The application of nanoscale zero-valent iron (nZVI) and dissimilatory iron-reducing bacteria for environmental remediation is restricted by nZVI passivation and reactive oxygen species (ROS)-induced bacterial inactivation. To address these challenges, a dual-enhancement strategy using natural dissolved humic substances (DHS) was developed, integrating metabolic priming and electron shuttling to establish a highly active MR-120/nZVI/DHS system. Under optimal conditions, this system completely eliminated 80 mg/L Cr(VI) within 48 h, demonstrating a 2.94-fold increase in the apparent reduction rate constant (kobs) versus the traditional MR-1/nZVI system. Mechanistically, DHS-induced metabolic priming stimulates substantial extracellular polymeric substances (EPS) secretion. EPS and exogenous DHS form an electroactive microlayer that physically shields cells from nanoparticles and Cr(VI) toxicity, and directly quenches extracellular hydroxyl radicals (·OH). Meanwhile, metabolic priming pre-activates intracellular antioxidant defense and enhances ROS scavenging, alleviating nZVI/Cr(VI) combined toxicity and greatly improving bacterial survival. Benefiting from the protective microenvironment, intracellular energy (ATP) and reductive power (NADH) levels enhanced by 1.74- and 2.78-fold, respectively. These surges triggered 8.88- and 48.64-fold upregulation of outer membrane cytochrome c (Cyt c) and endogenous riboflavin (RF), effectively activating a dual-channel electron transfer network and driving a 15.71-fold surge in overall electron transport system activity (ETSA). The enhanced electron flux accelerates dissolution of the nZVI surfaces passivation layer, continuously regenerating active biogenic Fe(II) to reduce Cr(VI) and drive its co-precipitation into stable FeCr2O4 spinels. This work presents an efficient approach for treatment of Cr(VI)-contaminated wastewater and offers new theoretical insights into synergistic detoxification at the bio-nanomaterial interface under heavy metal stress.

The practical application of Shewanella oneidensis MR-1 in heavy metal bioremediation is greatly restricted by its low extracellular electron transfer (EET) capacity. To address this bottleneck, we developed a high-efficiency "treating waste with waste" Cr(VI) bioreduction system using sludge-derived hydrothermal liquid (HTL) as a natural electron shuttle. Our results showed that HTL prepared at 210 °C (HTL210) increased the Cr(VI) bioreduction rate constant (kobs) by 20.33-fold, achieving complete reduction of 20 mg/L Cr(VI) within 48 h. Furthermore, a quantitative prediction model linking kobs to HTL spectral indices (SUVA, HIX) and electron transfer capacity (ETC) was established for non-destructive prediction of the system's reduction performance. Multi-scale analyses elucidated the underlying mechanisms of this enhancement. Extracellularly, HTL210 accelerates EET and induces massive extracellular polymeric substance (EPS) secretion, driving rapid Cr(VI) reduction and in-situ immobilization. Intracellularly, it alleviated Cr(VI) toxicity by scavenging reactive oxygen species (ROS) and drives bioenergetic reprogramming to boost ATP/NADH supplies. Crucially, combined transcriptomic profiling and RT-qPCR validation indicated that HTL210 induces adaptive transcriptional reprogramming of the EET pathway, suggesting a potential shift of electron flow from the energy-intensive mtrC pathway to a quinone-adapted mtrF network. This study reveals the intrinsic molecular mechanisms of HTL-enhanced EET, providing a novel paradigm for sludge HTL valorization and low-cost heavy metal-contaminated water remediation.

