Application of dirty-acid wastewater treatment technology in non-ferrous metal smelting industry: Retrospect and prospect.

Dirty-acid wastewater (DW) originating from the non-ferrous metal smelting industry is characterized by a high concentration of H2SO4 and As. During the chemical precipitation treatment, a significant volume of arsenic-containing slag is generated, leading to elevated treatment expenses. The imperative to address DW with methods that are cost-effective, highly efficient, and safe is underscored. This paper conducts a comprehensive analysis of three typical methods to DW treatment, encompassing technical principles, industrial application flow charts, research advancements, arsenic residual treatment, and economic considerations. Notably, the sulfide method emerges as a focal point due to its minimal production of arsenic residue and the associated lowest overall treatment costs. Moreover, in response to increasingly stringent environmental protection policies targeting new pollutants and carbon emissions reduction, the paper explores the evolving trends in DW treatment. These trends encompass rare metal and sulfuric acid recycling, cost-effective H2S production methods, and strategies for reducing, safely disposing of, and harnessing resources from arsenic residue.

Deep insight on mechanism and contribution of arsenic removal and heavy metals remediation by mechanical activation phosphogypsum.

Arsenic-containing wastewater and arsenic-contaminated soil can cause serious environmental pollution. In this study, phosphogypsum with partial mechanical activation of calcium oxide was used to prepare a new phosphogypsum-based passivate (Ca-mPG), and its remediation performance on arsenic-contaminated soil was evaluated in terms of both effectiveness and microbial response. The results showed that the optimum conditions for the preparation of the passivate were optimized in terms of single factor and response surface with a ball milling speed of 200 r/min, a material ratio of 6:4 and a ball milling time of 4 h. Under these conditions, the adsorption capacity was 37.75 mg/g. The leaching concentration of arsenic (As) in the contaminated soil after Ca-mPG modification decreased from 25.75 µg/L to 5.88 µg/L, which was lower than the Chinese national standard (GB/T 5085.3-2007); Ca-mPG also showed excellent passivation effect on other heavy Metals (copper, nickel, cadmium, zinc). In addition, As-resistant bacteria and passivators work together to promote the stabilization effect of contaminants during the remediation of As-contaminated soil. The mechanisms of Cu, As(III)/As(V), Zn, Cd, and Ni removal were related to ion exchange, electrostatic adsorption of substances on heavy metals, calcium binding to other substances to produce precipitation; and microbially induced stabilization of HMs, oxidized. Overall, this study demonstrates an eco-friendly "waste-soil remediation" strategy to solve problems associated with solid waste reuse and remediation of HM-contaminated soils.

Remediation of arsenic contaminated water and soil using mechanically (ball milling) activated and pyrite-amended electrolytic manganese slag.

With the development of industry, heavy metal (HM) pollution of soil has become an increasingly serious problem. Using passivators made of industrial by-products to immobilize HMs in contaminated soil is a promising in-situ remediation technology. In this study, the electrolytic manganese slag (EMS) was modified into a passivator (named M-EMS) by ball milling, and the effects of M-EMS on adsorption of As(V) in aquatic samples and on immobilization of As(V) and other HMs in soil samples were investigated under different conditions. Results demonstrated that M-EMS had a maximum As(V) adsorption capacity of 65.3 mg/g in the aquatic samples. Adding M-EMS to the soil reduced the leaching of As (from 657.2 to 319.8 µg/L) and other HMs after 30 d of incubation, reduced the bioavailability of As(V) and improved the quality and microbial activity of the soil. The mechanism for M-EMS to immobilize As in the soil are complex reactions, ion exchange reaction with As and electrostatic adsorption. This work provides new ideas of using waste residue matrix composites for sustainable remediation of Arsenic in the aquatic environment and soil.

Simultaneous immobilization of multiple heavy metals in polluted soils amended with mechanical activation waste slag.

