The transition of the mineral processing sectors, which depend mainly on various petroleum-origin chemicals, to the green industry based on the production of greener materials and the reduction of carbon footprints, is mandatory due to the growing concerns regarding the extensive environmental impact of the mining industry. In this context, biological ore beneficiation is spurring increasing interest from scientists and industrials. The latest scientific developments in bio-metallurgy have allowed it to exploit biotechnologies in the flotation process as a novel/cleaner approach for separating gangue materials from valuables minerals. Current fundamental research projects have focused on the progress of biotechnology and the employment of biopolymers, microorganisms, and their metabolites, as well as plant-derived biosurfactants in the emerging field of bioprocessing, particularly bioflotation. Almost all studies on flotation have focused on chemical and physical parameters, and the role of natural biosurfactants remains little explored. This review presents a thorough understanding of the bioflotation concept by assessing previous research on several elements of bioflotation, including the impacts of biotechnology and operating factors (pH, biomass, and biosurfactants concentration), probable mechanisms, and unexplored areas, but from various pragmatic viewpoints. Recent studies are summarized by categorizing microorganisms, plants, and biopolymers based on their bioflotation performances. Furthermore, the most recent research investigations on biological flotation of specific minerals (salt-type sulphide and oxide) are reviewed. Finally, critical challenges of bioflotation are discussed, and novel perspectives are provided to contribute to solving the difficulties faced in its implementation.Graphical abstract

Large-scale production of single-cell protein (SCP) has the potential not only to solve some of the food insecurity and water scarcity crises that plague a significant portion of our world today but also holds the promise to reduce the cost associated with the treatment of industrial and agricultural wastewater. Resource recovery of SCP from organic waste by microbes like yeast and microalgae is commonly documented. However, recently, a class of phototrophic bacteria, purple non-sulphur bacteria (PNSB), has emerged as a favourable option in terms of both wastewater treatment and resource recovery. PNSB are metabolically versatile and tolerant to a wide range of conditions, hence their ability to thrive in diverse waste streams. Besides its rich protein content, PNSB contains other nutritionally valuable bioproducts like carotenoids, coenzyme Q10, 5-aminolevulinic acid, and pantothenic acid. Recent evidence also indicates that PNSB-based aquafeed enhances growth and boosts immunity in certain aquaculture trials. It does not possess the same toxicity as most gram-negative bacteria due to its comparatively less potent lipopolysaccharide composition. With diverse promising prospects of PNSB-based SCP, it is critical to extensively examine the landscape from a holistic standpoint, highlighting the potential challenges large-scale SCP production may pose. Thus, this review explores the comparative advantages of utilizing PNSB for SCP production, essential components of PNSB-based SCP processing, and possible environmental and economic gains associated with the process. Current challenges with PNSB-based SCP production and future outlooks are also examined.

Microbial conversion of CO2 and CO into chemicals is a promising route that can contribute to the cost-effective reduction of anthropogenic green house and waste gas emissions and create a more circular economy. However, the biotechnological valorization of CO2 and CO into chemicals is still restricted by the limited number of model microorganisms implemented, and the small profit margin of the products synthesized. This perspective paper intends to explore the genetic potential for the microbial conversion of CO2 and CO into ectoines, in a tentative to broaden bioconversion platforms and the portfolio of products from C1 gas fermentations. Ectoine and hydroxyectoine can be produced by microorganisms growing at high salinity. They are high-value commodities for the pharmaceutical and medical sectors (1000–1200 €/kg). Currently microbial ectoine production is based on sugar fermentations, but expansion to other more sustainable and cheaper substrates is desirable. In this work, a literature review to identify halophilic microbes able to use CO2 and CO as a carbon source was performed. Subsequently, genomes of this poll of microbes were mined for genes that encode for ectoine and hydroxyectoine synthesis (ectABCD, ask, asd and ask_ect). As a result, we identified a total of 31 species with the genetic potential to synthesize ectoine and 14 to synthesize hydroxyectoine. These microbes represent the basis for the creation of novel microbial-platforms that can promote the development of cost-effective and sustainable valorization chains of CO2 and CO in different industrial scenarios.

