1 Introduction

Chromium (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 Cr-bearing particles pose to ecosystems and humans depend on the solid-phase 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 [Cr(III)] and hexavalent chromium [Cr(VI)]. Under anoxic conditions, cationic Cr(III) is typically the dominant form and is relatively stable and non-toxic at circum-neutral pH (6.5–8.5) due to the formation of insoluble hydroxide and oxide compounds and strong complexation with minerals and organic matter (Martello et al. 2007). Chromium(III) 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 Cr(VI) 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 Cr(VI) 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 worldwide.

Chromium occurs within urban environments around the globe as a result of abundant industrial and transport-related 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 air-borne particulate matter has been widely reported (Valerio et al. 1988; Yu et al. 2008) and speciation studies have generally observed Cr(III) 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 long-term 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 long-term behaviour of Cr in complex substances such as urban particulate matter, a synergistic and grain-specific approach is required that incorporates bulk and grain-specific chemical analysis and spectroscopic techniques such as scanning electron microscopy (SEM), x-ray powder diffraction (XRD) and synchrotron-based x-ray absorption spectroscopy (XAS) techniques such as x-ray 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 grain-specific 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 metal-rich glass grains that are major constituents of both terrestrial (Lottermoser 2002) and freshwater (Taylor and Boult 2007) urban particulates as a consequence of furnace-derived 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 2007).

In this paper, we adopt an integrated speciation and geochemical approach to describe the solid-phase 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 long-term geochemical mobility of Cr associated with urban particulates. This innovative approach could provide a strong evidence base for assessing the long-term risk of Cr to human health and ecosystems in urban environments.

2 Materials and methods

2.1 Study area and sampling procedures

The contaminated sediment described in this study is from the urban conurbation of Manchester, UK. The road dust sediment (RDS) samples were obtained from locations within Manchester city centre that comprised heavily urbanised environments with high traffic densities (Fig. 1a). Samples were collected by sweeping with a polyethylene dustpan and brush. High metal (Fe, Mn, Pb, Zn, Cu) concentrations have been reported in these sediments and sequential extraction analysis has shown some metals (Fe, Mn, Zn) are largely associated with the reducible fraction, suggesting changes in ambient pH and/or Eh could mobilise these metals (Robertson et al. 2003). Trace metals (Pb, Cu, Zn, Cr, Ni) in the RDS material are hosted mainly in iron oxides and iron-rich glass grains (Taylor and Robertson 2009). Speciation studies using x-ray absorption spectroscopy (XAS) have identified both Pb (Barrett et al. 2010) and Zn (Barrett et al. 2011) to exist in phases that are potentially harmful to human health. The aquatic sediment was sampled from the Salford Quays (Fig. 1b), a historically contaminated urban water body that received domestic and industrial sewage and road runoff prior to commencement of remediation activities in the late 1980s. Urban water bodies represent major sinks for contaminated sediment in urban centres worldwide (Taylor and Owens 2009). Sediment cores were retrieved from basin 9 of the Salford Quays in 2000 and 2001 using a stainless steel corer and contain a mix of natural and anthropogenic detrital grains, the latter dominated by alumino-silicate glass grains (Taylor and Boult 2007). Petrographic analysis indicated these glass grains were undergoing chemical dissolution supplying Fe and Zn to porewaters while bacterial Fe(III) and Mn(IV) reduction was hypothesised to supply Fe2+ and Mn2+ to porewaters. Whilst these previous studies (and others) have greatly increased our understanding of trace metal geochemistry in urban environments, they have tended to focus on elements (e.g. Fe, Mn, Zn, Pb, Cu) that represent, by mass, the bulk of metal contaminants found in urban environments. Here, we focus specifically on the trace metal Cr, an element which may display different geochemical and mobility characteristics to those more typically studied metals.

