ReviewElectrokinetic techniques, their enhancement techniques and composite techniques with other processes for persistent organic pollutants remediation in soil: A review
Graphical abstract
Introduction
Persistent organic pollutants (POPs) are toxic substances that are highly resistant to environmental degradation, which have become an important part of soil pollution [107], [115], which have photolysis resistance, chemical decomposition and biological degradation [6]. Polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are POPs commonly encountered in the terrestrial environment [101]. POPs have four characteristics including high toxicity (some POPs are endocrine disruptors and may be neurotoxic or immunosuppressive [100]), persistent biological accumulation (because of its lipophilic nature, contaminated food is easy to accumulate in fat tissue in the body when ingested by humans or animals [43]), long distance migration (POPs can be transported in the atmosphere over short and long distances in gaseous or particulate forms [22]), and persistence [44]. This storage in fat tissue allows compounds to be preserved in living organisms, which have a low metabolic rate. The n-octyl alcohol/water distribution coefficient (Kow) is the ratio of the concentration of a chemical in the n-octyl to the water phase, which is a good indicator of the lipophophilicity of many POPs. For example, the Log Kow value of chlordane is 5.8, hexachlorobenzene is 5.5, and methoxide is 5.08 (The data can be easily found in the database.) POPs are lipophilic chemicals that can be bioaccumulated in fat-rich tissues of the human body through biophospholipid membranes [82]. As a result, POPs can build up in the food chain [4]. Many studies have shown that many parts of the world today are contaminated with POPs. Yadav et al. showed that every environmental area in India is contaminated with persistent organic pesticides above the maximum residue limit [109]. Li et al. systematically analyzed the concentration of POPs in the surface soil of YanShan petrochemical district in Beijing [46]. The concentration of ∑PAHs ranged from 35.29 to 6120.22 ng/g with the mean concentration at 906.11 ng/g, ∑Organochlorine Pesticides (OCPs) varied from 4.13 to 7215.24 ng/g with a mean of 336.13 ng/g, and ∑PCBs ranged from 2.21 to 4008.47 ng/g with a mean of 486.12 ng/g. For these reasons, the study of POPs remediation techniques has become an important topic.
Various techniques have been widely investigated for POPs remediation in soil, including physical remediation, chemical remediation, bioremediation and united technologies [90]. Recently, electrokinetic (EK) remediation has proven to be a promising technology for the remediation of organic pollution in soil, especially suitable for remediation of low permeability soil [60], [74], [108]. The initial EK technology was mainly aimed at heavy metal pollution and successful in this field, but with the emergence of organic pollutants, EK technology began to be applied in this field [11], [37], [48]. The implementation of EK remediation involves inserting electrodes into the soil to surround the contaminated area and applying a low potential on the electrode, which induces the migration of pollutants to the electrode through the main transport processes of electromigration, electroosmosis and electrophoresis [65], [76]. The efficiency of EK methods depends on the molecular size of the pollutant, ion mobility, pollutant concentration and type, their solubility in a given soil, their charge, total ion concentration, their location and form in the soil, and the availability of organic matter in the soil [10].
EK remediation technology has been successfully used to remediate different types of soils and waste materials [32]. In Souza et al.’s study, the removal of pesticides (2,4-dichlorophenoxyacetic acid, 2,4-d) during a 15-day treatment process using electrokinetic soil flushing by direct current or solar panels was investigated [92]. After a 15-day long test, the removal rate of 2,4-d reached 90.2% and 73.6% respectively in dc power supply and solar panel power supply. Li et al. selected three commonly used soils of tetracycline (TCs) (oxomycin, chlortetracycline, and tetracycline) [47]. After 7 days of EK remediation, the average removal rate of TCs in different treatments was between 25 and 48%. The contribution of electrokinetics to TCs removal was 22–84%. Compared to traditional chemical treatment and biological treatment, EK has the following advantages: (i) good cost-effectiveness [56]; (ii) wide application range (in situ and ectopic restoration, available with saturated or unsaturated soil) [56], [116]; (iii) contact with less harmful substances, strong controllability, and generally fast and thorough treatment [35]. (iv) No damage to the original natural ecological environment (only deal with the pollutants between cathode and anode, without any other impact on the environment) [9]. However, the technology also needs to overcome some limitations, such as the low solubility of many organic compounds in water [8], and the inability to fully release organic pollutants attached to clay particles and organisms in soil and sediments [30]. EK technology will produce electrolysis of water during operation. In this process, the cathode produces OH− and the anode produces H+, which may lead to acidification and alkalization of the soil near the electrode [91]. High energy consumption and low mass transfer efficiency caused by side effects are also issues that need to be paid attention to in EK technology [106]. In view of this, the traditional EK remediation technology has been unable to meet the current needs, which gives birth to the enhancement technology and coupling technology of EK remediation technology. EK remediation technology is not only limited to the remediation of soil pollutants, but also applicable to the water environment, but the part of the water environment is beyond the scope of this paper.
In this review, we focused on the current development and application of EK remediation technology in soil POPs remediation. Firstly, we introduced the current situation of POPs pollution in soil. Secondly, we introduced the application of EK remediation technologies in the treatment of POPs. Thirdly, we specifically investigated the prospects of EK remediation for the treatment of POPs polluted soil, as well as some of its enhancement and coupling techniques. Finally, the conclusions and future challenges were presented in Fig. 1.
Section snippets
EK remediation of persistent organic pollutant
The major drivers of EK techniques for pollutant removal include electromigration, electrophoresis, and electroosmosis, as well as the movement of dissolved pollutants in pore water [98]. Electromigration and electrophoresis refers to the movement of charged particles toward an electrode with opposite charges, while electroosmosis is the movement of pore fluids in an electric field caused by surface charges, usually from the anode to the cathode, to enhance pollutant removal [3], [39], [87].
Enhanced EKs
As the problem of POPs pollution in soil becomes more and more serious, EK technology alone cannot meet the current treatment needs. The common EK enhancement techniques include controlling soil pH and increasing the solubility of pollutants.
EK coupling with other remediation technologies
At present, there are many methods for remediation of soil organic pollutants. Since it is easy to limit the scope of these methods by using them alone, the combined technique is the focus of current research. Combining EK remediation technology with other technologies can overcome the disadvantages of using EK remediation technology alone, improve the remediation efficiency and reduce the remediation cost. In this paper, we discuss the coupling of EK remediation technology with some other
Conclusions and perspectives
EK remediation technology is a new kind of remediation technology with low cost, wide application range, little contact with harmful substances, strong controllability, rapid and thorough treatment. At the same time, EK technology also has some shortcomings at present. It has a broad application prospect in the field of soil POPs remediation. This technology is affected by factors such as soil pH, soil and pollutant particles. In order to reduce the influence of these factors, enhancement
Conflict of 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.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The study was financially supported by the National Natural Science Foundation of China (Grant No. 51709103), National Key R&D Projects of China (Grant No. 2016YFC0403002), Natural Science Foundation of Hunan Province, China (Grant No. 2018JJ3242), China Postdoctoral Science Foundation (Grant No. 2018M630901), Training Program for Excellent Young Innovators of Changsha (Grant No. kq1802020), Hong Kong Scholars Program (Grant No. XJ2018029).
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