Characterization Of Heavy Metal Enrichment In Different Vegetable Species And Correlation With Root Dissolved Organic Matter

This paper investigates the enrichment and transport characteristics of heavy metals in different types of vegetables and the effects of Dissolved Organic Matter（DOM）from vegetable roots on heavy metal migration.A pot control experiment was conducted in a farmland with nearly 50 years of wastewater irrigation,where 20 vegetables（classified by plant organs:root vegetables,fruit vegetables,and leafy vegetables）were grown.The content of heavy metals（As,Cd,Cr,Cu,Pb,Zn）in each part of the vegetables（above ground,below ground,and fruit parts）and in the inter-root soil was measured to analyze the accumulation coefficient and Health Risk Index（HRI）of different types of vegetables.The results showed that fruit vegetables had the lowest accumulation coefficient and HRI of heavy metals and were the most suitable type of vegetables to grow in heavy metal-contaminated soil.The paper also analyzed the content,composition and spectral parameters of DOM from vegetable roots using UV-absorption spectroscopy and 3D fluorescence spectroscopy to explore the effects of vegetable types on root DOM content and optical properties.The correlation links between root DOM content and spectral parameters and heavy metals were tentatively explored.Specific findings are as follows:（1）The edible parts of vegetables had different levels of heavy metal content,with leafy vegetables having the highest,followed by fruit vegetables and root vegetables.The Bioconcentration Factor（BCF）and the Translocation Factors（TF）for Cd and Zn were higher in all three vegetable groups,while the correlation coefficients for As and Pb were lower.Root vegetables had a significantly higher bioconcentration factor of heavy metals than fruit and leafy vegetables（p<0.05）,whereas leafy vegetables had a significantly higher ability to transport As and Pb to their edible parts than root and fruit vegetables（p<0.05）.The health risk assessment showed that the non-carcinogenic risk of ingesting single Cr,Cu and Zn was within the safe range,but children faced a higher health risk than adults.The mean values of non-carcinogenic risk（HRI）of single heavy metals were ranked as follows:As>Cd>Pb>Cu>Zn>Cr.The total health risk index（THIR）of six heavy metals in three types of vegetables was:leafy vegetables>fruit vegetables>root vegetables,suggesting that root vegetables were more suitable for cultivation.In conclusion,choosing vegetables with low accumulation of contaminants based on the degree of soil contamination can effectively reduce the health risk of contaminant intake.（2）The vegetable species affected the DOM content,fractions and spectral parameters of their roots.The root source DOM content was highest in leafy vegetables,followed by root vegetables and fruit vegetables.The DOM fluorescence fractions consisted mainly of humic-like（C1）and protein-like（C2）substances.The specific absorbance SUVA₂₅₄（Specific Ultraviolet Absorbance at 254 nm:DOM aromaticity index）,spectral slope ratio S_R（Spectral Slope Ratio:size of DOM molecular weight）and humification index HIX（Humification Index:degree of humification）indicated that root vegetables had organic matter with high aromaticity,high humification and large molecular weight,whereas leafy vegetables had organic matter with low aromaticity,low humification and small molecular weight.The root DOM of the three vegetable groups was mainly composed of newly formed authigenic fractions.Solid ¹³C nuclear magnetic resonance spectroscopy(¹³C NMR)revealed that O-alkyl C was the dominant form of organic matter in the root samples of all three types of vegetables.（3）The root DOM fractions and aromaticity had a strong influence on the heavy metal contents in the soil-vegetable system.The correlation analysis showed that As,Cd,Cu,Pb and Zn contents in the below-ground parts of vegetables were related to the root DOM content and functional group abundance in root vegetables.The contents of Cd,Cu and Pb in all vegetable samples had negative correlations with O-alkyl C and positive correlations with aromatic and phenolic C in the root organic carbon,respectively.The specific absorbance per unit mass of organic matter(SUVA₂₅₄)and the ratios of each of the six heavy metals to the root dissolved organic matter content（[HM]/[DOM]）had significant positive correlations.Likewise,the humic-like fraction([Fmax _C1]/[DOM])and the protein-like fraction([Fmax _C2]/[DOM])of root DOM in all vegetable samples had positive correlations with the six[HM]/[DOM]ratios.The results indicated that both the DOM fraction and the aromaticity of DOM affected the content of the six heavy metals in vegetables.