Development of vegetative cover on the surface of polluted waste deposits provides a physical protective layer and an input of organic compounds via root exudation. The monocot yorkshire fog (Holcus lanatus) and eudicot red clover (Trifolium pratense) are plant species that have been observed to grow successfully in alkaline waste, managed with phytostabilization. In this work the effects of pH-stress conditions (pH similar to 10) on root exudation of both these species is studied in a hydroponic set up. Both species, when exposed to stress conditions, were able to neutralize the pH in the collection solution, with a reduction of similar to 2 pH units. For both yorkshire fog and red clover, the net exudation had no statistical difference between non-stressed and pH-stressed conditions (263 vs 220 mu g DOC (g-root)(-1)(hrs)(-1) for yorkshire fog and 158 vs 118 mu g DOC (g-root)(-1)(hrs)(-1) for red clover). GC-MS analysis of the exudate solutions, however, shows a shift from a sugars and sugar alcohols-dominated exudate solution in non-stressed conditions, to an exudate solution with upregulated organic acids in pH stressed yorkshire fog and organic acids plus amino acids in red clover. These results show a similar stress response for these two species, contrary to the general assumption that grass species are less efficient than eudicots in producing organic acids in response to stress. The prevalence of organic acids in exudates under stress conditions, could favour metal and nutrient mobilization in growth substrates, though the lack of an increase in overall exudation limits the extent of their potential impact.

Large volumes of concrete wastes are produced during building demolition that potentially can be reused as a secondary material. Reuse of crushed cementitious construction wastes as a fill material can result in leaching of reactive cement phases. The short-term dissolution kinetics of crushed concrete waste (CCW) from a UK nuclear site were studied in leaching tests using deionised water, acidic and alkaline leachants. The pseudo-steady state leaching rates for major and trace element constituents were determined as a function of leachate pH. At alkaline pH (>10), Ca and Si leaching rates were similar (2-6.5 x 10-11 mol m-2 s-1) producing leachates with Ca/Si ratios (1.1 ± 0.4) suggesting congruent leaching of the calcium silicate hydrate phases in the CCW (pre-leached Ca/Si = 0.9 ± 0.3). Below pH 10, the Ca/Si ratio in the leachate increased with decreasing pH because Ca leaching rates increased without much variation in the Si leaching rate. In this pH range leaching processes were dominated by initial rapid calcium carbonate dissolution and incongruent dissolution of calcium silicate hydrate phases, which produced Ca-depleted solids with overall higher rates of mass loss than in alkali-leached solids. The leaching rates of Mg, Fe and Mn were pH-dependent (with higher leaching rates at low pH) and solubility-controlled whilst the leaching rates of Cr, V, Pb, As, K and Zn were mostly pH independent. Trace element leaching rates were generally low relative to Ca or below detection between pH 3-13. This information can be used to help predict CCW leaching behaviour when disposed in scenarios where meteoric or groundwater flow through the waste is expected.

Environmental contamination by Cr(VI) leaching from chromite ore processing residue (COPR) legacy disposal sites can pose a threat to human health. Under iron-reducing conditions, microbial activity can convert mobile and toxic Cr(VI) to less mobile and less toxic Cr(III); however, COPR waste is a very hostile environment for microbial life. Microcosms using soil from beneath a COPR disposal site were challenged with Cr(VI) with and without acetate to stimulate microbial metabolism. Geochemistry showed that when the microbial populations were reducing iron, Cr(VI) was also reduced, and 16S rRNA gene sequencing showed that the community composition evolved over the course of the experiment. Meta-transcriptome data revealed ~3% of transcripts were differentially regulated (p = 0.01) between the acetate amended and unamended systems, with twice as many transcripts downregulated by acetate. Gene ontology (GO) terms for processes involving the cell wall, cell periphery, plasma membrane and encapsulating structures as well as catabolic processes, especially carbohydrate metabolism, were significantly enriched in the unamended microcosm meta-transcriptome. Transcripts for alternative sigma (σ) factors and anti-σ factors were prominent among the differentially regulated genes. The study provides insight into how the provision of acetate shapes metabolic processes and life history strategies in an alkaline Cr(VI) impacted environment.

