Oxidation reaction behavior of Cr-hosting spinels during heating of solid wastes containing Cr

Chromium speciation and mobility in contaminated coastal urban soils affected by water salinity and redox conditions

Chromium (Cr) is a redox-sensitive element in contaminated coastal urban soils. Sea level rise (SLR) with subsequent soil inundation may facilitate Cr transformation and mobilization through alterations in local redox conditions and porewater ion composition. We investigated the impact of water salinity and redox conditions on Cr chemistry in these environments. Synchrotron-based X-ray spectroscopy and wet chemical analyses revealed that the soils contained very high levels of Cr (up to 4320 mg kg-1) and that chromite (∼52%) and Fe-Cr hydroxide coprecipitates (∼44%) were the predominant Cr species. The abundance of these two components resulted in low Cr mobility under non-flooded conditions. Chromium(II) was identified in the soils, potentially derived from the waste parent material. Seawater and anoxic conditions resulted in lower Cr release compared to freshwater and aerobic conditions. Up to three to eight times more Cr was released under aerobic conditions versus anaerobic conditions in the freshwater versus saltwater, respectively, with total dissolved Cr values remaining below 0.02 mg L-1. The decrease in Cr release was likely due to Cr reduction by Fe(II) and sulfide. This work provides important information on how salinity and redox fluctuations impact Cr cycling which is likely to occur during SLR.

Formation kinetics and reaction behavior of pentavalent chromium formed in the cement kiln co-processing of solid waste

Cement kiln co-processing is becoming the main strategy to dispose of hazardous waste containing Cr. A newly-discovered pentavalent Cr compound, which was proved to be formed during cement kiln co-processing of solid waste, is partly responsible for the water-soluble Cr released from the cement. However, the formation characteristics and the solubility of Cr(V) are still unclear to date. In this study, the reaction kinetics and further redox reactions of Cr(V) at high temperature were examined, and its crystal structure and solubility were also explored. At the temperature range of 1000-1200 °C, the formation rate of Ca5(CrO4)3O0.5 reached over 90 % within 10 min, and then slowly increased to near 100 % from 10 min to 10 h. shows that Ca5(CrO4)3O0.5 is formed by interface reaction at an early period, and by diffusion at a later period. The kinetic analysis indicates that Ca5(CrO4)3O0.5 is initially formed through an interface reaction and subsequently through diffusion. Ca5(CrO4)3O0.5 was identified and assigned as hexagonal crystal group (P63/m). Approximately 0.55 g and 0.15 g of Ca5(CrO4)3O0.5 dissolve in neutral water at 100 °C and 50 °C, and the concentrations of Cr(V) in water reach 550 and 150 mg/L, respectively. Additionally, this study finds that at the temperature range of 400-700 °C Ca5(CrO4)3O0.5 can be oxidized into CaCrO4, and at the temperature higher than 1400 °C, it can be further converted into Ca3(CrO4)2 and reduced into CaCr2O4. This study gives a deep insight into Cr oxidation-reduction reaction during thermal treatment of solid waste. These insights provide a comprehensive understanding of Cr oxidation-reduction reactions during the thermal treatment of solid waste, offering valuable guidance for waste management strategies.

Immobilization of chromium in real tannery sludge via heat treatment with coal fly ash

The secure and harmless disposal for Cr-bearing tannery sludge (Cr-TS) has attracted an increasing concern, due to potentially adverse effect on ecosystem and human health. A greener alternative method about "waste treatment with waste" for thermally stabilizing real Cr-TS was developed via employing coal fly ash (CA) as dopants in this research. The co-heat treatment of Cr-TS and CA was carried out at the temperature range of 600-1200 °C to investigate the oxidation of Cr(III), immobilization of chromium and leaching risk of the sintered products, and the mechanism of chromium immobilization was further explored. The results indicate that the doping of CA can significantly inhibit the oxidation of Cr(III) and immobilize chromium by incorporating chromium into spinel and uvarovite microcrystal. At the temperature higher than 1000 °C, most of chromium can be converted into stable crystalline phases. Furthermore, a prolonged leaching test was conducted to study the leaching toxicity of chromium in sintered products, indicating that leaching content of chromium is much less than the regulatory limit. This process is a feasible and promising alternative for immobilization of chromium in Cr-TS. The research findings are supposed to offer a theoretical foundation and strategy choice for thermal stabilization of chromium, as well as safety and harmless disposal of Cr-containing hazardous waste.

