Assessment of the impact of cassava mill effluent (CME) on the microbial diversity, physicochemical parameters and heavy metal concentrations in the receiving soil

Abstract

Cassava mill effluent has become a major cause of environmental degradation which affects soil properties. This study revealed the impact of cassava mill effluent on microbial diversity, physicochemical parameters and heavy metal concentrations in the receiving soils collected in triplicate between April and June, 2015 from five sites in Ogwa, Edo State, Nigeria. Standard methods were used for the soil analysis. The mean heterotrophic, coliform and fungal counts ranged from 1.38 + 4.6x10⁸ CFU/g to 8.90 + 5.5x10⁸ CFU/g, 1.20 + 6.1x10⁸ CFU/g to 9.4 +  5.6x10⁸  CFU/g and 4.17 + 2.4x10⁸ CFU/g to 7.90 + 2.8x10⁸  CFU/g respectively (p<0.05). The microbial isolates identified in order of decreasing prevalence were Staphylococcus aureus (19.71%), Escherichia coli (12.87%), Proteus sp. (12.50%), Bacillus sp. and Klebsiella sp. (12.01%), Pseudomonas aeruginosa (11.30%), Penicillium sp. (7.21%), Aspergillus sp. (6.25%) and Rhizopus sp. (3.13%). The pH values ranged from 5.78 – 6.12, electrical conductivity (112 - 211 μS/cm), chloride(30.24 – 56.97 mg/kg), sulphate (1.23 – 2.32 mg/kg), nitrate (15.90 – 29.96 mg/kg), phosphate (10.98 – 20.68 mg/kg), total carbon (3.17 – 5.83%), total nitrogen (0.32 – 0.58%) and moisture   content (10.65 – 17.63%), Na⁺ (42.56 – 80.18 mg/kg), K⁺ (78.40 – 147.70 mg/kg), Ca²⁺ (9.07 – 17.09 mg/kg),  Mg²⁺ (11.09 – 20.89 mg/kg). The pH at the different sample locations were lower than the control, and other physicochemical parameters were higher (p<0.05). The concentrations of heavy metals in the contaminated    soils are in the following ranges : Fe²⁺ (7.95 - 14.98 mg/kg), Zn²⁺ (0.22 - 2.13 mg/kg), Mn²⁺ (0.35 - 1.18 mg/kg),  Cu²⁺(1.01 - 1.90 mg/kg), Ni²⁺(0.24 - 0.80 mg/kg), Cd ²⁺(0.001 - 0.60 mg/kg), V²⁺(0.19- 0.72 mg/kg), Cr⁶⁺ (0.26 -  1.03 mg/kg) and Pb²⁺ (0.04 - 0.35 mg/kg). These values are still below the permissible limits recommended in Nigeria by the Department of Petroleum Resources except for iron which was higher in all sampled sites and  significantly different from the control (p<0.05). The order of accumulation of heavy metals was Fe³⁺ > Cu²⁺ > Zn²⁺ > Mn²⁺ > Cr⁶⁺  > Ni²⁺ > V²⁺ > Cd²⁺ > Pb²⁺. The cassava mill effluent negatively affected the soil microbial diversity, physicochemical parameters and heavy metal concentrations in the impacted soils.

Key words: Cassava mill effluent, Microbial diversity, Physicochemical parameters, Heavy metals, Receiving soil.