BACKGROUND With the high specific surface area, rich functional groups and strong adsorption capacity, attapulgite (AT) and its modified materials have become a research hotspot in the field of heavy metal remediation of farmland soil.

OBJECTIVES To investigate the stabilization mechanism and effect of different modified AT for Cd-contaminated alkaline soil.

METHODS AT materials were modified by sodium hydroxide and ferric chloride. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and nitrogen adsorption/desorption test (BET-BJH) were used to analyze the microstructure and surface morphology of the modified AT. Combined with the characterization results, the passivation mechanism was analyzed, and the indoor simulated Cd-contaminated alkaline soil cultivation experiment and the lettuce potting experiment were carried out. Soil Cd content was determined by atomic absorption spectrophotometry. The effect differences between single application and compound application of two modified attapulgites in Cd-contaminated alkaline soil were investigated.

RESULTS Alkali treatment made the Si-O groups and structural negative charges increase, and the number of micropores and specific surface area of the iron-modified attapulgite (IM) also increased. The internal pore distribution and surface morphology of attapulgite can be significantly changed in two types of modification, which can also improve their adsorption capacity. Alkali-modified attapulgite (AM) can improve soil pH and cation exchange (CEC) by stronger chemical adsorption capacity, while IM has stronger physical adsorption capacity and strong chemical adsorption capacity, which makes soil pH decrease and CEC rise. To a certain extent, the compound application of the two materials can reduce soil pH and improve CEC, which can improve their stabilization effect. After using the passivation material of AM: IM ratio of 3:1 and 2.00% of the soil mass, the available DTPA-Cd content was reduced by 33.85%, and the BCF of lettuce was reduced by 24.49%. The treatment attained the best stabilization effect.

CONCLUSIONS IM has good stabilization effect on heavy metal Cd in alkaline soil. AM should be avoided for single application, and it can be recombined with other stabilization materials to achieve better reduction of soil Cd pollution while improving the quality of alkali soil.