Adsorption Of As(?) And Cd(?) And Effect On Their Contents In Rice By Ferromagnesium Oxides Loaded Humic Acids

Arsenic(As)and cadmium(Cd)are the two most typical inorganic pollutants in contaminated farmland soils whose chemical transformation behaviors are quite different.Present studies focus more on improving adsorption efficiency of As and Cd by new adsorbents and their adsorption mechanism.Thus,several ferromagnesium oxides loaded humic acids(FMHs)that were synthesized from three natural heavy metal adsorbents-humus,iron oxide and magnesium oxide were used to investigate their effects on the remediation of arsenic and cadmium in water and soil system.The effects of the novel adsorbent on the uptake of arsenic and cadmium in rice plants in contaminated farmland soils were also explored.The main results were as follows.1.Synthesis of ferromagnesium oxides loaded humic acids(FHM)and its adsorption of arsenic and cadmium in water.The FMH was synthesized by co-precipitation method.The results showed its high adsorption efficiency for As(?)and Cd(?)in both single and combined adsorption system.Furthermore,we studied the effects of several environmental factors including adsorbent dosage,solution intial pH and reaction time on the adsorption capacity of As(?)and Cd(?)by FMH.The results indicated FMH could adsorb more than 94%of As(?)and Cd(?)within 60 min under the condition of 0.25 g(FMH)L-1 and initial solution pH 6.0.The sorption kinetics and isotherms of As(?)and Cd(?)on FMH closely fitted Pseudo second order model(R2=0.95)and Langmuir isotherm model(R2=0.99)respectively,indicating that the sorption process was chemical monolayer adsorption.The maximum adsorption capacity of As(?)and Cd(?)by FMH was 41.1 mg g-1 and 199.2 mg g-1 respectively.SEM-EDX results showed that the FMH after adsorption exhibited amorphous and matrix interlaced morphology.The XPS results showed that there was an oxidation reaction during the adsorption process,and part of As(?)was oxidized to As(?).As(?),As(?)and Cd(?)were complexed with FMH through surface hydroxyl functional groups,in which the humic acid component mainly adsorbed Cd(?),and the iron-magnesium oxide component mainly adsorbed As(?)and As(?).2.The adsorption of arsenic and cadmium in soils as influenced by different ferromagnesium oxides loaded humic acids(FHMs).Different FHMs were synthesized according to the mass ratio of iron-magnesium oxide and humic acid(1:0,1:1,1:2,1:4,and 0:1).It was found that the effects of five materials added to paddy soil on available arsenic and cadmium in soil were significantly different.The effect of adsorbing materials on reducing the available arsenic and cadmium in soil followed the order of 1:1FMH>FMO>1:2FMH>1:4FMH>HA.3.Effect of ferromagnesium oxides loaded humic acids(FMH)on absorption of arsenic and cadmium in rice.In soil-rice system,non-rhizosphere soil pH and soil cadmium contents at tillering and harvesting stage were not affected by addition of FMH.Non-rhizosphere soil pH was higher than that of rhizosphere soil.Due to the release of root exudates and adsorption of iron plaque,arsenic and cadmium contents in rhizosphere soil were higher than that in non-rhizosphere soil.The residual and acid extractable fractions were the main forms of arsenic and cadmium in soil,respectively.FMH treatment could promote the growth of rice and increase the contents of iron plaque on the root surface of rice,which had higher adsorption capacity for arsenic.Thus,FMH treatment could reduce the content of arsenic in rice roots,stems,leaves and brown rice at maturity.However,the absorption of cadmium by rice was not affected by FMH treatment.At maturity,the contents of arsenic and cadmium in different tissues of rice followed the order of root>stem and leaf>brown rice.