Nitrogen loss in karst sloping farmland will lead to declining land productivity and environmental pollution, in which the nitrogen loss through underground pore fissures will directly lead to groundwater pollution. The characteristics of total nitrogen (TN) production were studied by simulating the “dual structure” microenvironment of sloping farmland in a karst region using an artificial rainfall simulation method. The results show that rainfall was the main driving factor of TN loss in karst sloping farmland. TN was mainly lost through underground pore fissures when the rainfall intensity was ≤ 30 mm · h⁻¹. TN was lost at the surface and underground when the rainfall intensity was ≥ 50 mm · h⁻¹, TN loss on the surface accounted for a large proportion, and the surface flow was the main carrier of TN loss. The TN loss underground is easily ignored because it is hidden underground. Therefore, TN loss belowground in karst sloping farmland should receive increased attention. It would be interesting to explore the influences of connectivity and type of underground pore fissure system on TN loss in karst sloping farmland. The prevention and control of TN loss in karst sloping farmland should be considered both at the surface and underground. Reducing the formation of slope flows and slowing rainwater filtration by increasing slope vegetation coverage can be considered to reduce TN loss. The results of this study provide a theoretical reference for agricultural non-point source pollution control in a karst region. Graphical abstract

This study was aimed to investigate the adsorption and fixation effects of hydroxyapatite (HAP) on lead-contaminated soil. According to the experimental results, the microstructure of hydroxyapatite was observed by a scanning electron microscope (SEM). Fourier transform infrared spectroscopy (FTIR) showed that OH⁻ and PO₄³⁻ were the main functional groups in HAP. Optimum adsorption conditions of Pb²⁺ were obtained: 0.2 g/L, adsorbent; initial solution pH of 5.5; and contact time of 120 min. The kinetic adsorption experiments were carried out with the initial lead solution concentrations of 50 mg/L, 150 mg/L, and 250 mg/L. The kinetics fitting was consistent with the pseudo-second-kinetic model, which indicated that the main process of HAP adsorption of Pb²⁺ was mainly controlled by surface reactions and chemical reactions. The adsorption isotherms had a satisfactory fit with the Langmuir model, which indicated that the adsorption of Pb²⁺ by HAP was a monolayer adsorption. According to the experimental results, ion exchange, phosphorus supply, precipitate, and complexation are the main immobilization mechanisms for soil remediation with HAP. In remediation of Pb²⁺-contaminated soil experiments, the adsorption rate of Pb²⁺ by HAP was significantly higher than that of non-HAP soil with increasing immobilization days. With the increasing addition of HAP, the weak acid soluble (WA), reducible (RED), oxidizable (OX), and water soluble (WS) are transformed into residue (RES). The application of HAP in contaminated soil effectively reduced the leachable and exchangeable Pb²⁺, indicating that HAP is a potential material for remediating environmental pollution with Pb²⁺.

In this study, a high sensitive metamaterial sensor has been designed to detect quality of water for water treatment centre. The water samples have been obtained and electrical properties have been measured in microwave range to design the proposed sensor. These electrical properties have been assigned in microwave simulator. Water quality has been investigated by using parametric study and genetic algorithm approach to realize high sensitivity in terms of resonance frequency shift. Around 130 MHz frequency shift has been observed between the water samples. This shift is sufficient to detect instantaneously to the quality of water. This is the first study that metamaterial approach has been used to detect water quality in literature.

In this study, we investigated the effect of olive mill wastewater on selected soil physical and hydraulic properties. Olive mill wastewater was added to each column every week at different loading rates (0, 50, 100, and 200 m³ ha⁻¹). Physicochemical and hydraulic properties were determined for surface (0–8 cm) and subsurface layers (8–16 and 16–24 cm). The highest loading rate (200 m³ ha⁻¹) showed an increase in aggregate stability from 18% (control) to 31 and to 38%, penetration resistance from 1.8 kg cm⁻² (control) to 3.5 and to 4.5 kg cm⁻², hydraulic conductivity from 43 cm day⁻¹ (control) to 15.3 and 3.3 cm day⁻¹, and water repellency from < 5 s (control) to 120 and 261 s in the first and second months for the surface layer, respectively. The opposite was observed for the infiltration rate, where it decreased from 39.01 mm h⁻¹ (control) to 1.26 and 0.42 mm h⁻¹ for the first and second months, respectively. This study showed that application of olive mill wastewater deteriorated the physical and hydraulic properties of soil proportional to loading rates and more specifically at the surface layer.