Historical disposal of coal mine wastes in the coastal zone has left a significant environmental pollution legacy. Climate change is increasing the likelihood that erosion of these wastes will lead to release of metal(loid)s to coastal environments. Whilst previous research has focussed on the generation of acidic, metal-rich waters from coal mine wastes in freshwater environments, a comprehensive investigation of metal(loid) leaching from such wastes in the coastal zone has not been undertaken. This study investigated the leaching behaviour of coal mine wastes under freshwater and saline conditions and determined the impacts of spatial heterogeneity of waste composition on such behaviour. The degree of leaching varied considerably within and between sites due to the heterogenous nature of the waste. Leachate pH varied from 1.80 to 6.99 with acidic leachates particularly enriched in Fe (<= 17,000 mg/kg dry waste) and sulfate (<= 48,000 mg/kg dry waste) due to dissolution of acid sulfate phases. Dissolution of Fe and Mn oxides, hydroxides and oxyhydroxides also led to release of surface adsorbed metal(loid)s such as As (<= 21 mg/kg dry waste), Zn (<= 86 mg/kg dry waste) and Cu (<= 14 mg/kg dry waste). Adsorption of As to high surface area minerals was confirmed by X-ray Absorption Near Edge Spectroscopy (XANES) analysis. Metal(loid) release was typically lower in the presence of seawater than deionised water due to the greater pH buffering capacity of seawater. This research provides an insight into the considerable challenges faced by coastal managers globally as they seek to mitigate the risks from such legacy pollution.

The erosion of legacy coastal municipal solid waste landfill sites will result in the dispersion of particulate material into nearby ecosystems with potential for effects on marine populations. Information on the speciation and solid phase associations of metal(loid) contaminants will help to predict contaminant behaviour and better understand ecosystem risks. Here, we investigate the solid phase composition of, and metal(loid) leaching from, fine fraction materials recovered from three actively eroding coastal landfill sites. High concentrations of a range of potentially toxic elements (As, Cd, Cr, Cu, Pb, Ni and Zn) were present in multiple samples, but metal(loid) leaching rates were very low (≪1 wt%) in both deionised water and seawater solutions. Therefore, particulate dispersion is the most likely mode of contaminant transport occurring at these sites. The fine fraction materials were dominated by fine sand sized (63-180 μm) quartz grains and silt sized (<63 μm) matrix components, which were likely to be poorly retained on beaches and easily transported offshore. Four priority contaminants (As, Cu, Pb and Zn) were found to occur primarily in adsorbed or precipitate forms, as either coatings on other particles or as discrete <10 μm particles. Dilution of these fine-grained contaminated particles within natural pelitic sediments will likely reduce the overall ecosystems impacts; but the risks to filter and bottom feeding organisms, and the potential for biomagnification across trophic levels are poorly understood.

Risk management for agricultural use of digested sewage sludge requires better understanding of the behaviour and fate of contaminant metals in the plant root zone. A study employing rhizo-pot and plug-tray experiments was conducted to identify the zone near spring barley roots (Hordeum vulgare) where concentration and speciation of Cu and Zn are affected. Cu and Zn bonding environments in the root epidermis/cortex and vascular tissue were also identified. In the digested sludge-amended soil, spring barley absorbed Cu only from the immediate vicinity of the roots (<< 1 mm), but Zn was taken up from further afield (> 1 mm). In the rhizosphere Cu was predominately present as Cu(I) oxides or as Cu(II) absorbed/bonded to phosphate, whereas Zn was present as Zn(II) in inner-sphere complexes with metal oxide surfaces, as Zn(II) sulphides or Zn(II) bonded to/incorporated into carbonates. Cu taken-up by spring barley roots was largely sequestered in the root epidermis and/or cortex predominately in the coordination environments similar to those seen in the rhizosphere. Only a small proportion of the Cu was translocated into the vascular tissue (where it is in the same two bonding environments). Zn taken-up by spring barley roots was present as Zn(II) sulphides, Zn(II) absorbed to/incorporated into carbonates, or Zn(II)-organic complexes. Zn was readily translocated from roots to shoots. Better understanding of these differences in the mobility and uptake of Cu and Zn in sludge-amended agricultural soils could be used to undertake element specific risk assessments.

Climate change is expected to significantly affect the release of toxic elements from legacy waste landfills across coastal areas because it will enhance severe flooding, erosion, and extreme wetting-drying cycles. The UK’s industrial past resulted in a wide range of legacy wastes deposited in estuarine and coastal settings, for a total of 2550 sites. In this perspective, a NERC project is ongoing to provide a national-scale assessment of geochemistry and mobility of these wastes, which is necessary to establish environmental and ecological risks and to prioritize sites for management interventions. In the project framework, a mesocosm experiment was carried out to investigate the leaching processes that can occur in different types of legacy wastes (LW) disposed across the UK coasts. The experiment was designed to study how wetting and drying cycles affects the release of pollutants in the coastal areas. Four sites were selected, each one for a different type of LW: colliery (L), landfill municipal wastes (CB), metal smelter (CP), and mining-impacted sediments (HE). These four sites represent the range of LW geochemistry encountered across the UK. Each wetting/drying cycle (WDC) lasted 2 weeks. Artificial rainwater was used to inundate the columns. Water samples were collected weekly for Dissolved Organic Carbon (DOC), major ions and metals. EC, Eh, DO, and DOC showed a great variability both at inter- and intra-sites: the L samples were the most acid ones (pH = 3.3 – 5.1), whereas the other sites showed pH values ranging from 6.0 to 8.7. HE samples showed the largest variation ranges for EC and DOC (409 – 109000 µS/cm and 8.3 – 405.8 mg/l, respectively), whereas the CP ones showed the narrowest ranges for both parameters (176 – 592 µS/cm and 0.2 – 248.1 mg/l, respectively). The maximum variation for EC and pH occurred after the first WDC for all the sites but CP, which showed a progressive decrease in both parameters throughout the experiment. DO, Eh, and DOC varied more irregularly for all sites. For each site, Control samples showed wider ranges of variation than those that experienced the WDC but the trends were similar for both control and non-control ones. These preliminary data, together with trace metal data, suggest that leaching processes are more likely to be efficient at the end of prolonged dry periods when heavy storms occur. Extreme climate conditions are expected to become more frequent in the near future, therefore a proper management is mandatory to reduce the risk posed by the coastal legacy wastes.

