Electronic waste (e-waste) has been widely studied by scholars all over the world, but the research topics and development trends in this field are still unclear. This study aimed to explore the status quo, hot topics, and future prospects in the field of e-waste. Data of publications were downloaded from the Web of Science Core Collection. We used CiteSpace V, Histcite, and VOSviewer to analyze literature information. A total of 2800 papers in e-waste research were identified, and the number of publications increased rapidly after 2004. Six thousand five hundred seventy-three authors participated in the e-waste research, but 70.01% of the authors published only 1 article. The most productive country in this field was China (1146 publications), and the most productive institution was the Chinese Academy of Sciences (370 publications). The Waste Management (225 publications) was the most productive journal, and Environment Science & Technology (9704 co-citations) was the most co-cited journal. The main hot topics in e-waste field were management and recycling of e-waste in developing countries, health risk assessment after exposure to organic pollutants, degradation and recovery of waste metal materials, and impact of heavy metals on children’s health. The frontier topic was degradation.

Nowadays, widespread application of engineered nanoparticles (ENPs) inevitably leads to their release into the environment. Soils are regarded as the ultimate sink for ENPs. The study on mobility of ENPs in soils is important in the assessment of potential risks related to their toxicity. The behavior of ENPs is dependent not only on parameters of soil but also on exposure scenarios, namely, the amount of ENPs trapped in soil. In the present work, the mobility of cerium dioxide nanoparticles (nCeO₂) in soils at different exposure scenarios has been studied. The relationship between mobility of nCeO₂ and their concentration in soil in the range from 1 to 1000 μg g⁻¹ is evaluated. It is shown that the mobility of nCeO₂ decreases with decreasing their concentration in soil and attains the minimum value at the concentration of nCeO₂ below 10 μg g⁻¹. In relative terms, only about 0.1–0.2% of nCeO₂ at their concentration in soil 10–1000 μg g⁻¹ are mobile and can migrate in soil profile under saturated conditions. The major portion of nCeO₂ (about 99.8%) remains immobile in soil. Evidently, the vertical transport of nCeO₂ in soil profile should depend on volume of released suspensions. In the case of small or moderate wet deposition, nanoparticles will accumulate in upper soil horizons, where biological activity is highest, and affect the soil inhabitants (plant roots, earthworms, insects, microorganisms, etc.).

Increasing applications of rare earth elements (REEs) and improving technologies have led to increased demand. Because of their limited availability and depletion of most resources, the recovery of these elements from waste has become important. The use of cost-effective materials for this purpose and the high value that can potentially be recovered would be beneficial and attractive to many industries using REEs. In this study, natural zeolite and bentonite were used in batch studies to recover REEs (La, Y, Lu, Sm, Pr, Tm, Ce, Nd, Yb, Gd, Eu, Er, Ho, Dy, and Sc) from aqueous solutions. The effect of adsorbent dosage, pH, concentration, contact time, and competing ions on recovery was investigated. Desorption studies were conducted using ammonium sulphate. Adsorption onto zeolite was found to increase with pH, whereas uniform adsorption was observed for bentonite, except at pH 2 (16% less efficiency). The pH values of 6.2 and 3.2 were selected as the optimum for zeolite and bentonite, respectively. For zeolite, the average adsorption efficiencies for REEs at 0.5, 1.0, 2.0, 5.0, and 10 mg L⁻¹ were found to be 91, 96, 89, 40, and 20% respectively but, > 98% adsorption efficiencies were achieved with bentonite at all concentrations. The zeolite and bentonite adsorption data were better described by Langmuir though, for bentonite, the coefficients of determination (R² values) for the Freundlich and Dubinin-Radushkevich isotherm models were also significant. There was no significant difference (p > 0.05) on the adsorption of the elements in the presence of competing ions. Bentonite proved to perform better, most likely as a result of its higher surface area. Generally, the good adsorption performance of both adsorbents in their natural forms makes them an attractive and potential cheap option for the recovery of REEs from wastewaters.

