Trace metal nanoparticles in pyrite

Hydrothermal pyrite contains significant amounts of minor and trace elements including As, Pb, Sb, Bi, Cu, Co, Ni, Zn, Au, Ag, Se and Te, which can be incorporated into nanoparticles (NPs). NP-bearing pyrite is most common in hydrothermal ore deposits that contain a wide range of trace elements, especially deposits that formed at low temperatures. In this study, we have characterized the chemical composition and structure of these NPs and their host pyrite with high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), analytical electron microscopy (AEM), and electron microprobe analysis (EMPA). Pyrite containing the NPs comes from two types of common low-temperature deposits, Carlin-type (Lone Tree, Screamer, Deep Star (Nevada, USA)), and epithermal (Pueblo Viejo (Dominican Republic) and Porgera (Papua New-Guinea)).EMPA analyses of the pyrite show maximum concentrations of As (11.2), Ni (3.04), Cu (2.99), Sb (2.24), Pb (0.99), Co (0.58), Se (0.2), Au (0.19), Hg (0.19), Ag (0.16), Zn (0.04), and Te (0.04) (in wt.%). Three types of pyrite have been investigated: “pure” or “barren” pyrite, Cu-rich pyrite and As-rich pyrite. Arsenic in pyrite from Carlin-type deposits and the Porgera epithermal deposit is negatively correlated with S, whereas some (colloform) pyrite from Pueblo Viejo shows a negative correlation between As + Cu and Fe. HRTEM observations and SAED patterns confirm that almost all NPs are crystalline and that their size varies from 5 to 100 nm (except for NPs of galena, which have diameters of up to 500 nm). NPs can be divided into three groups on the basis of their chemical composition: (i) native metals: Au, Ag, Ag–Au (electrum); (ii) sulfides and sulfosalts: PbS (galena), HgS (cinnabar), Pb–Sb–S, Ag–Pb–S, Pb–Ag–Sb–S, Pb–Sb–Bi–Ag–Te–S, Pb–Te–Sb–Au–Ag–Bi–S, Cu–Fe–S NPs, and Au–Ag–As–Ni–S; and (iii) Fe-bearing NPs: Fe–As–Ag–Ni–S, Fe–As–Sb–Pb–Ni–Au–S, all of which are in a matrix of distorted and polycrystalline pyrite. TEM-EDX spectra collected from the NPs and pyrite matrix document preferential partitioning of trace metals including Pb, Bi, Sb, Au, Ag, Ni, Te, and As into the NPs. The NPs formed due to exsolution from the pyrite matrix, most commonly for NPs less than 10 nm in size, and direct precipitation from the hydrothermal fluid and deposition into the growing pyrite, most commonly for those > 20 nm in size. NPs containing numerous heavy metals are likely to be found in pyrite and/or other sulfides in various hydrothermal, diagenetic and groundwater systems dominated by reducing conditions.     

Trace metal dynamics in a seasonally anoxic lake

Selected results are presented from a detailed 12-month study of trace metals in a seasonally anoxic lake. Dissolved concentrations of Fe, Mn, organic carbon, Cd, Cu, Pb, Zn, and pH were determined in the water column and the interstitial waters on 39 occasions. Trace metal concentrations remained low throughout the year in both water column and pore waters. There was evidence for some remobilization at the sediment-water interface but sediments deeper than 3 cm acted as a sink throughout the year. Variations in the water concentrations were largely associated with increased loading during periods of heavy rainfall. During the summer, concentrations of Cu and Zn in the waters overlying the sediments were enhanced by release from decomposing algal material. Similarly, enhanced concentrations of Cd, Cu, Pb, and Zn were observed during periods of much reduced mixing during ice-cover. Although there were large seasonal variations in the concentrations of dissolved and particulate Fe and Mn, there were no comparable changes in the concentrations of trace metals.     

