Optimum pH

Adsorption experiments were performed over a range of pH starting from 2.00 ± 0.01 to 9.00 ± 0.01. Figure 3 shows that the adsorption of RR158 was highest atpH 2.00 ± 0.01 which was 51.51 ± 10.60% and as pH increased, the dye

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Concentration of dye {mg/L)

0 20 40 60 SO 100

Concentration of dye {mg/L)

Fig. 2 Graph of absorbance against concentration of dye

Fig. 3 Dye removal at the corresponding pH values

removal was in a range of 17.00 ± 10.60to33.00 ± 10.60%. Hence, it was observed that adsorption of dyes is highly pH dependent, which influences the adsorption of both anions and cations at the solid-liquid interface. The effect of pH on the adsorption capacity can be attributed to the chemical forms in the solution at a specific pH and also due to different functional groups on the adsorbent surface, which become active sites for the metal biding at a specific pH. Therefore, an increase in pH may cause either an increase or a decrease in the adsorption capacity, resulting

Fig. 4 Graph of removal of dye with respect to time. Experimental conditions: at room temperature and pressure and 200 rpm, pH: 2 and dosage of adsorbent: 7.5 g/L

in different optimum pH values dependent on the type of the adsorbent. The increase in the dye adsorbed at lower pH 2 could be well explained by protonation properties of the adsorbent. At low pH the negative charges at the surface of internal pores are neutralized and some more new adsorption sites are developed and as a result the surface provided a positive charge for the RR158 dye to get adsorbed thus increasing the uptake of dye. Above pH 7 the RR158 dye particles acquire a negative surface charge leading to a lesser dye uptake since dye molecule becomes neutral at that pH value. Also, in alkaline medium, precipitation tends to appear and interfere with the accumulation or adsorbent deterioration as the dyes particles tend to flocculate and formed a cake layer thus reducing adsorption efficiency [12].

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