Fig. 1
figure 1

Location of Manchester showing a RDS sampling sites in Manchester city centre and b aquatic sediment sampling site in the Salford Quays

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2.2 Elemental analysis of RDS and aquatic sediment

Petrographic and quantitative chemical data were obtained through the use of scanning electron microscopy and electron microprobe analysis. Air-dried samples of RDS and aquatic sediment were impregnated with epoxy resin and polished surface blocks were produced. The petrography of the samples was analysed with JEOL (Tokyo, Japan) 5600LV (for RDS) and JEOL JXA 8100 (for aquatic sediment) electron microprobes using backscattered electron imagery. Wavelength dispersive spectrometers (WDS) were used to obtain quantitative data on major and trace element composition of mineral grains. Analysis of SiO2, Al2O3, TiO2, CaO, Na2O, K2O, Fe2O3, MnO, MgO, P2O5, SO3 (all in wt%), and Cr (in ppm) were undertaken in WDS mode using an accelerating voltage of 15 kV, current of 2.5 nA, electron beam diameter of 1 μm and counting time of 10 s. The analyses were calibrated against standards of natural silicates, oxides and Specpure® (Waltham, MA, USA) metals with the data corrected using a ZAF program.

2.3 Molecular-scale analysis of RDS and aquatic sediment

XANES is sensitive to the oxidation state, continuation and bonding environment around a central atom of transition elements such as Cr, both in single and mixed complexes, for example, amorphous material, silicate glasses and particulate matter (Pantelouris et al. 2004; Farges 2009). XANES data were collected at station I18 at the Diamond Light Source, UK, in September 2010. XAS spectra were collected at the Cr K-edge (5989 eV). Operating conditions for the storage ring were 3 GeV and 200 mA. On I18, which is an undulator beamline, the x-rays are focused by a pair of Kirkpatrick-Baez (KB) mirrors after being monochromated by a Si (111) double-crystal monochromator. Experiments were performed at ambient temperature. Standard spectra were collected for model compounds [Cr(OH)3, Cr foil, Cr(III), Cr(VI)] and these were analysed in transmission mode. The urban particulate samples were presented to the beam in resin-impregnated polished blocks. In these, the distribution of Cr in individual grains was mapped using μXRF and Cr XAS data were collected in fluorescence mode for qualitatively high-Cr areas (μXRF did not quantify the Cr concentrations). Data processing was carried out using ATHENA (to convert from monochromator position in millidegrees to energy in eV and to average multiple spectra from individual standards and samples; Ravel and Newville (2005)) and PySpline (to carry out background subtraction and normalization; Tenderholt et al. (2007)). The fluorescence signal is calculated as the total windowed fluorescence counts divided by I o. The normalization was done by fitting a first-order polynomial to the pre-edge region and extrapolating this to the end of the spectrum, then subtracting. A second-order polynomial was fitted to the post-edge region and subtracted. The edge step was normalized to 1. No linear combination modelling was undertaken due to the uncertainties around the appropriate model compounds.

