In-situ Repair Of Cr(?) Pollution In Groundwater By Mineral Composites And Microorganism Synergistic Reaction

Hexavalent chromium[Cr(?)]has caused different degrees of pollution to groundwater in China and many parts of the world,which seriously threatens human health.As a kind of in-situ groundwater remediation technology,permeable reactive barriers(PRB_S)has the advantages of stable operation,low cost and long-term effect,and has become an important method and research hotspot of groundwater remediation in the world.But in the process of long-term operation,there are problems of reaction medium consumption and blockage.In this paper,a combination of two natural minerals,pyrite and magnetite,and eight microorganisms were used to screen a group of optimal iron ore-microbial synergistic systems.The removal mechanism of Cr(?)by this system was studied by shaking flask experiment.The iron ore is mixed with kaolin and goldenrod leaves in a certain ratio and then fired into ceramsite.The ceramsite was immobilized by biofilm and mixed with self-made slow-release nutrients as the reaction material for indoor PRB simulation experiment.The removal effect and mechanism of Cr(?)in this new mineral composite ceramsite in PRB were studied.The experimental results of this paper are as follows:(1)The best synergistic system is magnetite and iron reducing bacteria(Lysinibacillus sp.JLT12).When carbon source is glucose(8 g/L),nitrogen source is yeast powder(6 g/L),p H=9,the removal effect of Cr(?)is the best.(2)The shake flask experiment showed that the reduction rate of Cr(?)was enhanced by synergistic effect.The removal rate of Cr(?)was 97.5%at 72 h and the concentration was decreased from 120.5 mg/L to 2.9 mg/L.The kinetic fitting results show that the reaction rate of this system is significantly higher than that of the single action system,and the reaction rate is greater than the sum of the two.The results of X-ray diffraction(XRD)and Raman spectrum analysis showed that not only can magnetite and iron reducing bacteria adsorb and reduce Cr(?),but also iron reducing bacteria can reduce Fe(?)on the surface of magnetite and in the liquid phase to Fe(?),and Fe(?)can continue to reduce Cr(?),ensuring the durability of the reaction.(3)The ceramsite produced by the magnetite,kaolin and goldenrod leaves has the best performance in a ratio of 9:1:1,and the best mass ratio to the slow-release nutrient salt is 10:1.5.Scanning electron microscopy(SEM)and 16S r DNA high-throughput sequencing analysis showed that the ceramsite is rich in porosity,and the functional strain Lysinibacillus sp.JLT12 has been successfully immobilized on the surface of ceramsite.(4)After 30 days of PRB operation,the removal rate of Cr(?)reached 98%,and the emissions of total nitrogen(TN),total phosphorus(TP)and chemical oxygen demand(COD)were all within the control range of the effective standard.The results of X-ray photoelectron spectroscopy(XPS)showed that Cr(?)and Cr(?)existed on the surface of ceramsite,most of Cr(?)was reduced to Cr(?),and some of them were fixed in the form of Fe(?)-Cr(?)coprecipitation.The column filtration experiment shows that the permeability coefficient of ceramsite is almost three times that of magnetite powder.