Physicochemical characterization, heavy metal assessment, and bioremediation potential of wastewater from a tire manufacturing plant in Al-Diwaniyah, Iraq

Abstract

Industrial wastewater generated by tire manufacturing plants contains complex mixtures of organic chemicals, heavy metals, and inorganic salts that pose serious environmental risks if discharged without adequate treatment. Despite the well-documented hazards of such effluents globally, data on their physicochemical and microbiological characteristics in Iraq remain scarce. This study aimed to characterize the physicochemical properties and heavy metal contamination of wastewater from a tire manufacturing plant in Al-Diwaniyah, Iraq, and to identify indigenous bacterial species with potential bioremediation relevance. Wastewater samples were collected monthly from May to October 2025 at three designated sampling points. Physicochemical parameters including pH, electrical conductivity (EC), salinity, total dissolved solids (TDS), nitrate, sulfate, phosphate, chloride, and chemical oxygen demand (COD) were measured using standard APHA methods. Heavy metals (Pb, Cd, Ni, Zn, Cu) were quantified by atomic absorption spectrophotometry (AAS). Indigenous bacteria were isolated and subjected to Gram staining, Vitek-2 identification, and confirmation by 16S rRNA gene sequencing with phylogenetic analysis. Statistical analysis included one-way ANOVA and Pearson correlation. Results revealed substantial temporal variability: pH ranged from 6.8 to 9.5, EC from 597 to 2792 µS cm⁻¹, TDS from 1032 to 2717 mg L⁻¹, and COD up to 531 mg L⁻¹. Heavy metal concentrations frequently exceeded Iraqi and WHO discharge limits, particularly for Zn (2.09–5.18 mg L⁻¹) and Ni (0.11–3.03 mg L⁻¹). Five Bacillus species were identified: B. subtilis, B. licheniformis, B. tequilensis, B. velezensis, and B. persicus, with 97–99% 16S rRNA gene similarity to NCBI reference sequences. The identified Bacillus species exhibit known adaptations to extreme and pollutant-laden environments, suggesting their candidacy for future bioremediation strategies; however, direct experimental validation of pollutant removal capacity is required before practical applications can be claimed.