Effect of pore structure on the adsorption of aqueous dyes to ordered mesoporous carbons

• Pore shape has different degrees of impact on adsorption of dyes varying in size.
• Open pores favor adsorption especially for bulky dyes.
• Carbon with open pores has much faster adsorption rate than closed porous carbon.

Three ordered mesoporous carbons, CMK-3, CMK-5 and its carbon/silica composite Si-CMK-5, with different pore structures were investigated for the adsorption of aqueous Acid Red 73 and Reactive Black 5. Among the adsorbents, CMK-5 shows the highest adsorption amount and fastest adsorption rate towards the two dyes, resulting from its double pore systems and large specific surface area. After normalizing by the specific surface area of the adsorbents, the adsorption of relatively small Acid Red 73 on the three adsorbents is close to each other. However, the adsorption of Reactive Black 5 on CMK-3 is stronger than on CMK-5 and Si-CMK-5, reflecting that the open and interconnected pores of CMK-3 are more favorable to adsorption of bulky dyes compared with the relatively closed and isolated pores of Si-CMK-5. The two dyes adsorption over these adsorbents can be well described using Langmuir model. CMK-5 adsorption toward Acid red 73 or Reactive black 5 is well fitted with double Langmuir model. Though the pore size of CMK-3 is smaller than that of Si-CMK-5, for a given adsorbate, the adsorption rate of CMK-3 is more rapid than that of Si-CMK-5 because of its open and interconnected pore features. In addition, the simulated results using Weber–Morris modal confirm that the adsorption processes of the dyes on the three adsorbents are affected by external mass transfer and intraparticle diffusion. External mass transfer is the slow step during adsorption process based on the results of Boyd model.

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