Microwave-activated eggshell biochar enhances spinach growth and antioxidant defense by immobilizing aluminum under wastewater irrigation conditions

Aluminum (Al) stress in acidic soils and industrial wastes limits global agricultural productivity by affecting root and plant physiology especially in the surrounding of mining sites. The potential of microwave-enhanced eggshell biochar to ameliorate Al toxicity was tested in spinach ( Spinacia oleracea L.) A factorial pot experiment was designed with four concentrations of simulated Al wastewater (0%, 25%, 50%, and 100%) and two biochar concentrations (5 and 10 g kg⁻¹ soil) optimized by a 90-second microwave treatment. The findings showed that 100% wastewater with no biochar application caused a 45.4% and 47.0% reduction in plant height and leaf dry weight, respectively, while a 128.6% rise in root lipid peroxidation (MDA) was observed. In contrast, using 10 g biochar substantially enhanced the growth, with a 21.3% increase in plant height under control conditions and promoted recovery under stress. Biochar amendment also enhanced antioxidant enzyme activities, specifically root SOD and CAT, while reducing leaf Al uptake by 40.8%. Two-way ANOVA confirmed significant interactive effects between wastewater level and biochar rate ( p < 0.05) on most growth and stress parameters. These findings indicate that microwave-activated eggshell biochar effectively detoxifies the rhizosphere through CaCO₃-mediated Al precipitation and surface adsorption. This study provides a sustainable strategy for safe vegetable production in wastewater-irrigated and mining affected agroecosystem.