Digestate landspreading is a key aspect of the circular economy. However, organic matter (OM) stability in digestates is usually either poorly assessed or done through laborious methods. Spectroscopic methods are useful and easy to deploy alternatives to assess several aspects in anaerobic digestion (AD) studies such as process performance, waste classification and both OM composition and transformation. In these studies, a lack of agreement on analytical techniques, indicators and reference values is evident. This unclear scenario brings to the forefront the need for a meta-analytical study providing benchmarking values and trends. This review aimed to fill up this gap through the identification and evaluation of: (i) the most frequently applied techniques, their principles, deployment methods and limitations, (ii) the quantitative spectroscopic indices to define OM stability, (iii) the common trends of these parameters due to AD effect on the OM and (iv) the relevance of each technique based on the frequency of statistically significant results reported. Ultraviolet–visible and fluorescence spectroscopy have been identified as the most relevant techniques for aqueous phase study whereas mid-infrared and 13C cross-polarisation magic angle spinning nuclear magnetic resonance were the most appropriate for the solid phase. Their most applied indicators and their trends after AD have been summarised. Finally, the research studies that displayed statistically significant findings were described, the representativeness of the indices and the influence of sample preparation on their calculation were discussed and future research lines were suggested. Overall, this review demonstrates that spectroscopic methods provide relevant information for better digestate management.Graphical abstract

Water quality index (WQI) is one of the most used tools to describe water quality. It is based on physical, chemical, and biological factors that are combined into a single value that ranges from 0 to 100 and involves 4 processes: (1) parameter selection, (2) transformation of the raw data into common scale, (3) providing weights and (4) aggregation of sub-index values. The background of WQI is presented in this review study. the stages of development, the progression of the field of study, the various WQIs, the benefits and drawbacks of each approach, and the most recent attempts at WQI studies. In order to grow and elaborate the index in several ways, WQIs should be linked to scientific breakthroughs (example: ecologically). Consequently, a sophisticated WQI that takes into account statistical methods, interactions between parameters, and scientific and technological improvement should be created in order to be used in future investigations.

AbstractVarious industries around the world recognize the importance of achieving Net Zero Greenhouse Gases (GHG) emission by 2050. Industrial processes have been attributed to the increase in CO2 concentrations causing climate change. As the rate of emissions continues to increase, there becomes a pressing need to adopt new technologies that allow for a real shift towards emission reduction. In this paper, Carbon Capture (CC) technologies are explored and reviewed across various industries namely: cement, steel, ethanol and powerplants. A statistical representation of various CC implementations and facilities is presented with a supportive case study, including some evaluative discussions of energy demands, capture rates, and economic impacts of various technology adoptions. Moreover, the paper includes a comparative data analysis of various solvents and configurations used for carbon capture, to show their effectiveness in reducing CO2 and cost of implementation. The paper also discusses some crucial limitations and challenges of CC, including economic and technical aspects. However, with the success seen in CC applications across various industries, there remains scope for future work to address novel solvents, with reduced costs and minimum modifications to existing processes. A brief overview of life cycle assessment of carbon capture and storage is also presented.Graphical abstract

The Membrane bioreactor (MBR) technology enhances biomass retention with less residual sludge which is desirable for the treatment of strong wastewater such as landfill leachate. The technology has evolved to encompass anaerobic MBRs (AnMBRs) due to the benefit of producing renewable energy in the form of methane biogas and reducing the cost of aeration and sludge handling. This paper presents a critical review on the progress and application of the AnMBR technology for the treatment of landfill leachate in particular. We evaluate its performance and elaborate on potential operational limitations with emphasis on fouling and inhibitory factors that may suppress the development of microbial communities in AnMBR systems. We conclude by highlighting existing gaps and future directions and proposing a framework with a comprehensive experimental program towards improving the application of the AnMBR technology.

Manganese is extensively used in various advanced technologies. Due to high manganese demand and scarcity of primary manganese resources, extracting the metal from spent batteries is gaining increasing interest. The recycling of spent batteries for their critical metal content, is therefore environmentally and economically feasible. The conventional pyro- and hydrometallurgical extraction methods are energy-intensive or use hazardous chemicals. Bioleaching of manganese from spent batteries as secondary resource has been suggested to meet two objectives: reduce environmental footprint and turn waste into wealth. A bioleaching process can operate with less operating costs and consumption of energy and water, along with a simple process, which produces a reduced amount of hazardous by-products. Hence, this review discusses various approaches for bioleaching manganese from secondary resources using redoxolysis, acidolysis, and complexolysis. Candidate microbes for producing inorganic and organic biolixiviants are reviewed, along with the role of siderophores and extracellular polymeric substances as other effective agents in manganese extraction. The three main types of bioleaching are discussed, incorporating effective parameters with regard to temperature, pH, and pulp density, and future perspectives for manganese bioleaching and provided.Graphical abstract

