Effects Of Three Low Molecular Weight Organic Acids On Migration And Transformation Of Arsenic In Mangrove Sediments

Due to the combined effects of human activities and the characteristics of mangrove habitats,mangrove wetlands often become agglomerating region of a large nmber of heavy metal pollutants,potentially causing serious damage to the marine environment and biodiversity of mangrove.Low-molecular-weight organic acids(LMWOAs)are important constituent of plant root exudates.Under the heavy metal pollution conditions,on the one hand,the composition and content of LMWOAs secreted by the roots usually change significantly;on the other hand,LMWOAs can alter the soil pH;the rhizosphere microbial composition and complexation/chelation with heavy metal ions.What the most noticeable is that LMWOAs could affect the migration and transformation of heavy metal pollutants at the plant root-soil interface.Arsenic(As)contamination in mangrove wetlands have been reported and it have also been reported that plant would secret root exudates suffered from As stress which could affect rhizosphere microbial activities and community diversity,but the mechanism of migration and transformation of As in rhizosphere affected by root exudates remain unclear.In this study,typical mangrove sediments were used as experimental materials,and the study focused on As as the study object which is currently reported to be contaminated in several mangrove wetlands.The citric acid,malic acid,and oxalic acid were selected as represented LMWOAs of root exduates which play important roles in rhizosphere secreted by mangrove plants as report goes.The effects of LMWOAs on the migration of As in soil-water interface in mangrove sedimentary environment was simulated from four aspects:(1)Organic acid dosage;(2)Microbial roles of sediments;(3)Variation of LMWOAs pH;(4)Temporal dynamics.Here are the main conclusions:1.The results of LMWOAs dose-responsing study indicated that the content of As in the extracted supernatant showed a gradually increasing trend with the increase of three LMWOAs concentrations,that represents root exudates LMWOAs significantly affected the bioavailability of As;There are significant differences in the desorption effects of three LMWOAs on soil-As,and the order of desorption capacity of LMWOAs is as follows:citric acid>malic acid>oxalic acid;The content of iron species in the extracted supernatant of LMWOAs also changed significantly and there was a significant positive correlation between Fe and As.2.Sterilization of the soil and comparative analysis under LMWOAs treatments on microbiological contribution showed that there are significant differences in the As content of supernatant compared to the unsterilized control group and among LMWOAs treatments.The As content of supernatant under citric acid treatment and malic acid treatment average decreased by 68.31%and 42.14%,respectively,and oxalic acid treatment average increased by 29.59%compared to the control group.The results showed that composition and activity of microorganisms significantly affected the extraction and behavior of As in soil-root interface by LMWOAs.However,there are significant differences in the effects of different LMWOAs on the composition or activity of microorganisms.3.The effects of pH variation experiment showed that the pH of sediments changed significantly after the addition of LMWOAs,and the acidification of LMWOAs reduced the pH of sediments.The As content of LMWOAs supernatant was positively correlated with LMWOAs concentration and negatively correlated with organic acid pH under pH adjusted treatments,but less related to the acidity of organic acids(oxalic acid>citric acid>malic acid).The difference results of As in LMWOAs supernatant indicated that the migration and transformation of soil As were comprehensively affected by factors such as organic acid types,content and soil pH,however the acid strength is less important.4.The results of temporal dynamics showed that the As content of LMWOAs extracted supernatant have a decreasing trend with the increase of treatment time,which indicating the As of LMWOAs-sediment mixture reaction system shifted from liquid phase to solid phase over time.The content of As under citric acid treatment was 2.88 times that of malic acid treatment by average,and 4.17 times that of oxalic acid treatment by aversge.The pH of LMWOAs supernatant gradually increased with time,indicating that the decrease of pH had a significant effect on immobilized As in mangrove sediments and LMWOAs could activate soil-As to re-mobilize it back into the liquid phase.5.The effects of adding LMWOAs on the species transformation of sediment As showed:The organic As was not detected in this study;There was a significant difference in the variation of As speciation under LMWOAs treatments;The order of As(V)content in sediments was as follows:oxalic acid>malic acid>citric acid;The As species in the sediments increased gradually from As(III)to As(V)over time under LMWOAs treatments.