Strategies And Mechanisms For Controlling Chromium Superconcentration In Leersia Hexandra Swartz On Root Surface Iron Plaque

Cr pollution in water has become one of the urgent environmental problems to be solved worldwide.The Cr hyperaccumulator plant Leersia hexandra Swartz（L.hexandra）has excellent Cr enrichment ability,and using L.hexandra to repair Cr contaminated water is a green and highly promising technology for heavy metal pollution remediation.There is a reddish brown iron plaque（IP）attached to the surface of the roots of L.hexandra.But the accumulation,transport mechanism,and physiological tolerance of Cr by root surface iron film have been rarely reported domestically and internationally.This study utilized Fe SO4to artificially induce the formation of IP with different thicknesses on the root surface of L.hexandra.Modern instrument analysis methods such as scanning electron microscopy（SEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,combined with chemical extraction,were used to compare and analyze the morphologic characteristics and composition of the IP formed in natural environment（Nat IP）and the IP formed by artificial induction.Through short-term 9 day and long-term 45 day hydroponic experiments,the removal rate of Cr（Ⅵ）in the culture solution,the contents of Cr and Fe in L.hexandra tissues,and the key physiological indexes of L.hexandra were systematically studied.The effects of IP on Cr enrichment ability and Cr tolerance of L.hexandra were investigated.In this study,the main Fe minerals in the IP on the root surface of L.hexandra were synthesized and Cr（Ⅵ）adsorption experiments were carried out.The changes of synthetic minerals before and after Cr（Ⅵ）adsorption were proved by Fourier transform infrared spectroscopy（FTIR）and XPS.The binding mechanism of synthetic minerals and Cr（Ⅵ）was analyzed,and the external mechanism of the IP on the root surface affecting the Cr（Ⅵ）enrichment performance of L.hexandra was revealed.Research shows that:The iron plaque was unevenly distributed on the surface of the roots of L.hexandra.The iron plaque on the root surface of L.hexandra gradually decreased along the longitudinal direction from the root to the root tip,and a small amount of iron plaque was embedded in the root epidermal cells of L.hexandra.The iron plaque induced by 50 mg/L Fe SO₄（Fe50）was the closest to the DCB-Fe content of Nat IP.The results of XPS showed that Fe（III）and Fe（II）coexisted on both Nat IP and Fe50 IP,and Fe（III）was dominant.The results of XRD showed that the Fe（oxy）hydroxides in the two IP were mainly ferrihydrite,lepidocrocite and goethite.The proportion of crystalline Fe（oxy）hydroxide in Nat IP is as high as 64.16%,and the weak crystalline Fe phase in Fe50 accounts for 50.57%.Under 10 mg/L Cr（Ⅵ）stress,Fe0Cr10 without iron plaque on root surface had higher Cr（Ⅵ）removal rate at the the beginning of Cr stress.After 21 days of Cr stress,the Cr（Ⅵ）removal rate of Gramineae was higher than that of Fe0Cr10,and the thicker the IP induced by Fe SO₄,the higher the Cr（Ⅵ）removal rate.Compared with Nat IP,the Fe SO4 induced formation of iron films had stronger ability to transfer Cr and Fe from root to shoot.On the45 day of harvest,the Cr and Fe contents in the aboveground part of Fe50Cr10 increased by about 60.5%and 52.4%,respectively,compared with Fe0Cr10.The iron plaque can hinder the enrichment of Cr（Ⅵ）in the water environment by the roots of L.hexandra.Avoiding the rapid absorption of Cr（Ⅵ）in the roots of L.hexandra at the initial stage of Cr stress,delay the time of Cr poisoning in L.hexandra.It can alleviate the damage to photosynthesis after a large amount of Cr accumulation in leaves,which is conducive to the continuous enrichment of Cr in high concentration of Cr（Ⅵ）environment.In an aqueous solution with an initial p H of 6 and a Cr（Ⅵ）concentration of 10 mg/L,the Cr（Ⅵ）adsorption effect of synthetic minerals was ferrihydrite>lepidocrocite>goethite≈aged ferrihydrite,and the adsorption capacity of ferrihydrite for Cr（Ⅵ）reached 4160mg/kg at adsorption equilibrium.Fe50 had better adsorption effect on Cr（Ⅵ）than Nat IP.The XPS spectra showed that the peaks of Fe（II）were greatly reduced and converted to Fe（III）after the reaction of synthetic minerals with Cr（Ⅵ）,and Cr（III）peaks appeared on the 2p orbit of Cr,indicating that these minerals reduced Cr（Ⅵ）adsorbed on the surface to Cr（III）.The Cr（Ⅵ）reduction ability of the four Fe minerals was ferrihydrite>aged ferrihydrite>goethite>lepidocrocite,which was positively correlated with the Fe（II）content on the surface of the minerals.The iron plaque on the root surface could retain Cr（Ⅵ）on the root surface by electrostatic adsorption,ligand exchange and co-precipitation,which slowed down the rapid absorption of Cr by L.hexandra.