Assessing the addition of mineral processing waste to green waste-derived compost: an agronomic, environmental and economic appraisal

Changes in vineyard soil parameters after repeated application of organic-inorganic amendments based on spent mushroom substrate

The changes of physicochemical and biochemical parameters of a silty loam (S1) and sandy loam (S2) vineyard soils added with spent mushroom substrate (SMS) or SMS composted with ophite (OF) as rock dust (SMS + OF) were studied. Two doses of SMS or SMS + OF (25 and 100 Mg ha^-1) were applied for two consecutive years (2020-2021) and changes of soil physicochemical parameters, and dehydrogenase activity (DHA), respiration (RES), microbial biomass (BIO), and the phospholipid fatty acids (PLFAs) profile were assayed on a temporal basis. The results showed an increase in soil organic carbon (OC) content, total and mineralised N, P, and K, especially when the highest SMS dose was applied to soils. Repeated application caused OC content over time up to 2.3 times higher than initial content in the silty loam soil. This increase was not observed in sandy soil, possibly due to a higher bioavailability of OC, as indicated by the evolution of extractable humic acid/fulvic acid pools. In both soils, all biochemical parameters increased after amendment, being favoured both by the OC and by the presence of OF. Significant positive correlations were found between DHA, RES and BIO, and OC content especially in the first part and then levelled off after the second dose application. Total bacterial or fungal PLFAs patterns reflected the variation of BIO by SMS application. The higher growth of fungi vs. bacterial community in amended soils was recorded after the first SMS application, although the opposite effect occurred after the second application, with similar results in both soils. The findings indicate that the application of SMS or SMS + OF in vineyard soils could be an appropriate agronomic management practice for maintaining soil sustainability, although doses and application times of these amendments should first be evaluated depending on soil texture.

A review on ex situ mineral carbonation

The increased CO₂ quantities in the environment have led to many harmful effects. Therefore, it is very important to decrease the CO₂ levels in the environment. CO₂ capture along with safe and permanent storage using mineral CO₂ sequestration method can play an important role to reduce carbon emissions into the environment. Mineral sequestration is a stable storage method that provides long-term storage and an appropriate substitute for the more popular geological storage method. The process is most suited for places where there is a lack of underground cavities for underground geological storage. Minerals rich in Ca and Mg are used predominantly in carbonation reactions. In addition, those alkaline wastes that are rich in Mg and Ca such as cement waste, steel slag and many process ashes can also be employed in CO₂ sequestration. Mineral carbonation could be used for the sequestration of billions of tonnes of CO₂ every year. However, various drawbacks related to mineral carbonation still need to be addressed, such as resolving the slow rate of reactions, necessity of large amounts of feedstock, decreasing the high overall cost of CO₂ sequestration and reducing the huge energy requirements to accelerate the carbonation reaction. This study explores a number of carbonation methods, parameters that control the process and future potential applications of carbonated products.

Optimizing Inorganic Carbon Sequestration and Crop Yield With Wollastonite Soil Amendment in a Microplot Study

