Equipping for risk: Lessons learnt from the UK shale-gas experience on assessing environmental risks for the future geoenergy use of the deep subsurface

findings are presented from an investigation to improve understanding of the environmental risks associated with developing an unconventional-hydrocarbons industry in the UK. The EQUIPT4RISK project, funded by UK Research Councils, focused on investigations around Preston New Road (PNR), Fylde, Lancashire, and Kirby Misperton Site A (KMA), North Yorkshire, where operator licences to explore for shale gas by hydraulic fracturing (HF) were issued in 2016, although exploration only took place at PNR. EQUIPT4RISK considered atmospheric (greenhouse gases, air quality), water (groundwater quality) and solid-earth (seismicity) compartments to characterise and model local conditions and environmental responses to HF activities. Risk assessment was based on the source-pathway-receptor approach. Baseline monitoring of air around the two sites characterised the variability with meteorological conditions, and isotopic signatures were able to discriminate biogenic methane (cattle) from thermogenic (natural-gas) sources. Monitoring of a post-HF nitrogen-lift (well-cleaning) operation at PNR detected the release of atmospheric emissions of methane (4.2 ± 1.4 t CH4). Groundwater monitoring around KMA identified high baseline methane concentrations and detected ethane and propane at some locations. Dissolved methane was inferred from stable-isotopic evidence as overwhelmingly of biogenic origin. Groundwater-quality monitoring around PNR found no evidence of HF-induced impacts. Two approaches for modelling induced seismicity and associated seismic risk were developed using observations of seismicity and operational parameters from PNR in 2018 and 2019. Novel methodologies developed for monitoring include use of machine learning to identify fugitive atmospheric methane, Bayesian statistics to assess changes to groundwater quality, a seismicity forecasting model seeded by the HF-fluid injection rate and high-resolution monitoring of soil-gas methane. The project developed a risk-assessment framework, aligned with ISO 31000 risk-management principles, to assess the theoretical combined and cumulative environmental risks from operations over time. This demonstrated the spatial and temporal evolution of risk profiles: seismic and atmospheric impacts from the shale-gas operations are modelled to be localised and short-lived, while risk to groundwater quality is longer-term.

Biochar benefits carbon off-setting in blue-green infrastructure soils - A lysimeter study

Carbon sequestration with amendments in blue-green infrastructure soils could off-set anthropogenic greenhouse gas emissions to alleviate climate change. In this 3-year study, the effects of wheat straw and its biochar on carbon sequestration in an urban landscaping soil were investigated under realistic outdoor conditions using two large-scale lysimeters. Both amendments were carried out by incorporating pellets at 0-15 cm soil depth with an equivalent initial total carbon input of 2% of the dry soil weight. Soil carbon, carbon isotope ratios, dissolved carbon in leachates, CO₂-C emissions, carbon fixed in above ground vegetation, soil water content, soil bulk electrical conductivity, and water infiltration rates, were then compared between the 2 lysimeters. After 3 years, we observed that, despite having a 17.2% lower vegetation growth, soil organic and inorganic carbon content was higher by 28.8% and 41.5%, respectively, in biochar as compared to wheat straw amended soil. Carbon isotope analysis confirmed the greater stability of the added carbon in the biochar amended soil. Water content was on average 23.2% and 13.0% in the straw pellet and biochar amended soil, respectively, whereas water infiltration rates were not significantly different between the two lysimeters. Overall, the incorporation of wheat straw biochar into soil could store an estimated 30 tonnes of carbon per hectare in city blue-green infrastructure spaces. Interviews involving institution stakeholders examined the feasibility of this biochar application. Stakeholders recognized the potential of biochar as an environment-friendly means for carbon offsetting, but were concerned about the practicality of biochar production and application into soil and increased maintenance work. Consequently, additional potential benefits of biochar for environmental management such as improving the quality of polluted run-off in stormwater treatment systems should be emphasized to make biochar an attractive proposition in sustainable urban development.

Removal of atmospheric CO2 by engineered soils in infrastructure projects

The use of crushed basic igneous rock and crushed concrete for enhanced rock weathering and to facilitate pedogenic carbonate precipitation provides a promising method of carbon sequestration. However, many of the controls on precipitation and subsequent effects on soil properties remain poorly understood. In this study, engineered soil plots, with different ratios of concrete or dolerite combined with sand, have been used to investigate relationships between sequestered inorganic carbon and geotechnical properties, over a two-year period. Cone penetration tests with porewater pressure measurements (CPTu) were conducted to determine changes in tip resistance and pore pressure. C and O isotope analysis was carried out to confirm the pedogenic origin of carbonate minerals. TIC analysis shows greater precipitation of pedogenic carbonate in plots containing concrete than those with dolerite, with the highest sequestration values of plots containing each material being equivalent to 33.7 t C ha^-1 yr^-1 and 17.5 t C ha^-1 yr^-1, respectively, calculated from extrapolation of results derived from the TIC analysis. TIC content showed reduction or remained unchanged for the top 0.1 m of soil; at a depth of 0.2 m however, for dolerite plots, a pattern of seasonal accumulation and loss of TIC emerged. CPTu tip resistance measurements showed that the presence of carbonates had no observable effect on penetration resistance, and in the case of porewater pressure measurements, carbonate precipitation does not change the permeability of the substrate, and so does not affect drainage. The results of this study indicate that both the addition of dolerite and concrete serve to enhance CO₂ removal in soils, that soil temperature appears to be a control on TIC precipitation, and that mineral carbonation in constructed soils does not lead to reduced drainage or an increased risk of flooding.

