Environmental behaviour of iron and steel slags in coastal settings.

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.

Use of propionic acid additions to enhance zinc removal from mine drainage in short residence time, flow-through sulfate-reducing bioreactors.

The effectiveness of liquid carbon additions to enhance zinc removal in laboratory-scale short hydraulic residence time (19 h) compost bioreactors receiving synthetic mine water with a high influent zinc concentration (45 mg/L) was investigated. Effective removal of such elevated zinc concentrations could not be sustained by sulfate reduction and/or other attenuation processes without carbon supplementation. Propionic acid addition resulted in improved and sustained performance by promoting the activities of sulfate reducing bacteria, leading to efficient zinc removal (mean 99%) via bacterial sulfate reduction. In contrast, cessation of propionic acid addition led to carbon limitation and the growth of sulfur oxidising bacteria, compromising zinc removal by bacterial sulfate reduction. These research findings demonstrate the potential for modest liquid carbon additions to compost-based passive treatment systems to engineer microbial responses which enhance rates of zinc attenuation in a short hydraulic residence time, enabling remediation of highly polluting mine drainage at sites with limited land availability.

Predicting the Benefits of Mine Water Treatment under Varying Hydrological Conditions using a Synoptic Mass Balance Approach.

Geochemical and hydrological data from abandoned mine watersheds demonstrated that (1) point sources of pollution fail to account for total receiving watercourse metal load at higher flows and (2) an inverse relationship exists between river flow and pH due to peatland runoff. Quantifying the varying importance of point and diffuse pollution sources enabled prediction of treatment benefits for a major point source of pollution in one watershed. Instream zinc load increases with river flow (∼3 to 14 kg Zn/d) due to diffuse groundwater and surface runoff pollution sources at higher flows. Lab tests demonstrated that metal release from the streambed, driven by pH decreases at higher flows, also contribute to increased downstream metal loads. Predicting point source treatment benefits demonstrates major instream improvements at low flow (zinc decreases from >800 to 120 µg Zn/L). At higher flows treatment benefits diminish (Zn decreases from 240 to only 200 µg Zn/L) due to the greater influence of diffuse sources. A quantitative understanding of the variable importance of point and diffuse sources of pollution, and instream processes of metal attenuation and release, is crucial to evaluating the benefits of treatment to downstream water quality.

Biodesalination: a case study for applications of photosynthetic bacteria in water treatment.

Shortage of freshwater is a serious problem in many regions worldwide, and is expected to become even more urgent over the next decades as a result of increased demand for food production and adverse effects of climate change. Vast water resources in the oceans can only be tapped into if sustainable, energy-efficient technologies for desalination are developed. Energization of desalination by sunlight through photosynthetic organisms offers a potential opportunity to exploit biological processes for this purpose. Cyanobacterial cultures in particular can generate a large biomass in brackish and seawater, thereby forming a low-salt reservoir within the saline water. The latter could be used as an ion exchanger through manipulation of transport proteins in the cell membrane. In this article, we use the example of biodesalination as a vehicle to review the availability of tools and methods for the exploitation of cyanobacteria in water biotechnology. Issues discussed relate to strain selection, environmental factors, genetic manipulation, ion transport, cell-water separation, process design, safety, and public acceptance.