[Objective] The flotation purification of phosphogypsum is an important way for its resource utilization. To solve the problems of traditional flotation collectors such as sensitivity to slime, poor solubility, difficulty in degradation and instability, [Method] based on the phosphogypsum of a mining company in Yichang, Hubei Province, a self-made quaternary ammonium salt collector (LLSG-1) was developed. The experiments of reverse flotation purification, impurity removal and whitening of phosphogypsum were carried out. [Result] Using the raw ore with a purity of 88.14%, a whiteness of 33.88%, and a SiO2 content of 6.26%, at the condition of pH value ranging from 2.3 to 2.5, with LSG-1 as the collector and MIBC as the foaming agent, after three-stage flotation, a purity of 93.61% and a whiteness of 65.19% were obtained. Phosphogypsum concentrate with a SiO2 content of 1.87% was also obtained. [Conclusion] The quality of the concentrate meets the first-class standard for building gypsum as stipulated in GB/T 23456-2018 "Phosphogypsum". The research provides technical support for the efficient recovery of phosphogypsum resources.

Chromite ore processing residue (COPR), a by-product of chromate production, contains significant amounts of environmentally hazardous Cr(VI). This study presents a synergistic “electrochemical pre-reduction-geopolymer deep stabilization” (G-E-COPR) process to efficiently detoxify and achieve long-term stabilization of toxic Cr(VI) in COPR, addressing the inefficiencies of traditional direct immobilization techniques and the susceptibility of Cr(III) to re-oxidation post-detoxification. The results show that Fe(II), generated in situ during the electrochemical pre-reduction step, effectively reduces Cr(VI) to Cr(III), which is then stabilized as Cr(III)-Fe(III) hydroxide in the solid phase (E-COPR). This process effectively eliminates the inhibitory effect of Cr(VI) on subsequent geopolymerization reactions. Compared to directly solidified COPR (G-COPR), the G-E-COPR solidified monoliths exhibit a denser microstructure, with superior chromium immobilization and mechanical properties. After curing for 28 days, the unconfined compressive strength reached 8.13 MPa, and Cr(VI) leaching concentrations remained consistently below the EPA limit (0.5 mg/L), in compliance with Chinese mine backfilling standards, and the material also demonstrated excellent freeze-thaw resistance and acid rain erosion durability. Mechanistic analysis indicates that the electrochemical pre-reduction promotes the formation of a dense N-(A)-S-H (sodium aluminosilicate-hydrate) gel network, wherein Cr(III) is effectively immobilized through both physical encapsulation and chemical bonding mechanisms, such as isomorphic substitution of Cr(III) for [AlO4] tetrahedra. Life cycle assessment revealed that this process exhibits significantly lower global warming potential and fossil resource consumption compared to conventional immobilization methods, underscoring its low-carbon advantages. This study offers a technically feasible and environmentally friendly strategy for the detoxification and resource utilization of COPR.

Sulfate-reducing bacteria (SRBs) and dissimilatory iron-reducing bacteria (DIRBs) are recognized as significant contributors to the occurrence of elevated arsenic (As) levels in groundwater. However, the precise effects and underlying mechanisms of their interactions on As behavior within sediments remain poorly understood. In this investigation, we compared the impacts and mechanisms of DIRBs, SRBs, and mixed bacterial consortia on the migration behavior of As and Fe/S species. Our findings revealed that during the initial phase of the reaction (0-8 days, Stage 1), the mixed bacterial consortium facilitated As release by intensifying the reduction of Fe(III) and sulfate, resulting in a maximum As concentration 1.5 times higher than that observed with either DIRBs or SRBs in isolation. Subsequently, in the intermediate phase (8-20 days, Stage 2), the mixed consortium suppressed the synthesis of sulfate reductase and the secretion of toxic substances (e.g., o-Methyltoluene) associated with steroid degradation pathways. This inhibition consequently reduced the formation of secondary Fe minerals and the fixation of As. Finally, in the latter stage (20-30 days, Stage 3), the system responded to the threat of toxic substances by secreting significant amounts of organic acids to facilitate their decomposition. However, this process also led to the re-decomposition of iron oxides, resulting in the release of As. These observations shed light on the intricate interplay between DIRBs and SRBs within bacterial consortia, elucidating their coordinated actions in inducing the migration and transformation of arsenic.