Carbon dioxide (CO₂) is a major greenhouse gas, and its concentration in the atmosphere is increasing continuously, hence there is an urgent need to reduce its level in the atmosphere. Soils offer a large natural sink to store CO₂. This study focuses on sequestering CO₂ in the agricultural soils as inorganic carbon, which can be accomplished by adding alkaline-earth silicates. Wollastonite is used in this study as a soil amendment, to sequester CO₂ via the geochemical route of mineral carbonation. The first objective of the present study was to evaluate the effect of mixing a wide range of dosages of wollastonite, as a soil amendment, on the growth performance of two leguminous plants frequently used in agricultural sector: soybean and alfalfa. The plants were grown with different wollastonite dosages (3-20 kg·m^-2 for soybean and 3-40 kg·m^-2 for alfalfa), for a duration of 14 weeks in a microplot experiment in Ontario, Canada. The second objective was to find evidence of enhanced weathering of wollastonite in soil, in addition to the augmentation of inorganic carbon content in soil. For this, mineralogical assessment of the soils was performed using XRD and SEM-EDS analyses. Wollastonite increased the soybean yield by two-fold in the plot amended with 10 kg·m^-2. At all dosages, wollastonite increased the alfalfa growth in terms of height and above-ground biomass dry weight, as well as root biomass. The rate of CO₂ sequestration, at optimum wollastonite dosage, reached 0.08 kg CO₂·m^-2·month^-1. XRD and SEM-EDS analyses indicated accumulation of calcite in wollastonite-amended soil and formation of other weathering products. The results obtained from this study help to understand the impact of wollastonite soil amendment on agronomy, and will aid in implementing such negative emissions technology by informing farmers and industry alike that the use of wollastonite contributes toward global climate change mitigation while supporting crop yield. The findings of this study add to the existing body of knowledge on enhanced weathering as an atmospheric CO₂ removal technology, providing further evidence that wollastonite weathering in agricultural soils can lead to significant capacity for CO₂ sequestration as inorganic carbon, while concurrently promoting plant growth.

Biochar incorporation increased nitrogen and carbon retention in a waste-derived soil

The synthesis of manufactured soils converts waste materials to value-added products, alleviating pressures on both waste disposal infrastructure and topsoils. For manufactured soils to be effective media for plant growth, they must retain and store plant-available nutrients, including nitrogen. In this study, biochar applications were tested for their ability to retain nitrogen in a soil manufactured from waste materials. A biochar, produced from horticultural green waste, was added to a manufactured soil at 2, 5 and 10 % (by weight), then maintained at 15 °C and irrigated with water (0.84 mL m^-2 d^-1) over 6 weeks. Total dissolved nitrogen concentrations in soil leachate decreased by 25.2, 30.6 and 44.0 % at biochar concentrations of 2, 5 and 10 %, respectively. Biochar also changed the proportions of each nitrogen-fraction in collected samples. Three mechanisms for biochar-induced nitrogen retention were possible: i) increased cation and anion exchange capacity of the substrate; ii) retention of molecules within the biochar pore spaces; iii) immobilisation of nitrogen through microbial utilisation of labile carbon further supported by increased soil moisture content, surface area, and pH. Dissolved organic carbon concentrations in leachate were reduced (-34.7 %, -28.9 %, and -16.7 %) in the substrate with 2, 5 and 10 % biochar additions, respectively. Fluorescein diacetate hydrolysis data showed increased microbial metabolic activity with biochar application (14.7 ± 0.5, 25.4 ± 5.3, 27.0 ± 0.1, 46.1 ± 6.1 µg FL g^-1 h^-1 for applications at 0, 2, 5, and 10 %, respectively), linking biochar addition to enhanced microbial activity. These data highlight the potential for biochar to suppress the long-term turnover of SOM and promote carbon sequestration, and a long-term sustainable growth substrate provided by the reuse of waste materials diverted from landfill.

Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth

To lock atmospheric CO₂ at anthropogenic timescale, fast weathering silicates can be applied to soil to speed up natural CO₂ sequestration via enhanced weathering. Agricultural lands offer large area for silicate application, but expected weathering rates as a function of soil and crop type, and potential impacts on the crops, are not well known. This study investigated the role of plants on enhanced weathering of wollastonite (CaSiO₃) in soils. Using rooftop pot experiments with leguminous beans (Phaseolus vulgaris L.) and nonleguminous corn (Zea mays L.), CO₂ sequestration was inferred from total inorganic carbon (TIC) accumulation in the soil and thermogravimetric analysis, and mineral weathering rate was inferred from alkalinity of soil porewater. Soil amendment with wollastonite promoted enhanced plant growth: beans showed a 177% greater dry biomass weight and corn showed a 59% greater plant height and a 90% greater dry biomass weight. Wollastonite-amended soil cultivated with beans showed a higher TIC accumulation of 0.606 ± 0.086%, as compared to that with corn (0.124 ± 0.053%). This demonstrates that using wollastonite as a soil amendment, along with legume cultivation, not only buffers the soil against acidification (due to microbial nitrogen fixation) but also sequesters carbon dioxide (12.04 kg of CO₂/tonne soil/month, 9 times higher than the soil without wollastonite amendment).

