Fossil rootlet biopores as conduits for contaminant transport through clay horizons: a case study of DNAPL behaviour in Severn alluvium, UK

This paper presents a case study of several DNAPL contaminated sites around the Severn Estuary (UK) where a combination of detailed observations, core dissections and physicochemical characterisation of alluvial clay–silt horizons have revealed the presence of fossil rootlet biopores which act and have the potential to act as conduits for contaminant migration through up to 13 m of clay–silt. The biopores are shown to penetrate the low-permeability (K ~ 10−10 m/s) clay–silt matrix throughout its entire depth (up to 13 m) and provide a preferential transport pathway for DNAPLs from near surface to the underlying aquifer, with particularly high concentrations measured in the biopores themselves. Capillary rise experiments with coal tar distillate demonstrate that DNAPLs are drawn into biopores, with values of surface interfacial tension for the system calculated as 8 × 10−2 J/m2. Wicking by residual plant fibres was demonstrated qualitatively and is thought to be an important additional transport mechanism. The DNAPL contamination below and throughout the Severn alluvial clay–silt horizons demonstrates that the assumption that these horizons act as an effective seal protecting underlying aquifers from severe pollution from the legacy sites around the Severn is flawed and highlights the failure of current protocols for sampling of clay horizons for hydraulic conductivity assessments, since current methods can destroy delicate in situ biopore structures. The study demonstrates that the interplay between ecological and the geological depositional environment can, at a regional scale, result in a network of biopores that can potentially act as conduits for contaminant transport

Physicochemical characterization of sludge originating from vegetable oil–based cutting fluids

Vegetable oil–based cutting fluids are a relatively recent development in large-scale metal machining. A metal machining factory in Wales that switched from mineral oil-based to vegetable oil-based cutting fluids has experienced the occurrence of a problematic floating sludge within the settling and holding tanks at the on-site effluent treatment plant. Physicochemical analyses have found that the sludge is composed of on average 33% water, 20% oleic acid, and 18% palmitic acid, originating from the vegetable oil–based cutting fluids used at the factory. A solvent separation step was devised and used successfully to separate water inherent within the sludge so as to study the division of the inorganic elements within the water and organic phases of the sludge. It was found that only a minor constituent of the sludge can be accounted for by Ca-bonded fatty acids. Formation of the sludge is suspected to be due to the biologically induced hydrolysis and oxidation of the oils from esters to the free fatty acids and subsequent creaming, forming a layer of stable floating sludge on the surface of the effluent storage tanks

In situ resource recovery from waste repositories: exploring the potential for mobilization and capture of metals from anthropogenic ores

Wastes and the waste repositories in which they reside are becoming targets for resource recovery, both for legacy wastes and for future waste arisings as part of a desire to move toward a circular economy. There is an urgent requirement to explore concepts for practicable technologies that can be applied to these ends. This paper presents a synthesis of concepts concerning in situ technologies (developed from mining and contaminated land remediation industries) that have enormous potential for application to technospheric mining. Furthermore, potential target waste streams and their mineralogy and character are presented along with a discussion concerning lixiviant and metal capture systems that could be applied. Issues of preferential flow (critical to the success of in situ techniques) and how to control it with engineering measures are discussed in detail. It is clear that in situ recovery of metals from anthropogenic ores is a novel technology area that links new sustainable remediation approaches for contaminated materials and technospheric mining for closing material loops, and warrants the further research and development of technologies applicable to major waste streams

Conversion of coal mine drainage ochre to water treatment reagent: Production, characterisation and application for P and Zn removal