The study explores the relationship between ecological footprint, urbanization, and energy consumption by applying the ARDL estimation technique on data spanning 1965–2014 for South Africa. After applying the unit root test that accounts for a break in the data, the Bayer and Hanck (J Time Ser Anal 34:83–95, 2013) combined cointegration test affirms cointegrating relationship among the variables. Findings further reveal that economic growth and financial development exact a deteriorating impact on the environment in the short run. However, the same was not true for both energy use and urbanization. While urbanization and energy use promote environmental quality in the long run, financial development and economic growth degrade it further. The long-run findings of our study are confirmed to be robust as reported by the fully modified OLS (FMOLS), dynamic OLS (DOLS), and the canonical cointegrating regression (CCR) estimates. The direction of causality supports the energy-led growth hypothesis for South Africa. Policy outcomes and directions, and the possibility of promoting sustainable growth without degrading the environment are discussed.

In a rapid urbanization context, socio-economic development has caused large increases in carbon emissions. In this study, various techniques such as cointegration analysis, vector autoregression, and decoupling elastic function methods are applied to analyze the sequential collaborative relationship between economic development and carbon emissions in the process of urbanization in terms of the time-series lag relationship and the decoupling relationship. The main findings are as follows: (1) urbanization and carbon emissions displayed a temporal correlation relation with a lag of order 4, according to stability tests, and (2) the development of urbanization, economic growth, and changes in land use may be responsible for the time lag in carbon emissions. Furthermore, the mechanisms behind the effect of urbanization on carbon emissions are investigated to assist future carbon emissions reduction efforts. (3) From 1990 to 2014, carbon emissions and economic development showed a temporal evolution trend of “weak decoupling–expansionary coupling–weak decoupling” in the Pearl River Delta region, and there was an overall weak decoupling state: carbon emissions increased with growth in economic development, but the emissions growth rate was lower than the speed of economic development. (4) From 1990 to 2014, economic development showed a trend of sustained growth in the Pearl River Delta region, and differences were detected in the decoupling status between carbon emissions and economic development at different times. The overall decoupling status of the nine cities in the region was one of weak decoupling; however, the decoupling index, carbon emissions, and economic development levels displayed differences, whereby cities with high carbon emissions and high economic development levels were not necessarily the cities in which environmental pressures from economic development were the most severe. Our results have important theoretical and practical significance as they clarify the impact of economic development on carbon emissions in the process of urbanization, as well as the carbon emissions reduction work that must be undertaken in urban systems.

The present study reports the use of Citrus limetta (CL) residue for cultivating Chlorella sp. mixotrophically to augment production of biodiesel. The cultivation of Chlorella sp. using CL as media was carried out by employing a fed-batch technique in open tray (open tray+CL) and in software (BioXpert V2)–attached automated photobioreactor (PBR+CL) systems. Data showed the limit of nitrogen substituent and satisfactory organic source of carbon (OSC) in CL, causing > 2-fold higher lipid content in cells, cultivated in both the systems than in control. For the cells grown in both the systems, ≥ 3-fold enhancement in lipid productivity was observed than in control. The total fatty acid methyl ester (FAME) concentrations from lipids extracted from cells grew in PBR+CL and in open tray+CL techniques were calculated as 50.59% and 38.31%, respectively. The PBR+CL system showed improved outcomes for lipid content, lipid and biomass productivity, FAME characteristics and physical property parameters of biodiesel than those obtained from the open tray+CL system. The physical property parameters of biodiesel produced from algal cells grown in PBR+CL were comparable to existing fuel standards. The results have shown lower cold filter plugging point (− 6.57 °C), higher cetane number (58.04) and average oxidative stability (3.60 h). Collectively, this investigation unveils the novel deployment of CL as a cost-effective feedstock for commercialisation of biodiesel production.

