Search Results (177)

Heap bioleaching is widely used to extract copper from low-grade sulfide ores thanks to its operational simplicity, low cost, and environmental sustainability. However, current control strategies rely primarily on single-factor optimization and often overlook the synergistic interactions of multiple key parameters, such as ore particle size, pore structure, pH, temperature, microbial activity, and oxygen transfer efficiency. As a result, issues such as low recovery rates, extended leaching periods, and high operational costs persist. Moreover, the “gray-box” nature of heap systems impedes real-time monitoring of internal physical, chemical, and biological processes. In addition, empirical multi-parameter optimization is time-consuming and inadequate for capturing complex interdependencies. This review was conducted to systematically examine the key factors influencing heap bioleaching efficiency and critically evaluate recent advances in numerical simulation and intelligent control strategies. As a result, we identified a major research gap: the existing models—including microscale shrinking core models (SCMs), mesoscale pore-network models based on CT reconstruction, and macroscale continuum models—have inherent limitations. SCMs assume idealized spherical particles with uniform mineral distribution while neglecting pore structure evolution and biofilm dynamics. Mesoscale models offer detailed pore characterization but lack robust multi-physics coupling (thermal–hydro–mechanical–chemical–biological, or THMCB). Macroscale models rely on homogenization assumptions that oversimplify spatial heterogeneity and temporal variations in permeability. This analysis covers the relevant literature from 1985 to 2025, with a focus on three methodological scales (micro, meso, and macro) and their integration with machine learning approaches. A notable finding is that hybrid neural network models (e.g., BP and RBF architectures) outperform purely physics-based models in predicting leaching kinetics under varying operational conditions. However, their accuracy depends heavily on high-quality field data—a limitation rarely addressed in prior reviews. By clearly delineating these model-specific limitations and scale-dependent trade-offs, this review makes two unique contributions: a structured framework for selecting and coupling numerical methods according to process requirements and a roadmap for integrating artificial neural networks with multi-physics simulations to achieve real-time intelligent control of heap bioleaching. The findings offer both theoretical guidance and practical references for optimizing the processing of low-grade copper sulfide ores. Full article

The urban heat island effect, a typical rapid urbanization issue, arises from natural surfaces covered by impermeable layers via urban sprawl. To clarify its unclear response to urban expansion under human–land synergy, this paper proposes a multidimensional urban expansion model and a random forest–intelligence integrated method for high-precision large-region population mapping. Taking the Pearl River Delta urban agglomeration as a sample, its urban expansion is divided into five modes to explore thermal environment impacts. The results show: (1) The proposed random forest–intelligence method achieves 84% overall accuracy in 30 m resolution population mapping. (2) The Pearl River Delta urban agglomeration is dominated by vertical expansion, but all cities have population-shrinking regions, especially around Guangzhou and Shenzhen. (3) From 2010 to 2020, Pearl River Delta urban agglomeration impervious surface expansion and population growth were mismatched: impervious surface extended to fringes, while population grew in core areas. (4) The expansion of impervious surface does not always exacerbate the urban heat island effect; when the per-capita land area is less than 1.8 m², it can actually mitigate the effect. (5) Guangzhou–Foshan–Zhaoqing and Shenzhen–Dongguan–Huizhou integration reduces heat island intensity. Core cities driving surrounding areas via clustered, interconnected development alleviates this effect. Full article

Global gridded population datasets are widely used in urban analysis, risk assessment, and sustainability monitoring, including the calculation of indicators for the Sustainable Development Goals (SDGs). Despite their broad use, their behaviour at local scales in shrinking cities remains insufficiently understood. This study evaluates three global population datasets—WorldPop, GHS-POP, and GPWv4—in seven Croatian city cores using official census data as reference. Croatia represents a relevant case due to long-term population decline combined with relatively stable built-up extents. Population estimates were compared at the city-core level for the period 2001–2021, and spatial differences between datasets were examined using pixel-level residuals, built-up intensity metrics, and land-cover stratification. The results show that WorldPop and GHS-POP achieve similar accuracy in city-total estimates, with relative errors generally ranging between about 2% and 10%, but differ systematically in their spatial allocation of population. GHS-POP concentrates population within built-up areas, while WorldPop redistributes a substantial share into non-built-up land-cover classes, exceeding GHS-POP by approximately 290,000 inhabitants outside built-up areas, whereas GHS-POP concentrates over one million additional inhabitants within built-up zones. GPWv4 often shows the smallest city-level errors but produces spatially uniform population surfaces that limit its suitability for intra-urban analysis. The findings highlight that model choice can strongly influence spatial indicators used in SDG-related and sustainability assessments, highlighting the need for context-specific evaluation of global population datasets in shrinking urban environments. Full article

