Stimulation and Inhibition of Anaerobic Digestion by Nickel and Cobalt: A Rapid Assessment Using the Resazurin Reduction Assay

Synergetic anaerobic digestion of food waste for enhanced production of biogas and value-added products: strategies, challenges, and techno-economic analysis

The generation of food waste (FW) is increasing at an alarming rate, contributing to a total of 32% of all the waste produced globally. Anaerobic digestion (AD) is an effective method for dealing with organic wastes of various compositions, like FW. Waste valorization into value-added products has increased due to the conversion of FW into biogas using AD technology. A variety of pathways are adopted by microbes to avoid unfavorable conditions in AD, including competition between sulfate-reducing bacteria and methane (CH4)-forming bacteria. Anaerobic bacteria decompose organic matter to produce biogas, a digester gas. The composition depends on the type of raw material and the method by which the digestion process is conducted. Studies have shown that the biogas produced by AD contains 65-75% CH4 and 35-45% carbon dioxide (CO2). Methanothrix soehngenii and Methanosaeta concilii are examples of species that convert acetate to CH4 and CO2. Methanobacterium bryantii, Methanobacterium thermoautotrophicum, and Methanobrevibacter arboriphilus are examples of species that produce CH4 from hydrogen and CO2. Methanobacterium formicicum, Methanobrevibacter smithii, and Methanococcus voltae are examples of species that consume formate, hydrogen, and CO2 and produce CH4. The popularity of AD has increased for the development of biorefinery because it is seen as a more environmentally acceptable alternative in comparison to physico-chemical techniques for resource and energy recovery. The review examines the possibility of using accessible FW to produce important value-added products such as organic acids (acetate/butyrate), biopolymers, and other essential value-added products.

Abiotic and biotic roles of metals in the anaerobic digestion of sewage sludge: A review

Anaerobic digestion (AD) is a promising technique for sludge treatment and resource recovery. Metals are very important components of sludge and can have substantial effects on its complex nature and microbial activity. However, systematic reviews have not addressed how metals in sludge affect AD and how they can be regulated to improve AD. This paper comprehensively reviews the effects of metals on the AD of sludge from both abiotic and biotic perspectives. First, we introduce the contents and basic characteristics (e.g., chemical forms) of intrinsic metals in sewage sludge. Then, we summarise the main mechanism by which metals influence sludge properties and the methods for removing metals and thus improving AD. Next, we analyze the effects of both intrinsic and exogenous metals on the enzymes and microbial communities involved in anaerobic bioconversion, focusing on the types, critical concentrations and valence states of the metals. Finally, we propose ideas for future research on the roles of metals in the AD of sludge. In summary, this review systematically clarifies the roles of metals in the AD of sludge and provides a reference for improving AD by regulating these metals.

Trace elements' deficiency in energy production through methanogenesis process: Focus on the characteristics of organic solid wastes

Excessive or insufficient supplementation of trace elements (TEs) limits the progression of anaerobic digestion. The main reason for this is the lack of sufficient understanding of digestion substrate characteristics, which significantly affects the demand for TEs. In this review, the relationship between TEs requirements and substrate characteristics is discussed. We mainly focus on three aspects. 1) The basis for TE optimization and existing problems: The optimization of TEs often based on the total solids (TS) or volatile solids (VS) of substrates, does not fully consider substrate characteristics. 2) TE deficiency mechanisms for different types of substrates: nitrogen-rich, sulfur-rich, TE-poor, and easily hydrolyzed substrates are the four main types of substrates. The mechanisms underlying TEs deficiency in the different substrates are investigated. 3) Regulation of TE bioavailability: characteristics of substrates affect digestion parameters, which disturb the bioavailability TE. Therefore, methods for regulating bioavailability of TEs are discussed.