Adsorption plays an important role in removing cadmium (Cd²⁺) from water, and magnetic adsorbents are increasingly being used due to their ease of separation and recovery. Magnetic Fe₃O₄–coated hydroxyapatite (HAP) nanoparticles (nHAP-Fe₃O₄) were developed by co-precipitation and then used for the removal of Cd²⁺ from water. The properties of these nanoparticles were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and magnetization curves. Experiments were conducted to investigate the effects of adsorption and mechanisms. Results illustrated that kinetic data were well fitted by a pseudo-second-order model. The adsorption capacity of nHAP-Fe₃O₄ was 62.14 mg/g. The mechanisms for the adsorption of Cd²⁺ on nHAP-Fe₃O₄ included rapid surface adsorption, intraparticle diffusion, and internal particle bonding, with the ion exchange with Ca²⁺ and chemical complexation being the most dominant. The regeneration efficiency and recovery rate of nHAP-Fe₃O₄ eluted by EDTA-Na₂ after the fifth cycle were 63.04% and 40.2%, respectively. Results revealed that the feasibility of nHAP-Fe₃O₄ as an adsorbent of Cd²⁺ and its environmental friendliness make it an ideal focus for future research.

For a long-period comparative analysis of air pollution in coastal and inland cities, we analyzed the continuous Morlet wavelet transform on the time series of a 5274-day air pollution index in Shanghai and Lanzhou during 15 years and studied the multi-scale variation characteristic, main cycle, and impact factor of the air pollution time series. The analysis showed that (1) air pollution in the two cities was non-stationary and nonlinear, had multiple timescales, and exhibited the characteristics of high in winter and spring and low in summer and autumn. (2) The monthly variation in air pollution in Shanghai was not significant, whereas the seasonal variation of air pollution in Lanzhou was obvious. (3) Air pollution in Shanghai showed an ascending tendency, whereas that in Lanzhou presented a descending tendency. Overall, air pollution in Lanzhou was higher than that in Shanghai, but the situation has reversed since 2015. (4) The primary cycles of air pollution in these two cities were close, but the secondary cycles were significantly different. The aforementioned differences were mainly due to the impact of topographical and meteorological factors in Lanzhou, the weather process and the surrounding environment in Shanghai. These conclusions have reference significance for Shanghai and Lanzhou to control air pollution. The multi-timescale variation and local features of the wavelet analysis method used in this study can be applied to varied aspects of air pollution analysis. The identification of cycle characteristics and the monitoring, forecasting, and controlling of air pollution can yield valuable reference.

The quantification of heavy metal contents in soils and their sources are essential for contamination monitoring and the assessment of the potential risks to the ecosystems. This study aims to investigate the source of heavy metals and other elements in soils from a uranium-phosphate deposit using integrated multivariate and geostatistics techniques. For this, 50 soil samples in Itataia deposit, Northeastern, Brazil, were collected at 0–0.2-m depth for the determination of U, Fe, Al, Mn, Ti, Zn, Cu, Ni, Mo, Co, Cr, Cd, Pb, As, Se, V, B, and Zr. The Pb, Se, Ni, Cr, As, and Mo mean contents were closer or exceeded The Brazilian Environmental Council (CONAMA) prevention values for soils. Uranium content was about 500 times higher than the mean levels reported for Brazilian soils. The cluster analysis indicates three geochemical groups based on different contamination levels. The first principal component was associated with lithological origin, the second principal component may be related to anthropogenic sources, and the third and fourth principal components indicated a joined source (natural and anthropogenic), indicating different sources of contamination. Mo was not related to other heavy metals, being found independent in the area. The accumulation of heavy metals in soils is associated not only with the parent material but also with the minerals of the soil. In the area of study, calcareous soils favored alkaline conditions that influenced the dynamics of heavy metals. The multivariate and geostatistical analyses were able to provide preliminary information regarding the metal contents in soil for environmental management.

Yellow-lupin (Lupinus luteus L.) was grown on soils contaminated with heavy metals during two parallel studies. In the first one, the soil was contaminated by industrial activities whereas, in the second one, the soil was artificially contaminated with a single metal including Cd, Pb, Zn, Ni (in nitrate form), and Ag (in nitrate and nanoparticles form). The study was performed to assess a plant’s response to contamination including its antioxidative response and molecular mechanisms involved in metal detoxification through the expression level of metallothioneins (MTs). Overall, the study provided insights into identification and validation of housekeeping genes (HKG) in L. luteus under exposure to metal stress and showed the effects of selected heavy metals and silver nanoparticles on the expression of metallothioneins, the activity of guaiacol peroxidase (GPX) and bioaccumulation of metals in leaves of L. luteus. As such, HKG validation using BestKeeper, NormFinder, and geNorm software allowed for the selection of four most stable reference genes in a context metal contamination for the selected plant. Moreover, a significant increase in the expression levels of MT was observed in plants grown under heavy metal stress and none on plants grown on 25 mg kg⁻¹ of silver nanoparticles. Also, the GPX activity and MT expression showed statistically significant changes between different conditions and doses which means that they can be used as highly sensitive stress markers for planning the phytoremediation process on a large scale.

