Investigation of Adsorptive Removal of Chromium (VI) from Aquatic System Using Dust Black Tea Leaves

Abstract

Some heavy metals are highly toxic as ions or in form of compounds. Ions or compounds of heavy metals, which are soluble in water, can be readily absorbed into living organisms. Chromium (VI ) is a harmful heavy metal ion which is indiscriminately discharged to aquatic-body through the effluents of process-plants of leather tanning, metallurgy, electroplating, textile dyeing, paints, ink, etc. and thus aquatic environment is being severely polluted. These industrial effluents contain chromium (VI ), which is much higher than the tolerance limit; 0.5 mg·L -1 in industrial wastewater (EPA ). So, the removal of Cr (VI ) from wastewater is prerequisite prior to discharge of the toxic ion into aquatic environment. The present study was undertaken to investigate the efficiency of Dust Black Tea leaves (DBTL ), a waste of tea process plant, as an adsorbent to remove chromium (VI ) from aqueous solution. Dust black tea leaves (DBTL )was collected and washed repeatedly with hot water until the color materials of the leaves were entirely eliminated. Small surface area, heterogeneous surface morphology and some functional groups were observed in dried DBTL using BET surface analyzer, SEM-EDX and ATR-IR.Chromium (VI )in aqueous solution was analyzed colorimetrically by forming a violet complex of Cr (VI )with 1,5- Diphenylcarbazide using UV-visible Spectrophotometric method. In this process, Cr (VI ), due to its contact with DBTL, was reduced to Cr (III ) along with adsorption of Cr (VI )on DBTL. Concentration of Cr (VI ) and Cr (III ) in solution were quantitatively determined by the colorimetric method. Both adsorption and reduction were found to be dependent on pH of chromium (VI ) solution. Since the minimum reduction of Cr (VI ) to Cr (III ) and insignificant quantity of adsorption of Cr (III ) on DBTL were found at pH 2.0 of the solution, the pH 2.0 was considered as an optimum condition to obtain the maximum removal capacity of DBTL. In each experiment, reduced amount of Cr (VI )to Cr (III )was deducted from the total removal to receive the amount adsorbed only. Batch adsorption kinetic experiments were conducted to investigate the effects of initial concentration of Cr (VI ), pH of Cr (VI ) solution, process temperature and particle-size of DBTL. Related-experimental data were verified by applying to different models of kinetic equations, such as, simple first order rate equation, second order rate equation , pseudo first order rate equation, pseudo second order rate equation and Elovich equation. The highest value of regression co-efficient, R 2 (0.999 )supported that pseudo second kinetic equation is best-fitted for different initial concentrations, solution pH, temperatures and particle-size of DBTL. The equilibrium amount of Cr (VI )adsorbed (qe ), equilibrium concentration (Ce )of Cr (VI ) solution and the pseudo-second order rate constant, k2p were calculated from the best-fitted pseudo-second order kinetic plots for different initial concentrations at pH 2.0. The maximum adsorption capacity, qm = 303.03 mg·g -1 , was calculated from the above kinetic data (qe and Ce ), using best fitted Langmuir equation. For a fixed concentration of Cr (VI ), the equilibrium amount adsorbed (qe ), equilibrium concentration (Ce ) and rate constant (k2p ) were calculated from the best-fitted pseudo-second order kinetic plots for different (i ) temperatures at pH 2.0, (ii o ) pH at 30 C and (iii ) sizes of DBTL at pH 2.0 and 30 o C. The effect of temperature on adsorption kinetics showed that the equilibrium amount of Cr (VI )adsorbed decreased with increase of temperature. This behavior indicated

