Response Of Arsenic Migration In Mangrove Sediments To Oxalic Acid Secretion By Roots And Its Relationship With Iron And Phosphorus

Arsenic(As)is a heavy metal pollutant which has been widely reported in mangrove wetland in recent years.Its migration and transformation mechanism in mangrove root soil interface is one of the hot topics in recent years.Some recent studies have shown that the migration and transformation of As in the sedimentary environment are closely related to the biogeochemical processes of iron(Fe)and phosphorus(P).However,under the influence of mangrove root system,especially root secretion,the migration and transformation of As and its relationship with Fe and P are still unclear.Changes in composition and content of low molecular weight organic acids(LMWOAs)exudated by roots are one of the important characteristics of mangrove plants in response to heavy metal stress.Oxalic acid is one of the most abundant compounds in LMWOAs secreted by mangrove roots,which has good complexing properties and high reducing power to heavy metals.In this study,oxalic acid in LMWOAs secreted by mangrove roots under the condition of As pollution was taken as the representative,and mangrove sediments with different As contents were taken as the research materials.Through the indoor controlled experiment,the effects of oxalic acid on the migration of As,P and Fe at the solid-liquid interface were studied,and the coupling and competitive relationship between As migration and Fe,P under the action of oxalic acid was discussed,so as to provide reference for the analysis.The mechanism of mangrove root behavior affecting As migration and transformation,and the key process of As migration and transformation at the root soil interface of mangrove provide theoretical basis.The main results of this study are as follows:1.In this study,the migration of As in sediment(solid phase)and supernatant(liquid phase)was studied;Under the same As pollution treatment condition,the concentration of As in liquid phase increased with the increase of oxalic acid treatment concentration;The content of Asad,Ascab and Asamo decreased with the increase of oxalic acid concentration,which was an important reason for the increase of As content in liquid phase.The content of Asad,Ascab,Asamo and Asres increased significantly with the increase of As concentration,while AScry decreased significantly.This may explain why the concentration of As in the liquid phase increased significantly with the increase of As content in the sediment under the same oxalic acid treatment.Oxalic acid treatment has a significant time effect on the distribution of As in solid-liquid phase.With the extension of reaction time,the concentration of As in liquid phase decreases gradually;Asamo,Ascab and Ascry increased first and then decreased with the increase of Asad content.2.Under the same As pollution level,the concentrations of Fe,Fe2+and Fe3+in the liquid phase increased with the increase of oxalic acid treatment concentration,while the content of amorphous iron(Feamo)in the sediment decreased significantly,indicating that the dissolution of Feamo caused by oxalic acid addition was an important reason for the increase of Fe content in the liquid phase.With the increase of As content in sediments,the content of Feamo increases,while the content of Fecry decreases,which indicates that the change of Fe speciations distribution in sediments is also an important factor affecting the distribution of As in solid-liquid phase.In addition,with the extension of reaction time,the content of Fe in liquid phase increased first and then decreased,while the proportion of Feamo and Fecry in sediment decreased first and then increased,and increased first and then decreased,respectively.In the absence of exogenous P,the concentration of P in liquid phase was lower,but the concentration of P increased significantly with the addition of oxalic acid;with the extension of reaction time,the concentration of P decreased gradually.Fe,Al bound P(NaOH-P)and Ca bound P(HCl-P)are the main modes of occurrence of inorganic P in sediments,and the content of exchangeable P(NaHCO3-P)is relatively low.With the increase of oxalic acid concentration,the contents of NaHCO3-P and NaOH-P decreased significantly,which was the direct reason for the increase of P concentration in liquid phase with the increase of oxalic acid concentration.3.Correlation analysis shows that there is a significant between the concentrations of As,P and Fe in liquid phase,and the speciation of Fe has an important influence on the migration of As and P.On the one hand,ASamo and NaOH-P are important binding forms of As and P in sediments,and their contents change with the change of oxalic acid treatment concentration and As pollution level;on the other hand,there is a significant positive correlation between their contents and Feamo content under experimental conditions,and the change of Feamo content is an important reason for the change of As and P in liquid phase.In addition,the pH value of the solution decreased significantly with the increase of oxalic acid concentration,but the concentration of As treatment had no significant effect on the pH value of the solution,indicating that the soil acidification caused by oxalic acid secreted by roots was one of the important factors for oxalic acid to affect the migration of As,P and Fe.4.The migration of As,P and Fe in the reaction system was significantly affected by exogenous P input.With the increase of exogenous P concentration,the contents of As,P and Fe in liquid phase increased,and there was a significant correlation.On the one hand,there were significant differences in As forms in sediments after exogenous P input.The content of Asad and Asres decreased gradually,which may be the main reason for the increase of As content in liquid phase.On the other hand,physical characterization analysis shows that exogenous P competes with As for adsorption sites on Fe oxide surface,and the proportion of As element in sediment decreased gradually,which makes As concentration in liquid phase increase.This may also be related to the occurrence form of iron,which needs further study.At the same time,the increase of exogenous P concentration can promote the transformation of As(Ⅲ)to As(Ⅴ).The adsorption behaviors of P on As bearing sediments are as follows:(1)rapid adsorption on the surface of sediments,mainly combining with the adsorption sites on the surface of Fe,Al compounds;(2)slow diffusion to the interior,mainly chemical adsorption.Under different pH conditions,P had a significant effect on As release(p<0.001).The release of As from the liquid phase increased with the increase of pH.Higher pH value will increase the concentration of OH-in soil solution,weakening the adsorption of oxygen anions and releasing as from sediment,which increases the ecological risk.