Stabilization of simulated lead sludge with iron sludge via formation of PbFe₁₂O₁₉ by thermal treatment

Effect of Fe-based substance on Cr leaching behavior in hazardous waste incineration fly ash after thermal treatment

This study investigated the influence of the interaction between Fe-based substances and thermal treatment parameters on the leaching behavior of Cr in hazardous waste incineration fly ash (HWIFA) after thermal treatment. The results revealed that the interaction between the addition of Fe-based substance and the thermal treatment parameters and their effects on static and dynamic leaching behaviors of Cr had significant differences when Fe₂O₃, Fe₃O₄, and Fe were added, respectively. Specifically, when Fe₂O₃ or Fe was added, the thermal treatment temperature was the most significant factor affecting the static leaching of Cr in thermal treated HWIFA, and the interaction effect of other factors was not significant. The most important influence on the dynamic leaching behavior of Cr was the interaction between the thermal treatment temperature and the addition of Fe₂O₃. Different from the addition of Fe₂O₃, the effect of the addition of Fe₃O₄ on the static leaching of Cr in thermal treated HWIFA was more significant than that of thermal treatment temperature; meanwhile, the interaction between the thermal treatment temperature and the addition of Fe₃O₄ was also significant. However, when Fe₃O₄ was added, the effect of interaction between factors on the dynamic leaching of Cr in thermal treated HWIFA was consistent with that when Fe₂O₃ was added.

Pb Stabilization by a New Chemically Durable Orthophosphate Phase: Insights into the Molecular Mechanism with X-ray Structural Analysis

The rapid progression of piezoelectric technology and the upgradation of electronic devices have resulted in a global increase in Pb-based piezoelectric ceramic materials. In this study, the feasibility of incorporating Pb into a PbZr(PO₄)₂ double orthophosphate structure was evaluated by investigating the interaction mechanism of the perovskite with phosphate. The unique combination of X-ray absorption spectroscopy, selected area electronic diffraction, and Pawley refinement revealed that Pb was incorporated into a hexagonal structure and tetra-coordinated with oxygen in the phosphate-treated product. The chemical durability was enhanced through the structural alterations via Zr-O-P and Pb-O-P bond linkages. The stable phase encapsulating both Pb and phosphate showed effectiveness not only in stabilizing Pb but also in inhibiting P release as a secondary pollution risk within a wide pH range (1 ≤ pH ≤ 13). Despite the excellent chemical durability of the robust PbZr(PO₄)₂ crystalline phase, the increased Ti doping amounts at the Zr site resulted in a slight decrease in the lattice parameters and further enhanced the Pb stabilization effect through the formation of PbZr_xTi_(1-x)(PO₄)₂ solid solutions. This study demonstrates that the newly robust crystalline structure, developed through a well-designed thermal treatment scheme, provides an effective strategy for the treatment of Pb frequently encountered in electronic wastes.

Effectively immobilizing lead through a melanotekite structure using low-temperature glass-ceramic sintering

This work evaluated the feasibility of using low-temperature thermal immobilization based on the reaction mechanism forming the melanotekite (Pb2Fe2Si2O9) crystal phase to stabilize lead (Pb) containing waste. X-ray diffraction demonstrated that Pb could be incorporated into the melanotekite structure at easily attainable treatment temperatures (less than 500 °C) by magnetite and SiO2 precursors. The γ-Fe2O3 intermediate was found to play a key role in initializing melanotekite crystallization at a much lower temperature than that in traditional thermal immobilization techniques. Although a higher sintering temperature may increase Pb incorporation efficiency, amorphization occurred at temperatures higher than 950 °C. In addition, Pb was found to partition more in the amorphous phase of the SiO2-rich matrix. The results of the prolonged toxicity characteristic leaching procedure revealed a substantial improvement in the acid resistance of the targeted crystallized product sintered at 850 °C compared with the amorphous product and the other oxide products. The results of batch adsorption and subsequent thermal treatment verified the possibility of using the melanotekite structure to stabilize aqueous Pb with the Fe3O4@SiO2 residue. The study demonstrated that the melanotekite structure can be used to immobilize both solid and aqueous Pb through low-temperature thermal stabilization.

Formation of lead ferrites for immobilizing hazardous lead into iron-rich ceramic matrix

A strategy of immobilizing lead in the framework of ferrite-ceramic matrix, to reduce its environmental hazard was explored in this study. The mechanisms of incorporating lead into lead ferrites (δ-phase (2PbO·Fe₂O₃), γ-phase (PbO·(2-2.5)Fe₂O₃) and β-phase (PbO·(5-6)Fe₂O₃)) was revealed by observing the phase transformation in the products. The δ-phase was dominantly formed at low temperature of 700-800 °C at Pb/Fe of 1/1-1/3. The significant growth of γ-phase was observed at 750-850 °C and Pb/Fe of 1/4-1/7. The β-phase substantially formed at 900-1000 °C with Pb/Fe of 1/7-1/12. The transformation of δ-phase to γ-phase and/or β-phase indicated the destruction of δ-phase unit and reconstruction of γ-phase and β-phase units during sintering process. However, the transformation of γ-phase into β-phase suggested a structure conversion process, local structural changes arose as a consequence of the addition of Fe₂O₃. When comparing the leaching ability of δ-, γ- and β-phase, the results showed the superiority of β-phase for lead immobilization over the longer leaching period.

