Hydrothermal carbonization (HTC) is a thermochemical conversion technique which is attractive due to its ability to transform wet biomass into energy and chemicals without predrying. The solid ...product, known as hydrochar, has received attention because of its ability to prepare precursors of activated carbon in wastewater pollution remediation, soil remediation applications, solid fuels, and other carbonaceous materials. Besides the generally lignocellulose biomass used as sustainable feedstock, HTC has been applied to a wide range of derived waste, including sewage sludge, algae, and municipal solid waste to solve practical problems and generate desirable carbonaceous products. This review presented the critical hydrothermal parameters of HTC, including temperature, residence time, heating rate, reactant concentration, and aqueous quality. The chemical reaction mechanisms involved in the formation of hydrochar derived from single components and representative feedstock, lignocellulose, and sludge termed as N-free and N-rich biomass, were elucidated and summarized to better understand the hydrochar formation process. Specifically, hydrochar physicochemical characteristics such as surface chemistry and structure were investigated. Current knowledge gaps, and new perspectives with corresponding recommendations were provided to further exploit the great potential of the HTC technique and more practical applications for hydrochar in the future.
•Hydrothermal carbonization of biomass waste receives great deal attention.•Hydrothermal conditions during HTC process are critical to hydrochar production.•The chemical reaction mechanisms involved in hydrochar formation are reviewed.•Hydrochar physicochemical characteristics from biomass waste are summarized.
•A comprehensive review of recent oily sludge treatment methods is provided.•Oily sludge treatment is divided into oil recovery and sludge disposal approaches.•The advantages and limitations of each ...treatment method are discussed.•Issues of both petroleum hydrocarbons and heavy metals in oily sludge are discussed.
Oily sludge is one of the most significant solid wastes generated in the petroleum industry. It is a complex emulsion of various petroleum hydrocarbons (PHCs), water, heavy metals, and solid particles. Due to its hazardous nature and increased generation quantities around the world, the effective treatment of oily sludge has attracted widespread attention. In this review, the origin, characteristics, and environmental impacts of oily sludge were introduced. Many methods have been investigated for dealing with PHCs in oily sludge either through oil recovery or sludge disposal, but little attention has been paid to handle its various heavy metals. These methods were discussed by dividing them into oil recovery and sludge disposal approaches. It was recognized that no single specific process can be considered as a panacea since each method is associated with different advantages and limitations. Future efforts should focus on the improvement of current technologies and the combination of oil recovery with sludge disposal in order to comply with both resource reuse recommendations and environmental regulations. The comprehensive examination of oily sludge treatment methods will help researchers and practitioners to have a good understanding of both recent developments and future research directions.
Chlorinated volatile organic compounds (Cl-VOCs), including polychloromethanes, polychloroethanes and polychloroethylenes, are widely used as solvents, degreasing agents and a variety of commercial ...products. These compounds belong to a group of ubiquitous contaminants that can be found in contaminated soil, air and any kind of fluvial mediums such as groundwater, rivers and lakes. This review presents a summary of the research concerning the production levels and sources of Cl-VOCs, their potential impacts on human health as well as state-of-the-art remediation technologies. Important sources of Cl-VOCs principally include the emissions from industrial processes, the consumption of Cl-VOC-containing products, the disinfection process, as well as improper storage and disposal methods. Human exposure to Cl-VOCs can occur through different routes, including ingestion, inhalation and dermal contact. The toxicological impacts of these compounds have been carefully assessed, and the results demonstrate the potential associations of cancer incidence with exposure to Cl-VOCs. Most Cl-VOCs thus have been listed as priority pollutants by the Ministry of Environmental Protection (MEP) of China, Environmental Protection Agency of the U.S. (U.S. EPA) and European Commission (EC), and are under close monitor and strict control. Yet, more efforts will be put into the epidemiological studies for the risk of human exposure to Cl-VOCs and the exposure level measurements in contaminated sites in the future. State-of-the-art remediation technologies for Cl-VOCs employ non-destructive methods and destructive methods (e.g. thermal incineration, phytoremediation, biodegradation, advanced oxidation processes (AOPs) and reductive dechlorination), whose advantages, drawbacks and future developments are thoroughly discussed in the later sections.
•Chlorinated volatile organic compounds (Cl-VOCs) are ubiquitous contaminants.•The sources, human health impacts and remediation methods of Cl-VOCs are reviewed.•Future directions on risk and exposure level evaluations of Cl-VOCs are pointed.•State-of-the-art remediation technologies of Cl-VOCs are thoroughly discussed.
Exposure to mercury ions can damage the human brain, the nervous system, the endocrine system, and other biological systems. Much effort has therefore been made to develop real-time monitoring of ...mercury variations, and many mercury-ion sensors have been reported recently. In this review, mercury-ion sensors reported since 2008 are described and discussed. The sensors are classified as molecular, nanomaterial based, and others. Molecular sensors are based on chemical and hydrogen bond formation, and the other types are based on changes in the materials used.