2.4 Geochemical analysis of alumino-silicate glass grains

Alumino-silicate glass grains were identified as a major constituent of terrestrial and aquatic sediments and were, therefore, subjected to further geochemical analysis to determine the environmental mobility of associated Cr. Glass grains were hand-picked from the aquatic sediment under a binocular microscope. Samples of sediment were washed in distilled water and the <63-μm fraction was sieved away leaving behind silt- and sand-sized fractions. The glass grains were generally coarse sand in size and were also dark and shiny, allowing easy separation. Pseudo-total Cr concentrations of the glass grains were determined by aqua-regia digestion in closed vessels using a CEM MARS Xpress microwave apparatus (Matthews, NC, USA). A three-step sequential extraction procedure (SEP) recommended by the Standards, Measurements and Testing programme (SM&T) of the European Union (Rauret et al. 1999) was utilised to extract bioavailable Cr from the glass grains. The chemical procedure extracts metals bound to three specific geochemical phases: (1) acid-soluble, easily exchangeable, and bound to carbonates; (2) reducible, bound to iron, and manganese oxides and (3) oxidisable, bound to organic, and sulphide compounds. The SM&T procedure is the only SEP with a certified reference material (CRM 701) (Quevauviller et al. 1997) for all three phases of extraction allowing validation of the procedure and comparison of results between studies. The SEP included two sample duplicates, one blank and the certified reference material. The Cr content in the residual fraction of the glass grains was estimated as the difference between pseudo-total content (microwave extraction) and the sum of the contents of the other fractions obtained by SEP. The supernatants produced after both extraction procedures were acidified to pH 2 and stored at 4 °C prior to analysis. All solutions were analysed within a month using inductively coupled plasma (ICP) optical emission spectroscopy (OES) (PerkinElmer Optima 2100DV) (Waltham, MA, USA). Detection limits of the ICP-OES were 0.05 ppm Cr. High precision (±10 %) of the control standards is reported and calibration curves had a coefficient value better than 0.99. The relative standard deviation was less than 2 % above 0.05 ppm Cr. Recovery rates for Cr in the CRM 701 were 95 % for step 1, 91 % for step 2, and 101 % for step 3.

3 Results

3.1 Elemental analysis of RDS and aquatic sediment

Cr-bearing grains within RDS and aquatic sediments are characterized primarily by iron oxides and alumino-silicate glass grains derived from industrial waste. Aquatic sediment is dominated by glass grains comprising Fe-rich and Fe-poor varieties with inclusions of metal-rich blebs and dark crystalline areas. Electron microprobe analysis indicates the Fe-rich glasses have high concentrations of Cr (mean 347 μg g−1), Fe (mean 38.5 % FeO) and Al (mean 7.93 % Al2O3) (Table 1). The Fe-poor glasses contain lower concentrations of Fe (mean 9.15 % FeO), Cr (mean 226 μg g−1) and other metals (Table 1). Dark glass inclusions in the Fe-rich glasses contain Cr-rich areas up to 133,360 μg g−1 (mean 69,800 μg g−1). Cr-bearing grains within the RDS are predominantly iron oxide grains but Cr-bearing silicate glass grains are also present. Electron microprobe analysis indicates concentrations of Cr up to 3300 μg g−1 (mean 425 μg g−1) in these grains (Table 1). Examples of Cr-bearing grains in the RDS samples are shown in Fig. 2. WDS analysis of these grains shows them to be Fe oxides ((a), (b), (e) and (f)), a mixed Fe-Al-silicate and Fe-Mg-silicate (c) and a Fe silicate (d) (Table 1). All grains except (d) are texturally and chemically heterogeneous.

Table 1 Mean elemental composition of glass grains in aquatic sediment and RDS determined by electron microprobe analysis
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Fig. 2
figure 2

Scanning electron microscope photomicrographs of Cr-bearing grains from RDS sample. Scale bar in each photomicrograph is 100 μm. a, b, e, and f Cr-bearing Fe oxides. c Mixed Cr-bearing Fe-Al silicate and Cr-bearing Fe-Mg-silicate. d Cr-bearing Fe silicate

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3.2 Molecular-scale analysis of RDS and aquatic sediments

The influence of oxidation state on the XANES spectra is revealed in differences in the shape and the position of the edge, as well as the presences of pre-edge and multi-scattering resonances (MSR). As illustrated by the four model compounds in Fig. 3, a shift in the edge position of the Cr K-edge XANES to higher energies is generally discerned with increasing oxidation state. Such a shift may be used as a qualitative indicator of the presence of Cr(III), Cr(VI) or mixtures of different Cr oxidation states. Table 2 summarizes the energy values of pre-edge resonances, shoulder, absorption edge, edge crest and multi-resonance structures in XANES spectra at Cr K-edge for model compounds and environmental samples.

