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 S0bio, Se0bio and Te0bio effectively removed Ga and Zn. The highest removal efficiency (Õ²) was obtained for Ga onto the adsorbent Te0bio (69 ± 0.4%), with an adsorption capacity (q) of 74 mg Ga (g Te0bio)-1, followed by Zn (Õ² = 40 ± 0.7%) with 43 mg Zn (g Te0bio)-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 (Õ² = 59.9 ± 2.6%) and Zn (Õ² = 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.

Pre-treatment and temperature effects on the use of slow release electron donor for biological sulfate reduction.

Lignocellulosic materials can be used as slow release electron donor (SRED) for biological sulfate reduction, potentially enhancing the subsequent metal sulfide precipitation. Lignocellulosic materials require a pre-treatment step in other biotechnological applications, but pre-treatment strategies for its use as a SRED for biological sulfate reduction have not yet been tested. Three pre-treatments strategies (mechanical, acid, and mechanical followed by acid pre-treatment) were tested to enhance electron donor release from brewery spent grain (BSG), and compared to a non-pre-treated control. Mechanical pre-treatment provided the highest sulfate removal rate (82.8 ± 8.8 mg SO42-.(g TVS.day)-1), as well as the highest final sulfide concentration (441.0 ± 34.4 mg.L-1) at mesophilic conditions. BSG submitted to mechanical pre-treatment was also assessed under psychrophilic and thermophilic conditions. Under mesophilic and psychrophilic conditions, both sulfate reduction and methane production occurred. Under psychrophilic conditions, the sulfate reduction rate was lower (25 ± 2.0 mg SO42-.(g TVS.day)-1), and the sulfide formation depended on lactate addition. A metal precipitation assay was conducted to assess whether the use of SRED enhances metal recovery. Zinc precipitation and recovery with chemical or biogenic sulfide from the BSG batches were tested. Sulfide was provided in a single spike or slowly added, mimicking the effect of SRED. ZnS was formed in all conditions, but better settling particles were obtained when sulfide was slowly added, regardless of the sulfide source.

Anoxic Microbial Community Robustness Under Variation of Hydraulic Retention Time and Availability of Endogenous Electron Donors.

The ADNMED (Anaerobic Digestion, Nitrification, and Mixotrophic Endogenous Denitrification) system comprises a triple chamber configuration that was shown to provide high-quality effluent regarding carbon, nitrogen, and sulfide. Hydraulic retention time (HRT) was 7 h in the anaerobic and anoxic chambers, and 5 h in the aerobic chamber (stage A). Sewage was directly added to the anoxic chamber to provide extra organic electron donors for denitrification (stage B) to improve the nitrogen removal efficiency (stage A 47 ± 19%). The addition of sewage at a flow rate equivalent to 10% of the feed flow increased nitrogen removal efficiency to 61 ± 12%. Illumina® sequencing revealed a restructuring of the microbial community in the anoxic chamber, according to the availability of the endogenous electron donors for denitrification. At stage A, denitrification was related to the decay of biomass, while the addition of sewage during stage B stimulated the establishment of fermentative bacteria.