Potential promotion of activated carbon supported nano zero-valent iron on anaerobic digestion of waste activated sludge

Influence of zero-valent iron nanoparticles on anaerobic digestion of swine manure: effects on methane yield

This study evaluated the effect of zero-valent iron nanoparticles (NZVI) on the anaerobic digestion of swine manure. A wide range of doses of NZVI was evaluated (5, 10, 15, 20, 25, 50, and 100 mgFe°/gVS). The maximum methane yield of 0.4506 L/gVSremoved was obtained with the concentration of 10 mgFe°/gVS representing an increase of 58.99% than the control system with 0.2834 L/gVSremoved, indicating that Fe° improves the methanogenic activity. However, when using doses greater than 20 mgFe°/gVS, there were decreases in the methane yield of 34.4-47.98%. Also, to observe the effect of NZVI in anaerobes was evaluated the activity in the electron transport system (ETS), where the control reactor showed an activity of 31.91 μg INTred/gVS•h, while in reactors with NZVI showed values of 39.48 μg INTred/gVS•h (10 Fe°mg/gVS), observing a stimulation of Fe° in microbial activity. However, the dose of 100 mgFe°/gVS showed the greatest decrease in methane yield (0.1474 L/gVSremoved) and a reduction in ETS was observed by 8.5% compared to the control. The effect on the composition of the volatile fatty acids was observed, where the control system obtained a maximum production of acetic acid of 639 mg/L, which was exceeded with the dose of 10 mg Fe°/gVS by 215% and a decrease of 41.15% with the inhibitory concentration of 100 mg Fe°/gVS. As a result, higher doses of NZVI affect the metabolic activity of anaerobes as well as the acetoclastic pathway causing a decrease in the methane production.

A novel approach to enhance methane production during anaerobic digestion of waste activated sludge by combined addition of trypsin, nano-zero-valent iron and activated carbon

A novel approach with a combination of trypsin, nano-zero-valent iron (NZVI) and activated carbon (AC) was conducted to promote the methane production of waste activated sludge (WAS) during the anaerobic digestion (AD) processes. Results showed that the combined addition of trypsin-NZVI-AC exhibited the synergistic effect during different AD stages. Trypsin mainly facilitated the hydrolysis process and the acetic acid conversion, while NZVI-AC enhanced the substrate metabolism and the electronic transfer to subsequently produce methane. A dose of 1000 mg/L trypsin was optimal to maximize this synergistic effect. Metagenomic analysis showed that trypsin-NZVI-AC addition effectively improved the relative abundance of acetyl-CoA carboxylase, and then strengthened both acetoclastic methanogenesis (M00357) and hydrogenotrophic methanogenesis (M00567). Hydrogenotrophic methanogens such as Methanobacterium, Methanoculleus, and Methanosarcina were greatly enriched with trypsin-NZVI-AC compared with trypsin or NZVI-AC addition. Moreover, electroactive bacteria G. sulfurreducens and G. metallireducens were also enriched by this method to conduct direct interspecies electron transfer among methanogens, leading to the better improvement of methane production. These findings supply a promising way to optimize the enzyme pretreatment technology and elevate the methanogenic efficiency of WAS.

Fabrication of the Ordered Mesoporous nZVI/Zr-Ce-SBA-15 Composites Used for Crystal Violet Removal and Their Optimization Using RSM and ANN–PSO

Crystal violet (CV), a triphenylmethane dye, is widely used in the textile, printing, paper, leather, and cosmetics industries. However, due to its higher chemical stability and lower biodegradability, CV has teratogenic and carcinogenic toxic effects on animals and humans. Therefore, the objective of the present study was to investigate whether or not the as-prepared nZVI supported on an ordered mesoporous Zr-Ce-SBA-15 composite (nZVI/Zr-Ce-SBA-15) had more potential for CV removal from simulated wastewater in comparison with Zr-Ce-SBA-15. Meanwhile, the parameters of CV adsorption onto nZVI/Zr-Ce-SBA-15 composites were optimized by a response surface methodology (RSM) and an artificial neural network combined with particle swarm optimization (ANN–PSO). According to XRD, FTIR, SEM, and TEM, N2 adsorption, and thermogravimetric analyses, nZVI was supported successfully on Zr-Ce-SBA-15 composites, becoming an ordered mesoporous material. The results of RSM indicated that the order of the effects of the four parameters on CV removal was, successively, initial pH, contact time, temperature, and initial CV concentration. ANN–PSO was more suitable, in comparison to RSM, to optimize the experimental parameters for CV removal from simulated wastewater using ordered mesoporous nZVI/Zr-Ce-SBA-15 composites. The optimized removal rate of CV was 93.87% under an initial pH of 3.00, a contact time of 20.00 min, an initial CV concentration of 261.00 mg/L, and a temperature of 45. Pseudo-second-order kinetics can better describe the behavior of CV adsorption onto nZVI/Zr-Ce-SBA-15 composites. The process of CV adsorption onto Zr-Ce-SBA-15 composites was followed by the Langmuir model, and its maximum adsorption capacity was 105 mg/g in 213 K. It was indirectly confirmed that the maximum adsorption capacity of nZVI/Zr-Ce-SBA-15 exceeded this value because the removal efficiency of CV using nZVI/Zr-Ce-SBA-15 was obviously higher than that of using Zr-Ce-SBA-15. The thermodynamics results indicated that CV adsorption onto nZVI/Zr-Ce-SBA-15 was a spontaneous, endothermic, and entropy-driven process. The dissolution of Fe ions and light/dark experiments confirmed nZVI/Zr-Ce-SBA-15 was simultaneously of adsorption and catalysis in the process of CV removal. The effect of removal CV was still maintained in the first four experiments (removal rate > 78%), and our suggestion is that nZVI/Zr-Ce-SBA-15 is a potential adsorbent for CV remediation from wastewater compared to Zr-Ce-SBA-15 and other adsorbents.