Photocatalytic and Biological Activities of Engineered Maize Biochar to Remove Aqueous Pollutants, Immobilize Soil Metals and Improve Plant Growth

Abstract

Industrial and agricultural processes produce different types of dyes, metals, and pesticides in water and soil. Removal of these contaminants is essential to elude environmental and health issues. Biochar is a pyrolyzed black color mass containing a high amount of carbon, and it has been reported to enhance soil fertility and improve plant growth. In this study, ZnO-loaded maize biochar nanocomposite (MB-ZnO) and Trichoderma harzianum loaded maize biochar (MBT) have been synthesized and used to remove different aqueous pollutants, immobilize toxic metals of soil, and improve the growth of a fast-growing model plant (Sesbania sesban). In the first part of this study, biochar with unique physicochemical characteristics, like increased surface area, high carbon contents, and electron-conductive behaviors was synthesized from maize straw, using free ball-milling technology. Synthesized MB-ZnO was characterized via thermogravimetric analysis (TGA), ultraviolet-visible (UV) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy. The average size of the MB-ZnO nanocomposite was determined (43 nm) by means of the X-ray line broadening technique. FTIR spectroscopic results depicted the presence of different functional groups on MB-ZnO to confirm its reduction and successful formation. Successfully produced MB-ZnO was applied as a photocatalyst to degrade pollutants present in aqueous medium pollutants in different light conditions. The degradation potential (adsorption potential and photocatalytic performance) of MB-ZnO nanocomposite was analyzed against Mancozeb (MC) and Safranin O (Saf) in a closed container in the dark and under various light sources including visible and UV light. The reaction kinetics were calculated by applying the pseudo-first-order kinetic study model to elaborate the procedure of MC and Saf amputation from the solution. MB-ZnO composite showed different photocatalytic activities to degrade Saf invisible light (83.5 %), UV radiation (81.0 %), and dark environments (78 %) in a time duration of 60 minutes. The highest degradation of MC via MB-ZnO was determined in visible light (56.5 %), followed by UV radiation (27.5 %) and dark environment (25.2 %). These results proved that the prepared nanocomposite can be applied as an efficient catalyst to eliminate pollutants from aqueous medium, in both light sources and dark environments. In the second section of this study, MB-ZnO nanocomposite was assessed in different in vitro biological activities, including anti-inflammatory assay, biocompatibility activity against RBCs and macrophages, cytotoxicity assays of leishmanial parasites, enzymes inhibition assay against protein kinase, alpha-amylase, antifungal assay, and antioxidant assays. Results of this study revealed that MB-ZnO did not create any harm in the biocompatibility and the cytotoxic activity, and it achieved better in the antioxidant and anti-inflammatory assays. MB-ZnO nanocomposite produced modest enzyme inhibition and was more powerful against fungal pathogens. These findings directed that MB-ZnO might be useful as an efficient catalyst in different practices. This study has provided the latest and most valuable information to researchers and readers working on nanocomposites and biopolymers. This study was further extended to apply MB-ZnO nanocomposite for the control of a novel fungal disease of the Kiwi plant. Kiwi worked as an excellent natural source of vitamin C and has multiple uses. During the month of October-November 2021, tiny brown spots were observed on the leaves of Kiwi. For diagnosis, the affected leaf samples were collected and placed on potato dextrose agar (PDA) nutrient media. Morphological and anatomical characterization has shown this disease-causing pathogen to be Rhizopus oryzae. For the molecular study of the isolated pathogen, partial sequences of elongation factor (EF-1α) and inter-transcribed sequence (ITS) were amplified and sequenced. BLAST analyses of the resultant ITS sequence revealed >99% similarity with R. oryzae (MT603964.1), whereas the EF-1α sequence exhibited 100% resemblance with the elongation factor-1α gene of R. oryzae (MK510718.1). The attained ITS sequence was deposited to the NCBI database (MW603842.1). Koch’s postulates confirmed the pathogenicity of the isolated pathogen and verified that R. oryzae was the leaf spot pathogen of Kiwi. For eco-friendly control of Kiwi leaf spot, MB-ZnO nanocomposite was used. MB-ZnO nanocomposite revealed considerable mycelial growth inhibition and the maximum of 79% inhibition was examined at 19 mg/mL dose rate of MB-ZnO nanocomposite. These results highlighted the efficacious use of MB-ZnO for the control of plant pathogens. In the fourth part of this study, simple maize straw biochar (MB) and fungus (Trichoderma harzianum) loaded biochar (MBT) were used at various rates, for the immobilization of Cd and Cu from polluted soil. Throughout the remediation time of 90 days, the dynamic effects of MB and MBT on the physiochemical properties and function of the soil were observed. The findings of this study showed that the application of 5% MBT considerably increased soil pH at an early stage of incubation, which decreased later to a neutral-alkaline level. The application of MBT promoted residual bound Cu-Cd fractions and decreased carbonate and exchangeable bound fractions in the treated soil. These bindings reduce the bio-accessibility of plants, animals, and humans to heavy metals. The addition of MBT also enhanced catalase and urease activities of soil, in the later phase of the experiment, indicating the retrieval function of soil for the post-stabilization of metal. The findings of this research offered new understandings of the production of functional substances and skills for the sustainable reclamation of heavy metal-polluted soil with the amalgamation of biochar and functional microbes. In the last part of this study, both types of engineered biochar (MB-ZnO nanocomposite and MBT) were applied to influence the growth of a fast-growing model plant (Sesbania sesban). In a greenhouse, S. sesban plants were grown in pots, containing Cd-Cu-contaminated soil. These plants were sprayed with various doses (0, 50, 75, 100 mg/L) of MB-ZnO nanocomposite and 1.0% (w/w) MBT. The findings of these applications showed that the combined application of MB-ZnO nanocomposite and MBT enhanced the physiological, biochemical, anatomical, and antioxidant enzyme activities of S. sesban and diminished Cd and Cu meditations. Foliar application of 100 mg/L MB-ZnO nanocomposite significantly reduced Cd and Cu content in the shoots of S. sesban by 30% and 31%, respectively. Combined application of MB-ZnO nanocomposite and MBT diminished Cd and Cu content by 39% and 38%, respectively, and increased the pH of soil from 8.03 to 8.23 units. These results signify the importance of the application of engineered biochar for plant growth under a metal-stress environment. Conclusive findings of this study proved that the application of engineered biochar (MB-ZnO nanocomposite and MBT) is an environment-friendly and efficient way to remove aqueous pollutants, immobilize toxic metals from the soil, control plant foliar diseases, and improve plant growth. There is a need to channelize the large-scale production and application of these products for a sustainable ecosystem.