Nanoscale Zero Valent Metals (NZVMs)

Nanoscale zero valent metals (NZVMs), i.e. Fe, Ni, Cu, Co, Ag, Au, etc., with significantly strong chemical reducibility, high efficiency, and large specific surface have the most promising applications in environmental processes compared to zero valent metals (ZVMs) . In fact, the percentage of atoms located at the surface of particle increases with the decrease in particle size, which can raise its tendency to adsorb, interact, and react with other atoms, molecules, and complexes As mentioned before, AD is carried out by different groups of microorganisms. The flow of electrons within these communites governs which reactions occur and their rates . In methanogenisis step, interspecies electron transfer between syntrophic bacteria and methanogenic archaea plays a vital role in improving AD efficiency . Mediated interspecies electron transfer (MIET) based on exchange of metabolites (i.e. H2, formate) between two microorganisms is one of the earliest discovered mechanisms of external electron transfer .

In MIET, the electron-donating microorganism reduces protons to Hydrogen and the electron-accepting microorganism oxidizes the Hydrogen with the reduction of an electron acceptor [46]. MIET is feasible only at low enough concentrations of H2/formate metabolites due to the thermodynamic constraints [47]. Intrinsically, MIET via diffusive cariers (i.e. H2 and formate) is a slow mechanism due to slow diffusion rate of metabolites in media, therefore methanogenisis from organic matters is known to be a rate limiting step in many AD processes [48]. Recently, researchers showed that direct interspecies electron transfer (DIET) via electrically conductive materials, i.e. nano zero valent iron (NZVI) and carbonous materials, energetically have more advantageous than MIET, because DIET does not require the multiple enzymatic steps to produce hydrogen as an electron carrier. Addition of electrically conductive materials, to anaerobic digesters is an engineered approach to stimulate DIET and subsequently promote biogas production.

NZVI has been widely investigated as a nanoscale zero valent metallic additive in the field of biogas production. Suanon et al. showed that the application of 100 mg/L NZVI enhanced the methane production from sewage sludge up to 25.2% . The results were in accordance with previous findings that application of NZVI in anaerobic digestion bioreactor increases methane production. Su et al. reported the promotion of biogas and methane production up to 30.4% and 40.4%, respectively, after addition of 0.1 wt% NZVI (10 mg/L) in AD process of sewage sludge during 17 days of digestion. The effects of NZVI NPs on the methanogenic bacteria during AD of raw manure (feces and urine) were also investigated by Abdelsalam et al.. They declared addition of 20 mg/L of NZVI NPs to biodigester under mesophilic temperature stimulate the methanogenic bacteria and increase its activity and subsequently increase biogas and methane production up to 45 and 59%, respectively, in comparison with control.

On the contrary, Yang et al. showed NZVI NPs with minimum concentration of 1 mM (56 mg/L) inhibited methanogenesis process by more than 20% during AD of digested sludge. They declared that the inhibition was due to the disruption of cell integrity in exposure of NZVI NPs. In addition, rapid H2 production was observed in the presence of NZVI NPs under anaerobic conditions (Fe + 2H2O → Fe2+ + H2 + 2OH−) as reported by Liu et al. [55]. The produced H2 take part in bacterially controlled hydrogenotrophic processes (i.e., 2CO2 + 4H2→ CH3COOH + 2H2O) in AD process. Addition of 30 mM (1680 mg/L) of NZVI to biodigester during AD process caused a significant increase of soluble COD (due to microorganism cell disruption) and volatile fatty acids in the bioreactor medium, resulting in a 69% decrease of methane production.

Abelsalam et al. also declared that introducing 2 mg/L Ni, 20 mg/L NZVI or 1 mg/L Co NPs to AD process of raw manure increase the biogas yield by 1.7, 1.8, and 1.5 times in comparison with control, respectively. The methane yield significantly increased by 2, 2.17, and 1.67 times more than the methane volume of the control, respectively. Ni, Co, and NZVI NPs applied in this study improved various biological processes involved in biogas and methane production. This observation is in accordance with the results obtained by Gustavsson et al. who stated that Ni and Co concentrations in digester substrates, organic matters, promote biogas production by increasing methane production and maintaining the stability of AD process.

Wang et al. investigated the effects of zero valent metal NPs (NZVI and Ag) on methane production during the AD of waste activated sludge at 35 °C. the results showed that 10 mg/g total suspended solid (TSS) NZVI increased methane production up to 120% of control, while 500 mg/g TSS Ag NPs produced lower amounts of methane (26.48% lower than that of the control). These results indicated that low concentrations of NZVI NPs promoted the number of microorganisms and activities of key enzymes but that higher concentrations of Ag NPs inhibited them. Quantification of microorganisms showed that 10 mg/g TSS NZVI produced more active bacteria, archaea, α-proteobacteria, β-Proteobacteria, bacteroidetes and methanosaeta. α-proteobacteria, β-proteobacteria, and bacteroidetes are bacteria, which may affect hydrolysis and acidogenesis. The genus methanosaeta belongs to the Archaea domain, members of which play a key role in methanogenesis. With 500 mg/g TSS Ag NPs, there were lower levels of active α -proteobacteria, β-proteobacteria, and bacteroidetes.