Characterising the biophysical, economic and social impacts of soil carbon sequestration as a greenhouse gas removal technology

To limit warming to well below 2°C, most scenario projections rely on greenhouse gas removal technologies (GGRTs); one such GGRT uses soil carbon sequestration (SCS) in agricultural land. In addition to their role in mitigating climate change, SCS practices play a role in delivering agroecosystem resilience, climate change adaptability and food security. Environmental heterogeneity and differences in agricultural practices challenge the practical implementation of SCS, and our analysis addresses the associated knowledge gap. Previous assessments have focused on global potentials, but there is a need among policymakers to operationalise SCS. Here, we assess a range of practices already proposed to deliver SCS, and distil these into a subset of specific measures. We provide a multidisciplinary summary of the barriers and potential incentives towards practical implementation of these measures. First, we identify specific practices with potential for both a positive impact on SCS at farm level and an uptake rate compatible with global impact. These focus on: (a) optimising crop primary productivity (e.g. nutrient optimisation, pH management, irrigation); (b) reducing soil disturbance and managing soil physical properties (e.g. improved rotations, minimum till); (c) minimising deliberate removal of C or lateral transport via erosion processes (e.g. support measures, bare fallow reduction); (d) addition of C produced outside the system (e.g. organic manure amendments, biochar addition); (e) provision of additional C inputs within the cropping system (e.g. agroforestry, cover cropping). We then consider economic and non-cost barriers and incentives for land managers implementing these measures, along with the potential externalised impacts of implementation. This offers a framework and reference point for holistic assessment of the impacts of SCS. Finally, we summarise and discuss the ability of extant scientific approaches to quantify the technical potential and externalities of SCS measures, and the barriers and incentives to their implementation in global agricultural systems.

Passive CO2 removal in urban soils: Evidence from brownfield sites

Management of urban brownfield land can contribute to significant removal of atmospheric CO₂ through the development of soil carbonate minerals. However, the potential magnitude and stability of this carbon sink is poorly quantified as previous studies address a limited range of conditions and short durations. Furthermore, the suitability of carbonate-sequestering soils for construction has not been investigated. To address these issues we measured total inorganic carbon, permeability and ground strength in the top 20 cm of soil at 20 brownfield sites in northern England, between 2015 and 2017. Across all sites accumulation occurred at a rate of 1-16 t C ha^-1 yr^-1, as calcite (CaCO₃), corresponding to removal of approximately 4-59 t CO₂ ha^-1 yr^-1, with the highest rate in the first 15 years after demolition. C and O stable isotope analysis of calcite confirms the atmospheric origin of the measured inorganic carbon. Statistical modelling found that pH and the content of fine materials (combined silt and clay content) were the best predictors of the total inorganic carbon content of the samples. Measurement of permeability shows that sites with carbonated soils possess a similar risk of run-off or flooding to sandy soils. Soil strength, measured as in-situ bearing capacity, increased with carbonation. These results demonstrate that the management of urban brownfield land to retain fine material derived from concrete crushing on site following demolition will promote calcite precipitation in soils, and so offers an additional CO₂ removal mechanism, with no detrimental effect on drainage and possible improvements in strength. Given the large area of brownfield land that is available for development, the contribution of this process to CO₂ removal by urban soils needs to be recognised in CO₂ mitigation policies.

Evaluation of raw material extraction, processivng, construction and disposal of cement and concrete products: datasets and calculations

To evaluate the material flows associated with construction and demolition in different countries it is necessary to have a consistent set of data. However, data collected by regulators and governments differ and this study used concrete as a case in point. Concrete is a significant man-made material in construction whose use reflects socio-economic variation between countries. Flows of natural components, cement and aggregates, are investigated from extraction to final disposal following demolition (Tangtinthai et al., 2019). The housing sector dominates the use of concrete in urbanized areas and greatly reflects socio-economic and resource extraction issues. To compare concrete stock, use and policies of contrasting countries the data from Thailand and Great Britain (GB) are considered, but as reported they differ for each country. We present here the results of the calculations required to generate an internally consistent database for Great Britain and for Thailand that enables an informed materials flow analysis to be undertaken on materials consumed and generated during construction and demolition of concrete structures. The research methodology and calculations for national cement and concrete production (including clinker, cement kiln dust, gypsum, and aggregates) and the resulting datasets help to make projections that shape policy requirements for Thailand and other emerging economies as reported in (Tangtinthai et al., 2019).

