Abstract

Membrane chromatography is mainly used for the separation and purification of proteins and biological macromolecules in the downstream processing process, also applications in sewage disposal. Membrane chromatography is recognized as an effective alternative to column chromatography because it significantly improves chromatography from affinity, hydrophobicity, and ion exchange; the development status of membrane chromatography in membrane matrix and membrane equipment is thoroughly discussed, and the applications of protein capture and intermediate purification, virus, monoclonal antibody purification, water treatment, and others are summarized. This review will provide value for the exploration and potential application of membrane chromatography.

Introduction

Traditional column chromatography has high adsorption capacity and separation accuracy for protein [1, 2]; so far, the downstream processing of biological agents has been highly dependent on the use of packed bed resin columns [3, 4]. However, traditional column chromatography generally suffers from a high-pressure drop; the intraparticle diffusion leads to the accumulation of solute molecules, which made the processing time increases, low ligand utilization, and long treatment time, which limits its productivity [5, 6]; meanwhile, column chromatography is expensive; more than 60% of the cost of the biopharmaceutical production process is concentrated in the downstream process of purification and recovery, in which the downstream purification of virus accounts for 70% of the total production cost [7, 8].

Compared with traditional column chromatography, membrane chromatography is more suitable for large proteins of separation and purification (MR > 250,000); such proteins rarely enter the pores of chromatographic particle matrix [9]. For viruses and macromolecules with obvious diffusion restrictions in conventional chromatographic media, it is particularly important [10]. Although the equilibrium binding capacity is generally low in the membrane, solutes in the membrane pores are mainly transported to the binding sites by convection, reducing the mass transfer resistance of the chromatography process (Fig. 1). Due to the advantage in mass transfer, membrane chromatography is an effective method for extracting trace proteins from large capacity feed, which can maintain their natural conformation by reducing the time in contact with adsorbents, maintaining the biological activity of the required biomolecules while removing impurities [11, 12]. Moenster et al. [13] used the SartobindQ exchange membrane adsorber method to separate and purify penicillin G amidase from cell lysate in one step, compared with previous multiple purification steps, which reduced the operating units and significantly improved the downstream processing efficiency.©

Summary and outlook

As an effective alternative to chromatographic column packing, membrane chromatography has been widely studied in the past to explore the use of virus purification; purified viruses have been used for gene treatment and production of vaccines. Ion-exchange membrane chromatography is currently used for the polishing of monoclonal antibody production processes in the biopharmaceutical industry; hydrophobic interaction chromatography membrane adsorbers are usually in flow mode and are rarely used. At present, the chromatographic process of membrane chromatography combined with column packing or monolithic column has been applied in the field of biological production processes, and has shown good application potential in the treatment of trace harmful matters such as heavy metal ions or dyes in water.

Given the relatively high raw material manufacturing cost of membrane materials, the purification capacity of membrane chromatography must reach a relatively high value, or have a high cycle of reuse to ensure a low commercial cost, to have a relatively high economic value. Predictable and high-precision theoretical models are developing to better clarify the potential mass transfer phenomena and thus improve the separation performance of membranes. Disposable membrane sorbent methods are a small market branch for chromatographic packing at present, from an adoption and evaluation perspective, in the downstream processing technology, which development is more advanced.

Abstract

Compound separation plays a key role in producing and analyzing chemical compounds. Various methods are offered to obtain high-quality separation results. Liquid chromatography is one of the most common tools used in compound separation across length scales, from larger biomacromolecules to smaller organic compounds. Liquid chromatography also allows ease of modification, the ability to combine compatible mobile and stationary phases, the ability to conduct qualitative and quantitative analyses, and the ability to concentrate samples. Notably, the main feature of a liquid chromatography setup is the stationary phase. The stationary phase directly interacts with the samples via various basic mode of interactions based on affinity, size, and electrostatic interactions. Different interactions between compounds and the stationary phase will eventually result in compound separation. Recent years have witnessed the development of stationary phases to increase binding selectivity, tunability, and reusability. To demonstrate the use of liquid chromatography across length scales of target molecules, this review discusses the recent development of stationary phases for separating macromolecule proteins and small organic compounds, such as small chiral molecules and polycyclic aromatic hydrocarbons (PAHs).

Introduction

Separation is a critical process to isolate a particular compound, whether it is a natural product or a synthetic product. Studies of a compound’s characteristics and elucidation structure provides reliable results for pure compounds because there is no interference from other compounds. The primary source of difficulty in a separation process is the high similarity between two or more compounds, such as racemic and homologous mixtures. Liquid chromatography has proven to be an effective solution to those problems. The key to liquid chromatography separation is a sustainable retention and elution process. Stationary phases essential for separating compounds in liquid chromatography. Various liquid chromatography columns of both preparative and quantitative types have been used and continue to develop. For this reason, multiple studies and publications related to liquid chromatography can be found and accessed easily.