Heavy metal soil contamination has become an increasingly serious problem in industrial development. However, industrial byproducts used for remediation are one aspect of green remediation that can contribute to sustainable practices in waste recycling. In this study, electrolytic manganese slags (EMS) were mechanically activated and modified into a passivator (M-EMS), and the heavy metal adsorption performance of M-EMS, heavy metal passivation ability in soil, dissolved organic matter (DOM) change and its effect on the microbial community structure of soil were investigated. The findings revealed that the maximum adsorption capacities of As(V), Cd2+, Cu2+ and Pb2+ were 76.32 mg/g, 301.41 mg/g, 306.83 mg/g and 826.81 mg/g, respectively, indicating that M-EMS demonstrated remarkable removal performance for different heavy metals. The Langmuir model fits Cd2+, Cu2+ and Pb2+ better than the Freundlich model, and monolayer adsorption is the main process. Surface complexation played a major role in the As(V) adsorption's on the surface of metal oxides in M-EMS. The passivation effect was ranked as Pb > Cr > As>Ni > Cd > Cu, with the highest passivation rate of 97.59 % for Pb, followed by Cr (94.76 %), then As (71.99 %), Ni (65.17 %), Cd (61.44 %), and the worst one was Cu (25.17 %). In conclusion, the passivator has the effect of passivation for each heavy metal. The addition of passivating agent can enhance the diversity of microorganisms. Then it can change the dominant flora and induce the passivation of heavy metals through microorganisms. XRD, FTIR, XPS and the microbial community structure of soil indicated that M-EMS can stabilize heavy metals in contaminated soils through four main mechanisms: ion exchange, electrostatic adsorption, complex precipitation and the microbially induced stabilization. The results of this study may provide new insights into the ecological remediation of multiple heavy-metal-contaminated soils and water bodies and research on the strategy of waste reduction and harmlessness by using EMS-based composites in combination with heavy metals in soil.

Preparation of phosphogypsum-copper smelting slag-based consolidating body with high compressive strength.

Phosphogypsum (PG) is an industrial waste residue produced during the production of phosphoric acid through the wet process. With strong acidity and a large amount of toxic impurities, PG is difficult to reuse. In this study, the solidified body (PG-S) was made by mechanical compression of the mixture of PG, copper smelting slag (CSS), CaO, NaOH, and water. Results indicate that the composition of the material phases in the PG-S samples changed with hydrated calcium silicate and amorphous silicate derivatives were formed during the reaction; Fe and Ca in the material were transformed; and the prepared geopolymer material had a dense internal structure with the materials being cemented to each other. The highest compressive strength of PG-S cured for 28 days could reach 21.3 MPa with a fixation efficiency of PO43-and F-reaching 99.81 and 94.10%, respectively. The leaching concentration of heavy metals of the PG-S cured for 28 days met the requirements of the Comprehensive Wastewater Discharge Standard (GB 8978-1996). The simulation results of the geochemical model verified the feasibility of the whole immobilization process from the thermodynamic point of view. This work directly uses copper smelting slag and phosphogypsum for coupled immobilization/stabilization treatment not only to achieve the immobilization of pollutants in both solid wastes but also to obtain colloidal masses with certain compressive strength, which also provides a new option for resource utilization of phosphogypsum and copper smelting slag. This work also shows great potential in turning the actual mine backfill into cementitious material.

Uptake of arsenic(V) using iron and magnesium functionalized highly ordered mesoporous MCM-41 (Fe/Mg-MCM-41) as an effective adsorbent.

Mesoporous silica (MCM-41) is widely used as a supporting material due to its large specific surface area and good stability, but it cannot remove heavy metals due to the lack of adsorption active sites. In this study, the MCM-41 (a mesoporous SiO2 material) decorated with iron and magnesium oxide (Fe/Mg-MCM-41) was found to be an excellent adsorbent to remove arsenic(V) from water. FTIR, BET, TEM-EDS, XRD, XPS, etc. were applied for characterization analysis. Adsorption isotherms were fitted well by the Langmuir model and the experimental maximum adsorption capacity of Fe/Mg4-MCM-41 (magnesium accounts for 4%) was 71.53 mg/g at pH = 3. Thermodynamics analysis suggested exothermic nature of adsorption behavior. Kinetic process was well described by the pseudo-second-order model and adsorption rate was controlled by intraparticle diffusion and film diffusion. Moreover, the adsorption behavior of As(V) onto Fe/Mg4-MCM-41 was investigated under different reaction conditions, such as pH, temperature, Mg-doping and competing ions. The results showed that loading a certain amount of magnesium can significantly improve arsenic removal efficiency. Additionally, Fe/Mg4-MCM-41 exhibits high arsenic(V) removal in the wide pH range of 3-10. The Fe/Mg4-MCM-41 can be regenerated and used after four consecutive cycles. The high arsenic(V) sorption capacity, wide range of pH applications, ability to regenerate, and reusability of Fe/Mg4-MCM-41 confirmed that this adsorbent is promising for treating As-contaminated wastewater.