Role of policy in managing mined resources for construction in Europe and emerging economies

Rapid urbanisation, with associated housing and infrastructure demands, leads to increased mining and use of non-renewable mineral raw materials needed for the construction industry including concrete and cement. In an emerging economy, like Thailand, which is part of Association of Southeast Asian Nations (ASEAN), current environmental management policies are insufficient to reduce raw material requirements or waste from demolition by generating inputs to construction through reuse or recycling. As part of the European Union (EU), Great Britain has successfully implemented integrated policies and achieved high rates of recycled aggregates in construction (29%) and a 70% reuse and recycling target for construction and demolition (C&D) waste. In this paper, Material Flow Analysis (MFA) of cement/concrete materials is combined with an interpretation of related policies to provide a deeper understanding how to achieve more sustainable management of natural resources. A comparative MFA for the construction industry in Great Britain and Thailand (representing an ASEAN country) has been developed that quantifies raw material inputs, buildings and infrastructure outputs, so that the practices in the two countries can be contrasted. We report domestic cement production and import/export data, and calculate the raw materials needed for cement and its calcination process for concrete production. Considering the most relevant policies and taxation in Great Britain, we identify possible ways forward for Thailand by introducing new policies and taxation that will have positive effects on raw material extraction, processing, construction and disposal practices and disposal behaviors. Following the MFA and policy analysis, we believe that similar benefits apply to other emerging economies.

Testing the ability of plants to access potassium from framework silicate minerals

The availability of K, essential for plant growth, from syenite (a silicate rock in which potassium feldspar is the dominant mineral; >90wt%), and phlogopite mica has been demonstrated using carefully designed plant growth pot experiments in which the only added source of K was the mineral of interest, with no loss of nutrients through drainage. Using pure quartz sand as a soil, both growth (increase in diameter) of leek plants and K-content of the plant material showed a dose-dependent positive response to the application (114-43000mgK/pot) of milled syenite with increases in plant diameter of 0.5-0.7mm/week, increasing with application rate. Phlogopite mica (114-6000mgK/pot) supported the highest observed increase in diameter (approx. 1mm/week) and plant K-content, both similar to that observed for a positive control (KCl). These experiments demonstrate that plants can obtain K for growth from milled syenite, in which feldspar is the dominant K-bearing mineral, and confirm previous observations that micas can be an effective source of K.

Evaluating an anaerobic digestion (AD) feedstock derived from a novel non-source segregated municipal solid waste (MSW) product

In many nations industrial scale AD of non-agricultural waste materials (such as MSW) has not yet reached its full potential, often constrained by the lack of secure, inexpensive, high quality AD feedstocks, and markets for the resulting digestate material. We tested the output material of a high throughput novel industrial process to define its potential as an AD feedstock (based on quality and consistency). This process, designed to circumvent the constraints of source segregation while still generating segregated waste streams, resulted in the production of a temporally homogenous fibrous material with: an average moisture content of 44.2 (±2.33)%; C:N ratio of ∼32.9:1 (±3.46:1), C:P ratio of ∼228:1 and gross calorific value of 17.4 (±0.29)MJ/kg_(DM). This material provided a CH₄ yield of between 201 and 297m³ CH₄/tonne_(DM) (271-401m³CH₄/tonne_(vs)) comparable to commonly used AD feedstocks. Material contaminant levels were temporally consistent (P>0.05), (average values being Cd 0.63 (±0.19), Cu 56.3 (±7.45), Cr^tot 51.4 (±4.41), Hg<0.3, Ni 28.9 (±5.17), Pb 79.2 (±23.71), Zn 202 (±44.5), total polyaromatic hydrocarbons (PAH) 2.2 (±0.3), and total polychlorinated biphenyls (PCB) (<0.2)mg/kg_(DM)). Calculated digestate contaminant levels were below the median contaminant threshold limits for anaerobic digestates of all countries within the European Union i.e. of Cd 3.35, Cu 535, Cr^tot 535, Hg 8.15, Ni 185, Pb 397.5, Zn 2100mg/kg_(DM). We suggest that novel high throughput processes that produce high quality AD feedstocks, may have a place in further diversion of waste from landfill.

An in situ FTIR spectroscopic and thermogravimetric analysis study of the dehydration and dihydroxylation of SnO2: the contribution of the (100), (110) and (111) facets

Nanoparticulate SnO2 produced by a hydrothermal method was characterised by BET, XRD, TGA-MS and in situ variable temperature diffuse reflectance infra red spectroscopy (DRIFTS) to determine the surface behaviour of water. For the (100) facets, hydrogen bonding does not occur, and water adsorption is less strong than for the (111) and (110) facets where hydrogen bonding does occur. Reversible uptake of oxygen was observed. These findings have implications for other surface-gas reactions in which Ni and Sb co-doped SnO2 (NATO) anodes are used for ozone generation. BET showed the relatively high surface area and nanometer scale of the SnO2 particles, whilst XRD confirmed the nano dimension of the crystallites and showed only the cassiterite phase. TGA analysis indicated four temperature regions over which mass loss was observed. These and the in situ DRIFTS studies revealed the existence of various forms of water associated with specific crystal facets of the SnO2, as well as the existence of isolated O-H groups and adsorbed oxygen species. Electronic absorptions were also observed and the data rationalised in terms of the existence of both free electron absorptions, and absorptions from oxygen vacancy states. The role of adsorbed molecular oxygen in electrochemical ozone generation at Ni and Sb co-doped SnO2 (NATO) anodes was strongly suggested by this work.

