Sepiolite and industrial waste-water purification: Removal of Zn2+ and Pb2+ from aqueous solutions

The interaction between sepiolite and Zn2+ and Pb2+ aqueous solutions has been studied in order to verify the heavy-metal cation sorption-desorption rates. To this end, weighed amounts of sepiolite were placed in two conventional chromatographic columns and percolated at constant temperature, flow rate, pH and percolating solution concentration (0.5 x 10(-2) M, with Zn2+ or Pb2+, Mg2+ and 10(-2) M Na+, respectively). The amount of cations retained and released by the sepiolite bed in the effluent solution was analyzed by atomic absorption and UV-Visible spectrophotometries. The experimental results indicate that: (i) heavy-metal cation sorption can occur on the surface of broken edges, in channels and at specific sites according to crystal chemical affinity: Mg2+ sites are mostly affected by Zn2+-treatment and Ca2+ sites by Pb2+- treatment (Zn2+-treated bed, Mg2+/Ca2+ = 0.627; Pb2+-treated bed, Mg2+/Ca2+ = 0.475); (ii) repeated treatments with heavy-metal cation solutions produce a decrease in adsorption and/or exchange capacity of the sepiolite bed; however, even when sepiolite is treated with alkali and earth alkali cations, it is able to retain a heavy-metal cation fraction, the retention of Zn2+ being greater than that of Pb2+. In order to investigate the chemical variation induced in sepiolite crystals by interaction between Zn2+ and Pb2+ solutions, a natural sample was treated with 1 M Zn2+ and 1 M Pb2+ solutions and then analyzed by X-ray powder diffraction and by differential thermal and thermogravimetric techniques.