Cadmium (Cd) is absorbed readily by rice plants and is transferred to humans when contaminated rice is consumed. Adding selenium (Se) to the plant nutrient solutions reduces the accumulation of Cd in the rice (Oryza sativa L.) seedlings. However, as the relevant underlying mechanism remains unclear, the aim of our study was to improve our understanding of the Se-mediated resistance to Cd stress in rice. We conducted hydroponic experiments to study the effects of selenite or selenate on Cd subcellular distribution and xylem transport in rice seedlings under Cd stress, and we investigated the antioxidative defense responses in the rice plants. We found that the supplementation of both Se forms decreased the Cd accumulations in the roots and shoots of the rice plants. The selenite addition significantly decreased the Cd contents in different subcellular fractions of the rice roots, increased the proportion of Cd distributed to soluble cytosol by 23.41%, and decreased the Cd distribution in the organelle by 28.74% in contrast with the treatment with Cd only. As regards the selenate addition, only the Cd distribution ratio of cytosol was increased by 13.07%. After adding selenite, a decrease of 55.86% in the Cd concentration in xylem sap was observed, whereas little change was found after treatment co-applied with selenate. The hydrogen peroxide (H₂O₂) and malondialdehyde(MDA) contents in the rice roots were elevated under Cd stress, and the addition of selenite and selenate decreased the H₂O₂ levels by 77.78% and 59.26%, respectively. Co-exposure to Cd and Se elevated the glutathione (GSH) accumulations in the rice shoots and roots, with the degree of increase being the following: co-applied with selenite > co-applied with selenate > Cd alone treatment. Exposure to Cd increased the catalase (CAT) activity in the roots significantly, whereas it decreased in the shoots. After selenite or selenate supplementation, the CAT activity in the rice roots increased compared with applying only Cd. Compared with the control, the addition of Cd or Se had no significant effect on the activities of peroxidase (POD) or ascorbate peroxidase (APX). Our results showed that Se affected the Cd accumulation in rice seedlings by altering the Cd subcellular distribution and decreasing the ROS induced by Cd stress. Such effects were more significant in the selenite than in the selenate applied treatment.

The Sub-Saharan Africa (SSA) is far lag behind the sustainable targets that set out in the United Nation’s Sustainable Development Goals (SDGs), which is highly needed to embark the priorities by their member countries to devise sustainable policies for accessing clean technologies, energy demand, finance, and food production to mitigate high-mass carbon emissions and conserve environmental agenda in the national policy agenda. The study evaluated United Nation’s SDGs for environmental conservation and emission reduction in the panel of 35 selected SSA countries, during a period of 1995–2016. The study further analyzed the variable’s relationship in inter-temporal forecasting framework for the next 10 years’ time period, i.e., 2017–2026. The parameter estimates for the two models, i.e., CO₂ model and PM₂.₅ models are analyzed by Generalized Method of Moment (GMM) estimator that handle possible endogeneity issue from the given models. The results rejected the inverted U-shaped Environmental Kuznets Curve (EKC) for CO₂ emissions, while it supported for PM₂.₅ emissions with a turning point of US$5540 GDP per capita in constant 2010 US$. The results supported the “pollution haven hypothesis” for CO₂ emissions, while this hypothesis is not verified for PM₂.₅ emissions. The major detrimental factors are technologies, FDI inflows, and food deficit that largely increase carbon emissions in a panel of SSA countries. The IPAT hypothesis is not verified in both the emissions; however, population density will largely influenced CO₂ emissions in the next 10 years’ time period. The PM₂.₅ emissions will largely be influenced by high per capita income, followed by trade openness, and technologies, over a time horizon. Thus, the United Nation’s sustainable development agenda is highly influenced by socio-economic and environmental factors that need sound action plans by their member countries to coordinate and collaborate with each other and work for Africa’s green growth agenda.

Transport sector contributed numerous carbon emissions in China. It is important to promote low-carbon bicycle-sharing using in China. This paper aims to analyze the influencing factors of the satisfaction and engagement of bicycle-sharing in China. An extended model of customer satisfaction is established, which considers both customer satisfaction theory and customer engagement theory. We explore the different effect of convenience, health, safety, and facility on satisfaction of bicycle-sharing using. We also explore the role of satisfaction on three different dimensions of engagement: enthusiasm, participation, and social interaction. Multi-group structure equation model is employed to explore the different roles of gender on the determinants of satisfaction and engagement of bicycle-sharing using. The findings reveal that facilities had the larger effect on satisfaction of bicycle-sharing using than safety and health for male. Safety had the largest influence on satisfaction of bicycle-sharing using for female, followed by facility, convenience, and health.