Black Carbon Contribution to Organic Carbon Stocks in Urban Soil

Soil holds 75% of the total organic carbon (TOC) stock in terrestrial ecosystems. This comprises ecosystem-derived organic carbon (OC) and black carbon (BC), a recalcitrant product of the incomplete combustion of fossil fuels and biomass. Urban topsoils are often enriched in BC from historical emissions of soot and have high TOC concentrations, but the contribution of BC to TOC throughout the urban soil profile, at a regional scale is unknown. We sampled 55 urban soil profiles across the North East of England, a region with a history of coal burning and heavy industry. Through combined elemental and thermogravimetic analyses, we found very large total soil OC stocks (31-65 kg m(-2) to 1 m), exceeding typical values reported for UK woodland soils. BC contributed 28-39% of the TOC stocks, up to 23 kg C m(-2) to 1 m, and was affected by soil texture. The proportional contribution of the BC-rich fraction to TOC increased with soil depth, and was enriched in topsoil under trees when compared to grassland. Our findings establish the importance of urban ecosystems in storing large amounts of OC in soils and that these soils also capture a large proportion of BC particulates emitted within urban areas.

Rapid Removal of Atmospheric CO2 by Urban Soils

The measured calcium carbonate content of soils to a depth of 100 mm at a large urban development site has increased over 18 months at a rate that corresponds to the sequestration of 85 t of CO2/ha (8.5 kg of CO2 m(-2)) annually. This is a consequence of rapid weathering of calcium silicate and hydroxide minerals derived from the demolition of concrete structures, which releases Ca that combines with CO2 ultimately derived from the atmosphere, precipitating as calcite. Stable isotope data confirm an atmospheric origin for carbonate carbon, and 14C dating indicates the predominance of modern carbon in the pedogenic calcite. Trial pits show that carbonation extends to depths of ≥1 m. Work at other sites shows that the occurrence of pedogenic carbonates is widespread in artificially created urban soils containing Ca and Mg silicate minerals. Appropriate management of fewer than 12000 ha of urban land to maximize calcite precipitation has the potential to remove 1 million t of CO2 from the atmosphere annually. The maximal global potential is estimated to be approximately 700-1200 Mt of CO2 per year (representing 2.0-3.7% of total emissions from fossil fuel combustion) based on current rates of production of industry-derived Ca- and Mg-bearing materials.

Historical and technical developments of potassium resources

The mining of soluble potassium salts (potash) is essential for manufacturing fertilizers required to ensure continuous production of crops and hence global food security. As of 2014, potash is mined predominantly in the northern hemisphere, where large deposits occur. Production tonnage and prices do not take into account the needs of the farmers of the poorest countries. Consequently, soils of some regions of the southern hemisphere are currently being depleted of potassium due to the expansion and intensification of agriculture coupled with the lack of affordable potash. Moving away from mined salts towards locally available resources of potassium, such as K-bearing silicates, could be one option to improve this situation. Overall, the global potash production system and its sustainability warrant discussion. In this contribution we examine the history of potash production and discuss the different sources and technologies used throughout the centuries. In particular, we highlight the political and economic conditions that favored the development of one specific technology over another. We identified a pattern of needs driving innovation. We show that as needs evolved throughout history, alternatives to soluble salts have been used to obtain K-fertilizers. Those alternatives may meet the incoming needs of our century, providing the regulatory and advisory practices that prevailed in the 20th century are revised.

Ground gas monitoring: implications for hydraulic fracturing and CO2 storage

Understanding the exchange of carbon dioxide (CO2) and methane (CH4) between the geosphere and atmosphere is essential for the management of anthropogenic emissions. Human activities such as carbon capture and storage and hydraulic fracturing ("fracking") affect the natural system and pose risks to future global warming and to human health and safety if not engineered to a high standard. In this paper an innovative approach of expressing ground gas compositions is presented, using data derived from regulatory monitoring of boreholes in the unsaturated zone at infrequent intervals (typically 3 months) with data from a high frequency monitoring instrument deployed over periods of weeks. Similar highly variable trends are observed for time scales ranging from decades to hourly for boreholes located close to sanitary landfill sites. Additionally, high frequency monitoring data confirm the effect of meteorological controls on ground gas emissions; the maximum observed CH4 and CO2 concentrations in a borehole monitored over two weeks were 40.1% v/v and 8.5% v/v respectively, but for 70% of the monitoring period only air was present. There is a clear weakness in current point monitoring strategies that may miss emission events and this needs to be considered along with obtaining baseline data prior to starting any engineering activity.