This review will discuss recent developments in liquid chromatography stationary phase technology for compound separation. Analytes that were once inseparable can now be separated and appropriately analyzed. The discussion focuses on the development of column material and separation interactions. This development aims to optimize and overcome the lack of existing columns. This review will discuss three types of analytes to demonstrate the use of liquid chromatography across length scales of target molecules, which are (1) larger biomolecules, particularly the proteins, (2) small chiral molecules, and (3) polycyclic aromatic hydrocarbons (PAHs).

Outlook

As liquid chromatography continues to be one of the main tools in the preparation and analysis of chemical compounds, various developments have been made to improve the binding selectivity and reusability of the columns. The surface chemistry of the stationary phases of silica, polysaccharides, and synthetic polymer matrixes has been adjusted and modified to result in better separation across length scales. The stationary phases can be chemically decorated with desired functional groups to ensure a stable and selective binding to the target molecule. For selective separation of chiral molecules, chiral stationary phases (CSPs) have been prepared using modified polystyrene, cellohydrobiolase, cyclodextrins, chiral metal–organic frameworks (MOF), and ionic liquids as chiral selectors. Hydrophobic surfaces are an essential component of liquid chromatography for PAH binding. Several hydrophobic chemical modifiers have been covalently anchored at silica surfaces to increase their affinities towards PAHs. Moreover, hydrophobic MOF and particle-based acrylics have also been developed as stationary phases to capture PAHs efficiently. In the case of protein capture and purification, affinity chromatography is commonly established through metal–protein complexation as well as ligand–protein complexation. Furthermore, by adjusting the conditions of elution, particularly the pH and ionic strength, hydrophobic charge-induction chromatography (HCIC) and mixed-mode chromatography (MMC) have also been investigated to achieve better protein purification.

Abstract

     Two dimensional nanomaterials, such as graphene and MXene, have been widely used to fabricate high performance nanocomposite films owing to their unique structure and excellent physiochemical properties, showing enormous potential applications in many areas. However, the existing voids and wrinkling in the nanocomposite films inevitably impedes realizing the high performance of the nanocomposite films during the assembly process of graphene and MXene nanosheets. Nacre provides an inspiration for fabricating high performance graphene and MXene nanocomposite films through constructing a high alignment structure and strong interface interactions. In this review, we summarize the preparation approaches of graphene and MXene nanocomposite films and discuss the mechanical properties. Then, several effective strategies for assembling graphene and MXene nanosheets have been concluded and discussed. Meanwhile, we also summarize and discuss the applications of graphene and MXene nanocomposite films including electromagnetic interference (EMI) shielding and thermal management. Finally, outlooks and challenges are proposed to promote their progress and applications in the future.

Introduction

     Since the first discovery of graphene in 2004, graphene has gradually aroused widespread attention [1]. Due to the outstanding mechanical, electrical and thermal properties, graphene has been applied in flexible electronic devices, aerospace and thermal management fields in the past decade [2], [3], [4], [5], [6]. For instance, the single-layer graphene has a Young’s modulus of 1.0 TPa [7] and a superhigh thermal conductivity of about 5300 W/mK [8] at room temperature, demonstrating enormous practical potentiality in the above mentioned fields. Before realizing these applications, it is crucial to assemble graphene nanosheets into high performance graphene nanocomposites including one dimensional (1D) fiber, two dimensional (2D) film and three dimensional (3D) bulk. Especially, high performance graphene nanocomposite films have gained enormous attention. However, it is difficult to achieve high performance nanocomposite films due to weak interaction between adjacent graphene nanosheets. Graphene oxide (GO) is a usual precursor for obtaining graphene with plentiful surface terminating groups, which can easily interact with various materials for preparing nanocomposite films [9,10]. Meanwhile, in recent years, MXene, a novel two dimensional (2D) materials, shows promising electrical and thermal properties, which shows promising application in aerospace field [11], [12], [13], [14], [15], [16]. Indeed, compared with graphene, the intrinsic mechanical and thermal conductivity properties and stabilities of MXene is relatively poor. Furthermore, the easily oxidizable characteristics and thermal vulnerability of MXene may restrict its practical applications. However, compared with graphene, MXene has plentiful surface functional groups, which is an ideal building block for fabricating nanocomposite films without chemical modification any more.