Insight into the role and mechanism of polysaccharide in polymorphous magnesium oxide nanoparticle synthesis for arsenate removal.

The low cost and non-toxic of magnesium oxides make it a potential eco-friendly material for arsenic removal. Polysaccharide is a kind of green modifier to obtain nanoscale MgO particles with a higher adsorption affinity. In this study, the impact of chain structures of polysaccharides on the morphology features and arsenate removal efficiency of MgO-NPs were investigated. Pullulan and starch facilitated the synthesis of flower-like MgO-NPs, and pectin facilitated the synthesis of plate-like ones. Although the two kinds of flower-like MgO-NPs undergone similar time to reach equilibrium, the one obtained from the starch-synthesis route showed a higher arsenate adsorption capacity (98 mg g-1), due to that their bushy and smaller petals on the surface provide more active sites for arsenic adsorption. The pectin-synthesis route also produced MgO-NPs with higher arsenate adsorption capacity (101 mg g-1), ascribed to stacking of nano-plates on their surfaces facilitated to form defect surfaces. However, due to their lower BET area, the plate-like MgO-NPs took twice times to reach equilibrium for arsenic adsorption compared with the others. In the stage for the hydrolysis of MgO, hydroxyl groups on the polymer chain provide active sites to physically trap or bond with MgO particles and then to produce hydrolyzed precursors. The poly chain containing inter- and intra-hydroxyl groups directed MgO molecular growing into hydroxide crystals with 3D frameworks during their nucleation and growth. However, pectin only provides inter-hydroxyl groups and directs to form hydroxides with 2D frameworks. Furthermore, the rapid-nucleation vs. slow-growth model in the stage of pyrolysis of hydroxide crystals successfully interprets the thinner petals and complex chemical phases of the final nanoparticles obtained from the pullulan-synthesis route. This work may provide direction and perspectives for the rational design of well-performing MgO materials for arsenate removal.

Strategy of optimizing anaerobic digestion of cassava distiller wastewater using a novel automatic biological incubation system.

Due to the drawbacks of using fossil fuels and the need to mitigate global warming caused by increasing greenhouse gas emissions, agricultural biomass for bioenergy production is gaining great interest around the world. This work presented a study at a biochemical plant in Lianyungang, Jiangsu Province, China to maximize methane production from cassava distiller wastewater. The plant's annual production of cassava distiller wastewater is more than 3 million tons and currently was treated using a series of 5000 m3 Internal Circulation (IC) reactors. Modification was applied at No.19 IC reactor by connecting it to two 1 m3 automatic biological incubators called Information Bio-Booster (IBB). The effluent of the IC reactor was fed into the IBBs and iron, cobalt and nickel were added directly in the IBBs. The function of the IBBs was to regulate the microbial community. Afterwards, the microorganisms in the IBBs were pumped back into the IC reactor to participate in the methane production reaction. Daily net increase of methane content and COD removal reached 8.02% and 33% respectively in No.19 IC reactor comparing to the unadjusted reactors. Preliminary lab experiments found that improvements of biogas production, enhanced COD removal and VS removal was closely related to the enhancement of anaerobic microbial communities' diversity and the promotion of enzyme activity through the addition of the metal salts. Daily economic value could be estimated to be $218 which indicated the application potential of using the proposed system to enhance anaerobic digestion at industrial plants for bioenergy production.

Highly efficient removal of Cr(VI) from aqueous solution by pinecone biochar supported nanoscale zero-valent iron coupling with Shewanella oneidensis MR-1.