Microbial degradation of crude oil hydrocarbons on organoclay minerals

The role of organoclays in hydrocarbon removal during biodegradation was investigated in aqueous clay/oil microcosm experiments with a hydrocarbon degrading microorganism community. The clays used for this study were Na-montmorillonite and saponite. These two clays were treated with didecyldimethylammonium bromide to produce organoclays which were used in this study. The study indicated that clays with high cation exchange capacity (CEC) such as Na-montmorillonite produced an organomontmorillonite that was inhibitory to biodegradation of the crude oil hydrocarbons. Extensive hydrophobic interaction between the organic phase of the organoclay and the crude oil hydrocarbons is suggested to render the hydrocarbons unavailable for biodegradation. However, untreated Na-montmorillonite was stimulatory to biodegradation of the hydrocarbons and is believed to have done so because of its high surface area for the accumulation of microbes and nutrients making it easy for the microbes to access the nutrients. This study indicates that unlike unmodified montmorillonites, organomontmorillonite may not serve any useful purpose in the bioremediation of crude oil spill sites where hydrocarbon removal by biodegradation is desired within a rapid time period.

Effect of interlayer cations of montmorillonite on the biodegradation and adsorption of crude oil polycyclic aromatic compounds

Cation exchange capacity, surface acidity and specific surface area are surface properties of clay minerals that make them act as catalysts or supports in most biogeochemical processes hence making them play important roles in environmental control. However, the role of homoionic clay minerals during the biodegradation of polycyclic aromatic compounds is not well reported. In this study, the effect of interlayer cations of montmorillonites in the removal of some crude oil polycyclic aromatic compounds during biodegradation was investigated in aqueous clay/oil microcosm experiments with a hydrocarbon degrading microorganism community. The homoionic montmorillonites were prepared via cation exchange reactions by treating the unmodified montmorillonite with the relevant metallic chloride. The study indicated that potassium-montmorillonite and zinc-montmorillonite did not enhance the biodegradation of the polycyclic aromatic hydrocarbons whereas calcium-montmorillonite, and ferric-montmorillonite enhanced their biodegradation significantly. Adsorption of polycyclic aromatic hydrocarbons was significant during biodegradation with potassium- and zinc-montmorillonite where there was about 45% removal of the polycyclic aromatic compounds by adsorption in the experimental microcosm containing 5:1 ratio (w/w) of clay to oil.

Effect of modified montmorillonites on the biodegradation and adsorption of biomarkers such as hopanes, steranes and diasteranes

The effect of modified montmorillonites on the biodegradation and adsorption of selected steranes, diasteranes and hopanes was investigated in aqueous clay/oil microcosm experiments with a hydrocarbon degrading microorganism community. The unmodified montmorillonite was treated with didecyldimethylammonium bromide, hydrochloric acid and the relevant metallic chloride to produce organomontmorillonite, acid activated montmorillonite and homoionic montmorillonite respectively which were used in this study. The study indicated that organomontmorillonite, acid activated montmorillonite and potassium montmorillonite did not support the biodegradation of the selected steranes, diasteranes and hopanes as alteration of the biomarkers via biodegradation varied from a paltry 2-6 %. The adsorption of the selected biomarkers on acid activated montmorillonite and organomontmorillonite was also poor. However, adsorption of the biomarkers on potassium montmorillonite was relatively high. Sodium montmorillonite and unmodified montmorillonite appear to stimulate the biodegradation of the selected biomarkers moderately (30-35 %) with adsorption occurring at low level. Calcium montmorillonite and ferric montmorillonite effected significant biodegradation (51-60 %) of the selected biomarkers.

Compositional changes of crude oil SARA fractions due to biodegradation and adsorption on colloidal support such as clays using Iatroscan

The compositional changes of saturates, aromatics, resins and asphaltenes (SARA) fractions in aqueous clay/oil microcosm experiments with a hydrocarbon-degrading microorganism community were analysed using Iatroscan. The clay mineral samples used in this study were organomontmorillonite, acid-activated montmorillonite and K, Ca, Zn and Cr montmorillonites produced by modifying the original montmorillonite sample. The evaluation and quantification of biodegradation and adsorption were carried out using a combination of the Iatroscan and gravimetric analysis. The SARA compositions in the presence of organomontmorillonite and acid-activated montmorillonite after incubation follow the same pattern in which the aromatic fraction is higher than the other fractions unlike in the presence of unmodified, K, Ca and Zn montmorillonites, where the saturates fraction is higher than the other fractions. Changes in SARA fractions due to biodegradation seemed to occur most in the presence of unmodified and calcium montmorillonites; hence, the removal of SARA fractions due to biodegradation was significant and enhanced in the presence of these two clay samples. However, biodegradation in the presence of organomontmorillonite and acid-activated and Cr montmorillonites was hindered. The study indicated that Cr montmorillonite adsorbed resins most, whereas Zn and K montmorillonites adsorbed aromatics most after incubation.

