Adsorption Behavior of Arsenate at Transition Metal Cations Captured by Amino-Functionalized Mesoporous Silicas

Abstract

We show that Fe 3+, Co 2+, Ni 2+, and Cu 2+ fixed by diamino-functionalized MCM-41 and MCM-48 work as adsorption centers for arsenate ions. The stoichiometry of the N/metal cation was determined as well as the As/metal cation for the adsorption of arsenate. The effectiveness of the removal of arsenate from a dilute solution is ordered according to Fe 3+ > H + ∼ Co 2+ > Ni 2+ ∼ Cu 2+, which is slightly different from the adsorption capacity: Fe 3+ > Co 2+ ∼ H + > Ni 2+ > Cu 2+ . The distribution coefficients, K d, exceed 200 000 at [arsenate] < 100 ppm in Fe/NN-MCM-41. The Fe center binds nearly three arsenate ions, which allows the adsorption capacity of Fe/NN-MCM-48 to be one of the largest at 2.5 mmol (g of adsorbent) - 1 among the adsorbents of arsenate. The difference between the meso-framework structures appears in their adsorption capacity; cation/NN-MCM-48 adsorbs a larger amount of arsenate than cation/NN-MCM-41. This is mainly caused by the difference of cation content in these functionalized mesoporous silicas. A sudden decrease in K d for Fe/NN-MCM-41 at a coverage around θ = 0.3 suggests a difference in the stabilization constant between the first and the second arsenates while K d (θ) in Ni and Cu/NN-MCM-41 decreased gradually. The equilibrium constants, b (= k a / k d ), were plotted against the coverage (Langmuir plots), where Fe 3+ showed a minimum at θ = 0.65 while b in the Ni 2+ and Cu 2+ sites was almost constant. The inhibition by coexisting anions such as SO 4 2 - and Cl - was evaluated by the suppression of the adsorption capacity. Differences between the suppression among the cations were found. We analyzed the local structure of the adsorption centers in Fe and Co captured in NN-MCM-41 by EXAFS spectroscopy.