Feasibility of biohydrogen production from Gelidium amansii

The feasibility of hydrogen production from red algae was investigated. Galactose, the main sugar monomer of red algae, was readily converted to hydrogen by dark fermentation. The maximum hydrogen production rate and yield of galactose were 2.46 L H₂/g VSS/d and 2.03 mol H₂/mol galactose_added, respectively, which were higher than those for glucose (0.914 L H₂/g VSS/d and 1.48 mol H₂/mol galactose_added). The distribution of soluble byproducts showed that H₂ production was the main pathway of galactose uptake. 5-HMF, the main byproduct of acid hydrolysis of red algae causes noncompetitive inhibition of H₂ fermentation. 1.37 g/L of 5-HMF decreased hydrogen production rate by 50% compared to the control. When red algae was hydrolyzed at 150 °C for 15 min and detoxified by activated carbon, 53.5 mL of H₂ was produced from 1 g of dry algae with a hydrogen production rate of 0.518 L H₂/g VSS/d. Red algae, cultivable on vast tracts of sea by sunlight without any nitrogen-based fertilizer, could be a suitable substrate for biohydrogen production.     

Simultaneous utilization of galactose and glucose by Saccharomyces cerevisiae mutant strain for ethanol production

Red algal biomass is a promising alternative feedstock for bioethanol production, due to several advantages including high carbohydrate content, growth rate, ethanol yield, and CO₂ fixation ability. However, it has been known that most yeast strains can not utilize galactose, the major sugar of red algae, as efficiently it can utilize glucose. The authors report a novel ethanogenic strain capable of fermenting galactose, Saccharomyces cerevisiae. This mutant yeast strain exhibited exceptional fermentative performance on galactose and a mixture of galactose and glucose. At 120 g/L of initial galactose concentration, ethanol concentration reached 6.9% (v/v) within 36 h with 88.3% of theoretical ethanol yield (0.51 g ethanol/g galactose). The ethanol concentration and yield were higher than that for glucose at the same initial concentration. In a mixed sugar (galactose + glucose) condition, the existence of glucose retarded galactose utilization however, 120 g/L of the mixed sugar was completely consumed within 60 h at any galactose concentration. The critical inhibitory levels of formic acid, levulinic acid and 5-hydroxymethylfurfural (5-HMF) on ethanol fermentation were 0.5, 2.0, and 10.0 g/L; respectively. From this result, the ethanol fermentation efficiency of the novel S. cerevisiae strain using the galactose base of red algae was superior to the fermentation efficiency when using the wild type strain, and the novel strain was found to have resistance to the major inhibitors generated during the saccharification process.     

Mixtures of 5-hydroxymethylfurfural,levulinic acid,and formic acid have different impact on H2-producing Clostridium strains

Fermentative hydrogen production allows the use of renewable biomasses as feedstocks. However, biomass saccharification results not only in carbohydrates, but also in products that can inhibit fermentation. Although biomass hydrolysates contain mixtures of inhibitors, most studies are performed with a single inhibitor. This study evaluates how 5-hydroxymethylfurfural (HMF, 0.60 g/L), levulinic acid (LA, 2.10 g/L), and/or formic acid (FA, 0.80 g/L) mixtures affect two H₂-producing clostridia, Clostridium beijerinckii Br21 and Clostridium acetobutylicum ATCC 824. Fermentation assays with and without (control) the inhibitors helped to calculate the specific H₂ production, substrate consumption, and bacterial cell growth rates for Clostridium beijerinckii Br21 or Clostridium acetobutylicum ATCC 824. HMF + AL, HMF + AF, AL + AF, and HMF + AL + AF mixtures inhibited H₂ production by C. beijerinckii Br21 by 58.7, 60.0, 46.9, and 83.0%, respectively, and by C. acetobutylicum ATCC 824 by 68.1, 71.4, 58.2, and 89.0%, respectively. Clostridium acetobutylicum ATCC 824 metabolized HMF more efficiently. However, organic acids and their combination with HMF inhibited H₂ production by C. beijerinckii Br21 to a lesser extent, which highlighted that this microorganism is robust for H₂ production from biomass hydrolysates.     