A Dye-Assisted Paper-Based Assay to Rapidly Differentiate the Stress of Chlorophenols and Heavy Metals on Enterococcus faecalis and Escherichia coli

Biological toxicity testing plays an essential role in identifying the possible negative effects induced by substances such as organic pollutants or heavy metals. As an alternative to conventional methods of toxicity detection, paper-based analytical device (PAD) offers advantages in terms of convenience, quick results, environmental friendliness, and cost-effectiveness. However, detecting the toxicity of both organic pollutants and heavy metals is challenging for a PAD. Here, we show the evaluation of biotoxicity testing for chlorophenols (pentachlorophenol, 2,4-dichlorophenol, and 4-chlorophenol) and heavy metals (Cu²⁺, Zn²⁺, and Pb²⁺) by a resazurin-integrated PAD. The results were achieved by observing the colourimetric response of bacteria (Enterococcus faecalis and Escherichia coli) to resazurin reduction on the PAD. The toxicity responses of E. faecalis-PAD and E. coli-PAD to chlorophenols and heavy metals can be read within 10 min and 40 min, respectively. Compared to the traditional growth inhibition experiments for toxicity measuring which takes at least 3 h, the resazurin-integrated PAD can recognize toxicity differences between studied chlorophenols and between studied heavy metals within 40 min.

Utilization of nanoparticles for biogas production focusing on process stability and effluent quality

Abstract

One of the most important techniques for converting complex organic waste into renewable energy in the form of biogas and effluent is anaerobic digestion. Several issues have been raised related to the effectiveness of the anaerobic digestion process in recent years. Hence nanoparticles (NPs) have been used widely in anaerobic digestion process for converting organic wastes into useful biogas and effluent in an effective way. This review addresses the knowledge gaps and summarizes recent researchers’ findings concentrating on the stability and effluent quality of the cattle manure anaerobic digestion process using single and combinations nanoparticle. In summary, the utilization of NPs have beneficial effects on CH4 production, process optimization, and effluent quality. Their function, as key nutrient providers, aid in the synthesis of key enzymes and co-enzymes, and thus stimulate anaerobic microorganism activities when present at an optimum concentration (e.g., Fe NPs 100 mg/L; Ni NPs 2 mg/L; Co NPs 1 mg/L). Furthermore, utilizing Fe NPs at concentrations higher than 100 mg/L is more effective at reducing H2S production than increasing CH4, whereas Ni NPs and Co NPs at concentrations greater than 2 mg/L and 1 mg/L, respectively, reduce CH4 production. Effluent with Fe and Ni NPs showed stronger fertilizer values more than Co NPs. Fe/Ni/Co NP combinations are more efficient in enhancing CH4 production than single NPs. Therefore, it is possible to utilize NPs combinations as additives to improve the effectiveness of anaerobic digestion.

Article highlights

Biomethane production from waste activated sludge promoted by sludge incineration bottom ash: The distinctive role of metal cations and inert fractions

Sludge incineration bottom ash (SA), a solid waste generated by incineration of waste activated sludge (WAS), has been demonstrated as an inexpensive additive to increase biomethane production from anaerobic digestion (AD) of WAS. However, how SA improved methanogenic performance of a WAS digester remains elusive. Here, we addressed this question by fractionating the SA into accessible leachate (SA-L) and inert residue (SA-R) and investigating their individual effects. The cumulative biomethane production was increased by 6.7%, 20.2% and 39.6% with addition of SA-L, SA-R and SA, respectively. Mechanistic study showed that release of organic-binding metals (Ca and Fe) from SA dissolution suppressed volatile fatty acids production by increasing the apparent activation energy (AAE) and decreasing the surface binding sites for hydrolytic/acidogenic enzymes during WAS hydrolysis-acidogenesis, while trace elements in SA-L promoted metabolism of methanogens (Methanothermobacter and Methanosarcina). In contrast, the gypsum/silicate-cored SA-R facilitated hydrolysis-acidogenesis with reduced AAE but drastically inhibited methanogenesis due to competition of sulfate-reducing bacteria Thermodesulfovibrio. The comparative analysis of KEGG-based functional genes indicated that the enhanced methane metabolism and reductive CO₂ fixation pathways with SA addition could result from the release of trace elements to support key enzyme activities.

