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The agricultural sector faces various challenges and barriers in transitioning from linear resource consumption to a circular economy. Based on the literature, four key areas of challenges are identified and discussed, conversion technology and research, business model and material flow in the supply chain, analytical tools for the circular economy, and stakeholder involvement. Among the reported studies, anaerobic digestion (AD) is a commonly-used organic waste conversion technology due to its applicability and cost-effectiveness. The success of AD depends on feedstock properties and operating conditions, which impact the quality and quantity of the digestate and biogas produced. Additional processes such as pretreatment, pyrolysis, and hydrothermal liquefaction can enhance biogas production and generate valuable byproducts. However, challenges remain in terms of feedstock composition, high operating temperatures, and the requirement for suitable infrastructure. The business model and material flow in the supply chain demonstrate another set of challenges. Efficient business models are essential for the circular economy in the agricultural sector. Complex circular business models, financial uncertainties, and stakeholders impede effective implementation. The establishment of a sustainable supply chain that ensures the availability of feedstock and byproducts in synchronization with demand is crucial. Suitable infrastructure, management expertise, and continuous training are also of specific importance. Analytical tools play a vital role in feasibility assessment for the circular economy. Life cycle assessment (LCA) is commonly used to quantify environmental impacts, and the related challenges include the limited availability of relevant data and the need to account for feedstock characteristics. Economic evaluation is another useful evaluation tool, but its application to agricultural waste management is limited. Stakeholder involvement, including government, farmers, and consumers, is another key to the success of a circular economy. Governments can incentivize the use of waste-derived products through policies and regulations. Farmers' willingness to adopt circular practices depends on their location, legislative rules, and available incentives. Overcoming these barriers requires collaborative efforts, innovative technologies, and effective communication among stakeholders. The integration of conversion technologies into existing infrastructure, designing sustainable supply chains, and developing appropriate analytical tools are key steps toward a successful agricultural circular economy. [Display omitted] • Transitioning to a circular economy in agriculture faces challenges and barriers. • Business models, analytical tools, and stakeholder involvement are critical. • AD is a widely used technology, but feedstock properties pose challenges. • Sustainable business models and supply chains are crucial for circular agriculture. • Collaboration, technologies, and policies are needed to overcome barriers. [ABSTRACT FROM AUTHOR]

The integrated system of astaxanthin and microalgal biomass production, and wastewater treatment is a promising process. Haematococcus pluvialis is a suitable microalgae for this coupled system to achieve this dual purpose in a single approach. In this study, biomass production and nutrient removal from bioethanol plant wastewater by H. pluvialis were investigated. An air-lift photobioreactor was used to utilize the wastewater as well as a CO2-enriched gas supply. The maximum biomass density and maximum specific growth rate achieved were 4.37 ± 0.007 g/L and 0.317 day−1, respectively, by culturing H. pluvialis in an air-lift photobioreactor, supplemented with 5% CO2 in air. Removal of 91.7% total nitrogen and 100% total phosphorous from the wastewater was achieved. The residual microalgal biomass, obtained after astaxanthin extraction (1.109 ± 0.009 mg/g DW), was characterized as a potential bioenergy feedstock due to its elevated higher heating value 15.6 ± 0.01 MJ/kg. Hence, integrating H. pluvialis cultivation with bioethanol effluent serves the combined purpose of wastewater treatment, CO2 utilization, and simultaneous production of astaxanthin, which has a potential market value of over USD 2000 per kg, and carbohydrate rich microalgal biomass, which can be applied to bioenergy production. [ABSTRACT FROM AUTHOR]

