Recent developments in industrialization have heightened the emphasis on soil contamination by heavy metals (Sarwar et al. 2017). The research to date about industrial chromium (Cr) pollution is considered to put an increasing burden on the environment and human health (Wang et al. 2019). That is attributed to its toxicity, persistence, and non-biodegradability gradually accumulating in the environment (Ayangbenro &Babalola 2017). Previously published studies have shown that Cr exists in nature mainly as two stable oxidation states, including hexavalent chromium and trivalent chromium (Sarin &Pant 2006). Cr(Ⅵ) usually occurs as anionic species such as CrO42−, HCrO4−, and Cr2O72−, which have high mobility in soil and groundwater, resulting in potential danger of toxicity and carcinogenicity (Eyvazi et al. 2019). Cr(Ⅲ) species, like Cr3+, Cr3(OH)45+, and Cr(OH)2+, are less toxic and more stable than Cr(Ⅵ) (Sarin &Pant 2006). Therefore, there is an urgent need to find an effective way to address the soil contamination problems caused by Cr.
Numerous methods have been employed to repair Cr-contaminated soil, such as chemical leaching, bioremediation technology, solidification/stabilization, electrokinetic remediation (EKR), etc. (Alidokht et al. 2021, Chen et al. 2021, Sarankumar et al. 2019, Zou et al. 2019). According to the previous literature, EKR, which possesses economic sustainability and satisfies the need to remove various contaminants covering organic pollutants and heavy metals, especially applicable to low permeability soils, is a promising in-situ soil remediation technology (Al-Hamdan &Reddy 2008, Li et al. 2012). Electromigration, electroosmosis, electrophoresis, and electrolysis have been instrumental in our understanding of the mechanism of EKR for heavy metal contaminated soils (Nasiri et al. 2020). However, there are still some limitations of traditional EKR that need to be addressed. For instance, Cr(VI) as the oxygen anion tends to accumulate in near-anode soil layers and is challenging to remove due to adsorption and potential flattening (Tang et al. 2021, Wen et al. 2021, Yu et al. 2020).
Combined EKR techniques to enhance the Cr(VI) remediation efficiency in near-anode soil layers have got more attention, such as the establishment of a main-auxiliary electrode system, approaching anode electrokinetic method, application of UV radiation and electrokinetic remediation, permeable reactive barrier (PRB) coupled with the electrokinetic process, etc. (Liu et al. 2020, Suzuki et al. 2014, Tang et al. 2021, Wang et al. 2019, Zhang et al. 2012, Zheng et al. 2021). Considerable literature has shown that the EKR/PRB system has the advantage of avoiding secondary contamination of electrolytes (Nasiri et al. 2020, Suzuki et al. 2014, Yeung &Gu 2011). During the EKR/PRB system, the barrier padded with reactive materials bonds with pollutants by reduction, precipitation, and adsorption to remove the anticipated contaminants (Nasiri et al. 2020).
The selection of fillers is central to the entire EKR/PRB, depending on the contaminant category. Active carbon, zeolite, and zero-valent iron are the common reagent medium (Zhou et al. 2021). Previously, a good deal of novel materials with excellent efficiency have been examined. For example, the CaAl-layered double hydroxides were used as a PRB filler for reparation of Cr-contaminated soil in the EKR/PRB system, the union of graphene oxide and fly ash as reaction media could reach a 92.6% removal rate for lead(II) from contaminated soil by EKR/PRB technology (Xu et al. 2016, Zhou et al. 2021). Conductive polymers appear in more studies due to their outstanding electrochemical performance, electrical conductivity, high carrier mobility, and re-utilization (Yuan et al. 2019). Polypyrrole (PPy), which has remarkable environmental stability and non-toxicity as an environmentally friendly polymer material, has recently caught the attention of researchers (Ghorbani et al. 2010, Hasani &Eisazadeh 2013, Hosseini et al. 2015). Under acidic conditions, PPy can protonate and generate electrostatic attraction with anion pollutants (Ting et al. 2021). Moreover, PPy could adsorb anions by carrying nitrogen atoms with positively charged and restore Cr(VI) to Cr(III) (Wei et al. 1993). However, there is a π-π force between the PPy molecular chains, and individual spherical PPy particles are prone to aggregate (Wang et al. 2020). It leads to a small specific surface area limiting their ability to remove Cr(VI) because of low binding sites (Amalraj et al. 2016, Ballav et al. 2014b, Bhaumik et al. 2011). Consequently, it is of great significance to modify PPy to reduce its agglomeration to improve the removal efficiency of Cr(VI).
Recently, Kera et al. have found that adding dopants during polymerization for chain alteration can serve as a suitable means of overcoming the phenomenon of aggregation (Amalraj et al. 2016, Fang et al. 2018, Kera et al. 2016). Furthermore, NH-containing polymers have drawn more attention because of their outstanding chemical reduction ability (Qiu et al. 2014). Based on the research achievement, the adsorptive capacity of modified PPy improves obviously with the number of amino groups within the chemical structure of the dopant (Ballav et al. 2014b, a, Ballav et al. 2012, Bhaumik et al. 2011, Chigondo et al. 2019, Karthik &Meenakshi 2015). They all tend to use modified PPy as an adsorbent to remove pollutants from water, making it possible to remediate contaminated soil with modified PPy as PRB near the anode in the PRB/EKR system.
This study intends to prepare three modified PPy materials, i.e., magnetic PPy (Fe3O4@PPy), arginine modified PPy (Arg@PPy), and arginine modified magnetic PPy (Arg/Fe3O4@PPy), which are based on PPy by polymerization of pyrrole monomer. The parametric effects on adsorption efficiency, including pH, initial Cr(VI) concentration, dosage, temperature, and contact time, were studied. Kinetics and isotherm studies were performed to research the capacity of adsorption. And then, these composites were used as the PRB fillers near the anode to remediate Cr-contaminated soil. During the EKR/PRB process, electric current, the characteristic of soil and electrolyte, and residual and leaching Cr(VI) content were measured to assess composites' enhancement.