Trace metal enrichments in Lake Tanganyika sediments: Controls on trace metal burial in lacustrine systems

We investigate the distributions of several key diagenetic reactants (C, S, Fe) and redox-sensitive trace metals (Mo, Cd, Re, U) in sediments from Lake Tanganyika, East Africa. This study includes modern sediments from a chemocline transect, which spans oxygenated shallow waters to sulfidic conditions at depth, as well as ancient sediments from a longer core (∼2 m) taken at ∼900 m water depth. Modern sediments from depths spanning ∼70-335 m are generally characterized by increasing enrichments of C, S, Mo, Cd, and U with increasing water depth but static Fe distributions. It appears that the sedimentary enrichments of these elements are, to varying degrees, influenced by a combination of organic carbon cycling and sulfur cycling. These modern lake characteristics contrast with a period of high total organic carbon (C_org), total sulfur (S_Tot), and trace metal concentrations observed in the 900 m core, a period which follows the most recent deglaciation (∼18-11 ky). This interval is followed abruptly by an interval (∼11-6 ky) that is characterized by lower C, S, U, and Mo. Consistent with other work we suspect that the low concentrations of S, Mo, and U may indicate a period of intense lake mixing, during which time the lake may have been less productive and less reducing as compared to the present. An alternative, but not mutually exclusive, hypothesis is that changes in the lake’s chemical inventory, driven by significant hydrological changes, could be influencing the distribution of sedimentary trace elements through time.     

Trace metal concentrations in lake and overbank sediments in southern Norway

 As, Be, Cd, Co, Cr, Cu, Hg, Ni, Pb, V, Se and Zn concentrations were determined and compared in lake and overbank sediments from 33 catchments without local pollution sources in southern Norway. There were no significant differences in concentrations of Be, Co, Cr, Cu, Ni, and V in overbank and pre-industrial lake sediments. In areas with shallow overburden, and significant influence from long-range atmospheric pollution, concentrations of As, Cd, Hg, Pb, Se, and Zn in overbank sediments were probably modified by vertical percolating water. In such areas, we suggest using lake sediments as a better sampling medium for mapping pre-industrial concentrations. Pre-industrial lake sediments yield natural concentrations of Hg and Se, which consist of both geogenic and natural atmospheric deposition. Important covariables like organic carbon content, Fe oxides, and fine mineral fraction were generally higher in pre-industrial lake sediments as compared to overbank sediments. By adjusting for such differences overbank sediments could be used as an alternative in mapping background concentrations of trace metals in regions with few lakes. Received: 19 February 1999 · Accepted: 17 April 1999     

Trace metal profiles in the varved sediment of an Arctic lake

Varved (annually-laminated) sediments offer a rare and physically undisturbed archive of past trace metal deposition and limnological conditions. Here, a high-resolution 1,300 year record of metal accumulation is presented from a varved lake sediment on Devon Island in the Canadian High Arctic. Down-core concentration profiles of Cd, Cu and Zn were positively correlated (P < 0.01) with organic C (Cd, Zn) or with leachable Fe (Cu), while distinct sub-surface peaks of these metals coincided with those of Fe, S and other redox-sensitive elements such as Co, Cr and U. The fluxes of these metals since 1854 were correlated with elements such as Ca, Al and La (P < 0.001) which are predominantly of local geological origin. Furthermore, the Cd, Cu and Zn patterns did not match concurrent records in Greenland Summit ice over the last century, nor global industrial emission histories. These facts suggest that inputs from local geological sources, coupled with some degree of post-depositional mobility or association with organic matter inputs, explain the metals’ sedimentary profiles, which were apparently not affected by long-range atmospheric metal pollution. Mercury concentrations were strongly correlated with total diatom abundance over the last 400 yrs, especially during the 20th Century when a two-fold increase in Hg concentrations and a four order-of-magnitude increase in diatoms occurred in tandem. Since 1854, 81% of the variation in Hg flux was associated with diatom and Ca fluxes. A similar correspondence between Hg and diatoms was found in a second lake nearby, confirming that the relationship was not unique to the main study lake. Recent Hg increases in Arctic and sub-Arctic lakes have been attributed to global anthropogenic Hg emissions. We propose an alternative hypothesis for High Arctic lakes: the recent Hg increases may be partly or entirely the product of elevated rates of Hg scavenging from the water column caused by markedly greater algal productivity, which in turn was driven by accelerating climate warming during the 20th Century. Given the important environmental assessment and policy implications if the alternative hypothesis is true, the possible effects of climate warming on sedimentary Hg fluxes in this region deserve further study.     

