title: Double shroud delivery of silica precursor for reducing hexavalent chromium in welding fume.
abstract: The welding process yields a high concentration of nanoparticles loaded with hexavalent chromium (Cr(6+)), a known human carcinogen. Previous studies have demonstrated that using tetramethylsilane (TMS) as a shielding gas additive can significantly reduce the Cr(6+) concentration in welding fume particles. In this study, a novel insulated double shroud torch (IDST) was developed to further improve the reduction of airborne Cr(6+) concentration by separating the flows of the primary shielding gas and the TMS carrier gas. Welding fumes were collected from a welding chamber in the laboratory and from a fixed location near the welding arc in a welding facility. The Cr(6+) content was analyzed with ion chromatography and X-ray photoelectron spectroscopy (XPS). Results from the chamber sampling demonstrated that the addition of 3.2 ≈ 5.1% of TMS carrier gas to the primary shielding gas resulted in more than a 90% reduction of airborne Cr(6+) under all shielding gas flow rates. The XPS result confirmed complete elimination of Cr(6+) inside the amorphous silica shell. Adding 100 ≈ 1000 ppm of nitric oxide or carbon monoxide to the shielding gas could also reduce Cr(6+) concentrations up to 57% and 35%, respectively; however, these reducing agents created potential hazards from the release of unreacted agents. Results of the field test showed that the addition of 1.6% of TMS carrier gas to the primary shielding gas reduced Cr(6+) concentration to the limitation of detection (1.1 μg/m(3)). In a worst-case scenario, if TMS vapor leaked into the environment without decomposition and ventilation, the estimated TMS concentration in the condition of field sampling would be a maximum 5.7 ppm, still well below its flammability limit (1%). Based on a previously developed cost model, the use of TMS increases the general cost by 3.8%. No visual deterioration of weld quality caused by TMS was found, although further mechanical testing is necessary.
authors.affiliation: Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure & Environment, University of Florida, Gainesville, Florida 32611-6450, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Wang
authors.forename: Mark
authors.initials: M
authors.lastname: Kalivoda
authors.forename: Jianying
authors.initials: J
authors.lastname: Guan
authors.forename: Alexandros
authors.initials: A
authors.lastname: Theodore
authors.forename: Jessica
authors.initials: J
authors.lastname: Sharby
authors.forename: Chang-Yu
authors.initials: CY
authors.lastname: Wu
authors.forename: Kathleen
authors.initials: K
authors.lastname: Paulson
authors.forename: Omar
authors.initials: O
authors.lastname: Es-Said
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry; D002274:Carcinogens, Environmental / Q000032:analysis* / Q000737:chemistry; D002852:Chromatography, Ion Exchange; D002857:Chromium / Q000032:analysis* / Q000737:chemistry; D006801:Humans; D053768:Metal Nanoparticles / Q000032:analysis* / Q000737:chemistry; D009569:Nitric Oxide / Q000737:chemistry; D016273:Occupational Exposure / Q000032:analysis* / Q000517:prevention & control; D011482:Protective Devices; D012822:Silicon Dioxide / Q000737:chemistry*; D014691:Ventilation; D014896:Welding / Q000295:instrumentation*
chemical_list: D002274:Carcinogens, Environmental; D002857:Chromium; C074702:chromium hexavalent ion; D009569:Nitric Oxide; D012822:Silicon Dioxide; D002248:Carbon Monoxide