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This study aims to utilize magnetite nanoparticles (MNP) embedded in granular activated carbon (GAC) originating from coconut shells as microbial support carriers in thermophilic biohydrogen production. MNP can facilitate intracellular electron transportation while providing essential nutrition for microbial growth. Response Surface Methodology (RSM) with a Central Composite Design was used to investigate the simultaneous effect of three variables; Ni:Fe (0.25–0.80), MNP:GAC (0.01–0.03) and type of GAC (GAC-O or GAC-C) on the hydrogen productivity rate (HPR). Biohydrogen content in the biogas to range from 22.25 to 64.71%. The quadratic model was well fitted (R-squared>0.80) with a confidence level higher than 90%. The optimum magnetite GAC was GAC-O as the preferred GAC at Ni:Fe (0.53) and MNP:GAC (0.02), with HPR of 20.33 ± 0.32 ml H 2 /L.h. Magnetite GAC exhibited a better biohydrogen productivity rate by 63.99% compared to non-magnetite GAC. The developed magnetite GAC shown a high potential to improve biohydrogen production. [Display omitted] • Optimisation of magnetite granular activated carbon using response surface methodology. • The performance of magnetite granular activated carbon in biohydrogen production. • Evaluation of mathematical derived model and kinetic modelling. • Physio-chemical characteristics of the optimum magnetite granular activated carbon. [ABSTRACT FROM AUTHOR]

In this study, we examined the formation of thermophilic microbial biofilm by self-attachment on microbial carrier of granular activated carbon (GAC) in five different micro-pore volumes 0.31, 0.41, 0.44, 0.48, and 0.50 cm 3 /g. It was found that the highest hydrogen production rate of 100.8 ± 3.7 mmol H 2 /l.d and yield of 1.01 ± 0.07 mol H 2 /mol sugar were obtained at 0.44 cm 3 /g volume size of GAC. The cellulolytic activity of attached-biofilm was further investigated using POME as a feedstock. The results showed that in all diluted POME substrate, the total sugar consumed by the microbes was found higher than that the amount of soluble monomeric sugar present in the POME medium. It is believe that the microbial biofilm was able to hydrolyse polymeric sugar of cellulosic fibre in the POME by performing enzymatic hydrolysis into simple monomeric sugar. The isolated biofilm bacteria that subjected to 16S rRNA gene analysis presented 99% high homology to the species of Thermoanaerobacterium thermosaccharolyticum which were guaranteed to perform a cellulosic degradation activity. [ABSTRACT FROM AUTHOR]

The production of biohydrogen can be improved by focusing on the nutrients needed by fermentative bacteria like iron. Iron reacts with the [Fe-Fe]-hydrogenase enzyme within the mixed bacteria culture for optimum hydrogen release. Iron nanoparticles (NPs) are attractive due to its unique properties and high reactivity. It can be produced through green synthesis, a more eco-friendly and relatively lower cost process, by using iron salt as precursor and green coconut shell extracted by deep eutectic solvent (DES) as reducing agent. The coconut shell extract consists of phytochemicals that help in producing polydisperse magnetic iron oxide nanoparticles at ∼75 nm in size. The addition of optimum concentration of 200 mg Fe/L magnetic iron NPs resulted in the maximum cumulative hydrogen production, glucose utilization and hydrogen yield of 101.33 mL, 9.12 g/L and 0.79 mol H 2 /mol glucose respectively. Furthermore, the kinetic analysis on Gompertz model using the optimum magnetic iron NPs concentration showed that the hydrogen production potential (P) and hydrogen production rate (R m) increased to 50.69 mL and 3.30 mL/h respectively and the lag phase time reduced about 7.12 h as compared with the control experiment (0 mg Fe/L). These results indicated the positive effects of magnetic iron NPs supplementation on fermentative biohydrogen production of mixed bacteria culture and proved the feasibility of adding the magnetic iron NPs as the micronutrient for enhancement of such hydrogen production system. [Display omitted] • Green synthesis of superparamagnetic iron nanoparticles (NPs) at ∼75 nm size was produced using plant extract. • Magnetic iron NPs at 200 mg Fe/L improved biohydrogen yield by 72.33%, enhanced sugar utilization, and shorten the lag phase. • Kinetics study using Gompertz and Monod model showed that added iron NPs have improved the kinetic parameters than control. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesis of the nickel–iron doped granular activated carbon originated from coconut shell. • The effect of sludge ratio towards the synthesized materials. • The performance of immobilization in the attached system of biohydrogen production. • Evaluation of optimal sugar loading using the Monod model. • Microbial identification of the organism involved in the biohydrogen production. Nickel-iron doped granular activated carbon (GAC-N) was used to enhance immobilization in biohydrogen production. The effect of the sludge ratio to GAC-N, ranged 1:0.5–4, was studied. The optimum hydrogen yield (HY) of 1.64 ± 0.04 mol H 2 /mol sugar consumed and hydrogen production rate (HPR) of 45.67 ± 1.00 ml H 2 /L.h was achieved at a ratio of 1:1. Immobilization study was performed at 2 d HRT with a stable HY of 2.94 ± 0.16 mol H 2 /mol sugar consumed (HPR of 83.10 ± 4.61 ml H 2 /L.h), shorten biohydrogen production from 66 d to 26 d, incrementing HY by 57.30 %. The Monod model resulted in the optimum initial sugar, maximum specific growth rate, specific growth rate, and cell growth saturation coefficient at 20 g/L, 2.05 h−1, 1.98 h−1 and 6.96 g/L, respectively. The dominant bacteria identified was Thermoanaerobacterium spp. The GAC-N showed potential as a medium for immobilization to improve biohydrogen production. [ABSTRACT FROM AUTHOR]

Biological hydrogen production by dark fermentation is an environmentally benign alternative to the conventional fossil-based hydrogen production process. However, low biological hydrogen yields remain a major constraint to commercial production. Unraveling the metabolic pathway for hydrogen production will unlock the potential to enhance biohydrogen yield. In this regard, the key fundamentals of various important dark fermentative hydrogen-producing metabolic pathways are scrutinized in this review, including those in strict and facultative anaerobic bacteria such as Clostridia , Enterobacter , Thermotoga , and Thermoanaerobacterium. Since the hydrogen metabolic pathway is governed by a series of redox reactions during fermentation, manipulation of the redox potential not only indicates the extent of an anaerobic condition, but also affects the growth of anaerobic bacteria. This article reviews the types of hydrogen-producing bacteria and their fundamental metabolic pathways, the effect of redox potential on metabolic fluxes towards growth and hydrogen production and discusses various important redox potential control strategies. In pure culture, strict anaerobes are more suitable to grow in a more reducing environment (lower redox potential), while facultative anaerobes thrive in the presence of oxygen where the redox potential is relatively higher. Redox potential control could minimize the carbon flow towards the propionate-producing pathway by avoiding the redox potential value of around −280 mV. Avoiding the propionate- and lactate-producing pathways results in a higher chance of producing hydrogen via the pyruvate decarboxylation process. Overall, the review provides an all-rounded investigation on the manipulation and impact of redox potential to achieve better hydrogen production for sustainable energy resources. [Display omitted] • Biological hydrogen production can be controlled by redox potential. • Different redox potential ranges are applied to strict and facultative anaerobes. • Strict anaerobes are more potent hydrogen producers than facultative anaerobes. • Redox potential regulates biomass growth and hydrogen-producing pathways. • Strict anaerobes require a lower redox potential than facultative anaerobes. [ABSTRACT FROM AUTHOR]