Summary and outlook

     In the past several years, two dimensional materials such as graphene and MXene have been widely studied owing to the excellent physiochemical properties. In this review, we have focused on the preparation methods, assembling strategies and mechanical properties of graphene and MXene nanocomposite films. Moreover, we have also discussed the applications of obtained graphene and MXene nanocomposite films such as EMI shielding and thermal conductivity. Various methods such as VAF, blade coating, drop casting and LbL have been used to prepare high performance graphene and MXene nanocomposite films. However, as aforementioned, the existing voids and wrinkling are harmful for the mechanical properties of graphene and MXene nanocomposite films. Hence, several strategies for fabricating high performance graphene and MXene nanocomposite films are proposed.

Abstract

Keywords

1. Introduction

2. DNA damage response

3. DDR and radiotherapy failure

4. DDR inhibitors decrease radio-resistance

5. DDR roles in chemotherapy failure

6. DDR inhibitors decrease chemo-resistance

7. Conclusions

Funding

Availability of data and material

Ethics approval and consent to participate

Consent for publication

Declaration of Competing Interest

Acknowledgements

References

Abstract

Up to now, many improvements have been made in providing more therapeutic strategies for cancer patients. The lack of susceptibility to common therapies like chemo- and radio-therapy is one of the reasons why we need more methods in the field of cancer therapy. DNA damage response (DDR) is a set of mechanisms which identifies DNA lesions and triggers the repair process for restoring DNA after causing an arrest in the cell cycle. The ability of DDR in maintaining the genome stability and integrity can be favorable to cancerous cells which are exposed to radiation therapy or are treated with chemotherapeutic agents. When DDR mechanisms are error-free in cancer cells, they can escape the expected cellular death and display resistance to treatment. In this regard, targeting different components of DDR can help to increase the susceptibility of advanced tumors to chemo- and radio-therapy.

1. Introduction

DNA-damaging agents originating whether from endogenous or exogenous resources are known to threat the viability of every cell of our body [1]. Besides to the cellular issues arising from the great number of mutations and genome instability, the lack of proper responses can establish generations of these defective cells and put our whole body in danger [1]. DNA damage response or DDR is a precise mechanism which contains sensing the lesions, establishing signals, and finally initiating repair processes by the means of these signals [2]. During DDR, cell cycle is also affected by the mediators of DDR and is temporarily stopped in order to inhibit the replication of a defective DNA [3]. In some cases, DNA deteriorations are irreparable and thus, cell death is the only option on the table [3].

In addition to normal cells, cancerous cells are also able to use the same mechanisms for their chemotherapy- and/or radiotherapy-induced damaged DNA. Until DDR is working properly in a cancerous cell, DNA damages would not lead to cell death and thus, treatment would not be considered successful [1]. This means that despite all the advantageous impacts of DDR, it stands in the way of overcoming a great challenge in cancer therapy: resistance to our common methods, chemo- and radio-therapy. Recently, a great number of researchers have worked on targeting distinct components of DDR for enhancing the efficacy of cytotoxic treatments [4]. In this paper, we review the process of DDR briefly and explain the role of its key players in decreasing the susceptibility of cancer cells to common therapeutic procedures. Additionally, the usage of DDR inhibitors on different types of cancer would be discussed for giving an insight for the treatment of advanced stages of this lethal disease.

1. Introduction

2. Insights on contaminants of emerging concern (CECs)

3. Treatment technologies

4. Process intensification

5. Conclusions

Acknowledgments

References

Abstract

Urban wastewater treatment plants (WWTPs) are among the major sources of contaminants of emerging concern (CECs) release to surface water. This is mainly associated with the low performance of conventional technologies (e.g. activated sludge, sand filtration and UV disinfection processes) for the removal of residual amounts (ng L-1 – μg L-1) of CECs. In this review, a list of CECs commonly detected in the aquatic environments is presented. Light-driven chemical/catalytic processes for CECs removal, namely UV/oxidant and photocatalysis, are presented with discussion of their process, advantages, drawbacks, and the main attainments. In this context, process intensification has been gaining high relevance for wastewater treatment purposes due to its potential to overcome the main treatment limitations. Accordingly, an overview of advances to overcome the limitations in light-driven chemical/catalytic processes for CECs mitigation was carried out, focusing mainly on the design of novel reactors and devices. Some aspects related to reactor configuration, removal mechanism, devices applied, illumination system, comparison criteria, and catalyst immobilization through implementation of novel reactors designs are also discussed. Regarding the reactor design, several devices exhibit a satisfactory mass transfer due to a large surface-tovolume ratio or enhanced mixing conditions. However, improvements in photon transfer are still the biggest challenge to be overcome. The irradiation of the entire reaction solution and/or catalyst surface has shown to be challenging in many reactor configurations. Nevertheless, the reactor design and treatment process to be applied for CECs removal must be selected according to the site-specific conditions, in order to accomplish CECs mitigation and attain the best reactor performance.

