Influence of carbohydrate polymer shaping on organic dye adsorption by a metal–organic framework in water

A number of studies have been conducted to develop new metal–organic frameworks (MOFs) as adsorbents for the removal of contaminants from polluted water. However, few reports exist describing detailed and thorough examinations of the effects of shaping on the adsorption properties of MOFs. In this study, a thorough analysis and comparison was conducted of the Orange II and Rhodamine B dye adsorption properties of unshaped MIL-100(Fe) (MIL) particles and alginate polymer-shaped MIL beads (MIL-alg). The adsorption affinities of Orange II and Rhodamine B for unshaped MIL were observed to be higher than those for shaped MIL-alg because partial coating of the surface of MIL particles by alginate polymer weakens adsorption forces. Kinetic analysis using a two-compartment model indicates that the contribution of the slow step in the mechanistic pathway for adsorption is more pronounced in MIL-alg compared to MIL because slow dye diffusion takes place in the alginate polymer. We believe that these fundamental findings will have a beneficial impact on approaches to design shaped MOFs that display improved dye removal performance.


S4. N 2 adsorption/desorption isotherm and t-plot
The theoretical micropore volumes of dye adsorbed MIL assuming that all dye molecules are adsorbed to the outer surface of MIL were calculated using the following equation, where V theor is the theoretical micropore volume of dye adsorbed MIL (cm 3 /g), V MIL is the observed micropore volume in MIL (cm 3 /g) and m dye is the mass of adsorbed dye (g). The adsorption amounts of dyes were determined in the same procedure described in the main text.
V exp , which is the experimental micropore volume (cm 3 /g), was calculated from t-plots (Fig.   S5). The theoretical dye adsorption amounts were calculated using V theor and the dimensions of dye molecules shown in Fig. 5.

Table S2
Micropore volumes, theoretical micropore volumes and theoretical dye adsorption amounts.
The theoretical dye adsorption amounts (see Table S2) were lower than the experimental values (424 and 164 mg/g for Orange II and Rhodamine B, respectively) probably because of overestimated molecular dimensions of dyes and removal of adsorbed dyes during the washing process.
The external surface area of MIL was calculated from the t-plot. The theoretical monolayer adsorption amounts of dyes on external surface of MIL were calculated using the external surface area and maximum cross-sectional areas of dye molecules estimated from the dimensions in Fig. 5.

Table S3
External surface area of MIL and theoretical monolayer adsorption amounts of dyes on external surface of MIL.  Table S4. Weight percentage data obtained from the EDX analysis (Fig. S6).  Table S5. Weight percentage data obtained from the point analysis (Fig. S7).

S7. Comparison of adsorption amounts on various adsorbents for Orange II and
Rhodamine B Table S6. Orange II and Rhodamine B adsorption amounts on previously reported adsorbents.

S10. Adsorption isotherm models, affinity distribution function and adsorption kinetic models
The adsorption isotherms were fitting using the Langmuir-Freundlich (LF) model (equation S1), S17,S18 where q e is the adsorption amount (mg/g), q m is the theoretical maximum adsorption amount (mg/g), a is the affinity constant and n is the heterogeneity parameter with the range from 0 to 1 and C e is the equilibrium concentration (mg/L).
The affinity distribution analysis based on the LF model was performed using the equation S2, S17,S18 where N i is the number of adsorption sites and K i is the association constant. K i has the range between K max = 1/C min and K min = 1/C max , in which C min and C max are the maximum and minimum equilibrium concentrations, respectively.
The adsorption kinetic analysis was performed using the following pseudo first-order (eq S3), pseudo second-order (equation S4) and pseudo nth-order (equation S5) models, S19,S20 where q t is the adsorption amount at time t (mg/g), q e is the equilibrium adsorption amount (mg/g), k 1 , k 2 and k are the kinetic rate constants and n is the reaction order.
The adsorption kinetic data were also analyzed using the following two-compartment kinetic model ( where q fast and q slow are the adsorption amounts of dye per gram of MIL (mg/g) at time t (min) of fast and slow adsorption, respectively, and k fast and k slow are the adsorption rate constants (1/min). (S6)