胶州湾底层水痕量元素组分存在形态及其生物有效性

利用同站位多介质同采的资料分析研究了各环境介质中的不同痕量金属对生物富集的贡献。提出底层水中Cu、Hg、As对生物体中同名组分富集的贡献较大。采用PHREEQC软件模拟了底层水中Cu、Hg、As三种元素的组分存在形态,结果表明:Cu(OH2)、HgCl3-、HAsO42-分别为底层水中Cu、Hg、As的优势态,但其生物有效形态依次为Cu2 、HgCl42-、HAsO42-,提出生物对其所处环境介质中痕量金属的富集并不取决于痕量金属的总量或其相应的优势态浓度的多寡,而是取决于痕量金属的生物有效态的浓度大小。     

Trace metal geochemistry of peat under a sanitary landfill-a reconnaissance

Core samples were taken at two sites from a peat deposit buried by a sanitary landfill operated by the city of Vancouver since the 1960s and from a third site where the same peat bed is not covered by landfill. Twenty-nine subsamples from the three cores were analyzed by a variety of techniques to determine the concentration of as many as 34 constituents. The content of heavy metals, the principal object of this investigation, is highest in the lower part of the peat succession, in which there is a significant amount of interbedded inorganic sediment, rather than in the upper clean bog peat. Individual layers as little as 2.5 cm thick can hold concentrations of heavy metals ten times that of the nearby layers. The heavy metal contents show a high positive correlation with those of iron and manganese and a very low correlation with sulfur. Iron from the landfill has been transported by downward percolating groundwater in solution or colloidal suspension into the lower layers of peat deposit where the passing heavy metals were sorbed. A comparison of the amounts of heavy metals stored in the peat alone with the amount leaving the whole landfill annually suggests that some metals, notably lead and arsenic, might be retained in the peat for very long periods, whereas other metals such as zinc and mercury might be quickly lost.     

Trace metal occurrence in a mineralised and a non-mineralised spodosol in northern Sweden

In order to determine the metal-bearing phases with special emphasis on Cu, a sequential extraction has been carried out on seven soil samples from a sulphide-bearing spodosol profile in Liikavaara Östra, close to the Aitik Copper Mine in northern Sweden. A reference spodosol profile with very low abundances of sulphides located far from anthropogenic emissions was also studied. Five fractions were selected for the extraction: (I) CH₃COONa-extractable (exchangeable/adsorbed/carbonate); (II) Na₄P₂O₇-extractable (labile organics); (III) 0.25 M NH₂OH·HCl-extractable (amorphous Fe oxyhydroxides/Mn oxides); (IV) 1 M NH₂OH·HCl-extractable (crystalline Fe oxides); and (V) KClO₃/HCl-extractable (organics and sulphides). The distribution of trace elements (Co, Cr, Cu, Ni, Pb and Zn) in the profile in Liikavaara Östra is different from that in the reference profile. Possible explanations for these differences are (i) the presence of sulphides in the soil, (ii) atmospheric deposition of dust derived from mining activities at the Aitik Copper Mine, and (iii) mineralogical heterogeneities inherited from the deposition of the till. There is no straightforward correlation between the amount of the extracted phases and the metal extractability in the soils. This fact indicates that other factors are important for the retention of trace metals as well. The data presented in this study suggest that Co, Cr and Ni, to a fairly large extent, are associated with the organic matter in the B-horizon in both profiles, while in the C-horizon in Liikavaara Östra, sulphides are probably the more important carriers of these elements. For Co and Ni, Fe oxyhydroxides seem to be important. Most of the Cr occurred in the residual remaining after the leaching procedure. Copper and Zn seem to be associated with the organic matter to some extent in the B-horizon. The concentration of Cu in the C-horizon in Liikavaara Östra is high (2310 ppm), but only a very small fraction is likely to be hosted by sulphides. It is concluded that the major part of Cu in the C-horizon and a prominent fraction in the B-horizon in Liikavaara Östra are associated with some secondary phase that is extractable during extractions III and IV. Possible candidates for this phase are goethite and inclusions of native Cu in weathered biotite.     

