Application of novel polypyrrole/melamine foam auxiliary electrode in promoting electrokinetic remediation of Cr(VI)-contaminated soil

Introducing hybrid electrokinetic-permeable reactive barrier-membrane systems for remediation of soil-slag mixtures contaminated with Pb and Zn

This study investigates the effectiveness of electrokinetic (EK) remediation for the removal of lead (Pb) and zinc (Zn) from a contaminated soil-slag mixture collected from the Isfahan Steel Industry. The mixture exhibited high buffering capacity, significant organic matter content, and elevated Pb and Zn concentrations. The electrokinetic setup, which was constructed from transparent plexiglass, consists of three main sections: a soil chamber, two electrolyte reservoirs and two stainless steel electrodes and they were placed at the ends of the soil chamber within electrode compartments. Nine experiments were performed under a constant voltage gradient of 1.5 V/cm over 120 h to evaluate different enhancement strategies, including (1) coupling EK with pistachio shell-derived activated carbon as a permeable reactive barrier (PRB) near the cathode and both electrolytes, (2) incorporating a cation exchange membrane (CEM) near the cathode, (3) conditioning the electrolyte with ethylenediaminetetraacetic acid (EDTA), both alone and in combination with PRB and CEM, and (4) pre-treating the mixture with EDTA. Visual Minteq modeling indicated that, in the absence of EDTA, the dominant Pb and Zn species were Pb3(OH)42⁺ and Zn(OH)2, while the presence of EDTA led to the formation of PbEDTA2- and ZnEDTA2-, enhancing metal mobility. The highest removal efficiencies were obtained in the experiment where EDTA (0.1 M) was used in the catholyte, PRB was placed at the center of the cell, and CEM was applied near the cathode, achieving 68.7% Pb and 48.3% Zn removal with lower energy consumption. SEM-EDS analysis confirmed significant Pb and Zn adsorption onto the activated carbon. The findings suggest that the integration of PRB, CEM, and EDTA-enhanced electrolyte conditioning improves electrokinetic remediation performance and offers a feasible approach for treating heavy metal-contaminated soil-slag mixtures.

Ball milling of mackinawite and oxalic acid to fabricate an efficient and stable reductive material for remediation of Cr(Ⅵ)-contaminated soil

Compared with zero-valent iron, iron sulfide has more diverse reactive species and higher reductivity, but it is still prone to be gradually deactivated due to various passivation factors. In this study, a novel reductive material (BMMW@OA) was prepared by ball milling of mackinawite (MW) as raw material and oxalic acid (OA) as modifier, so as to simultaneously improve its reductivity and stability by continuous releasing reductive species and maintaining freshness of the material surface. The BMMW@OA (w/w of MW/OA = 4/1) effectively removed Cr(Ⅵ) from water with wide pH adaptability. Compared to the BMMW obtained by direct ball milling of MW, BMMW@OA shows good resistance to air exposure, alkaline environment and scale-forming ions. The Freundlich model can accurately describes the Cr(Ⅵ) removal capacity of BMMW@OA. BMMW@OA continuously releases Fe2⁺ and S2⁻ into aqueous solution, thus maintaining the freshness and reactivity of the material surface. The homogeneous reduction by dissolved Fe2⁺ and the heterogeneous reduction by Fe⁰/Fe2⁺ on the surface of BMMW@OA are two primary mechanisms for the efficient removal of Cr(Ⅵ). Adding 2% BMMW@OA reduced the Cr(Ⅵ) content in soil from 989.826 mg/kg to 2.034 mg/kg within 2 h. The horizontal vibration, toxicity characteristic, and synthetic precipitation leaching procedure tests showed that the Cr(Ⅵ) concentration in the soil leachate was as low as 0.005-0.020 mg/L and remained stable in 30 d. This study developed a green and economic viable material for remediation of Cr(Ⅵ)-contaminated soil with high efficiency and stability.

A solar thermoelectric system by temperature difference for efficient removal of chromium (VI) in water and soil

In this work, we designed and developed a facile solar thermoelectric generator (STEG)-based system and a new electrokinetic remediation (EKR) system, which consists of main electrodes and unenergized auxiliary electrodes. The prepared nanocomposite was investigated for the effectiveness of the STEG+PANI-CNT/GF system in remediating Cr-contaminated. Photothermal performance test were applied in order to examine this STEG could export a power density of 365.56 mW/dm2 and output potential of 801 mV at the temperature difference of 50 ℃. Thus the STEG could be used as the power to construct a Cr(VI) removal system using polyaniline (PANI) film/carbon nanotubes (CNT) modified graphite felt (GF) electrode (PANI-CNT/GF) as cathode and graphite rod as anode. The as-prepared STEG+PANI-CNT/GF system exhibited a significant Cr(VI) removal efficiency (96.2 % in water) through electromigration, electro-adsorption and electroreduction. Moreover, a multi auxiliary electrodes (AEs) system (STEG+PANI-CNT/GF+AEs) with six PANI-CNT/GF auxiliary electrodes was constructed in remediating Cr(VI)-contaminated soil, showing Cr(VI) removal efficiency of 16.7-60.1 % higher than that of STEG+PANI-CNT/GF. The PANI-CNT/GF auxiliary electrodes could bind Cr(VI) and adjust electric field distribution, contributing to adsorption and reduction of Cr(VI). Consequently, this work provides a promoting approach for heavy metals removal in future application.

