10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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A Simulation Tool to Understand the Chemistry of Hexavalent Chromium in Airborne PM at pH 5 and pH9
MEHDI AMOUEI TORKMAHALLEH, Dinara Konakbayeva, Marios Fyrillas, Mirat Karibayev, Chemical and Aerosol Research Team, Nazarbayev University
Abstract Number: 1174 Working Group: Aerosol Modeling
Abstract Chromium is found in the atmosphere in two stable oxidation states, trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Cr(III) is an essential element for living organisms, mainly involved in the control of lipid and glucose metabolism. Conversely, Cr(VI) is a toxic element, and based on the exposure duration it causes adverse health effects such as perforation of the nasal septum, asthma, bronchitis, pneumonitis and lung cancer. Insoluble forms of Cr(VI) might be more toxic than soluble compounds. Soluble Cr can enter the blood and be excreted or be converted to Cr(III) in the blood stream. Animal studies demonstrated that slightly soluble and highly insoluble Cr(VI) particles such as the chromates of zinc, lead, strontium, barium, and sintered calcium consistently induced a tumor response, albeit with variable efficacy. Thus, it is imperative to investigate the soluble and insoluble fractions of Cr in the atmosphere to better understand its health effects. To obtain a more complete picture of atmospheric aqueous Cr speciation, Cr chemistry in the presence of a mixture of atmospheric oxidants and reductants including ambient PM matrix and dissolved gases needs to be investigated. Nevertheless, soluble and insoluble forms of the Cr(III) and Cr(VI) were not investigated in previous modeling studies. Currently, Cr(VI) sampling involves collecting PM on a filter at pH 9 and is integrated with wet chemical analysis. PM on the sampled filters is extracted using an alkaline or acidic solution that is then analyzed by Ion Chromatography (IC) or a combination of IC and ICPMS, respectively. However, wet analysis procedures may not extract the insoluble Cr(VI) and thus, may underestimate atmospheric Cr(VI) concentrations. Understanding the soluble and insoluble forms of Cr in the atmosphere using modeling studies will assist in better designing and integrating Cr(VI) sampling and analysis systems. The objective of this study was to develop a conceptual model for the aqueous chemistry of the atmospheric Cr where Cr is a non-limiting reactant, and to implement this model to better understand the soluble and insoluble forms of Cr(III) and Cr(VI) compounds under atmospheric and sampling conditions. This model is also applicable to locations where Cr emitting industries exist. The results of the present investigation will be helpful in developing an analytical method to quantify total Cr (soluble and insoluble) species in the atmospheric PM. Field measured concentrations of atmospheric species were utilized as model input in this study. The dominant form of Cr(VI) in the pH 5 solution was found to be Cr2O7 2-. At pH 5 chromate was produced by the dissolution of Na2CrO4(s) and K2CrO4(s) available in the solid core, but a considerable portion of the CrO4 2-precipitated as (NH4)2CrO4(s), CaCrO4(s), BaCrO4(s) and PbCrO4(s). Cr(OH)3 was found to be soluble, and the insoluble form of Cr(III) was Cr2(SO4)3. Conversion of Cr(VI) to Cr(III) was higher than the conversion of Cr(III) to Cr(VI). The simulation results agree with the field measurements near Cr industries. At pH 9, CrO4 2- was the dominant soluble form of Cr on the sampling filter while (NH4)2CrO4, CaCrO3, BaCrO4, PbCrO4 were the dominant form of insoluble Cr(VI). Reduction of Cr (VI) to Cr (III) was higher than the oxidation of Cr (III) to Cr (VI).