Electrocatalytic hydrogenation of indigo by NiMoS: energy saving and conversion improving

An NiMo alloy bonded with sulfur (NiMoS) exhibits enhanced surface affinity toward water and organic molecules, thereby enhancing electrocatalytic hydrogenation (ECH) reactions through synergistic effects. In industrial processes, indigo, an ancient dye employed in the denim industry, is typically chemically reduced using sodium dithionite. However, this process generates an excess of toxic sulfide, which heavily contaminates the environment. ECH is a sustainable alternative for indigo reduction due to its reduced reliance on chemicals and energy consumption. In this study, carbon-felt (CF)-supported NiMoS was synthesized in a two-step process. First, the NiMo alloy was electrodeposited onto the CF surface, followed by sulfidation in an oven at 600 °C. NiMoS exhibits a larger electrochemically active surface area and a smaller charge transfer resistance compared to pure Ni and NiMo. Furthermore, NiMoS demonstrates excellent thermodynamic and kinetic properties for water splitting in strong alkaline solutions (1.0 M KOH). Additionally, optimal reaction conditions for the ECH of indigo were explored. Under the conditions of a 1.0 M KOH hydroxide medium with 10% methanol (v/v), an indigo concentration of 5 g L-1, a reaction temperature of 70 °C, and a current density of 10 mA cm-2, NiMoS/CF achieved remarkable improvements in both conversion (99.2%) and Faraday efficiency (38.1%). The results of this experimental work offer valuable insights into the design and application of novel catalytic materials for the ECH of vat dyes, opening up new possibilities for sustainable and environmentally friendly processes in the dye industry.

Effect of Fermentation Scale on Microbiota Dynamics and Metabolic Functions for Indigo Reduction

During indigo dyeing fermentation, indigo reduction for the solubilization of indigo particles occurs through the action of microbiota under anaerobic alkaline conditions. The original microbiota in the raw material (sukumo: composted indigo plant) should be appropriately converged toward the extracellular electron transfer (EET)-occurring microbiota by adjusting environmental factors for indigo reduction. The convergence mechanisms of microbiota, microbial physiological basis for indigo reduction, and microbiota led by different velocities in the decrease in redox potential (ORP) at different fermentation scales were analyzed. A rapid ORP decrease was realized in the big batch, excluding Actinomycetota effectively and dominating Alkalibacterium, which largely contributed to the effective indigo reduction. Functional analyses of the microbiota related to strong indigo reduction on approximately day 30 indicated that the carbohydrate metabolism, prokaryotic defense system, and gene regulatory functions are important. Because the major constituent in the big batch was Alkalibacterium pelagium, we attempted to identify genes related to EET in its genome. Each set of genes for flavin adenine dinucleotide (FAD) transportation to modify the flavin mononucleotide (FMN)-associated family, electron transfer from NADH to the FMN-associated family, and demethylmenaquinone (DMK) synthesis were identified in the genome sequence. The correlation between indigo intensity reduction and metabolic functions suggests that V/A-type H+/Na+-transporting ATPase and NAD(P)H-producing enzymes drive membrane transportations and energization in the EET system, respectively.

Environmental factors contributing to the convergence of bacterial community structure during indigo reduction

Indigo is solubilized through the reducing action of the microbiota that occurs during alkaline fermentation of composted leaves of Polygonum tinctorium L. (sukumo). However, the environmental effects on the microbiota during this treatment, as well as the mechanisms underlying the microbial succession toward stable state remain unknown. In this study, physicochemical analyses and Illumina metagenomic sequencing was used to determine the impact pretreatment conditions on the subsequent initiation of bacterial community transition and their convergence, dyeing capacity and the environmental factors critical for indigo reducing state during aging of sukumo. The initial pretreatment conditions analyzed included 60°C tap water (heat treatment: batch 1), 25°C tap water (control; batch 2), 25°C wood ash extract (high pH; batch 3) and hot wood ash extract (heat and high pH; batch 4), coupled with successive addition of wheat bran from days 5 to 194. High pH had larger impact than heat treatment on the microbiota, producing more rapid transitional changes from days 1 to 2. Although the initial bacterial community composition and dyeing intensity differed during days 2-5, the microbiota appropriately converged to facilitate indigo reduction from day 7 in all the batches, with Alkaliphilus oremalandii, Amphibacillus, Alkalicella caledoniensis, Atopostipes suicloalis and Tissierellaceae core taxa contributing to the improvement of when the dyeing intensity. This convergence is attributed to the continuous maintenance of high pH (day 1 ~) and low redox potential (day 2~), along with the introduction of wheat bran at day 5 (day 5~). PICRUSt2 predictive function profiling revealed the enrichment of phosphotransferease system (PTS) and starch and sucrose metabolism subpathways key toward indigo reduction. Seven NAD(P)-dependent oxidoreductases KEGG orthologs correlating to the dyeing intensity was also identified, with Alkalihalobacillus macyae, Alkalicella caledoniensis, and Atopostipes suicloalis contributing significantly toward the initiation of indigo reduction in batch 3. During the ripening period, the staining intensity was maintained by continuous addition of wheat bran and the successive emergence of indigo-reducing bacteria that also contributed to material circulation in the system. The above results provide insight into the interaction of microbial system and environmental factors in sukumo fermentation.

