Badr Ali Mohamed, PhD

The COVID-19 pandemic not only has caused a global health crisis but also has significant environmental consequences. Although many studies are confirming the short-term improvements in air quality in several countries across the world, the long-term negative consequences outweigh all the claimed positive impacts. As a result, this review highlights the positive and the long-term negative environmental effects of the COVID-19 pandemic by evaluating the scientific literature. Remarkable reduction in the levels of CO (3 - 65%), NO(2) (17 - 83%), NO(x) (24 - 47%), PM(2.5) (22 - 78%), PM(10) (23 - 80%), and VOCs (25 - 57%) was observed during the lockdown across the world. However, according to this review, the pandemic put enormous strain on the present waste collection and treatment system, resulting in ineffective waste management practices, damaging the environment. The extensive usage of face masks increased the release of microplastics/nanoplastics (183 to 1247 particles piece(-1)) and organic pollutants in land and water bodies. Furthermore, the significant usages of anti-bacterial hand sanitizers, disinfectants, and pharmaceuticals have increased the accumulation of various toxic emerging contaminants (e.g., triclocarban, triclosan, bisphenol-A, hydroxychloroquine) in the treated sludge/biosolids and discharged wastewater effluent, posing great threats to the ecosystems. This review also suggests strategies to create long-term environmental advantages. Thermochemical conversions of solid wastes including medical wastes and for treated wastewater sludge/biosolids offer several advantages through recovering the resources and energy and stabilizing/destructing the toxins/contaminants and microplastics in the precursors.

Actual agricultural practices produce about 998 million tonnes of agricultural waste per year. Therefore, converting lignocellulosic wastes into energy, chemicals, and other products is a major goal for the future circular economy. The major challenge of lignocellulosic biorefineries is to transform individual components of lignocellulosic biomass into valuable products. Here we review lignocellulosic biomasses such as coffee husk, wheat straw, rice straw, corn cob, and banana pseudostem. We present pretreatment technologies such as milling, microwave irradiation, acidic, alkaline, ionic liquid, organosolv, ozonolysis, steam explosion, ammonia fiber explosion, and CO2 explosion methods. These methods convert biomass into monomers and polymers. For that, the concoction pretreatment methods appear promising.

Bauxite residue (BR) was tested as a low-cost microwave absorber and catalyst for microwave-assisted pyrolysis for producing phenolic-rich bio-oil, along with magnetic biochar that can be used as an adsorbent for capturing excess nutrients, such as phosphates, from eutrophic water or wastewater. Mixing BR alone with switchgrass (SG) did not enhance SG microwave absorption, while mixing 10 wt% clinoptilolite and 20 wt% BR (10Clino–20BR) with SG increased the heating rate by 382% in comparison with 10Clino alone, due to the formation of good microwave absorbers (i.e. maghemite and magnetite) on the biochar surfaces, confirming their synergistic effects. The addition of BR reduced the acidity of the bio-oil by up to 47%, and increased the production of aromatics and alkylated phenols by up to 180% and 319%, respectively, compared with 10Clino alone. Biochar produced from 10Clino–20BR had magnetic properties, which are very important for facilitating the separation of spent biochar from P-contaminated aqueous solutions in industrial applications. Using the magnetic biochar for phosphorus adsorption, the highest adsorption capacity of 64.4 mg PO43−/g (21 mg PO43--P/g) was measured for the biochar produced using 10Clino–20BR at pH 4, and the adsorption isotherm was fitted by the Freundlich model very well. The magnetic biochar showed a high adsorption capacity for phosphate removal. In addition, the P-loaded biochar could be marketed as a balanced fertilizer to increase soil fertility, converting BR into a valuable product, instead of a waste material.

Microalgae show growth and fatty acids (FAs) accumulation up to a certain level which is dependent on species and type of cultivation conditions. Application of abiotic stress on microalgae can induce nutrient uptake and enhance biocomponent accumulation.

Bioplastics are alternatives of conventional petroleum-based plastics. Bioplastics are polymers processed from renewable sources and are biodegradable. This study aims at conducting an environmental impact assessment of the bioprocessing of agricultural wastes into bioplastics compared to petro-plastics using an LCA approach. Bioplastics were produced from potato peels in laboratory. In a biochemical reaction under heating, starch was extracted from peels and glycerin, vinegar and water were added with a range of different ratios, which resulted in producing different samples of bio-based plastics. Nevertheless, the environmental impact of the bioplastics production process was evaluated and compared to petro-plastics. A life cycle analysis of bioplastics produced in laboratory and petro-plastics was conducted. The results are presented in the form of global warming potential, and other environmental impacts including acidification potential, eutrophication potential, freshwater ecotoxicity potential, human toxicity potential, and ozone layer depletion of producing bioplastics are compared to petro-plastics. The results show that the greenhouse gases (GHG) emissions, through the different experiments to produce bioplastics, range between 0.354 and 0.623 kg CO2 eq. per kg bioplastic compared to 2.37 kg CO2 eq. per kg polypropylene as a petro-plastic. The results also showed that there are no significant potential effects for the bioplastics produced from potato peels on different environmental impacts in comparison with poly-β-hydroxybutyric acid and polypropylene. Thus, the bioplastics produced from agricultural wastes can be manufactured in industrial scale to reduce the dependence on petroleum-based plastics. This in turn will mitigate GHG emissions and reduce the negative environmental impacts on climate change.