Impact of waste-derived organic and inorganic amendments on the mobility and bioavailability of arsenic and cadmium in alkaline and acid soils

In agriculture, more and more frequently waste-derived amendments are applied to soil to improve physical and chemical properties. Nevertheless, in soils polluted by potentially toxic metal(loid)s, this agricultural practice may significantly affect the mobility and bioavailability of pollutants modifying the risks for biota and human health. This work was aimed to assess the influence of poultry manure, biochar and coal fly ash on the mobility and bioavailability of As and Cd spiked in two Australian soils with different pH and texture: Mount Gambier (MGB)-alkaline sandy clay loam and Kapuda (KPD)-acid loamy sand. After 4 weeks of incubation from spiking and another 4 weeks from amendment addition, the soils were analysed for pH and amounts of As and Cd in pore-water and following 1 M NH₄NO₃ extraction. Bioavailable amounts were assessed by plant uptake, using Zea mays L. as test crop. In the alkaline MGB soil, the availability of Cd was reduced, while that of As increased. An opposite behaviour was observed in the acid KPD soil. All amendments, when added to KPD soil, increased pH and consequently reduced the mobility of Cd and increased the mobility of As. In MGB, the amendment addition had an effect only on As mobility and bioavailability, which increased likely as a result of the increased competition for adsorption with DOC released by organic compounds. These trends were confirmed by the amounts of Cd and As uptaken by maize plants.

Ecotoxicity of arsenic contaminated sludge after mixing with soils and addition into composting and vermicomposting processes

Sludge coming from remediation of groundwater contaminated by industry is usually managed as hazardous waste despite it might be considered for further processing as a source of nutrients. The ecotoxicity of phosphorus rich sludge contaminated with arsenic was evaluated after mixing with soil and cultivation with Sinapis alba, and supplementation into composting and vermicomposting processes. The Enchytraeus crypticus and Folsomia candida reproduction tests and the Lactuca sativa root growth test were used. Invertebrate bioassays reacted sensitively to arsenic presence in soil-sludge mixtures. The root elongation of L. sativa was not sensitive and showed variable results. In general, the relationship between invertebrate tests results and arsenic mobile concentration was indicated in majority endpoints. Nevertheless, significant portion of the results still cannot be satisfactorily explained by As chemistry data. Composted and vermicomposted sludge mixtures showed surprisingly high toxicity on all three tested organisms despite the decrease in arsenic mobility, probably due to toxic metabolites of bacteria and earthworms produced during these processes. The results from the study indicated the inability of chemical methods to predict the effects of complex mixtures on living organisms with respect to ecotoxicity bioassays.

Can macrophyte harvesting from eutrophic water close the loop on nutrient loss from agricultural land?

Eutrophication is a major water pollution issue and can lead to excessive growth of aquatic plant biomass (APB). However, the assimilation of nutrients into APB provides a significant target for their recovery and reuse, and harvesting problematic APB in impacted freshwater bodies offers a complementary approach to aquatic restoration, which could potentially deliver multiple wider ecosystem benefits. This critical review provides an assessment of opportunities and risks linked to nutrient recovery from agriculturally impacted water-bodies through the harvesting of APB for recycling and reuse as fertilisers and soil amendments. By evaluating the economic, social, environmental and health-related dimensions of this resource recovery from 'waste' process we propose a research agenda for closing the loop on nutrient transfer from land to water. We identify that environmental benefits are rarely, if ever, prioritised as essential criteria for the exploitation of resources from waste and yet this is key for addressing the current imbalance that sees environmental managers routinely undervaluing the wider environmental benefits that may accrue beyond resource recovery. The approach we advocate for the recycling of 'waste' APB nutrients is to couple the remediation of eutrophic waters with the sustainable production of feed and fertiliser, whilst providing multiple downstream benefits and minimising environmental trade-offs. This integrated 'ecosystem services approach' has the potential to holistically close the loop on agricultural nutrient loss, and thus sustainably recover finite resources such as phosphorus from waste.