The absolute concentration and bioavailability of trace elements: Two vital parameters affecting anaerobic digestion performance of chicken manure leachate

It is currently unclear whether trace elements (TEs) deficiency is due to low bioavailability or low absolute TEs concentrations, especially in high-pH anaerobic digestion (AD) systems. A mixed solution of TEs and EDTA-Na₂ were used separately in mono-AD of chicken manure (CM) leachate to investigate this research gap. The results showed relatively low bioavailability of Fe, Mn, and Zn. The bioavailability of all TEs remained stable along with a gradual increase in total ammonia nitrogen concentration. Both TE and EDTA-Na₂ supplementation improved the bioavailability of TEs, but TEs supplementation also gave a high proportion of soluble TEs. Adding TEs improved methane production efficiency (+38.3%) and decreased the H₂S content. The exchangeable fraction of specific TE (Mo) in H₂/CO₂ pathway was higher in the TEs treatment. TEs bioavailability and absolute concentrations of available TEs are critical aspects that need to be scrutinized to assess the risk of TEs deficiency.

Recent developments in monitoring technology for anaerobic digesters: A focus on bio-electrochemical systems

With the increasing popularity of waste to energy conversion, demand for large-scale operation of anaerobic digestors has emerged in the market. However, the process instabilities in anaerobic digestors limit the expansion of facilities to high loading rates. The irregularities in the process can be addressed directly by altering the feedstock characteristics provided an on-hand, robust, and sensitive monitoring device is available. In this context, the bioelectrochemical system has emerged as an excellent tool for monitoring and optimizing the anaerobic process within the reactor. This article reviews the gradual evolution in techniques and approaches for monitoring of anaerobic digestion (AD) process. An analysis of the recently popular biosensing techniques has been done with a focus on the bioelectrochemical monitoring system and its operation mode. A brief attempt to highlight the current challenges in the field of bioelectrochemical process monitoring for AD has also been made, which can be supportive of future research.

A simple paper-based colorimetric analytical device for rapid detection of Enterococcus faecalis under the stress of chlorophenols

Bacteria detection and toxicity measurement are essential in many aspects. Becoming increasingly popular in recent years, paper-based analytical devices (PADs) have proven to be cost-effective, portable and eco-friendly with quantitative diagnostic results. In this work, by a straightforward soaking-drying method, a resazurin-deposited PAD has been developed for rapid bacteria detection and biotoxicity measurement. The colorimetric response on the PAD was generated from metabolic reduction of resazurin by Enterococcus faecalis, a facultative anaerobic bacterial strain. After recording and quantifying the colorimetric response with Hue value by a smartphone, the bioassay on PAD enables the detection of resazurin reduction kinetics difference among bacteria at various densities in 10 min. Thereby, the bioassay on PAD was applied to study the toxicity of two chlorophenols, i.e. pentachlorophenol (PCP) and 4-chlorophenol (4-CP), to E. faecalis. Compared to growth-based inhibition test, which takes 5 h, this assay shows higher efficiency, i.e. in 30 min, the biotoxicity difference between PCP and 4-CP can be identified.

The effects of solid lignin on the anaerobic digestion of microcrystalline cellulose and application of smoothing splines for extended data analysis of its inhibitory effects

Lignocellulose is an abundant substrate for biogas production; however, for efficient utilization, proper pre-treatment is required to enhance the biomethane yield and hydrolysis rate significantly. Phenolic compounds from dissolved lignin, produced during alkali pre-treatment, have inhibitory effects on the anaerobic digestion; however, the possible inhibitory effects of solid lignin have not gathered enough interest. Especially, the effect of solid lignin on methanogenesis remains a knowledge gap. In this study, kraft lignin was used as a model solid lignin substrate for its co-digestion with microcrystalline cellulose. A new approach of modelling biomethane production curves using smoothing splines was developed to describe the long-term inhibitory effects of solid lignin on hydrolysis and methanogenesis. The method gives possibility to describe long-term inhibitory effects by using batch instead of continuous test data. Results revealed that kraft lignin showed mild inhibitory effects on methanogens. However lignin impact combined with volatile fatty accumulation can prolong hydrolysis and reactor recovery start-up by 47.3% and 75.3%, respectively. For small dosages of solid lignin adaptation of methanogens is possible.