Hydrogen (H₂) was produced from recycled aluminum microparticles (180–250, 300–425, and 425–500 μm) via alkaline hydrolysis using a 1.0 M NaOH solution to enhance oxide layer removal and aluminum dissolution. Maximum hydrogen flow rates of approximately 13, 15, and 19 mL·min⁻¹ were obtained, confirming that smaller particle sizes promote faster reaction rates due to increased specific surface area. The hydrogen evolution exhibited two-stage kinetic behavior: an initial stage characterized by rapid aluminum dissolution and increasing H₂ production, followed by a gradual decline associated with the formation of a passivating Al(OH)₃ layer. Despite the higher reaction rates observed for smaller particles, the maximum cumulative hydrogen production was obtained for the intermediate particle size (363 µm, 132 mL), compared to 106 mL and 102 mL for 215 µm and 463 µm, respectively, indicating a trade-off between surface area and passivation effects. Kinetic analysis based on the shrinking core model showed excellent agreement (R² = 99.94–99.97%), with rate constants of 0.137, 0.064, and 0.050 min⁻¹. The relationship k ∝ d⁻ⁿ (n ≈ 1.4) suggests a mixed kinetic regime involving both surface reaction and diffusion through the Al(OH)₃ layer. These findings indicate that hydrogen generation can be modulated by particle size; however, the relatively low flow rates and yields limit its immediate practical applicability. Full article

Copper anodic slime is often smelted with lead to improve silver and gold recovery, generating a fine lead-rich fly ash that contains notable amounts of selenium and tellurium. Due to its high lead content and sub-micron particle size, this residue poses significant environmental and occupational health risks. This study evaluates sodium carbonate (Na₂CO₃) leaching as an environmentally benign pre-treatment aimed at partially removing selenium and tellurium while stabilizing lead through carbonate formation. The goal is detoxification rather than maximum metal recovery, enabling safer disposal or subsequent recycling. A central composite design (CCD) in Design-Expert software (Version 12) was used to assess the effects of Na₂CO₃ concentration, temperature, solid-to-liquid ratio, and time on selenium and tellurium dissolution. Selenium recovery reached up to 53.9%, while tellurium recovery peaked at approximately 33.9%. Scanning electron microscopy showed the dust to consist mainly of semi-spherical and elongated particles, with lead carbonate forming preferentially on particle surfaces during leaching. Energy-dispersive spectroscopy confirmed conversion of lead sulfate phases to lead carbonate, which increasingly restricted selenium and tellurium dissolution. Kinetic evaluation suggested selenium leaching follows mixed control involving both surface reaction and diffusion through product layers, whereas tellurium dissolution lacked consistent kinetic behavior. Thermodynamic calculations supported the stabilization of lead as cerussite (PbCO₃), indicating improved environmental safety. The overall dissolution trends were successfully represented using an apparent Shrinking Core Model (SCM) based on measurements collected at 20 °C, 60 °C, and 100 °C. Full article

Urban shrinkage has become a critical constraint on China’s pursuit of high-quality economic development. As a core driver of new-quality productive forces, digital technological innovation warrants systematic examination for its role in mitigating urban shrinkage. Given the current lack of research on multidimensional measures of urban shrinkage and the mechanisms through which digital technologies influence this phenomenon, this study utilizes panel data from 269 prefecture-level and higher cities in China from 2014 to 2022. By employing two-way fixed-effects models, mediation models, and threshold regression models, the study systematically examines the impact, mechanisms, and nonlinear characteristics of digital technology innovation on urban shrinkage. The empirical results demonstrate that digital technological innovation has a significant mitigating effect on urban shrinkage; this conclusion holds even after conducting a series of robustness tests, including replacing the core explanatory variable, accounting for lag effects, using SYS-GMM estimation, and adjusting the sample range. Heterogeneity analysis indicates that the mitigating effect is more pronounced in shrinking cities, peripheral cities, resource-based cities, and cities with lower educational attainment. Mechanism analysis reveals that agricultural-related innovation acts as a mediating channel, whereas rural entrepreneurship exhibits a “partial masking effect” in the relationship between digital technological innovation and urban shrinkage. Moderation analysis further shows that higher levels of ecological environmental resilience amplify the inhibitory effect of digital technological innovation. Finally, threshold regression results identify a significant double-threshold effect, with the mitigating impact of digital technological innovation emerging only after exceeding the first threshold value of 5.690. Based on these findings, this study recommends implementing differentiated digital-technology-driven innovation strategies, with agriculture-related innovation serving as a strategic entry point to stimulate regional innovation and entrepreneurial vitality. At the same time, strengthening ecological resilience should be promoted to support coordinated green and digital transformation. These findings provide empirical evidence for the formulation of differentiated urban digital transformation policies aimed at mitigating urban shrinkage. Full article