Quantification of leuco-indigo in indigo-dye-fermenting suspension by normal pulse voltammetry

Quantification of leuco-indigo is most important for Aizome, Japanese indigo-dyeing; however, there has been no convenient quantitative method. This study demonstrated that normal pulse voltammetry under quiescent conditions can be used to detect leuco-indigo. As a result of quantification of leuco-indigo in the depth direction in fermenting suspensions, the steady-state concentrations of leuco-indigo showed sigmoidal profiles in the depth direction. The steady state is caused by competitive reactions of microbial reduction of indigo and autoxidation of leuco-indigo by O₂ dissolved from the air interface of the suspension. In addition, we investigated the effects of stirring the suspension and adding some nutrients to the concentration profile. The weakened activity was partially recovered by the addition of ethanol and remarkably recovered by the addition of hipolypepton or glucose. Knowledge is essential for the proper management of indigo-dye-fermenting suspensions.

Isolation and characterization of indigo-reducing bacteria and analysis of microbiota from indigo fermentation suspensions

In natural indigo dyeing, the water-insoluble indigo included in the composted indigo leaves called sukumo is converted to water-soluble leuco-indigo through the reduction activities of microorganisms under alkaline conditions. To understand the relationship between indigo reduction and microorganisms in indigo-fermentation suspensions, we isolated and identified the microorganisms that reduce indigo and analyzed the microbiota in indigo-fermentation suspensions. Indigo-reducing microorganisms, which were not isolated by means of a conventional indigo carmine-reduction assay method, were isolated by using indigo as a direct substrate and further identified and characterized. We succeeded in isolating bacteria closely related to Corynebacterium glutamicum, Chryseomicrobium aureum, and Enterococcus sp. for the first time. Anthraquinone was found to be an effective mediator that facilitated the indigo-reduction activity of the isolated strains. On analysis of the microbiota in indigo-fermentation suspensions, the ratio of indigo-reducing bacteria and others was found to be important for maintaining the indigo-reduction activity.

Indigofera tinctoria leaf powder as a promising additive to improve indigo fermentation prepared with sukumo (composted Polygonum tinctorium leaves)

Being insoluble in the oxidize form, indigo dye must be solubilized by reduction for it to penetrate textile. One of the procedures is the reduction by natural bacterial fermentation. Sukumo, composted leaves of Polygonum tinctorium, is a natural source of indigo in Japan. Although sukumo has an intrinsic bacterial seed, the onset of indigo reduction with this material may vary greatly. Certain additives improve indigo fermentation. Here, we studied the effects of Indigofera tinctoria leaf powder (LP) on the initiation of indigo reduction, bacterial community, redox potential (ORP), and dyeing intensity in the initial stages and in aged fermentation fluids prepared with sukumo. I. tinctoria LP markedly decreased ORP at day 1 and stabilised it during early fermentation. These effects could be explained by the phytochemicals present in I. tinctoria LP that act as oxygen scavengers and electron mediators. Using next generation sequencing results, we observed differences in the bacterial community in sukumo fermentation treated with I. tinctoria LP, which was not influenced by the bacterial community in I. tinctoria LP per se. The concomitant decrease in Bacillaceae and increase in Proteinivoraceae at the onset of fermentation, increase in the ratio of facultative to obligate anaerobes (F/O ratio), or the total abundance of facultative anaerobes (F) or obligate anaerobes (O) (designated F + O) are vital for the initiation and maintenance of indigo reduction. Hence, I. tinctoria LP improved early indigo reduction by decreasing the ORP and hasten the appropriate transitions in the bacterial community in sukumo fermentation.