Effects of foam nickel supplementation on anaerobic digestion: Direct interspecies electron transfer

Stimulating direct interspecies electron transfer with conductive materials is a promising strategy to overcome the limitation of electron transfer efficiency in syntrophic methanogenesis of industrial wastewater. This paper assessed the impact of conductive foam nickel (FN) supplementation on syntrophic methanogenesis and found that addition of 2.45 g/L FN in anaerobic digestion increased the maximum methane production rate by 27.4 % (on day 3) while decreasing the peak production time by 33 % as compared to the control with no FN. Cumulative methane production from day 2 to 6 was 14.5 % higher with addition of 2.45 g/L FN than in the control. Levels of FN in excess of 2.45 g/L did not show benefits. Cyclic voltammetry results indicated that the biofilm formed on the FN could generate electrons. The dominant bacterial genera in suspended sludge were Dechlorobacter and Rikenellaceae DMER64, whereas that in the FN biofilm was Clostridium sensu stricto 11. The dominant archaea Methanosaeta in the FN biofilm was enriched by 14.1 % as compared to the control.

Enhanced anaerobic digestion of primary sludge with additives: Performance and mechanisms

Anaerobic digestion of primary sludge with different additives, namely nano magnetite, graphite powder, activated carbon powder and NiCl₂/CoCl_2, were evaluated by biomethane potential tests, kinetics modelling and microbial community analysis. Specific methane yields increased from 136 mL/g VS for primary sludge to 146 mL/g VS, 151 mL/g VS, and 152 mL/g VS for the addition of nano magnetite, graphite powder, and activated carbon powder at optimal dosages, respectively. The first order hydrolysis constant k_h increased from 0.488 d^-1 to 0.526 d^-1, 0.622 d^-1, and 0.724 d^-1, respectively. Microbial community analysis revealed that the abundance of key bacterial and archaeal populations was positively correlated with hydrolysis and methane production. The enhanced methane production with activated carbon powder was due to shifting methane formation pathway from acetoclastic to hydrogenotrophic methanogenesis. In contrast, nano magnetite and graphite powder additives enhanced the direct interspecies electron transfer evidenced by increased abundance of Methanosaeta and Methanolinea.

Biodegradation of hexabromocyclododecane by Rhodopseudomonas palustris YSC3 strain: A free-living nitrogen-fixing bacterium isolated in Taiwan

The persistent organic pollutant, brominated flame retardant, hexabromocyclododecane (HBCD), identified as an emerging contaminant has been detected in various environmental matrix. The increased level of this toxic organic compound in the environment has been associated with serious human health risks. The results obtained from this study revealed that various Rhodopseudomonas palustris strains isolated from paddy soil in Taiwan possessed good HBCD biodegradation capability when they were cultured aerobically. Among these strains, YSC3 was considered as one of the most potential isolates for HBCD degradation. The optimum HBCD biodegradation occurred at neutral pH and at 35 °C in all our pH and temperature tests at an initial HBCD concentration of 1 ppm. HBCD degradation kinetics generally decreased with the increase of initial HBCD concentration. The study also suggested that the cultivation temperature played a vital role on YSC3 for its initiation of cellular HBCD degradation. The relative-molar ratio of the released bromide ions during the biodegradation of HBCD was observed in the range between 1 and 3.5, suggesting that the debromination reactions occurred. Concomitant with the loss of HBCD, there was a concurrent production of two metabolites, pentabromocyclododecanol and pentabromocyclododecene, which were determined by liquid chromatography and mass spectrometry techniques. On the basis of the obtained results, the possible biodegradation pathways were also proposed in this study.

