Biotreatment of acidic zinc- and copper-containing wastewater using ethanol-fed sulfidogenic anaerobic baffled reactor

Metal Sulfide Precipitation: Recent Breakthroughs and Future Outlooks

The interest in metal sulfide precipitation has recently increased given its capacity to efficiently recover several metals and metalloids from different aqueous sources, including wastewaters and hydrometallurgical solutions. This article reviews recent studies about metal sulfide precipitation, considering that the most relevant review article on the topic was published in 2010. Thus, our review emphasizes and focuses on the overall process and its main unit operations. This study follows the flow diagram definition, discussing the recent progress in the application of this process on different aqueous matrices to recover/remove diverse metals/metalloids from them, in addition to kinetic reaction and reactor types, different sulfide sources, precipitate behavior, improvements in solid–liquid separation, and future perspectives. The features included in this review are: operational conditions in terms of pH and Eh to perform a selective recovery of different metals contained in an aqueous source, the aggregation/colloidal behavior of precipitates, new materials for controlling sulfide release, and novel solid–liquid separation processes based on membrane filtration. It is therefore relevant that the direct production of nanoparticles (Nps) from this method could potentially become a future research approach with important implications on unit operations, which could possibly expand to several applications.

Recent advances in heavy metal recovery from wastewater by biogenic sulfide precipitation

Biological sulfide precipitation by sulfate reducing bacteria (SRB) is an emerging technique for the recovery of heavy metals from metal contaminated wastewater. Advantages of this technique include low capital cost, ability to form highly insoluble salts, and capability to remove and recover heavy metals even at very low concentrations. Therefore, sulfate reduction under anaerobic conditions has become a suitable alternative for the treatment of wastewaters that contain metals. However, bioreactor configurations for recovery of metals from sulfate rich metallic wastewater have not been explored widely. Moreover, the recovered metal sulfide nanoparticles could be applied in various fields such as solar cells, dye degradation, electroplating, etc. Hence, metal recovery in the form of nanoparticles from wastewater could serve as an incentive for industries. The simultaneous metal removal and recovery can be achieved in either a single-stage or multistage systems. This paper aims to present an overview of the different bioreactor configurations for the treatment of wastewater containing sulfate and metal along with their advantages and drawbacks for metal recovery. Currently followed biological strategies to mitigate sulfate and metal rich wastewater are evaluated in detail in this review.

Fluidized bed membrane bioreactor achieves high sulfate reduction and filtration performances at moderate temperatures

The study explored the potential of an up-flow sulfate reducing fluidized-bed membrane bioreactor (SR-FMBR) for biogenic sulfide generation at room temperature together with evaluation of filtration and fouling characteristics developed under various operational conditions. The SR-FMBR was tested at different COD/sulfate (mg/mg) ratios for a total of 127 days, initially at 35 °C and then at 23 °C. SR-FMBR was able to achieve COD oxidation and sulfate reduction efficiencies up to 98%, and allowed for biogenic sulfide generation up to 600 mg/L (97% of theoretical value) at room temperature. Alkalinity was generated as a result of sulfate reduction and averaged around 1900 mgCaCO₃/L in the permeate. Hence, starting the bioreactor operation at 35 °C and then decreasing it to 23 °C did not adversely affect the process performance. High filtration fluxes up to 9.3 L/m²/h (LMH) could be maintained at employed hydraulic retention times between 24 h and 6 h. Observing relatively high filtration performance was due to keeping a high fraction of biomass attached to the carrier material, which decreased the cake formation potential on the membrane surface compared to conventional MBR operation. The SR-FMBR performance may further be tested for heavy metal removal under sulfidogenic conditions for acid mine drainage treatment.

Sequential hydrotalcite precipitation and biological sulfate reduction for acid mine drainage treatment

Hydrotalcite precipitation is a promising technology for the on-site treatment of acid mine drainage (AMD). This technology is underpinned by the synthesis of hydrotalcite that can effectively remove various contaminants. However, hydrotalcite precipitation has only limited capacity to facilitate sulfate removal from AMD. Therefore, the feasibility of coupling biological sulfate reduction with the hydrotalcite precipitation to maximize sulfate removal was evaluated in this study. AMD emanating from a gold mine (pH 4.3, sulfate 2000 mg L^-1, with various metals including Al, Cd, Co, Cu, Fe, Mn, Ni, Zn) was first treated using the hydrotalcite precipitation. Subsequently, biological treatment of the post-hydrotalcite precipitation effluent was conducted in an ethanol-fed fluidized bed reactor (FBR) at a hydraulic retention time (HRT) of 0.8-1.6 day. The hydrotalcite precipitation readily neutralized the acidity of AMD and removed 10% of sulfate and over 99% of Al, Cd, Co, Cu, Fe, Mn, Ni, Zn. The overall sulfate removal increased to 73% with subsequent FBR treatment. Based on 454 pyrosequencing of 16S rRNA genes, the identified genera of sulfate-reducing bacteria (SRB) included Desulfovibrio, Desulfomicrobium and Desulfococcus. This study showed that sulfate-rich AMD can be effectively treated by integrating hydrotalcite precipitation and a biological sulfate reducing FBR.