Introduction

About 71 percent of the Earth’s surface is water-covered, however less than 3 percent of this amount can be considered as freshwater [1]. In addition, this limited resource is under threat from the  pollution, mainly caused by human factors. Power generation, industrial, agriculture and mining activities among others are some of the contributors to the aquatic compartments contamination, 6 which affects directly life on Earth [2]. Approximately two-thirds of the world’s population experiences water shortage for at least one month in a year [3]. Additionally, the World Health 8 Organization (WHO) reported that, in the year of 2017, 2 billion people living in various regions all over the world used drinking-water source contaminated with faeces [4]. In fact, it is estimated that almost 1.8 billion people worldwide might face moderate or severe water scarcity by 2025 [5].  Commonly, materials and chemicals used in daily human activities are continuously introduced into the environment. These so-called Contaminants of Emerging Concern (CECs) are one of the main causes of water pollution and, hence, being often related to the risks to human and biota health [6]. Therefore, as water safety and quality are fundamental to human development and well-being, the scientific community has been applying great efforts to the development of efficient technologies for water purification.

Keywords

1. Introduction

2. Graphene Oxide

3. Aerogels

4. Supercapacitor and Energy Storage

5. Supercapacitor applications of graphene and graphene oxide based aerogels

6. Conclusion

References

Abstract

Graphene and graphene-based materials have a high potential, especially in energy storage technology. Thanks to the three-dimensional (3D) structures developed with this material, their importance in the production and application of energy storage devices has increased. Studies on supercapacitor applications of graphene-based aerogels have begun to arouse interest in recent years. In this study, recent studies on aerogel supercapacitors, in which researchers have shown great interest, have been compiled and collected. In this study, production methods and properties of graphene oxide, properties and production of aerogels, production and applications of graphene/graphene oxide aerogels are discussed. In this way, the data is presented and discussed in an organized way for the researchers who study or who want to study in this field.

Introduction

Today’s rapidly depleted energy resources and the deteriorating ecological balance prioritized the need for energy storage systems. Moreover, the growing popularity of telecommunications equipment, backup storage devices, portable electronic devices, cardiac pacemakers and hybrid electric vehicles has aroused great interest towards energy storage devices. In this context, supercapacitors, which are mostly preferred among various energy storage devices, are electrochemical energy storage devices that cover the gap between batteries and dielectric capacitors with their high-power density, high specific energy and long life (>100,000 cycles) [1]. Furthermore, flexible solidstate supercapacitors are important for wearable electronic devices [[2], [3]].

Graphene, which come into prominence with wide specific surface area (SSA) (2630 m2 g−1 ), high electrical conductivity and chemical stability, as well as excellent mechanical, thermal and optical properties, has become popular especially for supercapacitors. However, a serious loss in surface area is observed due to the re-stacking of graphene layers during chemical reduction. The conversion of graphene nanotubes to graphene aerogel having a stable three-dimensional network solves this problem. The re-stacking problem resulting from π-π interactions between graphene layers and the Van der Waals forces are solved and the layers are chemically bonded together. Thus, the structure is intended to provide better performance in supercapacitors with much higher electronic conductivity, rapid charge transport between the layers and the resulting high surface area. The most interesting feature of GOs is that they are well protected by a hexagonal structure having functional groups such as hydroxyl, epoxide, carboxyl and some randomly distributed alkyl groups in the plane, together with a clear AB stacking order. Hydroxyl and epoxide functional groups are located close to each other in the basal planes, whereas carboxyl and alkyl groups are located at the edges of GO flakes. With longer oxidation times, hydroxyl groups are converted to more epoxide groups in the basal plane.

Abbreviations

1. Introduction

2. Method and analysis section

3. Petrochemical wastewater pretreatment and advanced treatment

4. Future perspectives

Acknowledgements

Appendix A. Supplementary data

References

Abstract

Petrochemical wastewater is a major industrial source of pollution that produces a variety of refractory and toxic organic pollutants that are detrimental to animals and plants in natural water bodies; it is especially harmful to biological treatment systems. Regardless of these threats, studies on these specific organic pollutants are limited at present. Consequently, it is extremely essential to promote relevant problem solving efficiently. Currently, only limited processes are available for pretreatment of high-concentration wastewater, and advanced treatment of secondary effluent as compared to the conventional treatment are introduced by current studies. Therefore, in this review, we have systematically generalized the characteristics of petrochemical wastewater; we have particularly summarized and compared different methods (recent developments, influencing factors, etc.) which are applied in pretreatment and advancement of current methods. Additionally, the interaction mechanisms of microbes under a wide range of concentrations of specific organic pollutants and associated degradation pathways have been described comprehensively. Moreover, we have analyzed bioenergy recovery from the degradation/removal of specific organic pollutants with environmental-friendly and economically methods because it can help realize the goal of circular resource utilization during the process of detoxification and minimization.