Environmental availability of trace metals in a fired brick elaborated from a marine dredged sediment

Each year, hundreds of millions of tons of sediments are dredged around the world. Alternatively to sea or land disposal, the reuse of these sediments as raw material in various civil engineering applications is developing. In this context, the French SEDIBRIC project (valorisation de SEDIments en BRIQues et tuiles) aims to replace, in the preparation of clay-fired bricks, a part of natural clays by harbor dredged sediments. The present study focuses on the fate of some potentially toxic elements (Cd, Cr, Cu, Ni, Pb, and Zn) that are initially present in the sediments. A fired brick is elaborated exclusively from one dredged sediment, after a desalination step. The total content of each element of interest is evaluated by ICP-AES, after a microwave-assisted acid (aqua regia) digestion, in the raw sediment and in the brick. Then, single extractions (H₂O, HCl, or EDTA as reactant) and one sequential extraction procedure (according to Leleyter and Probst, Int J Environ Anal Chem 73(2): 109-128 1999) are applied to the raw sediment and to the brick, in order to assess the environmental availability of the elements of interest. For Cu, Ni, Pb, and Zn, the results obtained with the various extractions procedures applied are consistent and confirm that the firing process induces their stabilization in the brick. The availability however increases for Cr and remains unchanged for Cd.

Electroplating sludge-derived metal and sulfur co-doping catalyst and its application in methanol production by CO2 catalytic hydrogenation

Electroplating sludge is a hazardous waste and its recycling is a hot topic. Electroplating sludge usually contains plenty of transition metals and multi-hetero atoms, which are potential resources. For the first time, this work synthesized spinel catalyst from Zn- and Cr-containing electroplating sludges by a simple calcination method, and applied the obtained catalysts in CH₃OH production by CO₂ catalytic hydrogenation. The spinel was doped by various heteroatoms, including Fe, Mn, Cu, and even S. According to detailed characterizations, the metal doping increased the low-temperature conversion efficiency of CO₂ but decreased the CH₃OH selectivity at the same time. After a further doping of S, although CO₂ conversion efficiency was slightly decreased, the selectivity of CH₃OH was significantly increased. After all, the optimized catalyst attained a conversion efficiency of 8.6% (CO₂) as well as a selectivity of 73.3% (CH₃OH) at 250 °C and 3 MPa. As a result, above results realized high-value-added utilization of hazardous waste and producing valuable product at the same time, which was in favor of circular development.

Oxidation and reduction reactions of (Al/FexCr1-x)2O3 caused by CaO during thermal treatment of solid waste containing Cr

Solid solutions of (Al_xCr_1-x)₂O₃ and (Fe_xCr_1-x)₂O₃ are predominant compounds containing Cr in solid waste and are frequently formed during thermal treatment of solid waste. (Al_xCr_1-x)₂O₃ and (Fe_xCr_1-x)₂O₃ have superior thermomechanical properties and excellent corrosion resistance. However, oxidation and reduction reactions of the Cr in these solid solutions seriously affect their chemical stabilities and the environmental risks posed by the final products. In this study, first the reaction behaviors of (Al_xCr_1-x)₂O₃ and (Fe_xCr_1-x)₂O₃ at high temperatures were analyzed and whether the incorporation of Cr(III) in solid solutions can prevent Cr(III) from being oxidized was determined. Both (Al_xCr_1-x)₂O₃ and (Fe_xCr_1-x)₂O₃ without the presence of CaO exhibit good thermal stability at high temperatures. However, the participation of CaO induces Cr(III) oxidation in (Al_xCr_1-x)₂O₃ and (Fe_xCr_1-x)₂O₃ at 500-1000 °C. Cr(III) oxidation in these solid solutions is accompanied by the formation of CaCrO₄ and Fe₂O₃ or Al₂O₃. Al₂O₃ combines with CaCrO₄ and further forms a more stable Cr(VI) compound (e.g., Ca₄Al₆O₁₂CrO₄). While Fe₂O₃ combines with CaCrO₄ at 1000-1200 °C. This is accompanied by the formation of CaCr₂O₄ and CaFe₂O₄, which effectively promotes the reduction of Cr(VI). Moreover, part of the CaCr₂O₄ transforms into a more stable phase (i.e., FeCr₂O₄) at 1200-1300 °C. Although the incorporation of Cr(III) in these solid solutions cannot prevent Cr(III) oxidation completely at high temperatures, the Cr(III) oxidation in these solid solutions is still suppressed compared with Cr₂O₃. The results of this study provide further insights into the oxidation and reduction reactions of Cr-hosting compounds during thermal treatment of solid waste.