Diversity patterns of benthic bacterial communities along the salinity continuum of the Humber estuary (UK)

Sediments from intertidal mudflats are fluctuating environments that support very diverse microbialcommunities. The highly variable physicochemical conditions complicate the understanding of the environmental controls on diversity patterns in estuarine systems. This study investigated bacterial diversity and community composition in surface (0-1 cm) and subsurface (5-10 cm) sediments along the salinity gradient of the Humber estuary (UK) using amplicon sequencing of the 16S rRNA gene, and it correlates variations with environmental variables. The sediment depths sampled were selected based on the local remobilisation frequency patterns. In general, bacterial communities showed similar composition at the different sites and depths, with Proteobacteria being the most abundant phylum. Richness of operationally defined taxonomic units (OTUs) was uniform along the Microbial Diversity of the Humber Estuary salinity gradient. However, Hill numbers, as bacterial diversity measures, showed that the common and dominant OTUs exhibited a decreasing trend from the inner towards the outer estuary sites. Additionally, surface and subsurface bacterial communities were separated by NMDS analysis only in the mid and outer estuary samples, where redox transitions with depth in the sediment profile were more abrupt. Salinity, porewater ammonium concentrations and reduced iron concentrations were the subset of environmental factors that best correlated with community dissimilarities. The analysis of the regional diversity indicated that the dataset may include two potentially distinct communities. These are a near surface community that is the product of regular mixing and transport which is subjected to a wide range of salinity conditions, and thus contains decreasing numbers of common and dominant OTUs seawards, and a bacterial community indigenous to the more reducing subsurface sediments of the mid and outer mudflats of the Humber estuary

Vanadium removal and recovery from bauxite residue leachates by ion exchange

Bauxite residue is an important by-product of the alumina industry, and current management practices do not allow their full valorisation, especially with regard to the recovery of critical metals. This work aims to test the efficiency of ion exchange resins for vanadium (V) removal and recovery from bauxite residue leachates at alkaline pH (11.5 and 13). As an environmental pollutant, removal of V from leachates may be an obligation of bauxite residue disposal areas (BRDA) long-term management requirements. Vanadium removal from the leachate can be coupled with the recovery, and potentially can be used to offset long-term legacy treatment costs in legacy sites. Kinetics studies were performed to understand the adsorption process. The rate kinetics for the V adsorption was consistent with the pseudo-first-order kinetic model, with a higher adsorption rate for pH 11.5 (1.2 min(-1)). Adsorption isotherm data fitted better to Freundlich equations than to the Langmuir model. The maximum adsorption capacity (Langmuir value q max) was greatest for pH 13 (9.8 mg V g(-1) resin). In column tests, breakthrough was reached at 70 bed volumes with the red mud leachate at pH 13, while no breakthrough was achieved with the effluent at pH 11.5. In regeneration, 42 and 76 % of V were eluted from the resin with 2 M NaOH from the red mud leachate at pH 13 and 11.5, respectively. Further optimization will be needed to upscale the treatment

Leaching behaviour of co-disposed steel making wastes: effects of aeration on leachate chemistry and vanadium mobilisation

Steelmaking wastes stored in landfill, such as slag and spent refractory liners, are often enriched in toxic trace metals (including V). These may become mobile in highly alkaline leachate generated during weathering. Fresh steelmaking waste was characterised using XRD, XRF, and SEM-EDX. Batch leaching tests were performed under aerated, air-excluded and acidified conditions to determine the impact of atmospheric CO2 and acid addition on leachate chemistry. Phases commonly associated with slag including dicalcium silicate, dicalcium aluminoferrite, a wüstite-like solid solution and free lime were identified, as well as a second group of phases including periclase, corundum and graphite which are representative of refractory liners. During air-excluded leaching, dissolution of free lime and dicalcium silicate results in a high pH, high Ca leachate in which the V concentration is low due to the constraint imposed by Ca3(VO4)2 solubility limits. Under aerated conditions, carbonation lowers the leachate pH and provides a sink for aqueous Ca, allowing higher concentrations of V to accumulate. Below pH 10, leachate is dominated by periclase dissolution and secondary phases including monohydrocalcite and dolomite are precipitated. Storage of waste under saturated conditions that exclude atmospheric CO2 would therefore provide the optimal environment to minimise V leaching during weathering