Nanoscale zero-valent iron (nZVI) has been extensively used to remove various pollutants. However, the rapid deactivation due to aggregation and surface passivation severely limits its practical application. In this study, a novel composite with nZVI supported by pinecone biochar (nZVI-PBC) was successfully synthesized and used for the removal of high concentration Cr(VI) from aqueous solution in the presence of Shewanella oneidensis MR-1 (MR-1). The results showed that the nZVI-PBC coupling with MR-1 (nZVI-PBC/MR-1) exhibited an excellent removal performance for high concentration Cr(VI) compared to the nZVI-PBC alone. Under optimal conditions, 100 mg/L Cr(VI) could be removed completely by nZVI-PBC/MR-1 within 48 h, while only 39.50% of Cr(VI) was removed by nZVI-PBC alone. The improvement of Cr(VI) removal is due to the dissolution of the surface passivation layer of nZVI-PBC, formation of sorbed Fe(II) in the presence of MR-1, and an important role of extracellular polymeric substance (EPS) derived from MR-1. X-ray photoelectron spectroscopy (XPS) and Cr K-edge X-ray absorption near-edge structure spectra (XANES) confirmed that most Cr(VI) was reduced to insoluble Cr(III) and formed Cr2O3, CrxFe1-x(OH)3 and FeCr2O4 precipitates, and a small amount of unreduced Cr(VI) was immobilized through adsorption and complexation. The results suggest that nZVI-PBC/MR-1 can effectively overcome the limitations of nZVI and achieve highly efficient removal of high concentration Cr(VI).

Electrolytic manganese residue-based cement for manganese ore pit backfilling: Performance and mechanism.

Slag backfilling with electrolytic manganese residue (EMR) is an economical and environmentally-friendly method. However, high ammonium-nitrogen and manganese ions in EMRs limit this practice. In this study, a method of highly efficient simultaneous stabilization/solidification of ultrafine EMR by making EMR-based cementitious material (named EMR-P) was proposed and tested via single-factor and response surface optimization experiments. Results show that the stabilization efficiency of NH4+ and Mn2+ were above 95%, and the unconfined compressive strength of the EMR-P was 18.85 MPa (megapascal = N/mm2). The mechanistic study concluded that the soluble manganese sulfate and ammonium sulfate in EMR were converted into the insoluble precipitates of manganite (MnOOH), gypsum (CaSO4), MnNH4PO4·H2O, and struvite (MgNH4PO4∙6 H2O), leading to the stabilization of NH4+ and Mn2+ in the EMR-P. Leaching tests of EMR-P indicated that NH4+, Mn2+, and others heavy metals in the leachate were within the permitted level of the GB/T8978-1996. The novelty of this study includes the addition of phosphate and magnesium ions to precipitate ammonium-nitrogen and the combination between calcium ions (from CaHPO4∙2 H2O) and sulfate (from the EMR) to form calcium sulfate to improve the stability and unconfined compressive strength of cementitious materials (EMR-P).

Mechanochemical activation on selective leaching of arsenic from copper smelting flue dusts.

A novel application, including mechanochemical pre-treating and alkali leaching, for arsenic selective leaching from copper smelter flue dusts (CSFUs) was developed to overcome the disadvantages of hydrometallurgical methods. Compared with raw CSFU powders, the mechanical-activated ones showed higher maximum arsenic leaching efficiency (increased by ~20%), and lower apparent activation energy (decreased by ~7 kJ·mol-1). Furthermore, this novel process only consumed half of alkali and sulfides and needed one-third of the leaching time to compare with the ones used in the traditional alkali leaching process. The promoting effect of mechanical force on arsenic leaching firstly relied on the physical property changes of CSFU powders, including a decrease of particle sizes and an increase of the specific surface. Secondly, mechanochemical force converted As5+ species into reduced phases (e.g. As2O3, NaAsO2), and thio-arsenates (e.g. AsO2S23-, AsO3S3-), which could spur its leaching due to their stronger mobilities in the alkali solution within sulfides. Finally, mechanochemical activation could be facilitated to separate discrete soluble arsenic species or incorporated ones from sulfate minerals in the CSFUs. This work may have important implications for the development of new eco-friendly technologies for purifying arsenic-bearing materials.

