Chemical fractionation and potential health risks assessment of particulate matter bound heavy metals in Bhopal, Central India 

Abstract

Fine particulate matter, PM2.5 - bound toxic metals upon uptake by the human body, plants and animals by various mechanisms pose health risks.  Additionally, they also play a role in altering the biogeochemical cycles of various species following dry/wet deposition onto soils, sediments and water bodies. Assessing the bioavailability of these metals using the total concentrations makes simplifying assumptions about the chemical states that these species are present in and the findings of risk assessments using such models/estimates are likely to have high uncertainties. Literature suggests that the sequential extraction procedure (SEP) is a versatile analytical method to examine the chemical fractionation of metals in particulate matter. Tessier’s SEP approach was used in this study to determine the extent of reactivity of metals in four fractions using specific solvents. Weekly composite of 24 hr integrated ambient PM2.5 samples (n=52+12 field blanks) collected onto Teflon filters, every other during 2019 were used.  These samples were collected at a regionally representative location in Bhopal, central India, as part of the COALESCE ambient aerosol measurement campaign.  Filter samples were utilized to quantify the metal fractions (K, V, Ti, Mn, Fe, Pb, Zn, and Cu) using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The internal check of the recovery of metals was determined by comparing the sum of metal fractions concentration with total metal concentration determined using a ED-XRF (Energy Dispersive X-Ray Fluorescence Spectrometer) on the same filters, prior to their leaching. The total sum of fractional concentrations were in good agreement with the bulk metal concentration (slope = 0.24-57.53, r2 = 0.92 -0.99) and the internal recovery ranged from 85%-110% for different analytes. Annual mean concentration of metal fractions of K, Ti, Pb and Zn were higher in the two bioavailable fractions including soluble, exchangeable fraction (F1) and carbonate, oxide and reducible fraction (F2) (43- 67 % of total concentration). Relatively higher proportions of Fe, Mn, and V were less bioavailable and were present in oxidizable fraction (F3) (40- 73 % of total concentration) indicating its high association with organic matter and inorganic sulfides. Cu was strongly bound to its silicate fraction and had its highest proportion present in the residual fraction (F4) (91 % of total concentration). As expected, application of the United States Environmental Protection Agency (USEPA) health risk assessment equations to the measured fractions revealed that the route of exposure for bioavailable metals was highest via the inhalation pathway, followed by dermal contact and ingestion. Total potential non-carcinogenic health risk indicator, the Hazard Quotients were below but close to the safe level of 1 for all bioavailable metal fractions. The cancer risk from bioavailable metal fractions were also within the USEPA acceptable limits for the three pathways. Overall, this work provides a database for bioavailable ambient PM2.5 heavy metals and evaluates potential health risks.  In the future, this work will be extended to assess the impacts of these metals on perturbing the biochemical cycles of these species in this regional environment.