Passive sequestration of atmospheric CO2 through coupled plant-mineral reactions in urban soils

Photosynthetic removal of CO(2) from the atmosphere is an important planetary carbon dioxide removal mechanism. Naturally, an amount equivalent to all atmospheric carbon passes through the coupled plant-soil system within 7 years. Plants cycle up to 40% of photosynthesized carbon through their roots, providing a flux of C at depth into the soil system. Root-exuded carboxylic acids have the potential to supply 4-5 micromoles C hr(-1)g(-1) fresh weight to the soil solution, and enhance silicate mineral weathering. Ultimately, the final product of these root-driven processes is CO(2), present in solution as bicarbonate. This combines with Ca liberated by corrosion associated with silicate mineral weathering to enter the soil-water system and to produce pedogenic calcium carbonate precipitates. Combining understanding of photosynthesis and plant root physiology with knowledge of mineral weathering provides an opportunity to design artificial soils or to plan land use in ways that maximize removal and sequestration of atmospheric CO(2) through artificially enhanced pedogenic carbonate precipitation. This process requires relatively low energy and infrastructure inputs. It offers a sustainable carbon dioxide removal mechanism analogous to the use of constructed wetlands for the passive remediation of contaminated waters, and is likely to achieve wide public acceptance.

Investigating carbonate formation in urban soils as a method for capture and storage of atmospheric carbon

This paper investigates the potential for engineered urban soils to capture and store atmospheric carbon (C). Calcium (Ca) and magnesium (Mg) bearing waste silicate minerals within the soil environment can capture and store atmospheric C through the process of weathering and secondary carbonate mineral precipitation. Anthropogenic soils, known to contain substantial quantities of Ca and Mg-rich minerals derived from demolition activity (particularly cement and concrete), were systematically sampled at the surface across a 10 ha brownfield site, Science Central, located in the urban centre of Newcastle upon Tyne, U.K. Subsequent analysis yielded average carbonate contents of 21.8±4.7% wt CaCO(3). Isotopic analysis demonstrated δ(18)O values between -9.4‰ and -13.3‰ and δ(13)C values between -7.4‰ and -13.6‰ (relative to Pee Dee Belemnite), suggesting that up to 39.4±8.8% of the carbonate C has been captured from the atmosphere through hydroxylation of dissolved CO(2) in high pH solutions. The remaining carbonate C is derived from lithogenic sources. 37.4 kg of atmospheric CO(2) has already been captured and stored as carbonate per Mg of soil across the site, representing a carbon dioxide (CO(2)) removal rate of 12.5 kg CO(2) Mg(-1) yr(-1). There is the potential for capture and storage of a further 27.3 kg CO(2) Mg(-1) in residual reactive materials, which may be exploited through increased residence time (additional in situ weathering). Overall, the Science Central site has the potential to capture and store a total of 64,800 Mg CO(2) as carbonate minerals. This study illustrates the potential for managing urban soils as tools of C capture and storage, an important ecosystem service, and demonstrates the importance of studying C storage in engineering urban anthropogenic soils.

Contaminant mobility and carbon sequestration downstream of the Ajka (Hungary) red mud spill: The effects of gypsum dosing

A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assessed the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis and stable isotope analysis. There was a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing were used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process was particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO(2). The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3-4% of the direct CO(2) emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.

Silicate production and availability for mineral carbonation

Atmospheric carbon dioxide sequestered as carbonates through the accelerated weathering of silicate minerals is proposed as a climate change mitigation technology with the potential to capture billions of tonnes of carbon per year. Although these materials can be mined expressly for carbonation, they are also produced by human activities (cement, iron and steel making, coal combustion, etc.). Despite their potential, there is poor global accounting of silicates produced in this way. This paper presents production estimates (by proxy) of various silicate materials including aggregate and mine waste, cement kiln dust, construction and demolition waste, iron and steel slag, and fuel ash. Approximately 7-17 billion tonnes are produced globally each year with an approximate annual sequestration potential of 190-332 million tonnes C. These estimates provide justification for additional research to accurately quantify the contemporary production of silicate minerals and to determine the location and carbon capture potential of historic material accumulations.

