Remediation of soil polluted with Pb and Cd and alleviation of oxidative stress in Brassica rapa plant using nanoscale zerovalent iron supported with coconut-husk biochar.

Accumulation of toxic elements by plants from polluted soil can induce the excessive formation of reactive oxygen species (ROS), thereby causing retarded plants' physiological attributes. Several researchers have remediated soil using various forms of zerovalent iron; however, their residual impacts on oxidative stress indicators and health risks in leafy vegetables have not yet been investigated. In this research, nanoscale zerovalent iron supported with coconut-husk biochar (nZVI-CHB) was synthesized through carbothermal reduction process using Fe2O3 and coconut-husk. The stabilization effects of varying concentrations of nZVI-CHB and CHB (250 and 500 mg/kg) on cadmium (Cd) and lead (Pb) in soil were analyzed, and their effects on toxic metals induced oxidative stress, physiological properties, and antioxidant defence systems of the Brassica rapa plant were also checked. The results revealed that the immobilization of Pb and Cd in soil treated with CHB was low, leading to a higher accumulation of metals in plants grown. However, nZVI-CHB could significantly immobilize Pb (57.5-62.12%) and Cd (64.1-75.9%) in the soil, leading to their lower accumulation in plants below recommended safe limits and eventually reduced carcinogenic risk (CR) and hazard quotient (HQ) for both Pb and Cd in children and adults below the recommended tolerable range of <1 for HQ and 10-6 - 10-4 for CR. Also, a low dose of nZVI-CHB significantly mitigated toxic metal-induced oxidative stress in the vegetable plant by inhibiting the toxic metals uptake and increasing antioxidant enzyme activities. Thus, this study provided another insightful way of converting environmental wastes to sustainable adsorbents for soil remediation and proved that a low-dose of nZVI-CHB can effectively improve soil quality, plant physiological attributes and reduce the toxic metals exposure health risk below the tolerable range.

Systematic understanding of char-volatile evolution and interaction mechanism during sewage sludge pyrolysis through in-situ tracking solid-state reaction and products fate.

The complexity of sludge components and the heterogeneity of pyrolysis products make it challenging to trace char-volatile evolutions and interaction mechanisms during pyrolysis. Herein, we systematically dissected the solid-state reactions and volatile dynamic variations via in-situ infrared/mass spectral probes coupled signal amplification techniques. The identification of hidden reactions was further enhanced by comparing the discrepancies in the pyrolysis of three systems: raw sludge, sludge-extracted organics, and pseudo-components of organics. A three-stage sludge pyrolysis of bond cleavage (α = 0.2-0.5), intermediates diffusion (α = 0.5-0.7), and interface interaction (α = 0.7-0.8) was proposed through solid-state reaction tracing, and the pyrolysis reaction was found to be dominated by the first two stages. The generation of reactive intermediates accelerated the collision frequency between reactants, which increased the order of solid-state reactions and raised the energy barrier from 148 to 180-261-297 kJ/mol. The temperature-response sequence of the major pyrolysis volatiles was H2O/CO2/furans/alcohols (<250 °C), amine-N/acids/ketones/esters (250-350 °C), heterocyclic-N/phenols/C2-3 (300-400 °C), CH4/aromatics/nitrile-N (350-450 °C), and CO/HCN (>450 °C). The temperature-dependent evolution of these volatiles was consistent with the variations of chars in terms of pyrolysis behaviors, reaction models, and surface characteristics. The comprehensive understanding of the staged pyrolysis pathways and the char-volatile interaction mechanisms may provide critical information for pyrolysis procedure design and product targeted regulation.