Study on the Effectiveness of Sulfate-Reducing Bacteria Combined with Coal Gangue in Repairing Acid Mine Drainage Containing Fe and Mn

In view of the characteristics of the high content of SO42−, Fe2+ and Mn2+ in acid mine drainage (AMD) and low pH value, based on adsorption and biological methods, coal gangue was combined with sulfate-reducing bacteria (SRB). On this basis, four dynamic columns, including Column 1 (SRB combined with spontaneous combustion gangue from the Gaode coal mine), Column 2 (SRB combined with spontaneous combustion gangue from Haizhou), Column 3 (SRB combined with gangue from Haizhou), and Column 4 (SRB combined with gangue from Shanxi), were constructed. The efficacy of four columns was compared by the inflow of AMD with different pollution load. Results showed that the repair effect of four columns was: Column 3 > Column 2 > Column 1 > Column 4. In the second stage of the experiment, the repair effect of Column 3 was the best. The average effluent pH value and oxidation reduction potential (ORP) value were 9.09 and –262.83 mV, the highest removal percentages of chemical oxygen demand (COD) and SO42− were 84.41% and 72.73%, and the average removal percentages of Fe2+, Mn2+ were 98.70% and 79.97%, respectively. At the end of the experiment, when deionized water was injected, the fixed effect of AMD in the four columns was stable and no secondary release appeared.

Preparation of metal-resistant immobilized sulfate reducing bacteria beads for acid mine drainage treatment

Novel immobilized sulfate-reducing bacteria (SRB) beads were prepared for the treatment of synthetic acid mine drainage (AMD) containing high concentrations of Fe, Cu, Cd and Zn using up-flow anaerobic packed-bed bioreactor. The tolerance of immobilized SRB beads to heavy metals was significantly enhanced compared with that of suspended SRB. High removal efficiencies of sulfate (61-88%) and heavy metals (>99.9%) as well as slightly alkaline effluent pH (7.3-7.8) were achieved when the bioreactor was fed with acidic influent (pH 2.7) containing high concentrations of multiple metals (Fe 469 mg/L, Cu 88 mg/L, Cd 92 mg/L and Zn 128 mg/L), which showed that the bioreactor filled with immobilized SRB beads had tolerance to AMD containing high concentrations of heavy metals. Partially decomposed maize straw was a carbon source and stabilizing agent in the initial phase of bioreactor operation but later had to be supplemented by a soluble carbon source such as sodium lactate. The microbial community in the bioreactor was characterized by denaturing gradient gel electrophoresis (DGGE) and sequencing of partial 16S rDNA genes. Synergistic interaction between SRB (Desulfovibrio desulfuricans) and co-existing fermentative bacteria could be the key factor for the utilization of complex organic substrate (maize straw) as carbon and nutrients source for sulfate reduction.

Mechanisms of Cu2+ migration, recovery and detoxification in Cu2+-, SO4(2-) -containing wastewater treatment process with anaerobic granular sludge

In this study, anaerobic granular sludge with sulphate-reducing bacteria (SRB) was applied to treat Cu2+-, SO4(2-) -containing wastewater in an expanded granular sludge bed reactor. The migration and enrichment of copper in anaerobic granular sludge were envaluated. By analysing the sludge with X-ray diffraction, copper was determined to be present as covellite (CuS) in the sludge. Observations at the microscopic level showed that CuS precipitates were absorbed onto granules and gradually migrated from the outer to the interior layer of the granule over time and finally accumulated in the core of the granular sludge. Because of the migration of the CuS precipitates and the protection of the extracellular polymeric substances matrix, SRB were able to tolerate copper concentrations up to 10 mg/L. A copper removal efficiency of about 96% was observed at a steady state for 3 months, and copper was enriched in the granular sludge.

Arsenic removal in a sulfidogenic fixed-bed column bioreactor

In the present study, the bioremoval of arsenic from synthetic acidic wastewater containing arsenate (As(5+)) (0.5-20mg/L), ferrous iron (Fe(2+)) (100-200mg/L) and sulfate (2,000 mg/L) was investigated in an ethanol fed (780-1,560 mg/L chemical oxygen demand (COD)) anaerobic up-flow fixed bed column bioreactor at constant hydraulic retention time (HRT) of 9.6h. Arsenic removal efficiency was low and averaged 8% in case iron was not supplemented to the synthetic wastewater. Neutral to slightly alkaline pH and high sulfide concentration in the bioreactor retarded the precipitation of arsenic. Addition of 100mg/L Fe(2+) increased arsenic removal efficiency to 63%. Further increase of influent Fe(2+) concentration to 200mg/L improved arsenic removal to 85%. Decrease of influent COD concentration to its half, 780 mg/L, resulted in further increase of As removal to 96% when Fe(2+) and As(5+) concentrations remained at 200mg/L and 20mg/L, respectively. As a result of the sulfidogenic activity in the bioreactor the effluent pH and alkalinity concentration averaged 7.4 ± 0.2 and 1,736 ± 239 mg CaCO3/L respectively. Electron flow from ethanol to sulfate averaged 72 ± 10%. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses were carried out to identify the nature of the precipitate generated by sulfate reducing bacteria (SRB) activity. Precipitation of arsenic in the form of As2S3 (orpiment) and co-precipitation with ferrous sulfide (FeS), pyrite (FeS2) or arsenopyrite (FeAsS) were the main arsenic removal mechanisms.