Effect of nickel, cobalt, and iron on methanogenesis from methanol and cometabolic conversion of 1,2-dichloroethene by Methanosarcina barkeri

Methanogens are responsible for the last step in anaerobic digestion (AD), in which methane (a biofuel) is produced. Some methanogens can cometabolize chlorinated pollutants, contributing for their removal during AD. Methanogenic cofactors involved in cometabolic reductive dechlorination, such as F₄₃₀ and cobalamin, contain metal ions (nickel, cobalt, iron) in their structure. We hypothesized that the supplementation of trace metals could improve methane production and the cometabolic dechlorination of 1,2‐dichloroethene (DCE) by pure cultures of Methanosarcina barkeri. Nickel, cobalt, and iron were added to cultures of M. barkeri growing on methanol and methanol plus DCE. Metal amendment improved DCE dechlorination to vinyl chloride (VC): assays with 20 µM of Fe³⁺ showed the highest final concentration of VC (5× higher than in controls without Fe³⁺), but also in assays with 5.5 µM of Co²⁺ and 5 µM of Ni²⁺ VC formation was improved (3.5–4× higher than in controls without the respective metals). Dosing of metals could be useful to improve anaerobic removal of chlorinated compounds, and more importantly decrease the detrimental effect of DCE on methane production in anaerobic digesters.

Effects of adding EDTA and Fe2+ on the performance of reactor and microbial community structure in two simulated phases of anaerobic digestion

The uptake of trace elements can be impeded by precipitation in the presence of carbonates and sulfates. The objective of this study was to investigate whether ethylenediaminetetraacetic acid (EDTA) enhances the performance of anaerobic digestion by forming dissolved complexes with Fe²⁺. Batch experiments were performed in this study and acidogenic and methanogenic phases were artificially simulated. EDTA was added to both of phases to examine its effects on Fe bioavailability, metabolic parameters and microbial community structure. The results showed that EDTA significantly accelerated the digestion process in both phases because its addition changed the Fe sorption law and increased Fe-bioavailability. The microbial community structure changed following by the change of Fe-fractions which was determined by EDTA. This study demonstrated that EDTA as ligand could increase the Fe-bioavailability and then reduced or replaced the addition of Fe.

A mediator-free whole-cell electrochemical biosensing system for sensitive assessment of heavy metal toxicity in water

A bioelectrochemical sensing system (BES) based on electroactive bacteria (EAB) has been used as a new and promising tool for water toxicity assessment. However, most EAB can reduce heavy metals, which usually results in low toxicity response. Herein, a starvation pre-incubation strategy was developed which successfully avoided the metal reduction during the toxicity sensing period. By integrating this starvation pre-incubation procedure with the amperometric BES, a sensitive, robust and mediator-free biosensing method for heavy metal toxicity assessment was developed. Under the optimized conditions, the IC50 (half maximal inhibitory concentration) values for Cu²⁺, Ni²⁺, Cd²⁺, and Cr⁶⁺ obtained were 0.35, 3.49, 6.52, 2.48 mg L^-1, respectively. The measurement with real water samples also suggested this method was reliable for practical application. This work demonstrates that it is feasible to use EAB for heavy metal toxicity assessment and provides a new tool for water toxicity warning.

Nickel spiking to improve the methane yield of sewage sludge

The presence of micro-nutrients can be stimulatory for the anaerobic digestion (AD) of hardly degradable wastes and thus, improve process performance. Among the essential trace elements, nickel is involved in multiple important enzymes necessary for efficient AD. The present study investigates the effect of nickel spiked sewage sludge on batch and continuous mode operation. Metal spiking was conducted in the form of nanoparticles (Ni-NPs) and salt (NiCl₂·6H₂O). Results from batch assays showed that 5 mgNi-Salt/kgVS in the presence of Nitrilotriacetic acid (NTA) enhanced the methane yield by ∼10% compared to the untreated sample. The impact of Ni-NPs in the AD process was also positive, but slightly lower compared to the effect of NiCl₂·6H₂O. The stimulatory impact of Ni was also revealed in continuously fed digester boosting the methane yield by ∼8%. Overall, the improved methane production indicated that methanogenic archaea were favoured by the simultaneous supplementation of Ni and NTA.

