Leaves of orange jasmine (Murraya paniculata) as bioindicators of airborne heavy metal pollution in Bangkok area

Abstract

Orange jasmine (Murraya paniculata) leaves, topsoil and particulate matter (PM10) were collected from three different sites in the area of Bangkok and in Pathumthani as reference site. The leaf samples were differently treated and divided into two groups: washed and unwashed leaves. All types of samples were digested and the concentrations of Cu, Fe, Pb, Mn, Ni, Cr and Zn were then quantified by using ICP-AES. The SEM study reveals that airborne particles are higher in density on the adaxial (upper surface) than abaxial (lower surface). The particles are heterogeneously distributed in forms and diameter lengths. The characteristics of stomata appeared on abaxial of old and young leaf in forms, sizes, and numbers are not different. Metal concentrations in PM10 are found insignificantly different between sites while the soil results indicate all metals are higher in urban areas relative to the reference site. All metals found in both types of leaves are higher in the sampling urban sites than the reference site. Pearson’s correlation coefficient is used to determine the degree of relationship in the study. There is not significant correlation of the same metal concentrations between washed leaves and soils, and washed leaves and PM10. The enrichment factor (EF) using Fe as a reference element is employed to metals found in leaf and air particle samples to separate the anthropogenic source from the crustal source. The patterns of EF values of all sample types are similar. The EF values of Cu, Ni, Pb and Zn indicate that the dominant sources of these elements are not natural. On the contrary, the Cr and Mn are found their origin from natural source. The statistical t-test is used to test the effects of washing leaves, leaf ages and seasons (wet and dry) on metal accumulation in leaves. Washing the leaves reduces the concentrations of metals significantly. Fe is most removed by washing. The levels of Cu and Fe concentrations in leaf increase significantly in the course of time while the Mn and Zn are remained constant. Metal contents accumulated in both types of leaves are insignificantly different between wet and dry seasons. The study on source identification of air metal pollution using leaves show all three sampling sites are polluted with metals compared with the reference site. The highest mean concentration of the studied metals is Fe while Pb is the lowest. The correlation analysis shows that there is a high correlation coefficient between Cu-Mn, Cu-Zn, Cu-Pb, and Mn-Zn. However, Fe is not correlated to other metals. The ANOVA testing reveals that there is not a significant difference in Pb contents among sites. The significant difference in metals could be attributed to different anthropogenic activities among sites. The principal component analysis (PCA) shows two main factors according to the sources of metals found. Factor 1 and 2 are anthropogenic (traffic) and natural (soil) sources, respectively. Based on the overall study, it can be concluded that the airborne particles found in Bangkok are strongly impacted by anthropogenic sources especially traffic. The results also show that Orange jasmine (Murraya paniculata) leaves can be used as bioindicators for metal air pollution.