In the past few decades, pollution from microplastics has emerged as an important issue on a global scale. These plastic particles are mainly the result of anthropogenic activities. Urban sprawl, industrialization, indiscriminate use and poor waste management of plastic products are the main factors responsible for the accumulation of microplastics in different ecosystems of the environment. The presence of microplastics in the soil matrix is considered an emerging threat to agroecosystems. Since most of the studies on microplastics have been done in the aquatic environment. The understanding of the ecotoxicological effects of these contaminants in terrestrial ecosystems is still limited, especially in agroecosystems. The negative effects of microplastics on the physical, chemical and biological properties of soil are now revealing. But the effects of microplastics on plant growth and yield are largely unexplored. Microplastic contamination in the soil can alter the functioning of plants by affecting the microbial community of the rhizosphere and disturbing the homeostasis of the agroecosystem. Furthermore, it may transfer into the plant system through nutrient and water absorption channels and affect plant physiology. The pervasive nature of microplastics in the soil is considered a barrier to sustainable agriculture and ecosystem functioning. The present review gives an overview of the sources, dissipation and effects of microplastics with reference to the soil–plant system, highlights the research gaps, and deciphers the possible future threats to agroecosystems. Graphical abstract

Soil pollution by potentially toxic elements (PTEs) is a serious threat to human health and ecosystem function. Soil washing using EDTA is one of the permanent disposal options available to remove PTEs from soil. Based on published studies, this paper summarized the current progress of remediation techniques using EDTA to mobilize and remove PTEs from contaminated soils. Firstly, the key factors to control EDTA washing were discussed, such as the concentration of EDTA, the pH of the washing solution, the washing time, multiple washing, and the liquid/solid ratio. Afterwards, the complicated changes in soil properties after washing were discussed. The change of soil properties is inevitable, so some measures need to be taken to reduce the damage to the soil by washing. Finally, the current improvements were summarized for the problems existing in the EDTA washing process. EDTA could be used in combination with other agents such as other chelating agents, reducing agents, acid compounds, and surfactants to improve efficiency of EDTA washing. The purpose of this paper is to provide a reference for further research and practical application of remediation of PTEs contaminated soil by EDTA washing.Graphic abstract:

The growing global population and higher living standards instantly demand the transition in the direction of a sustainable food system. A substantial section of means and agricultural lands are presently committed to protein-rich feed production to rear livestock for human consumption. Conversely, accelerated farming activities and the food industry have rendered a drastic increase in waste which impair the economic and environmental sustainability of the ecosystem. This situation emerges the need for developing an integrated technology for waste management and to improve sustainability footprints. Microbial protein (MP) production based on renewable electron and carbon sources has the potential as a substitute protein source. MP production for animal feed use is growing fast and is derived from bacteria, algae, and fungi including yeast. MP produced from all types of microbes is currently commercialized and in use. However, novel methods and processes are also under investigation to make MP production more economical and sustainable. Current research on MP has concentrated on the valorization of waste materials by using high protein content-containing microorganisms, which can then be used in animal feed. Using such kind of integrated approach, the agroindustry waste resources upcycling can contribute towards finding sustainable, cheaper, and environment-friendly protein sources. This review first describes the potential waste feedstock for MP production and summarizes the recent progress in the application of MP-producing microorganisms including fungus, yeast, bacteria, and phototrophic microbes. Bioprocesses, and production technology advances for MP production have been explored and discussed in detail. Finally, the MP application as animal feed, its challenges, and future perspectives in research have been evaluated.

Renewable resources have gained significant attention and are being evaluated as a base material for developing biodegradable packaging materials. A massive quantity of agricultural/food processing waste has been employed alone or in combination to make biodegradable packaging material with good barriers and functional capabilities. Onion is extensively used in dishes and condiments preparation. To meet consumer demand, the onion processing industry focuses on onion-based value-added products such as peeled onion, onion paste, onion powder, etc., that generate a considerable amount of solid waste (OSW). This OSW cannot be fit for animal fodder, landfill, and incineration due to their high carbon content and phytopathogens. OSW was found to be explored in the packaging sector because of high cellulose, lignin, and polyphenols. The biopolymers and pigments in OSW can be effectively used in active/smart packaging, which will aid in monitoring packaged food quality. The pigments and bioactive components in OSW can serve as indicators, maintaining the freshness of packaged foods in real-time by visual examination and reducing food loss and waste. The present review provides an approach of OSW for packaging material development and their potential application for food quality monitoring.Graphical Abstract

As the demand for metal resources increases and the quality and availability of rich ore resources decline, the focus is shifting to low-cost, eco-friendly bioleaching technologies that can effectively utilize low-grade minerals. The stress on bioleaching microorganisms due to high concentrations of metal ions in the bioleaching system is one of the most important factors limiting the effectiveness of bioleaching. Using the common copper-bearing ore bioleaching process as an example, the copper ion concentration reached 6 g/L. An in-depth examination of the copper defensive mechanism of bioleaching microorganisms will help elucidate the physiological mechanism of such extremophiles and pave the way for future genetically engineered highly efficient strains. We elaborate on the copper tolerance mechanism of extremophiles through the lens of biofilms, cell membranes, metal transport mechanisms, intracellular buffer mechanisms, and energy metabolism.

