Simulated Desorption Kinetics of Lead by the Dominant Plant Roots Released Low Molecular Weight Organic Acids from the Water-Level-Fluctuating Zone in the Three Gorges Reservoir

Abstract

Vegetations grow vigorously in the water-level-fluctuating zone (WLFZ) of Three Gorges Reservoir (TGR) during the non-flooded period. Low molecular weight organic acids (LMWOAs) excreted from the roots of these plants may active the heavy metals in soil. In order to investigate the desorption kinetics of Pb in the vegetation covered soil of TGR, the LMWOAs released from Cynodon dactylon and Echinochloa crusgalli roots, which are the dominant species in WLFZ, were analyzed both qualitatively and quantitatively. Furthermore, single and mixed species of LMWOAs were also used in the simulation. The results showed that the variety and content of LMWOAs secreted from Cynodon dactylon roots were larger and higher than those of Echinochloa crusgalli. Citric acid, malonic acid, acetic acid and malic acid were the same LMWOAs released from the two types of plants. The contents of acetic acid were the highest among all LMWOAs, which were 0.765 and 0.261 mmol·kg-1 in Cynodon dactylon and Echinochloa crusgalli roots, respectively.Citric acid, malonic acid and acetic acid were the same LMWOAs detected from their rhizosphere soils. In the citric acid, malonic acid and acetic acid desorption test, the desorption captivities of Pb were inhibited when the concentration of LMWOAs was lower than 1 mmol·L-1. The desorption ability displayed a decreasing order of malonic acid > citric acid > acetic acid. However, the three LMWOAs promoted the desorption of Pb in soil when their concentrations were higher than 1 mmol·L-1, respectively. The desorption ability displayed a decreasing order of citric acid > malonic acid > acetic acid. During the entire process of desorption, 0-240 min was the fast reaction stage, and the remaining time was the slow reaction stage. The pseudo-second order kinetics equation could fit best for the kinetic process.