Photocatalytic decomposition of formic acid and methyl formate on TiO2 doped with N and promoted with Au. Production of H2

The photo-induced vapor-phase decompositions of formic acid and methyl formate were investigated on pure, N-doped and Au-promoted TiO₂. Infrared (IR) spectroscopic studies revealed that illumination initiated the decomposition of adsorbed formate formed in the dissociation of formic acid and located mainly on TiO₂. The photocatalytic decompositions of formic acid and methyl formate vapor on pure TiO₂ occurred to only a limited extent. The deposition of Au on pure or doped TiO₂ markedly enhanced the extent of photocatalytic decomposition of formic acid. The main process was dehydrogenation to give H₂ and CO₂. The formation of CO occurred to only a very small extent. Addition of O₂ or H₂O to the formic acid decreased the CO level from ∼0.8% to ∼0.088%. Similar features were experienced in the photocatalytic decomposition of methyl formate, which dissociated in part to give surface formate. Experiments over Au deposited on N-doped TiO₂ revealed that the photo-induced decomposition of both compounds occurs even in visible light.     

Catalytic conversion of Helianthus tuberosus L. to sugars, 5-hydroxymethylfurfural and levulinic acid using hydrothermal reaction

In this study, the efficient conversion of Helianthus tuberosus L. (Jerusalem artichoke) to sugar, 5-hydroxymethylfurfural (5-HMF), and levulinic acid (LA) were investigated using a Brønsted acid-catalyzed hydrothermal reaction. From this approach, the optimal amounts and reaction conditions for total reducing sugar (TRS), 5-HMF, and LA were as follows: 444 g kg⁻¹ of TRS yield (150.4 °C, 14.6 kg m⁻³ sulfuric acid, and 9.8 min), 193 g kg⁻¹ of 5-HMF yield (166.0 °C, 4.6 kg m⁻³, 30.8 min), and 323 g kg⁻¹ of LA yield (185 °C, 30.8 kg m⁻³, 33.7 min). In terms of combined severity, the TRS and 5-HMF concentrations decreased linearly with increasing combined severity, whereas, the LA concentration was increased at a high combined severity factor. The H. tuberosus L. may be useful as a significant feedstock for the production of platform chemicals such as 5-HMF and LA.     

Efficient sugar production from sugarcane bagasse by microwave assisted acid and alkali pretreatment

Sugarcane bagasse represents one of the best potential feedstocks for the production of second generation bioethanol. The most efficient method to produce fermentable sugars is by enzymatic hydrolysis, assisted by thermochemical pretreatments. Previous research was focused on conventional heating pretreatment and the pretreated biomass residue characteristics. In this work, microwave energy is applied to facilitate sodium hydroxide (NaOH) and sulphuric acid (H₂SO₄) pretreatments on sugarcane bagasse and the efficiency of sugar production was evaluated on the soluble sugars released during pretreatment. The results show that microwave assisted pretreatment was more efficient than conventional heating pretreatment and it gave rise to 4 times higher reducing sugar release by using 5.7 times less pretreatment time. It is highlighted that enrichment of xylose and glucose can be tuned by changing pretreatment media (NaOH/H₂SO₄) and holding time. SEM study shows significant delignification effect of NaOH pretreatment, suggesting a possible improved enzymatic hydrolysis process. However, severe acid conditions should be avoided (long holding time or high acid concentration) under microwave heating conditions. It led to biomass carbonization, reducing sugar production and forming ‘humins’. Overall, in comparison with conventional pretreatment, microwave assisted pretreatment removed significant amount of hemicellulose and lignin and led to high amount of sugar production during pretreatment process, suggesting microwave heating pretreatment is an effective and efficient pretreatment method.     

