A STUDY ON THE TRACE METALS IN VEGETATION AND SOIL

Abstract

This thesis discusses the development of several analytical methods for the analysis of some trace metals in vegetation and soil. Based on the test methods and application areas, the thesis is divided into three parts. Part one of the thesis is on the study of Al in natural waters and teas. Aluminum has been implicated in neurotoxicity associated with mental diseases. The toxicity of Al is related to its species. The purpose of the study was to develop a method to classify the Al species in tea infusions. A simple, selective and sensitive HPLC method for the determination of Al in waters was developed using fluorimetric and spectrophotometric detections with lumogallion as a precolumn reagent. Then, the developed method was applied to the determination of Al species in estuarine and coastal sea waters after fractionation using the "Driscoll" method. Using different sample treatment and analytical methods, the Al species in tea infusions were successfully categorized. The results indicated that the compositions of Al species in tea infusion vary with the methods of tea production. About 21 % Al was leached into the water in the first tea infusion and a total of 27 % in four subsequent infusions. Part two of the thesis focuses on the methodology development for the analysis of metals based on microwave digestion. A systematic study on the sample preparation methods using microwave digestion or extraction was carried out. The parameters of microwave digestion were optimized for the determination of trace metals in sediments. These parameters include the ratio of the acid mixtures HCl:HNO3, the power output, the pressure setting, the digestion time and the addition of HF in the acid mixtures. The effect of addition of HCIO4, HF, H3BO3 in the acid mixtures of HCl and HNO3 was further evaluated in the digestion of soil sample. The comparisons of different microwave digestion methods were carried out for the determination of total mercury, selenium and arsenic in sediments and soils. In the biological samples, microwave extraction technique was used to improve on the dilute nitric acid extraction method. Using quick digestion program, most of the metals in 0.3 g biological samples could be completely extracted into 5 ml 14 % (v/v) nitric acid. The microwave digestion technique was also used in the treatment of biological samples using tetramethylammonium hydroxide and ethylenediaminetetraacetic acid. The digestion time was reduced from 4 h to 30 min. Part three of the thesis presents the results of the survey of trace metals in soil and vegetation in Singapore. The survey was carried out twice. In the first survey, only five metals in soil samples from three representative areas were analyzed. In the second survey, seven metals were analyzed from a total of 149 vegetation and soil samples collected from five different areas in Singapore. The five areas represented the industrial, housing estate, road sides with heavy traffic, NUS campus and nature reserve. The leaves of Angsana and Rain Tree which are commonly found in Singapore, were chosen for the study. Other species of plants were also studied. The results indicated that the metal pollution is low in Singapore and the main metal pollution sources are from vehicular exhaust and industrial activities.