Trends in research on characterization, treatment and valorization of hazardous red mud: A systematic review.

Meta-analysis of red mud-related literature in English published from 1976 to 2022 and in Chinese from 1990 to 2022 was performed to support critical analysis and evaluation of the available literature based on the following aspects of red mud research: (a) characterization, (b) treatment for harmfulness minimization, (c) recovery of valuable metals, (d) environmental applications, and (e) uses as construction materials. It was found that (a) sinter red mud tended to contain more silica and calcium, and less iron, sodium and aluminium compared to Bayer red mud; (b) gypsum was the most frequently used agent for harmfulness reduction treatment of red mud, followed by flue gas/CO2; (c) the mean optimal pH for adsorption of major anionic pollutants was 8.42 ± 1.13 (arsenite), 3.73 ± 0.68 (arsenate), 3.50 ± 2.38 (phosphate), 4.43 ± 1.04 (fluoride) and 3.80 ± 1.54 (chromate); (d) wastewater treatment has attracted more attention compared to contaminated soils and waste gases; (e) recovery of iron and scandium has attracted more attention compared to other metals; (f) cement making has been the focus in construction uses. Most of the research findings were based on laboratory-scale experiments that focused on efficacy rather than efficiency. There was a lack of integrated approaches for research in red mud valorization.

Strategies for cost-effective remediation of widespread oil-contaminated soils in Kuwait, an environmental legacy of the first Gulf War.

The Kuwaiti oil fire during the first Gulf War resulted in the formation of approximately 300 "oil lakes" of varying sizes that covered over 110 km2 of the desert land. This threatens the fragile desert ecosystems and human health. Following the award of over US$2 billion to the State of Kuwait by the United Nations, large-scale remediation of the oil-contaminated soils has now been on the agenda. However, how to implement the remediation program in a cost-effective way represents a major challenge. In this study, cost-effective remediation strategies were developed based on field and laboratory investigations in a typical oil lake area. Overall, most of the lighter petroleum hydrocarbons (PHCs) were lost due to evaporation. Long-chain aliphatic PHCs dominated the PHCs in the investigated oil lake area. This has implications for developing remediation strategies. Toxicity assessment results showed that the majority of soils pose a low environmental risk with a hazard index <1. Therefore, intensive treatment of these PHCs may not be necessary for these soils. Although active treatment methods are needed to remove the contaminants as soon as practical for the relatively small areas of high contamination, more cost-effective passive methods should be considered to minimize the remedial costs for the larger area of the non-hotspot areas. Given the extremely low risk in terms of groundwater contamination by the contaminated soils, it may not be necessary to remove the soils from the contaminated sites. A low-cost capping method should be sufficient to minimize human exposure to the PHC-contaminated soils.

Factors affecting N2O fluxes from heavy metal-contaminated mangrove soils in a subtropical estuary.

A 1-year field monitoring program was carried out to observe seasonal variation in N2O fluxes at two typical mangrove wetlands in a subtropical estuary. The soils in the island-type mangrove wetland had a higher level of heavy metal(loid) contamination and a lower level of salinity compared to the small bay-type mangrove wetland. While there was a high level of similarity in the seasonal variation pattern of N2O fluxes between the two investigated sites with both being significantly higher in summer than in other seasons, the average of N2O fluxes in the island-type mangrove wetland was 7.19 µg·m-2·h-1， which tended to be lower compared to the small bay-type mangrove wetland (15.63 µg·m-2·h-1). Overall, N2O flux was closely related to soil-borne heavy metal(loid)s, showing a trend to decrease with increasing concentration of these heavy metal(loid)s. The N2O fluxes increased with decreasing abundance of either denitrifiers or nitrifiers. But the opposite was observed for the anammox bacteria present in the soils. The anammox bacteria were more sensitive to heavy metal(loid) stress but more tolerated high salinity encountered in the investigated soils compared to the denitrifiers or nitrifiers. It appears that anammox reactions mediated by anammox bacteria played a key role in affecting the spatial variation in N2O fluxes from the mangrove soils in the study area. And an increased level of ammonium in soils tended to promote the activity of anammox bacteria and consequently enhanced N2O emission from the mangrove soils.

Optimizing carbon emission forecast for modelling China's 2030 provincial carbon emission quota allocation.