Biochar is a solid material obtained from the pyrolytic carbonization of biomass in an oxygen-free/limited environment. Sulfur-containing biochar has a wide range of applications, such as adsorptive removal of pollutants (e.g., Hg, Cd, and Ni) and acting as a solid acid catalyst or as an electrode of Li-S battery. To date, many methods have been developed to strengthen the function of biochar by introducing sulfur-containing groups to promote the application and commercialization of biochar. This review aims to present the formation, analysis, engineering, and application of sulfur-functional groups in biochar. The sulfur-functional groups such as organic sulfur (e.g., C–S, –C–S–C–, CS, thiophene, and sulfone) and inorganic sulfur (e.g., sulfate, sulfide, sulfite, and elemental S) can be determined through Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES). The sulfur-functional groups can be obtained through selecting biomass composition, pyrolysis process, S-doping, and post-treatment of biochar, but the engineering is challenging. The positive effect of sulfur-functional groups in the application is also analyzed in this paper, such as the complexation and electron transfer between sulfur-functional groups and heavy metal (e.g., Hg, Cd, and Ni) on improving biochar adsorption capacity. However, there are still challenges in directional synthesis, precise determination, and regulation of application performance. Based on the research gaps identified, future prospective investigation directions on analysis, engineering, and application of biochar S-functional groups were presented in this review.

This study explores the recovery of resources and energy from sewage sludge through the production of sludge-based activated carbon (SBAC) considering circular economy principles. The SBAC production costs were estimated under three scenarios considering various sludge dewatering/drying schemes to determine the production feasibility and its role in the circular economy. SBAC was tested in the removal of a mixture of nine commonly detected poly- and perfluoroalkyl substances (PFASs) in environmentally relevant concentrations of ∽50 μg/L in comparison to commercially available activated carbon (AC) using 5 mg of sorbent and 5 mL of a nine-PFAS mixture in deionised water. SBAC can be produced at approximately 1.2 US $/kg, which is substantially lower than the average production cost of commercial AC of >3 US $/kg. A net revenue ranging from 2 to 7 US $/kg SBAC was estimated by recycling the produced non-condensable gases and bio-oil to produce energy and selling the SBAC. Batch adsorption tests showed that the PFASs removal of SBAC was superior to that of granular AC and similar to that of powdered AC, reaching >91% to below the detection limit. The kinetics tests revealed that adsorption by SBAC and AC occurred within 15 min. The overall results demonstrate the potential of SBAC as an effective sorbent for PFASs, achieving waste-to-resources circular economy via resource and energy recovery from sewage sludge, eliminating sludge disposal and contaminant-leaching to the environment, and in enhancing the quality of wastewater effluent before discharge.

This study aimed to determine the feasibility of pyrolysis scenarios as sewage sludge treatment processes through cradle-to-gate life-cycle assessment and additional energy consumption, carbon emission, and economic benefit analyses, considering circular economy principles. The examined pyrolysis scenarios include slow pyrolysis with various residence times to produce activated carbon (AC) or biochar and fast pyrolysis to produce bio-oil, all with internal gas energy recovery. The functional unit (FU) in this study comprises 1000 kg of dried sludge entering the pyrolysis reactor. The overall evaluation and new product application routes address gaps in current studies on sludge treatment via pyrolysis. Environmentally, the bio-oil (-0.31 kg CO2-eq/kg FU) and biochar (-0.05 kg CO2-eq/kg FU) scenarios show considerable improvement over contemporary pyrolysis and other conventional sludge treatment methods. The AC scenarios have higher toxicity but lower carbon emissions (1.50–1.70 kg CO2-eq/kg FU) than contemporary AC production processes. Chemical reagent usage has significant effects on the environmental burden of AC production processes. The biochar and bio-oil pyrolysis scenarios achieve net energy recovery through product applications. Although the AC scenarios still require energy input, this demand can be significantly reduced by optimising moisture removal processes. Operating cost analysis indicates that the examined pyrolysis scenarios are potentially profitable. Primary product yield and market value are significant factors determining the net profit of these pyrolysis scenarios, but further assessment of capital costs is required. This study shows that bio-oil and biochar pyrolysis are eco-friendly sewage sludge treatment methods.