Behaviour of carbon-14 containing low molecular weight organic compounds in contaminated groundwater under aerobic conditions

Short chain carbon-14 (14C) containing organic compounds can be formed by abiotic oxidation of carbides and impurities within nuclear fuel cladding. During fuel reprocessing and subsequent waste storage there is potential for these organic compounds to enter shallow subsurface environments due to accidental discharges. Currently there is little data on the persistence of these compounds in such environments. Four 14C labelled compounds (acetate; formate; formaldehyde and methanol) were added to aerobic microcosm experiments that contained glacial outwash sediments and groundwater simulant representative of the Sellafield nuclear reprocessing site, UK. Two concentrations of each electron donor were used, low concentration (10-5 M) to replicate predicted concentrations from an accidental release and high concentration (10-2 M) to study the impact of the individual electron donor on the indigenous microbial community in the sediment. In the low concentration system only ~5% of initial 14C remained in solution at the end of experiments in contact with atmosphere (250-350 hours). The production of 14CO2(g) (measured after 48 hours) suggests microbially mediated breakdown is the primary removal mechanism for these organic compounds, although methanol loss may have been partially by volatilisation. Highest retention of 14C by the solid fractions was found in the acetate experiment, with 12% being associated with the inorganic fraction, suggesting modest precipitation as solid carbonate. In the high concentration systems only ~5% of intial 14C remains in solution at the end of the experiments for acetate, formate and methanol. In the formaldehyde experiment only limited loss from solution was observed (76% remained in solution). The microbial populations of unaltered sediment and those in the low concentration experiments were broadly similar, with highly diverse bacterial phyla present. Under high concentrations of the organic compounds the abundance of common operational taxonomic units was reduced by 66% and the community structure was dominated by Proteobacteria (particularly Betaproteobacteria) signifying a shift in community structure in response to the electron donor available. The results of this study suggest that many bacterial phyla that are ubiquitous in near surface soils are able to utilise a range of 14C-containing low molecular weight organic substances very rapidly, and thus such substances are unlikely to persist in aerobic shallow subsurface environments

Coprecipitation of 14C and Sr with carbonate precipitates: The importance of reaction kinetics and recrystallization pathways

This study investigated the simultaneous removal of Sr2+ and 14CO32- from an alkaline (pH >12) Ca(OH)2 solution by the precipitation of calcium carbonate. Initial Ca2+:CO32- ratios ranged from 10:1 to 10:100 (mM: mM). Maximum removal of 14C and Sr2+ both occurred in the system containing 10 mM Ca2+ and 1 mM CO32- (99.7% and 98.6% removal, respectively). A kinetic model is provided that describes 14C and Sr removal in terms of mineral dissolution & precipitation reactions. The removal of 14C was achieved during the depletion of the initial TIC in solution, and was subsequently significantly affected by recrystallization of a calcite precipitate from an elongate to isotropic morphology. This liberated >46% of the 14C back to solution. Sr2+ removal occurred as Ca2+ became depleted in solution and was not significantly affected by the recrystallization process. This reaction could form the basis for low cost remediation scheme for 90Sr and 14C in radioactively contaminated waters (<$0.25 reagent cost per m3 treated)

In situ disposal of crushed concrete waste as void fill material at UK nuclear sites: Leaching behavior and effect of pH on trace element release

The leaching behavior of stockpiled crushed concrete waste is important in determining its suitability for in situ disposal at UK nuclear sites. Sand sized particles from surface (0–0.1 m) and subsurface (2.5–2.7 m) samples were composed of silica and calcite grains in a matrix of calcium alumina-silicate hydrate (C-(A)-S-H) with Ca/Si ratios of 0.5 ±0.3 and 0.9 ±0.3 respectively. Calcite content was also higher in surface samples indicating a greater degree of weathering and carbonation. This resulted in lower leachate pH for the surface samples (pH 8–9.6) compared to subsurface samples (pH 10–11.3). The waste displayed a high acid buffering capacity but low alkaline buffering capacity. Element release as a function of pH was similar for surface and sub-surface samples and between different size fractions. Leaching of contaminant metals was close to minimum values at the pH values produced by the crushed concrete but increased by several orders of magnitude at pH 12 (for Al and Pb). Weathering and carbonation during long-term stockpiling, therefore, has a positive impact by producing a waste with stable pH and low metal leaching potential suitable for in-situ disposal as a void fill material

