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.
