Solidification of chromium-containing sludge with attapulgite combined alkali slag.

To solve the harm caused by hazardous chromium-containing sludge (CCS, chromium-containing sludge) waste to humans and the environment, this study used attapulgite to strengthen alkali slag to prepare cementitious materials to solidify/stabilize CCS. Single-factor and orthogonal experiments were used to optimize the preparation parameters of alkali slag cementitious materials. The compressive strength, heavy metal leaching toxicity, and microscopic characterization of a CCS solidified body were tested to investigate the solidification effect and mechanism of CCS formation. The best attapulgite content was 4%; the solidified body after the treatment of chromium-containing sludge had a good performance of heavy metal leaching and mechanical properties. The addition of attapulgite enhanced the compressive strength. Compared with the original CCS, the leaching concentration of heavy metals in the solidified body was significantly reduced. Among them, the solidified efficiency of chromium is stable above 90%. The changes in the results of XRD and FTIR for each component were studied. It indicated that the solidified body may solidify/stabilize heavy metals through physical encapsulation of the amorphous form and chemical immobilization. This research recognized the use of waste to treat waste, realized the combined effect of solidification/adsorption, and indicated the possibility of application of attapulgite and its solidified products in construction.

Preparation of Glass-Ceramics via Cosintering and Solidification of Hazardous Waste Incineration Residue and Chromium-Containing Sludge.

Residues from the incineration of hazardous wastes are classified as hazardous byproducts because they contain heavy metals. Chromium-containing sludge (CCS) is industrial sludge produced during the electroplating process and includes heavy metals, such as Cr, Pb, and Cu. These heavy metals can infiltrate natural ecosystems and cause significant environmental damage. To limit the toxicity of leached products, hazardous waste incineration residues (HWIRs) can be repurposed as raw materials for producing glass-ceramics. In this study, we designed an orthogonal experiment to optimize the heat treatment process, yielding glass-ceramics with excellent properties and realizing heavy metal solidification. The toxic characteristic leaching procedure was used to determine the leaching toxicity of the cosintered solidified heavy metals, revealing that their solidification efficiencies exceed 90%. Moreover, X-ray diffraction analysis indicates that certain heavy metals participate in the formation of heavy-metal-containing crystal lattices (FeCr2O4 and PbFe12O19), thereby reducing their leaching concentration. These results show that cosintering HWIR and CCS is an effective approach for heavy metal solidification and provides valuable insights into its utilization for producing building materials.

Efficient electrokinetic remediation of heavy metals from MSWI fly ash using approaching anode integrated with permeable reactive barrier.

During electrokinetic remediation (EKR) of heavy metals (HMs) (Pb, Zn, Cu, and Cd) from municipal solid waste incineration (MSWI) fly ash enhanced by a permeable reactive barrier (PRB), the nearer to the anode, the higher the concentration of H+ ions and the greater the remediation effect. Therefore, a potentially new method of PRB-enhanced EKR using an approaching anode (A-EKR + PRB) was studied to help H+ ions to quickly migrate to the sample near the cathode. Consequently, the HM leaching and total concentrations were reduced, while an energy reduction of nearly 40% was achieved. The results showed that the best remediation ability was obtained when MSWI fly ash was treated for 16 days at a voltage gradient of 2.5 V/cm, the approaching anode was moved after 4 days, and the PRB contained 10 g of activated carbon. After remediation, the environmental risk analysis showed that A-EKR + PRB reduced all the fractions of HMs, especially the acid extractable and oxidizable fractions, which might have been due to the enhancement of acid dissolution and oxidation by the approaching anode. In addition, the environmental risks of the remaining HMs were reduced, and the results indicated that A-EKR + PRB is an advisable choice for remediation of MSWI fly ash.

Synergism of citric acid and zero-valent iron on Cr(VI) removal from real contaminated soil by electrokinetic remediation.

This study focused on enhanced electrokinetic remediation of Cr(VI) in real contaminated soil. The citric acid (CA) as the electrolyte and Fe(II) released from zero-valent iron (ZVI) under anoxic conditions functioned as the main reducer. They were used for overcoming the high insoluble Cr(VI) fraction in real contaminated soil and high Cr(VI) residue in acidic soil near the anode simultaneously. The synergism of CA and ZVI is that CA helps the release of Cr(VI) to react with the generated Fe(II) and alleviates the hindrance of Fe and Cr co-precipitates in electromigration of Cr; meanwhile, the end product Fe(III) from ZVI catalyzed the Cr(VI) reduction by CA. The removal of Cr(III) and Cr(VI) was significantly improved in real contaminated soil. The optimum result (82.86%) was obtained at a voltage gradient of 2.5 V/cm after 12-day remediation with a 10 g ZVI dose when the catholyte and anolyte were 0.2 mol/L and 0.1 mol/L CA, respectively. This configuration has a significant improvement in overcoming the current obstacles for Cr(VI) electrokinetic remediation from real contaminated soil and prospects for large-scale practical applications.

The Solidification of Lead-Zinc Smelting Slag through Bentonite Supported Alkali-Activated Slag Cementitious Material.

The proper disposal of Lead-Zinc Smelting Slag (LZSS) having toxic metals is a great challenge for a sustainable environment. In the present study, this challenge was overcome by its solidification/stabilization through alkali-activated cementitious material i.e., Blast Furnace Slag (BFS). The different parameters (water glass modulus, liquid-solid ratio and curing temperature) regarding strength development were optimized through single factor and orthogonal experiments. The LZSS was solidified in samples that had the highest compressive strength (after factor optimization) synthesized with (AASB) and without (AAS) bentonite as an adsorbent material. The results indicated that the highest compressive strength (AAS = 92.89MPa and AASB = 94.57MPa) was observed in samples which were prepared by using a water glass modulus of 1.4, liquid-solid ratio of 0.26 and a curing temperature of 25 °C. The leaching concentrations of Pb and Zn in both methods (sulfuric and nitric acid, and TCLP) had not exceeded the toxicity limits up to 70% addition of LZSS due to a higher compressive strength (>60 MPa) of AAS and AASB samples. While, leaching concentrations in AASB samples were lower than AAS. Conclusively, it was found that the solidification effect depends upon the composition of binder material, type of leaching extractant, nature and concentration of heavy metals in waste. The XRD, FTIR and SEM analyses confirmed that the solidification mechanism was carried out by both physical encapsulation and chemical fixation (dissolved into a crystal structure). Additionally, bentonite as an auxiliary additive significantly improved the solidification/stabilization of LZSS in AASB by enhancing the chemical adsorption capacity of heavy metals.

Application of iron-loaded activated carbon electrodes for electrokinetic remediation of chromium-contaminated soil in a three-dimensional electrode system.

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.

Waste solidification/stabilization of lead-zinc slag by utilizing fly ash based geopolymers.

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.

Solidification/stabilization of chromite ore processing residue using alkali-activated composite cementitious materials.

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.