Thiosulfate is a lixiviant with potential applications for extraction of precious metals with lower environmental impact. As an alternative leaching reagent to cyanide, thiosulfate has promising gold extraction efficiency with much lower risk to operators and the environment. Thiosulfate is often produced at high temperatures via processes utilizing sulfide or sulfur and an oxidant. However, certain microorganisms can produce thiosulfate as the final product of their metabolism. This represents potential for lower emissions and costs in the manufacture of gold leaching reagents. Biotechnological applications of these processes have not been reported in the past and need to be investigated in depth. This review serves as a study of microorganisms to collect and analyze the reported species for potential utilization of biogenic thiosulfate in industrial applications, with a specific focus on precious metals extraction. Bacteria were identified and compared with respect to thiosulfate producing ability, feasibility for the mining industry, and cost of substrates. The future applications of biogenic thiosulfate and further direction of research on the topic have been identified.

Although coastal ecosystems such as mangroves have substantial productive and protective rules, this ecosystem is threatened due to inorganic and organic contaminants including polycyclic aromatic hydrocarbons (PAHs). PAHs are lipophilic, persistent, carcinogenic, mutagenic and considered as a global concern. We reviewed the occurrence, distribution and sources of PAHs in the mangrove ecosystem, providing a comprehensive discussion on this information and giving recommendations for future research. Through systematic literature search, this review considered existing studies on PAHs in the different compartments (water, sediment, aquatic fauna and plants) of mangrove system collected from field investigations. Little information is available for the levels and sources of PAHs in the water compartment of the mangrove systems. PAHs in the mangrove sediments are reported for 18 countries, and most of the levels of PAHs in mangrove sediments are considered as being low (0—100 ng g−1 dry weight, DW) to moderate (100–1000 ng g−1 DW). Different diagnostic ratios have been applied in order to determine the potential source of PAHs in the mangrove sediments, that are mainly attributed to mixed sources (pyrogenic and petrogenic). Studies have documented the biomonitoring of PAHs in mangrove systems, the majority of which use bivalves. Additionally, there are published studies for PAHs levels in 12 species of mangrove plants; showing a general tendency of residual PAHs accumulation in the leaves, if compared to root samples (leaves > roots). As a result of atmospheric PAH accumulation in leaf surfaces, leaves have higher concentrations of PAHs; implying that mangrove leaves can be used to monitor air quality relative to PAH pollution in coastal environments. This review has implications for future research in this field as well as coastal environmental management.Graphical abstract

The physical/chemical abatement of gas pollutants creates many technical problems, is costly and entails significant environmental impacts. Biological purification of off-gases is a cheap and ecologically safe way of neutralization of gas pollutants. Despite the recent advances, the main technological challenge nowadays is the purification of volatile organic compounds (VOCs) of hydrophobic character due to their low solubility in water. Among all known biological methods of air purification, the most cost-effective biodegradation of hydrophobic VOCs is conducted by biotrickling filters. In this context, fungi have gained an increasing interest in this field based on their ability to biodegrade hydrophobic VOCs. In addition, biotrickling filtration using fungi can support a superior hydrophobic VOC abatement when compared to the bacterial biofilters. This paper aims at reviewing the latest research results concerning biocatalytic activity of fungi and evaluating the possibilities of their practical application in biofiltration systems to remove hydrophobic VOCs.

Iron (Fe) oxides can rapidly recrystallize in the presence of aqueous Fe(II) (Fe(II)aq) under anoxic conditions. Since different Fe oxides have diverse affinities and redox reactivities for metal(loid)s, nutrients, and organic matters, recrystallization of Fe oxides significantly alters their speciation and environmental behavior. Therefore, the major reaction steps, rates, and influencing factors of Fe(II)aq-catalyzed recrystallization have gained increasing attention. This paper aims to review the latest advances, especially in redox cycling between Fe(II)aq and Fe oxide and in the kinetics of Fe atom exchange. The mineralogical recrystallization pathways and intermediate processes of different Fe oxides when exposed to Fe(II)aq are discussed. The influencing factors such as morphological natures of Fe oxides and typical environmental substances governing the kinetics of isotopic exchange between Fe(II)aq and Fe oxides are summarized. Several major analytical methodologies in this realm are also illustrated. Finally, some unsolved issues and future research directions in the field of Fe(II)aq-catalyzed Fe oxide recrystallization are outlined.Graphical abstract

