Heterogeneity of humic acid and clay minerals derived from different parent rock materials and their effects on Cd forms in paddy soils

At present, there is still a lack driving mechanism for how the biogenetic characteristics of different parent soils affect soil cadmium (Cd) fraction. In this study, the Cd2+ adsorption mechanisms by the clay minerals, iron oxides, soil acidity and organic matter heterogeneity in different parent soils (purple sandy shale (hereby defined as P), quaternary red clay soil (Q), granite soil (G), river sandy mud (R), yellow mud soil (Y), stucco field (S)) were analyzed. The results showed the kaolinite and hematite contents in Q were relatively high, R had a high montmorillonite, Y had a high illite. The soil latent acids content in Q was 1–9 times that of other parent soils. Y had the lowest latent acid content, which was 0.46 cmol·kg−1. With the increase of pH from 4 to 8.5, the removal efficiency of Cd2+ by these soils and main minerals reached up to 98.82 %, and the solid-liquid partition coefficient to 4.19 L·g−1. With the molecular weight increase of the extracted HA in the soils from 3600 to 57339, the complexation stability constant and the maximum coordination number increased from 2.09 and 0.93–5.54 and 1.72, respectively. The Cd2+ different adsorption pathways in soils resulted in certain differences in the intensity of the absorption peaks as indicated by FTIR. For example, the C-O stretching vibration was most pronounced at 1032 cm−1 in soil R. Through the above studies, the microinterface characteristics and differences of Cd adsorption and binding speciation of different parent paddy soils were revealed.