Potential CO2 intrusion in near-surface environments: a review of current research approaches to geochemical processes

A comprehensive review of remediation strategies for mitigating salt precipitation and enhancing CO2 injectivity during CO2 injection into saline aquifers

Geological CO2 sequestration is a proven method for mitigating climate change by reducing atmospheric CO2 levels. However, CO2 injection often induces salt precipitation, leading to decreased formation permeability, which in turn limits CO2 injectivity and storage capacity. Conventional approaches, such as freshwater and low-salinity water injection, have been employed to mitigate salt precipitation. Despite their widespread use, these methods provide only temporary improvement and can be ineffective in some scenarios, resulting in long-term issues such as salt recrystallization and clay swelling. Given the complexity and significance of this issue, a comprehensive review of salt precipitation mechanisms and remediation techniques is essential. This paper critically examines the processes of salt precipitation during CO2 injection in saline aquifers and evaluates various remediation techniques aimed at improving CO2 injectivity. The paper reviews the influence of CO2 flow dynamics, geochemical reactions, and fluid properties on salt precipitation and pore throat accumulation, assessing the efficacy and limitations of existing mitigation methods. Additionally, the paper explores alternative techniques with potential for long-term CO2 sequestration, analyzing their advantages and drawbacks. Based on insights from the reviewed sources, the paper recommends exploring alternative treatment measures and the integration of hybrid solutions to enhance CO2 injectivity. The findings presented serve as a valuable reference for advancing research and practice in this critical area, offering a deeper understanding of the challenges and potential solutions for effective CO2 sequestration in saline aquifers.

Sustaining ex-mining lake-converted constructed wetlands as nature-based solutions: A comprehensive assessment of the carbon-water nexus in Paya Indah Wetlands, Malaysia

Water and carbon, essential for Earth's well-being, face imminent threats from human activities that fuel climate change. This study investigates nature-based solutions, focusing on the carbon-water nexus of ex-mining lake-converted constructed wetlands, specifically in Malaysia's Paya Indah Wetlands (PIW). Addressing research gaps, it assesses the ecosystem services of these wetlands, emphasising integrated evaluations for informed land management and employing a top-down conservation approach. Methodologically, spatial assessments, soil and water sampling, carbon quantification, water quality index calculations, land cover classification and stakeholder surveys were conducted. Results underscore the significant carbon sequestration and water quality improvement potential of constructed wetlands, with soil and sediment carbon accumulation reaching 1553.11 Mg C ha-1 (equivalent to 5700 Mg CO2 ha-1), translating to an annual sequestration capacity of 67.5 Mg C ha-1 year-1. Water quality index values ranged from 58 to 81 (Classes II to III). PIW's establishment led to a reduction of over 90% in barren land, with increases in water bodies (36%) and vegetation-covered land (38%), boosting wildlife populations by 30%. Spatial variations in organic carbon density and water quality underscore the complexity of the carbon-water nexus and its impacts on ecosystem health and water security. Despite land use changes, PIW demonstrates resilience, contributing to climate change mitigation. Stakeholder perceptions vary, emphasising the need for adaptive strategies. The study proposes transdisciplinary conservation initiatives and adaptive plans, stressing the pivotal role of ex-mining lake-converted constructed wetlands in enhancing climate resilience.

Synthesis and characterisation of K2CO3-activated carbon produced from distilled spent grains for the adsorption of CO2

Activated carbon was prepared from distilled spent grains (DSG) using K2CO3 activation and chitosan modification. The effects of activator dosage, activation temperature, and the incorporation of chitosan as a nitrogen source on the adsorption performance were studied in this paper. The activated carbons were characterised by scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen and carbon dioxide gas adsorption. Under the optimal conditions, the BET-specific surface area, total pore volume, and microporous volume of the activated carbon were as high as 1142 m2/g, 0.62 cm3/g, and 0.40 cm3/g, respectively. Chitosan was used as the nitrogen source, and surface modification was carried out concurrently with the K2CO3 activation process. The results revealed a carbon dioxide adsorption capacity of 5.2 mmol/g at 273.15 K and 1 bar without a nitrogen source, which increased to 5.76 mmol/g after chitosan modification. The isosteric heat of adsorption of CO2 all exceed 20 kJ/mol, hinting at the coexistence of both physisorption and chemisorption. The adsorption behaviour of the DSG-based activated carbon can be well-described by the Freundlich model.

The impacts of climate change on groundwater quality: A review

Groundwater has been known as the second largest freshwater storage in the world, following surface water. Over the years, groundwater has already been under overwhelming pressure to satisfy human needs for artificial activities around the world. Meanwhile, the most noticeable footprint of human activities is the impact of climate change. Climate change has the potential to change the physical and chemical properties of groundwater, thereby affecting its ecological functions. This study summarizes existing research affiliated with the possible effects of a changing climate on the quality of groundwater, including changes in water availability, increased salinity and pollution from extreme weather events, and the potentiality of seawater intrusion into coastal aquifers. Previous works dealing with groundwater-induced responses to the climate system and climate impacts on groundwater quality through natural and anthropogenic processes have been reviewed. The climate-induced changes in groundwater quality including pH, dissolved oxygen level, salinity, and concentrations of organic and inorganic compounds were assessed. Some future research directions are proposed, including exploring the potential changes in the occurrences and fate of micropollutants in groundwater, examining the relationship between the increase of microcystin in groundwater and climate change, studying the changes in the stability of metals and metal complexation, and completing studies across different regional climate regions.