Highly efficient removal of Cu(II), Zn(II), Ni(II) and Fe(II) from electroplating wastewater using sulphide from sulphidogenic bioreactor effluent

A bench-scale, stirred-tank batch precipitator was used to assess the selective removal of Cu2+, Zn2+, Ni2+ and Fe2+ from acidic electroplating wastewater using sulphide from a sulphidogenic bioreactor effluent. At pH approximately 1.7, >99% of Cu was selectively precipitated, over Zn, Ni and Fe, from the wastewater as pure CuS by recycling H2S from the bioreactor effluent via N2 sparging, resulting in a Cu effluent concentration <0.4 mg/L. The rate of Cu precipitation increased from 1.6 to 6.4 mg Cu/(L x min) when the pH of the bioreactor effluent decreased from 7.5 to 5.5. Experiments focusing on the precipitation of Zn, Ni and Fe from the wastewater devoid of Cu (at pH approximately 1.7), using sulphide-rich bioreactor effluent, achieved approximately 85-97% precipitation efficiency for Zn, approximately 25-92% for Ni, and approximately 2-99% for Fe, depending on the initial sulphide/metal molar ratio. The sulphide/metal ratio of 1.76 was found to be optimal for the precipitation of Zn, Ni and Fe with sulphides and, to a lesser extent, with hydroxides, resulting in residual metal concentrations of 1 mg Zn/L, 3 mg Ni/L, and 0.5 mg Fe/L. These findings suggest the potential of waste biogenic sulphides for the selective recovery of valuable metals from acidic metal-rich industrial wastewaters.

Performance of a sulfide-oxidizing, sulfur-producing membrane biofilm reactor treating sulfide-containing bioreactor effluent

Sulfide-containing waste streams are generated in mining, petrochemical plants, tanneries, viscose rayon manufacture, and the gasification of coal. Colorless sulfur bacteria can oxidize sulfide to elemental sulfur (S°), which can be recovered, when oxygen is their electron acceptor. This study evaluated sulfide oxidation and S° recovery in an oxygen-based membrane biofilm reactor (MBfR) treating the effluent from a sulfidogenic anaerobic baffled reactor. Sulfide oxidation efficiency (37-99%) and S° recovery (64-89% of oxidized sulfide) could be controlled by manipulating the sulfide loading, oxygen pressure to the fibers, and hydraulic retention time (HRT). For example, too-low oxygen pressure decreased S° recovery due to decreased sulfide oxidation, but too-high oxygen pressure lowered S° recovery due to its oxidation to sulfate. Most importantly, high sulfide oxidation (>98%) and conversion to S° (>75%) could be achieved together when the sulfide loading was less than 1.7 mol/m²·d and the O₂ pressure was sufficient to give an O₂ flux of at least 1.5 mol/m²·d. However, higher sulfide loading could be compensated by a higher O₂ pressure, and the best performance occurred when the sulfide loading was high (2 molS/m²·d), the O₂ pressure was high (∼1 atm), and the HRT was short (1.9 h). Membrane fouling caused a low O₂ flux, which led to low sulfide-oxidation efficiency, but fouling could be reversed by mild acid washing.

Sulfidogenic fluidized bed treatment of real acid mine drainage water

The treatment of real acid mine drainage water (pH 2.7-4.3) containing sulfate (1.5-3.34 g/L) and various metals was studied in an ethanol-fed sulfate-reducing fluidized bed reactor at 35°C. The robustness of the process was tested by increasing stepwise sulfate, ethanol and metal loading rates and decreasing feed pH and hydraulic retention time. Highest sulfate reduction rate (4.6g/L day) was obtained with feed sulfate concentration of 2.5 g/L, COD/sulfate ratio of 0.85 and HRT of 12 h. The corresponding sulfate and COD removal efficiencies were about 90% and 80%, respectively. The alkalinity produced in sulfidogenic ethanol oxidation neutralized the acidic mine water. Highest metal precipitation efficiencies were observed at HRT of 24 h, the percent metal removal being over 99.9% for Al (initial concentration 55 mg/L), Co (9.0 mg/L), Cu (49 mg/L), Fe (435 mg/L), Ni (3.8 mg/L), Pb (7.5 mg/L) and Zn (6.6 mg/L), and 94% for Mn (7.21 mg/L).