Study on Adsorption of Malachite Green by Date Palm Fiber

In this study, the adsorption of malachite green (MG) onto date palm fibers (DPF) was examined in aqueous solutions. The Langmuir and Freundlich adsorption isotherm models were employed to describe the equilibrium adsorption data. The Freundlich isotherm model was found to fit the adsorption data The removal percentage of MG dye from aqueous solutions via adsorption onto DPF adsorbent varied with pH and temperature, showing that these factors play a key role in the adsorption process. The removal percentage was found to increase with decrease in the solution temperature and in alkaline media. The adsorption kinetic data were fitted with a pseudo-secondorder model.


INTRODUCTION
Color removal from industrial effluents has been a subject of investigation because of its potential toxicity.Malachite green (MG) is one of the dyes that is extensively used in the textile industry.It exhibits toxic properties that are known to cause carcinogenesis, mutagenesis, teratogenesis and respiratory toxicity 1,2 .In recent times, adsorptiontechniques for wastewater-treatment have become very popular because of to their improved efficiency in the removal of pollutants too stable for biological methods 3 .Research in the past few years has been focused on using agricultural wastes as adsorbent materials because they are inexpensive, eco-friendly and renewable.For instance, pumpkin seeds 4 , the husk of mango seeds 5 , cotton plant 6 , coconut bunch wastes 7 , orange peels 8 , and pineapple stem 9 have been studied as potential adsorbents.Date palm is an important fruit crop in Saudi Arabia as well as around the world.It is estimated that the annual date palm agricultural wastes comprise more than 20 kg of dry leaves and fibers for each date palm tree 10 .Thus, utilizing the date palm waste as a potential adsorbent for wastewater treatment via adsorption is of considerable interest.In this study, the adsorption of MG from aqueous solutions was studied using the brownish fibrous sheath that surrounds the trunk of the date palm (known as leef) as an adsorbent.In addition, we studied the process kinetics.The removal percentage was calculated to determine the influence of pH and temperature on the adsorption.

MATERIALS AND METHOD
The date palm fibers (DPF) wastes were obtained a local from a farm in the southern region of Riyadh city in Saudi Arabia.They were cut, ground and sieved to a size ≤ 63 µm.A typical date palm fiber can be seen in Fig. 1 10 .MG was received from Techno Pharmchem (India) was used as an adsorbate.The dye content is at least 90%.Test solutions of desired concentrations are prepared from the stock solution using distilled water.
Adsorption tests were carried out by using batch experiments.For each test, 0.5 g of the adsorbent was placed in screw-capped Erlenmeyer flasks containing 25 mL of MG solution.The flasks were shaken constantly for a sufficient period to achieve equilibrium using an orbital shaker (J.P. Selecta, Spain) at 100 rpm and 25°C.Then the solution was filtered using Whatman filter paper (125 mm Ø, cat.No. 1001 125).The dye uptake was monitored with a spectrophotometer (Jenway model 6800 UV/VIS) by measuring the absorbance at l max = 616 nm.The adsorption at the equilibrium, q e (mol/g), was calculated using the following equation: where C o and C e (mol/L) are the initial and the equilibrium liquid-phase concentrations of the dye, respectively.V is the volume of the solution (in liters), and m is the mass of the dry adsorbent (in grams).The equilibrium adsorption data were fitted with two isotherm models, the Langmuir and Freundlich models.
The effect of pH was studied by preparing suitable adsorbent-adsorbate solutions at room temperature with fixed adsorbent dosage and initial dye concentration at different pH.The pH was adjusted by adding NaOH (1 M) or HCl (1 M) solutions.The temperature effect on the adsorption process was studied by varying the adsorption temperature with a constant initial concentration of the solution.The removal percentage was calculated using Eq.(2).
The kinetic study was also performed at room temperature with a flask that was identical to the one used for batch equilibrium tests.The flask was shaken constantly to achieve equilibrium and the aqueous samples were collected at presettime intervals, and concentrations of MG were measuresed.

