Product and Metal Stocks Accumulation of China's Megacities: Patterns, Drivers, and Implications

Insights into the Mechanism of Selective Removal of Heavy Metal Ions by the Pulsed/Direct Current Electrochemical Method

Heavy metal pollution treatment in industrial wastewater is crucial for protecting biological and environmental safety. However, the highly efficient and selective removal of heavy metal ions from multiple cations in wastewater is a significant challenge. This work proposed a pulse electrochemical method with a low-/high-voltage periodic appearance to selectively recover heavy metal ions from complex wastewater. It exhibited a higher recovery efficiency for heavy metal ions (100% for Pb2+ and Cd2+, >98% for Mn2+) than other alkali and alkaline earth metal ions (Na+, Ca2+, and Mg2+ were kept below 3.6, 1.3, and 2.6%, respectively) in the multicomponent solution. The energy consumption was only 34-77% of that of the direct current electrodeposition method. The results of characterization and experiment unveil the mechanism that the low-/high-voltage periodic appearance can significantly suppress the water-splitting reaction and break the mass-transfer limitation between heavy metal ions and electrodes. In addition, the plant study demonstrates the feasibility of treated wastewater for agricultural use, further proving the high sustainability of the method. Therefore, it provides new insights into the selective recovery of heavy metals from industrial wastewater.

Gridded material stocks in China based on geographical and geometric configurations of the built-environment

Material stocks have created alternative perspectives in many environmental and climate studies. Their significance nonetheless may be under-explored, partially due to scarcity of more precise, timely and higher-resolution information. To address this limitation, our present study developed a gridded material stocks dataset for China in Year 2000 and 2020, by examining the geographical distribution and geometric configurations of the human-made stock-containing environment. The stocks of twelve materials embodied in five end-use sectors and 104 products and constructions were assessed at a resolution of 1 × 1 km grid. Material intensity in each product or construction component was carefully evaluated and tagged with its geometric conformation. The gridded stocks aggregately are consistent with the stock estimation across 337 prefectures and municipalities. The reliability of our assessment was also validated by previous studies from national, regional, to grid levels. This gridded mapping of material stocks may offer insights for urban-rural disparities, urban mining opportunity, and climate and natural disaster resilience.

Material stock analysis of urban road from nighttime light data based on a bottom-up approach

In recent years, there has been an increasing focus on the dynamics of material stock, that is, the basis of material flow in the entire ecosystem. With the gradual improvement of the global road network encryption project, the uncontrolled extraction, processing, and transportation of raw materials impose serious resource concerns and environmental pressure. Quantifying material stocks enable governments to formulate scientific policies because socio-economic metabolism, including resource allocation, use, and waste recovery, can be systematically assessed. In this study, OpenStreetMap road network data were used to extract the urban road skeleton, and nighttime light images were divided by watershed to construct regression equations based on geographical location attributes. Resultantly, a generic road material stock estimation model was developed and applied to Kunming. We concluded that (1) the top three stocks are stone chips, macadam, and grit (total weight is 380 million tons), (2) the proportion of asphalt, mineral powder, lime, and fly ash is correspondingly similar, and (3) the unit area stock decreases as the road grade declines; therefore, the branch road has the lowest unit stock.

High-resolution mapping and evolution of steel stocks and waste in civil buildings: a case study of Changsha, China

Systematic estimation of steel stocks and waste in urban areas and analysis of its historical evolution pattern is crucial for urban buildings steel recycling and environmental sustainability. However, it is a challenging task to collect big data from different sources and estimate accurately with high resolution. In this study, we proposed a novel hybrid approach (GMB model) to estimate building steel stocks and the annual waste rate through combining Geographic Information System, Material Flow Analysis, and Big Data Mining techniques. We estimated the civil-building steel stocks and amount of waste in Changsha urban area from 1985 to 2020 based on the GMB model, and analyzed the historical evolution pattern of steel stocks by using standard deviation ellipse and kernel density. The results showed that the cumulative steel stock in civil buildings grew from 0.66 million tons in 1985 to 8.26 million tons in 2020. The amount of waste increased by 2557 times. The spatiotemporal analysis showed that variations in distribution of the steel stocks are mainly concentrated in the central city, indicating a "central-peripheral" distribution, with a southward trend in the standard deviation ellipse and a southeast-northwest direction in the center of gravity of the steel stocks. There is low-high and high-low spatial aggregation patterns. We also compared the experimental results with the observed data to determine the feasibility of the GMB model. Our study can promote the management of steel resources recycling and aid to achieve the green and low-carbon goals in sustainable development policies.

