Speciation and potential longterm behaviour of ch

Authors: Patrick Byrne Kevin G Taylor Karen A HudsonEdwards Judith E S Barrett

Publish Date: 2016/09/26

Volume: 17, Issue: 11, Pages: 2666-2676

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Abstract

Chromium a potentially harmful element occurs commonly within the urban sediment cascade as a result of abundant industrial and transportrelated sources The risks that Crbearing particles pose to ecosystems and humans depend on the solidphase chemical speciation of Cr and its environmental mobility In this study we adopt an integrated geochemical approach to investigate and determine the longterm fate of Cr in the urban sediment cascadeWe use bulk chemical digests sequential chemical extraction analysis electron microscopy electron microprobe and microfocus XANES analysis to describe the solidphase speciation geochemical characteristics and potential longterm behaviour of Cr in urban particulate matter from both aquatic sediment and road dust sediment RDS in Manchester UKCrbearing grains within RDS and aquatic sediment are predominantly iron oxides and aluminosilicate glass grains Electron microprobe analysis indicates Cr concentrations up to 3300 and 133400 μg g−1 in the RDS and aquatic grains respectively XANES analysis indicates that CrIII is the dominant oxidation state with only trace amounts of CrVI Importantly Cr speciation does not appear to have changed between sedimentary environments and the dominance of CrIII suggests limited bioavailability or toxicity under predominant environmental anoxic and neutral pH conditions in the aquatic sediment sink Furthermore geochemical analyses suggest the environmental mobility of Cr in the aquatic sediment sink is low compared to other toxic metals due to its association mainly with aluminosilicate glass grains and its inclusion as an integral part of the glass structureIndustrial glass grains are a major component of urban sediment worldwide The speciation and geochemical investigations performed in this study suggest most Cr within the urban sediment cascade may be resistant to environmental processes that could mobilise other toxic metalsChromium Cr has long been known to be a potentially harmful element within the environment Nriagu and Nieboer 1988 Humans and ecosystems can be exposed to Cr through natural and anthropogenic pathways in water and particulate soil sediment and aerosol matter Werner et al 2007 However the risks that Crbearing particles pose to ecosystems and humans depend on the solidphase chemical speciation and environmental mobility of Cr in the particles Kotas and Stasicka 2000 In the environment Cr exists primarily in two oxidation states—trivalent chromium CrIII and hexavalent chromium CrVI Under anoxic conditions cationic CrIII is typically the dominant form and is relatively stable and nontoxic at circumneutral pH 65–85 due to the formation of insoluble hydroxide and oxide compounds and strong complexation with minerals and organic matter Martello et al 2007 ChromiumIII is an essential element for organisms and in small quantities is required for sugar and lipid metabolism Anderson 1989 Broadway et al 2010 Under oxic conditions anionic CrVI tends to dominate it is soluble and can be mobile across the full pH range There is clear evidence of the toxicological risk and carcinogenic properties of CrVI in humans and aquatic ecosystems Broadway et al 2010 Stern 2010 Ohgami et al 2015 As such Cr is widely recognized as a potentially harmful element and listed as a contaminant of serious concern in environmental legislation worldwideChromium occurs within urban environments around the globe as a result of abundant industrial and transportrelated sources Owens et al 1999 Important sources are vehicle tyres and brake linings steel production combined sewer overflows CSOs municipal discharges and industrial effluents including chromite ore processing residue COPR and tannery effluent Chromium contamination of urban freshwater sediments road dust sediment and airborne particulate matter has been widely reported Valerio et al 1988 Yu et al 2008 and speciation studies have generally observed CrIII to dominate Martello et al 2007 Werner et al 2007 Broadway et al 2010 Swietlik et al 2011 Landrot et al 2012 This information has proved useful for assessing the potential toxicity of Cr in urban particulate matter assuming stable environmental conditions However environmental change driven by the dynamic urban environment and/or climatic forces has the potential to affect the longterm environmental mobility and toxicity of Cr in these sediments Calmano et al 1993 Butler 2009 Knott et al 2009 In order to understand the potential longterm behaviour of Cr in complex substances such as urban particulate matter a synergistic and grainspecific approach is required that incorporates bulk and grainspecific chemical analysis and spectroscopic techniques such as scanning electron microscopy SEM xray powder diffraction XRD and synchrotronbased xray absorption spectroscopy XAS techniques such as xray absorption near edge structure XANES Xia et al 1997 Manceau et al 2002 Berry and O’Neill 2004 Wei et al 2007 Barrett et al 2010 2011 Chen et al 2010 The major advantage of grainspecific analyses over bulk sediment analyses is that the major host species for contaminants can be identified and subjected to geochemical analyses to determine environmental mobility A case in point are the metalrich glass grains that are major constituents of both terrestrial Lottermoser 2002 and freshwater Taylor and Boult 2007 urban particulates as a consequence of furnacederived slag from steel production These are known to contain high concentrations of trace elements Pb Cu Cr Zn Ni and have been identified in urban particulate matter worldwide Kida and Sakai 2001 Lind et al 2001 Reich 2003 Saffarzadeh et al 2009 Taylor and Robertson 2009 Following deposition in urban water bodies the ultimate sink for contaminated urban particulates these glass grains have been observed to undergo dissolution and early diagenetic reactions potentially releasing Cr to interstitial and overlying waters Taylor and Boult 2007In this paper we adopt an integrated speciation and geochemical approach to describe the solidphase speciation and environmental mobility of Cr in urban particulate matter from the Greater Manchester urban conurbation UK To the authors’ knowledge this is the first time such an integrated approach has been adopted to study toxic element cycling through urban sediment systems The specific objectives of this study were to 1 investigate the association of Cr with terrestrial and aquatic urban particulates 2 confirm the speciation of Cr in these particulates and 3 investigate the potential longterm geochemical mobility of Cr associated with urban particulates This innovative approach could provide a strong evidence base for assessing the longterm risk of Cr to human health and ecosystems in urban environments

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