Effects And Mechanisms Of Humic Acid On The Availability Of Cadmium And Arsenic In Typical Co-Contaminated Paddy Soil

Cadmium and arsenic are two highly toxic inorganic pollutants that closely related to food safety and human health.Humic acid,as the main component of soil organic matter,not only has a very important effect on the soil fertility and the availability of heavy metals in soil,but also plays an important role in the process of plant uptake of heavy metals.HA could change the soil physiochemical and biochemical properties and interact with solid components in soil so as to affect the speciation and availability of heavy metals.HA can directly interact with heavy metals through complexation and precipitation and may influent the key processes of plant absorption,transportation and accumulation.The effect of humic acid on the availability of heavy metals in the soil has been studied for decades.However,existing studies have mainly focused on the relationship between soil humus and the availability of heavy metals.Whether exogenous humic acid has an effect on the availability and speciation of heavy metals in co-contaminated soil is still under debate.In this study,the Cd/As co-contaminated paddy soil and the straw derived（BFA）and mineral derived（HA）humic acid were selected.Combined with methods such as chemical adsorption characterization,soil incubation and pot experiment,and used modern spectral characterization FTIR,SEM-EDS,XPS,EEM and chemical detection techniques,systematically studied the chemical properties of the BFA and HA and the adsorption characteristics of two humic acids on Cd and As.Meanwhile,the effects of the two humic acids on the speciation,availability and plant absorption of Cd and As and their differences and mechanisms.The main results are as follows:1)BFA and HA had similar chemical composition and structure,and had similar functional groups,such as sulfur-containing groups and hydroxyl and carboxyl groups.In addition,BFA had organic components such as polysaccharides,amino acids,and a large amount of sulfur and nitrogen.The adsorption process of Cd and As by BFA and HA was mainly chemical adsorption,and the functional groups involved were hydroxyl and carboxyl groups.In addition,a heterogeneous diffusion adsorption process had also been found.Solution p H and ionic strength were important factors that affected the adsorption of BFA and HA on Cd and As.The two humic acids did not change their morphological characteristics after adsorbed Cd and As,and the adsorption sites of Cd and As were evenly distributed on the surface of the material.2)Both BFA and HA reduced Cd availability and increased As availability in soil.The main mechanism is that humic acid affected soil Eh and then changed the composition of soil iron oxides.The two humic acids promoted the transformation of Cd from high activity to low activity speciation,and reduced the Cd（CaCl₂-Cd）by 12.66%–93.13%;promoted the transformation of arsenic from steady state to unsteady state,and increased the As（KH₂PO₄-As）by 18.81%–71.38%.The fixation effect of HA on Cd was better than that of BFA,and the activation effect of BFA on As was higher than that of HA.The application of the two humic acids changed the soil pH,reduced the soil Eh,and thereby promotes the redox of iron oxides,then increased the proportion of amorphous iron oxide（Feo）,and reduced the proportion of crystalline iron oxide（Fed）.Redundancy analysis and structural equation modeling showed that the reduction in soil Eh and Fed and increase in Feo were closely related to the Cd and As availability.3)Both the two humic acids could effectively reduce the Cd contents in grains,and the Cd contents in rice roots,shoots,and leaves were also reduced accordingly.The reduction in the availability of Cd and the translocation coefficient of Cd in rice parts significantly reduced the Cd contents in grains.The reduction of DCB-Cd with the two humic acids might also help reduce the absorption of Cd by rice.The application of the two humic acids reduced DCB-Cd.The reduction in soil pe+pH and available Cd might be the main factors that reduced the DCB-Cd.At the same time,the inhibition in iron plaque formation on the root surface also reduced the DCB-Cd.4)HA increased the total As（TAs）contents in grains,while the BFA slightly reduced the TAs in grains.The two humic acids all increased the content of organic arsenic（OAs）in grains,and increased with the increase in application rates.The difference in DCB-As and the As retention by the root would be the main reason for the difference in TAs in grains with the BFA and HA.The distribution of As in rice root subcellular with the two humic acids was opposite,which might affect the absorption and accumulation of As in brown rice.The BFA reduced DCB-As,while the HA increased DCB-As.The reduction in soil pe+pH and iron plaque on root surface might be the main factors for the reduction of DCB-As by BFA.In short,the addition of HA-Na and BFA,particularly combined with flooding,could effectively reduce the available Cd in co-contaminated soil.However,both the two humic acid can increase the risk of As in soil.The reduction in soil available Cd,DCB-Cd and TF of Cd in rice can effectively reduce the accumulation of Cd in grains after application of the BFA and HA.The addition of HA may increase the risk of As in grains,especially at the minimum addition rate,while no risk of As accumulation in grains with the BFA was observed.