Product, building, and infrastructure material stocks dataset for 337 Chinese cities between 1978 and 2020

Reliable city-level product, building, and infrastructure material stocks data are essential for understanding historical material use patterns, benchmarking material efficiency, and informing future recycling potentials. However, such urban material stocks data are often limited, due primarily to unavailable, inconsistent, or noncontinuous city-level statistics. Here, we provided such an Urban Product, Building, and Infrastructure Material Stocks (UPBIMS) dataset for China, a country that has undergone a remarkable urbanization process in the past decades, by collating different official statistics and applying various gap-filling methods. This dataset contains the stock of 24 materials contained in 10 types of products, buildings, and infrastructure in all 337 prefecture-level cities in China from 1978 to 2020. This quality controlled and unified dataset is the first of its kind with such a full coverage of all prefecture-level Chinese cities and can be used in a variety of applications, for example in urban geography, industrial ecology, circular economy, and climate change mitigation. Every piece of data is tagged with its source and the dataset will be periodically updated.

Spatiotemporally Explicit Mapping of Built Environment Stocks Reveals Two Centuries of Urban Development in a Fairytale City, Odense, Denmark

The urban built environment stocks such as buildings and infrastructure provide essential services to urban residents, and their spatiotemporal dynamics are key to the circular and low-carbon transition of cities. However, spatiotemporally explicit characterization of urban built environment stocks remains hitherto limited, and previous studies on fine-grained mapping of built environment stocks often focus on an urban area without consideration of temporal dynamics. Here, we combined the emerging geospatial data and historical maps to quantify the spatially and temporally refined stocks of buildings and infrastructure and developed a novel indexing method to track the construction, demolition, and renovation for each building across various historical snapshots, with a case study of Odense, Denmark, from 1810 to 2018. We show that built environment stock in Odense increased from 80 t/cap in 1810 to 279 t/cap in 2018. Their dynamics appear overall in line with urban development of Odense over the past two centuries and well reflect the combined effects of industrialization, infrastructure development, socioeconomic characteristics, and policy interventions. Such spatiotemporally explicit stock mapping offers a physical and resource perspective for measuring urbanization and provides the public and government insight into urban spatial planning and related resource, waste, and climate strategies.

Tracking Three Decades of Global Neodymium Stocks and Flows with a Trade-Linked Multiregional Material Flow Analysis

Neodymium (Nd), an essential type of rare earth element, has attracted increasing attention in recent years due to its significant role in emerging technologies and its globally imbalanced demand and supply. Understanding the global and regional Nd stocks and flows would thus be important for understanding and mitigating potential supply risks. In this work, we applied a trade-linked multiregional material flow analysis to map the global and regional neodymium cycles from 1990 to 2020. We reveal increasingly complex trade patterns of Nd-containing products and a clearly dominant but slightly weakening role of China in the global Nd trade (for both raw materials and semi- and final products) along the life cycle in the last 30 years. A total of 880 kt Nd was mined accumulatively and flowed into the global socioeconomic system, mainly as NdFeB permanent magnets (79%) in semi-products and conventional vehicles and home appliances (together 48%) in final products. Approximately 64% (i.e., 563 kt Nd) of all the mined Nd globally were not recycled, indicating a largely untapped potential of recycling in securing Nd supply and an urgency to overcome the present technological and non-technical challenges. The global Nd cycle in the past three decades is characterized by different but complementary roles of different regions along the global Nd value chain: China dominates in the provision of raw materials and semi- and final products, Japan focuses on the manufacturing of magnets and electronics, and the United States and European Union show advantages in the vehicle industry. Anticipating increasing demand of Nd in emerging energy and transport technologies in the future, more coordinated efforts among different regions and increased recycling are urgently needed for ensuring both regional and global Nd supply and demand balance and a common green future.

An Electrochemical Strategy for Simultaneous Heavy Metal Complexes Wastewater Treatment and Resource Recovery

Heavy metals chelated with coexisting organic ligands in wastewater impose severe risks to public health and the ambient ecosystem but are also valuable metal resources. For sustainable development goals, the treatment of heavy metal complexes wastewater requires simultaneous metal-organic bond destruction and metal resource recovery. In this study, we demonstrated that a neutral pH electro-Fenton (EF) system, which was composed of an iron anode, carbon cloth cathode, and sodium tetrapolyphosphate electrolyte (Na₆TPP), could induce a successive single-electron activation pathway of molecular oxygen due to the formation of Fe(II)-TPP complexes. The boosted •OH generation in the Na₆TPP-EF process could decomplex 99.9% of copper ethylene diamine tetraacetate within 8 h; meanwhile, the released Cu ions were in situ deposited on the carbon cloth cathode in the form of Cu nanoparticles with a high energy efficiency of 2.45 g kWh^-1. Impressively, the recovered Cu nanoparticles were of purity over 95.0%. More importantly, this neutral EF strategy could realize the simultaneous removal of Cu, Ni, and Cr complexes from real electroplating effluents. This study provides a promising neutral EF system for simultaneous heavy metal complexes wastewater treatment and resource recovery and sheds light on the importance of molecular oxygen activation in the field of pollutant control.

