Elemental sulfur biorecovery from phosphogypsum using oxygen-membrane biofilm reactor: Bioreactor parameters optimization, metagenomic analysis and metabolic prediction of the biofilm activity.

This work investigated elemental sulfur (S0) biorecovery from Phosphogypsum (PG) using sulfur-oxidizing bacteria in an O2-based membrane biofilm reactor (MBfR). The system was first optimized using synthetic sulfide medium (SSM) as influent, then switched to biogenic sulfide medium (BSM) generated by biological reduction of PG alkaline leachate. The results using SSM had high sulfide-oxidation efficiency (98 %), sulfide to S0 conversion (∼90 %), and S0 production rate up to 2.7 g S0/(m2.d), when the O2/S ratio was ∼0.5 g O2/g S. With the BSM influent, the system maintained high sulfide-to-S0 conversion rate (97 %), and S0-production rate of 1.6 g S0/(m2.d). Metagenomic analysis revealed that Thauera was the dominant genus in SSM and BSM biofilms. Furthermore, influent composition affected the bacterial community structure and abundances of functional microbial sulfur genes, modifying the sulfur-transformation pathways in the biofilms. Overall, this work shows promise for O2-MBfR usage in S0 biorecovery from PG-leachate and other sulfidogenic effluents.

Optimizing Autotrophic Sulfide Oxidation in the Oxygen-Based Membrane Biofilm Reactor to Recover Elemental Sulfur.

Biological sulfide oxidation is an efficient means to recover elemental sulfur (S0) as a valuable resource from sulfide-bearing wastewater. This work evaluated the autotrophic sulfide oxidation to S0 in the O2-based membrane biofilm reactor (O2-MBfR). High recovery of S0 (80-90% of influent S) and high sulfide oxidation (∼100%) were simultaneously achieved when the ratio of O2-delivery capacity to sulfide-to S0 surface loading (SL) (O2/S2- → S0 ratio) was around 1.5 (g O2/m2-day/g O2/m2-day). On average, most of the produced S0 was recovered in the MBfR effluent, although the biofilm could be a source or sink for S0. Shallow metagenomic analysis of the biofilm showed that the top sulfide-oxidizing genera present in all stages were Thauera, Thiomonas, Thauera_A, and Pseudomonas. Thiomonas or Pseudomonas was the most important genus in stages that produced almost only S0 (i.e., the O2/S2- → S0 ratio around 1.5 g of the O2/m2-day/g O2/m2-day). With a lower sulfide SL, the S0-producing genes were sqr and fccAB in Thiomonas. With a higher sulfide SL, the S0-producing genes were in the soxABDXYZ system in Pseudomonas. Thus, the biofilm community of the O2-MBfR adapted to different sulfide-to-S0 SLs and corresponding O2-delivery capacities. The results illustrate the potential for S0 recovery using the O2-MBfR.

Effect of alkaline leaching of phosphogypsum on sulfate reduction activity and bacterial community composition using different sources of anaerobic microbial inoculum.

Phosphogypsum (PG), a by-product of the phosphate industry, is high in sulfate, (SO42-), which makes it an excellent substrate for sulfate-reducing bacteria (SRB) to produce hydrogen sulfide. This work aimed to optimize SO42- leaching from PG to achieve a high biological reduction of SO42- and generate high sulfide concentrations for subsequent use in the biological recovery of elemental sulfur. Five SRB consortia were isolated and enriched from: IS (Industrial sludges), MS (Marine sediments), WC (Winogradsky column), SNV (petroleum industry sediments) and PG (stored Phosphogypsum). The five consortia showed reduction activity when using PG leachate (with water) as source of SO42- and lactate, acetate, or glucose as the electron donor. The highest reduction rate (81.5 %) was registered using lactate and the IS consortium (81.5 %) followed by MS (79 %) and PG (71 %). To enhance the concentration of leached SO42- from PG for future utilization with the isolated consortia, PG was treated with NaOH solutions (2 % and 5 %). SO42- release of 97 % was achieved with a 5 % concentration and the resulting leachate was further diluted to target a SO42- concentration of 12.4 g·L-1 for utilization with the isolated consortia. Compared to water leachate, a significantly higher reduction rate was registered (2 g·L-1 of SO42) using the IS consortium, demonstrating limited inhibition effect of sulfide- concentration on SRB functionalities. Moreover, metagenomic analysis of the consortia revealed that using PG as a source of SO42- increased the abundance of Deltaproteobacteria, including known SRB like Desulfovibrio, Desulfomicrobium, and Desulfosporosinus, as well as novel SRB genera (Cupidesulfovibrio, Desulfocurvus, Desulfococcus) that showed, for the first time, significant potential as novel sulfate-reducers using PG as a SO42- source.

Elaboration of geopolymer package derived from uncalcined phosphate sludge and its solidification performance on nuclear grade resins loaded with 134Cs.

