Interactions of phosphates and selected organic molecules with palygorskite and sepiolite
Palygorskite and sepiolite are silicate clay minerals that commonly occur in soils of and semiarid regions. Despite the significance of these minerals in soil agrochemistry and in a variety of industrial applications, the sorptive behavior of palygorskite and sepiolite is not fully elaborated. The objectives of this work were to: (i) study the interactions of orthophosphate (Pi) and inositol hexaphosphate (IHP) with palygorskite and sepiolite in pure systems and in the presence of calcite and (ii) quantify the contribution of different sites in sorption of monovalent organic cations, crystal violet (CV) and methylene blue (MB), on palygorskite and sepiolite. The Pi sorption capacity of sepiolite and palygorskite amounted at about 0.7 and 1.5 cmol kg-1, respectively, which is higher than other crystalline silicate clays. Sepiolite has the potential to be a carrier for orthophosphate compounds. The IHP sorption capacity was 24 and 15 cmol kg'-'1 for sepiolite and palygorskite, respectively. The large surface area and Al-Mg contents, may explain the P sorptive behavior of these minerals. The availability of sorbed P by palygorskite and sepiolite in the presence of calcite was compared with that of a montmorillonite, using a sequential extraction technique. About 90% of total Pi sorbed on a sepiolite-calcite mixture was recovered over seven extractions, while the recovery of sorbed Pi was about 20%, 12%, and 2% for palygorskite-calcite, montmorillonite-calcite, and pure calcite, respectively. Application or presence of sepiolite in calcareous soils may favor the phosphate availability in these soils. The maximum sorption of MB and CV greatly exceeded the cation exchange capacity of these minerals. In sorption maxima, the neutral sites showed the highest contribution to the sorption of organic cations as compared to the free and complexed negative sites. This part of the study may help the modification of clay surfaces for different industrial applications.