Phosphogypsum flotation tailings (PFT) pose significantly greater environmental risks than raw phosphogypsum, making their effective treatment critical for improving the resource utilization efficiency of phosphogypsum via reverse flotation technology. This study employed calcium carbide residue (CCR) and red mud (RM) as co-immobilization agents to pre-immobilize soluble phosphorus and fluoride in PFT via mechanochemical enhancement, then optimizing the gelling conditions to produce stable and safe mine pit backfill materials. Results indicated that the alkali-mechanical coupling effect significantly pre-immobilization efficiency, reducing soluble phosphorus and fluorine in PFT to 0.34 and 8.11 mg/L, respectively. Immobilization mechanisms for soluble phosphorus and fluoride in pre-immobilization residue (BM-RM-CCR-PFT) include the formation of stable calcium-aluminum phases, ion exchange, and physical encapsulation. G-BM-PRC backfill materials, prepared from BM-RM-CCR-PFT, exhibit superior fluorine and phosphorus immobilization efficiency and mechanical properties compared to those made directly from raw PFT (G-PRC), effectively achieving fluorine stabilization. The contents of unstable phosphorus and fluorine in G-BM-PRC were significantly lower than in G-PRC, primarily due to pre-immobilization treatment, which promoted the formation of hydration products. This process facilitated the generation of hydroxyapatite and stable fluorides, thereby promoting the transformation of unstable phosphorus and fluorine into more stable forms. DFT calculations further confirmed that PO43-/F-can be adsorbed onto hydration product surfaces and subsequently substituted for SiO4 4-/OH-. Consequently, deep stabilization mechanisms include chemical precipitation, gel encapsulation, and physical adsorption. Additionally, SPLP leaching and freeze-thaw cycle tests validated the long-term stability of G-BM-PRC. This work provides a viable strategy for the resource utilization of PFT.

Aggregation and oxidative passivation critically compromise the adsorption and reduction capabilities of nano zero-valent iron (nZVI), which diminishes its environmental remediation efficacy. In this study, natural dissolved humus extracts (DHE) were used to modify nZVI (DHE-nZVI) for superior Cr(VI) clarification. Experimental results showed that the unmodified nZVI only removed 63.33 % of 20 mg/L Cr(VI) within 60 min, whereas DHEnZVI achieved complete Cr(VI) removal within 30 min. DHE-nZVI exhibited an 8.53-fold increase in the apparent Cr(VI) removal rate constant relative to unmodified nZVI. Concurrently, DHE-nZVI exhibited superior stability, oxidation resistance, and reusability, maintaining 94.17 % removal efficiency after 7 days of aging and 56.67 % after three reuse cycles. Characterization analysis revealed that the DHE modification: (i) amplified the Kirkendall effect in DHE-nZVI, facilitating Fe atoms/electrons outward migration and Cr(VI) inward diffusion; (ii) significantly increased surface Fe(0) and Fe(II) content from 0.81 % to 8.04 % and 51.63 % to 57.44 %, respectively, accelerating interfacial electron transfer and Cr(VI) reduction rates; (iii) strengthened Cr(VI) adsorption affinity, evidenced by reduced adsorption energy (-5.14 eV vs. -3.20 eV for nZVI). Notably, the DHE modifier is derived from naturally abundant humus, and the resulting DHE-nZVI composite exhibits high Cr(VI) removal efficiency in both simulated and actual water matrices. This study not only proposes novel strategies for nZVI modification and Cr(VI) pollutant remediation, but also unveils new environmental functions of humus fractions, offering fresh perspectives on ecosystem carbon cycling and carbon utilization.

