Adsorption behavior of arsenate at transition metal cations captured by amino-functionalized mesoporous silicas

We show that Fe3+, Co2+, Ni2+, and Cu2+ 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 Fe3+ > H+ similar to Co2+ > Ni2+ similar to Cu2+, which is slightly different from the adsorption capacity: Fe3+ > Co2+ similar to H+ > Ni2+ > Cu2+. 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 theta = 0.3 suggests a difference in the stabilization constant between the first and the second arsenates while K-d(theta) 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 Fe3+ showed a minimum at theta = 0.65 while b in the Ni2+ and Cu2+ sites was almost constant. The inhibition by coexisting anions such as SO42- 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.