The integration of nature values and services in the nature-based solution assessment framework of constructed wetlands for carbon-water nexus in carbon sequestration and water security

As the climate change impacts are expected to become increasingly disruptive, affecting water security, environmental health and ecosystem, constructed wetlands receive attention for their functions in delivering various life-sustaining services to human and environmental systems. In this article, a systematic review was conducted through the Preferred Reporting Items for Systematic Reviews and Meta-Analyses standard to identify the current research on constructed wetlands' nature values and services from 2011 to 2020 of two databases, namely Scopus and Web of Science. The criteria of assessment focus on holistic deliberation of subject matters, namely carbon sequestration and water security as regulating and provisioning services, as well as nature values of constructed wetlands, namely instrumental and intrinsic values. As a result, 38 articles were selected and comprehensively examined. As the lack of an interdisciplinary approach makes data and information integration difficult, this study derived an integrated classification of constructed wetlands' services and mapped with its nature values, guided by the Millennium Ecosystem Assessment framework. Besides, mechanisms and factors affecting carbon sequestration and water security were also discussed. The carbon-water nexus was then conceptualised as interlinkages between engineered and natural physicochemical processes at the interface between carbon and water cycles. To fill the gaps, based on the carbon-water nexus concept, a new framework was synthesised at the end of the deliberation for constructed wetlands in regulating local climate through carbon sequestration and ensuring water security through water treatment and purification as well as influencing socio-cultural values, which needs an integrated approach that is the novelty of this work. The framework integrates the dichotomy of the instrumental-intrinsic nature values of constructed wetlands to evaluate the importance and benefit of the carbon-water nexus. The framework that reveals the vitality of nature values provided by constructed wetlands can help improve the decision-making to prioritise ecosystem services and conservation efforts, particularly in the sustainable management of constructed wetlands.

Assessment of geochemical modeling applications and research hot spots-a year in review

Geochemical modeling has been employed in several fields of science and engineering in recent years. This review seeks to provide an overview of case studies that applied geochemical modeling in the 2019 year, which includes over 250 articles. This review is intended to inform new users on the possibilities that geochemical modeling brings, while also informing existing and past users on its latest developments. The survey of studies was conducted with an emphasis on the modeling techniques, the objective of studies, the prevalent simulated variables and the use of specific software packages. The analysis showed that geochemical modeling is still predominantly employed in experimental projects and in the form of equilibrium modeling. PHREEQC and Visual MINTEQ were recognized as the most popular software packages for simulating a wide range of processes, using equilibrium or other geochemical modeling forms. The study of fluid-rock interactions and pollution and remediation processes can be regarded as the principal geochemical modeling objectives, constituting 37% and 36% of the reviewed studies, respectively. Focusing on fluid-rock interactions, hydrogeochemical processes, carbon capture and storage and enhanced oil recovery have been the main topics examined with geochemical modeling. Assessments of the toxicity of metals in terms of leachate and mobilization, as well as their removal from soil and water systems, have been major topics investigated with the aid of geochemical modeling in terms of pollution and remediation research. It was found that the scholars benefit from geochemical modeling in their research both as a main technique and as an accessory tool. Saturation index, elemental concentration and speciation, mineral mass and composition and pH were among the most common variables modeled in reviewed studies. Geochemical modeling has gained a wider user base in recent years, and many research groups have used it in consecutive studies to deepen knowledge. However, much potential for further dissemination still remains.

A worksheet-based tool to implement reactive transport models in COMSOL Multiphysics

The increasing needs for modelling of reactive transport phenomena in different areas of environmental modelling have led to the development of many numerical codes. However, many of them suffer from a lack of flexibility, which hinders the adaptation of the codes to new problems. Moreover, in many cases, changes can be done by a very reduced group of people, and often by a single person, the main developer. Implementation platforms based on multiphysics modelling removes these barriers, although until now within that programming environments has been only possible the coupling of geochemical codes to transport equations using operator splitting techniques. This paper presents the EE4MGM tool, a MS Excel worksheet, provided in supplementary material, for the edition and complete implementation of reactive transport models in COMSOL. The tool automatically generates the code needed to solve the desired reactive transport problem by selecting only which species make up the geochemical system. This way, the numerical model will be completely adapted to the idealisation to be applied, being able to choose easily and effortlessly from a wide range of different levels of conceptual complexity. The organization of data input and the equation libraries obtained for the implementation in the multiphysics COMSOL environment are first described. Afterwards, two examples, in one and two-dimensional domains, to check the utility of the tool are presented.

