Conversion of clay minerals to photocatalysts for CrVI reduction and salicylic acid decomposition

This study investigates the conversion of biotite, a subgroup of clay minerals, into photocatalysts through heat treatment with CaCl₂. The resulting reaction products were analyzed in terms of their composition, structure, optical properties, and photocatalytic activity against Cr^VI and salicylic acid (SA). Biotite and CaCl₂ mixtures could be heated to 600 °C while retaining the biotite crystal structure, whereas heating to 700 °C resulted in transformation to octahedral wadalite crystals. The band gap of the wadalite obtained after heat treatment was approximately 3.10 eV. The photocatalytic reduction rate per unit surface area increased markedly with increasing heat-treatment temperature, and the Cr^VI reduction and SA degradation rates of the sample calcined at 800 °C were approximately 18 and 9 times greater, respectively, than those of the sample calcined at 500 °C. Leaching tests of the reaction products revealed that the elution of Ca²⁺ and Cl⁻ was particularly significant. Even the samples that retained the biotite structure after heat treatment displayed some photocatalytic activity, indicating that this method may be applicable to preparing photocatalysts from mica and related minerals with similar structures.