Functionalized clay heterostructures for reducing cadmium and lead uptake by plants in contaminated soils

Natural and synthetic clay minerals have been transformed into Si-pillared mesoporous structures and then grafted with SH functions, for heavy metal ion trapping in polluted soils. The techniques and the limits encountered were detailed, with assessments of transformations by FTIR spectrometry and X-ray diffraction analyses. Adsorption isotherms of Cd2+ and Pb2+ were measured after 1 day shaking in calcium nitrate suspensions. Tests of Cd and Pb plant uptake were pursued by ray-grass cultivation on polluted soil amended or not with a functionalized synthetic beidellite during 3 months. Natural beidellite, hectorite and vermiculite were transformed into SH-grafted pillared clays but difficulties were encountered to keep a regular and significant interlayer distance until the last step. The well crystalline high-charge vermiculite of Santa Olalla (Spain), after Li-saturation, exchange with alkyl-ammonium cations, formation and grafting of interlayer Si-pillars, showed the highest capacity of Cd2+ and Pb2+ fixation among the 3 functionalized tested clay minerals. The cadmium uptake by ray-grass grown on polluted soil has been quite depressed by amendments with 0.17 or 0.5% of functionalized synthetic beidellite, but these amendments were inefficient for lead.