Fig. 3
figure 3

Representative Cr K-edge XANES spectra for sediment particulates and Cr K-edge XANES spectra for model compounds. Note the dominance of Cr(III) and only small amounts of Cr(VI) in aquatic sediment particulates

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Table 2 Energy values of pre-edge resonances, shoulder, absorption edge, edge crest and multi-resonance structures in XANES spectra at chromium K-edge for model compounds and environmental samples (values reported on eV)
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Consideration of the environmental samples finds that the aquatic grain XANES spectra display similar profiles. The lack of an intense peak in the pre-edge region coupled with two small pre-edge resonances indicates that Cr in such samples is present mainly in the octahedrally coordinated trivalent form, and that the contribution of tetrahedrally coordinated Cr(VI) is estimated to be ≤5 % (Huggins et al. 1999). Comparison of the pre-edge and edge position energies of the two aquatic samples spectra show comparability to the XANES spectra of the Cr(III) model compounds. In particular, the equivalence noted between the energies of the pre-edge resonance, edge crest and second MSR reported for Aquatic 1-51028 and those of Cr(OH)3 suggests that Cr(OH)3 is likely to be a main contributor to the Cr speciation of this sample. Differences in the energies of the pre-edge resonance and the second MSR reported for Aquatic 2-51029 may imply that Cr2O3 may also influence the Cr speciation of the Aquatic 2-51029.

Qualitative analysis of the XANES spectra of the RDS grains identifies a greater degree of variance between the individual spectra of the RDS. The edge position energies for the RDS grains, with the exception of RDS3-51050, appear to conform with values reported for the Cr(III) model compounds (6001.74 and 6002.43 eV) suggesting that Cr speciation is dominated by trivalent Cr species in the RDS samples. As with the aquatic samples, the presence of two low intensity pre-edge resonances identified in all the RDS samples, except RDS3-51050 and the absence of any sharp high intensity peak in the pre-edge region implies that Cr(VI) species play a secondary role to Cr(III) in the Cr speciation of RDS. Visual comparison of the five RDS spectra (Fig. 3) identifies strong similarities between RDS2-51044 and RDS5-51082 and to an extent, RDS1-51407. Consideration of the pre-edge resonance, edge and MSR energies determined for RDS2-51044 and RDS5-51082 finds agreement with those reported for Cr(OH)3 (Table 2). Although, RDS1-51407 spectra shows some comparability with that of Cr(OH)3, similarities to the edge position and MSR energies reported for Cr2O3 suggest that a combination of the two model compounds may be influential in this sample. The occurrence of shoulder structures in the XANES spectra of samples RDS3-51050 and RDS4-51059 infer that mixed oxidation states may exist in such samples (Berry and O’Neill 2004). The XANES spectrum of RDS3-51050 with the presence of a shoulder structure around 5990 eV and edge crest and MSR energies of 6007 and 6022 eV, respectively, displays characteristics of Cr in metallic and trivalent forms. By contrast, XANES spectrum of RDS3-51059, is characterized by the prominence of an absorption edge shoulder, which is likely to result from a 1 → 4 s transition indicative of Cr(III) being present and a MSR at approximately 6021 eV.

3.3 Geochemical analysis of alumino-silicate glass grains

Pseudo-total metal concentration results for glass grains in the aquatic sediment are summarized in Table 3. All of the metals investigated have very high concentrations and can be classified as grossly contaminated when compared to Environment Agency (of England) guidelines for bulk sediments. Iron exhibits the highest mean metal concentration of 119,700 μg g−1. Chromium shows the lowest mean metal concentration of 260 μg g−1 which is similar to values reported for electron microprobe analysis. Sequential extraction analysis results for the glass grains are summarized in Table 3 and are represented as percentages associated with each geochemical phase in Fig. 4. Metals are primarily associated with the residual phase, although there is considerable variation between metals in the percentage found in chemically defined form. Over 60 % of Zn is found in bioavailable forms, whereas less than 20 % of Cr is found in these fractions. In terms of bioavailable fractions, all of the metals are primarily associated with the reducible fraction. The next most common association for metals is the oxidisable fraction. Lead and Cr show only weak associations with the exchangeable phase. Zinc exhibits the greatest percentage in the exchangeable phase and is the most widely distributed metal between phases—exchangeable (12 %), reducible (29 %), oxidisable (24 %), and residual (35 %). In comparison, Cr shows the least distribution between phases—exchangeable (1 %), reducible (7 %), oxidisable (7 %), and residual (85 %).