Adsorption study
Adsor ption isother ms indicate how adsorbates are distributed between the liquid phase and the solid phase when the adsorption process reaches equilibrium 11 .The adsorption isotherm is important for understanding how the adsorbate interacts with the adsorbent, giveing an idea of the adsorption capacity of the adsorbent 12 .Fig. 2 illustrates the adsorption isotherm of MG onto DPF.The maximum experimental adsorption capacity for the dye onto DPF is approximately 0.2136 mol g -1 .
From the figure, we can be see that the adsorption isotherm is an S type (based on Giles et al. classification), ndicating that the adsorption becomes easier as the concentration increases 13 .In practice, the S curve of the adsorption isotherm indicates a strong competition between the solvent and the adsorbed species for the adsorbent surface sites 13 .Analyzing the isotherm data by fitting it to different isotherm models is important for identifying a suitable  10 model that can be used for design purposes 14 .The adsorption isotherm models of Langmuir (Eq.( 3)) and Freundlich (Eq.( 4)) were applied to describe the sorption data.The applicability of the isotherm equation to describe the adsorption process was judged in terms of the correlation coefficients (R 2 ) and the maximum adsorption capacities.
where C e is the equilibrium concentration, q e is the amount of adsorbate adsorbed per unit mass of adsorbent (in mol/g), and Q o and b are the Langmuir constants related to the adsorption capacity and rate of adsorption, respectively.In the Langmuir model, a plot of 1/C e against 1/q e yields a straight line with a slope of (1/bQ 0 ) and an intercept of (1/Q 0 ), as shown in Fig. 3.
where K f and n are Freundlich constants related to the adsorption capacity and adsorption intensity, respectively.The n value indicates the degree of nonlinearity between solution concentration and adsorption as follows: if n=1, then the adsorption is linear; if n > 1, then the adsorption is a physical process; and if n < 1, then the adsorption is a chemical process 15 .In the Freundlich model, a plot of log q e against log C e yields a straight line with a slope of 1/n and an intercept of log K f , as shown Fig. 4. The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity 16 .It becomes more heterogeneous as its value gets closer to zero; a value of 1/n below one indicates a normal Langmuir isotherm while that above one is indicative of cooperative adsorption 16 .
The constants in these models, i.e., Q o, b, K f , and n, and the correlation coefficients,R 2 are given in Table 1.According to the R 2 values, both models seemed to represent the equilibrium adsorption data.However, Langmuir isotherm is difficult to interpret because it is characterized with a negative q e value.Therefore, the adsorption of MG onto the adsorbent could be described by the Freundlich model.It was found that the n value in Freundlich equation is <1, indicating that the process is cooperative because the value of the slope 1/n is above unity.

Effect of pH
Fig. 5 illustrates the effect of pH on the removal of MG dye by DPF in the pH range 4-10.It is noticeable from the figure that the maximum removal is 95.725% at pH = 10 and the removal of dye increases with increasing pH.This result could be explained by the attraction forces between different charges; the adsorbent surface becomes negatively charged in alkaline media, resulting in an attraction between the positively charged dye molecule and the negatively charged adsorbent.In acidic media, the presence of excess H + ions competing with the positive charge of dye for the adsorption sites, and therefore the removal of dye slows down.
Effect of temperature Fig. 6 gives information about the percentage removal of the MG by DPF at different temperatures.As observed, the percentage removal decreases slightly with increase in temperature from 25 to 45 °C and the maximum removal is 94.3% at 25°C.A similar trend was reported by Chandra et al. 17 for the adsorption of Mb onto activated carbon prepared from durian shell; the researchers suggest that the physical bonding between the organic compound (including the dye) and the active sites on the adsorbent weakens as the temperature increases.In addition, the dye solubility also increases, which causes the interaction between the solute and the solvent to become stronger than that between the solute and adsorbent.Therefore, the solute is less likely to be adsorbed.The highest MG removal of 94.3% was achieved at 25°C.

Kinetic Study
Two models were used to analyze the adsorption kinetics of MG by DPF at 25°C.The first model was pseudo-first-order, and therefore the Lagergren equation was applied.It is given as follows 18 : ln(q e -q t ) =ln q e -K 1 t ... (5)   where q e is the adsorption capacity at equilibrium, q t is the adsorption capacity at time t and k 1 is the rate constant.A plot of ln(q e -q t ) versus t is linear and K 1 could be calculated from the slope while q e could be determined from the intercept of the plot (the figure is not shown).The adsorption process could not be described using the model because the value of correlation coefficient R 2 was low and the experimental q e value did not agree with the value calculated using the equation.
Second model was based on the linear pseudo-second-order equation.It is given as follows 19 : (t/q t ) = 1/(K 2 q e 2 ) + (1/q e )t ...( 6) where k 2 is the pseudo-second-order rate constant.The slope of the plot of t/q t versus t gives the value of q e , and the intercept can be used to calculate K 2 .The plot of t/q t versus t yields a straight line, as shown in Fig. 7.The calculated value of q e for the pseudo-second-order model is 0.080 (mol g -1 ), which agrees well with the experimental value at 25°C (= 0.0858 mol g -1 ).Moreover, the R 2 value for this model is 0.999, indicating an excellent fit.The value of K 2 is found to be 454.215(gmol -1 min -1 ).

Fig. 7 :
Fig. 7: pseudo-second-order model of the kinetic adsorption of MG dye by DPF