Reoxidation of estuarine sediments during simulated resuspension events: effects on nutrient and trace metal mobilisation

Estuarine environments are considered to be nutrient buffer systems as they regulate the delivery of nutrients from rivers to the ocean. In the Humber Estuary (UK) seawater and freshwater mixing during tidal cycles leads to the mobilisation of oxic surface sediments (0-1 cm). However, less frequent seasonal events can also mobilise anoxic subsurface (5-10 cm) sediments, which may have further implications for the estuarine geochemistry. A series of batch experiments were carried out on surface and subsurface sediments taken from along the salinity gradient of the Humber Estuary. The aim was to investigate the geochemical processes driving major element (N, Fe, S, and Mn) redox cycling and trace metal behaviour during simulated resuspension events. The magnitude of major nutrient and metal release was significantly greater during the resuspension of outer estuarine sediments rather than from inner estuarine sediments. When comparing resuspension of surface versus subsurface sediment, only the outer estuary experiments showed significant differences in major nutrient behaviour with sediment depth. In general, any ammonium, manganese and trace metals (Cu and Zn) released during the resuspension experiments were rapidly removed from solution as new sorption sites (i.e. Fe/Mn oxyhydroxides) formed. Therefore Humber estuary sediments showed a scavenging capacity for these dissolved species and hence may act as an ultimate sink for these elements. Due to the larger aerial extent of the outer estuary intertidal mudflats in comparison with the inner estuary area, the mobilisation of the outer estuary sediments (more reducing and richer in sulphides and iron) may have a greater impact on the transport and cycling of nutrients and trace metals. Climate change-associated sea level rise combined with an increasing frequency of major storm events in temperate zones, which are more likely to mobilise deeper sediment regions, will impact the nutrient and metal inputs to the coastal waters, and therefore enhance the likelihood of eutrophication in this environment

Enhanced Crystallographic incorporation of Strontium(II) ions to Calcite via Preferential Adsorption at Obtuse growth steps

Sr-containing calcium carbonates were precipitated from solutions containing Ca(OH)₂, SrCl₂ and Na₂CO₃ in a reactor where constant solution composition was maintained. The total concentration of divalent ions was same in all experiments, but the Sr/Ca ratio was varied between 0.002 and 0.86, and the pH value was between 12.02 and 12.25. All solutions were oversaturated with respect to calcite (SIcalcite = 1.2-1.5). Calcite was the only product formed at low Sr/Ca ratios, but at Sr/Ca ≥ 0.45 strontianite was detected in some systems. Sr-rich precipitate was observed in both a surface layer on (6.9-6 µm) rhombic calcite seed crystals and as smaller (> 3.64-1.96 µm) calcite crystals that were elongated along their C-axis. The degree of crystal elongation increased with the Sr/Ca ratio in those crystals. Precipitates recovered from low Sr/Ca ratio experiments exhibited an XRD spectrum identical to that of rhombic calcite, however the peaks attributed to Sr-containing calcite shifted progressively to lower 2θ values with increasing solution Sr/Ca ratio, indicating increased lattice volume. Sr K-edge EXAFS analysis of the precipitates showed that the shift in morphology and lattice volume is accompanied by a change in the local coordination of Sr²⁺ in calcite. The Sr-O bond lengths were similar to the Ca-O bond lengths in calcite, but Sr-O coordination increased from 6 fold in crystals containing 0.21 Wt. % Sr, to 8 fold in crystals containing 9.47 Wt. % Sr, and the Sr-Ca coordination decreased from 6 and 6 (for the first and second Sr-Ca shells respectively) to 4 and 1. It is suggested that Sr²⁺ undergoes preferential incorporation at obtuse (+) growth sites on the calcite surface due to its large ionic radius (1.13 Å), and this increases the growth rate parallel to the C-axis, resulting in the observed elongation in this direction