Nano zero-valent iron (nZVI) is a promising material for the remediation of Cr(VI) contamination. However, its inherent defects in easy agglomeration and passivation limit its large-scale application. To address these limitations, this study first introduced the environmentally ubiquitous natural dissolved humus substances (DHS) into the nZVI system. The results proved that DHS-mediated effectively improved the reactivity of nZVI, thereby promoting Cr(VI) removal. Physicochemical properties analysis revealed that DHS with a higher aromaticity and molecular weight was more favorable to Cr(VI) removal. Notably, the optimized nZVI/EL system could achieve up to 4.8 times enhancement in Cr(VI) removal rate compared to pristine nZVI. Three synergistic mechanisms of DHS promoted the removal of Cr(VI): (i) DHS promoted nZVI corrosion to generate H*; (ii) Surface-adsorbed DHS on nZVI enhanced its reactivity by promoting the dissolution of the surface passivation layer; (iii) the redox-active groups in DHS can promote the conversion of Fe(III) to Fe(II) via ligand-to-metal charge transfer (LMCT) pathways. Therefore, by exploring the new functions of DHS as an environmentally friendly material and organic carbon component, this study provided a fresh perspective for green remediation of heavy metal pollution and carbon recycling in ecosystems.

In the reverse flotation process for desilication and impurity removal from phosphogypsum (PG), quaternary ammonium salts are commonly used as cationic collectors, but their selectivity and stability under strongly acidic conditions are limited. In this study, for the first time, dodecyl-dimethyl-ethyl benzyl ammonium chloride (DDEA) and dodecyl-bis(2-hydroxyethyl)-methyl ammonium chloride (2HEAC-12), were used as collectors for the PG co-reverse flotation desilication and impurity removal. Flotation experiments show that both DDEA and 2HEAC-12 collectors exhibit good desilication and impurity removal performance for PG, whereas DDEA shows superior selectivity for quartz than that of 2HEAC-12. When DDEA and 2HEAC-12 were used as collectors, the purity of dihydrate gypsum in obtained PG concentrates was 94.88% and 93.47%, respectively, meeting the firstgrade standard for PG used as a building material (GB/T 23456-2018). Adsorption experiments indicate that the adsorption of both collectors on quartz and dihydrate gypsum follows the Langmuir model, suggesting monolayer physical adsorption. Both collectors exhibit a higher selectivity for quartz than dihydrate gypsum. The adsorption process was mainly accomplished by electrostatic interactions and hydrogen bonds. The adsorption process was mainly accomplished by electrostatic interactions and hydrogen bonds. This work provides a viable strategy for developing collectors for PG co-reverse flotation efficient desilication and impurity removal.

Impurities such as silicon, organic matter and fluoride restrict the utilization of phosphogypsum (PG). In this study, dodecyl dimethyl ethylbenzyl ammonium chloride (DDEA) was used as the reverse flotation collector to remove impurities from PG. At the natural pH (2.3-2.5) of the slurry, with pine oil and DDEA dosages of 300 g/t, slurry concentration of 30 %, and aeration of 0.20 m3/h, the content of SiO2, organic matter, and fluoride ions in PG can decreased from 6.87 wt%, 0.94 wt%, and 0.42 wt% to 1.90 wt%, 0.13 wt%, and 0.03 wt%, respectively, and the whiteness increased from 33.67 to 64.41 %. Correspondingly, the obtained CaSO4.2H2O purity is 94.44 %, meeting first grade standard of PG used as building materials (GB/T 23456-2018). Characterization analysis revealed that flotation primarily removed the quartz phase present in the fine particles. DDEA exhibited high selectivity for adsorption on the quartz surface by significantly affecting the electronegativity and hydrophobicity of the quartz surface. It was mainly adsorbed on the quartz surface through electrostatic attraction with SiO-and hydrogen bonding with Si-O bonds, without undergoing chemical reactions. Compared with traditional flotation methods, this process achieved the simultaneous removal of SiO2, organic matter, and fluoride, which has potential industrial application prospects.