The rapid proliferation of cyanobacteria in bodies of water has caused cyanobacterial blooms, which have become an increasing cause of concern, largely due to the presence of toxic secondary metabolites (or cyanotoxins). Cyanotoxins are the toxins produced by cyanobacteria that may be harmful to surrounding wildlife. They include hepatotoxins, neurotoxins and dermatotoxins, and are classified based on the organs they affect. There are also non-toxic secondary metabolites that include chelators and UV-absorbing compounds. This paper summarizes the optimal techniques for secondary metabolite extraction and the possible useful products that can be obtained from cyanobacteria, with additional focus given to products derived from secondary metabolites. It becomes evident that the potential for their use as biocides, chelators, biofuels, biofertilizers, pharmaceuticals, food and feed, and cosmetics has not yet been comprehensively studied or extensively implemented. [ABSTRACT FROM AUTHOR]

Coronavirus disease (COVID-19) has led to increasing demand for single-use plastic which aggravates the already existing plastic waste problem. Not only does the demand for personal protective equipment (PPE) increase, but also people shift their preference to online shopping and food delivery to comply with administrative policies for COVID-19 pandemic control. The used PPEs, packaging materials, and food containers may not be handled or recycled properly after their disposal. As a result, the mismanaged plastic waste is discharged into the environment and it may pose even greater risks after breaking into smaller fragments, which was regarded as the source of secondary microplastics (MPs, < 5 mm) or nanoplastics (NPs, < 1 μm). The main objective of this manuscript is to provide a review of the studies related to microplastic release due to pandemic-associated plastic waste. This study summarizes the limited work published on the ecotoxicological/toxicological effect of MPs/NPs released from PPE on aquatic organisms, soil organisms, as well as humans. Given the current status of research on MPs from COVID-related plastic waste, the immediate research directions needed on this topic were discussed. [Display omitted] • COVID-19 pandemic-generated plastic is adding to the worldwide plastic pollution. • Limited awareness of pandemic-generated plastic as a source of microplastic. • Risk mitigation measures include the use of eco-friendly plastic. • Include mandatory regulation regarding single-use plastic in waste management plan. • Microplastic risk to soil biota and human health needs extensive research. [ABSTRACT FROM AUTHOR]

• Ultrasound intensifies the recovery of secondary metabolites from microalgae. • Ultrasound aids cell wall disruption and enhanced release of secondary metabolites. • Ultrasonicated algal extracts tested effective for root-knot nematode suppression. Root-knot nematodes are one of the plant damaging nematodes in agriculture causing a projected annual yield loss of ∼12 % (∼$160 billion) worldwide. Conventional solutions to control these plant-parasitic nematodes involve chemical nematicides. To reduce the use of harmful chemicals, microalgal extracts can be used as greener alternatives for nematode management. Microalgae produce valuable metabolites, including cyanotoxins which can aid in nematode suppression. In this study, two microalgae species, Trichormus variabilis and Nostoc punctiforme , were treated with ultrasound for intensified recovery of secondary metabolites. Ultrasound results in cell wall disruption of the microalgal species, thus resulting in enhanced release of secondary metabolites. Microalgal biomass was treated with an ultrasound probe at 50 % amplitude, 20 kHz frequency, using water as the extraction medium, for 5–30 min. The extraction efficiency was determined in terms of the total chlorophyll (Chl) content of the extract. Microscopic images of the treated cells were also investigated to gain insight into the effect of the ultrasonication time on the cell morphology. Our results suggest that ultrasonication resulted in the intensified release of secondary metabolites, as established through the total chlorophyll content of the ultrasonicated microalgal samples as well as the microscopic images of the ruptured cells. The best extraction for Trichormus variabilis was achieved with 15 min extraction time where the Total Chl content increased by 29 times (compared to the non-ultrasonicated sample), and for the Nostoc punctiforme , 30 min extraction time gave the highest metabolite recovery of 6.4 times higher than the non-ultrasonicated sample. Ultrasonicated algal extracts were then tested for their nematicidal potential against root-knot nematode, Meloidogyne hapla, in infested field soil samples. Experimental study was conducted using different concentrations of each microalga, Trichormus sp. and Nostoc sp., individually, as well as in combination. The nematode count for the treated soil was compared with that of the control (untreated soil). Ultrasonicated microalgal extracts showed 66% to 100% inhibition on root-knot nematodes in the soil samples tested. [ABSTRACT FROM AUTHOR]

