BIOAUGMENTATION OF HEAVY METAL (Ni, Pb, Cr) CONTAMINATED SOIL USING HYDROXYAPATITE AND CITROBACTER spp.

The contamination of soils with heavy metals from anthropogenic activities like automobile repair poses a significant threat to ecosystem and human health. This study investigated the effectiveness of hydroxyapatite (HAp) and phosphate-solubilizing bacteria (PSB) for the remediation of heavy metals in a contaminated mechanic workshop soil in Ekosodin, Benin City, Nigeria. Soil samples were collected and subjected to four treatments: untreated soil (control), soil amended with PSB only, soil amended with HAp only, and a combined treatment of soil with both HAp and PSB. Hydroxyapatite was synthesized from cow bones via thermal calcination, and indigenous PSB were isolated from the contaminated soil. The concentrations of Chromium (Cr), Lead (Pb), and Nickel (Ni) were analyzed using Atomic Absorption Spectrophotometry, and microbial counts were monitored over the experimental period.The results demonstrated that the combined application of HAp and PSB was the most effective treatment, achieving the highest percentage removal efficiencies: 38.65% for Cr, 29.05% for Pb, and 33.12% for Ni. Individual applications of PSB or HAP showed significantly lower remediation effects. Furthermore, the combined treatment (Soil+PSB+HAp) supported a higher total heterotrophic bacterial count (2.75 ± 0.71 × 10⁶ CFU/g) compared to the PSB-only treatment (2.50 ± 0.00 × 10⁶ CFU/g), indicating improved soil microbial activity. The study concludes that the synergy between hydroxyapatite and phosphate-solubilizing bacteria enhances the immobilization of heavy metals and promotes a healthier soil microbiome, presenting a promising, eco-friendly strategy for remediating soils co-contaminated with heavy metals