The soda ash chromite ore processing residue (SA-COPR) is a hazardous solid waste generated inevitably in the production process of chromate, even if detoxified, it is prone to the reoccurrence of Cr(VI). Copper slag (CS) is mainly composed of Fe(II) rich mineral-fayalite and can be used as a potential reductant. In this study, the SACOPR detoxification assisted by CS under ball milling was investigated and the optimal results indicated a TCLP total Cr concentration of 3.65 mg/L, which met the US EPA standards (<5 mg/L). The detoxified SA-COPR was then solidified by montmorillonite for long-term stabilization. Results showed that the Cr in solidified body is further fixed in three forms: physical encapsulation, physical adsorption, and chemical bonding. The 28d compressive strength of the solidified body with 50% SA-COPR content is 11.83 +/- 0.83 MPa, meeting the GB/T 50081-2002 standard (>10 MPa). Moreover, the long-term stabilization of the solid body after 120 days was examined by TCLP and only yielded a total Cr concentration of 0.139 mg/L, which was significantly lower than the EPA standard. Overall, this complex reduction and immobilization of SA-COPR by copper slag-ball millmontmorillonite provide a new reference for the efficient reduction and "re-yellowing" inhibition as well as subsequent resource utilization.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous at relatively high concentrations by atmospheric deposition, and they are threatening to the environment. In this study, the toxicity of naphthalene on tall fescue and its potential responding mechanism was first studied by integrating approaches. Tall fescue seedlings were exposed to 0, 20, and 100 mg L−1 naphthalene in a hydroponic environment for 9 days, and toxic effects were observed by the studies of general physiological studies, chlorophyll fluorescence, and root morphology. Additionally, Ultra Performance Liquid Chromatography - Electrospray Ionization - High-Resolution Mass Spectrometry (UPLC-ESI-HRMS) was used to depict metabolic profiles of tall fescue under different exposure durations of naphthalene, and the intrinsic molecular mechanism of tall fescue resistance to abiotic stresses. Tall fescue shoots were more sensitive to the toxicity of naphthalene than roots. Low-level exposure to naphthalene inhibited the electron transport from the oxygen-evolving complex (OEC) to D1 protein in tall fescue shoots but induced the growth of roots. Naphthalene induced metabolic change of tall fescue roots in 12 h, and tall fescue roots maintained the level of sphingolipids after long-term exposure to naphthalene, which may play important roles in plant resistance to abiotic stresses.

The development of sustainable methods for the control and bio-stabilization of arsenic in sediments, without generating secondary pollution, is an urgent technological need. In this study, we utilized three types of natural carbon-containing biomass (NCCB) to explore the stabilization of arsenic through the synergistic action of native sediment microbiomes. We also examined the metabolic pathways of microorganisms following the introduction of NCCB into high-arsenic sediments, aiming to elucidate the biological processes critical for arsenic bio-stabilization. Our findings indicate that humic acid (HA) and soil organic matter (SOM) are effective in preventing the leaching of As(III) from sediments, while fulvic acid (FA) and SOM can significantly reduce the leaching of As(V). Furthermore, the introduction of NCCB into the system altered the biological metabolic processes, with notable upregulation of metabolites such as 8-hydroxyondansetron, 1,2,3,5,6,8-hexathionane, and citric acid. These results hold promise for the application of these findings in the management of arsenic in natural sediments.

Copper smelting slag (CSS) are waste slag obtained from smelters after reusing sulphur smelting slag. This study explores the potential of CSS to serve as a resource in cement mortar construction. Specifically, the study investigates the use of mechanical and chemical methods to enhance the volcanic ash activity of CSS, enabling them to replace up to 30 % of the cement content in cement mortar. The modified CSS was analyzed in terms of particle size and (Toxicity Characteristic Leaching Procedure) TCLP testing, while cement mortar specimens were subjected to a battery of tests including compressive strength, Freeze-thaw experiment, TCLP testing and cement stability testing. The results showed that compared with the unmodified CSS material, the copper smelting slag cement material with CaCO3 3 meets the requirements of GB/T 1596-2017 on the standard compressive strength of OPC 42.5 grade, with a compressive strength of 38.88 MPa at 10 % CaCO3 3 admixture, among which the CSS cement material with 10 % CaCO3 3 is the best and meets the leaching toxicity standard. Moreover, the modified CSS reduced energy consumption by 7.15 %, CO2 2 emissions by 27.41 %, and cost by 19.84 %. XRD, FTIR and SEM analysis showed that the mechanical activation of CaCO3 3 doping more drastically damaged the crystal structure of CSS, and local lattice distortion occurred, which induced the transformation of CSS from crystalline phase to amorphous phase and destroyed the ordered structure of minerals, resulting in the volcanic ash activity increased. Overall, this study demonstrates that CSS can serve as a viable raw material in cement mortar samples, reducing environmental impact and achieving resourceful use of slag.

