Contribution of Cross-Linker and Silica Morphology on Cr(VI) Sorption Performances of Organic Anion Exchangers Embedded into Silica Pores

Abstract

Removal of Cr(VI) from the environment represents a stringent issue because of its tremendous effects on living organisms. In this context, design of sorbents with high sorption capacity for Cr(VI) is getting a strong need. For this purpose, poly(vinylbenzyl chloride), impregnated into porous silica (PSi), was cross-linked with either N,N,N',N'-tetramethyl-1,2-ethylenediamine (TEMED) or N,N,N',N'-tetramethyl-1,3-propanediamine, followed by the reaction of the free -CH₂Cl groups with N,N-diethyl-2-hydroxyethylamine to generate strong base anion exchangers (ANEX) inside the pores. The PSi/ANEX composite sorbents were deeply characterized by FTIR spectroscopy, SEM-energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and water uptake. The sorption performances of composites against Cr(VI) were investigated as a function of pH, contact time, initial concentration of Cr(VI), and temperature. It was found that the cross-linker structure and the silica morphology are the key factors controlling the sorption capacity. The adsorption process was spontaneous and endothermic and well described by pseudo-second-order kinetic and Sips isotherm models. The maximum sorption capacity of 311.2 mg Cr(VI)/g sorbent was found for the composite prepared with mesoporous silica using TEMED as cross-linker. The PSi/ANEX composite sorbents represent an excellent alternative for the removal of Cr(VI) oxyanions, being endowed with fast kinetics, equilibrium in about 60 min, and a high level of reusability in successive sorption/desorption cycles.

Keywords: anion exchanger; chromium (VI); cross-linker; porous silica; reusability; sorption isotherm; sorption kinetics.

Scheme 1

Synthesis of porous silica (PSi)/anion exchangers (ANEX) composite by impregnation first of poly(vinylbenzyl chloride) (PVBC) into PSi pores followed by cross-linking with TEMED, and then generation of the quaternary ammonium salt groups by the reaction of the free -CH₂Cl groups with N,N-diethyl 2-hydroxyethyl amine (DEHEA); RT-room temperature.

Figure 1

FTIR spectra of the PSi/ANEX composites before loading with Cr(VI) and of the composite PSi1/ANEX1 after loading with Cr(VI).

Figure 2

TG (a) and DTG (b) curves of the PSi/ANEX composite sorbents.

Figure 3

SEM images of the exterior (mag 1000×), and the interior morphology (mag. 4000×) of the PSi/ANEX composites before loading with Cr(VI) of the PSi1/ANEX1 (a₁ and a₂), PSi1/ANEX2 (b₁ and b₂), PSi2/ANEX2 (c₁ and c₂) composites, and after loading with Cr(VI) of PSi1/ANEX1 (a₃ and a₄), PSi1/ANEX2 (b₃ and b₄), and PSi2/ANEX2 (c₃ and c₄).

Figure 4

Potentiometric titrations of bare silica microspheres (PSi1 and PSi2) and of the PSi/ANEX composites. pH_PZC for PSi1/ANEX1 has been measured both before (empty triangle) and after loading with Cr(VI) (filled triangle).

Figure 5

Effect of pH on the sorption capacity (q_e) of the SiO₂/ANEX composite sorbents for Cr(VI) oxyanions: sorbent dose 1 g/L; temp. 23 °C; contact time 6 h; shaking rate 180 r.p.m.

Figure 6

(a) Sorption kinetics of Cr(VI) onto PSi/ANEX composite microspheres fitted with three kinetic models: (b) Fitting IPD model on the sorption kinetics data of Cr(VI) onto the PSi/ANEX composites: sorbent dose 1 g/L; temp. 23 °C; initial pH 4.0; initial concentration of Cr(VI) 100 mg/L.

Figure 7

Sorption isotherms of Cr(VI) onto PSi.1/ANEX1 (a), PSi.1/ANEX2 (b), PSi.2/ANEX2 (c), and removal efficiency as a function of number of sorption/desorption cycles (d); sorption conditions: sorbent dose 1 g/L, pH 4, temperature 23 °C, contact time 6 h; desorption conditions: 0.1 M NaOH, 6 h, pH 4.

Figure 8

Plot of ln K_c versus 1/T for the sorption of Cr(VI) ions onto the porous silica/ANEX composite sorbents; sorption conditions: sorbent dose 1 g/L, pH 4, C_in = 100 mg/L, contact time 6 h.