Utilization of coal fly ash (CFA) and oyster shells (OS) for remediation of polluted coastal sediment

Abstract

Marine sediments serve as vital and dynamic reservoirs for both organic matter and mineral particles, playing a significant role in the biogeochemical cycling of elements within the Earth's oceans (Wang et al., 2018). These sediments create essential habitats for a wide variety of microbial communities, including bacteria, and they exert considerable influence on the chemical makeup of sediments and the surrounding water column (Wang et al., 2018).However, sediments rich in fine particles and organic matter have experienced degradation due to the build-up of contaminants, leading to reduced permeability and lower oxygen levels; thus, exacerbating anaerobic conditions within the sediment (Jeong & Kim, 2022). The coastal environment faces threats from rising levels of nutrients and pollutants stemming from terrestrial runoff related to industrial and agricultural wastewater, along with discharged municipal sewage (Jeong & Kim, 2022). This surge in nutrient loading within coastal ecosystems surpasses the concentrations necessary for ecosystem support (Jeong & Kim, 2022).The diminishing sustainability of coastal ecosystems, resulting from this unstable nutrient balance, triggers environmental issues such as harmful algal blooms and oxygen-depleted waters. To mitigate the declining health of coastal regions, regulations have been established to limit pollutant discharge, alongside an increase in wastewater treatment facilities (Jeong & Kim, 2022). Nonetheless, coastal environments remain susceptible to eutrophication (Pourabadehei, 2017), and the emergence of various environmental challenges persists (Jeong & Kim, 2022). One contributing factor to these environmental issues is the leaching of excessive nutrients trapped in coastal sediments (Jeong & Kim, 2022). Consequently, eutrophication, which refers to the escalation of aquatic primary production due to nutrient enrichment, has emerged as a significant concern for coastal regions worldwide (Patil et al., 2022). Eutrophication is associated with numerous coastal water problems, including reduced water clarity, increased oxygen demand in bottom sediments from heightened organic loading, declines in biodiversity, alterations in trophic dynamics, and the proliferation of unpleasant and harmful algal blooms (Patil et al., 2022). Overall, reducing nutrients in coastal sediments is crucial for alleviating eutrophication across permeable coastal areas. Several methods, such as dredging, have been suggested for the restoration of aquatic environments. Dredging is an effective technique for the rapid removal of contaminated sediment (Knox & Paller, n.d.), but it necessitates post-treatment of the excavated sediments and appropriate disposal sites(Chatterjee, 2017). Furthermore, dredging has been restricted to protect the benthic environment (Jeong & Kim, 2022). Various in-situ capping materials have been examined to address contaminated sediments, aiming to maintain a thriving ecosystem in coastal regions. Granulated coal ash (GCA) is an effective in-situ capping material (CM) in coastal ecosystems. However, cement included in GCA could cause environmental problems such as heavy metal elution. Therefore, a new approach is required to replace cement to minimize the possibility of adverse environmental problems.(Jeong & Kim, 2022). The industrial by-products, specifically Coal Fly Ash (CFA) and oyster shells (OS), were synthesized employing various mixing ratios. The formulation of OS-CFA incorporated granulated coal ash along with pyrolyzed oyster shells, resulting in the emergence of primary chemical constituents that are comparable to those found in cement. Coal Fly Ash (CFA) contains about 51.21% SiO2, while oyster shells (OS) consist of approximately 95.20% CaO (Jeong & Kim, 2022), which are essential components of cement. Coal Fly Ash (CFA) and oyster shells (OS) were chosen as both aggregate and binder for the development of capping material (CM). Globally, approximately 484 Mt of CFA and 3.08 Mt of OS are produced each year (Jeong & Kim, 2022). The OS-CFAs primarily consist of CaO, CO₃, CaCO₃, SiO₂, Al₂O₃, Fe₂O₃, and MgO, alongside quartz and aluminosilicate crystal phases. Subsequently, Coal Fly Ash (CFA) and oyster shells (OS) augmented the permeability coefficient of the sediment by sevenfold, transforming its characteristics from standard clayey silt to silt, in accordance with soil classification criteria based on permeability coefficients (Kim & Kim, 2020). Moreover, Coal Fly Ash (CFA) and oyster shells (OS) have also been utilized as CM to mitigate elevated concentrations of PO₄-P and NH₃-N. Consequently, OS-CFA demonstrates potential for phosphate absorption from wastewater due to its advantageous physical and chemical attributes, including its high specific surface area and calcium oxide content.(Kim et al., 2020). In addition, phosphate can be eliminated by Coal Fly Ash (CFA) and oyster shells (OS) via precipitation or adsorption mechanisms. However, only the adsorption process effectively facilitates phosphate removal when using acid-treated Granulated Coal Fly Ash (CFA) and oyster shells (OS). Ultimately, it was determined that Coal Fly Ash (CFA) and oyster shells (OS) serve as an efficient in-situ capping material for the remediation of eutrophic coastal marine sediments.