Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 185

Figure 5.25 Effect of pH on adsorption of RB and BB onto untreated coffee residues [116].
adsorption temperatures for both of the dyes due to the increased penetra-
tion of the dye molecules in the porous structure of the adsorbent. Also,
the agitation rate was found to have a considerable impact on the removal
capacity of the adsorbent. According to Figure 5.26, as the agitation speed
increased, the boundary layer resistance decreased, the mobility of the dye
molecules increased and the removal percent was increased. Desorption
experiments for RB and BB were carried out at extreme alkaline and acidic
media, respectively. The results showed the RB and BB desorption of 90
and 94%, respectively, in the aforementioned media. In order to investigate
the possibility of the reuse of the adsorbent, the adsorption-desorption
experiments were conducted in many cycles and the reduction in adsorp-
tion percentage was revealed to be only 7% for BB and 10% for RB (see
Figure 5.27). This reduction is sufficiently low enough to make it a suit-
able adsorbent for further reuse. The decrease in the adsorption capacity
after several cycles was attributed to a progressive saturation of the surface
active sites of the adsorbent, pore blockage by the impurities or the degra-
dation of the material at extreme pH conditions.
Reffas et al. [117] used a chemical activation method to prepare acti-
vated carbon from coffee grounds. The washed coffee grounds were
impregnated with different amounts of phosphoric acid as the activating