Rational allocation of carbon quotas is the fundamental premise for the orderly operation of carbon markets. To achieve the set target of carbon peak by 2030, there is an urgent need to establish China's 2030 provincial carbon quota allocation scheme. Although some proposed schemes have been formulated, there are problems with the methods used for carbon emission forecasting and evaluating the rationality of a proposed allocation scheme. This study aimed to optimize carbon emission forecast by incorporating terrestrial carbon sinks into the mechanism for building China's 2030 provincial carbon emission quota allocation schemes. Aquila Optimizer's Double Support Vector Regression (AO-based TWSVR) that has the advantages in solving problems associated with small sample size, nonlinear and high-dimensional pattern recognition with fast training speed and insensitivity to noise was adopted to predict the net carbon emission. The results show that the application of AO-based TWSVR model allows satisfactory forecast of the net carbon emission in China for the period from 2021 to 2035. This allowed terrestrial carbon sequestration being incorporated into the mechanism to formulate China's 2030 provincial carbon quota allocation schemes. Comparison of the three provincial carbon quota allocation schemes using social network analysis suggests that the equity-based carbon quota allocation scheme is more suitable for China's national conditions compared to the efficiency-based scheme and the combined principle-based scheme. The findings obtained from this study have implications for optimizing the scheme of China's 2030 provincial carbon quota allocation.

Trend in Research on Characterization, Environmental Impacts and Treatment of Oily Sludge: A Systematic Review.

Oily sludge is a hazardous material generated from the petroleum industry that has attracted increasing research interest. Although several review articles have dealt with specific subtopics focusing on the treatment of oily sludge based on selected references, no attempt has been made to demonstrate the research trend of oily sludge comprehensively and quantitatively. This study conducted a systematic review to analyze and evaluate all oily sludge-related journal articles retrieved from the Web of Science database. The results show that an increase in oily sludge-related research did not take place until recent years and the distribution of the researchers is geographically out of balance. Most oily sludge-related articles focused on treatment for harmfulness reduction or valorization with limited coverage of formation, characterization, and environmental impact assessment of oily sludge. Pyrolytic treatment has attracted increasing research attention in recent years. So far, the research findings have been largely based on laboratory-scale experiments with insufficient consideration of the cost-effectiveness of the proposed treatment methods. Although many methods have been proposed, few alone could satisfactorily achieve cost-effective treatment goals. To enable sustainable management of oily sludge on a global scale, efforts need to be made to fund more research projects, especially in the major oil-producing countries. Pilot-scale experiments using readily available and affordable materials should be encouraged for practical purposes. This will allow a sensible cost-benefit analysis of a proposed method/procedure for oily sludge treatment. To improve the treatment performance, combined methods are more desirable. To inform the smart selection of methods for the treatment of different oily sludge types, it is suggested to develop universally accepted evaluation systems for characterization and environmental risk of oily sludge.

Managing soils of environmental significance: A critical review.

Globally, environmentally significant soils (ESSs) mainly include acid sulfate, heavy metal(loid)-contaminated, petroleum hydrocarbon-contaminated, pesticide-contaminated, and radionuclide-contaminated soils. These soils are interrelated and have many common characteristics from an environmental management perspective. In this review, we critically evaluate the available literature on individual ESSs, aiming to identify common problems related to environmental quality/risk assessment, remediation approaches, and environmental regulation for these soils. Based on these findings, we highlight the challenges to, and possible solutions for sustainable ESS management. Contaminated land has been rapidly expanding since the first industrial revolution from the industrialized Western countries to the emerging industrialized Asia and other parts of the world. Clean-up of contaminated lands and slowdown of their expansion require concerted international efforts to develop advanced cleaner production and cost-effective soil remediation technologies in addition to improvement of environmental legislation, regulatory enforcement, financial instruments, and stakeholder involvement to create enabling environments. Two particular areas require further action and research efforts: developing a universal system for assessing ESS quality and improving the cost-effectiveness of remediation technologies. We propose an integrated framework for deriving ESS quality indicators and make suggestions for future research directions to improve the performance of soil remediation technologies.

Arsenic in leafy vegetable plants grown on mine water-contaminated soils: Uptake, human health risk and remedial effects of biochar.

Field investigation and microcosm experiment were conducted to examine the uptake of arsenic by vegetable plants grown on the soils contaminated by acidic mine water and evaluate the human health risk from consuming the vegetables. Plant uptake of arsenic was related to the ratio of phosphorus to arsenic in soil solution for the same vegetable species. Bioaccumulation coefficient (BAC) of arsenic was highly variable amongst the different vegetable species with water spinach (white stem) and sweet potato leaf being identified as major vegetable species with high BAC. There was a reasonably good relationship between the gastric phase-bioaccessible arsenic and the gastrointestinal phase-bioaccessible arsenic. Consumption of the vegetables grown in the investigated area poses a significantly potential human health risk with a hazard quotient (HQ) of 2.7. Application of biochar significantly inhibited the uptake of arsenic by the vegetable plant due to protonation of biochar surfaces under acidic conditions, which favoured adsorption of arsenic. The bioaccessibity of arsenic in the edible part of vegetable was also reduced due to biochar application. The HQ of the test vegetable plant (Gynura cusimbua) after soil amendment by biochar was reduced to 2 from 6 for the unamended soil.