Quantitative assessment of deep-seated CO2 leakage around CO2-rich springs with low soil CO2 efflux using end-member mixing analysis and carbon isotopes

This study examined a mountainous area with two hydrochemically distinct CO₂-rich springs to understand the origin, flow, and leakage of CO₂, which may provide implications for precise monitoring of CO₂ leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO₂-rich water (δ¹³C_DIC) and those of soil CO₂ (δ¹³C_CO2) indicated a deep-seated CO₂ supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO₂-rich springs separated by 7 m, despite similar δ¹³C_DIC and partial pressure of CO₂, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO₂ preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO₂-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO₂ efflux was low compared to that in other natural CO₂ emission sites, and in particular it was noticeably low near the CO₂-rich springs, whereas δ¹³C_CO2 was high close the CO₂-rich springs. The dissolution of CO₂ in the near-surface water body seemed to decrease the deep-seated CO₂ leakage through the soil layer, while δ¹³C_CO2 imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO₂ to the low soil CO₂ efflux by assuming that atmospheric CO₂ and soil CO₂ (by respiration) as well as deep-seated CO₂ contribute to the soil CO₂ efflux. For each end-member, characteristic δ¹³C_CO2 and CO₂ concentrations were defined, and then their apportionment to soil CO₂ efflux was estimated. The resultant proportion of deep-seated CO₂ was up to 8.8%. Unlike the spatial distribution of high soil CO₂ efflux, high proportions exceeding 3% were found around the CO₂-rich springs along the east-west valley. The study results indicate that soil CO₂ efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO₂ leakage detection because CO₂ dissolution in the underground water body may blur leakage detection on the surface. The implication of this study is the need to quantitatively assess the contribution of deep-seated CO₂ using the soil CO₂ concentration, soil CO₂ efflux, and δ¹³C_CO2 at each measurement site.

CO2 capture through electro-conductive adsorbent using physical adsorption system for sustainable development

The most critical energy and environmental challenge that our planet is facing today is to minimize the dependence on fossil fuels. Carbon dioxide may be of utmost significance as a solution of this issue through realization of carbon neutral energy cycle. Potentially, this could be achieved through the carbon dioxide capture as the urgent response to ongoing climate change. Activated carbon (AC) adsorption is one the most effective, environment friendly and techno-economic process for the carbon capture. In the current research, an electro-conductive-activated carbon was prepared by mixing powdered activated carbon (PAC) with an electro-conductive polymer (ECP). Different ratios of 0, 25, 50, 75 and 100 wt% of ECP with PAC were used for the different analyses of activated carbons in a gas mixture of CO₂/N₂ using a physical adsorption system. Adsorption and desorption analyses, capacities of the process and desorption effects were examined. Electro-conductive polymers (ECP) were mixed with AC samples, where breakthrough time was increased up to 400% when mixed with the PAC for CO₂ adsorption. Following adsorption analysis, desorption of activated carbons was conducted with different potentials. It was revealed that mixing could help the PAC sample to overcome the packing issue to increase the breakthrough capacity and the volumes before and after the breakthrough adsorption in the packed bed systems. The desorption rates of the PAC sample were also enhanced, and fast desorption was observed when mixed with ECP. It is envisioned that this method is very much promising carbon capture method for the techno-economic feasibility and sustainable development of the environment.

Leakage of CO2 from geological storage and its impacts on fresh soil-water systems: a review

Leakage of CO₂ from the geological storage is a serious issue for the sustainability of the receiving fresh soil-water systems. Subsurface water quality issues are no longer related to one type of pollution in many regions around the globe. Thus, an effort has been made to review studies performed to investigate supercritical CO₂ (scCO₂) and CO₂ enrich brine migration and it's leakage from geological storage formations. Further, the study also reviewed it's impacts on fresh soil-water systems, soil microbes, and vegetation. The first part of the study discussed scCO₂/CO₂ enrich brine migration and its leakage from storage formations along with it's impact on pore dynamics of hydrological regimes. Later, a state-of-the-art literature survey has been performed to understand the role of CO₂-brine leakage on groundwater dynamics and its quality along with soil microbes and plants. It is observed in the literature survey that most of the studies on CO₂-brine migration in storage formations reported significant CO₂-brine leakage due to over-pressurization through wells (injections and abandoned), fracture, and faults during CO₂ injection. Thus, changes in the groundwater flow and water table dynamics can be the first impact of the CO₂-brine leakage. Subsequently, three major alterations may also occur-(i) drop in pH of subsurface water, (ii) enhancement of organic compounds, and (iii) mobilization of metals and metalloids. Geochemical alteration depends on the amount of CO₂ leaked and interactions with host rocks. Therefore, such alteration may significantly affect soil microbial dynamics and vegetation in and around CO₂ leakage sites_. In-depth analysis of the available literature fortifies that a proper subsurface characterization along with the bio-geochemical analysis is extremely important and should be mandatory to predict the more accurate risk of CO₂ capture and storage activities on soil-water systems.