Grape is one of the most well-known fruits worldwide, being transformed into distinct food products, one of which is wine. A significantly large portion of grape production is exclusively destined for winemaking which, as an industrial process, produces significant amounts of by-products. Grape pomace is considered the major by-product of winemaking and consists of skins, seeds and stems that are left behind after the stage of grapes’ pressing. According to the Waste Framework Directive (Directive 2008/98/EC) of the European Parliament, “waste prevention should be the first priority of waste management and re-use and material recycling should be preferred to energy recovery from waste”. This review aims to approach the recent advances over the valorization of grape pomace as a value-adding material, exploring its potential uses in the industry. The main methods for this purpose are discussed, including traditional methods (such as distillates production, animal feed, and soil fertilizer), its use as a technological aid in different industrial processes (e.g., adsorption, immobilization, food packaging and cosmetics), its addition in food products, highlighting its potential effects on physicochemical, functional and sensory attributes of these products, and as a raw material for bioenergy production. The large number of strategies that have been employed for the possible use of grape pomace show its high revalorization potential, with studies being conducted on several different food products of different categories, such as dairy, meat, fish and bakery products or even wine itself. The main positive effect of this addition consists the increase of the antioxidant activity of the medium, contributing towards enhanced shelf-life, both microbiologically and physicochemically. Given the fact that a linear production model is not satisfactory for sustainable development and focus has been shifted towards circular models, it appears that grape pomace consists a major value-adding component that can promote such economy schemes, as it can be reutilized in numerous ways across a number of industrial categories, including the winery that it came from. However, this study highlighted also the absence of a comprehensive legislative framework covering the addition of grape pomace in foods, of studies analyzing completely the cost of grape pomace revalorization, and of specific research on green/food-grade extraction methods of valuable compounds from grape pomace.Graphic Abstract

Remediation of persistent organic pollutants in soil especially total petroleum hydrocarbon (TPH) is of global concern due to its toxicity and health implications. Soil flushing has been considered a promising technique among in-situ technologies for treating non-volatile TPH-contaminated soils because it weakens the interaction between hydrocarbons and soil particles to enhance pollutant mobilization efficiency. It is still challenging to optimize the soil flushing treatment because the overall efficacy significantly depends on the environmental characteristics of the subsurface. Advanced soil flushing strategies (e.g., integrating with oxidation, air sparging, and nanoparticles) and novel flushing solutions are discussed to overcome the limitations of the existing process during the remediation of soil systems contaminated with recalcitrant TPH. The flushed-out toxic chemicals comprise a large amount of waste solution, creating another pollutant. The present review summarizes the enhanced soil flushing techniques, and critically discusses their advantages and disadvantages, and addresses follow-up remediation of the generated wash solution containing toxic substances for its safe discharge. Fundamental information on soil flushing is discussed to overcome the challenges encountered during field application such as poor efficiency, high operating cost, and a large amount of generated secondary wastewater. Graphical abstract

Plastic pollution is a global concern due to the long half-life and high resistance of many synthetic plastics to natural biodegradation. Therefore, great effort is required to avoid littering. However, the challenge of managing the ever-increasing quantities of plastic waste is daunting. The biodegradation of synthetic plastics, such as polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polyurethane (PUR) by microorganisms is either slow or under investigation as to whether it occurs at all in different environmental niches (e.g., soil, aquatic systems). There is an urgent need to complement the existing knowledge on the biodegradation and biotransformation of synthetic plastics to enable effective bioremediation strategies to mitigate the effects of environmental plastic contamination. Therefore, the aim of this review is to highlight current fundamental and applied research regarding the most promising biodegradation processes for synthetic plastics, the synthesis and applications of the most effective plastic-degrading enzymes, successful biotechnological strategies to improve degradation, such as enzyme engineering and novel reactor designs, and plastic waste bioconversion into value-added products. In addition, this review is intended to depict indications for techno-economic analyses toward the valorization of plastic biodegradation processes and the environmental impacts of synthetic plastic biodegradation. Combining strategies, such as enzymatic plastic degradation followed by microbial biotransformation with the broad array of available pretreatment methods and abiotic factors, can contribute, under confined conditions, to the end-of-life utilization of plastics, consequently leading to more efficient biorecycling processes, and hence, to a circular plastic economy.