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•Microalgae based constructed wetlands (CWs) are effective for swine wastewater treatment.•Duckweeds based CWs have advantages in swine wastewater (SW) treatment.•Characteristics of ...microalgae and duckweeds based CWs in SW treatment were reviewed.•Mechanisms of and factors affecting microalgae and duckweeds based CWs were discussed.•Challenges and future perspectives of microalgae and duckweeds based CWs were proposed.
Constructed wetlands for swine wastewater treatment have been one of the most exciting research topics. Usually hydrophytes based constructed wetlands could not adapt well to high concentration of ammonia nitrogen in swine wastewater, while microalgal and duckweed based constructed wetlands are promising for the nutrient removal. In this critical review, the important roles of microalgae and duckweeds played in wastewater treatment in constructed wetlands were first summarized. Performances including biomass growth, nutrient removal capacities and mechanisms of microalgal and duckweed based constructed wetlands were reviewed for swine wastewater treatment. Challenges for the applications of constructed wetlands including microalgal and duckweed based ones were discussed which includes a better understanding and utilization of synergistic effects among microalgae and duckweeds, difficulty and costs in harvesting biomass, applications in various field conditions including low temperatures, and selections of various types of microalgal and duckweed species. Future research needs were also proposed accordingly.
The rapid development of plastic industrials has created a variety of plastic products, causing revolutionary progress in chemistry, physics, biology, and medicine. Large-scale production and ...applications of plastics increase their possibility of entering the environment. Previous environmental impact studies typically focused on the toxicity, behavior and fate; limited attention was paid on greenhouse gas emissions and climate change. With the increase of plastic waste, the threat of plastic pollution to the earth’s climate has been gradually taken seriously. Evidence showed that greenhouse gas emissions occur at every stage of the plastic life cycle, including extraction and transportation of plastic raw materials, plastic manufacturing, waste treatment and entering the environment. The oil and gas industries used to make plastics are the main sources of greenhouse gas emissions (from the extraction of raw materials to the manufacture of plastics). Emissions of greenhouse gases during manufacture are mainly controlled by the production facilities themselves, usually depending on the efficiency, configuration and service life of equipment. Additionally, there are some unintended impacts, including transport requirements, pipeline leakage, land use, as well as impeding forests as natural carbons sinks. Recycling of plastic waste energy seems to be a good way to deal with waste plastics, but this process will release a lot of greenhouse gases. With this energy conversion occurring, the incineration of plastic packing waste will become one of the main sources of greenhouse gas emissions. Furthermore, plastics released into the environment also slowly release greenhouse gases, and the presence of (micro)plastics in the ocean will seriously interfere with the carbon fixation capacity of the ocean. In its current form, greenhouse gas emissions from cradle to grave of plastics will reach 1.34 gigatons per year by 2030 and 2.8 gigatons per year by 2050. This will seriously consume the global remaining carbon budgets, thereby threatening the ability of the global community to keep global temperatures rising by below 1.5 °C even 2 °C by 2100. In order to achieve this goal, the total global greenhouse gas emissions must be kept within the remaining carbon budget of 420–570 gigatons. The accumulative greenhouse gas emissions from cradle to grave of plastics may exceed 56 gigatons by 2050 (approximately accounting for 10%–13% of the total remaining carbon budget). As the plastic industry plans to expand production on a large scale, the problem will worsen further. The World Economic Forum forecasted that by 2030, the production and use of plastics will grow at an annual rate of 3.8%, and this growth rate will fall to 3.5% per year from 2030 to 2050. However, there are significant challenges and uncertainties in this estimation, and challenge and uncertainty factors come from all aspects. Recently, several organizations and researchers have started to discern the relationship between greenhouse gas emissions and plastic industrials, but relevant research on these impacts is still in its infancy. Consequently, the contribution of plastic pollution to greenhouse gas emissions and climate change should be given immediate attention and it needs to further explore the impact of plastic pollution on greenhouse gas emission and climate change. The implementation of measures to solve or alleviate the (micro)plastic crisis was critical necessary and proposed: (1) production control of global plastics; (2) improving the treatment and disposal of plastic waste; and (3) assessment of the impact of global environmental (micro)plastics on climate.
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•A crisis that plastic life cycle affects GHG emission and climate change is raised.•GHG emissions from cradle to grave of plastics will reach 1.34 Gt per year by 2030.•Accumulative GHG emission from cradle to grave of plastics may exceed 56 Gt by 2050.•GHG emissions from plastic life cycle seriously threaten remaining carbon budget.•Perspectives and challenges on plastic industry and policy are put forward.
