Solidification/stabilization (S/S) is recognized as an effective technology for solid waste treatment. In S/S, the application of geopolymers synthesized by industrial waste (rich in active silicon and aluminum) to immobilize hazardous waste is a research focus. In this article, a fly ash based geopolymer was used to immobilize lead–zinc slag containing Pb, Ni, Zn and Mn. A fly ash based geopolymer with good mechanical strength was obtained through single factor experiments and the compressive strength of the geopolymer reached 29.72 MPa. The effects of immobilizing lead–zinc slag in the fly ash based geopolymer were discussed by means of compressive strength, leaching test and speciation analysis. The solidification/stabilization mechanism was further investigated using XRD, FTIR and SEM. The mechanical properties of the fly ash based geopolymer were negatively affected by addition of lead–zinc slag, and compressive strength decreased to 8.67 MPa when 60% lead–zinc slag was added. The geopolymer has the ability to reduce toxicity of lead–zinc slag by immobilizing heavy metals (Pb, Ni, Zn and Mn), but the ability was not unlimited. The migration of heavy metals to residual form indicates that heavy metals may either be bonded into the geopolymer matrix via the T–O bond (T = Si, Al) or captured in framework cavities to maintain the charge balance. The NASH (Na₂O–Al₂O₃–SiO₂–H₂O) gel structure observed by XRD, FTIR and SEM can physically encapsulate the contaminants during geopolymerization. It is finally concluded that heavy metals were immobilized in the fly ash based geopolymer through a combination of chemical bonding and physical encapsulation.

Copper-mine tailings are the residual products after the extraction of precious copper metal from copper ores, and their storage can create numerous environmental problems. Many researchers have used copper-mine tailings for the preparation of geopolymers. This paper studies the enhancement of the cementitious activity of copper-mine tailings in geopolymer systems. First, copper-mine tailings are activated through mechanical grinding activation. Then, the mechanically activated copper-mine tailings are further processed through thermal activation and alkaline-roasting activation. The cementitious activity index of copper-mine tailings is characterized through the degree of leaching concentration of Si and Al. It was observed that the Si and Al leaching concentration of mechanically activated tailings was increased by 26.03% and 93.33%, respectively. The concentration of Si and Al was increased by 54.19% and 119.92%, respectively. For alkaline-roasting activation, roasting time, temperature and the mass ratio of copper-mine tailings to NaOH (C/N ratio) were evaluated through orthogonal tests, and the best condition for activation was 120 min at 600 °C with a C/N ratio of 5:1. In this study, scanning electron microscopy (SEM), X-ray diffraction (XRD) and infra-red (IR) analysis show that mechanical, thermal and alkaline-roasting activation could be used to improve the cementitious activity index of copper-mine tailings.

Hexavalent chromium from industrial residues is highly mobile in soil and can lead to the contamination of groundwater through runoff and leaching after rainfall. This paper focuses on the three-dimensional (3D) electrokinetic remediation (EKR) of chromium-contaminated soil from an industrial site. Activated carbon particles coupled with Fe ions (AC-Fe) were used as the third electrode. The optimum dose ratio of the electrode particles and remediation time were selected on the basis of single-factor experiments. X-ray photoelectron spectroscopy (XPS) analysis was carried out to explore the reduction of Cr(VI) on the surface of the electrode particles (AC-Fe). The results showed that AC-Fe had a positive effect on Cr(VI) reduction with a removal rate of 80.2%, which was achieved after 10 d by using a 5% dose of electrode particles. Finally, it was concluded that the removal mechanism combined the processes of electromigration, electrosorption/adsorption and reduction of Cr(VI) in the 3D EKR system.

Improper disposal of chromium (Cr) and its compounds, especially hexavalent chromium (Cr(VI)), results in soil and ground water pollution and is consequently harmful to human health. In this study, three-dimensional electro-kinetic remediation of Cr-contaminated soil is investigated by coupling a two-dimensional electrode with a permeable reactive barrier (PRB) with a graphite electrode as the third electrode. Mixed zero-valent iron and zeolite are used as filling materials in the PRB. Moreover, three experimental conditions, i.e. two-dimensional electro-kinetic remediation with and without PRB and three-dimensional electro-kinetic remediation with PRB, are investigated herein. The results are evaluated based on the removal rate and leaching efficiency both in the pre- and post-experiments. Upon comparing the three conditions, the results show that three-dimensional electro-kinetic remediation with PRB has a better effect on both leaching efficiency and removal rate of contaminated soil. Single and multifactor experiments were designed to explore the optimum conditions on the basis of three-dimensional remediation. Graphite particles with a 5% dosage, resulted from the single-factor experiments, are used in the multi-factor experiments. The results show that the best remediation efficiencies are achieved after 12 d using 0.05 mol L⁻¹ citric acid and a voltage gradient of 1.5 V cm⁻¹ in three-dimensional electro-kinetic remediation coupled with PRB.