Table 3 Mean concentrations (μg g−1) of metals in industrial glass particles from Salford Quays sediment (n = 3). Results are shown for pseudo-total metals and each geochemical phase of the sequential extraction
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Fig. 4
figure 4

Mean sequential extraction and residual Pb, Zn, Cu, Ni, Cr, Fe and Mn concentrations (n = 3) from glass grains. Results are shown as percentages associated with each phase

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4 Discussion

4.1 Speciation of chromium

Chromium is present in high concentrations in urban particulate matter in Manchester and generally well above what is considered safe by soil and sediment quality guidelines. XANES analysis of Cr speciation suggests the dominance of relatively stable and non-toxic Cr(III) in both of the major grain types under study, with some trace amounts of Cr(VI) also detected. One of the key sources of Cr-rich grains in the Salford Quays sediment is historically deposited slag. The findings of the present study that Cr exists in such material in the Cr(III) form, concurs with research carried out (Chaurand et al. 2006, 2007) on the Cr speciation of basic oxygen furnace steel slag, using XANES spectroscopy; Cr in BOF material was found to be present mainly in the octahedrally coordinated trivalent form. The dominance of Cr(III) species in RDS samples, may reflect the usage of by-products from industrial processes, in which Cr predominantly as exists in the trivalent form, in road construction (Chaurand et al. 2006). Experimental XANES spectra for the present study show strong similarities with those reported by Huggins et al. (2000) and Werner et al. (2007) for chromite and Cr–Fe spinel, respectively, in ambient air particulate matter (AAPM). Potential sources of Cr–Fe spinel to RDS include combustion by-products of Cr–Fe containing fuel, road aggregate, as well as a soil component (Werner et al. 2007). Given the significant contribution Cr0, Cr2O3, and Cr(OH)3 to the Cr speciation in urban PM2.5 (Huggins et al. 2000; Werner et al. 2007), this suggests that RDS may be a potential and key source of Cr in AAPM. The dominance of Cr(III) in both the Manchester RDS and aquatic sediment suggests there might be limited biogeochemical weathering and opportunity for speciation change as the grains are transferred through the urban sediment cascade. However, it is impossible to confirm this hypothesis without undertaking similar analyses on sediment collected from pathways linking RDS and aquatic sinks.

Electron microbeam analysis of the particulate matter has found Cr to exist predominantly in Fe-rich and Fe-poor glass grains in aquatic sediment and in both Fe oxides and Fe-rich glass grains in RDS. This supports earlier work on Manchester sediment (Taylor and Boult 2007; Taylor and Robertson 2009) and urban sediment worldwide (Kida and Sakai 2001; Lind et al. 2001; Reich 2003; Saffarzadeh et al. 2009) that suggests Fe oxides, derived from vehicular wear and tear, and Fe-rich glass grains, derived from metal smelting and concrete, are major hosts for contaminant metals in urban particulates. Iron oxides and Fe(III) oxyhydroxides are known to be important Cr scavengers and Cr(III) can readily substitute for Fe(III) in metal oxides (Frommer et al. 2009). Cr(III) can also co-precipitate with goethite (FeOOH) to form an (Fe,Cr)OOH phase, due to structural similarities between the host Fe(III) mineral and the pure Cr surface precipitate phase (CrOOH) (Charlet and Manceau 1992; Hansel et al. 2003). Cr(III) has also been found to dominate in hematite (Fe2O3)-bearing red mud from the Ajka (Hungary) tailings dam spill (Burke et al. 2012).