Iron and steel slags have a long history of both disposal and beneficial use in the coastal zone. Despite the large volumes of slag deposited, comprehensive assessments of potential risks associated with metal(loid) leaching from iron and steel by-products are rare for coastal systems. This study provides a national-scale overview of the 14 known slag deposits in the coastal environment of Great Britain (those within 100 m of the mean high-water mark), comprising geochemical characterisation and leaching test data (using both low and high ionic strength waters) to assess potential leaching risks. The seaward facing length of slag deposits totalled at least 76 km, and are predominantly composed of blast furnace (iron-making) slags from the early to mid-20th Century. Some of these form tidal barriers and formal coastal defence structures, but larger deposits are associated with historical coastal disposal in many former areas of iron and steel production, notably the Cumbrian coast of England. Slag deposits are dominated by melilite phases (e.g. gehlenite), with evidence of secondary mineral formation (e.g. gypsum, calcite) indicative of weathering. Leaching tests typically show lower element (e.g. Ba, V, Cr, Fe) release under seawater leaching scenarios compared to deionised water, largely ascribable to the pH buffering provided by the former. Only Mn and Mo showed elevated leaching concentrations in seawater treatments, though at modest levels (<3 mg/L and 0.01 mg/L, respectively). No significant leaching of potentially ecotoxic elements such as Cr and V (mean leachate concentrations <0.006 mg/L for both) were apparent in seawater, which micro-X-Ray Absorption Near Edge Structure (μXANES) analysis show are both present in slags in low valence (and low toxicity) forms. Although there may be physical hazards posed by extensive erosion of deposits in high-energy coastlines, the data suggest seawater leaching of coastal iron and steel slags in the UK is likely to pose minimal environmental risk.

Strontium-90 (90Sr) is a major contaminant at nuclear legacy sites. The mobility of 90Sr is primarily governed by sorption reactions with sediments controlled by high surface area phases such as clay and iron oxides. Sr2+ adsorption was investigated in heterogeneous unconsolidated aquifer sediments, analogous to those underlying the UK Sellafield nuclear site, with grainsizes ranging from gravels to clays. Batch sorption tests showed that a linear Kd adsorption model was applicable to all grainsize fractions up to equilibrium [Sr] of 0.28 mmol L-1. Sr2+ sorption values (Kd; Langmuir qmax) correlated well with bulk sediment properties such as cation exchange capacity and surface area. Electron microscopy showed that heterogeneous sediments contained porous sandstone clasts with clay minerals (i.e. chlorite) providing an additional adsorption capacity. Therefore, gravel corrections that assumed that the > 2 mm fractions are inert were not appropriate and underestimated Kd(bulk) adsorption coefficients. However, Kd (<2 mm) values measured from sieved sediment fractions, were effectively adjusted to within error of Kd (bulk) using a surface area dependant gravel correction based on particle size distribution data. Amphoteric pH dependent Sr2+ sorption behaviour observed in batch experiments was consistent with cation exchange modelling between pH 2-7 derived from the measured cation exchange capacities. Above pH 7 model fits were improved by invoking a coupled cation exchange/surface complexation which allowed for addition sorption to iron oxide phases. The overall trends in Sr2+ sorption (at pH 6.5-7) produced by increasing solution ionic strength was also reproduced in cation exchange models. Overall, the results showed that Sr2+ sorption to heterogeneous sediment units could be estimated from Kd (<2 mm) data using appropriate gravel corrections, and effectively modelled using coupled cation exchange and surface complexation processes.