Transformation of hazardous lead into lead ferrite ceramics: Crystal structures and their role in lead leaching

This study quantitatively determined the transformation of lead into lead ferrite ceramics and examined the influence of structural defects in lead ferrites (i.e. Pb₂Fe₂O₅, PbFe₄O₇ and PbFe₁₂O₁₉) on lead leaching. Mechanisms of metal incorporation were examined from quantifying the phase compositions of lead ferrites in the products of sintering lead oxide with hematite. At low-temperature of 700°C, Pb was preferentially incorporated into the Pb₂Fe₂O₅ crystals, and the incorporation efficiency ranged from 25.7 to 97.5% depending on different Pb/Fe molar ratios. By increasing temperatures to 750-850°C, Pb₂Fe₂O₅ was subsequently reacted with hematite for the formation of PbFe₄O₇ and PbFe₁₂O₁₉ in Pb/Fe of 1/4 and 1/12 systems. PbFe₁₂O₁₉ was found to be the high-temperature (1000°C) stable phase for incorporating lead, and the incorporation efficiency ranged from 28.6 to 92.1% by different Pb/Fe molar ratios. Leaching tests demonstrated that PbFe₁₂O₁₉ was more resistant to acid attack than Pb₂Fe₂O₅ and PbFe₄O₇. The crystal structural defects in Pb₂Fe₂O₅ and PbFe₄O₇ were determined to be the factors influencing their intrinsic phase durability. On the other hand, PbFe₁₂O₁₉ was relatively free of structural defects and was found to be the preferred stabilization product to reduce the environmental hazard posed by lead.

The mechanisms of heavy metal immobilization by cementitious material treatments and thermal treatments: A review

Safe disposal of solid wastes containing heavy metals is a significant task for environment protection. Immobilization treatment is an effective technology to achieve this task. Cementitious material treatments and thermal treatments are two types of attractive immobilization treatments due to that the heavy metals could be encapsulated in their dense and durable wasteforms. This paper discusses the heavy metal immobilization mechanisms of these methods in detail. Physical encapsulation and chemical stabilization are two fundamental mechanisms that occur simultaneously during the immobilization processes. After immobilization treatments, the wasteforms build up a low permeable barrier for the contaminations. This reduces the exposed surface of wastes. Chemical stabilization occurs when the heavy metals transform into more stable and less soluble metal bearing phases. The heavy metal bearing phases in the wasteforms are also reviewed in this paper. If the heavy metals are incorporated into more stable and less soluble metal bearing phases, the potential hazards of heavy metals will be lower. Thus, converting heavy metals into more stable phases during immobilization processes should be a common way to enhance the immobilization effect of these immobilization methods.

Influence of an iron-rich amendment on chemical lability and plant (Raphanus sativus L.) availability of two metallic elements (As and Pb) on mine-impacted agricultural soils

Variation of the chemical extractability and phytoavailability of two metallic elements (e.g., As and Pb) on amendment-treated soils was investigated. Four mine-impacted agricultural soils contaminated with both As (174-491 mg kg^-1) and Pb (116-357 mg kg^-1) were amended with an iron-rich sludge at the rate of 5 % (w/w). After a 4-, 8-, and 16-week incubation, the extractability of metallic elements was assessed by sequential extraction procedure (SEP; F1-F5). The control without amendment was also run. In amended soils, the labile element mass (i.e., F1 + F2) promptly decreased (15-48 % of As and 5-10 % of Pb) in 4 weeks, but the decrement was continued over 16 weeks up to 70 and 28 % for As and Pb, respectively. The labile mass decrement was quantitatively corresponded with the increment of F3 (bound to amorphous metal oxides). In plant test assessed by radish (Raphanus sativus) grown on the 16-week soils, up to 57 % of As and 28 % of Pb accumulation was suppressed and 10-43 % of growth (i.e., shoot/root elongation and fresh weight) was improved. For both the control and amended soils, element uptake by plant was well correlated with their labile soil concentrations (r ² = 0.799 and 0.499 for As and Pb, respectively). The results confirmed that the iron-rich material can effectively suppress element uptake during R. sativus seedling growth, most likely due to the chemical stabilization of metallic elements in growth medium.

Formation of lead-aluminate ceramics: Reaction mechanisms in immobilizing the simulated lead sludge

We investigated a strategy of blending lead-laden sludge and an aluminum-rich precursor to reduce the release of hazardous lead from the stabilized end products. To quantify lead transformation and determine its incorporation behavior, PbO was used to simulate the lead-laden sludge fired with γ-Al2O3 by Pb/Al molar ratios of 1/2 and 1/12 at 600-1000 °C for 0.25-10 h. The sintered products were identified and quantified using Rietveld refinement analysis of X-ray diffraction data from the products generated under different conditions. The results indicated that the different crystallochemical incorporations of hazardous lead occurred through the formation of PbAl2O4 and PbAl12O19 in systems with Pb/Al ratios of 1/2 and 1/12, respectively. PbAl2O4 was observed as the only product phase at temperature of 950 °C for 3h heating in Pb/Al of 1/2 system. For Pb/Al of 1/12 system, significant growth of the PbAl12O19 phase clearly occurred at 1000 °C for 3 h sintering. Different product microstructures were found in the sintered products between the systems with the Pb/Al ratios 1/2 and 1/12. The leaching performances of the PbO, Pb9Al8O21, PbAl2O4 and PbAl12O19 phases were compared using a constant pH 4.9 leaching test over 92 h. The leachability data indicated that the incorporation of lead into PbAl12O19 crystal is a preferred stabilization mechanism in aluminate-ceramics.