The effect of Fe2NiO4 and Fe4NiO4Zn magnetic nanoparticles on anaerobic digestion activity

Two types of magnetic nanoparticles (MNPs), i.e. Ni ferrite nanoparticles (Fe₂NiO₄) and Ni Zn ferrite nanoparticles (Fe₄NiO₄Zn) containing the trace metals Ni and Fe, were added to the anaerobic digestion of synthetic municipal wastewater at concentrations between 1 and 100 mg Ni L^-1 in order to compare their effects on biogas (methane) production and sludge activity. Using the production of methane over time as a measure, the assays revealed that anaerobic digestion was stimulated by the addition of 100 mg Ni L^-1 in Fe₂NiO₄ NPs, while it was inhibited by the addition of 1-100 mg Ni L^-1 in Fe₄NiO₄Zn NPs. Especially at 100 mg Ni L^-1, Fe₄NiO₄Zn NPs resulted in a total inhibition of anaerobic digestion. The metabolic activity of the anaerobic sludge was tested using the resazurin reduction assay, and the assay clearly revealed the negative effect of Fe₄NiO₄Zn NPs and the positive effect of Fe₂NiO₄ NPs. Re-feeding fresh synthetic medium reactivated the NPs added to the anaerobic sludge, except for the experiment with 100 mg Ni L^-1 addition of Fe₄NiO₄Zn NPs. The findings in this present study indicate a possible new strategy for NPs design to enhance anaerobic digestion.

Effects of molybdenum, selenium and manganese supplementation on the performance of anaerobic digestion and the characteristics of bacterial community in acidogenic stage

The addition of trace elements to aid anaerobic digestion has already been widely studied. However, the effects of rare trace elements on anaerobic digestion remain unclear. In this study, the effects of Mo, Se and Mn on anaerobic digestion of rice straw were explored. The results showed the methane yield increased by 59.3%, 47.1% and 48.9% in the first 10 days following addition of Mo (0.01 mg/L), Se (0.1 mg/L) and Mn (1.0 mg/L), respectively. Toxic effects and the accumulation of volatile fatty acids (VFAs) were observed when the Se, Mo and Mn concentrations were greater than 100, 1000 and 1000 mg/L, respectively. The half-maximal inhibitory concentrations (IC₅₀) for Se, Mn and Mo were 79.9 mg/L, 773.9 mg/L and 792.3 mg/L, respectively. The addition of trace elements has changed the bacterial structure of the bacteria, which in turn has affected the digestion performance.

Optimization of Fe2+ supplement in anaerobic digestion accounting for the Fe-bioavailability

Fe is widely used as an additive in anaerobic digestion, but its bioavailability and the mechanism by which it enhances digestion are unclear. In this study, sequential extraction was used to measure Fe bioavailability, while biochemical parameters, kinetics model and Q-PCR (fluorescence quantitative PCR) were used to explore its mechanism of stimulation. The results showed that sequential extraction is a suitable method to assess the anaerobic system bioavailability of Fe, which is low and fluctuates to a limited extent (1.7 to -3.1wt%), indicating that it would be easy for Fe levels to be insufficient. Methane yield increased when the added Fe²⁺ was 10-500mg/L. Appropriate amounts of Fe²⁺ accelerated the decomposition of rice straw and facilitated methanogen metabolism, thereby improving reactor performance. The modified Gompertz model better fitted the results than the first-order kinetic model. Feasibility analysis showed that addition of Fe²⁺ at ≤50mg/L was suitable.