Investigation of the interaction between intermediates from gasification of biomass in supercritical water: Formaldehyde/formic acid mixtures

Supercritical water gasification (SCWG) is a promising technology for converting wet biomass and waste into renewable energy. While the fundamental mechanism involved in SCWG of biomass is not completely understood, especially hydrogen (H₂) production produced from the interaction among key intermediates. In the present study, formaldehyde mixed with formic acid as model intermediates were tested in a batch reactor at 400 °C and 25 MPa for 30 min. The gas and liquid phases were collected and analyzed to determine a possible mechanism for H₂ production. Results clearly showed that both gasification efficiency (GE) and hydrogen efficiency (HE) increased with addition of formic acid, and the maximum H₂ yield reached 17.92 mol/kg with a relative formic acid content of 66.67% in the mixtures. The total organic carbon removal rate and formaldehyde conversion rate also increased to 67.33% and 89.81% respectively. The reaction pathways for H₂ formation form mixtures was proposed and evaluated, formic acid promoted self-decomposition of formaldehyde to generate H₂, and induced a radical reaction of generated methanol to produce more H₂.     

Kinetic study of formic acid oxidation on carbon supported Pd electrocatalyst

The oxidation of formic acid at the Pd/C catalyst electrode is a completely irreversible kinetic process with the reaction order of 1.0. The oxidation rate of formic acid is increased with increasing the concentration of formic acid and is decreased with increasing H⁺ concentration. The apparent negative reaction order with respect to H⁺ is about −0.18 or −0.04 in H₂SO₄ or HClO₄ solution respectively, because bisulfate anions would inhibit formic acid oxidation at some extent. The kinetic parameters, charge transfer coefficient and the diffusion coefficient of formic acid were obtained under the quasi steady-state conditions.     

Effect of furans and linoleic acid on hydrogen production

The effects of furans (furfural and 5-hydroxymethylfurfural (HMF)) on hydrogen (H₂) production using mixed anaerobic cultures were evaluated by conducting batch experiments. Two mixed anaerobic cultures (culture A and B) fed furans plus glucose and treated with and without linoleic acid (LA) at pH 5.5 were maintained at 37 °C. In the LA inhibited cultures A and B fed 0.75 g L⁻¹ furfural and 0.25 g L⁻¹ HMF, the maximum H₂ yields observed were 1.89 ± 0.27 mol mol⁻¹ glucose and 1.75 ± 0.22 mol mol⁻¹ glucose, respectively. In cultures with maximum H₂ yields, Clostridium sp. and Flavobacterium sp. were dominant. Acetate, butyrate and ethanol were the major soluble metabolites detected in cultures A and B whereas propionate was also dominant in culture B. A canonical correspondence analysis based on the byproducts and the relative abundance of the terminal-restriction fragments revealed less variation between cultures treated with LA and low correlation value between the factors and the species composition.     

An intensified reaction technology for high levulinic acid concentration from lignocellulosic biomass

An intensified reaction technology was investigated to achieve high concentrations of levulinic acid (LA) from eucalyptus wood. In the new process, the woody biomass was first pretreated in mild conditions to hydrolyze hemicellulose and recover xylose. The pretreated wood with a high content of cellulose was further treated under relatively severe conditions to produce LA and formic acid. The reaction solution was reused in treatment of new batches of biomass, resulting in high concentrations of LA (105 g/L) and formic acid (39 g/L), respectively. The lignin and humins were recovered as solid byproducts, which had an estimated high heating value of 25–27 MJ/kg in comparison with 18 MJ/kg of original wood. In the sequential repeated batch reactions, however, the LA yield deteriorated batch by batch. It was found out that the old LA in the reaction solution inhibited formation of new LA from glucose. In addition, some LA at high concentrations was lost due to its interactions with other intermediates and byproducts.     