In this paper, an adsorbent composite called S-PEMR, which consists of electrolytic manganese residues (EMR)silicate minerals-based polyacrylic acid-polyacrylamide double-network hydrogels, has been developed. The successful synthesis was confirmed by the characteristic peaks of COO, amide I, C--O, -CH2, C-N group vibration in FTIR spectra as well as amorphous carbon in XRD patterns. This composite tackles the issue of harnessing EMR, which consists of layered silicate minerals and additional elements. These elements have potential value but are difficult to be effectively and safely utilized. It was observed that the adsorption process agreed with the quasi-secondary model and the Langmuir isotherm, the adsorption process is controlled by a chemisorption mechanism with the occurrence of electron sharing or electron transfer (establishment of covalent bonds) between the S-PEMR and the adsorbate. The S-PEMR was found to be recyclable up to four times. In the initial experiment, the concentrations of Ni, Cu, Cd, and Pb in a natural wastewater were recorded as 645.84, 906.21, 378.15, and 8.29 mu g/L, respectively. The composites used in the study exhibited high removal efficiency for Ni (42.06 %), Cu (93.14 %), Cd (89.68 %), and Pb (77.02 %). The S-PEMR can be recycled up to four times with only a moderate reduction in adsorption performance. The removal mechanism primarily involved chelation or coordination, ion exchange, functional groups containing elements such as O and N, as well as the interaction of Si-O and Al-O with the heavy metal ions. This research establishes S-PEMR as a highly effective adsorbent for removing harmful contaminants from water.

Electrolytic manganese slag (EMS), a bulk waste generated in industrial electrolytic manganese production, can be a cost-effective adsorbent for heavy metals removal after appropriate modification. In this study, EMS was activated by NaOH and then used to make the EMS-based double-network hydrogel (an EMS/PAA hydrogel) via a one-pot method. The results showed that the EMS/PAA hydrogel exhibits a high selective adsorption capacity of 153.85, 113.63 and 54.35 mg·g-1 for Pb (II), Cd (II) and Cu (II), respectively. In addition, Density Functional Theory (DFT) suggests that the adsorption energies (Ead) of Pb, Cd and Cu on SiO2/PAA of the EMS/PAA gels are - 4.15, - 1.96, and - 2.83 eV, respectively, and SiO2/PAA, with a strong affinity to Pb2+, is one of the reasons for the selective adsorption capacity of EMS/PAA gel for Pb2+. The removal efficiency of the EMS/PAA gel for Pb2+, Cd2+, Cu2+ decreased after four adsorption-desorption cycles by 20.00 %, 24.56 % and 46.56 %, respectively. Mechanism studies suggested that the elimination of the heavy metals by EMS/PAA gels mainly involves electrostatic attraction, inner-sphere complexation, and coordination interactions. The EMS/PAA hydrogels not only have high adsorption capacity, but are also easy to prepare and circulate, making them ideal for practical applications.

磷石膏(PG)是磷肥工业中一种含有稀土元素的废弃物,稀土元素含量低但储量大,从磷石膏中回收稀土元素具有较大的经济价值.本研究重点是磷石膏的深入表征以及浸出过程研究,采用微波辅助酸浸磷石膏实现稀土元素浸出的方法.结果表明:在微波辅助条件下,当硝酸浓度为2.0 mol/L,固液比为6:100(g/mL),浸出温度为 80℃,浸出时间为 30 min 时,磷石膏稀土总元素的浸出率为96.52%.借助SEM、XRD探究微波酸浸机理,进一步表明在微波酸浸过程中由于微波、高温等相互作用会破坏晶体结构使颗粒细化,从而促使微波浸出效果比水浴浸出效果高12.3%.

