The effects of different electrode materials on seed germination of Solanum nigrum L. and its Cd accumulation in soil

Abstract

The effects of different electrode on Solanum nigrum L. seed germination were determined. The result showed that germination percentage (GP) of seeds in treatment T2 (titanium electrode) was 26.6% higher than in control (CK, without electric field). High potassium and calcium concentrations were beneficial for seed enzymatic activity in treatment T2, which could partly explain the increase in GP. Cd accumulation (μg/pot) in S. nigrum treated with any electric field was significantly higher (p<0.05) than in CK without electric field. Specifically, Cd accumulation under the treatment T3 (stainless steel electrode) was the highest both in roots and shoots; this accumulation in shoots and roots were 74.7 % and 67.4 % higher for stainless steel than in CK. This increase must have been associated with a higher Cd concentration in plants and did not exert a significant effect on the biomass. In particular, Cd concentrations in roots and shoots under stainless steel treatment were both significantly higher than in CK (p<0.05), which had to be related to the higher available Cd concentration in the soil in the middle region. Furthermore, it could be attributed to altered soil pH and other soil properties. Moreover, none of the biomasses were significantly affected (p<0.05) by different electrode materials compared to CK.

Introduction

Soil pollution has currently become a worldwide problem, and the issue is becoming increasingly serious due to the uncontrolled rapid industrialization and expanding population (Xu et al., 2020a). Heavy metal contamination is one of the most important forms of soil pollution that has a huge impact on human health and ecosystem (Raklami et al., 2021). And cadmium (Cd) is one of the most hazardous soil pollutants among heavy metals (Yang et al., 2020). Although many research studies have been conducted in recent years and various technologies have been developed, e.g. immobilization, soil washing or electric remediation (Han et al., 2020; Cameselle et al., 2013), there is still no standard or reliable method of handling individual contaminated soils due to economic and technical reasons (Xu et al., 2020a; Cameselle et al., 2013). For example, traditional physicochemical technology is associated not only with high costs of soil remediation, but also with the destruction of soil structure and the effects on the physical and chemical soil properties (Luo et al., 2019).

Phytoremediation, with its green and sustainability characteristics (Cameselle et al., 2013), low cost (Yang et al., 2020) and environmental disturbance (Acosta-Santoyo et al., 2017), shows good application prospects and attracts more and more attention (Yang et al., 2020). This term includes the processes of phytoextraction, translocation, detoxification and hyperaccumulation (Han et al., 2020). Furthermore, phytoremediation has been often used on a large scale in soils with low heavy metal concentrations, especially in farmland (Zehra et al., 2020). Hyperaccumulator plants species are often selected for the process of phytoremediation based on their heavy metal accumulation mainly in shoots (Yang et al., 2020), which is convenient for post-harvest landfill or incineration (Cameselle et al., 2013). The use of these types of plants can improve the efficiency of phytoremediation (Sanz-Fernandez et al., 2017). While the application of phytoremediation is promising, there are several limitations of the technology, such as the problem of low remediation efficiency due to lower biomass, longer remediation cycle, small rhizosphere operating range etc. (Guo et al., 2020). An ideal way of improving remediation efficiency should not only promote plant biomass, but also increase Cd concentration in plants (Mahar et al., 2016). Research has also been carried out on several aspects involving the chelating agent addition (EDTA, NTA, etc., Guo et al., 2020), fertilizer application (Yang et al., 2020), chromosomal engineering technology (Feng et al., 2019) or combinations with other remediation technologies (Cameselle et al., 2013; Luo et al., 2019).

