模拟三峡库区消落带优势植物根系低分子量有机酸对土壤中铅的解吸动力学

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

三峡库区消落带落干期时会生长大量植被,其根系分泌的低分子量有机酸会对消落带土壤中重金属的活化带来一定影响,为此,本研究通过水培法以及利用去离子水直接浸提其根际土测出三峡库区优势植物狗牙根和稗草根系分泌的低分子量有机酸种类和含量,并模拟优势植物所分泌的单一有机酸和混合有机酸分别对消落带土壤中的铅(Pb)进行解吸,探究其对消落带土壤中铅的解吸动力学.结果表明,狗牙根根系分泌的低分子量有机酸种类和含量均大于稗草,其中柠檬酸、丙二酸、乙酸、苹果酸是两种植株根系均分泌的有机酸,且乙酸含量最高,分别为0.765 mmol·kg-1、0.261 mmol·kg-1,而在狗牙根和稗草根际土中均检测到柠檬酸、丙二酸、乙酸;选取柠檬酸、丙二酸、乙酸对消落带土壤中铅进行解吸时,发现当有机酸浓度≤1 mmol·L^(-1)时,有机酸解吸量由高到低为:丙二酸>柠檬酸>乙酸;而当有机酸浓度>1 mmol·L^(-1)时,有机酸解吸量由高到低为:柠檬酸>丙二酸>乙酸,且低分子量有机酸在土壤中含量低时,抑制土壤对Pb的解吸;含量高时,促进土壤对Pb的解吸.在解吸过程中,0~240 min为快反应阶段,240 min后为慢反应阶段,准二级动力学方程拟合效果最好.

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.