Removal of Cr(VI) from Aqueous Solution by Polypyrrole/Hollow Mesoporous Silica Particles.

Abstract: The removal of Cr(VI) in wastewater plays an important role in human health and environment. In this work, polypyrrole/hollow mesoporous silica particle (PPy/HMSNs) adsorbents have been newly synthesized by in-situ polymerization, which prevent the aggregation of pyrrole in the process of polymerization and exhibit highly selective and powerful adsorption ability for Cr(VI). The adsorption process was in good agreement with the quasi-second-order kinetic model and the Langmuir isotherm model. And the maximum adsorption capacity of Cr(VI) was 322 mg/g at 25 °C. Moreover, the removal rate of Cr(VI) by PPy/HMSNs was ~100% in a number of binary systems, such as Cl-/Cr(VI), NO3-/Cr(VI), SO42-/Cr(VI), Zn2+/Cr(VI), Fe3+/Cr(VI), Sn4+/Cr(VI), and Cu2+/Cr(VI). Thus, the PPy/HMSNs adsorbents have great potential for the removal of Cr(VI) in wastewater.

where Qe is the mass of Cr(VI) absorbed by adsorbents per unit mass (mg·g -1 ), C0 is the initial Cr(VI) concentration (mg·L -1 ), Ce is the Cr(VI) concentration after adsorption (mg·L -1 ), V is the volume of the solution (mL) and m is the weight of the adsorbent (mg).

Adsorption properties of PPy/HMSNs at different pH
The adsorbents (25 mg) was added to the Cr(VI) solution (25 mL) with an initial concentration of 100 mg/L at different pH (2.0~10) for 24 h at 25 °C. Then, the mixture was filtered to measure the content of Cr(VI). And the removal rate of Cr(VI) was calculated using Eq. 2 to determine the optimum pH.

Adsorption properties of PPy/HMSNs with different dose
The PPy/HMSNs (10~100 mg) was added to the Cr(VI) solution (25 mL) with an initial concentration of 400 mg/L at pH 2.0 for 24 h at 25 °C. Then, the mixture was filtered to measure the content of Cr(VI). And the removal rate and adsorption capacity of Cr(VI) was calculated using Eq. 1and Eq. 2 to determine the optimum dose.

Influence of co-existing ions for the Cr(VI) adsorption
The PPy/HMSNs (20 mg) was added to 20 mL of the binary solution system (the concentration of Cr(VI), Cl -, NO3 -, SO4 2-, Zn 2+ , Fe 3+ , Sn 4+ and Cu 2+ were all 100 mg/L) at pH 2.0 for 24 h at 25 °C. Then, the mixture was filtered to measure the content of Cr(VI). And the removal rate of Cr(VI) was calculated using Eq. 2.

Cyclic adsorption
The PPy/HMSNs (50 mg) was added to the Cr(VI) solution (20 mL) with an initial concentration of 100 mg/L at pH 2.0 and 25 °C. After adsorption, the mixture was filtered. Then, the adsorbent was desorbed using NaOH solution (0.1 mol/L) for 3 h and washed using H2O. Finally, the adsorbent was dried at 60 °C for 6 h, and the adsorption-desorption experiments were repeated 5 times.

Adsorption kinetics
The adsorbents (30 mg) was added to the Cr(VI) solution (200 mL) with different initial concentration (25, 50 and 70 mg/L) at pH 2.0 and 25 °C. Then, some mixture solutions were collected and filtered to measure the concentration of Cr(VI) at time t. And the adsorption capacity (Qt) of Cr(VI) at time t was calculated using Eq. 3. Finally, the data were fitted by quasi-first-order kinetic (Eq. 4), quasi-second-order kinetic (Eq. 5) and particle diffusion (Eq. 6) models.
where Qt is the mass of Cr(VI) absorbed by adsorbents per unit mass (mg·g -1 ) at time t, Ct is the Cr(VI) concentration after adsorption (mg·L -1 ) at time t, K1 is the pseudo-first-order kinetic adsorption rate constant (min -1 ), K2 is the pseudo-second-order kinetic adsorption rate constant (g·mg -1 ·min -1 ), Kip is the particle diffusion model adsorption rate constant (mg·g -1 ·min 0.5 ), and C is the intercept related to the boundary layer thickness.

Adsorption isotherms
The adsorbents (25 mg) was added to the Cr(VI) solution (50 mL) with different initial concentration (50~800 mg/L) at pH 2.0 and different temperature (25, 35 and 45 ℃). Then, some mixture solutions were collected and filtered to measure the concentration of Cr(VI). And the adsorption data were fitted by Langmuir (Eq. 7) and Freundlich (Eq. 9) models.
where Qm is the maximum adsorption capacity (mg·g -1 ), b is the adsorption free energy constant (L·mg -1 ), RL is a nondimensional factor, the Freundlich constant (K) indicates the relative adsorption capacity of the adsorbents (mg·g -1 ), 1/n is the adsorption strength.

Adsorption thermodynamics
The Gibbs free energy change (ΔG 0 ), enthalpy change (ΔH 0 ) and entropy change (ΔS 0 ) are the major parameters in the adsorption process. The parameters are calculated by the Eqs. 10-12.