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Introduction

Discharge of coloured wastewaters into natural water bodies is not desirable, as they are aesthetically displeasing and prevent reoxygenation in receiving waters by cutting off penetration of sunlight. In addition many dyes are toxic to aquatic organisms, mammals and humans.^1-5Contamination of water resources with dyes, mainly in the surrounding areas of dyeing and textile industries, has caused great concern among environmentalists. For example, dyeing industry wastewater is one of the major environmental problems in Coimbatore district of Tamil Nadu.⁶

Various treatment methods have been used for the removal of dyes from aqueous solutions including chemical coagulation, ozonization,⁷membrane filtration,⁸electrolysis,⁹and microbial degradation.¹⁰These established technologies often unable to adequately reduce contaminants concentrations to desired and/or legislated levels¹¹or are associated with some practical difficulties. This has initiated a search for more effective and economic treatment techniques to offer significant reduction in capital costs than for example filtration and biological processes. Adsorption is by far the most effective and widely used technique for the removal of dyes from aqueous solution. In recent years several investigators have concentrated their work on low-cost, non-conventional adsorbent materials¹²to achieve the economically feasible and effective treatment of wastewater containing dyes.

The present study is undertaken to evaluate the efficiency ofTamarindus Indicafruit shell (TIFS), for the removal of two basic dyes- Rhodamine B (RB) and Malachite Green (MG). Malachite green is a common basic dyestuff of triphenylmethane series used for dyeing silk and wool directly and cotton mordanted with tannin to deep green. Rhodamine B is also widely used in dyeing industries.Tamarindus Indica(tamarind tree) is one of the common and most important trees of India. A full-grown tree yields 180-225 Kg of fruit per season. On the average the pod is composed of 55% pulp, 34% seed and 11% shell and fibre.¹³

Table 1: Isotherm parameters for the adsorption of dyes at 30°C Click here to view table

Experimental

Adsorbent

The adsorbent used in this study was tamarind fruit shell collected from fruits of a single tree. The shells were washed with water to remove the adhered pulp and dust, air dried, ground and sieved to get particles of size 150-250 µm.

Table 2: RL values for adsorption of dyes at 30°C Click here to view table

Analysis of Dyes

The dyes were analyzed by monitoring their absorption in the visible region, 555nm for RB and 620nm for MG, using Spectronic 20D+ spectrophotometer (Spectronic Instruments, USA). Calibration graphs were prepared (1-6mg/L for RB and 1-8mg/L for MG) and concentrations of sample aliquots were established by referring to the respective calibration graph.

Figure 1. Kinetics curves for the adsorption of dyes on TIFS Click here to view figure

Batch Adsorption Experiments

Each equilibrium adsorption experiment comprised three replicate 100mL glass-stoppered bottles containing appropriate amount of adsorbent and 50mL of dye solutions of selected concentrations. Control flasks without the adsorbents also prepared simultaneously. Mixtures were maintained in a rotary shaker (Orbitek, Chennai, India) at constant temperature (30, 45 or 60°C). After the attainment of equilibrium the contents of each flask were filtered through a Whatmann No. 41 filter paper. The filtered samples were then analyzed for unadsorbed solutes. The equilibrium data obtained were analyzed using the following three isotherm equations, namely, Freundlich, Langmuir and Redlich- Peterson:

Figure 2. t vs Bt for the adsorption of dyes on TIFS Click here to view figure

Freundlich
q_e=K_FC_e^(1/n)...(1)

q_e= K_LC_e/(1+ b C_e)= q_mb C_e/(1+ b C_e) ...(2)

Redlich- Peterson
q_e=K_RC_e/(1+ b_RC_e^β)...(3)

whereq_e是the adsorption capacity (mg/ g);C_e, equilibrium concentration of the adsorbate (mg/L);K_F(mg/g) andn, Fruendlich constants;K_Landb(L/mg), Langmuir constants;q_m, Langmuirmonolayer adsorption capacity (mg/g) andK_R,b_Rand β are Redlich-Peterson isotherm constants.