Conjunctive and mineralization impact of municipal solid waste compost and inorganic fertilizer on lysimeter and pot studies

Objectives of the present study were to investigate the physico-chemical properties of municipal solid waste (MSW)-enriched compost and its effect on nutrient mineralization and subsequent plant growth. The enrichment of MSW compost by inorganic salts enhanced the humification rate and reduced the carbon nitrogen (C/N) ratio in less time than control compost. The chemical properties of compost, C/N ratio, humic acid, fulvic acid, degree of polymerization and humification index revealed the significant correlation amid properties. A laboratory-scale experiment evaluated the conjunctive effect of MSW compost and inorganic fertilizer on tomato plants in a pot experiment. In the pot experiment five treatments, Inorganic fertilizer (T1), enriched compost (T2), enriched compost 80% + 20% inorganic fertilizer (T3), enriched compost 60% + 40% inorganic fertilizer (T4) were defined including control (Ts), applied at the rate of 110 kg-N/ha and results revealed that all treatments significantly enhanced horticultural production of tomato plant; however T4 was most effectual as compared with control, T1, T2 and T3. Augmentation in organic matter and available phosphorus (P) potassium (K) and nitrogen (N) were also observed in compost treatments. The leachability and phytoavailability of phosphorus (P), potassium (K) and nitrogen (N) from sandy soil, amended with enriched, control compost and inorganic fertilizer at rates of 200, 400 and 600 kg-N/ha were evaluated in a lysimeter study. Results illustrated that concentration of mineral nitrogen was elevated in the leachate of inorganic fertilizer than enriched and control composts; therefore compost fortifies soil with utmost nutrients for plants' growth.

Food waste composting: its use as a peat replacement

We successfully co-composted catering waste with green waste and shredded paper to yield two high-nitrogen composts for use in horticulture. Sunflowers (Helianthus annuus L.) were grown in various mixtures of the compost and a commercially available peat-based compost to assess the efficacy of catering waste-based composts for peat replacement. Height, head diameter, seed mass and above-ground biomass were measured, with all mixtures giving a significant increase in yield or size over the commercially available peat-free control compost. We conclude that differences in physical structure governed sunflower growth over substrate chemistry, and none of the compost mixtures were nutrient deficient. We recommend that catering waste co-compost can be substituted to at least 75% within Sphagnum-based traditional growing media, providing a viable replacement for a large proportion of peat used as a growth medium in the horticulture industry. Our catering waste compost yielded similar seed head, seed mass and above-ground biomass values to 100% peat-based compost in all food waste compost blends tested in this study.

Critical evaluation of municipal solid waste composting and potential compost markets

Mechanical biological treatment (MBT) of mixed waste streams is becoming increasingly popular as a method for treating municipal solid waste (MSW). Whilst this process can separate many recyclates from mixed waste, the resultant organic residue can contain high levels of heavy metals and physical and biological contaminants. This review assesses the potential end uses and sustainable markets for this organic residue. Critical evaluation reveals that the best option for using this organic resource is in land remediation and restoration schemes. For example, application of MSW-derived composts at acidic heavy metal contaminated sites has ameliorated soil pollution with minimal risk. We conclude that although MSW-derived composts are of low value, they still represent a valuable resource particularly for use in post-industrial environments. A holistic view should be taken when regulating the use of such composts, taking into account the specific situation of application and the environmental pitfalls of alternative disposal routes.