Indigo-Mediated Semi-Microbial Biofuel Cell Using an Indigo-Dye Fermenting Suspension

Aizome (Japanese indigo dyeing) is a unique dyeing method using microbial activity under anaerobic alkaline conditions. In indigo-dye fermenting suspensions; microorganisms reduce indigo into leuco-indigo with acetaldehyde as a reductant. In this study; we constructed a semi-microbial biofuel cell using an indigo-dye fermenting suspension. Carbon fiber and Pt mesh were used as the anode and cathode materials, respectively. The open-circuit voltage (OCV) was 0.6 V, and the maximum output power was 32 µW cm−2 (320 mW m−2). In addition, the continuous stability was evaluated under given conditions starting with the highest power density; the power density rapidly decreased in 0.5 h due to the degradation of the anode. Conversely, at the OCV, the anode potential exhibited high stability for two days. However, the OCV decreased by approximately 80 mV after 2 d compared with the initial value, which was attributed to the performance degradation of the gas-diffusion-cathode system caused by the evaporation of the dispersion solution. This is the first study to construct a semi-microbial biofuel cell using an indigo-dye fermenting suspension.

The Mechanism Underlying of Long-Term Stable Indigo Reduction State in Indigo Fermentation Using Sukumo (Composted Polygonum tinctorium Leaves)

Indigo fermentation fluid maintains its indigo-reducing state for more than 6 months under open-air. To elucidate the mechanism underlying the sustainability of this indigo reduction state, three indigo fermentation batches with different durations for the indigo reduction state were compared. The three examined batches exhibited different microbiota and consisted of two phases. In the initial phase, oxygen-metabolizing-bacteria derived from sukumo established an initial network. With decreasing redox potential (ORP), the initial bacterial community was replaced by obligate anaerobes (mainly Proteinivoraceae; phase 1). Approximately 1 month after the beginning of fermentation, the predominating obligate anaerobes were decreased, and Amphibacillus and Polygonibacillus, which can decompose macromolecules derived from wheat bran, were predominantly observed, and the transition of microbiota became slow (phase 2). Considering the substrate utilization ability of the dominated bacterial taxa, the transitional change from phase 1 to phase 2 suggests that this changed from the bacterial flora that utilizes substrates derived from sukumo, including intrinsic substrates in sukumo and weakened or dead bacterial cells derived from early events (heat and alkaline treatment and reduction of ORP) to that of wheat bran-utilizers. This succession was directly related to the change in the major substrate sustaining the corresponding community and the turning point was approximately 1 month after the start of fermentation. As a result, we understand that the role of sukumo includes changes in the microbial flora immediately after the start of fermentation, which has an important function in the start-up phase of fermentation, whereas the ecosystem comprised of the microbiota utilizing wheat bran underpins the subsequent long-term indigo reduction.

Voltammetric in-situ monitoring of leuco-indigo in indigo-fermenting suspensions

Cyclic voltammetry was successfully applied to in-vivo monitoring of leuco-indigo in indigo-fermenting suspensions under quiescent conditions without deoxygenation; the working and counter electrodes were kept on the surface of each suspension by a polyethylene vinyl alcohol tube holder. The anodic peak current was used as a measure of the leuco-indigo concentration. The voltammetric wave shape suggested partial solubilization of the indigo with some macromolecules in the fermenting suspensions, which lead to an in-situ method without any electrode surface pretreatment. The anodic peak current well reflected the dyeing activity of a suspensions. The results obtained for laboratory-level fermentation systems clarified the number of days required for dye fermentation, the effectiveness of addition of old suspension as an additive for preparing fresh fermenting suspensions, and the importance of addition of a nitrogen-based nutrient as well as a glucose-based one to recover the indigo-reducing activity. The method can also be applied to determine the amounts of indigo in used dye suspensions and extracts of fermented indigo leaves (sukumo) by adding a chemical reduction pretreatment.