Nuclear-grade Spent Organic Resin (SOR) contains high concentrations of radioactive nuclides and metal contaminants, while phosphate sludge contains high amount of fine clayey particles and CO32-, both posing a major threat to the biosphere. In this study, a novel geopolymer package (GP) was proposed to directly solidify SOR loaded with 134Cs by incorporating uncalcined phosphate sludge (UPS) as feedstocks, activated by NaOH/KOH. The results showed that alkali-mixed reagents-activated GP is more advantageous in terms of chemical stability and mechanical properties than NaOH-activated GP, recording compressive strength values greater than the waste acceptance criteria and OPC. The 28-day compressive strength of solidified packages can exceed 31 MPa at the highest amount of 42 wt% UPS. The addition of NaF powder into the solidified packages generates more hybrid type gels, which are more conducive to partial dissolution and bonding UPS particles, thereby producing stable and stronger GP. Leaching results of solidified GP in presence of up to 13 wt% SORs showed that only 0.15 % of total 134Cs was leached, even under aggressive solutions. Solidification mechanism revealed that activation of UPS-MK blend forms N,K-A-S-H, (N,K,C)-A-S-H/C-S-H gels coexisting with unreacted particles, thereby solidify/stabilize metal contaminants and Cs+ by a synergetic immobilization action of hydration products via substitution and encapsulation. This study provides a promising paradigm for effective solidification of nuclear-grade resins and synergetic harmless treatment of industrial/phosphate mine solid wastes.

Efficient leaching process of rare earth, alkali and alkaline earth metals from phosphogypsum based on methanesulfonic acid (MSA) as green & eco-friendly lixiviant.

The leaching of rare earth elements (REEs) from secondary resources is exponentially increasing to supply the widespread range of high-tech applications of these elements including phosphors lighting materials, catalysis and permanent magnets. Phosphate fertilizer byproducts including phosphogypsum (PG) were identified as a potential alternative resource of REEs, not only to face the expansion of market demand, but also to achieve a sustainable management of REE resources. This study reports the leaching of REEs from PG using methanesulfonic acid (MSA) as a green organo-sulfonic acid in comparison with other acids such as p-toluenesulfonic acid (PTSA) and hydrochloric acid (HCl). MSA achieved the highest leaching efficiency of 78% with low solubility of PG under the operating conditions of 3 M, solid to liquid ratio (S/L) of 1/8, 120 min and 25 °C. The optimized leaching process was also modeled using shrinking core theory to assess the kinetics behavior of the system and to enable the determination of the predominant mechanisms. It was demonstrated that the leaching is governed by a product layer diffusion-controlled model with an activation energy of 2.73 kJ mol-1. The cleaned PG after leaching could greatly meet the quality requirements of the building materials industry.

Microbial transformations by sulfur bacteria can recover value from phosphogypsum: A global problem and a possible solution.

Rising global population and affluence are increasing demands for food production and the phosphorus (P) fertilizers needed to grow that food. Essential are new approaches for managing the growing amount of phosphogypsum (PG) that is a by-product of phosphoric-acid production from phosphate rock. Today, only ~15% of the worldwide production of PG is recycled, mainly for agriculture and road construction. This review addresses microbial valorization of PG through strategies that apply sulfur-transforming bacteria: sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB). The focus is on recovering elemental sulfur (S0), which can be used to make the sulfuric acid needed to produce phosphoric acid from rock phosphate. Our review provides in-depth understanding of the microbiological, chemical, and technological bases for microbial reclamation of S0 from PG. The review presents the principles and practices for sulfate leaching from PG, reduction of sulfate to sulfide by SRB, and oxidation of sulfide to S0 by SOB. The choice of electron donor for SRB, control of oxygen delivery to SOB, and nutrient requirements are emphasized. Although microorganism-based technologies for PG reclamation are far from mature, the efficiency of such SRB- and SOB-based processes has been documented at laboratory and industrial scales. This review should spur biotechnological advances toward recovering value from PG.

Sustainable coating material based on chitosan-clay composite and paraffin wax for slow-release DAP fertilizer.

The coating of fertilizers by polymers is one of the most efficient tools for their slow and control release into soil. This strategy avoids excessive use of the fertilizers and increases their availability to the crops needs. In the present paper, hydro-soluble diammonium phosphates (DAP) fertilizer was double coated following the dip-coating process by chitosan-clay composites as inner coating and paraffin wax as an outer coating. The chitosan composite preparation and characterization were deeply investigated. The montmorillonite (MMT) clay incorporation as filler improves the water barrier diffusion, mechanical properties, and thermal stability of the composite. The combination of the swelling behavior of the chitosan-clay composite (inner coating) and the hydrophobic property of paraffin wax (outer coating) was confirmed by the water holding capacity evaluation and the phosphorus release essays in water and soil. Indeed, the phosphorus dissolution from the coated DAP granules was significantly delayed compared to the uncoated DAP. Moreover, the biodegradation study of composite material in soil and the biochemical oxygen demand (BOD) tests revealed that the coating system proposed could be considered as a carbon source for microorganisms after the fertilization process, which confirms its sustainability.