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.

Fly ash from incinerated municipal solid waste is a source of secondary pollutants (Cu, Zn, Pb and Cd). The inappropriate management of fly ash leads to the contamination of soil and ground water. This paper is based on the electro-kinetic removal of heavy metals from municipal solid waste incinerated fly ash by using a three-dimensional electrode in orthogonal single and multi-factor experiments to obtain the optimal experimental conditions by varying the leaching toxicity removal rate for the heavy metals in the groups. The optimal dimensions (6 × 6 mm) of the particle electrodes to achieve high removal rates for heavy metals are found by using various measurements in single-factor orthogonal experiments. In addition, the multi-factor orthogonal experiment is based on three factors: (a) the particle electrode ratio, (b) the voltage gradient and (c) the repair time, while keeping the optimum specification (6 × 6 mm) for particle electrodes constant. The result showed that a high removal rate for heavy metals was obtained by applying a voltage gradient of 9 V for 5 days (repair time) and a 5% dosing ratio.

•The optimum ratio of BFS and FA for preparing alkali-activated binder was obtained.•Alkali-activated composite cementitious material was applied to immobilize COPR.•The product as an available construction material has a high compressive strength.•The immobilization effect of alkali-activated binder is much better than that of OPC.Chromite Ore Processing Residue (COPR) produced in chromium salt production process causes a great health and environmental risk with Cr(VI) leaching. The solidification/stabilization (S/S) of COPR using alkali-activated blast furnace slag (BFS) and fly ash (FA) based cementitious material was investigated in this study. The optimum percentage of BFS and FA for preparing the alkali-activated BFS-FA binder had been studied. COPR was used to replace the amount of BFS-FA or ordinary Portland cement (OPC) for the preparation of the cementitious materials, respectively. The immobilization effect of the alkali-activated BFS-FA binder on COPR was much better than that of OPC based cementitious material. The potential for reusing the final treatment product as a readily available construction material was evaluated. X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR) and scanning electron microscope with energy dispersive spectrometer (SEM-EDS) analysis indicated that COPR had been effectively immobilized. The solidification mechanism is the combined effect of reduction, ion exchange, precipitation, adsorption and physical fixation in the alkali-activated composite cementitious material.

Chromite ore processing residue (COPR) is an industrial waste produced in the chromic salts production process and contains a small portion of leached Cr(VI), which is highly toxic and is listed as a hazardous waste. The immobilization of COPR using a blast furnace slag-based geopolymer has been investigated in this study. The optimum parameters for preparing the blast furnace slag-based geopolymer using an orthogonal experiment were obtained. COPR was used to replace the amount of blast furnace slag for the preparation of the geopolymer. The COPR-bearing blast furnace slag-based geopolymer has potential application as a construction material and for geological disposal. The combined effect of physical fixation, adsorption and ion exchange in the geopolymeric and CSH (calcium silicate hydrate) gel is considered to be the main mechanism, and the reduction of S2− in the blast furnace slag played a significant role in the solidification of the COPR.

This paper demonstrated a biocementation technology for chromium slag by strain GM-1, a calcifying ureolytic bacterium identified as Microbacterium, based on microbially induced calcium carbonate. The characterization of Microbacterium sp. GM-1 was assessed to know the growth curve in different concentrations of Cr(VI). Microbacterium sp. GM-1 was tolerant to a concentration of 120 mg/L Cr(VI). Chromium waste forms were prepared using chromium, sand, soil and bacterial culture. There we had three quality ratios (8:2:1; 8:1:1; 8:2:0.5) of material (chromium, sand and soil, respectively). Bacterial and control chromium waste forms were analyzed by thermal gravimetric analyzer. All bacterial forms (8:2:1; 8:1:1; 8:2:0.5 J) showed sharp weight loss near the decomposition temperature of calcium carbonate between 600 and 700 °C. It indicated that the efficient bacterial strain GM-1 had induced calcium carbonate precipitate during bioremediation process. A five step Cr(VI) sequential extraction was performed to evaluate its distribution pattern in chromium waste forms. The percentage of Cr(VI) was found to significantly be decreased in the exchangeable fraction of chromium waste forms and subsequently, that was markedly increased in carbonated fraction after biocementation by GM-1. Further, compressive strength test and leaching test were carried out. The results showed that chromium waste forms after biocementation had higher compressive strength and lower leaching toxicity. Additionally, the samples made of 8:1:1 (m/m/m) chromium + sand + soil were found to develop the highest compressive strength and stand the lowest concentration of Cr(VI) released into the environment.