Applications of stable isotope ratio mass spectrometry in cattle dung carbon cycling studies

Understanding the fate of dung carbon (C) in soils is challenging due to the ubiquitous presence of the plant-derived organic matter (OM), the source material from which both dung-derived OM and soil organic matter (SOM) predominantly originate. A better understanding of the fate of specific components of this substantial source of OM, and thereby its contribution to C cycling in terrestrial ecosystems, can only be achieved through the use of labelled dung treatments. In this short review, we consider analytical approaches using bulk and compound-specific stable carbon isotope analysis that have been utilised to explore the fate of dung-derived C in soils. Bulk stable carbon isotope analyses are now used routinely to explore OM matter cycling in soils, and have shown that up to 20% of applied dung C may be incorporated into the surface soil horizons several weeks after application, with up to 8% remaining in the soil profile after one year. However, whole soil delta(13)C values represent the average of a wide range of organic components with varying delta(13)C values and mean residence times in soils. Several stable (13)C isotope ratio mass spectrometric methods have been developed to qualify and quantify different fractions of OM in soils and other complex matrices. In particular, thermogravimetry-differential scanning calorimetry-isotope ratio mass spectrometry (TG-DSC-IRMS) and gas chromatography-combustion-IRMS (GC-C-IRMS) analyses have been applied to determine the incorporation and turnover of polymeric plant cell wall materials from C(4) dung into C(3) grassland soils using natural abundance (13)C isotope labelling. Both approaches showed that fluxes of C derived from polysaccharides, i.e. as cellulose or monosaccharide components, were more similar to the behaviour of bulk dung C in soil than lignin. However, lignin and its 4-hydroxypropanoid monomers were unexpectedly dynamic in soil. These findings provide further evidence for emerging themes in biogeochemical investigations of soil OM dynamics that challenge perceived concepts of recalcitrance of C pools in soils, which may have profound implications for the assessment of the potential of agricultural soils to influence terrestrial C sinks.

An evaluation of the reactivity of synthetic and natural apatites in the presence of aqueous metals

Metal removal from contaminated effluents was examined following reaction with natural apatites of biological and geological origin or a synthetic hydroxylapatite (HAP). Mammalian meat and bone meal (MBM), a by-product from meat industry, was the biological apatite source. The effect of incineration on metal removal capacity of MBM and HAP was also examined. The reactivity of apatites for all tested metals (Pb, Cd, Cu and Zn) followed the general order: synthetic > biological > mineral. For all apatites tested, Pb was removed best and preferentially from multi-metal solutions. MBM and HAP (0.5 g solid) removed Pb completely from both highly concentrated single metal solutions (50 ml, 1000 mg/L Pb) and from multi-metal solutions (50 ml) with 100 mg/L each of Cd, Cu and Zn in addition to Pb. The incineration of MBM (725 degrees C and 850 degrees C) reduced significantly its capacity for removal of Zn (by 47%, from 56 mg/g to 9 mg/g) and Cd (by 38%, from 53 mg/g to 13 mg/g) in particular and to a lesser extent for Cu (by 14%, from 61 mg/g to 46 mg/g) while the removal of Pb was not affected (100 mg/g). The same pattern was observed for incinerated HAP. SEM and XRD analysis indicated that HAP reacted with the metals by precipitation of pure metal phosphates--Pb hydroxylapatite, Zn phosphate (hopeite), a Cd phosphate (identified only by ED-SEM) and Cu phosphate (libenthenite).

Where does all the helium that we use come from?

Helium occurs naturally in association with geological fluids and is regarded by economists as a mineral commodity. Commercially, helium is obtained as a by-product from natural gas reservoirs that typically have concentrations of up to 1% He by volume (STP). Existing sources of helium within the USA are expected to decline in parallel with declining natural gas production. Huge reserves of helium occur in Algeria, Qatar and Russia, which are likely to dominate production for the next several decades. In this context, political and economic factors that affect hydrocarbon gas production may influence the availability and price of helium.

Influence of recent vegetation on labile and recalcitrant carbon soil pools in central Queensland, Australia: evidence from thermal analysis-quadrupole mass spectrometry-isotope ratio mass spectrometry

The effect of a recent vegetation change (<100 years) from C(4) grassland to C(3) woodland in central Queensland, Australia, on soil organic matter (SOM) composition and SOM dynamics has been investigated using a novel coupled thermogravimetry-differential scanning calorimetry-quadrupole.mass spectrometry-isotope ratio mass spectrometry (TG-DSC-QMS-IRMS) system. TG-DSC-QMS-IRMS distinguishes the C isotope composition of discrete SOM pools, showing changes in labile, recalcitrant and refractory carbon in the bulk soil and particle size fractions which track the vegetation changes. Analysis of evolved gases (by QMS) from thermal decomposition, rather than observed weight loss, proved essential in determining the temperature at which SOM decomposes, because smectite and kaolinite clays contribute to observed weight losses. The delta(13)C analyses of the CO(2) evolved at different temperatures for bulk soil and particle size-separates showed that most of the labile SOM under the more recent woody vegetation was C(3)-derived carbon whereas the delta(13)C values in the recalcitrant SOM showed greater C(4) contributions. This indicated a shift from grass (C(4))- to tree (C(3))-derived carbon in the woodland, which was also supported by the two-phase (13)C enrichment with depth, i.e. C(3) vegetation dominated the top soil (0-10 cm), but the C(4) contribution increased with depth (more gradual). This is perturbed by the inclusion of charcoal from forest fires ((14)C age incursions) and by the deep incorporation of C(3) carbon due to root penetration.

