A novel strategy for the efficient decomposition of toxic sodium cyanate by hematite

Quantitative Contribution of Oxygen Vacancy Defects to Arsenate Immobilization on Hematite

Hematite is a common iron oxide in natural environments, which has been observed to influence the transport and fate of arsenate by its association with hematite. Although oxygen vacancies were demonstrated to exist in hematite, their contributions to the arsenate immobilization have not been quantified. In this study, hematite samples with tunable oxygen vacancy defect (OVD) concentrations were synthesized by treating defect-free hematite using different NaBH4 solutions. The vacancy defects were characterized by positron annihilation lifetime spectroscopy, Doppler broadening of annihilation radiation, extended X-ray absorption fine structure (EXAFS), thermogravimetric mass spectrometry (TG-MS), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The results revealed that oxygen vacancy was the primary defect type existing on the hematite surface. TG-MS combined with EPR analysis allowed quantification of OVD concentrations in hematite. Batch experiments revealed that OVDs had a positive effect on arsenate adsorption, which could be quantitatively described by a linear relationship between the OVD concentration (Cdef, mmol m-2) and the enhanced arsenate adsorption amount caused by defects (ΔQm, μmol m-2) (ΔQm = 20.94 Cdef, R2 = 0.9813). NH3-diffuse reflectance infrared Fourier transform (NH3-DRIFT) analysis and density functional theory (DFT) calculations demonstrated that OVDs in hematite were beneficial to the improvement in adsorption strength of surface-active sites, thus considerably promoting the immobilization of arsenate.

Treatment of thiocyanate-containing wastewater: a critical review of thiocyanate destruction in industrial effluents

Thiocyanate is a common pollutant in gold mine, textile, printing, dyeing, coking and other industries. Therefore, thiocyanate in industrial wastewater is an urgent problem to be solved. This paper reviews the chemical properties, applications, sources and toxicity of thiocyanate, as well as the various treatment methods for thiocyanate in wastewater and their advantages and disadvantages. It is emphasized that biological systems, ranging from laboratory to full-scale, are able to successfully remove thiocyanate from factories. Thiocyanate-degrading microorganisms degrade thiocyanate in autotrophic manner for energy, while other biodegrading microorganisms use thiocyanate as a carbon or nitrogen source, and the biochemical pathways and enzymes involved in thiocyanate metabolism by different bacteria are discussed in detail. In the future, degradation mechanisms should be investigated at the molecular level, with further research aiming to improve the biochemical understanding of thiocyanate metabolism and scaling up thiocyanate degradation technologies from the laboratory to a full-scale.

Analysis of thermal decomposition of acidified sediments in gold plants and harmless disposal of it

In the present work, thermal decomposition of ASs in air was characterized by a combination of TG-DSC, XRD, and TG-FTIR. The treatment of generated toxic (CN)₂ gas was investigated as well. The result showed that the decomposition of Zn₂Fe(CN)₆ in ASs preferentially reacted with CuSCN leading to the early decomposition of ASs, in which a part of CuSCN decomposed into Cu₅FeS₄ or Cu₂S followed by being oxidized to sulfates and oxides as the temperature increased to 420 °C. For Zn₂Fe(CN)₆·3H₂O in ASs, the decomposition products below 500 °C include ZnS, ZnSO₄, Cu_xFe_yS_z, iron oxides and Zn(CN)₂; instead, Fe₃O₄, ZnSO₄ and ZnFe₂O₄ were formed. The FTIR and chemical quantitative analysis showed that nitrogen-containing gaseous products predominately contained (CN)₂, HCN and small amounts of NH₃ and NO_x. In view of toxic gases released, catalytic oxidation employing in-situ generation of roasting slag at 600 °C (AS1) can be effectively used for the conversion of (CN)₂ to N₂ under the optimal conditions of airflow rate of 0.7 L/min and AS1/ASs mass ratio of 0.5. Significantly, the ZnFe₂O₄ phase in AS1 completely disappeared and was converted to ZnSO₄ after the experiment, which facilitated the subsequent acid leaching, thereby achieving the synergistic treatment of exhaust gases and slag.

Insights into the improving mechanism of defect-mediated As(V) adsorption on hematite nanoplates

The fate of As(V) in subsurface environments is strongly affected by ubiquitous iron oxides. Defects are commonly present in natural hematite, while the impacts of defects on the active sites and complexation mechanism of hematite for As(V) remain poorly understood. In this study, the defect-rich hematite was employed to investigate the surface charge characteristics and As(V) adsorption behavior using potentiometric acid-base titration and CD-MUSIC model in comparison with corresponding defect-poor hematite. The total arsenate-active site density (5.7 sites/nm²) on defective hematite includes 1.2 sites/nm² of original sites and 4.5 sites/nm² of Fe vacancy-induced sites. The result revealed that the vacant Fe³⁺ sites in defective hematite was compensated by the protons in solution, thus resulting in a considerable increase in site density as well as positive charge. The CD-MUSIC modeling results demonstrated that the presence of Fe vacancies in hematite is beneficial to the improvement in affinity constants for both monodentate and bidentate arsenate complexes. The high adsorption capacity of defective hematite (2.60 μmol/m²) compared to defect-free hematite (1.33 μmol/m²) is attributed to its large affinity constants as well as its more active surface sites, thereby playing a vital role in reducing the threats of heavy metals in the environment.

Calcination of Calcium Sulphoaluminate Cement Using Pyrite-Rich Cyanide Tailings

Pyrite-rich cyanide tailings (CTs) are industrial hazardous solid wastes arising from the gold mining industry. Every year, hundreds of millions of tons of cyanide tailings are produced and discharged to tailings dams. It is of great significance to dispose of cyanide tailings harmlessly and resourcefully. The feasibility of calcination of calcium sulphoaluminate (CSA) cement clinker using pyrite-rich cyanide tailings as Fe2O3 and SO3 sources was investigated for this paper. The behavior of pyrite during the calcination of cyanide tailings under various calcination conditions and the properties of calcium sulphoaluminate cement clinker were examined. The results show that it is feasible to produce calcium sulphoaluminate cement clinker using pyrite-rich cyanide tailings. The optimal conditions for the calcination of calcium sulphoaluminate cement using pyrite-rich cyanide tailings are confirmed. During the calcination process, the cyanides decompose into carbonate, CO2, and N2. The pyrite decomposes into Fe2O3 and SO2, and they react with CaO and Al2O3 to form the intermediates of CaSO4, 2CaO·Fe2O3, and CaO·2Al2O3, which further react to form 3CaO·3Al2O3·CaSO4, 4CaO·Al2O3·Fe2O3, and 12CaO·7Al2O3. The calcium sulphoaluminate cement prepared by pyrite-rich cyanide tailings exhibits excellent mechanical properties and meets the compressive strength criteria of 42.5 grade calcium sulphoaluminate cement.