Is crystalline chromium phosphate environmentally stable? A study on the formation, dissolution and oxidation risk of CrPO4·6H2O

Hexavalent chromium (Cr(VI)) contamination in soil and groundwater is usually remediated via reduction techniques. The formation of crystalline chromium phosphate (CrPO4·6 H2O) occurs as a byproduct during Cr(VI) remediation processes in the presence of phosphate, yet its stability in the environment has received limited attention. In this study, the formation conditions, structure, properties, and risks associated with the dissolution and oxidation of CrPO4·6 H2O were comprehensively assessed. Results showed that crystalline CrPO4·6 H2O was formed under pH 5 - 7 at room temperature. CrPO4·6 H2O exhibits higher dissolution risk compared to Cr(OH)3·3 H2O due to a long Cr-P bond (4.2 Å). H+ and OH- increased the risk of dissolution at pH 5 and 11, respectively, owing to the formation of CrH2PO42+ and Cr(OH)4-. In addition, under faintly acidic conditions, the high solubility of CrPO4·6 H2O increases the risk of oxidation; under neutral and weakly alkaline conditions, the presence of positively charged Cr(H2O)63+ structures on the surface elevates its susceptibility to contact and oxidation by δ-MnO2 compared to Cr(OH)3·3 H2O. Specifically, at pH 11, the conversion of CrPO4·6 H2O to Cr(OH)3·3 H2O results in similar oxidation risks for both Cr(III) precipitates.

Electrokinetic remediation for the removal of heavy metals in soil: Limitations, solutions and prospection

Electrokinetic remediation (EKR) technology is a promising method to remove heavy metals from low permeability soil, because it is environmentally friendly, efficient and economical, and can realize in-situ remediation. In this paper, the basic principles and related physical and chemical phenomena of EKR are systematically summarized, and three limiting problems of EKR technology are put forward: the weak ability of dissolving metals, focusing effect, and energy consumption. There are many methods to solve these technical problems, but there is a lack of systematic summary of the causes of problems and solutions. Based on various enhanced EKR technologies, this paper summarizes the main ideas to solve the limiting problems. The advantages and disadvantages of each technology are compared, which has guiding significance for the development of new technology in the future. This paper also discusses the dissolution of residual heavy metals, which is rare in other articles. The energy consumption of EKR and the remediation effect are equally important, and both can be used as indicators for evaluating the feasibility of new technologies. This paper reviews the influence of various electric field conditions on power consumption, such as renewable energy supply, new electrode materials and electrode configurations, suitable voltage values and functional electrolytes. In addition, a variety of energy consumption calculation methods are also introduced, which are suitable for ohmic heat loss, energy distribution when there is non-target ion competition, and power consumption of specific ions in various metal ions. Researchers can make selective reference according to their actual situations. This paper also systematically introduces the engineering design and cost calculation of EKR, lists the research progress of some engineering cases and pilot-scale tests, analyzes the reasons why it is difficult to apply EKR technology in large-scale engineering at present, and puts forward the future research direction.

PANI/MCM-41 adsorption for removal of Cr(VI) ions and its application in enhancing electrokinetic remediation of Cr(VI)-contaminated soil

In this study, a polyaniline/mesoporous silica (PANI/MCM-41) composite material that can be used as a filler for permeable reactive barrier (PRB) was prepared by in situ polymerization. Firstly, the adsorption capacity of PANI/MCM-41 on Cr (VI) in solution was investigated. The results show that the prepared PANI/MCM-41 exhibits a significant Cr (VI) adsorption capacity (~ 340 mg/g), and the adsorption process is more accurately described by the Langmuir isotherm and pseudo-second-order kinetic model. The thermodynamic functions evidenced that the Cr(VI) adsorption was an endothermic spontaneous process. In addition, adsorption-desorption cycle experiments proved the excellent reusability of the material. Subsequently, the material was utilized as a filler in the PRB for the remediation of Cr(VI)-contaminated soil using electrokinetic-permeable reactive barrier (EK-PRB) technology. The results show that compared with traditional electrokinetic remediation, the use of PANI/MCM-41 as an active filler can enlarge the current during remediation and enhance the conductivity of soil, which increases the removal rates of total Cr and Cr(VI) in soil (17.4% and 10.2%).

Coupling electrokinetic remediation with flushing using green tea synthesized nano zero-valent iron/nickel to remediate Cr (VI)

This study focuses on a flushing-electrokinetic remediation technology of hexavalent chromium from the chromium slag dump site. A suspension of nanoscale zero-valent iron/nickel fabricated from green tea (GT-nZVI/Ni), was employed as an eluent to degrade Cr (VI) and enhance the remediation effectiveness of a single EK. The removal efficiency of Cr (VI) was compared under different voltages, electrode spacings and pH values of the anolyte. The results demonstrated that the combined flushing and EK achieved a removal rate of Cr (VI) in the soil throughout all the experiments ranging from 83.08 to 96.97% after 120 h. The optimal result was obtained when the voltage was 28 V, the pH value of anolyte was 3 and the electrode spacing was 15 cm. The removal of Cr (VI) reached 91.49% and the energy consumption was 0.32606 kW·h·g-1. The underlying mechanisms responsible for the removal of Cr (VI) by GT-nZVI/Ni flushing-EK primarily involved electromigration, reduction and adsorption co-precipitation processes. The fractionation analysis of Cr (VI) concentration in the soil after remediation showed that the presence of GT-nZVI/Ni facilitated the conversion of Cr (VI) into oxidizable and residual states with low mobility and toxicity. The results of toxicity characteristic leaching procedure (TCLP) indicated that the leaching concentration of Cr (VI) was below 1 mg·L-1, complying with the standards set by the Environmental Protection Agency. Additionally, the phytotoxicity testing revealed that the germination index (GI) of the remediated soil reached 54.75%, indicating no potential harm to plants.