Iron-rich palygorskite： A potential clay material for contaminated environment remediation

Palygorskite is a natural clay mineral characterized by a porous crystalline structure containing tetrahedral layers alloyed together by longitudinal sideline chains. Due to its sorptive and rheological properties, palygorskite is widely used in different industrial fields. In particular, palygorskite clays depict a high susceptibility to ion exchange, large special surface area, considerable porosity and thermal stability. The structural formula of the iron-rich palygorskite is established as： （Si 7.66 Al 0.34） （Al 1.08 Fe 0.81 Mg 1.99 Ti 0.05-1.07） 02 0（OH）2（OH2）4. The SEM images （Figure 1） reveal the fibers of high-iron palygorskite have the short-stick shape and the fibers can occur as the bundles of agglomerate needle-like structures. The interspaces of the structures increase the special surface area evidently. Compared to the iron-rich sample, the low-iron palygorskite fibers present as the compact stack without many interspaces and some fibers are flexural in shape. Tunnel-like rectangular microspores with the channels approximately 3.7-5.4 A in the crystal structure should also be considered. The iron-rich palygorskite has a normally higher special surface area （180-200 m^2/g） than other palygorskites （110-130 m^2/g） and the pore size is distributed from micropore （〈2 nm） to macropore （〉 100 nm）, which could extend the range of the adsorbate size. After acid leaching, the special surface area will increase up to 250-300 m^2/g. Previous studies proposed maximum sorption values of 37.2 mg/g for lead, 17.4 mg/g for copper, 7.11 mg/g for zinc and 5.83 mg/g for cadmium at pH 5-6. For excellent adsorption capacity of organic molecules, the clay has been applied in treatment of dye wastewater, decoloring edible oils and absorption of odorant molecules recently.