Electrokinetic remediation is a promising method for Cr(VI) treatment. Electrokinetic remediation operates based on a low-intensity electric field across the soil, which mobilizes contaminants towards the electrodes. Some previous studies tried to improve the mobility and availability of contaminants for extraction. However, the technology faces some challenges such as interference from non-target contaminants. It is thus necessary to develop the electrokinetic remediation with other technologies like chemical agents and permeable reactive barriers. Some minerals can be used to remediate Cr(VI) due to their properties like surface effect and redox effect. The existing researches on minerals for Cr(VI) remediation mainly focus on zero-valent iron (ZVI) materials. ZVI usually requires loading materials to minimize agglomeration. In this paper, a magnetite/pyrrhotite composite material (i.e., pyrrhotite naturally dispersed in magnetite) was introduced to the electrokinetic remediation systems, mitigating shortage of electron donors in Cr(VI) contaminants and stabilizing Cr in the form of Cr-containing minerals. The impacts of mineral filling position and potential gradient on the Cr(VI) remediation performance were investigated, and the Cr-containing products were analyzed.

The mineral enhanced electro-remediation devices consisted of acrylic reactors (length 100 mm, diameter 50 mm), and plate-shaped electrodes (length 80 mm, width 50 mm). The minerals were filled on the both sides or in the middle. The applied potential gradient was 0.2 V/cm or 0.4 V/cm, provided by a model CHI 1000C Electrochemical workstation. In the remediation process, the samples were taken at 12, 24, 36, 48, 72, 96, 120, 144 h, and 168 h, respectively. The concentrations of Cr(VI) were determined by a model Thermo Fisher EVOLUTION 220 UV visible spectrophotometer based on the national standard GB7467-87. The total Cr (TCr) concentrations were determined by a model SPECTROBLUE SOP inductively coupled plasma optical emission spectrometer, in accordance with HJ 776-2015 method. The mineral phase was tested by a model PANalytical X-pert3 powder X-ray diffractometer (XRD), and the element analysis was performed on a model Thermo ARL ADVENT XP+X-ray fluorescence spectrometer (XRF). The scanning electron microscopy images and energy dispersive X-ray spectroscopy mapping of Cr-containing products were performed by a model FEI Quattro S environmental scanning electron microscope (ESEM) at 15 kV. The Raman spectra were determined on a model Renishaw inVia Reflex Laser micro-Raman spectrometer with a 532 nm laser. After remediation, the reaction devices were maintained at room temperature for six months. Subsequently, the contaminated simulated soil underwent a second washing with ultrapure water to mimic leaching of harmful components into the environment under specific conditions. The concentrations of Cr(VI) and TCr were measured to evaluate the stability of the remediation. The electrodes were removed from the devices, weighed, and placed in 250 mL beakers. The volume of leaching agent and water was determined at a liquid-to-solid ratio of 10∶1 (L/kg). After adding the leaching agent, the beakers were sealed with a film and mounted vertically in a horizontal oscillation device (THZ-D). The oscillation settings were adjusted to 120 cycles per minute with an amplitude of 40 mm. The leachates were then filtered through a 0.45 μm microporous membrane, collected and analyzed for Cr(VI) and TCr concentrations. This process assessed the contribution of electrodes adsorption to Cr(VI) removal, following the guidelines of HJ 557-2010.

The XRF and XRD analysis demonstrate that the minerals in magnetite/pyrrhotite composite are mainly composed of 70% magnetite and 26% pyrrhotite. Moreover, the mineral material does not contain other heavy metals. The mineral and electrokinetic remediation both are effective in the treatment of Cr(VI)-contaminated simulated soil. However, reducing the Cr(VI) concentration below 200 mg/L proves a challenge. In contrast, the mineral-enhanced electro-remediation system reduces the Cr(VI) concentration to 200 mg/L within 24 h and progressively approaches zero at seventh day. Also, the removal efficiency of Cr(VI) at both the anode and cathode doubles, compared to electrokinetic remediation. The effects of potential gradient and mineral filling position on the remediation of Cr(VI) were investigated. The results reveal that the experimental systems with a higher potential gradient exhibit the superior removal efficiency for TCr, compared to those with a lower potential gradient. The removal efficiency of TCr in a high potential group exceeds that in a low potential group by more than 10% under identical mineral filling conditions. Moreover, within the same potential gradient, the mineral filling position exerts a minimal influence on the remediation outcome. The stability analysis of Cr-containing products demonstrates a reduction in the Cr(VI) re-dissolution ratio by over 54% in the mineral-enhanced electro-remediation group, compared to the electrokinetic remediation group, indicating that the generated Cr-containing products are relatively stable and resistant to re-dissolution. Based on the analysis by Raman spectroscopy, XRD, and ESEM, Cr in stable mineral phases is correlated to Fe distribution.

Mineral-enhanced electro-remediation was the most effective approach for Cr(VI) removal. After 7 d operation of the device filled with minerals on both the sides and with a potential gradient of 0.4 V/cm, the overall removal efficiency of Cr(VI) and TCr was 100.0% and 95.7%, respectively. A potential gradient is the main factor affecting Cr(VI) treatment. The overall efficiency of total Cr was increased by more than 10% when the potential gradient was increased from 0.2 V/cm to 0.4 V/cm. The results of stability experiment showed that the amount of Cr(VI) re-dissolved from the electrodes, and the simulated soil in the mineral-enhanced remediation was close to zero, indicating that the remediation method had a long-term effect on the Cr(VI) treatment. The replacement of Fe³⁺ and SO₄^2- by Cr(III) and Cr(VI) in minerals was the main reason for maintaining the long-term effect of Cr(VI) treatment. This study could demonstrate the enhanced effect of magnetite/pyrrhotite composite mineral on the Cr(VI) removal, providing an effective approach for the long-term and stable remediation of Cr(VI)-contaminated sites.