Some metal concentrations in the edible parts of Tridacna maxima, Red Sea, Egypt

The concentrations of Ca, some essential (Fe, Zn, Mn, and Cu) and non-essential nutritive elements (Cd, Pb, and Ni) were measured in the edible parts (mantle and adductor muscles) of Tridacna maximx collected from south Quseir City (Red Sea). The general trend of metal contents of the different parts follows the order; Ca > Fe > Zn > Pb > Mn > Cu > Cd > Ni. The tissues before cooking recorded the highest average concentrations of Cu, Pb, Fe, Cd, Mn, and Ni (2.658, 5.250, 34.375, 1.464, 3.207, and 0.886 ppm, respectively) relative to tissues after cooking and the water of cooked tissues (WCT). The total cooked tissues recorded higher average contents of Zn and Ca (17.282 and 1,114.679 ppm) than the uncooked tissues. Calcium recorded the highest concentration in the ECT of adductor and mantle muscles (2,081.126 ± 177.39 and 1,893.326 ± 394.28 ppm). Mantle recorded higher concentrations of Pb, Mn, Ni, and Ca (7.489 ± 4.65, 4.241 ± 1.13, 0.980 ± 0.60, and 1,039.362 ± 177.42 ppm, respectively) than adductor muscle before cooking. Ca concentration levels in the WCT increased after cooking tissues especially in adductor muscles. This may attributed to the liberation of larger amount of calcium in ionic form in water. The clams may have intracellular regulatory mechanisms to keep their concentrations in equilibrium, subsequently; the recorded metal concentrations are in the safe limits for human consuming, where these concentrations did not exceed the clam’s capacity of regulation.     

Trace metal behaviour in riverine sediments: Role of organic matter and sulfides

Three sediment cores were collected in the Scheldt, Lys and Spiere canals, which drain a highly populated and industrialized area in Western Europe. The speciation and the distribution of trace metals in pore waters and sediment particles were assessed through a combination of computational and experimental techniques. The concentrations of dissolved major and trace elements (anions, cations, sulfides, dissolved organic C, Cd, Co, Fe, Mn, Ni, Pb and Zn) were used to calculate the thermodynamic equilibrium speciation in pore waters and to evaluate the saturation of minerals (Visual Minteq software). A sequential extraction procedure was applied on anoxic sediment particles in order to assess the main host phases of trace elements. Manganese was the most labile metal in pore waters and was mainly associated with carbonates in particles. In contrast, a weak affinity of Cd, Co, Ni, Pb and Zn with carbonates was established because: (1) a systematic under-saturation was noticed in pore waters and (2) less than 10% of these elements were extracted in the exchangeable and carbonate sedimentary fraction. In the studied anoxic sediments, the mobility and the lability of trace metals, apart from Mn, seemed to be controlled through the competition between sulfidic and organic ligands. In particular, the necessity of taking into account organic matter in the modelling of thermodynamic equilibrium was demonstrated for Cd, Ni, Zn and Pb, the latter element exhibiting the strongest affinity with humic substances. Consequently, dissolved organic matter could favour the stabilization of trace metals in the liquid phase. Conversely, sulfide minerals played a key role in the scavenging of trace metals in sediment particles. Finally, similar trace metal lability rankings were obtained for the liquid and solid phases.     

Trace metal adsorption potential of phases comprising black coatings on stream pebbles

A sequential extraction of Cu and Zn was completed on 8 samples of black coatings on coarse stream alluvium before and after immersion in prepared metal-rich solutions. Such immersion resulted in a substantial change in the partitioning of the metals among the various phases present.Most of the samples were collected from small streams in areas of the Valley and Ridge, Blue Ridge, and Piedmont provinces of north-central Virginia without known mineral deposits, and partitioning relationships were determined by a five-step selective extraction procedure. The following order of partitioning was determined in the samples:     

Trace elements levels in sediments of the Czech part of the Elbe river

Large segments of the world's people have elected to live where water meets land, or very near to those places. Water has a multitude of attractions to people, such as transport, power, recreation, and industrial and domestic supply. Most importantly it is essential to life. Our water supply must be fresh (non-salt), safe (free of contaminants), and be available in an adequate supply. This lesson is based on the scientific article by Philip P. Micklin entitled Man and the Water Cycle: Challenges for the 21st Century in this issue of GeoJournal. These challenges include the provision of a safe and reliable water supply throughout the world. The global issue of safe water, and access to that water is the main focus of this activity.     