Toward carbon neutrality: Uncovering constraints on critical minerals in the Chinese power system

China has set up its ambitious carbon neutrality target, which mainly relies on significant energy-related carbon emissions reduction. As the largest important contributing sector, power sector must achieve energy transition, in which critical minerals will play an essential role. However, the potential supply and demand for these minerals are uncertain. This study aims to predict the cumulative demand for critical minerals in the power sector under different scenarios via dynamic material flow analysis (DMFA), including total demands, supplies and production capacities of different minerals. Then, these critical minerals are categorized into superior and scarce resources for further analysis so that more detailed results can be obtained. Results present that the total minerals supply will not meet the total minerals demand (74260 kt) in 2060. Serious resource shortages will occur for several key minerals, such as Cr, Cu, Mn, Ag, Te, Ga, and Co. In addition, the demand for renewable energy will be nearly fifty times higher than that of fossil fuels energy, implying more diversified demands for various minerals. Finally, several policy recommendations are proposed to help improve the overall resource efficiency, such as strategic reserves, material substitutions, and circular economy.

Component-Based Model for Building Material Stock and Waste-Flow Characterization: A Case in the Île-de-France Region

Building demolition is one of the main sources of waste generation in urban areas and is a growing problem for cities due to the generated environmental impacts. To promote high levels of circular economy, it is necessary to better understand the waste-flow composition; nevertheless, material flow studies typically focus on low levels of detail. This article presents a model based on a bottom-up macro-component approach, which allows the multiscale characterization of construction materials and the estimation of demolition waste flows, a model that we call the BTP-flux model. Data mining, analytical techniques, and geographic information system (GIS) tools were used to assess different datasets available at the national level and develop a common database for French buildings: BDNB. Generic information for buildings in the BDNB is then enriched by coupling every building with a catalog of macro-components (TyPy), thus allowing the building’s physical description. Subsequently, stock and demolition flows are calculated by aggregation and classified into 32 waste categories. The BTP-flux model was applied in Île-de-France in a sample of 101,320 buildings for residential and non-residential uses, representative of the assessed population (1,968,242 buildings). In the case of Île-de-France, the building stock and the total demolition flows were estimated at 1382 Mt and 4065 kt, respectively. For its inter-regional areas—departments—, stock and demolition waste can vary between 85 and 138 tons/cap and 0.263 and 0.486 tons/cap/year, respectively. The mean of the total demolition wastes was estimated at 0.33 tons/cap/year for the region. Results could encourage scientists, planners, and stakeholders to develop pathways towards a circular economy in the construction sector by implementing strategies for better management of waste recovery and reintegrating in economic circuits, while preserving a maximum of their added value.

China material stocks and flows account for 1978-2018

As the world's top material consumer, China has created intense pressure on national or global demand for natural resources. Building an accurate material stocks and flows account of China is a prerequisite for promoting sustainable resource management. However, there is no annually, officially published material stocks and flows data in China. Existing material stocks and flows estimates conducted by scholars exhibit great discrepancies. In this study, we create the Provincial Material Stocks and Flows Database (PMSFD) for China and its 31 provinces. This dataset describes 13 materials' stocks, demand, and scrap supply in five end-use sectors in each province during 1978-2018. PMSFD is the first version of material stocks and flows inventories in China, and its uniform estimation structure and formatted inventories offer a comprehensive foundation for future accumulation, modification, and enhancement. PMSFD contributes insight into the material metabolism, which is an important database for sustainable development as well as circular economy policy-making in China. This dataset will be updated annually.