Influence of impregnation with lactic acid on sugar yields from steam pretreatment of sugarcane bagasse and spruce, for bioethanol production

Lignocellulosic biomass can be utilized to produce ethanol, a promising alternative energy source produced through fermentation of sugars. However, in order to achieve high sugar and ethanol yields, the lignocellulosic material must be pretreated before the enzymatic hydrolysis and fermentation. Dilute acid pretreatment, using SO₂, is one of the most promising methods of pretreatment for softwood and agricultural residues. However, handling the high acidity of the slurry obtained from pretreatment and difficulty in recycling/degradation of the impregnating agent are some of the drawbacks of the dilute acid processes. In the present study the influence of utilization of a weak organic acid (lactic acid), as impregnating agent, on the sugar yield from pretreatment, with and without addition of SO₂, was investigated. The efficiency of pretreatment was assessed by enzymatic hydrolysis of the slurry obtained by pretreatment, using sugarcane bagasse and spruce, stored for one and two months in the presence of lactic acid separately, as feedstocks. Pretreatment of bagasse after storage with 0.5% lactic acid resulted in an overall glucose yield, i.e. after enzymatic hydrolysis, of 79% of theoretical based on the amount available in the raw material. This was as good as pretreatment using SO₂ as impregnating agent. However, storage of spruce with lactic acid before pretreatment, with and without addition of SO₂, was not efficient and resulted in lower sugar yields than pretreatment using SO₂ only.     

Enrichment of the hydrogen-producing microbial community from marine intertidal sludge by different pretreatment methods

To determine the effects of pretreatment on hydrogen production and the hydrogen-producing microbial community, we treated the sludge from the intertidal zone of a bathing beach in Tianjin with four different pretreatment methods, including acid treatment, heat-shock, base treatment as well as freezing and thawing. The results showed that acid pretreatment significantly promoted the hydrogen production by sludge and provided the highest efficiency of hydrogen production among the four methods. The efficiency of the hydrogen production of the acid-pretreated sludge was 0.86 ± 0.07 mol H₂/mol glucose (mean ± S.E.), whereas that of the sludge treated with heat-shock, freezing and thawing, base method and control was 0.41 ± 0.03 mol H₂/mol glucose, 0.17 ± 0.01 mol H₂/mol glucose, 0.11 ± 0.01 mol H₂/mol glucose and 0.20 ± 0.04 mol H₂/mol glucose, respectively. The result of denaturing gradient gel electrophoresis (DGGE) showed that pretreatment methods altered the composition of the microbial community that accounts for hydrogen production. Acid and heat pretreatments were favorable to enrich the dominant hydrogen-producing bacterium, i.e. Clostridium sp., Enterococcus sp. and Bacillus sp.. However, besides hydrogen-producing bacteria, much non-hydrogen-producing Lactobacillus sp. was also found in the sludge pretreated with base, freezing and thawing methods. Therefore, based on our results, we concluded that, among the four pretreatment methods using acid, heat-shock, base or freezing and thawing, acid pretreatment was the most effective method for promoting hydrogen production of microbial community.     

Evaluation of process performance on biohydrogen production in continuous fixed bed reactor (C-FBR) using acid algae hydrolysate (AAH) as feedstock

In this study, a novel inoculation method to mitigate the inhibition of 5-hydroxymethylfurfural (5-HMF) is proposed. Acid algae hydrolysate containing 1.5 g 5-HMF/L and 15 g hexose/L hexose was fed to a continuous fixed bed reactor (C-FBR) partially packed with hybrid-immobilized beads. The inoculation method enabled a high rate of H₂ production, due to the reduction of 5-HMF inhibition and enhanced biofilm formation. Maximum hydrogen production was achieved at a hydraulic retention time of 6 h with a hydrogen production rate (HPR) of 20.0 ± 3.3 L H₂/L-d and a hydrogen yield (HY) of 2.3 ± 0.4 mol H₂/mol hexose _added. Butyrate and acetate were the major soluble metabolic products released during fermentation. Quantitative real-time polymerase chain reaction analysis revealed that Clostridium butyricum comprised 94.3% of the total bacteria, which was attributed to the high rate of biohydrogen production.     

