Recovery of bio‑sulfur and metal resources from mine wastewater by sulfide biological oxidation-alkali flocculation: A pilot-scale study

Mine wastewater treatment using bio-sulfate reduction technology forms sulfur-containing wastewater that comprises sulfides (HS^- and S^2-) and metal ions. Bio‑sulfur generated by sulfur-oxidizing bacteria in such wastewater is usually negatively charged hydrocolloidal particles. However, bio‑sulfur and metal resource recovery are difficult using traditional methods. In this study, the sulfide biological oxidation-alkali flocculation (SBO-AF) method was investigated to recover the above resources, and to provide a technical reference for mine wastewater resource recovery and heavy metal pollution control. Specifically, the performance of SBO in forming bio‑sulfur and the key parameters of SBO-AF were explored and then applied in a pilot-scale process to recover resources from wastewater. Results show that partial sulfide oxidation was achieved under a sulfide loading rate of 5.08 ± 0.39 kg/m³·d, dissolved oxygen of 2.9-3.5 mg/L and temperature of 27-30 °C. The average sulfide oxidation rate and sulfur selectivity ratio were 92.86 % and 90.22 %, respectively. At pH 10, metal hydroxide and bio‑sulfur colloids co-precipitated through the precipitation catching and adsorption charge neutralization effect. The average manganese, magnesium and aluminum concentrations and turbidity in the wastewater were 53.93 mg/L, 522.97 mg/L, 34.20 mg/L and 505 NTU, respectively, and decreased to 0.49 mg/L, 80.65 mg/L, 1.00 mg/L and 23.33 NTU, respectively, after treatment. The recovered precipitate mainly contained sulfur, along with metal hydroxides. The average sulfur, manganese, magnesium and aluminum contents were 45.6 %, 29.5 %, 15.1 % and 6.5 %, respectively. Economic feasibility analysis and the above results show that SBO-AF has obvious technical and economic advantages in the recovery resources from mine wastewater.

Recovery of rare earth elements from mine wastewater using biosynthesized reduced graphene oxide

Recycling rare earth elements (REEs) from sources of secondary waste such as REEs mine wastewater has emerged as a sustainable approach with both waste reuse and wastewater processing. In this study, green reduced graphene oxide (G-rGO) was prepared utilizing green tea extract with the advantages of being environmentally friendly, sustainable, and low cost. To understand how G-rGO functions, it was compared to commercial reduced graphene oxide (rGO), and the efficiencies in adsorbing Y(III) were 91.6% and 11.9%, respectively. This indicated there is a synergistic adsorption between the capping layer of G-rGO and rGO alone. G-rGO and rGO were characterized before and after exposure to Y(III). This comparison indicated that Y(III) was adsorbed on the surface of G-rGO through complexation and electrostatic interaction. The adsorption kinetics best fit the pseudo-second-order model and the Langmuir model isotherm model, with adsorption capacities of 24.54 mg g^-1. A probable adsorption mechanism of Y(III) by G-rGO was proposed, involving electronic complexation, electrostatic adsorption and ion exchange. Furthermore, the adsorption efficiencies of G-rGO for Y(III), Ce(III) and Zn(II) in mine wastewater were 22.1%, 89.1% and 14.6%, respectively. These results demonstrate that G-rGO has great potential in the recovery of REEs from mine wastewater.

Biosynthesis of bionanomaterials using Bacillus cereus for the recovery of rare earth elements from mine wastewater

The growing demand for rare earth elements (REEs) increasingly requires secondary resources such as mine wastewater containing high concentrations of REEs, to be used as a source of REEs. The current challenge is how to efficiently recover REEs from this feed source. In this paper, a functional bionanomaterial (FeNPs-EPS) was biosynthesized using Bacillus cereus as a possible means of recovering REEs. This composite was composed of both synthesized iron nanoparticles (FeNPs) and extracellular polymeric substances (EPS). Synthesis of the FeNPs-EPS composite via a one-step biosynthesis was confirmed by materials characterization. The peak in the material's UV-Vis spectra at 511 nm demonstrates the formation of FeNPs-EPS, where 3D-EEM showed that FeNPs-EPS was wrapped predominantly with tryptophan protein-like and humic acid-like substances. In addition, while FTIR indicated that the functional groups present in EPS where virtually identical to those observed in FeNPs-EPS, XPS demonstrated that Fe and O were the major elemental present as both FeO and Fe₂O₃. Zeta potential measurements indicated that FeNPs-EPS had good stability under different pH conditions, where BET analysis supported multilayer adsorption. Finally, on exposure to high concentrations of Eu(III) and Tb(III) in mine wastewater, the synthesized FeNPs-EPS demonstrated strong potential to remove two cations from the wastewater and hence a potentially practical way to efficiently recover REEs from such waste streams.

