Effects Of Wastewater Containing Hexavalent Chromium On Characterization Of Organic Matter And Chemical Transformation Of Chromium In Coix Lacryma-jobi L.

In recent years,with the development of industrialization,chromium pollution of arable land due to substandard discharge of chromium-containing wastewater is also an increasingly frequent phenomenon,which seriously endangers the environmental health of arable land and the safety of food production.The use of constructed wetlands has a better purification effect on chromium-containing wastewater and can reduce soil chromium pollution.In this study,1/2 Hoagland’s nutrient solution spiked with glucose as irrigation water and vertical flow artificial wetland（VC）and horizontal submerged flow artificial wetland（HC）planted with Coix lacryma-jobi L.were constructed,0,10,and 20 mg·L^-1of Cr⁶⁺（added with K₂Cr₂O₇）was added to irrigation water to study effects of wastewater containing hexavalent chromium on soil organic matter and chromium transformation during root decomposition in Coix lacryma-jobi L.The results of the study are as follows:1.The fresh dry weight of above-and below-ground Coix lacryma-jobi L.in each treatment decreased significantly with increasing Cr⁶⁺treatment concentration,but increased with increasing Cr⁶⁺treatment time,and 10 and 20 mg·L^-1Cr⁶⁺treatments significantly inhibited the increase of stem diameter;the biomass of above-and below-ground Coix lacryma-jobi L.in HC was greater than that in VC treatment;the malondialdehyde（MDA）content of leaves increased with increasing Cr⁶⁺concentration,and the malondialdehyde content of leaves in VC treatment was greater than that in HC treatment.The leaf malondialdehyde content of Coix lacryma-jobi L.increased with the increase of Cr⁶⁺concentration,and the malondialdehyde content of VC-treated leaves was greater than that of HC-treated leaves.The total chromium content of all parts of Coix lacryma-jobi L.increased with increasing concentration of Cr⁶⁺treatment,and the size of chromium content of different organs was root>leaf>stem in order,and the accumulation of chromium in all parts of VC-treated Coix lacryma-jobi L.was greater than that of HC treatment.2.The residual rate of dry matter during root decomposition of Coix lacryma-jobi L.decreased with increasing time of Cr treatment,showing a fast-slow trend change,and the root decomposition rate of HC treatment was greater than that of VC dissolved organic carbon（DOC）and microbial quantity carbon（MBC）content of HC treatment substrate both increased with increasing Cr⁶⁺concentration,and DOC content of HC substrate was greater than that of VC treatment,while MBC content of VC substrate was greater than that of HC treatment.3.The particulate organic matter（POM）consisted mainly of residues decomposed by plant roots and carbohydrates,etc.,and carbohydrates were the main adsorption sites for chromium,and the chromium content of POM was greater in the VC treatment than in the HC treatment.the results of the POM spectral characteristics indicated that the differences in the characteristic peaks might be influenced by the plant roots.4.The dissolved organic matter（DOM）content in the effluent of both wetlands was highest under the 10 mg·L^-1Cr⁶⁺treatment and lowest under the 20 mg·L^-1Cr⁶⁺treatment.The DOM content in the effluent increased with increasing time of Cr⁶⁺treatment,and the DOM content in VC treatment was greater than that in HC treatment.the DOM was mainly composed of carbohydrates,sugars,organic acids,etc.the molecular weight of DOM in HC was less than that in VC treatment,while the degree of aromatization and humification level of DOM in HC treatment was greater than that in VC treatment;the DOM in the effluent was mainly composed of endogenous DOM,and the authigenic index and humification level of HC was greater than that of VC treatment.5.The pH and redox potential（Eh）of the substrate decreased with increasing Cr⁶⁺concentration,where Eh increased with increasing Cr⁶⁺treatment time,and the pH and Eh of the VC-treated substrate were greater than those of the HC-treated counterpart.10 and 20 mg·L^-1Cr⁶⁺substrates showed an increase in total chromium content and chromium content of each form with increasing Cr⁶⁺treatment time and concentration,and the total chromium content of the HC substrate was greater than that of the VC.The magnitudes of different forms of Cr content were as follows:residue state>Fe-Mn oxide-bound state>organic matter-bound state>exchangeable state>carbonate-bound state,where the Cr content of residue state and Fe-Mn oxide-bound state was greater than that of other forms.6.The common characteristics of bacteria were significantly greater than those of fungi in all treatments,but chromium stress had a greater effect on fungi.The abundance and diversity of microbial communities decreased with the increase of Cr⁶⁺stress time,and the abundance of microbial communities in HC treatment was greater than that in VC treatment.the abundance of ascomycetes,cryptomycetes,tamoxifera,tephritidomycetes and potamycetes in HC treatment was greater in fungi,and the abundance of cyanobacteria and green curvularia in VC treatment decreased with the increase of treatment concentration.The correlation analysis with environmental factors revealed that pH was more influenced by microorganisms than each other factor,and there was a negative correlation between pH and each other environmental factor,while there was a positive correlation between total chromium content and each form of chromium content.The abundance of most bacterial species in the matrix was positively correlated with pH and Eh,and highly significantly negatively correlated with DOM,total chromium amount,and matrix chromium morphological content.Among the fungi,Rhizobium and Trapomyces showed significant positive correlations with DOM content,total chromium content and chromium content of each morphology;Peridiomycetes and Monomycetes showed significant negative correlations with DOM content,total chromium content and chromium content of each morphology;Potylobacter showed significant negative correlations with Eh and DOM content.7.The content of chromium in all forms in VC-treated buried roots and in the residual and 80%ethanol-extracted forms in HC-treated roots increased with increasing time and concentration of Cr⁶⁺treatment.The percentage of residual state was always the largest among all forms.Except for the control,the content of Cr in all forms in VC roots was greater than that in HC roots.8.The Cr⁶⁺removal rate of both artificial wetland effluents under Cr⁶⁺treatment was high,up to 98%,and decreased with the extension of Cr⁶⁺treatment time,and the removal rate of Cr⁶⁺in HC was greater than that in VC.