Effect of concentration and contact time on heavy metal uptake by three bacterial isolates

摘要

The effect of heavy metal concentration and contact time (exposure period) on heavy metal up take by pure cultures of three bacteria (Pseudomonas,BacillusandAeromonas) isolated from a crude oil impacted brackish aquatic system in the Niger Delta were investigated. Heavy metals employed included metals found in this Bonny light crude oil (Fe, Zn, Cd, Cu, Ni and Pb). Accumulation of these metals was gradual and the amount increased in direct proportion to initial metal concentration up to an extent that ranged from 1 - 100 (mg/l) after which uptake remained either constant or declined. Maximum uptake of Fe, Pb, Cd, Cu, Zn and Ni were obtained at initial concentrations of 10, 10, 10, 100, 100 and 10 (mg/l), respectively and the values were 8.75, 0.01, 1.3, 0.06, 4.2 and 0.001 milligram per gram dry weight (mg/g dry wt) ofBacilluscells. ForPseudomonassp. initial metal concentration that resulted in maximum uptake were 10 mg/l (Fe), 1 mg/l (Zn,) 10 mg/l (Pb), 0.1 mg/l (Cd), 10 mg/l (Cu) and 10 mg/l (Ni). Values accumulated at these concentrations were 14, 0.7, 1.0, 0.08, 0.3 and 0.11 (mg/g dry wt), respectively. Whereas, maximum amounts accumulated byAeromonassp.were 1.65, 0.1, 0.001, 0.9, 0.2 and 0.013 (mg/g dry wt) respectively. The respective initial concentration that yielded these uptake values was 100, 10, 1.0, 10, 10 and 10 (mg/l). Contact duration increased the amount of metal bioconcentrated by each test organism. At all tested concentrations maximum uptake of Fe, Zn, Cu, Cd, Pb and Ni byBacilluswere at the 8th, 8th, 8th, 4th, 4thand 4thhours of exposure respectively. Slight decreases in uptake were noticed on further incubation beyond these durations. Maximum accumulation of Fe, Zn, Cu, Cd, Pb and Ni byPseudomonassp. were obtained at incubation durations of 8th, 12th, 12th, 8th, 2ndand 24thhours. 12th, 12th, 12th, 4th, 8thand 4thh were the incubation periods that resulted in maximum bioconcentration of Fe, Zn, Cu, Cd, Pb and Ni byAeromonassp. The three test organisms presented distinct uptake capacities which decreased thus:Pseudomonassp. Bacillussp.≥Aeromonassp. Affinities ofBacillussp.,Pseudomonassp.andAeromonassp. for the various heavy metals followed the pattern Fe Zn Cd Cu Ni Pb, Fe Pb ≥ Zn Cu Ni ≥ Cd and Fe Cd Cu ≥ Zn Ni Pb respectively. Results showed that heavy metal concentrations between 10 – 100 mg/l and exposure periods of between 4 - 12 h depending on the metal and the test organism rapidly promoted accumulation in heavy metal polluted sites.