Potassium supplement enhanced cadmium removal in a Microcystis aeruginosa photobioreactor: Evidence from actual and simulated wastewater

In this study, a Microcystis aeruginosa-based photobioreactor (M. aeruginosa-based PBR) was developed for the removal of cadmium (Cd²⁺) from diluted actual mine wastewater (DW) and Cd²⁺-contained simulated wastewater (SW), with a uniform Cd²⁺ concentration of 0.5 mg/L. For the DW and SW, both K⁺ -abundant (DWA & SWA) and K⁺-insufficient (DWB & SWB) treatments were conducted. It was found that continuous supplementation of K⁺ benefited Cd²⁺ removal. The Cd²⁺ removal efficiency in SWA reached 70% during the 41 days of operation, which was 20% higher than that in the SWB. The K⁺ addition triggered great higher Cd²⁺ removal efficiency (90%) in the DWA in comparison to the SWA. The Cd²⁺ assimilation by M. aeruginosa and Cd²⁺ retention on M. aeruginosa surface were the primary processes involved in the PBR system. The K⁺ starvation triggered a 45% and 43% loss of M. aeruginosa biomass in the DWA and the DWB, respectively. Hence, the Cd²⁺ removal efficiency in DWB increased significantly, and this was attributed to the increased abundance of non-living cells and enhanced bioretention of Cd²⁺. The results revealed that continuous K⁺ supplementation enhanced the Cd²⁺ removal efficiency in the M. aeruginosa-based PBR jointly by prompting algal cell growth, Cd²⁺ assimilation and biosorption, as well as Cd²⁺ retention on the algal cells.

Acid mine drainage (AMD) treatment by neutralization: Evaluation of physical-chemical performance and ecotoxicological effects on zebrafish (Danio rerio) development

Acid mine drainage (AMD) represents a major problem in the mining industry worldwide due to the risk of water and soil pollution. Its active treatment involves the addition of alkaline reagents such as NaOH or Ca(OH)₂ to increase the pH and precipitate the dissolved metals, although substantial amounts of dissolved ions might persists. Under a remediation approach, the aim of this work was to assess the chemical and physical characteristics of treated effluent and to evaluate its ecotoxicological effects on zebrafish (Danio rerio) embryonic and larval stages, through developmental, functional, morphological, and behavioral end-points. The studied AMD sample, highly associated with pyrite, presented high sulfate and dissolved metal ions content and was submitted to the following treatment conditions: NaOH - pH 7.0 and 8.7, and Ca(OH)₂ - pH 7.0 and 8.7. All neutralizing treatments resulted in a satisfactory reduction of the metals concentration, with best results achieved using Ca(OH)₂ at pH 8.7; although Mn and As still remained above or very near the discharge maximum limits according to Brazilian legislation. Therefore, an additional step was employed to Mn and As adsorption by algal biomass. Regarding in-vivo toxicological assays, no significant lethality was recorded in all treated AMD groups, although adverse effects were observed in all endpoints analyzed. Ca(OH)₂ groups performed closer to control than NaOH-treated groups. The additional polishing stage treatment with the algae Scenesmus sp. allowed tenuous improvements in terms of removal of residual amounts of As and Mn but not in the toxicological characteristics of treated AMD.

A facile one-step synthesized epsilon-MnO2 nanoflowers for effective removal of lead ions from wastewater

