Elementary sulfur in effluent from denitrifying sulfide removal process as adsorbent for zinc(II)

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.

Selective removal and recovery of gallium and germanium from synthetic zinc refinery residues using biosorption and bioprecipitation

The depletion of primary ores, the environmental concerns related to mining activities, and the need to promote circular economy has drawn attention to the recycling of metallic compounds. Bio-based technologies are suitable for metal recovery, as they operate under mild conditions (ambient temperature and pressure) and are ideal for treating low-concentration waters. This study compared the effectiveness of adsorption and precipitation for the removal and recovery of gallium, germanium and zinc. Adsorption of the metallic ions on elemental forms of sulfur (S⁰), selenium (Se⁰) and tellurium (Te⁰), both of chemical and biological sources, was tested. Biosorption onto elemental forms of S⁰_bio, Se⁰_bio and Te⁰_bio effectively removed Ga and Zn. The highest removal efficiency (ղ) was obtained for Ga onto the adsorbent Te⁰_bio (69 ± 0.4%), with an adsorption capacity (q) of 74 mg Ga (g Te⁰_bio)^-1, followed by Zn (ղ = 40 ± 0.7%) with 43 mg Zn (g Te⁰_bio)^-1. Precipitation with chemical and biogenic sulfide at different metal to sulfide (Me/S) ratios was also assessed. Biologically produced sulfide was more efficient for Ga and Zn compared to chemical sulfide. Precipitation with biogenic sulfide was efficient for the removal of Ga (ղ = 59.9 ± 2.6%) and Zn (ղ = 44.2 ± 3.0%). The lowest ratio between metal to sulfide (Me/S = 0.2) achieved higher zinc removal efficiencies, whereas gallium removal was more efficient at Me/S = 1.5. None of the tested methods allowed for recovery of Ge. Biosorption and bioprecipitation gave nevertheless high removal and recovery of Ga and Zn.

Advances in elemental sulfur-driven bioprocesses for wastewater treatment: From metabolic study to application

Elemental sulfur (S0) is known to be an abundant, non-toxic material with a wide range of redox states (-2 to +6) and may serve as an excellent electron carrier in wastewater treatment. In turn, S0-driven bioprocesses, which employ S0 as electron donor or acceptor, have recently established themselves as cost-effective therefore attractive solutions for wastewater treatment. Numerous related processes have, to date, been developed from laboratory experiments into full-scale applications, including S0-driven autotrophic denitrification for nitrate removal and S0-reducing organic removal. Compared to the conventional activated sludge process, these bioprocesses require only a small amount of organic matter and produce very little sludge. There have been great efforts to characterize chemical and biogenic S0 and related functional microorganisms in order to identify the biochemical pathways, upgrade the bioprocesses, and assess the impact of the operating factors on process performance, ultimately aiming to better understand and to optimize the processes. This paper is therefore a comprehensive overview of emerging S0-driven biotechnologies, including the development of S0-driven autotrophic denitrification and S0-based sulfidogenesis, as well as the associated microbiology and biochemistry. Also reviewed here are the physicochemical characteristics of S0 and the effects that environmental factors such as pH, influent sulfur/nitrate ratio, temperature, S0 particle size and reactor configurations have on the process. Research gaps, challenges of process applications and potential areas for future research are further proposed and discussed.

Recent Advances in Autotrophic Biological Nitrogen Removal for Low Carbon Wastewater: A Review

Due to carbon source dependence, conventional biological nitrogen removal (BNR) processes based on heterotrophic denitrification are suffering from great bottlenecks. The autotrophic BNR process represented by sulfur-driven autotrophic denitrification (SDAD) and anaerobic ammonium oxidation (anammox) provides a viable alternative for addressing low carbon wastewater. Whether for low carbon municipal wastewater or industrial wastewater with high nitrogen, the SDAD and anammox process can be suitably positioned accordingly. Herein, the recent advances and challenges to autotrophic BNR process guided by SDAD and anammox are systematically reviewed. Specifically, the present applications and crucial operation factors were discussed in detail. Besides, the microscopic interpretation of the process was deepened in the viewpoint of functional microbial species and their physiological characteristics. Furthermore, the current limitations and some future research priorities over the applications were identified and discussed from multiple perspectives. The obtained knowledge would provide insights into the application and optimization of the autotrophic BNR process, which will contribute to the establishment of a new generation of efficient and energy-saving wastewater nitrogen removal systems.

Sulfate-reduction, sulfide-oxidation and elemental sulfur bioreduction process: modeling and experimental validation

This study describes the sulfate-reducing (SR) and sulfide-oxidizing (SO) process using Monod-type model with best-fit model parameters both being reported and estimated. The molar ratio of oxygen to sulfide (ROS) significantly affects the kinetics of the SR+SO process. The S(0) is produced by SO step but is later consumed by sulfur-reducing bacteria to lead to "rebound" in sulfide concentration. The model correlated well all experimental data in the present SR+SO tests and the validity of this approach was confirmed by independent sulfur bioreduction tests in four denitrifying sulfide removal (DSR) systems. Modeling results confirm that the ratio of oxygen to sulfide is a key factor for controlling S(0) formation and its bioreduction. Overlooking S(0) bioreduction step would overestimate the yield of S(0).

Facultative autotrophic denitrifiers in denitrifying sulfide removal granules

The denitrifying sulfide removal (DSR) process applied autotrophic and heterotrophic denitrification pathways to achieve simultaneous conversion of nitrate to N, sulfide to elementary sulfur, and organic substances to CO. However, autotrophic denitrifiers and heterotrophic denitrifiers have to grow at comparable rates so the long-term DSR stability can be maintained. This work assessed the autotrophic and heterotrophic denitrification activities by 16 isolates from anaerobic granules collected from a DSR-expanded granular sludge bed reactor. A group of strains with closest relatives as Pseudomonas sp. (89.9-98.3% similarity), Agrobacterium sp. (94.6% similarity) and Acinetobacter sp. (96.6% similarity) were identified with both autotrophic and heterotrophic denitrification capabilities. These facultative autotrophic denitrifiers can be applied as potential strains for lifting the limitation by balanced growth of two distinct bacterial groups in the DSR reactor.

Simultaneous biological removal of nitrogen-sulfur-carbon: recent advances and challenges

Biological removal of carbon, nitrogen and sulfur is drawing increasing research interest in search for an efficient and cost-effective wastewater treatment. While extensive work on separate removal of nitrogen and sulfur is well documented, investigation on simultaneous denitrifying sulfide removal has only been reported recently. Most of the work on denitrifying sulfide removal has been focusing on bioreactor performance, loading and operating conditions. Nonetheless, underlying principles elucidating the biochemical reactions and the mechanisms of the microbial degradation are yet to be established. In addition, unstable denitrifying sulfide removal which is a major operating problem that hinders practical application of the process, is yet to be resolved. This paper provides a review on the state-of-the-art development of simultaneous biological removal of sulfur, nitrogen and carbon. Research on bioreactor operation and performance, reactor configurations, mechanisms and modeling work including the use of mass balance analysis and artificial neural networks is delineated. An in-depth discussion on the microbial community and functional consortium is also provided. Challenges and future work on simultaneous biological removal of nitrogen-sulfur-carbon are also outlined.