Heavy metal pollution increases CH4 and decreases CO2 emissions due to soil microbial changes in a mangrove wetland: Microcosm experiment and field examination.

Mangrove plays an important role in modulating global warming through substantial blue carbon storage relative to their greenhouse gas emission potential. The presence of heavy metals in mangrove wetlands can influence soil microbial communities with implications for decomposition of soil organic matter and emission of greenhouse gases. In this study, field monitoring and a microcosm experiment were conducted to examine the impacts of heavy metal pollution on soil microbial communities and greenhouse gas fluxes. The results show that heavy metal pollution decreased the richness and diversity of the overall soil microbial functional groups (heterotrophs and lithotrophs); however, it did not inhibit the activities of the methanogenic communities, possibly due to their stronger tolerance to heavy metal toxicity compared to the broader soil microbial communities. Consequently, the presence of heavy metals in the mangrove soils significantly increased the emission of CH4 while the emission of CO2 as a proxy of soil microbial respiration was decreased. The soil organic carbon content could also buffer the effect of heavy metal pollution and influence CO2 emissions due to reduced toxicity to microbes. The findings have implications for understanding the complication of greenhouse gas emissions by heavy metal pollution in mangrove wetlands.

Effect of soluble calcium on enhancing nitrate retention by biochar.

Batch experiments were conducted to test the hypothesis that nitrate (NO3-) could be immobilized by biochar via adsorption of CaNO3+ to the negatively charged biochar surfaces. The results show that addition of soluble Ca in both aqueous and soil systems enabled NO3- retention by the biochar material. Increase in the added Ca enhanced the retention rate and the optimal NO3- retention was gained at a Ca/NO3 molar ratio of 2 for the aqueous system. For the soil system, the Ca/NO3 molar ratio required to attain the optimal NO3- retention was much greater due to competition of other soil-borne ligands and soil colloids for the available Ca. At the same level of added Ca, the amount of NO3- being retained tended to increase with increasing dose of the biochar. More NO3- was retained in the soil system than in the aqueous system at the same dosage level of biochar due to additional adsorption of CaNO3+ by negatively changed soil organic and inorganic colloids. The findings obtained from this study have implications for developing effective methods for reducing NO3- leaching from soils.

Biochar-driven reduction of As(V) and Cr(VI): Effects of pyrolysis temperature and low-molecular-weight organic acids.

Batch experiments were conducted to examine the differential effects of biochar pyrolysis temperature and low-molecular-weight organic acids on the reduction of As(V) and Cr(VI) driven by Pennisetum hydridum biochar. The results showed that pyrolysis temperature significantly affected the reducing strength of the biochar. Biochar produced at 500 °C had a stronger electron-donating capacity than did the biochars produced at 300 and 700 °C. In the co-presence of the biochar and a low-molecular-weight organic acid, arsenic and chromium behaved differently. Oxalic acid and malic acid tended to have better effects on enhancing biochar-driven Cr(VI) reduction, as compared to citric acid while the opposite was observed for biochar-driven As(V) reduction. Biochar produced at 300 °C was more favourable for Cr(VI) reduction, as compared to the higher-temperature biochars while the opposite was observed for As(V) reduction in the presence of low-molecular-weight organic acids. This may make the lower-temperature biochar ideal for remediating contaminated soils containing both As(V) and Cr(VI) since it could maximize Cr(VI) reduction while minimizing As(V) reduction.

Tungsten- and cobalt-dominated heavy metal contamination of mangrove sediments in Shenzhen, China.

A baseline investigation into heavy metal status in the mangrove sediments was conducted in Shenzhen, China where rapid urban development has caused severe environmental contamination. It is found that heavy metal contamination in this mangrove wetland is characterized by the dominant presence of tungsten and cobalt, which is markedly different from the neighboring Hong Kong and other parts of the world. The vertical variation pattern of these two metals along the sediment profile differed from other heavy metals, suggesting an increasing influx of tungsten and cobalt into the investigated mangrove habitat, as a result of uncontrolled discharge of industrial wastewater from factories that produce or use chemical compounds or alloys containing these two heavy metals. Laboratory simulation experiment indicated that seawater had a stronger capacity to mobilize sediment-borne tungsten and cobalt, as compared to deionized water, diluted acetic, sulfuric and nitric acids.