This study presents a transient mathematical model and its numerical solution for the moving-boundary problem related to substrate diffusion and reaction in enzymatic catalytic particles. The main focus is on bioreactor startup, where the initial substrate concentration inside the particles is zero, forming a dead core that shrinks over time and makes the catalytic effectiveness factor time-dependent. The substrate mass balance leads to a partial differential equation with a moving boundary, solved using the method of lines coupled with Newton’s method (MLN), implemented in Wolfram Mathematica (WM). The proposed approach was validated for zero- and first-order kinetics at steady state, whose analytical solutions are available. Compared to the method of orthogonal collocation on finite elements, the MLN offers advantages such as not requiring an initial concentration profile and simple implementation in WM. The results demonstrate that the proposed method provides accurate and physically consistent solutions, contributing to a better understanding of dead-core dynamics and supporting the design of heterogeneous bioreactors with immobilized enzymes. Full article

This study investigated the extraction of aluminum from aluminum silicate-rich coal ash from the ash-slag waste of the Almaty CHP-2 power station using microwave-assisted alkaline leaching. The high chemical stability of the quartz and mullite phases in the ash leads to high energy consumption during conventional acid–base treatment. To improve the kinetic parameters of the leaching process, amorphous graphite was therefore used as an active additive, which effectively absorbs microwave energy. The experiments were conducted in the temperature range of 50–200 °C, in 1–6 M NaOH solution, and over a period of 5–30 min. The amount of amorphous graphite varied between 5 and 20 wt%. The proportion of amorphous graphite varied between 5 and 20 wt%. Upon microwave irradiation, the graphite-free ash reached a temperature of 200 °C within approximately 12 min, whereas this temperature was reached in the system with 15% amorphous graphite after only 8–9 min. At low alkali concentrations (1–2 M NaOH), the aluminum transfer into solution in the graphite-free system was approximately 18%–35%. With increasing NaOH concentrations to 3–4 M, the aluminum removal efficiency increased to 38%–58%. Under the same temperature conditions, the leaching process was significantly accelerated by the addition of amorphous graphite; thus, at temperatures near 200 °C and in a 5–6 M NaOH solution, 70%–72% of aluminum was removed. The leaching kinetics were analyzed using the shrinking core model. The results showed that the apparent activation energy of the reaction decreased from 54 kJ/mol to 32 kJ/mol in the presence of graphite. These results suggest that microwave-assisted alkaline leaching in the presence of amorphous graphite is an energy-efficient and promising method for aluminum recovery from coal ash. Full article

