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Biochar is a carbonaceous and porous material with limited adsorption capacity, which increases by modifying its surface. Many of the biochars modified with magnetic nanoparticles reported previously were obtained in two steps: first, the biomass was pyrolyzed, and then the modification was performed. In this research, a biochar with Fe3O4 particles was obtained during the pyrolysis process. Corn cob residues were used to obtain the biochar (i.e., BCM) and the magnetic one (i.e., BCMFe). The BCMFe biochar was synthesized by a chemical coprecipitation technique prior to the pyrolysis process. The biochars obtained were characterized to determine their physicochemical, surface, and structural properties. The characterization revealed a porous surface with a 1013.52 m2/g area for BCM and 903.67 m2/g for BCMFe. The pores were uniformly distributed, as observed in SEM images. BCMFe showed Fe3O4 particles on the surface with a spherical shape and a uniform distribution. According to FTIR analysis, the functional groups formed on the surface were aliphatic and carbonyl functional groups. Ash content in the biochar was 4.0% in BCM and 8.0% in BCMFe; the difference corresponded to the presence of inorganic elements. The TGA showed that BCM lost 93.8 wt% while BCMFe was more thermally stable due to the inorganic species on the biochar surface, with a weight loss of 78.6%. Both biochars were tested as adsorbent materials for methylene blue. BCM and BCMFe obtained a maximum adsorption capacity (qm) of 23.17 mg/g and 39.66 mg/g, respectively. The obtained biochars are promising materials for the efficient removal of organic pollutants.

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The biotransformation of the SARS-CoV-2 antiviral drugs, ribavirin and tenofovir, was studied in methanogenic bioreactors. The role of iron-rich minerals, recovered from a metallurgic effluent, on the biotransformation process was also assessed. Enrichment of anaerobic sludge with recovered minerals promoted superior removal efficiency for both antivirals (97.4 % and 94.7 % for ribavirin and tenofovir, respectively) as compared to the control bioreactor lacking minerals, which achieved 58.5 % and 37.9 % removal for the same drugs, respectively. Further analysis conducted by liquid chromatography coupled to mass spectroscopy revealed several metabolites derived from the biotransformation of both antivirals. Interestingly, tracer analysis with 13CH4 revealed that anaerobic methane oxidation coupled to Fe(III) reduction occurred in the enriched bioreactor, which was reflected in a lower content of methane in the biogas produced from this system, as compared to the control bioreactor. This treatment proposal is suitable within the circular economy concept, in which recovered metals from an industrial wastewater are applied in bioreactors to create a biocatalyst for promoting the biotransformation of emerging pollutants. This strategy may be appropriate for the anaerobic treatment of wastewaters originated from hospitals, as well as from the pharmaceutical and chemical sectors.

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Metals are key materials extensively employed in several industries to produce technological and daily-life products. The mining industry that produces such commodities generates Tons of waste that if not remediated can be transferred to the surrounding environment, thus representing a water, air, and soil pollution threat. In this work, we evaluated the feasibility of microbial sulfate reduction (SR) as a management strategy for this waste. Mine tailings were sampled from two abandoned mining sites located in Sonora (northwestern Mexico) and treated in anaerobic microcosms under SR conditions using anaerobic sludge as the inoculum at two different tailing:inoculum ratios (TIR). Major TIR's were found to be the triggering factor for the highest SR activities observed (73.6 ± 8.8 mg SO42- L-1 day-1). This stimulation was linked to the dissolution of sulfate bearing minerals (anglesite, jarosite, and gypsum) which provided additional sulfate for microbial activity. However, under this condition, longer lag phases for SR were observed, which was potentially due to pH inhibition at early incubation stages (pH ~3.7). Despite this, all biologically SR performing treatments presented important sulfide precipitation which was associated to changes in the mineralogy of the mine tailings. Metals of environmental concern such as As, Cd, Co, Cr and, Pb were detected to have shifted from the aqueous extractable phase to the bound to Fe and Mn oxides and residual phases. This finding was in accordance with the non-detectable concentrations of these metals in the aqueous phase by the end of the biological treatment which proved the effectiveness of this approach. This study provides insights into the promising potential of anaerobic microbes for the environmental management of mine tailings.

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Geobacter sulfurreducens is capable of reducing Pd(II) to Pd(0) using acetate as electron donor; however, the biochemical and genetic mechanisms involved in this process have not been described. In this work, we carried out transcriptome profiling analysis to identify the genes involved in Pd(II) reduction in this bacterium. Our results showed that 252 genes were upregulated while 141 were downregulated during Pd(II) reduction. Among the upregulated genes, 12 were related to energy metabolism and electron transport, 50 were classified as involved in protein synthesis, 42 were associated to regulatory functions and transcription, and 47 have no homologs with known function. RT-qPCR data confirmed upregulation of genes encoding PilA, the structural protein for electrically conductive pili, as well as c-type cytochromes GSU1062, GSU2513, GSU2808, GSU2934, GSU3107, OmcH, OmcM, PpcA, and PpcD under Pd(II)-reducing conditions. ΔpilA and ΔpilR mutant strains showed 20% and 40% decrease in the Pd(II)-reducing capacity, respectively, as compared to the wild type strain, indicating the central role of pili in this process. RT-qPCR data collected during Pd(II) reduction also confirmed downregulation of omcB, omcC, omcZ, and omcS genes, which have been shown to be involved in the reduction of Fe(III) and electrodes. The present study contributes to elucidate the mechanisms involved in Pd(II) reduction by G. sulfurreducens. Graphical Abstract KEY POINTS: • Transcriptome analysis provided evidence on Pd(II) reduction by G. sulfurreducens. • Results indicate that electrically conductive pili is involved in Pd(II) reduction. • G. sulfurreducens was not able to grow under Pd(II)-reducing conditions. • The study contributes to a better understanding of the mechanisms in Pd(II) reduction.