Log Mean Divisia Index Decomposition Analysis of the Demand for Building Materials: Application to Concrete, Dwellings, and the U.K

Dwellings are material intensive products. To date, material use in dwellings has been investigated mainly using economic (exogenous) or dwelling (endogenous) drivers, with few studies comprehensively combining both. For the first time, we identify a comprehensive set of such drivers of demand for building materials and analyze them using the logarithmic mean divisia index (LMDI) method. We combine the LMDI method, the concept of dynamic material flow analysis, and physical and monetary flows to decompose the demand for building materials into the following six effects: material intensity, floor area shape, dwelling type, dwelling intensity, economic output, and population. We analyze these six effects on demand for concrete in new dwellings in the U.K. from 1951 to 2014, classified into six dwelling types and four subregions. Of these six effects, the material intensity effect is the most important, overall contributing to increasing concrete demand by +79 Mt from 1950 to 2014, while the dwelling intensity effect plays an opposite role, overall reducing concrete demand from 1950 to 2014 by -56 Mt. The economic output effect is also significant (+38 Mt from 1950 to 2014). A comparative analysis of the six effects in the four U.K. nations reveals that most of the effects arise from England, while the other nations have minor effects due to their smaller populations. Our results show that changes to the demand for concrete in the U.K. fluctuate and have mainly remained between ±30 Mt year^-2 from 1950 to 2014, and thus the inflows of concrete into the in-use stock of dwellings have experienced neither entirely increasing or decreasing trends during this period. This study contributes to understanding changes in resource demand due to social, economic, and technological factors and thus improves the capability to reliably and quantitatively model the use of materials in the built environment.

Byproduct Surplus: Lighting the Depreciative Europium in China's Rare Earth Boom

Europium (Eu) is often regarded as a critical mineral due to its byproduct nature, importance to lighting technologies, and global supply concentration. However, the existing indicator-based criticality assessments have limitations to capture Eu's supply chain information and thus fall short of reflecting its true criticality. This study quantified the flows and stocks of Eu in mainland China from 1990 to 2018. Results show that: (1) China's Eu demand decreased by 75% from 2011 to 2018, as a result of the lighting technology transition from fluorescent lamps to light-emitting diodes, which significantly reduced Eu's importance; (2) the supply of Eu mined as a byproduct kept increasing together with the growing rare earth production, which caused a substantial supply surplus being ≈1900 t by 2018; (3) despite the leading role of China in global Eu production, Eu mined in China was exported mainly in the form of intermediate and final products, and ≈90% Eu embedded in domestically produced final products was used for export recently. This study indicates that Eu's criticality is not as severe as previously assessed and highlights the necessity of material flow analysis for a holistic and dynamic view on the entire supply chain of critical minerals.

High-Resolution Mapping of the Urban Built Environment Stocks in Beijing

Improving our comprehension of the weight and spatial distribution of urban built environment stocks is essential for informing urban resource, waste, and environmental management, but this is often hampered by inaccuracy and inconsistency of the typology and material composition data of buildings and infrastructure. Here, we have integrated big data mining and analytics techniques and compiled a local material composition database to address these gaps, for a detailed characterization of the quantity, quality, and spatial distribution (in 500 m × 500 m grids) of the urban built environment stocks in Beijing in 2018. We found that 3621 megatons (140 ton/cap) of construction materials were accumulated in Beijing's buildings and infrastructure, equaling to 1141 Mt of embodied greenhouse gas emissions. Buildings contribute the most (63% of total, roughly half in residential and half in nonresidential) to the total stock and the subsurface stocks account for almost half. Spatially, the belts between 3 and 7 km from city center (approximately 5 t/m²) and commercial grids (approximately 8 t/m²) became the densest. Correlation analyses between material stocks and socioeconomic factors at a high resolution reveal an inverse relationship between building and road stock densities and suggest that Beijing is sacrificing skylines for space in urban expansion. Our results demonstrate that harnessing emerging big data and analytics (e.g., point of interest data and web crawling) could help realize more spatially refined characterization of built environment stocks and highlight the role of such information and urban planning in urban resource, waste, and environmental strategies.

Dynamic Assessment of Construction Materials in Urban Building Stocks: A Critical Review

There is a lack of understanding on the different types of dynamics of building stocks, in real life and in models. Moreover, there is now a particular interest in the embodied impacts of construction materials, since with the increasing efficiency of buildings operation, embodied impacts gain more importance in the overall building life cycle. This critical review wants to advance the understanding of the type of dynamics, methods, and tools used. The well-known IPAT equation is adapted for building stocks and three dynamics are defined: spatial, evolutionary temporal, and spatial-cohort dynamic. A framework is defined that can help researchers choose a method, tool, and dynamics of input parameters depending on their research goal, case study, and data. Moreover, generally valid conclusions are drawn, including MFA is useful to model spatially dynamic material flows; GIS is needed to include spatial dynamics. Retrofit, compared to construction and demolition, is understudied and usually analyzed through top-down methods. Material intensity and emission intensity are rarely modeled in a dynamic way. Overall, scholars seem to perform increasingly data intensive and complex studies tailored to a specific case study. However, there are big differences in the quality depending on the dynamic of input parameters.