The efficient recovery of rare earth elements from associated rare-earth-bearing phosphate ores is of paramount importance for expanding the supply of rare earth resources. In contrast to conventional studies that focus on extracting rare earths either from phosphate concentrates or from phosphogypsum generated during the sulfuric acid wet-process, this study takes as its subject the rare-earth-enriched residue—an intermediate product obtained after the selective leaching of phosphorus via the hydrochloric acid route—from a rare-earth-bearing phosphate ore in Zhijin, Guizhou Province. The occurrence states, leaching behavior, and kinetic mechanisms of rare earth elements within this residue were systematically elucidated. Analyses using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) and aberration-corrected scanning transmission electron microscopy (STEM) reveal that rare earth elements are hosted in residual fluorapatite and newly formed CaF₂ through isomorphic substitution. The substitution of REE³⁺ for Ca²⁺ induces lattice contraction in CaF₂, with the interplanar spacing decreasing from 0.27 nm to 0.26 nm. Through single-factor experiments and response surface methodology (RSM) optimization, the optimal leaching conditions were determined to be a temperature of 80 °C, a leaching time of 120 min, a hydrochloric acid dosage of 160% of the theoretical requirement, a solid–liquid ratio of 1:6, and a agitation speed of 500 r·min⁻¹. Under these conditions, the leaching efficiency of rare earth elements reached as high as 92.69%. Kinetic analysis indicates that the leaching process follows the shrinking-core model, with the rate controlled by diffusion through the solid product layer. The apparent activation energy was calculated to be 37.2 kJ·mol⁻¹, characteristic of a diffusion-controlled process. Furthermore, response surface analysis of variance confirms that leaching temperature and time are the most significant factors influencing rare earth leaching. This study elucidates, from multiple perspectives, the leaching mechanism of rare earth elements from enriched residues within a hydrochloric acid system, thereby providing important theoretical support for the efficient recovery and process optimization of rare earth resources from associated phosphate ores. Full article

Compact city policies have been promoted as a mechanism for improving commuting efficiency through higher density and spatial concentration. However, their effectiveness in small and medium-sized cities that experience population decline, such as in small and medium-sized cities in South Korea remains unclear. This study examines how urban compactness and employment spatial structure influence commuting time across different urban contexts in South Korea, with particular attention to contrasts between the Seoul Capital Region and non-capital cities. Using the 2021 Korean Individual Travel Survey, we examine multilevel mixed-effects models that link individual commuting trips to neighborhood-level built environment characteristics and city-level employment spatial structure. The findings reveal systematically different effects of residential and employment density on commuting times. Higher residential density is generally associated with longer commuting times, whereas higher workplace employment density reduces commuting time only in non-capital regions. In the Seoul Capital Region where urban form is already highly compact, further employment densification does not improve commuting efficiency and may even increase commuting time. Instead, shorter commutes are observed primarily where job–housing balance is relatively high and employment is strongly concentrated in a dominant center. Moreover, the contrasting effects of employment Moran’s I and the employment concentration index indicate that employment dominance and spatial clustering capture distinct dimensions of urban spatial structure, with commuting efficiency depending critically on the internal configuration of employment clusters rather than density alone. These findings suggest that, in shrinking cities, compact city policies should be reframed not as strategies of residential densification, but as strategies of functional consolidation, focusing on sustaining viable employment cores and aligning them with transport networks and residential areas. Full article

Carbon sinks have been widely recognized as critical components of climate change mitigation and achieving carbon neutrality. With rapid urbanization, protecting and optimizing urban carbon sinks remain major challenges. This study uses Zhengzhou as a case study and analyzes 2000–2023 land-use data with the InVEST model to quantify carbon stocks and identify high-value carbon-sink areas. Circuit theory was further integrated to delineate ecological security patterns and inform optimization strategies. The results show a net decrease of 19.12 × 10⁶ t in carbon storage from 2000 to 2023, with the most rapid decline occurring during 2015–2020. Spatially, high-value carbon storage clustered in forested, high-elevation areas in the southwest, whereas low values predominated in the urban core. Carbon-sink source areas continued to shrink: fragmentation increased in the east, the west remained relatively stable, and the central area was highly fragmented. Corridor analysis indicated that the mean corridor length first increased and then decreased, accompanied by an expansion of pinch points and barrier areas. The study developed a systematic optimization framework that establishes a “Two Cores, Five Carbon-Sink Areas, Multiple Corridors” security pattern and proposes targeted conservation measures. The proposed methodology and findings offer a transferable basis for managing urban carbon sinks in rapidly developing regions and support both ecological security and climate-change mitigation, promoting sustainable urban development. Full article

Digital village (DV) construction is the core driving force for high-quality development of the rural economy, and is a key strategy for achieving coordinated progress in urban development and rural revitalization. This study empirically analyzes the direct effect and enhancement mechanisms of DV construction on the high-quality development of grain production (HDGP) by panel data from 170 cities in China’s major grain-producing areas spanning 2013–2022; this study uses the CRITIC-EWM combined evaluation, two-way fixed effects, mediating effect and moderating effect model. The results show that: (1) HDGP appears more sluggish compared to the orderly growth of DV construction, but the level of DV construction and the level of HDGP are mismatched in spatial distribution. (2) DV construction has a significant promoting effect on HDGP, and the digitalization of economy and digitalization of life play more efficiently motivating role in HDGP. (3) This promoting effect is stronger in the population-shrinking regions than in the non-population-shrinking regions. (4) Approximately 8% of the promoting impact of DV construction on the HDGP is achieved indirectly through the scale of new agricultural business entities. (5) Government innovation planning exerts a significant enhancing moderating effect on the influence of DV construction on HDGP. Full article