Table 3: Pseudo- first order parameters for adsorption of dyes at 30°C Click here to view table

For kinetic studies, a series of bottles with fixed amounts of adsorbent and dye solutions were taken. One bottle was taken out for the determination of unadsorbed dye at time intervals of 5, 10, 15, 20, 25, 30, 45, 60, 120 and 180 minutes. To describe the adsorption kinetics, the pseudo-fist order model proposed by Lagergren¹⁴and the pseudo-second order model proposed by Ho and McKay¹⁵were used in the following forms:

First order rate equation

log(q_e-q_t) = logq_e(1)–k₁t...(4)

Second order rate equation

t/q_t= (1/h) + (1/q_e(2))t...(5)

where,q_t(mg/g) is the amount adsorbed at timet(min);q_e, amount adsorbed at equilibrium (mg/g);q_e(1), adsorption capacity predicted by the I order model (mg/g);k₁, first order rate constant (min^-1);h(=k₂/q_e(2)²), initial sorption rate (mgg^-1min^-1); andq_e(2), adsorption capacity predicted by the II order model (mg/g).

Table 4: Pseudo- second order parameters for adsorption of dyes at 30°C Click here to view table

For determining the effect of pH on adsorption, dye solutions adjusted to different pH values using dilute NaOH or HCl solutions were taken with the chosen adsorbent dose.

Results and Discussion

Table 5: Effective pore diffusion coefficients for adsorption of dyes Click here to view table

Equilibrium Adsorption Studies

The equilibrium data obtained for each system was fitted to the three isotherm equations (for RB adsorption 0.25g and for MG adsorption 0.1g adsorbent were used per 50mL of adsorbate solutions at the solution pHs) and the isotherm constants are listed in Table 1. It is to be noted that the Langmuir monolayer adsorption capacity of the adsorbent towards MG is far greater than that of RB. This could be due to the larger molecular size the RB dye molecule, which will restrict the entry of them into micro and mesopores present in the carbon surface.

Theq_mvalues for the dyes adsorption on TFS are quite high, it seems likely, therefore, that there could be some specific forces involved between the dyes and TFS or that a multilayer surface coverage would have occurred. This view is also supported by the Langmuirbvalues (Table 1) that are measures of adsorbent-adsorbate interaction forces or strengths.

Table 6: Isotherm constants for adsorption of Dyes at 45°C Click here to view table

Theb值用来计算th进一步决定e dimensionless separation factor, R_L,¹⁶defined as

R_L= 1/(1 +bC_i) ...(6)

where C_i是the initial solute concentration. The magnitude of R_Lvalue gives an idea about the nature of adsorption equilibrium, favourable when it lies in the range 0-1.

In all the systems studied, R_Lvalues were comprised between 0 and 1 (values listed in Table 2) indicating favourable adsorption of the dyes on TIFS.

Figure 3. Effect of pH on the adsorption of dyes on TIFS Click here to view figure

Adsorption Kinetics

The kinetic curves obtained (conditions: for RB adsorption 0.25g TIFS/50 mL of 25mg/L RB solution and for MG adsorption 0.10g TIFS/50mL of 50mg/L MG solution) are shown in figure 1 and the results of kinetic analyses were presented in tables 3 and 4. The high correlation coefficients and the good agreement between the theoreticalq_eand experimentalq_evalues for the II order model suggest that the sorptions are better described by this model. Such a betterment of the II order model over I order model has been observed for many adsorption processes.¹⁵

Table 7: Isotherm constants for adsorption of Dyes at 60°C Click here to view table

Attempt has been made to find whether film or particle diffusion of the dye molecules determine the overall order assuming them ions. According to Boydet al.¹⁷film diffusion will be rate-determining if a graph of time versus ln(1-F) yields a straight line and particle diffusion control the overall adsorption rate if a plot oft^0.5againstF(expressed in equation 7) produces a straight line. Such linear relationships do exist for the systems under study (figures not shown) and an additional quantitative treatment proposed by Boyd¹⁷and Reichenberg¹⁸as adapted others^{19, 20}was followed. The sorption dynamics can be represented by the following expressions:

