Study of pesticide-decomposing bacteria Bacillus megaterium immobilised on Oryza sativa (rice hull) biochar

Abstract

Pesticides are widely used to protect crops from pest damage. However, their persistence in the environment can lead to significant ecological and health risks. Profenofos, a commonly used organophosphorus insecticide in Sri Lanka, is classified as moderately hazardous and is known to inhibit acetylcholinesterase activity in humans. In response to the growing need for eco-friendly remediation methods, this research explored the use of Bacillus megaterium immobilized on rice hull biochar (RHB) as a sustainable bioremediation strategy for degrading Profenofos residues in contaminated soil. Scanning electron microscope (SEM) analysis confirmed the successful colonization of the porous biochar surface by dense bacterial biofilms. Periodic measurements of optical density at 600 nm wavelength (OD600) and viability testing on agar plates were carried out over 50 days for two treatments: B. megaterium in rice hull-saline medium (BM-RHSM) and B. megaterium in saline medium (BM-SM). The OD600 measurements of BM-RHSM indicated consistent bacterial growth over time compared to the control, BM-SM. Shelf-life studies demonstrated that BM-RHSM maintained viability for 45 days, with higher and more stable growth compared to BM-SM. The neutral pH (7.25) and high carbon content (54.2%) of RH biochar provided favorable conditions for bacterial growth and long-term survival. Uncontaminated soil was collected from 20 cm depth in a dense forest area in Samanalawewa area, Imbulpe Divisional Secretariat, Sabaragamuwa Province. Pesticide degradation experiments involved four treatments: uncontaminated soil sample, profenofoscontaminated soil (200 ppm) (PCS) sample, PCS with RHB (SRH) and PCS with textitB. megaterium immobilised RHB (SBMRH). Gas chromatography–mass spectrometry (GC-MS) analysis showed that SRH reduced Profenofos retention from 100% in the control to approximately 63%, primarily through adsorption. In contrast, SBMRH treatment achieved reductions as low as 1.57%, highlighting the synergistic effects of adsorption by biochar and biodegradation by immobilised bacteria. This study demonstrated that RHB is an effective immobilisation matrix that enhances bacterial growth, extends viability, and significantly improves pesticide degradation in soil. The approach offers a low-cost, eco-friendly solution that can be implemented on-site using locally produced biochar and bacterial strains. Future research should focus on field-scale trials and biochar production optimisation to maximise its remediation potential.