Thallium is one of the most toxic elements on the planet, and one alternative method for its precipitation is through jarosite-type compounds. Therefore, in this work, the kinetics of thallium jarosite were evaluated in an alkaline medium (NaOH and Ca(OH)₂). Experiments were conducted to assess the effect of medium concentration from 0.03 M to 5.5 × 10⁻⁴ M and the effect of temperature from 20 °C to 60 °C. The sigmoidal curves showed an induction period, during which there was no release of sulfur or thallium ions into the solution, nor the formation of solid byproducts, according to the X ray diffraction (XRD) results. Similarly, a progressive conversion period was observed, evidenced by the release of sulfur and thallium ions into the solution and the formation of amorphous solids. Finally, a stability zone is reached, indicating that the decomposition reaction has ended, as there are no changes in the concentration of sulfur and thallium ions in the solution. The reaction was monitored by determining S using Inductively Coupled Plasma (ICP). The experimental results for the progressive conversion period show a better fit to the chemically controlled shrinking core kinetic model. The reaction order for the kinetics in NaOH medium is 1.09 for the induction period and 0.89 for the progressive conversion period, while for Ca(OH)₂ medium it is 0.78 for the induction period and 0.47 for the progressive conversion period. The activation energies for the progressive conversion period in the two proposed media are 91.87 kJ mol⁻¹ in NaOH and 71.14 kJ mol⁻¹ in Ca(OH)₂, indicating that the controlling mechanism in both systems is the chemical reaction. For the induction period, the activation energies are 101.52 kJ mol⁻¹ and 79.45 kJ mol⁻¹, respectively, indicating that the chemical reaction also controls the initiation of the reactions. The high activation energy in both reaction media suggests that high concentrations of OH⁻ and high temperatures are required to initiate the decomposition reaction. Thallium jarosite precipitates a large amount of thallium and requires high energy to decompose, so it could be a viable alternative in thallium retention. Full article

In the rapidly urbanizing Global South, megacities face a perplexing “paradox of idleness”: acute land scarcity in the urban core coexisting with inefficient rural homesteads in the hinterland. Using Shanghai as a representative case, this study integrates spatial autocorrelation analysis with Geographical Detector modeling to quantify the spatial differentiation patterns and driving mechanisms of this phenomenon. The results reveal a distinct core-periphery gradient, with vacancy density increasing from the inner suburbs to the remote hinterland. Four regional typologies were identified: dispersed-inefficient, high-density accumulation, sparse-stable, and intensive-efficient. Quantitative analysis identifies demographic aging and low agricultural efficiency as dominant drivers. Counter-intuitively, the study finds that top-down institutional pilots alone exert a negligible direct impact. Instead, interaction analysis confirms a significant policy-bundling effect, in which institutional tools promote revitalization only when coupled with economic and locational incentives. These findings expose a mechanism of “involuntary vacancy” trapped by institutional rigidity, distinct from the market-driven abandonment seen in shrinking or remote Western contexts. Consequently, a gradient-based governance framework is proposed to transition from “one-size-fits-all” regulation to targeted spatial restructuring pathways. Full article

In this paper, the time-dependent properties of the elastic modulus of fly ash concrete under sustained compressive load were studied. An experiment was conducted and showed an increment of elastic modulus for two types of fly ash concrete (20% and 40% fly ash replacement) under sustained load. The mechanisms of this increment were analyzed, and two Representative Volume Elements (RVEs) were established to represent the micro-heterogeneous space of binder and concrete based on continuum mechanics. The shrinking core models of hydration and pozzolanic reaction were adopted to quantify the volume fraction of each phase within the binder RVE. A prediction model was proposed by incorporating the effects of extra hydration and time-dependent aggregate concentration rate under sustained load. Finally, parameter analysis including the influences of initial loading age and the loading level was conducted. Full article