Trace metal bioaccumulation in the shells of mussels and clams at deep-sea hydrothermal vent fields

The bioaccumulation of trace metals in the carbonate shells of mussel and clams was investigated at seven hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, Rainbow, and Broken Spur) and the Eastern Pacific (9°N and 21°N at the East Pacific Rise and the southern trough of Guaymas Basin). Mineralogical analysis showed that the carbonate skeletons of the mytilid mussel Bathymodiolus sp. and the vesicomyid clam Calyptogena m. are composed mainly of calcite and aragonite, respectively. The first data were obtained for the content of a variety of elements in the bivalve carbonate shells from various hydrothermal vent sites. The analysis of the chemical compositions (including Fe, Mn, Zn, Cu, Cd, Pb, Ag, Ni, Cr, Co, As, Se, Sb, and Hg) of 35 shell samples and 14 water samples from the mollusk biotopes revealed the influences of environmental conditions and some biological parameters on the bioaccumulation of metals. Bivalve shells from hydrothermal fields with black smokers are enriched in Fe and Mn by a factor of 20–30 relative to the same species from the Menez Gwen low-temperature vent site. It was shown that the essential elements Fe, Mn, Ni, and Cu were more actively accumulated during the early ontogeny of the shells. The high concentration factors of most metals (n × 10²−n × 10⁴) indicate an efficient accumulation function of bivalve carbonate shells. Passive metal accumulation owing to adsorption on the shell surface was estimated to be no higher than 50% of the total amount, varying from 14% for Fe to 46% for Mn.     

Trace metal concentrations in euryhaline fish species from Chilika lagoon: human health risk assessment

Five trace metals, viz. copper (Cu), chromium (Cr), iron (Fe), mercury (Hg) and zinc (Zn), in three organs (muscle, gills and liver) of four selected fish species (Mugil cephalus, Eleutheronema tetradactylum, Etroplus suratensis and Daysciaena albida) from Chilika lagoon were studied monthly to assess the concentration level and human health risk via consumption. The average and range of metal concentrations (μg g^?1) in flesh of the fish species were found to be: Cu (4.15, 3.47–5.03), Cr (0.25, 0.04–0.63), Fe (19.72, 14.51–27.30), Hg (0.07, 0.05–0.08) and Zn (8.09, 4.41–11.36). The metal concentrations measured in the edible muscles of all fish species were found lower as compared to the contents in liver and gill. Analysis of variance revealed a significant difference in trace metal accumulation among the organs (p < 0.001), but the difference among the seasons and fish species was insignificant (p > 0.05), with the exception of Zn accumulation (p < 0.05). The estimated daily intake (EDI) of the trace metals through the consumption of fish as a dietary component varied widely from 0.48 to 21.33% with respect to the corresponding permissible tolerable daily intake (PTDI). The target hazard quotients (THQs) calculated considering these EDIs for individual metals indicated that the Cr and Cu metals dominantly contributed (avg. THQ = 0.198) to represent the hazard index (HI) than other metals (avg. THQ = 0.029). The average HI determined for all the species was <1 (0.484, ranged 0.255–0.605), indicating that these fish species of Chilika are safe for human consumption.     

Trace metal deposition and mobility in the sediments of two lakes near Sudbury,Ontario

The accumulation and mobility of Fe, Mn, Al, Cu, Ni and Pb in the sediments of two lakes (Clearwater, pH 4.5; and McFarlane, pH 7.5) near Sudbury, Ontario have been investigated. The Al, Cu and Ni concentrations are expectedly relatively high in the overlying waters of Clearwater Lake and much lower for Al and Cu in McFarlane Lake. The low trace metal concentrations found in the anoxic porewaters of Clearwater Lake could be explained by a sharp increase in porewater pH concomitant with SO₄² reduction and H₂S production within the first 1–2 cm of the sediments, which has conceivably led to the precipitation of mineral phases such as AL(OH)₃, NiS, and CuS. In both lakes, Fe concentrations in anoxic porewaters appear to be controlled by FeS and/or FeCO₃ formation. Solubility calculations also indicate MnCO₃ precipitation in McFarlane Lake. In Clearwater Lake, however, both porewater and total Mn were relatively low, a possible result of the continuous loss of Mn(II) through the acidic interface. It is suggested that upwardly decreasing total Mn profiles resulting from the removal of Mn from the top sediment layers under acidic conditions may constitute a reliable symptom of recent lake acidification.The downward diffusion of AI, Cu and Ni from the overlying water to the sediments has been estimated from their concentration gradients at the interface and compared to their total accumulation rates in the sediments. In both lakes the diffusion of Al is negligible compared to its accumulation rate. However, diffusion accounts for 24–52% of the accumulation of Cu in the sediments of Clearwater Lake, but appears negligible in McFarlane Lake. The downward diffusive flux of Ni is important and may explain 76–161% of the estimated Ni accumulation rate in Clearwater Lake, and 59% in McFarlane Lake. The porewater Cu and Ni profiles suggest that the subsurface sedimentary trace metal peaks observed in Clearwater Lake (as in other acid lakes) may not be caused by sediment leaching or by a recent reduction in sedimentation but may have a diagenetic origin instead. Diffusion to the sediments thus appears to be an important and previously overlooked trace metal deposition mechanism, particularly in acid lakes.     

