Spirulina Platensis for Decontamination of Chromium Laden Aqueous Effluent

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Spirulina platensis, a blue-green microalga, grows in high saline (13 to 35 g Cl- L-1) and alkaline (pH 8 to 10.5) condition. The acid-base characteristic of a biomaterial is an important factor determining the efficiency of metal removal. The quality and quantity of acidic sites of algal species strongly depend on its cultivation conditions. This study investigates the applicability of Spirulina platensis for removal of both Cr(III) and Cr(VI) in batch reactor. The acidic sites of dead Spirulina platensis biomass were determined from potentiometric titration by employing a simple proton binding model. The concentration of functional groups was in the order of phosphatic>> carboxyl > amine. Maximum Cr(III) sorption was noted at equilibrium pH of 6.2. A modified metal binding model was tested and Cr(III) binding constants of 80.9, 3.9×103 and 3.5×104 L mol-1 were found for carboxylic, phosphatic and amine groups, respectively. Cr(III) uptake model showed dual sites coverage of Cr(III) on Spirulina surface with predominant occupancy to phosphatic sites. The amine group however, according to metal binding model, was less significant in Cr(III) binding. The overall Cr(III) binding constant was found as 1.13×1010 L mol-1. pH and temperature exhibited major influence on Cr(III) uptake capacity of dead biomass. [Cr(III)] < 10 mg L-1 showed negligible effect on Spirulina growth, while [Cr(III)] > 50 mg L-1 was resulted in decrease of cell density in Zarrouk media composition (ZMC) because of Cr(OH)3 precipitates. The active functional groups are capable to detoxify Cr(VI) to Cr(III). A 2nd order kinetic model was developed in terms of concentration of protonated acidic groups of dead Spirulina sp. biomass for the reduction of Cr(VI) into Cr(III). Cr(VI) reduction model was validated over a broad range of pH, temperature and anionic strength. Lower pH favoured Cr(VI) reduction reaction and the experimental results well fitted to the kinetic model. The overall rate constant ( t k ) decreased logarithmically from 22.7 to 2.8 mM-1 s-1 with rise of pH from 0.5 to 6.0. Whereas, , increased nearly by 23% with elevation temperature from 25 to 45 C. Higher concentration (> 0.235 mM) of background anions (Cl-, SO42- and NO3 -) was resulted in decreases in values in case of dead biomass. Cr(VI) inhibition on cell growth in ZMC was almost at the same extent of Cr(III) with 12 h photo period per day. Concentration of Cr(VI) in solution was almost unaffected at pH 8.5. Exhausted chrome tanning stream typically contains upto 15 g L-1 of Cl-. The feasibility of precipitated chrome tanning effluent (PCTE) for the cultivation of Spirulina sp. was investigated to utilize its Cl- resource. Hydroxide precipitation although reduced its Cr(III) loading but Cl- concentration was unchanged. The kinetics of cell growth were recorded in ZMC with addition of Cr(III), Cr(VI) and PCTE. The inhibition on growth was in the order of (ZMC+PCTE)>> (ZMC+Cr(VI))> (ZMC+Cr(III). Spirulina sp. growth rate was about three fold less in PCTE+ZMC as compared to native Spirulina in ZMC. ZMC+ PCTE cultivated Spirulina was resulted in decrease of formation of carbohydrate and chlorophyll b by 58 and 70%, respectively. Whereas, chlorophyll a formation increased by 40% in comparison to ZMC cultivation. A kinetic model was tested by incorporating the rate of photosynthesis, nutrient uptake and respiration for Spirulina cell growth in ZMC, ZMC+Cr(III), ZMC+Cr(VI) and ZMC+PCTE. The growth parameters were found to be in the close proximity of the earliest reports with other biomaterials. The minimum amount of nutrient required to support Spirulina growth in ZMC+ PCTE was lower than ZMC+Cr(III) and ZMC+Cr(VI).
Supervisor: Animes Kr. Golder