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Metal-humic acid complexes were synthesized and immobilized by a granulation process in anaerobic sludge for their application as solid-phase redox mediators (RM) in the biotransformation of iopromide. Characterization of Ca- and Fe-humic acid complexes revealed electron accepting capacities of 0.472 and 0.556milli-equivalentsg(-1), respectively. Once immobilized, metal-humic acid complexes significantly increased the biotransformation of iopromide in upflow anaerobic sludge blanket (UASB) reactors. Control UASB reactor (without humic material) achieved 31.6% of iopromide removal, while 80% was removed in UASB reactors supplied with each metal-humic acid complex. Further analyses indicated multiple transformation reactions taking place in iopromide including deiodination, N-dealkylation, decarboxylation and deacetylation. This is the first successful application of immobilized RM, which does not require a supporting material to maintain the solid-phase RM in long term operation of bioreactors. The proposed redox catalyst could be suitable for enhancing the redox conversion of different recalcitrant pollutants present in industrial effluents.

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The capacity of anaerobic granular sludge to reduce Pd(II), using ethanol as electron donor, in an upflow anaerobic sludge blanket (UASB) reactor was demonstrated. Results confirmed complete reduction of Pd(II) and immobilization as Pd(0) in the granular sludge. The Pd-enriched sludge was further evaluated regarding biotransformation of two recalcitrant halogenated pollutants: 3-chloro-nitrobenzene (3-CNB) and iopromide (IOP) in batch and continuous operation in UASB reactors. The superior removal capacity of the Pd-enriched biomass when compared with the control (not exposed to Pd) was demonstrated in both cases. Results revealed 80 % of IOP removal efficiency after 100 h of incubation in batch experiments performed with Pd-enriched biomass whereas only 28 % of removal efficiency was achieved in incubations with biomass lacking Pd. The UASB reactor operated with the Pd-enriched biomass achieved 81 ± 9.5 % removal efficiency of IOP and only 61 ± 8.3 % occurred in the control reactor lacking Pd. Regarding 3-CNB, it was demonstrated that biogenic Pd(0) promoted both nitro-reduction and dehalogenation resulting in the complete conversion of 3-CNB to aniline while in the control experiment only nitro-reduction was documented. The complete biotransformation pathway of both contaminants was proposed by high-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis evidencing a higher degree of nitro-reduction and dehalogenation of both contaminants in the experiments with Pd-enriched anaerobic sludge as compared with the control. A biotechnological process is proposed to recover Pd(II) from industrial streams and to immobilize it in anaerobic granular sludge. The Pd-enriched biomass is also proposed as a biocatalyst to achieve the biotransformation of recalcitrant compounds in UASB reactors.

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Palladium(II) reduction to Pd(0) nanoparticles by Geobacter sulfurreducens was explored under conditions of neutral pH, 30 °C and concentrations of 25, 50, and 100 mg of Pd(II)/L aiming to investigate the effect of solid species of palladium on their microbial reduction. The influence of anthraquinone-2,6-disulfonate was reported to enhance the palladium reaction rate in an average of 1.7-fold and its addition is determining to achieve the reduction of solid species of palladium. Based on the obtained results two mechanisms are proposed: (1) direct, which is fully described considering interactions of amide, sulfur, and phosphoryl groups associated to proteins from bacteria on palladium reduction reaction, and (2) quinone-mediated, which implies multiheme c-type cytochromes participation. Speciation analysis and kinetic results were considered and integrated into a model to fit the experimental data that explain both mechanisms. This work provides elements for a better understanding of direct and quinone-mediated palladium reduction by G. sulfurreducens, which could facilitate metal recovery with concomitant formation of valuable palladium nanoparticles in industrial processes.

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The present study is the first report on the ability of Geobacter sulfurreducens PCA to reduce Pd(II) and produce Pd(0) nano-catalyst, using acetate as electron donor at neutral pH (7.0 ± 0.1) and 30 °C. The microbial production of Pd(0) nanoparticles (NPs) was greatly enhanced by the presence of the redox mediator, anthraquinone-2,6-disulfonate (AQDS) when compared with controls lacking AQDS and cell-free controls. A cell dry weight (CDW) concentration of 800 mg/L provided a larger surface area for Pd(0) NPs deposition than a CDW concentration of 400 mg/L. Sample analysis by transmission electron microscopy revealed the formation of extracellular Pd(0) NPs ranging from 5 to 15 nm and X-ray diffraction confirmed the Pd(0) nature of the nano-catalyst produced. The present findings open the possibility for a new alternative to synthesize Pd(0) nano-catalyst and the potential application for microbial metal recovery from metal-containing waste streams.

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