A Comparative Study On Insoluble Cadmium Mobilized By Various Rhizosphere Organic Acids

The interaction of rhizosphere organic acids and heavy metals is one of the important factors that influence the migration and transformation of heavy metals in the environment.At present,many studies have focused on the mobilization of Cadmium in soil rhizosphere acidification caused by the secretion of organic acids,and lacked the evalution of the mobilization ability of all kinds of rhizophere organic acids.In this paper,Based on the metabolomics of rhizosphere secretions of amaranth,cabbage,spinach and other vegetables in the early stage of the research group,44 kinds of rhizosphere-secreted organic acids were identified,and the actual concentration of rhizosphere was simulated.The mobilization experiment of carbonate-bound cadmium was carried out,and the activation ability was evaluated respectively.The mobilization experiments of humic acid bound Cadmium and palygorskite bound Cadmium were studied by using 5 kinds of organic acids(citric acid,oxalic acid,tartaric acid,malic acid and lactic acid)with good mobilization effect.The differences in the mobilization of humic acid bound Cadmium and palygorskite bound Cadmium were studied.In addition,the above five kinds of organic acids were added to low-concentration cadmium-contaminated farmland to evaluate the release of Cadmium from low-concentration cadmiumcontaminated soil.In order to understand the actual rhizosphere environment of vegetables mobilized different forms of Cadmium to provide a theoretical basis.The main contents and conclusions are as follow:(1)44 kinds of organic acids on the mobilization of Cadmium Carbonate,the organic acid species and concentration will affect the mobilization of Cadmium Carbonate.The concentration of organic acids has a good linear relationship with the mobilized Cadmium Carbonate,and the mobilized Cd by citric acid was 22.73 mg·L-1,followed by malic acid and cis-aconitic acid,which were 14.55 mg·L-1 and 12.75 mg·L-1 respectively.The effect of PAs mobilization is not obvious.Among AAs,the mobilization effect of aspartic acid,histidine and glutamate was relatively strong and the rest were not significant.The mechanism of different organic acids on the mobilization effect of Cadmium carbonate is mainly the effect of hydrogen ions and acid ions.At the equimolar concentration,the citric acid mobilized carbonic acid is the strongest,and the main contribution is the chelation of functional groups.(2)The adsorption of Cd on humic acid and palygorskite were mainly based on chemical adsorption.At the equimolar concentration,the mobilization of five kinds of organic acids was different in three forms,but the mobilization trend was the same,citric acid>malic acid> oxalic acid,tartaric acid> lactic acid.The mobilization of Cadmium carbonate,humic acid bound Cadmium and palygorskite bound Cadmium were 76.90%,73.59% and 69.31%,respectively,compared with lactic acid.The effect of organic acid on the mobilization of Cadmium in three forms is Cadmium Carbonate> humic acid bound Cadmium> palygorskite bound Cadmium.The results are in line with the difficulty of Tessier continuous extraction in three forms(Cadmium Carbonate,organic bound Cadmium and silicate-bound Cadmium).(3)The mobilization of Cadmium in the actual soil was also different,and the mobilization trend was the same,which was citric acid> malic acid> oxalic acid,tartaric acid> lactic acid.The addition of 50 mg·L-1 citric acid(near the concentration of rhizosphere secretion)to the soil cadmium mobilization can make the edible part of the crop Cd content exceeds the food safety national standard,reaching the risk level.