Color has always been very important in nature and environment and it continues to play a very significant role in imparting interesting hues to plants, animals and in the lives of mankind. After the dyeing process the residual and unspent substances are usually discharged into the environment. Amongst the different industrial wastewaters with different types of color-causing substances, synthetic textile organic dye wastes occupy a prominent position [1]. The colloidal matter can often be carcinogenic; show allergic reactions; interfere with photosynthesis; clog the pores of the soil; be a breading ground for bacteria and viruses. It is important to remove these pollutants from the waste waters before their final disposal [2-4]. Adsorption of organics from solutions on activated carbon is one of the oldest and widespread applications of this material. Earlier studies of activated carbon adsorption were carried out on fatty acids and later extended to a large number of dyes [5].

Nevine Kamal Amin [6] investigated the use of activated carbons prepared from pomegranate peel for the removal of direct blue dye from aqueous solution. Azza Khaled et al. [7]. studied the removal of Direct N Blue-106 from textile dye effluent using activated carbon from agricultural waste material: orange peel. Equilibrium adsorption isotherm for the removal of basic dye (Methylene Blue) from aqueous solution using bituminous coal-based activated carbon has been examined by Emad N et al. [8]. The effect of experimental parameters, namely, pH and adsorbent particle size were studied and the maximum adsorptive capacity was determined. B. H. Hameed [9] researched on activated carbon prepared from non-wood forest product waste using it as adsorbent for the removal of methylene blue dye from an aqueous solution.

Abhiti Purai et al. [10] tested the ability of cow dung ash, an eco friendly and low cost adsorbent, without any pretreatment to remove color from textile dyes. The adsorption was achieved under different pH and adosrbate concentration.

The present work forms a part of continuing study to compare the adsorptive capacity of different samples of activated carbon and low cost adsorbents.

Samples of Granular Activated Carbon C₁ and C₂ used were obtained from Brillex Chemical Ltd. Punjab and Industrial Carbon Pvt. Ltd., Gujarat. Surface area of GAC C₁ used in the study was 950 m²/g and that of GAC C₂ was 600 m²/g. Bulk density of the two was 500~550 and 600~650 g/L respectively. The ash content was 6% in C₁ and 5% in C₂. From the stock solution of the dyes dilutions were made with distilled water to make different concentrations. Optical density of all the solutions was measured on a spectrophotometer (ELICO make, wavelength range 200~

900 nm). One gram of activated carbon was placed in each 50 mL solution of 10 to 100 ppm. The solutions were shaken and kept in a thermostat for 24 h. The samples were then filtered and analyzed spectrophotometrically.

Table 1 and Figs. 1~3 show the amount q_e of Reactive Dye Blue 221, N Blue RGB and Acid Blue MTR adsorbed by C₁ and C₂ samples of granulated activated carbon at various pH values and various ppm concentrations. It can be seen that q_e on GAC C₁ and C₂ was higher at lower ppm concentration as compared to the adsorption at higher ppm concentration with the adsorbent dose being kept constant for all the three dyes. At 10 ppm concentration and initial (slightly acidic) pH of 5.49 C₁ adsorbed 0.645 mg/g of the dye Reactive Blue 221 and C₂ adsorbed 0.495 mg/g. At 100 ppm concentration the adsorption of Reactive Blue 221 was 2.978 mg/g, 2.470 mg/g, on C₁ and C₂ respectively at the same pH. This pattern of higher adsorption on lower ppm concentrations was observed for all the three reactive dyes. C₁ showed better adsorption as compared to adsorption on C₂.

Adsorption of the three dyes has been studied at different pH for carbon sample C₂ and at one pH for C₁. C₂ was greatly effected by change in pH of the aqueous solution as can be seen in Table. 1 and Figs. 1~3. Amount of dye adsorbed q_e on C₂ in acidic pH is higher at all ppm concentrations as compared to adsorption in a lesser acidic and an alkaline environment. At an acidic pH of 3.43 at 10 ppm concentration C₂ adsorbed 5.21 mg/g of the dye Reactive Blue 221. In an alkaline pH of 9.27 the same C₂ adsorbed only 3.90 mg/g leaving behind a good number of free adsorption sites on the adsorbent. The same pattern could be observed on N Blue RGB and Acid Blue MTR. At 10 ppm concentration C₂ adsorbed 5.40 mg/g and 5.10 mg/g of N Blue RGB and Acid Blue MTR at acidic pH. At alkaline pH the adsorption was only 4.10 mg/g and 3.80 mg/g respectively at the same 10 ppm concentration of N Blue RGB and Acid Blue MTR.

C₁ as compared to C₂ shows higher adsorption at initial pH .It also shows better adsorption than C₂ did in acidic environment.

It can be seen from Figs. 1~3 that the experimental data fitted well to polynomial equation of the type:

q_e=A+B₁C_e+B₂C_e²+B₃C_e³

The constants and standard deviation are given in Table 2. Comparative adsorption of all the dyes on C₁ & C₂ at various pH values is shown in Figs. 1~3. Maximum dye removal of N Blue RGB was observed on C₁ at acidic pH 5.2. Least adsorption efficiency of all the three dyes was shown by C₂ at alkaline pH values. On the whole C₁ exhibited most favourable adsorption at initial pH values as compare to C₂. C₂ showed maximum adsorption at acidic pH.

The experimental data was fitted to linear form of

Langmuir isotherm for all the pH values of the three dyes and for the two samples of GAC.

Q and b the Langmuir constants were calculated from the straight line slope and intercept of linear plot between 1/q_e and 1/C_e as shown in Fig. 4 to 6.

Table 3 gives the values of Langmuir constants Q and b along with the values of r² and SD for all the three dyes and their varied pH values and for the two samples of GAC, C₁ and C₂. Value of constants Q b signify good adsorption of all

the three dyes on C₁ as compared to C₂ at initial pH. It also indicates a better adsorption at acidic pH on C₂.

The linear plots of log q_e and log C_e for Freundlich isotherm are shown in Figs. 7~9. Table 4 shows the Freundlich constants KF and n calculated from the slope and intercept of log q_e and log C_e along with R² and SD. K_F (parameter relative to adsorption capacity) and n (process intensity) were calculated. Values of constant K_F indicate higher adsorption capacity on Carbon sample C₁ as compared to C₂.

Granulated Activated Carbon sample C₁ and C₂ can be effectively used for the removal of dyes from wastewater by adsorption. The present study shows that there is a decrease in percentage removal of dye of carbon per gram with increase in ppm concentration of the dye. Change in pH values showed higher adsorption at acidic pH. The three dyes obeyed Langmuir and Freundlich isotherms. Langmuir isotherm gave a better fit. Carbon sample C₁ showed high adsorption for all the three dyes.