Catalytic activity,stability and impedance behavior of PtRu/C,PtPd/C and PtSn/C bimetallic catalysts toward methanol and formic acid oxidation

PtRu, PtPd and PtSn with weight ratios of (2:1) on carbon black (Vulcan XC-72) supported bimetallic catalysts were prepared by using microwave method via chemically reduction of H₂PtCl₆·6H₂O, RuCl₃, PdCl₂ and SnCl₂·2H₂O precursors with ethylene glycol (EG). These prepared catalysts were systematically investigated and obtained results were compared with commercial Pt black, PtRu black catalysts and with each other. The catalysts were characterized with XRD, ICP-MS, EDS and TEM. The electrocatalytic activities, stability and impedance of the catalysts were investigated in sulfuric acid/methanol and sulfuric acid/formic acid mixtures using electrochemical measurements. The results showed that PtSn/C catalyst showed comparable activity and durability with commercial Pt/C catalyst toward methanol oxidation. The synthesized PtRu/C catalyst was found to completely oxidize methanol and it showed more catalytic activity than commercial PtRu catalyst. Bimetallic PtPd/C catalyst gave better activity than both commercial Pt black and synthesized Pt/C catalyst for oxidation of formic acid. Higher electrochemical active surface areas were obtained with supported bimetallic catalysts.     

Efficient photocatalytic hydrogen production from formic acid on inexpensive and stable phosphide/Zn3In2S6 composite photocatalysts under mild conditions

Developing an efficient, stable and low-cost photocatalytic hydrogen production from formic acid is a daunting challenge and has attracted the intense interest of many of researchers. In this paper, we report the synthesis of novel composite photocatalysts (Ni₂P/Zn₃In₂S₆ (ZIS6) and MoP/ZIS6) and their catalytic performance for H₂ production reaction from formic acid under visible light irradiation, in which Ni₂P and MoP were used as cocatalysts to enhance hydrogen generation activity of ZIS6. The photocatalytic hydrogen production rates of the optimized 1.5 wt% Ni₂P/ZIS6 (45.73 μmol·h⁻¹) and 0.25 wt% MoP/ZIS6 (92.69 μmol·h⁻¹) were 3.5 times and 7.2 times higher than that of the pure ZIS6 (12.88 μmol·h⁻¹), respectively. The apparent quantum efficiency at wavelength λ = 400 ± 10 nm for the two photocatalysts was about 1.8% and 6.4%, respectively. Significantly, it was found that the remarkable improvement of hydrogen production performance is attributed to the introduction of the phosphide cocatalysts, which can serve as a charge separation center and an active site for photocatalytic hydrogen production from the decomposition of formic acid. The reaction mechanism of photocatalytic hydrogen production from formic acid was also proposed.     

Biohydrogen and 5-aminolevulinic acid production from waste barley by Rhodobacter sphaeroides O.U.001 in a biorefinery concept

The production of biohydrogen and 5-aminolevulinic acid (5-ALA) by Rhodobacter sphaeroides O.U.001 was investigated in a biorefinery concept. Waste barley was used as a substrate after acid hydrolysis. The hydrolysate was analyzed in terms of its total simple sugar, organic acid, ammonium, element and total phenol contents. Four different growth media having 5 g/L, 7 g/L, 9 g/L and 11 g/L sugar content were prepared using the waste barley hydrolysate to produce biohydrogen and 5-ALA. The increased sugar concentrations resulted in higher cell density and hydrogen accumulation. Accordingly, the highest cell density (OD₆₆₀: 1.78) and hydrogen production (0.4 L H₂/L culture) were observed in the 11 g/L sugar-containing medium. A 67.4 μM 5-ALA was produced upon vitamin B₁₂ and levulinic acid additions. These results showed that waste barley can be used as a substrate for R. sphaeroides for biohydrogen and 5-ALA production within a biorefinery concept.     