Carbon isotope determination for separate components of heterogeneous materials using coupled thermogravimetric analysis/isotope ratio mass spectrometry

A gas-tight thermal analysis system (Netzsch STA 449C Jupiter) has been connected to an isotope ratio mass spectrometer (PDZ Europa 20-20) via an interface containing an oxidizing furnace, water trap, and gas-sampling valve. Using this system, delta(13)C has been measured for CO(2) derived from the thermal decomposition of carbonate and oxalate minerals and organic materials at temperatures that correspond to different decomposition events. There is close agreement between measured and published delta(13)C values for carbonate and oxalate minerals, which have simple decarbonation reactions on heating. Cellulose and lignin-rich materials show much more complex thermal decomposition, reflecting differences in their purity and structure, and measured delta(13)C values vary with the temperature of gas sampling. Provided that measurements are made at temperatures that correspond to the decomposition of cellulose and lignin (indicated by maximum weight loss), internally consistent data can be obtained. However, measurements for cellulose and lignin are systematically enriched in delta(13)C (by up to 1.4 per thousand) with respect to those reported for reference materials, possibly due to the slower combustion kinetics (compared with EA-IRMS). Thermogravimetric analysis/isotope ratio mass spectrometry (TG-IRMS) is ideal for materials and samples for which it is not possible to use other isotopic measurement techniques, for example because of sample heterogeneity.

Assessment of the ecological potential of mine-water treatment wetlands using a baseline survey of macroinvertebrate communities

A baseline survey of macroinvertebrate populations in two mine-water treatment wetlands, one treating a net acidic spoil heap discharge and one a net alkaline ferruginous pumped mine water, was undertaken to assess the potential of these systems to provide habitats for faunal communities. Both wetlands were found to be impoverished in comparison to natural wetlands but did sustain a macroinvertebrate community that could support higher organisms. Wetland size and water quality in terms of pH, conductivity and metal concentrations were found to be important factors in determining the quality of the populations supported. Direct toxicity to organisms was unlikely to be the main cause of lower diversity, but the smothering of organisms via the precipitation of iron hydroxides particularly in the early parts of the treatment systems affected macroinvertebrate communities. The presence of areas of open water within the planted systems was found to be important for providing habitats for macroinvertebrates and this should be both a future design and maintenance consideration for environmental managers.

Characterization of the sorption of an anthranilate fungicide in soil using thermal analytical and mineralogical techniques

The sorption of pesticides to soil particles has implications for their distribution and fate in the soil environment. A batch equilibrium technique was used to investigate sorption of the fungicide AEC623892 to intact and hydrogen-peroxide-treated whole soils and several particle-size fractions isolated from them. Sorption of AEC623892 to the soil as a whole was low. K(f oc) values measured in the whole soils were 169.2 and 41.9 ml g(-1) for Soil A and Soil B respectively. The highest values of K(f oc) were measured in soil particle-size fractions <53 microm (266.5 ml g(-1) in the 2-20 microm fraction of Soil A; 471.9 ml g(-1) in the 20-53 microm fraction of Soil B). Sorption was most irreversible in the 2-20 microm fractions. Overall, treatment of soil particle-size fractions with hydrogen peroxide resulted in lower values of K(f oc) (112.3 ml g(-1) in Soil A whole soil and 30.9 ml g(-1) in Soil B whole soil). In both soils, the maximum sorption among hydrogen-peroxide-treated samples was observed in the <2 microm fraction (166.6 and 311.0 ml g(-1) for Soil A and Soil B, respectively). Investigation of the mineralogical composition of the soils suggested that the clay mineralogy (dominated by kaolinite and illite) is less likely to account for the differences in sorption observed than differences in the characteristics of the soil organic matter. Thermal analysis of the different soil fractions indicated that hydrogen peroxide treatment preferentially removed aliphatic fractions of organic matter, but had less effect on lignin-like, aromatic fractions.

Application of simultaneous thermal analysis mass spectrometry and stable carbon isotope analysis in a carbon sequestration study

The simultaneous analysis of evolved gases and the determination of stable isotope composition (delta13C) as part of a thermal analysis experiment have been used to (a) distinguish bulk chemical hosts for carbon (C) and nitrogen (N) within a soil and (b) track labelled C within a soil sequestration experiment. C3 and C4 dung was applied to a pasture soil, and soil samples taken for analysis. The results of thermogravimetry-differential scanning calorimetry-quadrupole mass spectrometry-isotope ratio mass spectrometry (TG-DSC-QMS-IRMS) show that the proportion of more refractory C (lignin-like) is greater for the dungs than for the soil organic matter (SOM), and that this increases with time within the soil. Analysis of evolved gases shows that nitrogen is associated with the decomposition of more refractory C, and is not so strongly associated with the labile C component. IRMS analysis distinguished C3 and C4 dung, and allowed the amount of C from these sources to be estimated for the soil samples. Most dung C enters the refractory SOM fraction. This paper demonstrates the potential of TG-DSC-QMS-IRMS in the investigation of SOM.