The combined use of electrokinetic remediation and phytoremediation is one of the preferred methods employed to improve the process of phytoremediation (Putra et al., 2013). The combined technology can partially overcome the challenges encountered by individual technologies (Cameselle et al., 2013), such as problems of limited Cd availability in soil and the fact that plants can only act on heavy metals in root vicinity, and even be conducive to nutrient absorption by plants (Rostami and Azhdarpoor, 2019). Several studies have proved that the application of electric field can not only increase the concentration of available heavy metal in soil, which is beneficial to heavy metal accumulation by plants (Xiao et al., 2017; Putra et al., 2013), but also promote the growth of plants under certain electric field conditions (Luo et al., 2019). Acosta-Santoyo et al. (2019) reported that the application of low DC electric field had a clear benefit for ryegrass germination. Luo et al. (2019) found that electric field with a low and moderate voltage gradient significantly stimulated the growth of N. caerulescens. This could be related to the stimulating effect of a low-frequency electric field on maize superoxide dismutase (SOD) activity and the reduction of malondialdehyde (MDA) content (He et al., 2017). Moreover, H⁺ ion generation from the anode electrolyzed water under the action of DC electric field results in an acidic environment, and thus promote the activation of heavy metals and their migration due to the electrokinetic effect (Luo et al., 2019). There are several advantages of switching the polarity; on the one hand, it can neutralize hydrogen ions and hydroxide ions generated in the anode and cathode over time, avoiding the adverse effects of polar acid and alkaline environment on plant growth (Xiao et al., 2017); on the other hand, it can promote heavy metal migration from the electrode regions to the middle region, and thus positively affect root absorption and accumulation (Xu et al., 2020b). Electrode materials also play an important role in this combined technology. Electrode material should be sufficiently stable and should not release harmful ions to the fluid (Acosta-Santoyo et al., 2017). Acosta-Santoyo et al. (2019) reported that the titanium anode was not as efficient in increasing the germination rate as the IrO₂Ta₂O₅-modified titanium anode. Additionally, a lower voltage gradient (0.1V/cm and 0.2V/cm) with the IrO₂Ta₂O₅|Ti anode and Ti cathode was shown to promote plant growth. Previous study found no significant changes when using graphite and stainless steel electrodes, and no negative effect to the soil or plants was recorded. However, the aluminum electrode released Al³⁺ ions, and they exerted a negative effect on seed germination and plant growth, which could be caused by increased aluminum phytotoxicity in the acidic environment created by the anode (Acosta-Santoyo et al., 2017).

Plants commonly employed for phytoremediation show hyperaccumulator properties, increased biomass and rapid growth (Putra et al., 2013), and are also the best candidates for use in combination with electrokinetic remediation (Cameselle et al., 2013). In recent years, the studies have been conducted involving plants such as, Noccaea caerulescens (Luo et al., 2019), Poa pratensis L. (Putra et al.,2013) or Brassica juncea (Cang et al., 2012), but few focused on hyperaccumulators. Solanum nigrum L. belongs to the large genus Solanum of the diverse family Solanaceae (Han et al., 2020). It was first reported as a Cd hyperaccumulator in a systematic screen on the basis of a pot culture experiment and field pilot (Wang et al., 2015; Wei et al., 2005). Previous study demonstrated that the DC electric field with switching polarity was beneficial for Cd phytoextraction in S. nigrum (Xu et al., 2020a), and the highest Cd accumulation by S. nigrum was achieved at a voltage gradient of 1 V/cm (Xu et al., 2020b). Thus, the Cd hyperaccumulator S. nigrum was selected as the test plant species and the above conditions were crucial in the present study.

At present, there is few research concerning seed germination under electric field, it is rare to apply electric field at the seed germination stage, as in the current study, to assess the combined application of electrokinetic remediation and phytoremediation. It is well known that seed germination and seedling growth have a great influence on the subsequent plant growth. Therefore, it is necessary to study the application of an electric field starting from seed germination. The graphite, titanium, and stainless steel electrodes used in this study are stable and inexpensive electrode materials that have been shown to produce no toxic ions for plant growth (Acosta-Santoyo et al., 2019; Acosta-Santoyo et al., 2017). The aim of the present study was to identify the best electrode material for seed germination and Cd accumulation by S. nigrum, and then to demonstrate the feasibility of combined use of electrokinetic remediation and phytoremediation. We hypothesized that the different characteristics of electrode materials could significantly affect S. nigrum seed germination and its Cd accumulation in plants.