Effect Of Wastewater Containing Cr⁶⁺ On Cr Transformation And Migration During Plant Growth In Constructed Wetland

The non-compliant discharge of wastewater and waste residues of chromium-related enterprises is an important source of chromium（Cr）pollution in cultivated land.Constructed wetland can effectively remove Cr from wastewater,so as to protect farmland from the threat of Cr pollution.In order to study the effects of Cr containing wastewater under different water management methods on plant growth,physiological and biochemical indexes,and Cr transformation and migration in constructed wetland,small vertical flow constructed wetland（VFCW）and horizontal subsurface flow constructed wetland（HSCW）were constructed,and planted with Coix lacryma-jobi L.,which was drought tolerant and flood tolerant.1/2 Hoaglands nutrient solution supplemented was used to prepare 0,10 and 20 mg·L^-1Cr⁶⁺containing wastewater respectively.The results are as follows:1.With the increase of Cr⁶⁺treatment time and concentration,the growth rate of stem diameter of Coix lacryma-jobi L.significantly decreased,treated by HSCW was less inhibited than that of VFCW.The content of proline and malondialdehyde in leaves increased with the increase of Cr⁶⁺concentration,and the content and amplitude of malondialdehyde in leaves treated by VFCW were higher than that of HSCW.The activity of superoxide dismutase（SOD）decreased with the increase of Cr⁶⁺concentration,and increased first and then decreased with the prolongation of Cr⁶⁺treatment time.The activity of SOD enzyme in leaves of Coix lacryma-jobi L.treated by VFCW was higher than that of HSCW.2.The content of total chromium in roots,stems and leaves of Coix lacryma-jobi L.increased with the increase of Cr⁶⁺treatment time and concentration.The content of Cr in different organs was in the order of root>leaf>stem,and the accumulation of Cr in VFCW treatment was higher than that in HSCW treatment.The content of Cr in VFCW roots increased with the increase of Cr⁶⁺treatment time and concentration.The variation trend of Cr content in residual state（FRes）and 80%ethanol extraction state（FEth）in HSCW roots was similar to that in VFCW.The contents of Cr in deionized water（FH₂O）,Na Cl extraction state（FNa Cl）,2%HAc extraction state（FHAc）and HCl extraction state（FHCl）were firstly increased and then decreased with the increase of Cr⁶⁺treatment time and concentration.The Cr content of each form was in the order of FRes>FEth>FH₂O>FHAc>FHCl>FNa Cl.Except for the control,the content of Cr in roots of Coix lacryma-jobi L.treated by VFCW was higher than that of HSCW.3.The p H and Eh of the substrate decreased with the increase of Cr⁶⁺concentration and increased with the prolongation of Cr⁶⁺treatment time.The p H and Eh of the substrate treated by VFCW were higher than that of HSCW.Except for the control,the content of total Cr and various forms of Cr increased with the increase of Cr⁶⁺treatment time and concentration.The content of total Cr in VFCW substrate was higher than that in HSCW substrate.The content of different forms of Cr was in the order of residual state（F5）>iron manganese oxide combined with state（F3）>organic combining state（F4）>exchangeable（F1）>carbonate combined with states（F2）.The proportion of F4 and F5 decreased with the increase of Cr⁶⁺concentration.4.After ¹³C labeling,the ¹³C abundance of substrate decreased with the increase of Cr⁶⁺treatment time and concentration（except for VC20）,and the ¹³C abundance of labeled substrates in VFCW was higher than that in HSCW.The concentration of dissolved organic carbon（DOC）and microbial biomass carbon（MBC）in HSCW substrate increased with the increase of Cr⁶⁺concentration.The DOC concentration of HSCW substrate was higher than that of VFCW,and the MBC concentration of VFCW substrate was higher than that of HSCW except for 10 mg·L^-1Cr⁶⁺treatment.5.The proportion of particulate organic matter（POM）with diameter of 2～0.05 mm in the substrate was very small and increased with the increase of Cr⁶⁺concentration.The proportion of POM in VFCW treatment was higher than that in HSCW treatment.The proportion of C in POM was the largest,followed by H.The C/N ratio decreased with the increase of Cr⁶⁺concentration.POM was mainly composed of plant root residues,mainly containing C,O,B,N and other elements.Hydrocarbons were the main substances that constituted POM and adsorption points of Cr.The content of Cr in POM in VFCW was higher than that in HSCW.FTIR indicated that carboxyl and dehydroxylation functional groups may participate in the process of POM adsorption of Cr.6.Under 10 mg·L^-1Cr⁶⁺treatment,the content of dissolved organic matter（DOM）in the effluent of the two wetlands was the highest,while that under 20 mg·L^-1Cr⁶⁺treatment was the lowest.DOM content increased with the extension of Cr⁶⁺treatment time,and DOM content in VFCW was higher than that in HSCW.FTIR showed that DOM contained carbohydrates,aromatics and other substances,and contained amides,carboxyl groups and other groups.The aromatization degree,molecular weight,humification level and hydrophobic organic components of DOM increased with the increase of Cr⁶⁺treatment concentration,and the increase of these components in HSCW was higher than that in VFCW.7.With the extension of Cr⁶⁺treatment time,the common characteristics of bacteria in all treatments were significantly greater than that of fungi.The effects of Cr stress on fungi in the substrate were higher.Under the stimulation of Cr⁶⁺,the abundance and diversity of microbial community increased,but the long-term Cr stress would make it decrease.The abundance of microbial community in HSCW substrate was greater than that in VFCW.The relative abundances of Ascomycota,Basidiomycota,Mortierella and cryptomycota in fungi were relatively high.Firmicutes and Proteobacteria accounted for the largest proportion in the Cr⁶⁺treated wetlands.Microorganism and environmental factors of the correlation analysis showed that the abundance of most fungal species was negatively correlated with p H and Eh,while it was positively correlated with total Cr content,DOM concentration and Cr content of various chemical forms in substrate.Bacillus was positively correlated with p H and Eh,while DOM concentration,total Cr content in the substrate,various forms of Cr content were significantly negative correlation;Escherichia-Shigella was positively correlated with Eh,DOM concentration,total Cr content and various forms of Cr content.Other bacterial species were significantly positively correlated with DOM concentration,total Cr content and various forms of Cr content,and negatively correlated with p H.8.Under the treatment of Cr⁶⁺,the maximum removal rate of Cr⁶⁺in the effluent of the two constructed wetlands was 98%and decreased with the extension of Cr⁶⁺treatment time.The removal rate of Cr⁶⁺in HSCW treatment was higher than that in VFCW.