Structural and physicochemical properties of Mayan-inspired hybrid nanocomposites obtained by complexing palygorskite to lawsone and methyl orange

Novel hybrid nanocomposites were tentatively prepared by grinding, heating and purifying in Soxhlet preset amounts of lawsone and methyl orange dyes with the microporous palygorskite clay mineral, i.e., following the ancient recipe of the famed Maya Blue pigment. These composites were investigated in the various synthesis steps with a multi-analytical approach (XRPD, FE-SEM-EDS, BET-SSA, TGA-DSC, UV–Vis, ATR-FT-IR and Fluorescence spectroscopies), which allowed understanding their structural features and physicochemical properties, unveiling the nature and sites of the interactions existing between the host and the guest. Also, their stability was evaluated through harsh chemical attacks, monitoring colour changes and spectral features. The obtained outcomes proved that lawsone forms a stable hybrid composite after heating with palygorskite, establishing H-bonds between the C=O groups on its quinonoid ring and the structural/zeolitic water in the grooves carving the clay fibrils surface. Such a stability, coupled to an observed colour shift after pH fluctuations (yellow in acids; orange in alkalis), suggests that this composite might be used as a solid-state, reversible acid-base indicator, a colorimetric-electrochemical sensor for anions or for bio-reduction of pollutants in remediation activities. Methyl orange, on the other hand, undergoes systematic degradation when heated together with palygorskite, forming byproducts unable of efficiently binding to the host framework. Despite a presumed affinity of palygorskite for absorbing azo dyes, methyl orange is not firmly stabilized on the host structure due to an incipient mutual electrostatic repulsion that prevents bonding and shielding during grinding, paving the way for an inevitable decay of the molecule after heating.