exothermic nature of the adsorption at pH 2.0. Using the pseudo-second order rate constant, the activation energy Ea. of adsorption was determined from Arrhenius plot and the value was found to be 7.37 kJ·mol -1 which is very small compared to that (Ea = -1 65 - 250 kJ·mol )of chemical adsorption. Thermodynamic parameters: ΔG o o o , ΔH and ΔS for Cr (VI ) adsorption on DBTL were determined from the equilibrium adsorption constant (Kc ). The negative value (-27.38 -1 kJ mol o )of ΔH indicated the adsorption of chromium (VI ) on DBTL was exothermic, spontaneous and physical in nature. The positive values from + 2.03 to + 5.60 kJ mol -1 o of ΔG increased with increase of temperatures from 15 to 50 o C which indicated the process was slow and less feasible due to the enhancement of the reduction of Cr (VI ) at high temperature. The negative value of ΔS o -1 (-0.102 kJ·mol -1 K )predicted the decreased randomness through the adsorption of chromium (VI ) on DBTL. The effect of pH of solution showed that the equilibrium amount of Cr (VI )adsorbed linearly decreased with increase of pH from 2.0 to 6.0 and then became slow to the pH 8.0 indicating electrostatic interaction between anionic Cr (VI ) and positively charged protonated-DBTL-surface at low pH. The effect of particle-size shown that the equilibrium amount of chromium (VI ) adsorbed on DBTL decreased with increase of particle-sizes of DBTL due to the lowering of surface area. The rate of adsorption decreased with increase of particle-size of DBTL from 106 to 450 µm. The higher rate of Cr (VI ) uptake by smaller size of particle is due to the greater accessibility to pores and greater surface area per unit mass of small sizes of DBTL. The transfer mechanism of the adsorption of Cr (VI ) on DBTL was investigated using Weber and Morris’s intraparticle diffusion model equation towards the kinetic data for different concentrations, temperatures, pH and sizes of DBTL.The effects of concentration and temperature have shown that the film diffusion is dominative for the adsorption of chromium (VI ) on DBTL at pH 2.0 for all concentrations. Again, the effects of solution pH and particle-size revealed that the intraparticle diffusion is favorable at high pH of the solution and large size of DBTL-particle, but at low pH and small particle-size, the adsorption is dominated by film diffusion. Equilibrium adsorption isotherms at different temperatures were constructed using 6 hours as predetermined equilibrium time at pH 2.0. Different equations for adsorption isotherm, such as Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R ) equations were applied to the equilibrium adsorption data obtained experimentally to evaluate the feasibility of the process. The results indicated that the Langmuir equation fits the data better than any other of the above isotherm equations. The maximum monolayer adsorption capacity, qm was calculated and found to be 303.03 mg·g -1 at 30 o C which decreased with increase of temperature. The Langmuir constant, b was used to determine the separation factor, Rb and thermodynamic parameters to understand the mechanism of the process. At low concentration and low temperature, values of Rb were found to be near to 1. These results indicated that the adsorption was favorable at this condition. The negative value of ΔG o -1 (-19.79 kJ∙mol )indicated the uptake of chromium (VI ) on DBTL was spontaneous and the positive value of ΔH o -1 (15.3 kJ∙mol ) which is too insufficient to that required for occurring chemical adsorption and the value of ΔS o -1 (+ 0.11 kJ∙mol -1 ∙K )might be due to small amount of reduction of chromium (VI ) to chromium (III ), during the adsorption of chromium (VI ) on DBTL.

The mean adsorption energy, E at different temperatures were also calculated from the Dubinin-Radushkevich isotherm and the values were limited within the range of – 8.8 to -12.7 kJ·mol -1 which indicated that the adsorption of Cr (VI ) on DBTL might be controlled by physical in nature. The effects of pH and particle-size of DBTL on the adsorption isotherm were also investigated by applying different isotherms model equations. The maximum monolayer adsorption capacity was calculated using well fitted Langmuir isotherm equation. The results showed that the values were decreased with increase of both of solution pH and particle-size, like the same as the results obtained from kinetic study. A comparison of the Attenuated Total Reflectance Infra-red (ATR-IR )spectra of un- adsorbed and Cr (VI ) adsorbed DBTL shown that -OH, C=C, C-O and aromatic =C-H groups of DBTL were interacted (by shifted their positions) with Cr and a new peak appeared at 717.52 cm -1 for –C-Cr interaction. Scanning Electron Microscopic (SEM) microgram of Cr (VI ) adsorbed DBTL shown some spherical chromium particles on the surface which were confirmed by Energy Dispersive X-ray (EDX ) spectrum. Again, the deposition of chromium on DBTL surface are not homogeneously distributed which was determined by EDX point analysis of Cr (VI ) adsorbed DBTL surface. By treating with 2M sodium hydroxide solution, adsorbed chromium (VI )was fully recovered from the Cr (VI )-adsorbed-DBTL surface in order to eliminate the creation of secondary pollutant.