Study On The Biosorption Of Cationic Dyes By Spent White Rot Fungi Cultivation Substrate

This study focused on the possible use of the spent cottonseed hull substrate (SCHS), an agricultural waste was used after the cultivation of white rot fungus Pleurotus ostreatus (P. ostreatus), to adsorb three kinds of cationic dyes, Neutral Red (NR), Methylene Blue (MB) and Malachite Green(MG) from aqueous solutions. Meanwhile, the reliable maximum absorption wavelength (λmax) value for Congo Red (CR) dye at different pH values was researched in the paper.It was found that the λmax of CR is576nm at pH2.18-3.16,567nm at pH3.86, and496nm at pH>4.71, for the different structure of CR molecules at different pH. CR is very sensitive to the pH of solution and would change from red to blue, due to π-π transition in azo group shifts to higher wavelength because of protonation. This experiment revealed that at lower pH it becomes cationic and shows two tautomeric forms of protonated CR, i.e. ammonium rich variety and azonium variety. So the test proved that relevant papers about research of λmax of Congo Red published in some Journals are incorrect.Fourier transforms infrared (FTIR) spectroscopy and thermo-gravimeter analyses (TG-DTG) were used for the characterization of the biosorbents. For the pure SHCS adsorbents, the functional groups on the SCHS surface, such as-NH, OH, C=O, COO-and-P=O, have been considered to be potential adsorption sites for entrapment of dyes molecules. The pH at point of zero charge (pHPZC) of the adsorbent was determined by the titration method. The efficient adsorption process is ascribed to the massive functional groups on the biosorbent surface such as hydroxyl and carboxyl. The TG-DTG of the biomass proved that SCHS is mainly composed of hemicelluloses, cellulose and lignin, etc.A batch adsorption study was carried out with variable solution pH, adsorbent amount, adsorption time, temperature and initial concentration. It was found that dyes uptake was favorable at pH>pHzpc. Langmuir, Freundlich, Redlich-Peterson and Sips isotherm models were applied to the analysis of experimental equilibrium data of SCHS adsorption dyes. The effect of contact time at different temperatures was fitted to pseudo-first-order and pseudo-second-order kinetic models. Linear regressive method and nonlinear regressive method were used to obtain the relative parameters. The nonlinear method may be better with the absolute error as limited condition. The adsorption process was spontaneous and endothermic.Thermodynamic parameters of free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), obtained from biosorption of dyes ranging from different temperature, showed that the adsorption was a spontaneous and endothermic process. The negative value of ΔG°indicates the spontaneity and feasibility of the biosorption process. The decrease in ΔG°value with rising temperature reveals that adsorption of dyes onto SCHS becomes more favorable at higher temperature. By using micro-calorimetry C80to measure the heat change of dyes onto SCHS in the experiment, it is proved that the wetting heat plays a leading role in solid-liquid adsorption system and the wetting is exothermic process during action.At last, the adsorption potential of SCHS to remove dyes from aqueous solution was investigated using fixed-bed adsorption column. The adsorption data were fitted by two fixed-bed adsorption models namely, Thomas and BDST models. The results fitted well to the models at different conditions. The SCHS was shown to be suitable adsorbent for adsorption of dyes using fixed-bed adsorption column.The major innovative points of this paper were summarized as follows.First of all, the experiments proved that the chromophore structure of some dyes will change at different pH and λmax and its value would change with the changes of pHs value. So the tests proved that relevant papers about research of λmax of Congo Red published in some Journals are incorrect by some scholars in the previous study.Second, linear regressive method and nonlinear regressive method were used to obtain the relative parameters of the adsorption isotherm, kinetics model. The data reliability of non-linear regression is higher than the linear regression.The error analysis was conducted to prove that nonlinear method was better to predict the experimental results.Third, by using micro-calorimetry C80to measure the heat change of solid phase-liquid phase adsorption of dye in the experiments, it was proved that the heat change process is composed by the wetting heat of exothermic and the adsorption heat of endothermic. Above all, the wetting heat of the system plays a leading role and its value is much larger than the later. This is not reported in the previous literature.