Kinetic and Adsorption isotherms of cadmium from aqueous solution using some bioadsorbents

Disposal of wastewater containing heavy metals into terrestrial and aquatic environment, can cause deterioration in water quality and toxicity to human health. Heavy metals are not degradable, therefore the removal of them is important in terms of health of livings specimens. Cadmium (Cd) is a potentially toxic element to all organisms and is known as carcinogenic to humans. Biosorption of heavy metals plays an important role in removing metal ions from aqueous solutions. The main advantages of this method are the reusability of the bioadsorbent, low cost, short execution time, and no production of secondary compounds that may be toxic. Biosorbents with high capacity to remove heavy metals are recyclable, so they are widely used in wastewater treatment. These adsorbents include inorganic, organic or biological compounds. Feathers, as the most abundant waste from the poultry industry, are available in large quantities, so the disposal or recycling of these materials has always been a big challenge. Chicken feathers contain more than 90% of natural keratin with a high specific surface and functional groups that may have potential as bio-adsorbents to remove heavy metals. The use of agricultural residues are also a good option for removing heavy metals due to their unique chemical composition, availability, renewable, low cost and efficiency. Sugarcane bagasse obtained from the sugarcane industry can been used as a biosorbent to remove heavy metals from wastewater. Trametes pubescens is a basidiomycete fungus and has many functional groups responsible for the absorption of heavy metals. The Trametes pubescens is resistant to some heavy metals and is able to remove them from the polluted water environment. Therefore, the aim of this study is to investigate the impact of Cd on Trametes pubescens as biosrbent and using of feather and sugarcane bagasse as carrier for this fungus to remove Cd from aqueous solution.

The different concentrations of Cd2+ from the source of CdCl2 (0, 10, 25, 50, 100, 200 mg/L) in PDA medium were prepared and the growth of fungus on this medium was considered to 5 days. The inhibition of fungus growth was calculated from the difference of fungus growth in control medium and in the presence of Cd, thereafter the ED50 of Cd was determined. The influence of Cd2+ on laccase activity of this fungus was also evaluated in solution medium containing mentioned concentrations of Cd using ABTS as substrate for lacasse. The removal of Cd by some bioadsorbents including fungal biomass, fungus+sugarcane bagasse, fungus+feather was studied under shaking times (1 to 72 h). The adsorption isotherms and kinetic models for describing adsorption capacity and process were fitted on data. The used absorbents were removed from the Erlenmeyer flasks and they were dried at laboratory temperature after rinsing with distilled water to remove surface absorbed particles. The presence of fungus mycellium on sugarcane bagasse and chicken feathers were examined with scanning electron microscope and cadmium adsorption by exposed mycelium to 200 ppm of metal was determined by the Energy-dispersive X-rays analysis. The interaction of functional groups with Cd were examined with Fourier Transform Infrared spectroscopy (FTIR).

The obtained data demonstrated growth inhibition of fungus by an increase of Cd concentration. Concentrations of 50 and 100 mg/L of cadmium decreased fungus growth by 85 and 89%, respectively, without significant difference. EC50 and EC80 index values were calculated as 16.6 and 82.17 mg/L, respectively. Laccase activity during 120 hours of exposure to different concentrations of cadmium showed the decrease in the presence of cadmium. The highest inhibitory effect was related to the concentrations of 100 and 200 mg/L. The maximum absorption of cadmium by fungus, fungus+bagasse, fungus+feather was 66%, 72% and 46% at contact time of 72 hours. The highest correlation coefficients of metal absorption by fungus with the values of 99 and 98 %, respectively, were related to Elovich models and intraparticle diffusion models. The maximum correlation coefficients of Cd absorption by fungus + sugarcane bagasse by 97% and 95 %, respectively, were related to second-order models and intraparticle diffusion. The Elovich and intraparticle diffusion models were well fitted to the Cd biosorption data by fungus+feather. Based on the correlation coefficients, the Langmuir and Freundlich isotherms were more suitable for describing the equilibrium data of cadmium biosorption by three applied bioadsorbents. Scanning electron microscope images showed the presence of fungus in bagasse and chicken feather carriers. Also, the images of the scanning electron microscope of the fungal biomass with Cd showed an obvious difference in the morphology of the mycelium surface compared to non-exposed mycelia to Cd. Closed and twisted mycelium with some particles on the surface of the fungal cells was observed in the presence of cadmium. The elemental composition of mycelium exposed to metals in the EDX results showed 17.59% by weight of cadmium. The interaction of heavy metals with functional groups on the surface of white rot fungi may change the morphology of the mycelium surface. It seems that the changes in the surface of the mycelium, which depends on the type of metal and its concentration, are related to the intracellular detoxification of heavy metals. The accumulation of mycelium and ring formation is probably due to the metal interaction with functional groups on the fungal mycelium. The changes some peaks and appearance of new peaks indicated the interaction of functional groups with Cd confirmed via Fourier Transform Infrared spectroscopy analysis.

Filamentous fungi are effective microorganisms in removing heavy metals form wastewaters. Agricultral residue such as sugarcane bagasse can be used to massive production of fungus, and have efficiency for the removal of heavy metals form wastewater. In this study, Trametes pubescens as white rot fungus along with sugarcane bagasse had the highest removal of cadmium from aqueous solution. The removal of this metal was a function of the contact time of the adsorbent with the solution containing the metal, and the Elovich and intraparticle diffusion models were effective to describe the Cd adsorption process. Morphological changes of the fungus in the presence of cadmium indicated the response of the fungus to tolerate the toxicity of this metal.