Biosorption of heavy metal ions from aqueous solutions by activated sludge

Biosorption experiments were performed using different pretreated activated sludge as bioadsorbent for Cu2+ Cd2+ and Ni2+. Pretreatment with NaOH was found to improve the adsorption capacity of the sludge where as treatment with HCl reduces it. The treated and untreated sludge show great affinity towards Cu2+, Cd2+ and low affinity towards Ni2+. The maximum loading capacity of the NaOH treated sludge was 131.6 and 93.4 mg/g for Cu2+ and Cd2+, respectively. It was found that both Langmuir and Freundlich isotherm models fit the adsorption data with the different types of sludge and ions. The value of R-2 exceeds 0.95 in most cases. It was found that the maximum adsorption capacity drops 18.3% as the temperature increases from 25 to 45 degrees C. The adsorption capacity was found to increase as the adsorbent mass and mixing speed increase. The optimum pH corresponding corresponds to maximum adsorption efficiency was found to be about 5. Desorption experiments indicate that the desorption efficiency with 0.1 M H2SO4 solution reaches 94% in the first cycle and decreases to 8% in the 4th cycle after collecting 20,700 mg Cd2+/1 H2SO4.