Mega-nourishments, where large volumes of sediment are deposited on coastlines, are increasingly employed to manage shoreline erosion, yet our understanding of their long-term behaviour is limited by the fact that most current schemes are less than 15 years old. However, on the County Durham coast, 39 million m3 of coal spoil was tipped onto beaches between the late 1800s and 1993, acting as a de facto mixed sediment meganourishment. Our findings reveal key insights into the long-term dynamics of mega-nourishment schemes, including evidence of effective sediment dispersal around headlands into normally disconnected units of coast. Following cessation of tipping, shorelines retreated up to 12 m yr(-1), with 150 m overall retreat in 12 years. Subsequently, retreat slowed but the present-day shoreline remains seaward of its 1860 position and is subject to ongoing coastal recession. We document significant fining of the deposited material in the years post deposition through abrasion and chemical breakdown. Furthermore, we show that the highest erosion rates now occur downdrift from the initial dump site, indicating that nourishment impacts migrate through time. These findings highlight the need for holistic and adaptive management approaches to mega-nourishment schemes, showing the behaviour of the nourishment to continually change in both location and magnitude as the system evolves. We demonstrate that mixed sediment mega-nourishments can be a cost-effective and durable solution to mitigate erosive losses, even in the absence of a planned approach to the location or composition of deposited sediment. Our results suggest that lessons from this historical intervention can inform the design and management of future mega-nourishment schemes, particularly in mixed sediment environments.

Dumped Chromium Ore Processing Residue (COPR) at legacy sites poses a threat to health through leaching of toxic Cr(VI) into groundwater. Previous work implicates microbial activity in reducing Cr(VI) to less mobile and toxic Cr(III), but the mechanism has not been explored. To address this question a combined metagenomic and geochemical study was undertaken. Soil samples from below the COPR waste were used to establish anaerobic microcosms which were challenged with Cr(VI), with or without acetate as an electron donor, and incubated for 70 days. Cr was rapidly reduced in both systems, which also reduced nitrate, nitrite then sulfate, but this sequence was accelerated in the acetate amended microcosms. 16S rRNA gene sequencing revealed that the original soil sample was diverse but both microcosm systems became less diverse by the end of the experiment. A high proportion of 16S rRNA gene reads and metagenome-assembled genomes (MAGs) with high completeness could not be taxonomically classified, highlighting the distinctiveness of these alkaline Cr impacted systems. Examination of the coding capacity revealed widespread capability for metal tolerance and Fe uptake and storage, and both populations possessed metabolic capability to degrade a wide range of organic molecules. The relative abundance of genes for fatty acid degradation was 4× higher in the unamended compared to the acetate amended system, whereas the capacity for dissimilatory sulfate metabolism was 3× higher in the acetate amended system. We demonstrate that naturally occurring in situ bacterial populations have the metabolic capability to couple acetate oxidation to sequential reduction of electron acceptors which can reduce Cr(VI) to less mobile and toxic Cr(III), and that microbially produced sulfide may be important in reductive precipitation of chromate. This capability could be harnessed to create a Cr(VI) trap-zone beneath COPR tips without the need to disturb the waste.

Increasing municipal solid waste (MSW) production poses challenges for sustainable urban development. Modern energy-from-waste (EfW) facilities incinerate MSW, reducing mass and recovering energy. In the UK, MSW incineration bottom ash (MSW IBA) is primarily reused in civil engineering applications. This study characterizes UK-produced MSW IBA, examining its pH-dependent leaching behaviour and response to environmental lixiviants. Results show predominant components include a melt phase, primary glass and fine ash aggregations, and a chemical composition dominated by SiO2 (30-50 %), CaO (∼15 %), Fe2O3 (∼10 %), and Al2O3 (∼8%). X-ray absorption near edge structure (XANES) analysis shows that Zn and Cu are most likely oxygen-bound (adsorbed to oxy-hydroxides and as oxides) with some sulphur bound. Polychlorinated biphenyls (PCBs) and polychlorinated dibenzodioxins/furans (PCDD/Fs) are well below regulatory limits, and polycyclic aromatic hydrocarbons (PAHs) were undetectable. Leaching tests indicate trace elements mobilize at pHs ≤ 6. With a natural pH of 11.3 and high buffering capacity, significant acid inputs to the MSW IBA are required to reach this pH, which are improbable in the environment. Wood chip additions increase leachate's dissolved organic carbon (DOC) and reduce pH, but had minimal impact on metal-leaching behaviour. Synthetic plant exudate solutions minimally affect metal leaching at realistic concentrations, only enhancing leaching at ≥ 1500 mg l-1 DOC. This work supports MSW IBA's low-risk in specified civil engineering applications.

Metals such as Zn and Cu present in sewage sludge applied to agricultural land can accumulate in soils and potentially mobilise into crops. Sequential extractions and X-ray absorption spectroscopy results are presented that show the speciation changes of Cu and Zn sorbed to anaerobic digestion sludge after mixing with soils over three consecutive 6-week cropping cycles, with and without spring barley (Hordeum vulgare). Cu and Zn in digested sewage sludge are primarily in metal sulphide phases formed during anaerobic digestion. When Cu and Zn spiked sludge was mixed with the soil, about 40