Elevated salinity levels in wastewater usually have negative consequences to soil quality and crop growth if used directly in irrigation without proper treatment. Though many techniques have been ...developed to solve the problem, high capital costs or energy consumption have hindered their application in wider regions. One economical solution of reusing the wastewater of high salinity is applying natural or synthetic zeolites as ion exchanger and adsorbent. Chemically modified natural zeolites or synthetic zeolites have been utilized to reduce Na+ in various kinds of saline/sodic waters. In this paper, we reviewed the recent technical improvements of using natural or modified zeolites in the field of water salinity reduction. The mechanisms governing the salt removal process by zeolites are mainly ion exchange, adsorption, and salt storage. Factors such as zeolite's geochemical properties, pH, co-existing anions, concentration, valency, surface charge, and experimental conditions all influence the ion exchange process. Adsorption isotherm of Na+ ions on zeolites, mostly reported to follow either Langmuir or Freundlich isotherm, has a composition-dependent behaviour. The adsorption kinetics of Na+ on zeolites is mostly a pseudo-second-order type with an exothermic nature. Sodium removal by zeolites appears to be an effective water treatment technology for maximizing the beneficial use of poor-quality saline/sodic wastewater. However, challenges still remain and further work is required in areas of lowering operational cost and improving zeolite's regenerability. To overcome these challenges, researchers could make more efforts in technical improvements, including alterable surface properties and the incorporation of other approaches to achieve better salt removal outcome.
•Few review articles focus on zeolites for salinity/sodicity reduction in waters.•Mechanisms, influencing factors, challenges and opportunities are discussed.•Zeolites present great potential for salinity reduction in wastewater.
The pollutants classified as “persistent organic pollutants (POPs)”, are being subject to high concern among the scientific community due to their persistence in the environment. TiO2-based ...photocatalytic process has shown a great potential as a low-cost, environmentally friendly and sustainable treatment technology to remove POPs in sewage to overcome the shortcomings of the conventional technologies. However, this technology suffers from some main technical barriers that impede its commercialization, i.e., the inefficient exploitation of visible light, low adsorption capacity for hydrophobic contaminants, uniform distribution in aqueous suspension and post-recovery of the TiO2 particles after water treatment. To improve the photocatalytic efficiency of TiO2, many studies have been carried out with the aim of eliminating the limitations mentioned above. This review summarizes the recently developed countermeasures for improving the performance of TiO2-based photocatalytic degradation of organic pollutants with respect to the visible-light photocatalytic activity, adsorption capacity, stability and separability. The performance of various TiO2-based photocatalytic processes for POPs degradation and the underlying mechanisms were summarized and discussed. The future research needs for TiO2-based technology are suggested accordingly. This review will significantly improve our understanding of the process of photocatalytic degradation of POPs by TiO2-based particles and provide useful information to scientists and engineers who work in this field.
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•The limitations of TiO2-based technology for POPs degradation were discussed.•The approaches for improving the performance of photodegradation were summarized.•The mechanisms of various TiO2-based technologies for POPs removal were discussed.•The future research needs for TiO2-based technology are suggested.
Sediment can be applied on land as a soil conditioner. However, toxic substances such as heavy metals within the sediment often lead to soil contamination if no proper management is conducted prior ...to land application. In order to reduce the bioavailable portion of heavy metals such as Pb, Cu, Zn and Cd, zeolite as a kind of stabilizer was investigated on the effect of metal stabilization in sediment. Zeolite was firstly modified and screened to get the best condition for removal of heavy metals. Results showed that the granulated zeolite with NaCl conditioning had the highest CEC and metal sorption. Using BCR sequential extraction, the selected modified zeolite effectively stabilized Pb, Cu, Zn and Cd in sediment to different extents. It was most suitable for Cd stabilization by reducing its acid exchangeable fraction while increasing the contents of the reducible and residual fractions. Modified zeolite also immobilized Cu, Zn and Pb in sediment by enhancing one stable fraction while decreasing the acid exchangeable fraction.
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•Sediment stabilization prior to land use meets the demands of resource recovery and reuse.•NaCl modification greatly enhanced zeolite's CEC andmetal removals from solution.•Modified zeolite reduced the exchangeable fraction ofheavy metals in sediment, indicating a reduced mobility.•Modified zeolite most effectively stabilized Cd in sediment.
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Recently, Fe-based metal–organic frameworks (MOFs) have attracted increasing attention and been widely used. To date, however, it is unknown whether they can be employed to degrade ...tetracycline, one of the most widely used antibiotics. This work therefore aims to provide such support by comparing the performance of three Fe-based MOFs (namely, Fe-MIL-101, Fe-MIL-100, and Fe-MIL-53) in removing tetracycline. Experimental results showed that Fe-MIL-101 exhibited the best performance in tetracycline removal, with 96.6% of tetracycline being removed (initial tetracycline concentration at 50 mg/L) while Fe-MIL-100 and Fe-MIL-53 removed 57.4% and 40.6% under the same conditions. Additionally, the effects of adding dosage, adsorption time, and initial concentration of tetracycline on degradation efficiency were examined. It was found that the adsorption and photocatalytic degradation effect was better with the increase of time, the optimum dosage of Fe-MIL-101 was 0.5 g/L and the removal efficiency decreased with the increasing of initial tetracycline concentrations. Moreover, the trapping experiments and ESR tests indicated that O2−, OH and h+ were the main active species in photocatalytic degradation process of tetracycline. Due to its high removal efficiency and simple synthesis, it could be used as a potential catalyst for degradation of tetracycline and other antibiotics.