4.2 Control of Eh and pH on the long-term environmental mobility of chromium

Oxyanion-forming elements such as Cr have strong pH- and redox-dependent sorption behaviour. In anoxic conditions (e.g. in Salford Quays sediment), Cr(III) may be relatively stable as Cr(OH)3 or adsorbed to Fe oxides and glass grains. Phosphorus has been reported to scavenge trace metals (Zn, Cu, Pb) in the Salford Quays by incorporating them into the mineral precipitate vivianite (Fe3+(PO4)2 · 8H20) (Taylor and Boult 2007), although Cr was not considered in this previous study. Minor sulphate reduction has also been found in the Salford Quays which has been shown to be immobilizing Cu as an insoluble sulphide (Taylor and Boult 2007). However, Cr is not a divalent metal and will therefore not form an insoluble metal sulphide under reducing conditions. Chromium(III) solubility in anoxic sediments is further limited by complexation with solid-phase organic ligands which also facilitate the rapid reduction of Cr(VI) to Cr(III) (James 2002). The Salford Quays sediment is known to be rich in organic matter (up to 10 % TOC in the upper, metal-rich layers) (Taylor and Boult 2007).

Several studies have noted the oxidation of Cr(III) to Cr(VI) by Mn oxides in anoxic conditions Fendorf and Zasoski 1992; Apte et al. 2006; Tang et al. 2014; Kazakis et al. 2015). This natural oxidation is primarily related to the presence of Mn(III,IV) hydroxides which are considered as direct oxidising agents. Extensive experiments on the oxidation capacity of Cr(III) have shown that the Mn(IV) phase oxidizes the highest amount of Cr(III) (Landrot et al. 2012). Manceau and Charlet (1992) and Landrot et al. (2012), observed that Cr(III) was tightly sorbed as an inner-sphere complex to Mn(IV) in Cr(III)-reacted MnO2 analysed by EXAFS. Manganese hydroxides were not found to be a major constituent of the urban particulate matter in the Salford Quays sediment (mean 0.75 % MnO in Fe-rich glasses; mean 0.37 % MnO in RDS). However, these relatively low quantities of Mn can still make a significant difference to sorption behaviour (Jenne 1968) and the oxidation of Cr(III) to Cr(VI) by Mn oxides cannot be ruled out in urban aquatic sediment sinks.

Oxic conditions are not usual in sedimentary basins such as the Salford Quays due to limited water column mixing and high organic matter loading that maintains a high sediment oxygen demand. However, oxidation of canal, reservoir and harbour sediments can occur in response to dredging, flooding and bioturbation, leading to the phase transfer of some metals to more bioavailable species (Calmano et al. 1993; Zoumis et al. 2001). In the Salford Quays, Helixor pumps have been employed to oxygenate water in an effort to improve water quality and promote immobilization of cationic metals as hydroxides. Chromium(III) is still thermodynamically favourable even under mildly oxidizing conditions (Martello et al. 2007), but it is possible that the introduction of oxygen into the water column may promote oxidation of surface sediments leading to the oxidation of Cr(III) to Cr(VI). This could only occur, however, at Eh greater than 0.7 V (Takeno 2005). Enhanced Cr(VI) production has been reported previously in surface sediments of seasonally anoxic lakes (Achterberg and van den Berg 1997). However, in field experiments, Bloomfield and Pruden (1980) found both oxic and anoxic soil conditions increased the reduction of Cr(VI). Importantly, these experiments considered a typical soil pH of between 4 and 7, with Cr(VI) reduction being most efficient at pH <5. While Eh is an important control on Cr speciation and mobility in sediment systems, Cr(VI) can only exist in both an oxic and high pH environment.