Coal mine drainage ochre is a ferruginous precipitate that forms from mine water in impacted watercourses and during treatment. With thousands of tonnes per annum of such ochre arising from mine water treatment in the UK alone, management of these wastes is a substantive issue. This paper demonstrates that the ochre from both active and passive treatment of coal mine drainage can be transformed into an effective water treatment reagent by simple acid dissolution and that the reagent can be used for the removal of dissolved phosphorous from municipal wastewater and zinc from non-coal mine waters. Ochre is readily soluble in H2SO4 and HCl. Ochre is more soluble in HCl with solubilities of up to 100 g/L in 20% (w/w) HCl and 68 g/L in 10% (w/w) H2SO4. For four of the eight tested ochres solubility decreased in higher concentrations of H2SO4. Ochre compositional data demonstrate that the coal mine ochres tested are relatively free from problematic levels of elements seen by other authors from acid mine drainage-derived ochre. Comparison to British Standards for use of iron-based coagulants in drinking water treatment was used as an indicator of the acceptability of use of the ochre-derived reagents in terms of potentially problematic elements. The ochre-derived reagents were found to meet the ‘Grade 3’ specification, except for arsenic. Thus, for application in municipal wastewater and mine water treatment additional processing may not be required. There was little observed compositional difference between solutions prepared using H2SO4 or HCl. Ochre-derived reagents showed applicability for the removal of P and Zn with removals of up to 99% and 97% respectively measured for final pH 7–8, likely due to sorption/coprecipitation. Furthermore, the results demonstrate that applying a Fe dose in the form of liquid reagent leads to a better Fe:P and Fe:Zn removal ratio compared to ochre-based sorption media tested in the literature

Thermal degradation of monoethanolamine and its effect on CO2 capture capacity.

Amine scrubbing is a proven technology in the oil and gas industries. Its use in coal fired power plants is not fully understood and the likelihood of solvent degradation is high. Decreased absorption efficiency, undesirable by-products and the environmental impact of their disposal are the main consequences. In the present study, samples of monoethanolamine were thermally degraded, at 160 °C for between 2 and 8 weeks, and their CO2 removal capacity deterioration was determined. The findings show that thermal degradation at 160 °C for 8 weeks reduced monoethanolamine concentration by 95%, but the remaining solvent still retained 22% of its capacity to remove CO2, probably due to the capacity of some of the degradation products to remove CO2. Therefore, the requirement for monoethanolamine make-up in operational amine scrubbing systems may not be quite as serious as initially believed. A 20% higher MEA loss was determined in the samples with 0.37 initial CO2 loading (mol CO2/mol MEA). 2-Oxazolidone, N-(2-hydroxyethyl)-ethylenediamine and 1-(2-hydroxyethyl)-2-imidazolidinone were identified as the major monoethanolamine degradation products, the latter being indicated as the most stable product with concentrations of up to 17% (v/v). Corrosion (1.95 mm/year) of the stainless steel (type 316) equipment, used during the experiments, was also observed

Humidity cell tests for the prediction of acid rock drainage

This paper presents a study of various geochemical humidity-style weathering tests that were carried out on waste mine rock from Avoca, County Wicklow, Ireland. The aim of this paper is to present data that demonstrate some of the geochemical controls on weathering rates together with release rates from laboratory testwork. These data are used to determine the applicability of various interpretations of humidity cell data for prediction of acid rock drainage. Furthermore, within this context the paper offers opinion on common questions related to the use of such tests: should humidity cells be aerated? How long should the test be run for? Is pre-treatment of the samples required? Is inoculation of the samples with iron and sulfur oxidising microbes required? And should these tests really be considered to be accelerated weathering tests

New perspectives on the passive treatment of ferruginous circumneutral mine waters in the UK

This paper examines major physico-chemical processes during the passive treatment of ferruginous circumneutral drainage from abandoned coal mines in the UK. Data collected over several years of studies on mine water treatment systems shed new light on the relative importance of hydraulics, settling velocity, Fe(II) oxidation rates and cascade aeration, which, in turn, informs the design of future systems. This paper demonstrates that (1) the complex settling behaviour of Fe(III) precipitates may be described by a first-order volumetric process and that settling rate is different for different mine waters; (2) the hydraulic efficiency (ratio of time to peak tracer concentration to nominal residence time) of the settling ponds studied was widely variable at low flow rates in comparison to constructed wetlands; (3) aeration cascades contribute dissolved oxygen and lead to a rise in pH due to CO2 degassing, which are very important in reducing the required time for iron oxidation and removal; (4) for at least 10 of the 30 sites examined, modelling of the rates of Fe(II) oxidation and particulate settling reveals that removal of iron is primarily dependent on settling rate; and (5) that substantial increases in pH can be brought about by forced aeration of mine water over several hours. Findings of this study apply to the majority of coal mine water treatment sites in the UK and may have broader application to other ferruginous waters with circumneutral pH or after treatment to increase pH