminated with PAHs [9], [10], [11], [12]. However, the deactivated reversible surfactant still has the surface activity to solubilize PAHs, which leads to the high PAHs concentration in groundwater after remediation. In our previous studies, we synthesized a Gemini photoresponsive surfactant [13] and combined it with micro-nano bubbles (MNBs) [14] to further remove PAHs from contaminated groundwater. However, there were still 30% of PAHs remained in the groundwater. Further research is needed to completely remediate PAHs-contaminated groundwater.
To relieve the groundwater contamination led by PAHs, advanced oxidation processes (AOPs) (examples being electrochemical oxidation, ozone oxidation and photocatalytic oxidation) have been developed and applied broadly in industries [15]. In order to remove organic contaminants quickly and non-selectively, AOPs systems have generated highly reactive oxygen species (ROS) [16]. Peroxymonosulfate (PMS) is a strong oxidant (E0 = 1.82 V) and can get activated by heat, alkali, transition metals and electricity to degrade most organic pollutants [17]. Moreover, PMS is less harmful and more applicable for subsurface treatment, owning to its long lifetime in subsurface [18]. Owning to this, PMS has gained wide attention in the remediate of groundwater polluted by various emerging pollutants. Studies also report that halide ions can accelerate the degradation of contaminants by PMS-based oxidation/disinfection technology [19], [20], [21]. Last, chloride (Cl−) is proven a spur to the transformation of rhodamine B by PMS [19]. Zhou et al. find that the PMS-led degradation of steroid estrogens is significantly accelerated by the existence of chloride and bromide (Cl- and Br-) [22]. Therefore, surfactants containing Br- may activate persulfate while enhancing the solubility of PAHs, thereby achieving efficient removal of PAHs in groundwater. In the existing literature, the combination of surfactants, PMS and additional activation methods has shown great potential [23], [24], [25]. However, there is no report on the treatment of PAHs-contaminated groundwater by surfactant activated persulfate.
To achieve cost reduction and efficiency improvement in groundwater remediation, we, for the first time, apply photoresponsive surfactant (N1, N2-bis[4-[4-[(4-butylphenyl) azo] phenoxy] butyl]-N1, N2-tetramethylethane-1,2-diammonium bromide, AzoPBT) to activate PMS and subsequently remove PAHs in groundwater. The mechanism of the Br-/PMS process are explored through electron paramagnetic resonance (EPR) and radical quenching experiments. Based on the analysis of gas chromatography-mass spectrometry (GC–MS), we then propose possible degradation pathways of phenanthrene (Phe) and naphthalene (Nap). Next, to analyse the role of surfactant micelles in the oxidation process, we investigate the antioxidant capacity and surface properties of surfactants in the reaction. Finally, we explore the influence of the significant environmental factors (pH and inorganic salts) of natural groundwater on the removal of PAHs and examine the performance of this technology in removing PAHs from natural groundwater. Overall, the PMS/AzoPBT process effectively removes PAHs from groundwater because it both increases the solubility of PAHs and activates the PMS. It is therefore considered a promising method for remediating groundwater contaminated with PAHs.
Section snippets
Materials
Gemini photosensitive surfactant AzoPBT was synthesized in our previous research[13]. Under UV or visible-light irradiation, the azo group contained in AzoPBT undergoes photosensitive isomerization, resulting in changes in hydrophobicity (Fig. S1 and S2 in Supplemental Information). Phe (≥97%) and Nap (≥99%) were obtained from Adamas (Shanghai, China). PMS was purchased from Adamas (Shanghai, China, available as Oxone). HCl, NaOH, NaCl, Na2CO3, NaHCO3, Na2SO4, CaSO4, MgSO4, KCl, MeOH, EtOH, tert
Reaction process of PMS with AzoPBT, CTAB, NaBr and NaCl
Nap, with higher solubility in water than other PAHs, was used to test the activation ability of different halogen-containing substances to PMS. As shown in Fig. 1, different halogen-containing compounds (i.e., NaBr, CTAB, AzoPBT and NaCl) could activate PMS to degrade Nap and all reactions reached equilibrium at 60 min. The results show a minor loss of Nap (8.1 %) when only PMS was added. With the addition of NaCl and NaBr, the degradation of Nap in the solution increased significantly, which
Conclusion
For the treatment of PAHs in groundwater, the PMS/AzoPBT system was investigated. It was found that 1O2 made a significant contribution to the degradation of PAHs. According to the degradation products, a pathways of Phe degradation was proposed. Meanwhile, the role of surfactant micelles in PMS/AzoPBT system was further discussed. The positively charged micelles in the PMS/AzoPBT system promote the removal of PAHs by electrostatically polymerizing Br- and HSO5- in the solution. However, the
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
This work was supported by the National Key R&D Program of China (2019YFE0114900), the National Natural Science Foundation of China (42077175, 52270164), and “Shanghai Science and Technology Innovation Action Plan” Project (19230742400, 21230712100).
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