•Content, characteristics, transformation of heavy metals in coal and ash are discussed before and after combustion.•Modified calcium carbonate (CaCO3) is used as an additive to treat heavy metals.•The migration and release of heavy metals in coal is studied and tested in the experiments.Coal plays an important role in the Chinese energy source. The heavy metals released from combustion process stay in the environment and are not easily degraded. This paper examines 8 coal samples from China. It explores the content, characteristics, transformation and control of heavy metals through the microwave digestion, sequential chemical extraction, static burning test and X-ray diffraction (XRD) methods. The zinc (Zn) and cadmium (Cd) in coal mainly exist in exchangeable, sulfide, and residual forms. The arsenic (As) content is mainly present in the sulfide binding state. For these three elements, degrees of organic speciation are not considered to be important and the residue speciation mainly occurs in coal ash. After coal combustion, the heavy metals in coal ash are relatively enriched, and the harm to the environment cannot be ignored. Most of the Cd, As and Zn residues are released into the atmosphere and their volatility rates can be ordered as follows: As > Zn > Cd. There are significant differences in mineral composition between the main phases and mineral carriers of heavy metals in coal and ash. Based on these differences, modified calcium carbonate (CaCO3) is used as an additive. In orthogonal experiment, the optimal parameters for treating heavy metals are the temperature (1000 °C), metal salt (Al2(SO4)3), and ion ratio (γ = 15). Under this condition, the volatilization rates of heavy metals (As, Cd and Zn) decreased remarkably.

BackgroundNowadays, leaching-ore bacteria, especially Acidithiobacillus ferrooxidans is widely used to retrieve heavy metals, many researches reflected that extra adding microorganism could promote bioleaching efficiency by different mechanisms, but few of them discussed the interaction between microorganisms and based on growth model. This study aimed to provide theoretical support for the collaborative bioleaching of multiple microorganisms by using the Lotka–Volterra (L–V) model.ResultsThis study investigated the interaction of Acidithiobacillus ferrooxidans, Rhizobium phaseoli, and Rhodotorula sp. Results showed that the individual growth of the three microorganisms fit the logistic curves. The environmental capacities of A. ferrooxidans, R. phaseoli, and Rhodotorula sp. were 1.88 × 109, 3.26 × 108, and 2.66 × 108 cells/mL, respectively. Co-bioleaching showed mutualism between A. ferrooxidans and R. phaseoli with mutualism coefficients of α = 1.19 and β = 0.31, respectively. The relationship between A. ferrooxidans and Rhodotorula sp. could be considered as commensalism. The commensalism coefficient γ of the effect of Rhodotorula sp. on A. ferrooxidans was 2.45. The concentrations of A. ferrooxidans and R. phaseoli were 3.59 × 109 and 1.44 × 109 cells/mL in group E, respectively, as predicted by the model. The concentrations of A. ferrooxidans and Rhodotorula sp. were 2.38 × 109 and 2.66 × 108 cells/mL, respectively. The experimental peak values of the concentrations in microorganism groups E and F were detected on different days, but were quite close to the predicted values.ConclusionThe relationship among microorganisms during leaching could be described appropriately by Lotka–Volterra model between the initial and peak values. The relationship of A. ferrooxidans and R. phaseoli could be considered as mutualism, whereas, the relationship of A. ferrooxidans and R. phaseoli could be considered as commensalism.