Interactions between electrokinetics and rhizoremediation on the remediation of crude oil-contaminated soil

Highlights

• An electrokinetics-enhanced phytoremediation (EK-R) was constructed to remediate crude oil-contaminated soil.

• Removal of TPH and microbial activity were increased by use of EK-R.

• Absolute and relative numbers of bacteria in bulk soil of EK-R were similar to that in rhizosphere soil.

Abstract

An electrokinetics (EK)-enhanced phytoremediation system with ryegrass was constructed to remediate crude oil-polluted soil. The four treatments employed in this study included (1) without EK or ryegrass (CK-NR), (2) EK only (EK-NR), (3) ryegrass only (CK-R), and (4) EK and ryegrass (EK-R). After 30d of ryegrass growth, EK at 1.0 V·cm⁻¹ with polarity reversal (PR-EK) was supplied for another 30 d. The electric current was recorded during remediation. The pH, electrical conductivity, total petroleum hydrocarbon content (TPH), 16S rDNA, functional genes of AlkB, Nah, and Phe, DGGE, and dehydrogenase activity in soil were measured. The physical-chemical indexes of the plant included the length, dry mass, and chlorophyll contents of the ryegrass. Results showed that EK-R removed 18.53 ± 0.53% of TPH, which was higher than that of other treatments (13.34–14.31%). Meanwhile, the values of 16S rDNA, AlkB, Nah, Phe, and dehydrogenase activity in the bulk soil of EK-R all increased. Further clustering analysis with numbers of genes and DGGE demonstrated that EK-R was similar to the ryegrass rhizosphere soils in both EK-R and CK-R, while the EK treatment of EK-NR was similar to that of CK-NR without EK and ryegrass. These results indicate that the PR-EK treatment used in this experiment successfully enlarged the existing scale of the rhizosphere microorganisms, improved microbial activity and enhanced degradation of TPH.

Introduction

As petroleum hydrocarbons and their derivatives are important in the modern world, crude oil extraction, transport, refinement, and utilization are increasing. Concomitantly, the occurrence of accidental spillages has also increased, such that water, soil, and sediments have become contaminated, which can adversely affect both the environment and human health (Varjani and Upasani, 2017). Various techniques have been applied to remediate soils contaminated with oil, including the extraction of oil with solvents, chemical oxidation, thermal technologies, ultrasonication, and flotation with density gradients. As they are both cost-effective and “environmentally friendly”, less active methods of natural attenuation, including phytoremediation, have been applied to pollution hotspots (Feng et al., 2017).

Phytoremediation is defined as a “green technology” that uses plants and the associated microorganisms in their rhizosphere (roots) to degrade organic pollutants in soil (Feng et al., 2017). In this system, various exudates released by plants, such as carbohydrates, amino acids, organic acid anions, and secondary metabolites, provide substrates for bacteria that can then either internally or externally degrade petroleum hydrocarbons by co-metabolism (Martin et al., 2014; Sasse et al., 2018). These exudates can serve as carbon or energy sources, or provide micronutrients that support and/or enhance the growth and activity of microorganisms in the rhizosphere, and also activate functional genes that result in the co-metabolism of organic pollutants, including petroleum hydrocarbons (Martin et al., 2014; Sasse et al., 2018). Specific microbial communities associated with the rhizospheres of plants that can degrade chemicals that are potentially toxic to plants or animals have evolved, and can be used in phytoremediation to reduce the concentrations of pollutants (Alagić et al., 2015). The results of several studies have confirmed that phytoremediation could enhance the degradation of organic pollutants (Acosta-Santoyo et al., 2017; Ancona et al., 2017). However, in soils, the scale of this is limited and restricted to areas near the rhizospheres of plants. This, combined with the movement of exudates by passive diffusion, which is slow, and the immobility of microbes in soils, has restricted the effectiveness of phytoremediation for practical application to large-scale contamination (Corgie et al., 2004; Tu et al., 2017). To address these limitations, plants have been seeded to increase the density of the rhizosphere in a certain area, or nutrients have been added to soils to provide nitrogen, which often limits the growth of microbes (Jagtap et al., 2014; Liu et al., 2014). Furthermore, in some cases, specific microbes that can degrade oil have been added to soils, or soils have been tilled back into local soils to move microbes or provide aerobic environments (Tang et al., 2010). These amendments have increased the rates of degradation, but each has limitations. The addition of a nitrogen fertilizer can result in the formation of ammonia, which can be toxic to invertebrates in soils and sediments. Further, the benefits of adding specific microbes normally are typically only short-term due to their inability to compete with natural soil microflora or their loss of genetically modified capabilities through hybridization. Finally, these active augmentations of bioremediation by bacteria are time-consuming and costly. An alternative, less active, and less costly method of facilitating the diffusion of rhizosphere microorganisms from the rhizosphere zone into bulk soil to overcome the slow nature of this process is required.

By imposing weak electric fields on contaminated soils through electro-kinetic phenomena, such as electro-osmosis, electro-migration, and/or electrophoresis, electro-kinetic (EK) technologies can facilitate the movement of water, metals, microorganisms, terminal electron acceptors, and nutrients, as well as organic pollutants, through soils to promote degradation (Lima et al., 2017). This movement of materials is especially attractive when combined with other methods to maximize the effectiveness of those for degradation and/or the removal of contaminants, such as total petroleum hydrocarbon (TPH), as such a combination can overcome the inherent limitations of the formation of depleted zones due to slow diffusion (Lima et al., 2017). The results of previous studies that have used EK in combination with other bioremediation methods, including bio-stimulation and bio-augmentation, have improved the effectiveness of remediation (Wang et al., 2013; Hassan et al., 2017; Zhang et al., 2017). In those studies, EK mainly increased the bioavailability of organic pollutants by facilitating contact between microbes and nutrients/pollutants, however, the weak electric current may have also directly stimulated microbial activity (Kim et al., 2010; Velasco-Alvarez et al., 2011) or degraded some of the pollutants through an electrolytic reaction (Acimovic et al., 2017). EK was also successfully combined with phytoremediation to remove metals from soils (Jamari et al., 2014; Acosta-Santoyo et al., 2017; Luo et al., 2018a). In these studies, EK not only moved metals to plants, but may have also stimulated their growth and respiration.

Phytoremediation for treating organic polluted soils has been studied previously and combined with EK for the removal of metals (Cameselle et al., 2013; Putra et al., 2013; Jamari et al., 2014; Acosta-Santoyo et al., 2017). However, to our knowledge, there has been no previous research into joint effects of EK and phytoremediation for treatment of soils contaminated with organic pollutants. Based on previous studies, this combination is hypothesized to not only increase the mobility of contaminants and nutrients, but also enlarge the effective biodegradation zone by diffusing rhizosphere microorganisms into bulk soil to overcome the inherent limitations of phytoremediation. In this way, EK could improve both the spatial scale and speed of remediation by plants.

Thus, an EK-enhanced phytoremediation system was constructed for remediating crude oil-polluted soils. The effects of EK on remediation, as well as the growth of plants and responses of the microbial communities in bulk soil, were studied after 30 d of incubation.