A novel method for solidification/stabilization of Cd(II), Hg(II), Cu(II), and Zn(II) by activated electrolytic manganese slag.

This study was aimed at removing and stabilizing heavy metals (HgII, ZnII, CuII, and CdII). A novel material (named A-EMS) for heavy metal removal was proposed by ball grinding activated electrolytic manganese slag (EMS) with low content of sodium hydroxide. For different application scenarios, the two physical properties of the materials were developed: the powdery A-EMS (powder) was used to remove heavy metals from wastewater. In addition, the blocky A-EMS (porous brick) was used to build barrier walls for tailings ponds to prevent heavy metals from flowing out. The maximum removal amount of Hg(II) Cd(II), Zn(II), and Cu(II) by A-EMS were 475.35, 77.72, 259.70, and 203.20 mg/g in 30 min. The heavy metals ions were removed and fixed on A-EMS mainly through ion exchange and some forms of electrostatic adsorption and hydroxyl complexation. After consolidating the heavy metals, the compressive strength of the materials can reach 20 Mpa and the leaching toxicity met the national standard of China (GB/T 3838-2002) in 60 days. These excellent properties made A-EMS widely used to remove heavy metals in wastewater and to intercept and solidify heavy metals in mine wastewater.

Bio-leaching of manganese from electrolytic manganese slag by Microbacterium trichothecenolyticum Y1: Mechanism and characteristics of microbial metabolites.

The related microbial metabolomics on biological recovery of manganese (Mn) from Electrolytic Manganese Slag (EMS) has not been studied. This study aimed at open the door to the metabolic characteristics of microorganisms in leaching Mn from EMS by using waste molasses (WM) as carbon source. Results show Microbacterium trichothecenolyticum Y1 (Y1) could effectively leach Mn from EMS in combination with using waste molasses as carbon and energy sources. For the first time, Y1 was identified to be capable of generating and then metabolizing several organic acids or other organic matter (e.g., fumaric acid, succinic acid, malic acid, glyoxylic acid, 3-hydroxybutyric acid, glutaric acid, L(+)-tartaric acid, citric acid, tetrahydrofolic acid, and L-methionine). The production of organic acids by Y1 bacteria was promoted by EMS with the carbon source. This study demonstrated for the first time that metabolic characteristics and carbon source metabolic pathways of Y1 in bioleaching of Mn from EMS.

Species identification and mutation breeding of silicon-activating bacteria isolated from electrolytic manganese residue.

A strain of silicon-activating bacteria was isolated from electrolytic manganese residue (EMR); identified as a species of Ochrobactrum by integrated microscopic morphological characteristics, biochemical index determination, and clone analysis (i.e., results of 16S rRNA sequence); and temporarily named as Ochrobactrum sp. T-07 (T-07). The optimal growth conditions of the strain T-07 were obtained as follows: temperature of 30 °C, initial pH of 7.0, shaking speed of 180 rev. min-1, and loading volume of 100 mL. In order to enhance its activation activity of silicon, T-07 went through the ultraviolet (UV) mutagenesis and nitrosoguanidine (NTG) mutagenesis breeding, and the mutant strain T-07-B with higher activity was obtained. Under the optimal fermentation condition (leaching time of 20 days, temperature of 30 °C, initial pH of 7, pulp concentration of 5%, shaking speed of 180 rev. min-1, and particle diameter of EMR ≤ 180 µm), the available silicon content in the supernatant reached 98.8 mg L-1, which was 2.4 times of the original strain T-07. Therefore, T-07 can be used as a good backup in developing biological silicon fertilizer for plants.

Comparative transcriptome combined with metabolome analyses revealed key factors involved in nitric oxide (NO)-regulated cadmium stress adaptation in tall fescue.