The most favourable environment for the formation of Cr(VI) species is Eh >0.5 V and pH >8 (Takeno 2005). The typical pH range for Salford Quays sediment is 6.6–7.6, and although Eh has not been measured, predominant anoxic conditions are inferred from the pore water chemistry (Taylor and Boult 2007). Therefore, future large-scale oxidation of Cr(III) to Cr(VI) is unlikely to occur in the Salford Quays sediment. However, this may not be the case in other Cr-contaminated urban water bodies and careful investigation of the environmental processes that might drive Cr transformation is needed in order to comprehensively evaluate environmental risk. The formation of soluble Cr(III) species due to the dissolution of Cr(III) minerals and adsorbed species is the most likely mechanism by which Cr may mobilise from the sediments. The solubility of chromium hydroxide [Cr(OH)3] is low between pH 6 and 12 and under reducing and mildly oxic conditions (Takeno 2005). As long as anoxic conditions remain, Cr-bearing Fe oxides may also exist as insoluble sulphide minerals or, more likely, incorporated into the phosphate mineral vivianite. However, most Cr in the aquatic sediment was found to be associated with glass grains. Furnace-derived glasses in contaminated soils and sediments are known to be prone to chemical dissolution at sub-neutral pH due to organic acid generation as part of early sediment diagenesis Parsons et al. 2001; Lottermoser 2002). Taylor and Boult (2007) found evidence from petrographic observations of chemical dissolution occurring in Fe-rich glass grains and associated contaminant metal (Zn) release. However, Cr concentrations in pore waters were not measured so it remains unknown as to whether chemical dissolution may affect the cycling of Cr in urban aquatic sediment sinks. Evidence from the chemical extractions performed in the present study suggests Cr(III) has limited mobility and is not easily leached compared to other trace and toxic metals. However, the timeframe of exposure to low pH may be an important factor. Aqueous Cr species (e.g. Cr2+, Cr3+) can generally only exist at pH below 4 and while such low pH levels can occur in acidic soils, they are rarely encountered in sediments. Therefore, the release of organic acids during early diagenetic organic matter oxidation may not impact upon glass solubility and Cr mobility. However, it must be acknowledged that the presence of glass slag phases as major hosts of Cr is not reflected in grain-specific sequential chemical extraction methodologies (including BCR).

5 Conclusions

This study has provided important speciation and geochemical information that may have consequences for the long-term cycling of Cr in urban environments. Chromium has been found to exist primarily in two major grain types in urban particulate matter from the major urban conurbation of Manchester, UK. Chromium is mainly associated with Fe oxides and industrial glass grains in urban aquatic sediment. XANES analysis suggests this Cr exists primarily as the relatively non-toxic Cr(III) species. The predominant anoxic and neutral pH of the Salford Quays sediments suggests that this Cr(III) may be stable as long as these environmental conditions are maintained. While oxidation of the sediments may occur under a variety of scenarios, it is unlikely that sediment pH can be maintained at a high-enough level, in conjunction with oxic conditions, to mobilize Cr(III) as more soluble and toxic Cr(VI). Of course, this may not be the case in other urban aquatic sediment sinks that exhibit different environmental conditions to the Salford Quays. Aside from the issue of speciation, geochemical analyses have shown that most Cr is incorporated within the crystalline structure of industrial glass grains in the aquatic sediment and therefore exhibits limited environmental mobility in comparison to other trace metals. These glass grains may be prone to chemical dissolution at low pH; however, it is unlikely that sediment pH can reach a low-enough level to mobilize aqueous Cr species. Industrial glass grains are a major component of urban sediment worldwide suggesting most Cr within the urban sediment cascade may be resistant to environmental processes that could mobilize more bioavailable forms. Future research should, through experiments and geochemical modelling, address this potential mobility under a variety of environmental scenarios. These data would facilitate the development of environmental risk models for Cr mobility and cycling in urban environments.