Introduction

The petrochemical industry is a fundamental industry that plays an essential role 4 in any country’s national economy and provides support to many other sectors, such as agriculture, energy, transportation, etc. (Fig. 1) (Clews, 2016c; Jafarinejad 2017).  Petrochemical wastewater is generated via several routes from this industry, including effluent from raw materials, factory rainwater, cooling water, and domestic sewage. It has been reported that global petroleum production has reached 4.40 billion tons based on related industrial statistics (https://www.sohu.com/a/119515720_122917). Moreover, relevant studies have revealed that 3.00-3.50 m3 10 of petrochemical wastewater is generated per ton of petroleum refinery process (Zhang and Fan, 2016; 12 Siddique et al., 2017). For instance, China’s annual industrial wastewater discharge is 2.10 × 1010 13 t, of which the proportion of petrochemical wastewater was 3.00-5.00% in 14 2016. A report derived from China’s oil and chemical industry has shown that 2.00 15 billion tons of petrochemical wastewater were produced only in 2016, and the total 16 emission increased up to 4.00 billion during the 13th 5-year plan period (CMEP, 17 2015). Moreover, massive petrochemical wastewater was produced all over the world 18 during the same period that needed to be treated properly and urgently.

An overview of capacitive technologies based on carbon materials (energy storage in electrical double-layer capacitors (EDLCs), capacitive deionization (CDI), energy harvesting, capacitive actuation, and potential controlled chromatography) is presented. The review reveals the role of carbon for these scientific and industrial purposes with disclosing the benefits and limitations of various nanostructured carbons for a certain application. A special attention is placed on the electrical double-layer (EDL) formation mechanisms affected by the porous texture of carbon and the electrode architecture. The importance of a careful selection of the electrolytic solution for the EDL formation inside the intraparticle pores of carbon electrodes is also enlightened.

Introduction

During the last fifty years, the worldwide industrial development accompanied by improving wealth in emerging markets and growth of the human population have driven a steadily increasing demand for new energy and fresh water supplies. The emission of noxious gases during fossil fuel consumption and the depletion of their reserves have led to introduce renewables (sun and wind) in the energy mix. Unfortunately, due to their intermittent character, the renewable technologies are not able to ensure a real-time balance of electricity supply.[1] Therefore, in order to adapt the energy delivery to the demand and improve the energy efficiency, current researches are focused on introducing energy-storage technologies which would meet sustainability, environmental requirements and be cost-effective. What is more, the automotive industry faces the challenge to reduce the exhaust gases emission and amount of energy dissipated in urban areas by internal combustion engine powered vehicles. Encouraging efficient energy management should accelerate commercialization of the powertrain technologies to be applied in low carbon vehicles (LCV), fast-charging infrastructures in the stations, wireless monitoring systems or electricity load leveling in stationary and transportation systems. Apart from energy related issues, another critical problem worldwide concerns the shrinking access to potable water sources. It is commonly known that sea or brackish water is not suitable to drink without being desalinated. Therefore, alternative options to i.e. expensive vacuum distillation processes, reverse osmosis and cogeneration desalination plants, using either fossil fuels or nuclear power, are still highly sought in order to meet the global needs for clean water with reduced carbon footprint. Electrochemical technologies are able to bring some response to the issues related with efficient energy management, reduction of greenhouse gases emissions and water desalination by utilizing the concept of electrical double-layer (EDL) created at the surface of nanoporous electrodes.[2-4] When an electrode is polarized, the ions of opposite charge present in the electrolyte migrate to the electrode surface, where they create an EDL.[5] The double-layer capacitance depends on the electrode surface area able to accumulate electrical charge by pure electrostatic forces and on the charge separation distance imposed by dimensions of the charge carriers. Therefore, electrodes made from nanostructured carbon materials (also called carbons further in the text), characterized by a highly extended surface area and pores which match with the size of electrolyte ions, are generally considered to be capacitive.[4, 6-13] The capacitive technologies include: i) capacitive energy storage in electrical double-layer capacitors (EDLCs);[14, 15] ii) capacitive energy harvesting where small amount of ambient energy is captured, accumulated, and converted into electrical energy;[16, 17] and iii) capacitive deionization (CDI) of sea or brackish water where the salt ions are removed from the feed water, electrostatically held in the EDL and released in a brine stream during discharge.[18-20] Besides, the chargedependent ions separation can be also employed in a new type of liquid chromatography (called potentialcontrolled chromatography) to enhance the anolyte concentration or to introduce withholding of certain species.[21] What is more, the distribution of electric charges within the porosity of carbon material entails changes of geometric surface area of the electrodes triggered by charging and discharging, so-called capacitive actuation. The flexible capacitive devices where the electrodes deformation is controlled by a voltage stimulation can be used in soft robotics to make shape-changing robots.[22] In this context, the paper reveals the benefits and limitations of various nanostructured carbons, with a special attention to the charge storage mechanism in EDLCs affected by the porous texture of carbon electrodes. It then reviews the state-of-the-art of capacitive technologies based on carbon materials with discussing in some detail how the double-layer concept is being utilized for their development.