BACKGROUND: It has been reported that nitric oxide (NO) could ameliorate cadmium (Cd) toxicity in tall fescue; however, the underlying mechanisms of NO mediated Cd detoxification are largely unknown. In this study, we investigated the possible molecular mechanisms of Cd detoxification process by comparative transcriptomic and metabolomic approaches. RESULTS: The application of Sodium nitroprusside (SNP) as NO donor decreased the Cd content of tall fescue by 11% under Cd stress (T1 treatment), but the Cd content was increased by 24% when treated with Carboxy-PTIO (c-PTIO) together with Nitro-L-arginine methyl ester (L-NAME) (T2 treatment). RNA-seq analysis revealed that 904 (414 up- and 490 down-regulated) and 118 (74 up- and 44 down-regulated) DEGs were identified in the T1 vs Cd (only Cd treatment) and T2 vs Cd comparisons, respectively. Moreover, metabolite profile analysis showed that 99 (65 up- and 34-down- regulated) and 131 (45 up- and 86 down-regulated) metabolites were altered in the T1 vs Cd and T2 vs Cd comparisons, respectively. The integrated analyses of transcriptomic and metabolic data showed that 81 DEGs and 15 differentially expressed metabolites were involved in 20 NO-induced pathways. The dominant pathways were antioxidant activities such as glutathione metabolism, arginine and proline metabolism, secondary metabolites such as flavone and flavonol biosynthesis and phenylpropanoid biosynthesis, ABC transporters, and nitrogen metabolism. CONCLUSIONS: In general, the results revealed that there are three major mechanisms involved in NO-mediated Cd detoxification in tall fescue, including (a) antioxidant capacity enhancement; (b) accumulation of secondary metabolites related to cadmium chelation and sequestration; and (c) regulation of cadmium ion transportation, such as ABC transporter activation. In conclusion, this study provides new insights into the NO-mediated cadmium stress response.

Removal of cadmium and lead from aqueous solutions by thermal activated electrolytic manganese residues.

Electrolytic manganese residues (EMR) is produced from the electrolysis manganese industry. In this study, the thermal activated EMRs (T-EMR) were used to adsorb cadmium and lead from aqueous solution. X-ray diffractometer (XRD), scanning electron microscope-Energy Dispersive Spectrometer (SEM-EDS), X-ray photoelectron spectroscopy (XPS) were adopted to characterize EMR before and after the modification, and the performance and adsorption mechanisms of T-EMR for cadmium and lead were determined. Results show that the pH has a strong influence on the adsorption of cadmium and lead and the maximum adsorption capacity can be achieved at pH 6. The adsorption of Cd(II) can be better fitted by the Lagergren pseudo-first-order dynamic model, while that of Pb(II) fits the pseudo-second-order kinetic model better. The Freundlich isotherm model fits the adsorption of two metals better than Langmuir model. The thermodynamic results demonstrate that the adsorption of Cd(II) or Pb(II) on T-EMR is endothermic and spontaneous. As the nitric acid with pH 0.5 was used, nearly all of the adsorbed Cd(II) and 75% Pb(II) can be desorbed from the loaded T-EMR. It is concluded that the adsorption of Cd(II) and Pb(II) on T-EMR is in virtue of electrostatic attraction, ion-exchange and surface precipitation. The heavy metals are mainly adsorbed on ferric and manganese oxides and silicate minerals in T-EMR by electrostatic attraction. In addition, cadmium and lead also can be adsorbed via the ion exchange reaction. Moreover, some Pb(II) are adsorbed by forming lead sulfate. Thus, T-EMR may be an environmentally-friendly, effective adsorbent for the removal of heavy metals from aqueous solution.

Bioleaching of silicon in electrolytic manganese residue (EMR) by Paenibacillus mucilaginosus: Impact of silicate mineral structures.

Electrolytic manganese residue (EMR) is characterized by high silicon content, and thus, is an important silicon source. While considerable research has been conducted on bioleaching EMR for silicon recovery, sufficient information is not available on the impact of specific silicate mineral structures in EMR on silicon bioleaching. In the present study, the mineral composition of EMR was determined firstly, and then the leaching effect of Paenibacillus mucilaginosus on these different silicate minerals were investigated by shake flask experiments. Results showed that the silicon in EMR was mainly composed of quartz, sericite, muscovite, biotite, olivine and rhodonite; Paenibacillus mucilaginosus had a significantly different weathering and decomposition effects on different silicate minerals. Among them, sericite, muscovite and biotite with layered structure had the most obvious silicon leaching effect, followed by rhodonite with island structure, while silicon leaching from olivine with chained structure and quartz with frame structure was much more difficult. One can roughly judge the adaptability of bioleaching of silicon in EMR using Paenibacillus mucilaginosus if the main form of silicate minerals in EMR is determined.

