STUDIES ON THE BIOSORPTION O~F CHROMIUM AND NICKEL BY METAL TOLERANT STRAIN OF Aspergillus niger RH-19

Abstract

Industrial and mining waste waters are the major source of pollution by heavy metals. Metals can cause health hazards if their concentrations exceed permissible limits, since heavy metals are known to accumulate within biological systems. Recently, increasing awareness of the environmental impact of heavy metals has provoked a demand for the decontamination of industrial waste waters prior to discharge into natural waters. Therefore, the need for the production of cheaper adsorbents to replace costly conventional methods of wastewater treatment methods is attracting attention of scientists. Recently microorganisms like fungus, algae and bacteria have been successfully used as adsorbing agents for the removal of heavy metal ions. It has been found that both living and dead microbial cells can be used for this pmpose. The present piece of work was undertaken to study the capability of Aspergillus niger to remove nickel and chromium from aqueous solutions. A strain of Aspergillus species was isolated from the soil containing heavy metal ions. This strain was identified to be Aspergillus niger by sequencing the 18S rRNA gene. Maximum growth of the isolate was observed to be at temperature 3SoC and pH 6. A. niger RH-19 shows high tolerance against Ni2+ and Cr6 + (individually) exhibiting maximum resistance level upto 300 mg L-' . In growing conditions, the Cr6 + uptake was found to be maximum (33%) at metal concentration of SO mg L-'. At pH 6 the biosorbent exhibited the maximum metal uptake (37%). The strain also showed highest adsorption i.e. 33% at 3SoC. Oven-dried biomass showed maximum removal of 34% at initial metal concentration of SO mg L-'. 3SoC was found to be the optimum temperature for biosorption where maximum removal of 3S% was observed. At pH 6, the strain exhibited maximum uptake i.e. 27%. Similarly the growing biomass showed maximum Ni2 + uptake (87%) at initial metal concentration of SO mg L-'. At 3SoC, the metal uptake was maximum i.e. 97%. The biosorbent also exhibited highest adsorption value (94%) at pH 6. In oven-dried conditions, best removal rate (84%) was observed at SO mg L-'. RH-19 showed maximum adsorption i.e. 84% at 3SoC. Also at pH 6, maximum uptake (100%) was observed. The strain showed tolerance against Ni2+ and Cr6+ (mixture) amended SDA plates upto 2S0 mg L-'. RH-19 was subj ected to shake flask fermentation to check its ability to remove Ni2 + x Summary and Cr6 + in bimetal component system and maximum uptake was found to be 71 % and 75% at 12.5 mg L-1 concentration, 56% and 97% at 35°C, 95% and 71 % at pH 6 for these metals respectively. When the biosorption experiment conditions were scaled up to 1 L fermentor, the maximum individual Ni2 + and Cr6+ removal was 56 % and 60 % of 12.5 mg L-1 of each metal while maximum uptake in consortium was 37% and 36% respectively. Temperature, pH and weight of biomass were optimized by oven-dried biomass of RH-19 for the removal of Ni2 + and Cr6 + in mixture. It showed maximum removal of nickel and chromium salts at 35°C equal to 47% and 30%, at pH 6 equal to 96% and 23%, 46% and 63% with biomass weight equal to 1.2 g respectively. Nickel and chromium uptake in bi-metal system by nongrowing biomass of RH-19 was maximum i-e, 67% and 59% at 35°C and 60% and 66% at pH 6, respectively. Effect of presence of metals in a solution on biosorption potential of RH-19 was predicted by using equilibrium modeling. Cr6 + adsorption trends followed Langmuir isotherm (r2 =0.9982) and Ni2+ adsorption relatively conforms to Freundlich isotherm (r2=0.9906) in single metal removal system, but under bimetal condition chromium adsorption fitted better to Freundlich model and that of nickel followed Langmuir isotherm suggesting considerable change in behavior and interaction between biosorbent and metal ions. The mechanism of uptake of nickel and chromium by A. niger RH-19 was determined to be adherence to the cell wall as found out by Transmission Electron Microscopic studies. The metal ions were adsorbed on the cell surface and no bioaccumulation of these metals within the cell was observed. Furthermore, our results suggest that Aspergillus niger RH-19 is an effective biosorbent that can be used for the treatment of industrial effluents where the major contaminants are nickel and chromium.