1- Introduction

2- Chemistry of geopolymers

3- Method and analysis section

4- Synthesis of geopolymers and mechanisms involved

5- Use of industrial waste as filler for the fabrication of crystalline metakaolin-based geopolymer

6- Result and discussion on applications driven by the surface chemistry of geopolymers in water

7- Barriers to utilization

8- Future research and prospects

9- Conclusion

References

Abstract

Geopolymer is a ceramic material, most often amorphous; finds applications in fire- and heat-resistant coatings and adhesives, medicines, refractory ceramics and binders, and manufacturing of radioactive waste container. Over the last decade, new cement based on geopolymers has been developed. Most relevant to this review is the fact that its porous nature and chemical similarity to zeolites is being employed for applications pertaining to wastewater treatment. Most of the work has been on the adsorptive treatment of water. However, using chemical reasoning and literature available we show that geopolymers have relevance for further activity on additional areas of relevance to waste water treatment such as photocatalysis, disinfection, and H2-energy production from waste water etc. These applications would depend strongly on the properties of geopolymers, which in turn would rely on the precursors employed and the synthetic methods used. The relevance of geopolymers for cleaner production is also highlighted. The use of fly ashes and metakaolin composites for the fabrication and surface tailoring of geopolymers (perhaps using relevant surfactants) is suggested as a plausible step in the right direction. Given the critical analysis of the state of the art, and the plausible directions identified, this article will benefit environmental scientists, engineers and chemists interested in deploying geopolymers for environmental remediation purposes. However moving forward, barriers are to be anticipated for the large scale implementation of geopolymers. Several barriers (e.g. legal, economical, technocrats and synthetic challenges) that are likely to hinder future research and translation are highlighted.

Introduction

Heavy metal removal and organic molecule degradation from wastewater have become a major topic of the public interest, due to their relevance to health and environment. Heavy metal and hazardous organic molecules are known to be toxic and a precursor to ill-health of the populace (Jaishankar et al., 2014). They are primarily released into the environment through human/industrial activities (Azimi et al., 2017). Hence research on multifunctional materials which relates to simultaneous environmental remediation and energy production are relevant to contemporary technologies connected with cleaner production. Geopolymers are a promising category of materials, for the removal of toxic substances from industrial and household effluents. It is usually prepared from a rather simple and eco-friendly reaction between an alkali such as NaOH/Na2SiO3 or KOH; and Al and Si source (s), making it a material amenable to clean production. Geopolymers obtained thus are often applied for the removal of metal ions such as Cs+ , Pb2+, Cd2+ etc. from wastewater. Geopolymers have come a long way, since their first discovery four decades ago (Davidovits, 1976). It has gained attention primarily because of the ease with which it can be synthesized with little or zero emission of green house gases (CO2. SO2, NOx etc.) (Noor ul et al., 2016). This coupled with its properties which include toughness, fire and heat resistance (i.e. its refractory nature), radiation hardness (making it relevant for radioactive waste containment) and pozzolanic action (Carsten, 2013), make geopolymers truly multifunctional.