In order to investigate the impurity eliminations method of phosphogypsum in Hubei Province,improve the purity and whiteness,the automatic mineral analysis system(AMICS)was applied to systematically investigate the process mineralogy,including the mineral composition and characteristics,the elemental distribution,the particle size distribution,the embedded characteristics and dissociation of important minerals.The process mineralogy showed that the main minerals of the phosphogypsum sample were gypsum with a content of 80.46%,followed by quartz and feldspar,phosphate rock,and a small amount of muscovite,plagioclase and barite.The metallic minerals mainly included pyrite,limonite and rutile with low content.The results showed that wet ball milling significantly improved the whiteness of the flotation concentrate,but with the increase of grinding time,the contents of silicon,organic matter and soluble P2O5 were reduced.The phosphogysum concentrate indexes were the best when the grinding time was 8 min,its yield was 73.56%,whiteness was 73.89%,SiO2 content was 1.81%,soluble P2O5 content was 0.069%,soluble F-content was 0.032%,and organic matter content was 0.012%.

The traditional chromite ore processing residue (COPR) detoxification process is carried out in strong acid or anoxic high-temperature condition, which not only has a high input cost but leading difficulties for resource utilization of treated COPR. This study first reported the efficient reduction of Cr(VI) and recovery of Fe from COPR by waste molasses-assisted hydrothermal treatment. Under optimal conditions (i.e., the Na2CO3 dosing of 0.5 mol/L, the molasses/COPR mass ratio of 10%, hydrothermal treatment at 140 degrees C for 150 min), the Cr(VI) reduction efficiency reached 99.96 +/- 0.01%, and the leaching toxicity of total Cr met the USEPA regulatory limit of 5 mg/L. Moreover, Fe was recycled in the form of magnetic magnesioferrite with Fe2O3 grade of 52.58 +/- 0.90%, which can be used as raw material in steel industry. The Cr(VI) reduction reaction conforms to a pseudo-first-order kinetic model, and the reaction rate constants increase with increasing temperature (120 similar to 160 degrees C). As a potential mechanism of the reaction, the Mg-Al hydrotalcite (LDHs) structure collapses in the presence of Na2CO3 during the hydrothermal treatment, and the embedded Cr(VI) is released, which is then reduced to Cr(OH)3 by reducing substances such as fructose, sucrose, and glucose in waste molasses. From the standpoint of a circular economy, this study may provide a potential strategy for minimizing resource input and waste/emission production by harmless disposal and resource utilization of COPR and waste molasses.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Traditional wet and dry Chromite Ore Processing Residue (COPR) detoxification methods consume acid for pH adjustment and produce greenhouse gasses (CO2), yielding potential secondary pollution. It is imperative to develop an environmentally-friendly, efficient, and effective method for COPR treatment. This study reported the one-step natural pyrite-assisted mechanochemical detoxification of Cr(VI) in COPR. Under optimal conditions (i.e., the pyrite/COPR mass ratio of 5% and mechanochemical treatment at 600 rev./min for 2.5 h), Cr(VI) in COPR was efficiently reduced and immobilized, and the leaching concentration of total Cr met the regulatory limit of 5 mg/L. The X-ray absorption near-edge structure (XANES) analysis further verified that the Cr(VI) in treated COPR was 100% reduced to Cr(III). Potential mechanism could be that the enclosed non-exchangeable Cr(VI) in the chromite matrix was exposed to the COPR particle surface under the mechanical action; then, the redox reaction between pyrite and Cr(VI) were induced and enhanced through accumulated crystal defects and generated new surfaces with dangling bonds under mechanical forces. Life cycle assessment (LCA) results highlight that the high energy input is the major contributor eliciting environmental impacts for this current technology. More efficient and cleaner energy supplies should be sought alternatively to overcome the overall sustainability.