Abdelsalam et al. studied the effect of Ni and Co NPs (0.5, 1, and 2 mg/L) on AD process of manure slurry at mesophilic temperature. They showed that generally, using these NPs in the process led to reduction of lag phase and the time to achieve the maximum biogas and methane production. The results revealed, Ni and Co NPs have a strong biostimulating effect on the methanogenic activity during the AD of manure and increased methane percentage. The cumulative amount of biogas and methane production was directly proportional to the concentration of Ni NPs up to 2 mg/L. the results confirmed that the addition of 0.5, 1 and 2 mg/L Ni NPs and 1 mg/L NiCl2 to the substrate increased the biogas volume by 46, 72, 74 and 44% more than the biogas volume produced by the control.

These results agree with Demirel and Scherer who stated that the stimulatory concentrations of Ni for batch cultures of methanogens were in range of 0.012 to 5 mg/L. Introducing 1 mg/L of Co NPs increased the biogas production up to 54170 mL which is higher than the volume of biogas produced by the control, CoCl2 and 0.5 mg/L Co NPs, which produced 33006.7, 43823.3 and 45053.3 mL of biogas, respectively. Enhancement of biogas and methane production by using 1 mg/L Co NPs was in acordance with the results obtained by Banks et al. and Feng et al. who reported that Co is an important metal for methanogenesis. On the other hand, the addition of Co NPs with concentration more than 1 mg/L, i.e. 2 mg/L, showed a toxicity effect on the methanogenic bacteria which decreased 5% of biogas production and 12.7% methane, compared to the control assays, during 50 days of AD process. The undesired effect of 2 mg/L Co NPs on biogas and methane production are in agreement with those reported by Facchin et adeclaring that the supplementation of excess metals could have a negative effect on acetoclastic methanogenesis or on the other metabolic pathways of an AD process.

García et al. investigated the effects of Ag and Au NPs on activity of key microorganisms such as ordinary heterotrophic organisms (OHO), ammonia oxidizing bacteria (AOB), and thermophilic and mesophilic anaerobic bacteria that present in wastewater treatment plants. Based on their results, Ag NPs had an intermediate toxicity effect and inhibit 33-50% biogas production, whereas Au NPs showed zero or slight inhibition for all kind of tested substrates.

Gitipour et al. stabilized Ag NPs with different capping agents and used them in AD process of biosolid from wastewater treatment plant with concentration of 0.5, 1, 5, and 100 mg/L. Based on their results, negatively charged citrate-coated NPs (citrate-Ag NPs) and minimally charged polyvinylpyrrolidone coated Ag NPs (PVP-Ag NPs) showed no toxicity and inhibitory effects on anaerobic biogas production at any concentration. In the case of citrate-Ag NPs, the lack of toxicity might be caused by aggregation and subsequent settling and as a result, the NPs were less possibility to interact with the microorganisms. The minimum surface charge on PVP-Ag NPs may have resulted in lower attraction between the NPs and the negatively charged bacterial membrane thus exhibiting a low toxicity. Relatively high concentration (100 mg/L) of positively charged branched polyethyleneimine coated Ag NPs (BPEI-Ag NPs) demonstrate elevated toxicity, similar in magnitude to that of Ag+ ion, in comparison to PVP-Ag NPs and the anionic citrate-Ag NPs. Unsar et al. also investigated long and short term inhibition impacts of 20-40 nm Ag NPs on AD of waste activated sludge (WAS). They observed that Ag did not cause extreme impacts on AD process. In the long term test, introducing Ag NPs at highest dosage (1000 mg/gTS) caused 12.1% decrease in the methane production.

Inhibitory effects of Ag, Cu and Fe NPs on acetoclastic and hydrogenotrophic methanogenic activity of anaerobic granular sludge were investigated by Gonzalez-Estrella et al. Their results revealed that 1500 mg/L of Cu NPs (40-60 nm) were highly inhibitory to activity of acetoclastic and hydrogenotrophic methanogens due to the release of toxic Cu2+ ions caused by corrosion and dissolution of the NPs. In contrary, methanogens were not inhibited when exposed to high concentrations (1500 mg/L) of Ag and Fe NPs. This is in contrast with other researches that reported Ag NPs had toxic effects on methanogenesis and biogas production. Although Ag NPs are toxic to many microorganisms, relatively high concentrations of Ag NPs (40-43 mg /L) did not cause methanogenic inhibitio. Oxysulfidation and sulfidation of Ag NPs in anaerobic media caused precipitation of Ag NPs in form of AgS, a non-toxic form of Ag [65]. Therefore, the sulfide concentration in the various studies should be expected to have a considerable effect on the inhibitory potential of the Ag NPs.