Total (all forms of inorganic and organic) concentrations of mercury (Hg) and selenium (Se) were measured in dorsal muscle and eggs of wild fishes from two shallow lakes in China: Tai Lake (Ch: Taihu; TL) and Baiyangdian Lake (BYDL). Hazard quotients (HQs) were calculated by dividing concentrations of Se or Hg in muscle or eggs of fishes by threshold concentrations for effects expressed as tissue residue toxicity reference values (TR-TRVs). Concentrations of Hg in whole bodies of fishes were estimated by concentrations in muscle. Based on concentrations of Hg in whole body, HQs for fishes in TL and BYDL were less than 1.0, which suggests little to moderate potential for effects on these fishes and unaccepted adverse effects of Hg are unexpected for adult fishes. HQs of Se in muscle of common carp from TL were closed to 1.0, and 27% of HQs based on concentrations of Hg in eggs of fishes from BYDL exceeded 1.0. Potential hazard due to Hg on common carp in TL and reproductive effects of Se on fishes from BYDL exhibited need for concern. Ratios of molar concentrations of Se to Hg were greater than 1.0. Thus, there might be some protective effects of Se on effects of Hg on fishes in TL and BYDL.

Long-term agricultural development has led to agricultural non-point source (NPS) pollution. Ecological ditches (eco-ditch), as specific wetland systems, can be used to manage agricultural NPS water and achieve both ecological and environmental benefits. In order to understand which type of eco-ditch systems (Es, soil eco-ditch; Ec, concrete eco-ditch; Eh, concrete eco-ditch with holes on double-sided wall) is more suitable for plant nutrient balance meanwhile reducing NPS water (total nitrogen [TN], about 10 mg/L; total phosphorus [TP], about 1 mg/L), it is essential to evaluate the plant (Vallisneria natans) stoichiometry response to water in different types of eco-ditches under static experiment. The results indicated that there were no significant differences in TP removal efficiency among three eco-ditches, yet Eh systems had the best TN removal efficiency during the earlier experimental time. Addition of agricultural NPS water had varying effects on plants living in different types of eco-ditch systems. Plant organ stoichiometry of V. natans varied in relation to eco-ditch types. Plant stoichiometry (C:N, C:P, and N:P) of V. natans in Eh systems could maintain the homeostasis of nutrients and was not greatly affected by external changing environment. V. natans in Es systems can more easily modify the nutrient contents of organs with regard to nutrient availability in the environment. Our findings provide useful plant stoichiometry information for ecologists studying other specific ecosystems.

Effluents form the swine livestock industry contain a high concentration of pollutants and require complex treatment systems. The most recurrent form to treat Swine wastewater is by a conventional anaerobic–aerobic treatment. For example, an up-flow anaerobic blanket sludge reactor followed by an activated sludge reactor. However, in many countries, a high percentage of producers are small or medium-sized farms that can afford neither complex treatment systems nor specialized operations. The present study assessed the performance of a novel and different combinations of treatment processes, based on changing the anaerobic systems that require a specialized operation for one very simple to operate for farm owners. The assessed system is composed by a septic tank in combination with an up-flow anaerobic filter packed with volcanic rocks and an aerobic biofilter packed with waste wood chips. The effect of the hydraulic residence time and the volumetric organic loading in the septic tank and up-flow anaerobic filter and the effect of surface hydraulic loading in the aerobic biofilter were also evaluated. The system efficiently removed chemical oxygen demand (86–93%), total suspended solids (91–97%), volatile suspended solids (86–97%), and ammoniacal nitrogen (86–87%), showing a constant removal efficiency under a VOL of between 5 and 14.6 kg COD m⁻³ d⁻¹in the up-flow anaerobic filter. The advantages of this system are that the packing materials can be available in rural zones and are sustainable; the whole system is cost-effective and easy to handle; thus, farmers can operate and maintain it with their own means.