Photocatalytic H2 production from acetic acid solution over CuO/SnO2 nanocomposites under UV irradiation

The CuO/SnO₂ composites have been prepared by the simple co-precipitation method and further characterized by the XRD, FESEM and Raman spectroscopy. The photocatalytic H₂ production from acetic acid (HAc) solution over CuO/SnO₂ photocatalyst has been investigated at room temperature under UV irradiation. Effects of CuO loading, photocatalyst concentration, acetic acid concentration and pH on H₂ production have been systematically studied. Compared with pure SnO₂, the 33.3 mol%CuO/SnO₂ composite exhibited approximately twentyfold enhancement of H₂ production. The H₂ yield is about 0.66 mol-H₂/mol-HAc obtained under irradiation for prolonged time. The Langmuir-type model is applied to study the dependence of hydrogen production rate on HAc concentration. A possible mechanism for photocatalytic degradation of acetic acid over CuO/SnO₂ photocatalyst is proposed as well. Our results provide a method for pollutants removal with simultaneous hydrogen generation. Due to simple preparation, high H₂ production activity and low cost, the CuO/SnO₂ photocatalyst will find wide application in the coming future of hydrogen economy.     

Electrochemical impedance studies of electrooxidation of methanol and formic acid on Pt/C catalyst in acid medium

Cyclic voltammetry (CV), amperometric i − t experiments, and electrochemical impedance spectroscopy (EIS) measurements were carried out by using glassy carbon disk electrode covered with the Pt/C catalyst powder in solutions of 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ HCOOH at 25 °C, respectively. Electrochemical measurements show that the activity of Pt/C for formic acid electrooxidation is prominently higher than for methanol electrooxidation. EIS information also discloses that the electrooxidation of methanol and formic acid on the Pt/C catalyst at various polarization potentials show different impedance behaviors. The mechanisms and the rate-determining steps of formic acid electrooxidation are also changed with the increase of the potential. Simultaneously, the effects of the electrode potentials on the impedance patterns were revealed.     

Fermentative hydrogen production by Clostridium butyricum CGS5 using carbohydrate-rich microalgal biomass as feedstock

In this work, a carbohydrate-rich microalga, Chlorella vulgaris ESP6, was grown photoautotrophically to fix the CO₂. The resulting microalgal biomass was hydrolyzed by acid or alkaline/enzymatic treatment and was then used for biohydrogen production with Clostridium butyricum CGS5. The C. vulgaris biomass could be effectively hydrolyzed by acid pretreatment while similar hydrolysis efficiency was achieved by combination of alkaline pretreatment and enzymatic hydrolysis. The biomass of C. vulgaris ESP6 containing a carbohydrate content of 57% (dry weight basis) was efficiently hydrolyzed by acid treatment with 1.5% HCl, giving a reducing sugars (RS) yield of nearly 100%. C. butyricum CGS5 could utilize RS from C. vulgaris ESP6 biomass to produce hydrogen without any additional organic carbon sources. The optimal conditions for hydrogen production were 37 °C and a microalgal hydrolysate loading of 9 g RS/L with pH-controlled at 5.5. Under the optimal conditions, the cumulative H₂ production, H₂ production rate, and H₂ yield were 1476 ml/L, 246 ml/L/h, and 1.15 mol/mol RS, respectively. The results demonstrate that the C. vulgaris biomass has the potential to serve as effective feedstock for dark fermentative H₂ production.     

Investigating how biomass hydrolysis derivatives inhibit H2 production by an isolated Clostridium beijerinckii

This study evaluates how fermentation inhibitors derived from biomass, namely 5-hydroxymethylfurfural (HMF), levulinic acid (LA), and formic acid (FA), affect H2 production by a Clostridium beijerinckii strain. The specific fermentative H2 production rate (μH2), bacterial cell growth rate (μ), and substrate (glucose) consumption rate (μS) during fermentation helped to estimate which HMF, LA, and FA concentrations inhibited 50% of the rates (IC50). IC50 for μ was 2.4, 2.7, and 1.4 g/L for HMF, LA, and FA, respectively. HMF inhibited H2 production the most potently and favored the lactate and ethanol pathways. Butyric acid was the only metabolite to be detected in the presence of LA or FA, which attested that these inhibitors completely inhibited the acetate pathway. The glucose consumption rate was the least affected by the inhibitors, and FA was more potent than HMF and LA. This information should be useful for more appropriate biomass feedstock application in fermentative H2 production.