The increasing contamination of lead ions (Pb(II)) in groundwater has become a serious environmental issue, which provides the impetus for intense research on Pb(II) removal. ε-MnO₂ nanoflowers were successfully fabricated through a simple decomposition reaction. And the obtained ε-MnO₂ nanoflowers were employed to remove Pb(II) from water. The detailed microstructure and surface properties of ε-MnO₂ were systematically characterized. The results indicate that the pure ε-MnO₂ phase was obtained and the specific surface area is 96.33 m² g^-1. Batch adsorption experiments of Pb(II) were carried out, and the ε-MnO₂ nanoflowers exhibited outstanding adsorption performance. The maximum adsorption capacity for Pb(II) and Cd(II) achieved to 239.7 mg g^-1 and 73.6 mg g^-1 at the dosage of 0.2 g L^-1. Besides, the prepared ε-MnO₂ nanoflowers show much higher removal efficiency toward Pb(II) compared with commercial MnO₂. The XRD results reveal the stability of ε-MnO₂ nanoflowers, and the XPS results suggest that both the electrostatic interaction and structural tunnels are responsible for the removal mechanisms of Pb(II). This work finds a facile method to synthesize ε-MnO₂ nanoflowers, showing great potential for Pb(II) removal.

Cellulose-metallothionein biosorbent for removal of Pb(II) and Zn(II) from polluted water

Intake of toxic trace elements in drinking water can lead to adverse health effects. To remove toxic trace elements from water, we developed a novel biosorbent composed of cellulose and a fusion protein. The fusion protein was constructed from metallothionein (MT) and a carbohydrate-binding module (CBM), where CBM can bind to cellulose while MT can capture heavy metal ions in solution. In a batch experiment, the biosorbent had maximum biosorption capacities for Pb(II) and Zn(II) ions of 39.02 mg/g and 29.28 mg/g, respectively. Furthermore, the biosorbent could be used in a semi-continuous system and showed good regeneration and recyclability. Both cellulose and the MT-CBM are environmentally friendly and renewable materials, and this biosorbent has great potential for efficient removal of toxic trace elements from polluted water.

Effects of mine wastewater irrigation on activities of soil enzymes and physiological properties, heavy metal uptake and grain yield in winter wheat

In China, coal-mining industries are mainly located in the water shortage areas including arid or semiarid areas. Mine wastewater is used for irrigation of agricultural land in these areas. However, few studies have been conducted to address ecological and food safety risks caused by mine wastewater irrigation. In this research, a pot experiment was performed to examine the effects of mine wastewater irrigation on soil enzymes, physiological properties of wheat and potential risks of heavy metal contamination to wheat crop. Plants were subjected to three mine wastewater irrigation treatments: leacheate of coal gangue (T1), coal-washing wastewater (T2) and precipitated coal-washing wastewater (T3). Plants irrigated with well water were taken as the control (CK). The results showed that mine wastewater irrigation caused adverse effects on soil enzymes, physiological properties and grain yield of winter wheat. At anthesis, T1, T2 and T3 treatments significantly reduced the activities of soil enzymes (urease, sucrase and catalase), root activity and net photosynthetic rate of wheat compared to CK. At maturity, grain yield was decreased by 17.8%, 15.4% and 9.8% by T1, T2 and T3, respectively, as compared to that of CK. Importantly, mine wastewater irrigation resulted in accumulation of heavy metals (Cr, Pb, Cu and Zn) in wheat grain. Contents of these heavy metals in grains of winter wheat subjected to mine wastewater irrigation were significantly higher than those in CK. The comprehensive contamination indexes of wheat grain in T1, T2 and T3 all reached high pollution level. Our results showed that mine wastewater irrigation significantly increased the pollution risk of heavy metals, thus unsuitable for crop irrigation.

Effect of mine wastewater on nutrient removal and lipid production by a green microalga Micratinium reisseri from concentrated municipal wastewater

Effect of mine wastewater on the nutrient removal efficiency of a green microalga Micratinium reisseri from concentrated municipal wastewater (CMW) with simultaneous lipid production was investigated. Different dilution ratios (1-10%) of CMW either with mine wastewater (MWF) or mine wastewater without Fe (MWOF) were used. M. reisseri showed the highest growth (0.8gL(-1)) and nutrient uptake (35.9mgTNL(-1) and 5.4mgTPL(-1)) at 3% MWF ([Fe]tot=6.7mgL(-1)), and the highest lipid productivity (10.4mgL(-1)day(-1)) at 5% MWF ([Fe]tot=11.2mgL(-1)) after 15days. CMW supported the algal autoflocculation due to formation of phosphate, calcium and magnesium precipitates at a high suspension pH. Fatty acid methyl ester analysis revealed that the microalgal lipids possessed 79-82% of C16/C18 fatty acids. Application of mine wastewater improved the nutrient removal efficiency, growth and lipid productivity of M. reisseri cultivated in CMW.