1- Introduction

2- Application of nanoparticles in biofuel production processes

3- Factors affecting the performance of nanoparticles in biofuel production processes

4- Conclusions and future recommendations

References

Abstract

Biofuels are fast advancing as alternative sources of renewable energy due to their non-polluting features and cost-competitiveness in comparison to fossil fuels. However, in order to fast-track their development, focus is shifting towards the use of technologies that will maximize their yields. Nanoparticles are gaining increasing interest amongst researchers due to their exquisite properties, which enable them to be applied in diverse fields such as agriculture, electronics, pharmaceuticals and food industry. They are also being explored in biofuels in order to improve the performance of these bioprocesses. This review critically examines the various studies in literature that have explored nanoparticles in biofuel processes such as biohydrogen, biogas, biodiesel and bioethanol production, towards enhancing their process yields. Furthermore, it elucidates the different types of nanomaterials (metallic, nanofibers and nanotubes) that have been used in these bioprocesses. It also evaluates the effects of immobilized nanoparticles on biofuels such as biodiesel, and the ability of nanoparticles to effectively suppress inhibitory compounds under certain conditions. A short section is included to discuss the factors that influence the performance of nanoparticles on biofuels production processes. Finally, the review concludes with suggestions on improvements and possible further research aspects of these bioprocesses using nanoparticles.

Introduction

The depletion of hydrocarbon fuel reserves, the increase in environmental pollution, and unstable energy prices have triggered a search for clean and sustainable energy resources [1–5]. Biofuel development initiatives are widely being implemented in many countries in order to mitigate these challenges [6–9]. They are classified into two groups i.e. primary and secondary biofuels. The primary biofuels are directly produced from plants, forests, animal waste and crop residues [10–12]. Secondary biofuels are produced from a combination of biomass feedstocks and microorganisms, and are further categorized into three groups i.e. first generation, second generation, and third generation biofuels [13,14]. First generation biofuels are generated from edible crops such as corn, wheat, sugarcane, barley, sorghum, sunflower oil, etc. Second generation biofuels are synthesized from biomass residues such as wheat straw, grass, jatropha, miscanthus, cassava, corn cob, etc [15–17]. These biofuels are considered a viable option because they do not a pose a threat to food security, deforestation, water shortages, and other social challenges [18,19]. Meanwhile, the third generation biofuels use various types of microalgal species [15,20]. Over the last decade, research focusing on third generation biofuels has also intensified because microalgae can thrive under diverse growth conditions and can produce different types of renewable fuels such as biohydrogen, biodiesel, and biogas [21,22]. Biofuels such as biohydrogen, biodiesel, bioethanol and biogas are receiving increasing attention amongst researchers because they are environmentally friendly, use diverse feedstocks that are accessible, non-edible and cheap [23–25]. Biohydrogen is gaining increasing prominence over contending biofuel technologies due to its characteristics which include: (i) high energy content (120 kJ/g) that is approximately 3 times greater than that of fossil fuels i.e. sub-bituminous coal (25–35 kJ/g), gasoline (41.2 kJ/g), and diesel (42.9 kJ/g), (ii) its carbon-sequestration abilities, (iii) its ability to utilize diverse feedstocks including organic effluents, (iv) its ability to use diverse bacteria which are found in various environments, (v) it can be produced at ambient temperature and pressure, thus making it feasible for its largescale production, and (vi) the process offers the simplest way of producing hydrogen energy [26]. The process is highly dependent on various operational conditions such as substrate concentration, pH, temperature and hydraulic retention time [26].

1- Introduction

2- Water; one of the prerequisites for life as we know it

3- Isoscapes

4- Stable isotope ecology

5- Forensic isotope ecology

6- Caveats — a fishy example

7- Conclusions

8- The science bit: isotopologues, chemically identical yet not the same

References

Abstract

Stable isotope ecology and forensic isotope ecology are not only linked by name. More often than not, knowledge and insights gained through the former serve as a springboard for application focused work of the latter. This review aims to offer a glimpse into the fascinating world of both though with more emphasis on forensic isotope ecology. To this end a selection of past and recent published work is presented and discussed to highlight both potential and limitations of isotopic analytical approaches to the detection of illegal trade in plants and animals.

Introduction

Ecology, and its underpinning environmental studies, are two areas within natural and life sciences with a long history of applying stable isotope chemistry and bio-chemistry to gain insights into, for example, food webs, life cycles, or migration. It was only a matter of time before the methods that were developed, and the insights gained during ecological studies, were extended to wildlife forensics. For the appreciation of as wide an audience as possible, in addition to highlighting the wealth of information that stable isotope abundance signatures can provide, in Section 8 this review also provides some background information on the nature of stable isotopes, the terminology used, and why their relative abundance varies in the natural world. In-depth treatises on the subject of stable isotope chemistry in the context of ecological, geochemical, and forensic sciences, have been published in the form of several textbooks [1–5].

Introduction

Fundamentals and thermodynamics of photocatalysis

Advancements in photocatalysts for water splitting

Heterojunction construction

Factors that influence photocatalyst activity

Advancements in photoreactors

Conclusion and future perspectives

References

Abstract

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H2 evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H2 production have been discussed. In the mainstream, recent developments in TiO2 and g-C3N4 based photo-catalysts and their potential for H2 production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H2 production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H2 production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.

