Hydrochloric acid pretreatment combined with microwave-assisted oxalic acid leaching of natural red palygorskite-rich clay for efficiently change the color and properties

Abstract

The hydrochloric acid (HA) pretreatment combined with microwave-assisted oxalic acid (OA) leaching process was employed to treat natural red palygorskite-rich clay (Pal-R-C). The conventional HA treatment in advance was responsible for eliminating impurity minerals such as dolomite, calcite, and gypsum. Subsequent microwave-assisted OA leaching with a fast reaction rate and low OA consumption, as a key process for adjusting the color and properties of Pal-R-C, quickly decomposed the color-causing substance hematite and partially leached the metal ions in the framework of palygorskite and illite. As a result, Pal-R-C quickly changed from red to white, the dispersion of clay particles was remarkably improved, and the surface active sites were greatly increased. Moreover, there was almost no damage to the crystal structure of palygorskite and illite, and the shapes of palygorskite nano-rods and illite nano-laminas remained intact. Finally, a relatively purified white Pal-R-C (L* = 82.38, a* = 0.11, and b* = 6.63) with a high specific surface area of 129.35 m2/g was obtained after 3 mol/L HA pretreatment for 1 h combined with microwave-assisted OA leaching for 30 min at a 40 wt% mass ratio. The whitening product could be used as an inorganic nanofiller to significantly increase the tensile strength of the chitosan (CS) polymer film from 27.22 to 47.15 MPa and retained a good elongation at break of 36.17%, showing a better toughening and reinforcing potential for polymers compared with raw Pal-R-C and high-purity palygorskite.