Trace metal distribution in sediments of northern continental shelf of Crete Island,Eastern Mediterranean

The present study investigates the distribution of trace metals (Zn, Hg, Cd, Cu, and Pb), as indicators of pollution, in the surficial offshore shelf sediments along the northern coast of Heraklion Prefecture (Crete, Mediterranean Sea). The concentrations and the spatial distribution of the different trace metals, in relation to the sedimentological characteristics and the water circulation pattern of the entire continental shelf, are associated with human inshore sources of pollutants located along the coastline of the study area. Although the trace metal concentrations measured are higher than the background values, they are not considered to be dangerous to human health, as they are lower than the standard values given by the World Health Organisation, with only a few localised exceptions. Furthermore, results reveal the important role of local hydrodynamism that moves fine-grained material and associated trace metals offshore (seawards to wave breaking zone) and then transports them eastwards by entrapping them in the prevailing offshore shelf-water circulation.     

Marine scavenging: Trace metal adsorption by interfacial sediment from MANOP Site H

The adsorption of thirteen trace metals from seawater was studied on interfacial sediment from MANOP site H. The adsorption data indicate a long (~20 day) equilibration time for most metals, an increase in adsorption with an increase in pH and particle concentration, a dependence of adsorption on total metal concentration at high adsorption densities, and a lack of correlation between metal binding ability and metal hydrolysis in solution.Apparent equilibrium binding constants normalized to the total number of available sites on the solid were determined for metal binding with the interfacial sediment. The binding constants indicate that the affinity sequence for metal interactions with the interfacial sediment is: Pb > Fe > Sn ? Co≈ Mn > Cu > Be > Sc ? Zn > Ni > Cd ? Ba > Cs at pH 7.82 in seawater.A comparison of the binding constants for suspended particles, interfacial sediment, and surface sediment indicate that the composition of particles influences the binding ability of the particles. Biogenic particles tend to bind most metals more strongly than lithogenic or authigenic particles.Based on limited data, there is a strong positive correlation between the measured binding constants and the observed partitioning of metals between sediment and seawater.     

Trace metal remobilization following the resuspension of estuarine sediments: Saguenay Fjord,Canada

The Saguenay Fjord sediments are characterised by high trace metal, organic matter and Fe monosulfide (AVS) concentrations. The presence of important maritime activities, dredging operations around harbour installations, and natural slumpling events contribute to the resuspension of the Fjord sediments, their exposure to an oxidising environment, and the remobilization of a variety of contaminants to the water column.In order to simulate these processes, sediments collected from various depths in a box core recovered on the landward slope of the fjord's interior basin were resuspended in aerated seawater in the laboratory at room temperature. The slurries were sampled at various time intervals over a period of nearly 2000 h and the dissolved Fe, Mn and As concentrations were determined. Results of this study indicate that the amount of Fe and As released to the solution is strongly correlated to the acid volatile sulfide (AVS) content of the resuspended sediments. The release of Mn to the water phase, however, appeared to be mainly controlled by the dissolution of a solid phase, other than the Fe monosulfides but soluble in 1 N HCl, to which Mn (II) is associated. This phase could be a Mn carbonate or a mixed Mn–Ca carbonate.The dissolved metal concentrations measured at any time during the resuspension experiments reflect the reactivity of the carrier phases and the competing kinetics of the release and removal mechanisms. Upon their release to solution, Fe, Mn and As were scavenged by the newly precipitated Fe and Mn oxyhydroxides. The differential rate of formation of these phases and removal of associated metals in solution reflects their respective oxidation kinetics. The residence time of As in solution was very similar to that of Mn, implying that it was more efficiently removed with the slowly precipitated Mn oxyhydroxides. This observation is consistent with the faster oxidation kinetics of As (III) to As (V) in the presence of Mn oxyhydroxides. Nevertheless, results of As speciation analyses also suggest that a portion of the solubilized As was adsorbed and possibly oxidised by the rapidly precipitated Fe oxyhydroxides.