Risks of using EDTA as an agent for trace metals dosing in anaerobic digestion of olive mill solid waste

Low concentrations of trace elements in many organic wastes recommend their supplementation in order to avoid potential limitations. Different chelating agents have been used to ensure an adequate trace metal pool in the soluble fraction, by forming dissolved complexes. Ethylenediaminetetraacetic acid (EDTA) is probably the most common, although several negative effects could be associated with its usage. Biomethane potential tests were performed using Olive Mill Solid Waste as the substrate, supplementing different combinations of Fe, Co, Ni, Ba, always under the presence of EDTA. Results show that Ni and Co slightly recovered biodegradability. However, Ba supplementation resulted in worsening the methane yield coefficient in all cases. High concentration of EDTA led to decrease in the activity of anaerobic digestion. High availability of EDTA induces the capture of trace metals like Co or Ni, key trace metals for anaerobic biomass activity. While supplementing trace metals, the addition of Ba and/or EDTA must be carefully considered.

Effect of Nickel and Cobalt on Methanogenic Enrichment Cultures and Role of Biogenic Sulfide in Metal Toxicity Attenuation

Metals play an important role in microbial metabolism by acting as cofactors for many enzymes. Supplementation of biological processes with metals may result in improved performance, but high metal concentrations are often toxic to microorganisms. In this work, methanogenic enrichment cultures growing on H2/CO2 or acetate were supplemented with trace concentrations of nickel (Ni) and cobalt (Co), but no significant increase in methane production was observed in most of the tested conditions. However, high concentrations of these metals were detrimental to methanogenic activity of the cultures. Cumulative methane production (after 6 days of incubation) from H2/CO2 was 40% lower in the presence of 8 mM of Ni or 30 mM of Co, compared to controls without metal supplementation. When acetate was used as substrate, cumulative methane production was also reduced: by 18% with 8 mM of Ni and by 53% with 30 mM of Co (after 6 days of incubation). Metal precipitation with sulfide was further tested as a possible method to alleviate metal toxicity. Anaerobic sludge was incubated with Co (30 mM) and Ni (8 mM) in the presence of sulfate or sulfide. The addition of sulfide helped to mitigate the toxic effect of the metals. Methane production from H2/CO2 was negatively affected in the presence of sulfate, possibly due to competition of hydrogenotrophic methanogens by sulfate-reducing bacteria. However, in the enrichment cultures growing on acetate, biogenically produced sulfide had a positive effect and more methane was produced in these incubations than in similar assays without sulfate addition. The outcome of competition between methanogens and sulfate-reducing bacteria is a determinant factor for the success of using biogenic sulfide as detoxification method.

Poly(methyl methacrylate) Surface Modification for Surfactant-Free Real-Time Toxicity Assay on Droplet Microfluidic Platform

Microfluidic droplet reactors have many potential uses, from analytical to synthesis. Stable operation requires preferential wetting of the channel surface by the continuous phase which is often not fulfilled by materials commonly used for lab-on-chip devices. Here we show that a silica nanoparticle (SiNP) layer coated onto a Poly(methyl methacrylate) (PMMA) and other thermoplastics surface enhances its wetting properties by creating nanoroughness, and allows simple grafting of hydrocarbon chains through silane chemistry. Using the unusual stability of silica sols at their isoelectric point, a dense SiNP layer is adsorbed onto PMMA and renders the surface superhydrophilic. Subsequently, a self-assembled dodecyltrichlorosilane (DTS) monolayer yields a superhydrophobic surface that allows the repeatable generation of aqueous droplets in a hexadecane continuous phase without surfactant addition. A SiNP-DTS modified chip has been used to monitor bacterial viability with a resazurin assay. The whole process involving sequential reagents injection, and multiplexed droplet fluorescence intensity monitoring is carried out on chip. Metabolic inhibition of the anaerobe Enterococcus faecalis by 30 mg L^-1 of NiCl₂ was detected in 5 min.