This study aimed to determine the feasibility of pyrolysis scenarios as sewage sludge treatment processes through cradle-to-gate life-cycle assessment and additional energy consumption, carbon emission, and economic benefit analyses, considering circular economy principles. The examined pyrolysis scenarios include slow pyrolysis with various residence times to produce activated carbon (AC) or biochar and fast pyrolysis to produce bio-oil, all with internal gas energy recovery. The functional unit (FU) in this study comprises 1000 kg of dried sludge entering the pyrolysis reactor. The overall evaluation and new product application routes address gaps in current studies on sludge treatment via pyrolysis. Environmentally, the bio-oil (-0.31 kg CO2-eq/kg FU) and biochar (-0.05 kg CO2-eq/kg FU) scenarios show considerable improvement over contemporary pyrolysis and other conventional sludge treatment methods. The AC scenarios have higher toxicity but lower carbon emissions (1.50–1.70 kg CO2-eq/kg FU) than contemporary AC production processes. Chemical reagent usage has significant effects on the environmental burden of AC production processes. The biochar and bio-oil pyrolysis scenarios achieve net energy recovery through product applications. Although the AC scenarios still require energy input, this demand can be significantly reduced by optimising moisture removal processes. Operating cost analysis indicates that the examined pyrolysis scenarios are potentially profitable. Primary product yield and market value are significant factors determining the net profit of these pyrolysis scenarios, but further assessment of capital costs is required. This study shows that bio-oil and biochar pyrolysis are eco-friendly sewage sludge treatment methods.

Bauxite residue (BR) is a highly alkaline type of solid waste generated by the aluminum industry that poses a significant environmental risk upon disposal. However, BR is abundant in metals, especially iron, that offer the desired catalytic activity for microwave pyrolysis. Thus, this study aimed to use BR as a low-cost microwave absorber and catalyst for biomass microwave pyrolysis to obtain higher quality bio-oil and biochar. The addition of BR to switchgrass, the representative biomass, did not facilitate microwave absorption because most of the iron in the BR was in the form of goethite and hematite. However, the addition of an efficient microwave-absorbing catalyst (e.g., K3PO4 or clinoptilolite) to the BR triggered synergistic effects, increasing the microwave heating rate by ~ 346% compared to K3PO4 or clinoptilolite alone, which was attributed to the reduction of hematite and goethite to maghemite and/or magnetite. The addition of 10% BR to a mixture of 10% K3PO4 and 10% bentonite further triggered synergistic effects that resulted in the highest microwave heating rate of 439 °C/min, which was a 211% increase compared to using 10% K3PO4 and 10% bentonite without BR, and doubled the Brunauer-Emmett-Teller (BET) surface area of the biochar, reduced the bio-oil acidity by up to 71% compared to that obtained using a single catalyst, and increased the alkylated phenols contents in the bio-oil by 339% compared to that produced without a catalyst. These results demonstrated that the synergistic effects of BR can only be triggered when mixed with another efficient microwave-absorbing catalyst.

Pb, Ni, and Co are among the most toxic heavy metals that pose direct risks to humans and biota. There are no published studies on biochars produced at low temperatures (i.e., 300 °C), which possess high sorption capacity for heavy metal remediation and reclamation of contaminated sandy soils. This research studied the effect of catalytic microwave pyrolysis of switchgrass (SG) using bentonite and K3PO4 to produce biochar at low temperature (300 °C) with high sorption capacity for reducing the phytotoxicity of heavy metals, and investigated the synergistic effects of catalyst mixture on biochar sorption capacity. The quality of the biochars was examined in terms of their impacts on plant growth, reducing phytotoxicity and uptake of heavy metals in sandy soil spiked with Pb, Ni, and Co. All catalysts increased the micropore surface area and cation-exchange capacity of biochars, and resulted in biochars rich in plant nutrients, which not only decreased heavy metal phytotoxicity, but also boosted plant growth in the spiked soil by up to 140% compared to the sample without biochar. By mixing bentonite and K3PO4 with SG during microwave pyrolysis, the efficacy of biochar in reducing phytotoxicity and heavy metals uptake was further enhanced because of the highest micropore surface area (402 m2/g), moderate contents of Ca, Mg, K, and Fe for ion-exchange and moderate concentration of phosphorus for the formation of insoluble heavy metal compounds. Generally, the biochar created at 300 °C (300-30KP) showed similar performance to the biochar created at 400 °C (400-30KP) in terms of reducing heavy metal bioavailability.