Introduction

Natural resources such as coal and petroleum products as a source of energy are nearly exhausted [1]. The reduction of fossil fuel reserves has prompted substantial research efforts toward the usage of hydrogen (H2) as an environmentally friendly energy carrier for the post fossil fuel regime [2]. It is currently generally agreed that H2 may be the best option for tackling the triple issues of exhaustion, pollution and climate change effects [3]. One of the technologies for H2 production is photocatalytic water splitting, since it entails photonic energy, which is the most abundant energy resource on the Earth [4]. Previous research states that solar based H2 generation by photocatalysis provides near zero global warming and air pollutants [5], and can be stored easily [6]. Therefore, H2 is considered as a possible important energy in future, since it is free from toxic and it can produce high energy content from natural resources such as light (photon) energy and water, which are clean, long lasting sources of energy, and renewable resources [7]. Pioneer work as early as 1972 by Fujishima and Honda [8] reported water splitting for H2 production over TiO2 semiconductor. Since then, various types of semiconductors for photocatalytic H2 productions are under investigation. Among all, titanium dioxide (TiO2) with band gap 3.2 eV is a recognized photocatalyst and it has been extensively studied because of numerous advantages such as low cost, high photochemical stability and non-toxic [6,9]. On the other hand, wide band gap limits its applications under visible light and faster charges recombination rate lowers its photocatalytic activity [6,10]. Coupling TiO2 with visible light semiconductors can narrowing the band gap with faster charges separation, thus could enables enhanced photo-catalytic activity. Among the low band gap semiconductors, polymeric graphitic carbon nitride (g-C3N4) has attracted more attentions as metal-free polymeric semiconductor in photocatalytic water splitting. It is a visible light responsive with lower band gap and low cost semiconductor. It can be synthesized from cheap precursors such as melamine and urea by simple thermal approach. In addition, g-C3N4 has numerous advantages such as high thermal and chemical stability and appropriate band structure (2.7 eV) to absorb visible light irradiation [11]. Among the limitations, g-C3N4 has low surface area and small active sites for interfacial (photon) reaction, moderate oxidation reaction of water to Hþ and low charge mobility which disrupt the delocalization of electrons.

Abstract

The root growth of most crop plants is inhibited by soil salinity. Roots respond by modulating metabolism, gene expression and protein activity, which results in changes in cell wall composition, transport processes, cell size and shape, and root architecture. Here, we focus on the effects of salt stress on cell wall modifying enzymes, cellulose microfibril orientation and non-cellulosic polysaccharide deposition in root elongation zones, as important determinants of inhibition of root elongation, and highlight cell wall changes linked to tolerance to salt stressed and water limited roots. Salt stress induces changes in the wall composition of specific root cell types, including the increased deposition of lignin and suberin in endodermal and exodermal cells. These changes can benefit the plant by preventing water loss and altering ion transport pathways. We suggest that binding of Na+ ions to cell wall components might influence the passage of Na+ and that Na+ can influence the binding of other ions and hinder the function of pectin during cell growth. Naturally occurring differences in cell wall structure may provide new resources for breeding crops that are more salt tolerant.

Introduction

Plant evolution has resulted in a large array of mechanisms to tolerate the stresses associated with increased soil salinity. However, for most cereal crops the growth of roots is disrupted when soil salinity exceeds 4 dS/m, equivalent to about 40 mM NaCl. Increased soil salinity exposes plants to ionic sodium (Na+) and chloride (Cl−), which leads to a cascade of responses in the plant due to the ionic and osmotic components of salt stress [1,2].

Salt stress can indirectly affect cell wall properties by causing changes in gene expression, but Na+ can also physically interact with the cell wall components directly, and change their chemical properties [3]. An increase in soil salinity results in accumulation of Na+ in the apoplast, which can lead to an increase in interactions between Na+ and negatively charged sites within cell wall polymers, and also influence apoplastic pH. Salinity causes transient alkalinisation of the apoplast, and this could limit growth in the context of the acid growth theory [4,5]: Auxin activates plasma membrane H+-ATPases and protons are extruded into the apoplast, apoplastic acidification induces cell wall loosening by activating expansins and other remodelling enzymes resulting in loosening of the cell wall. Hence, growth could be limited by a decrease in free apoplastic protons causing a shift in the apoplastic pH away from the range that favors cell-wall loosening [4], although in maize the inhibition of growth as a result of salinity was not associated with the capacity of the epidermal cells to acidify their walls [6].