Using Electrolytic Manganese Residue to prepare novel nanocomposite catalysts for efficient degradation of Azo Dyes in Fenton-like processes.

In this study, Electrolytic Manganese Residue (EMR) was treated by EDTA-2Na/NaOH, ultrasonic etching, and hydrothermal reaction to obtain a novel nanocomposite catalyst (called N-EMR), which then was used, together with H2O2, to treat synthetic textile wastewater containing Reactive Red X-3B, Methyl Orange, Methylene blue and Acid Orange 7. Results indicated that the N-EMR had a nano-sheet structure in sizes of 100-200 nm; new iron and manganese oxides with high activity were produced. The mixture of a small amount of N-EMR (40 mg/L) and H2O2 (0.4 × 10-3 M) could removal about 99% of azo dyes (at 100 mg/L in 100 mL) within 6-15 min, much faster than many advanced oxidation processes (AOPs) reported in the literature. The elucidation of the associated mechanism for azo dyes degradation indicates that azo dyes were attacked by superoxide radicals, hydroxyl radicals, and electron holes generated within system. N-EMR was found to be reusable and showed limited inhibition by co-existing anions and cations. Moreover, high removal efficiency of azo dyes could happen in the system with a wide range of pH (1-8.5) and temperatures (25-45 °C), indicating that the process developed in this study may have broad application potential in treatment of azo dyes contaminated wastewater.

Chromium(VI) bioreduction and removal by Enterobacter sp. SL grown with waste molasses as carbon source: Impact of operational conditions.

A technology utilizes bacteria Enterobacter sp. SL grown in an anaerobic reactor with waste molasses as carbon source to bio-reduce hexavalent chromium [Cr(VI)] in wastewater and then remove total chromium has been developed. The performance was elucidated through different initial and operating experiments conditions, and the associated mechanism of Cr(VI) reduction was explained. Results show that Cr(VI) removal is 99.91% at 25 h in the anaerobic reactor initially containing bacteria of 5% (v/v), (NH4)2Fe(SO4)2·6H2O of 0.5 g·L-1, waste molasses of 2.5 g·L-1, Cr(VI) of 100 mg·L-1, pH of 6.0, and with the operational temperature of 45 °C. After 120 h reaction, Cr(total) removal reached 91.10%. The major reduction products [FeS, Cr2O3, Cr(OH)3, S0 granules] together with microbes was removed by sludge separation with Cr(VI) in the supernatant (0.027 mg·L-1) being much lower than that (not excess 0.2 mg·L-1) of Electroplating Pollutant Emission Standard.

Efficient removal of heavy metals by synergistic actions of microorganisms and waste molasses.

In this study, two bacteria strains (Enterobacter sp. SL and Acinetobacter sp. SL-1) and waste molasses (carbon source) were used to remove Zn(II), Cd(II), Cr(VI), and Cr(Total) in the liquid solution (87 mg·L). The results showed the removal efficiencies of Cr(Total) and Cr(VI) could reach over 98.00% after reaction, and the removal efficiencies of Zn(II) and Cd(II) were all about 90.00% by the synergistic actions of microorganisms and waste molasses. In this process, waste molasses provides nutrients for microorganisms and has the characteristics and capability of Cr, Zn, and Cd. Microorganisms mainly use biological adsorption (36.95% and 45.69%) and metabolism (24.37% and 17.05% by producing humic-acid and fulvic-acid like substances) to remove Zn(II) and Cd(II), while waste molasses could to remove Cr(Total) (81.24%) and Cr(VI) (75.90%). This study has potential application value for the treatment of wastewater containing high concentrations of heavy metals.