Effects Of Different Environmental Conditions On The Chemical Forms And Activities Of Heavy Metals In Vegetable-growing Soils

With the increase of arthropogenic activities, heavy metal contamination in soils especially in vegetable garden soils has been mainly concerned because of the food chain of soil~vegetables/animals. Although the total concentrations of heavy metals in soils can reflect the level of metal contamination, it is insufficient to indicate the metals'bioavailability and toxicity for plants. The different chemical forms of heavy metals in soils can provide more valuable information of the bioavailability. Therefore, this dissertation aims to investigate the total contents and different chemical forms of heavy metals in vegetable-growing soils. The effects of different environmental conditions on the metals'bioavailability are also studied. Meanwhile, the amendment by in-situ immobilization of heavy metals in soils has also been conducted through pot experiment.This dissertation mainly consists of three chapters as follows:In chapter one, the total contents and chemical forms of heavy metals (including Pb, Cd, Cu, Zn, Cr and As) in the vegetable-growing soils from six areas of Fujian Province were analyzed. The results of BCR sequential extraction procedures showed that Pb, Zn, As, and Cr were found to mainly present in residual forms. But Cu and Cd were found to largely present in the forms of sulfide bound organic matter and Fe-Mn oxides. A large scale of Cd was found to present in the acid-extractable fractions compared with that of Pb.In chapter two, the effects of the envronmental factors on the solubility of heavy metals in soils were investigated by simulating different environmental conditions (such as pH values, temperatures, ultraviolet radiation, ionic strength, etc.) coupled with BCR sequential extraction procedures. The results showed that the soluble metals increased under the conditions of lower pH and lower ionic strength compared with the control group. With the increase of temperature and the exposure of ultraviolet radiation, the heavy metals in vegetable garden soils may be activated because of the increase of soluble metals in soils. For example, after the ultraviolet radiation the soluble metals increased by 2.61% ~ 8.54% for Cd and 8.37% ~ 44.1% for Pb, respectively.In chapter three, several kinds of modifiers were employed to study the amendment by immobilizing in-situ the heavy metals in soils. Soils sampled from Fuzhou Jinshan (FZJS) were mixed with Pb at the concentrations of 0, 250, 500, 800μg/kg, and soils from Sanming Mingxi (SMMX) were mixed with Cd at the concentrations of 0, 0.3, 1, 3μg/kg. Four modifiers (including 3 mg/kg sodium sulfide, 2 mg/g humic acid, 10 mg/g zeolite and 4 mg/kg of calcium hydroxide) were added into the mixed soils before pot experiment. The contents of the uptake metals (Cd and Pb) in different parts of the planted vegetables were determinated after six weeks'cultivation in the mixed soils. The results showed that the four modifiers could effectively inhibit the absorption of heavy metals for the vegetables. Especially, zeolite presented more inhibition than humic acid for Cd absorption, and calcium hydroxide was found to be superior to sodium sulfide on the remediation of Pb in soil. The concentrations of Cd in root, leaf and stem of vegetables decreased by 37.4%, 30.1% and 14.0%, 17.6% after adding 10 mg/g zeolite and 2 mg/g humic acid in soils; while Pb in root, leaf and stem of vegetables decreased by 33.1%, 59.1% and 19.0%, 37.2% after adding 3 mg/kg sodium sulfide and 4 mg/kg calcium hydroxide.