Accumulation Characteristics Of Five Heavy Metals In Leafy Vegetables And Their Environmental Critical Values In The Pearl River Delta

In China, serious contamination of heavy metals in vegetables and farmlands poses a threat to human health. Leafy vegetable is one of the most widely planted, and largest consumed, but also the most vulnerable to heavy metal contamination catergory of vegetables with many varieties. Therefore, study on the accumulation characteristics of heavy metals in leafy vegetables and the soil critical values for food safety is desirable, which has an important guiding role in improving the quality and safety of agricultural products. In the present study, a pot experiment was conducted using pakchoi(Brassica chinensis L.) and flowering cabbage(Brassica campestris L.) and vegetable field soils with different heavy metal contamination gradients in the Pearl River Delta. We aimed, to study the enrichment characteristics of different heavy metals (Cd, Pb, As, Hg, Cr) in edible parts of leafy vegetables, to explore the effects of soil physicochemical properties on availability and absorption of heavy metals in vegetables, and to estimate the environmental critical values of heavy metals in field soils for leafy vegetable safety production based on the national standard for maximum levels of heavy metal contaminants in foods (GB2762-2005). The results were as follows:(1) After planting pakchoi and flowering cabbage, the contents of As and Hg in soil reduced significantly. There was a certain gradient relationship for As, while Hg had no obvious gradient. Concentration of Cr in flowering cabbage-growing soil also showed a few reduction, while Cd and Pb did not change significantly.(2) In pakchoi-growing soil, OM and CEC showed respectively positive correlations with total Hg in soil, CEC had negative correlation with the total Cr, pH can enhance the activity of soil Cr. In soil of planting flowering cabbage, OM had positive correlation with the content of available Hg in soil and Cd in flowering cabbage.(3) The contents of available Cd, Pb, As, Hg, Cr showed respectively positive correlations with that of total heavy metals in pakchoi-growing soil. The availability coefficients of heavy metals in pakchoi-growing soil ranked from highest to lowest as: Cd>Pb>As>Cr>Hg; the contents of Cd, Pb, As in pakchoi showed respectively positive correlation with the contents of total and available heavy metals; Pakchoi’s enrichment coefficients of heavy metals ranked from highest to lowest as:Cd>Cr>As>Pb. The contents of available Cd, Pb, As, Hg, Cr showed respectively positive correlation with that of total heavy metals in soil of growing flowering cabbage, the availability coefficients of heavy metals in soil of growing flowering cabbage ranked from highest to lowest as:Cd>Pb>As>(Cr、Hg); the contents of Cd, Pb, As, Cr in flowering cabbage showed respectively positive correlation with the contents of total heavy metals, and the contents of Cd, Pb, As in flowering cabbage showed respectively positive correlation with available contents of heavy metals; Flowering cabbage’s enrichment coefficients of heavy metals ranked from highest to lowest as:Cd>Cr>As>Pb.(4) The soil critical values of soil total heavy metals in the pakchoi field were calculated as follows:Cd:1.74mg/kg, Pb:494.78mg/kg, As:113.58mg/kg, and the critical values of soil available heavy metals were calculated as follows:Cd:0.83mg/kg, Pb:146.87mg/kg, As:21.94mg/kg. The soil critical values of soil total heavy metals in the flowering cabbage field were calculated as follows:Cd:1.18mg/kg, Pb:401.93mg/kg, As:204.96mg/kg, Cr:158.53mg/kg, and the critical values of soil available heavy metals were calculated as follows:Cd:0.64mg/kg, As:35.13mg/kg, Cr:4.93mg/kg. Pakchoi and flowering cabbage were classified as leafy vegetable, and then used Linear regression analysis. The soil critical values of soil total heavy metals in the leafy vegetable field were calculated as follows:Cd:1.46mg/kg, Pb:435.71mg/kg, As:139.85mg/kg, Cr:48.16mg/kg, and the critical values of soil available heavy metals were calculated as follows:Cd:0.74mg/kg, Pb:125.31mg/kg, As:24.78mg/kg. The field validation results indicated that the threshold values of Cd and As were accurate; the threshold of Pb was overestimated, while the threshold of Cr was underestimated.(5) The contents of Hg in pakchoi and flowering cabbage had nonsignificant correlation with the contents of total and available Hg in soil, but in this experiment, Hg contents in edible parts of pakchoi and flowering cabbage were much lower than the criterion(GB2762-2005). It indicated that both pakchoi and flowering cabbage accumulated low Hg. Therefore, leafy vegetables grown in dry land might had a low risk for Hg contamination.(6) In this experiment, vegetable-soil total content relationship of Cd, in the correlation coefficient and significant aspects had no obvious difference with vegetable-soil available state relationship; The correlation coefficient in vegetable-soil total content relationship of Pb was better than in vegetable-soil available state relationship; The correlation coefficient and significant aspects in vegetable-soil total content relationship of As was worse than in vegetable-soil available state relationship. The correlation coefficient in flowering cabbage-soil total content relationship of Pb was better than in flowering cabbage-soil available state relationship. Thus, The threshold values for Cd could be based on total and available content as indicators to assess Cd pollution status of vegetable fields, while Pb and Cr should choose the total content as an indicator, and As should select available content as an indicator.