The objectives of this study were to evaluate bioethanol wastewater streams (thin stillage, process condensate and scrubber/dryer samples) as potential 'green' media to culture microalgae Haematococcus pluuialis, and to assess the effectiveness of ultrasound in microalgal cell disruption and astaxanthin extraction. H. pluuialis was cultured in wastewater dilutions (20-80 times) and formulations. Astaxanthin was extracted using ultrasound with cell-disrupting chemicals (acetic acid, citric acid, hydrogen peroxide and sodium hydroxide) and astaxanthin extracting solvents (acetone, ethanol and methanol) at different extraction durations (5-35 min). The results demonstrated that 60 times as the optimum dilution factor of thin stillage for microalgal growth, and a formulation known as GroAst media (60% of 60 times diluted thin stillage, and 40% of process condensate) was proposed as the best media for astaxanthin production: 0.634 ± 0.009 mg/ml cyst density, a 10.5% improvement over standard media. Astaxanthin extraction using ultrasound intensified the process in terms of short extraction time (25 min), less chemical consumption (2 M NaOH), usage of GRAS solvent (methanol), high efficiency in cell disruption, high extraction yield (80.6 ± 0.9%), and suitability to extract thermolabile astaxanthin. [ABSTRACT FROM AUTHOR]

Wollastonite is a natural silicate mineral that can be used as an agricultural soil amendment. Once in the soil, this mineral undergoes weathering and carbonation reactions, and, under certain soil and field crop conditions, our previous work has shown that this practice leads to accumulation of inorganic carbon (calcium carbonate). Mineral carbonation is the carbon sequestration approach with the greatest potential for sequestration capacity and permanency. Agricultural lands offer vast areas onto which such minerals can be applied, while benefiting crops. This work illustrates a technique to separate wollastonite-containing soils into different fractions. These fractions are characterized separately to determine organic and inorganic content, as well as to determine the chemical and mineral composition. The aim is to detect the fate of wollastonite in agricultural soils, and the fate of weathering/carbonation products in the soil. The soils used in the study were collected from soybean and potato farmlands in Southern Ontario, and from an experimental pilot plot. Soil fractionation was done using sieving, and soil fractions were analyzed by a calcimeter, X-ray diffraction, and loss-on-ignition. Acid digested samples were measured by Inductively Coupled Plasma Mass Spectrometry. Carbonates and wollastonite were enriched by fractionation. [ABSTRACT FROM AUTHOR]

Extreme climate change due to heat-trapping gases, especially carbon dioxide, necessitates its mitigation. In this context, the carbon dioxide sequestration technology of enhanced weathering has for years been investigated, with a possible implementation strategy via alkaline mineral soil amendment being more recently proposed. Candidate materials for enhanced weathering include calcium and magnesium silicates, most notably those belonging to the olivine, pyroxene and serpentine groups of minerals, given their reactivity with CO2 and global availability. When these finely crushed silicate rocks are applied to the soil, the alkaline earth metal cations released during mineral weathering gradually react with carbonate anions and results in the formation of pedogenic carbonates, which, over time, and under the right conditions, can accumulate in the soil. This review paper critically reviews the available literature on alkaline mineral soil amendments and its potential to sequester enough CO2 to be considered a climate change 'stabilization wedge'. Firstly, evidence of how agricultural soil can serve as a carbon sink in discussed, based on the observed accumulation of inorganic carbon in alkaline mineral-amended soils. Secondly, the impact of alkaline minerals on agricultural soil and crops, and the factors determining the rate of the weathering process are assessed. Lastly, the CO2 sequestration potential via alkaline mineral soil amendment is quantified according to an idealized shrinking core model, which shows that it has the potential to serve as a climate change stabilization wedge. [ABSTRACT FROM AUTHOR]