F = q_t/ q_e...(7)

F = 1- 6/π²∑_n=1^∞1/n²exp (D_it π²n²/ r²) ...(8)

F = 1- 6/π²∑_n=1^∞1/n²exp (-n²Bt)…(9)

where,

B = D_iπ²r²= time constant...(10)

F= fractional attainment of equilibrium at timetD_i= effective diffusion coefficient of the ions in the adsorbent phase

r= adsorbent particle radius

n = 1,2,3,¼ are the integers defining the infinite series solution obtained by a Fourier type of analysis.

Btvalues were derived for eachFvalueby the use of Reichenberg’s table.¹⁸A plot oftversusBtwas employed to assess the contributions of filmand particle diffusion on rates of adsorption. Both thetversusBt情节是线性的(在图2)出于on over figure 2 reveals that dye adsorptions under study pass near the origin indicating that the rate-limiting step for these processes are predominately governed by particle diffusion constraints.

Table 8: Thermodynamic parameters for adsorption of dyes on TIFS Click here to view table

The effective particle diffusion coefficient values (D_i’s) are calculated by equation (10) whereB是the slope of thetversusBtplots. Since in thepresent study, adsorbents of particle sizes ranging

from 150 - 250 µm were used, the average of them, 200 µm was taken as the mean particle diameter, which would give 100 × 10^-4cm as the mean particle radius (r). TheBandD_ivalues calculated are listed in Table 5.

Increase in the pH of the dye solutions lead to increased adsorption for both the dyes Fig. 3. This is as expected for the exchange of any cationic dye. Increase of solution pH increases the surface charge of any adsorbent which will eventually result in greater tendency to attract positively charged species like MG and RB.

Effect of Temperature and Thermodynamics

Equilibrium adsorption studies were conducted at two more temperatures, namely 45 and 60°C apart from room temperature. The adsorptions increased with increase in the operating temperature suggesting that the processes are endothermic. This is also seen in the trends of Langmuirq_mandK_Lvalues. The former increases but the latter decreases with increase in temperature (Tables 6 and 7). The increased adsorption at higher temperatures can be due to acceleration of some originally slow step(s),²¹creation of some new activation sites on the adsorbent surface²²or decrease in the size of the adsorbing species as desolvation may occur at high temperatures.

Table 9: Comparison of removal efficiency of TIFS towards RB and MG with other adsorbents reported in the literature Click here to view table

Thermodynamic parameters such as Gibbs’s free energy change ( G), enthalpy change ( H) and entropy change ( S) were calculated using the following expressions:

K_C= C_ae/ C_e...(11)

G = - RT ln K_C...(12)

log K_C= [ S/ 2.303 R] - [DH/ 2.303 RT] ... (13)

where,

K_C= equilibrium constant for the distribution of dyes between the two phases
C_ae= solid phase dye concentration, mg/L
C_e= liquid phase dye concentration, mg/L
T = absolute temperature, °K
R = gas constant

Equation (13) was used to construct Van’t Hoff plots and H and S were calculated from the slope and intercept of the Van’t Hoff plot respectively. Thermodynamic parameters evaluated for varied dye concentrations are listed in Table 8.

The negative values of G obtained for the adsorptions reflect the spontaneity. The positive values of H indicate the endothermic nature and the positive values of S indicate increased randomness at the interface. The rather large S values speak for the large size of the species that are being adsorbed; a single dye molecule displaces a lot of water molecules from the adsorbent surface.

Comparison of removal efficiency of TIFS with other adsorbents

Finally an attempt has been made to compare the removal efficency of TIFS with other adsorbents reported in literature (Table 9). It is found that the capacity of TIFS in removing the dyes selected in this study is comparable to other adsorbents reported.

References