Study On The Chemical Forms Of Mercury And Their Bioavailability In Vegetable-growing Soils

With the development of agriculture and urbanization, the problem of mercury pollution invegetable-growing soils is becoming more and more serious, which arouse people’s highattention. There are various chemical forms of mercury in vegetable-growing soils; each mercuryform with specific bioavailability can cause specific biological effect on vegetables. In order tocontrol and reduce mercury contaminaton in vegetables, it is important to determine theconcentrations of mercury in vegetable-growing soils and vegetables, and study the chemicalforms of mercury and their bioavailability in soils. In this dissertation, an isotope dilutioninductively coupled plasma mass spectrometry （ID-ICP-MS） method for the determination oftrace Hg in vegetable-growing soils and vegetable samples was established. Moreover, in orderto study the bioavailability of mercury forms in a soil-vegetable system, the chemical forms ofmercury, determined with modified Tessier sequential extraction, were analyzed by isotopelabelling method.This dissertation consists of four chapters.In chapter one, the harm of mercury contamination was described in great detail. Thepresent situation of mercury contamination in vegetable-growing soils was reviewed. Theanalytical methods for determining the amounts and chemical forms of mercury in soils wereintroduced. Besides, the methods for estimating the bioavailability of mercury in soils werediscussed.In chapter two, an isotope dilution inductively coupled plasma mass spectrometry（ID-ICP-MS） method for the determination of trace Hg in vegetable-growing soils and vegetablesamples was established. The concentrations of Hg in soils and vegetables were calculated withID formula based on the accurate measurement of isotope ratio RHg(²⁰²Hg/²⁰⁰Hg) with ICP-MS.Instrumental parameters, amounts of spike isotopic reagent(²⁰²Hg>98%), mass bias, dead time,and isobaric and polyatomic interferences were discussed and optimized. An internal standardcontaining50μg/L of Tl was simultaneously measured for the correction of mass bias and thesignal fluctuation by the isotope ratio of205Tl/203Tl. After being digested with HNO3and HCl bymicrowave, the concentrations of Hg in poplar leaves （GBW07604） and soil （GBW07423） weremeasured with the established method. The recoveries of112%for poplar leaves and100%forsoil were obtained. The established method showed simple sample preparation, good accuracyand precision, and was suitable for the determination of trace Hg in soils and vegetables. In chapter three, the chemical forms of mercury and their bioavailability invegetable-growing soils were studied. In order to investigate the mercury forms and theirbioavailability, the pot experiment and modified Tessier sequential extraction procedures wereused to determine the concentrations of different mercury forms in rhizosphere soils during theplant cultivation. The results showed the sum of mercury form concentrations in soils throughTessier sequential extraction were less than the total mercury. Mercury in soils mainly presentedin residue-bound form, and the bioavailable concentrations of water soluble and exchangeableforms accounted for0.28%-1.55%of the total mercury. In the process of pot experiment, theconcentrations of the residue-bound form of mercury in soils didn’t change，which indicated themercury form was inert; the concentrations of the water soluble form of mercury in soilsincreased, which indicated this mercury form was easily bioavailable; humic acid-bound form ofmercury significantly decreased, which showed this mercury form was easily bioavailable andhad potential bioavailability; the concentrations of other chemical forms decreased to a certianextent, but their concentrations were low, which suggested they had bioavailability to somedegrees and the contribution of mercury absorbed by vegetables was small.In chapter four, the bioavailability of mercury forms in vegetable-growing soils was studiedby isotope labelling method. In order to evaluate the bioavailability of mercury forms, mercuryforms, which labeled by²⁰²Hg in rhizosphere soils, were extacted through modified Tessiersequential extraction during the plant cultivation. The results showed that isotope ratioRHg(²⁰²Hg/²⁰⁰Hg) of residue-bound form didn’t change, which indicated this mercury form wasextremely stable and could hardly be transformed into other forms; the RHg(²⁰²Hg/²⁰⁰Hg) of watersoluble and exchangeable forms significantly decreased by40.1%-75.7%, which suggested thetwo mercury forms were easily bioavailable; the RHg(²⁰²Hg/²⁰⁰Hg) of fulvic acid-boundsignificantly increased, and the RHg(²⁰²Hg/²⁰⁰Hg) of humic acid-bound forms significantlydecreased, which showed the two mercury forms were also easily bioavailable. theRHg(²⁰²Hg/²⁰⁰Hg) of other mercury forms changed to some degree, but the RHg(²⁰²Hg/²⁰⁰Hg) andthe concentrations of other mercury forms were low. Therefore, we infered they hadunsignificant bioavailability and and the contribution of mercury absorbed by vegetables wassmall.From the third chapter and the fourth chapter, with the isotope labelling method the resultsof water soluble, fulvic acid-bound and Fe-Mn oxide-bound forms of mercury in soils weredifferent compared with the results from AFS. The difference may result from the lowconcentrations of the mercury fractions that could not be measured accurately by AFS; even withthe condition of low concentrations, the measurement of the RHg(²⁰²Hg/²⁰⁰Hg) with ICP-MSremained accurate. Therefore, the dependeble and accuracy information of mercury forms, which were studied with isotope labelling method, could reflect the bioavailability of mercury formsduring the plant cultivation.In conclusion, isotope labelling method had the advantages of rapidity, directviewing andaccuracy, and suited to gain accurately the information of the bioavailability of mercury forms insoils. In order to provide scientific experimental basis for controlling and reducing mercurycontaminaton in vegetables, it is important to study the chemical forms of mercury and theirbioavailability in vegetable-growing soils.