Knowledge Mapping of the Phytoremediation of Cadmium-Contaminated Soil: A Bibliometric Analysis from 1994 to 2021.

Cadmium pollution of soil threatens the environmental quality and human health. Phytoremediation of cadmium-contaminated soil has attracted global attention in recent decades. This study aimed to conduct a comprehensive and systematic review of the literature on phytoremediation of cadmium-contaminated soil based on bibliometric analysis. A total of 5494 articles published between 1994 and 2021 were retrieved from the Web of Science Core Collection. Our knowledge mapping presented the authors, journals, countries, institutions, and other basic information to understand the development status of phytoremediation of cadmium-contaminated soil. Based on a keyword cluster analysis, the identified major research domains were "biochar", "Thlaspi caerulescens", "endophytic bacteria", "oxidative stress", "EDTA", and "bioconcentration factor". Overall, this study provided a detailed summary of research trends and hotspots. Based on the keyword co-occurrence and burst analysis, the core concepts and basic theories of this field were completed in 2011. However, the pace of theoretical development has been relatively slow. Finally, future research trends/frontiers were proposed, such as biochar addition, rhizosphere bacterial community manipulation, cadmium subcellular distribution, and health risk assessment.

Method on site-specific source apportionment of domestic soil pollution across China through public data mining: A case study on cadmium from non-ferrous industries.

The lack of emission data of major Cd-emitting enterprises has long limited the source apportionment of soil cadmium (Cd). Non-ferrous metal enterprises (NMEs) contribute the most Cd emissions in China in recent years. We estimated the cumulative Cd emission of 8750 NMEs across China through public data collection and material balance methods for the first time. The results showed that the total Cd emissions were estimated at 133,177 tons, of which 78.68% contributed by zinc primary smelting and mining. The emission hotspots are mainly concentrated in the south of the Yangtze River, such as Nanling Mountain areas, Nanpan River Basin, and Jincheng River Basin, as well as a few parts of the North and Northwest China. Then a significant positive spatial correlation was furtherly detected between NMEs and soil Cd, except for secondary smelting enterprises. Moreover, the hotspots of soil Cd pollution caused by NMEs were identified across China. By promoting the accounting calibrator from annual emission intensity of regional (mainly provincial) scale to the cumulative emission of site-specific enterprise in its entire life cycle, this study realized the finer description of the spatial heterogeneity of Cd emission from non-ferrous industry on a large scale and make it possible to refine the reliability of follow-up site-specific source apportionment, by introducing the emission intensity instead of the enterprise sites density. Finally, a modified approach for the regional source apportionment of soil pollution was proposed to obtain a more realistic and precise drawing. The results pointed out key NMEs subcategories and the affected hotspots which require continuous strengthening of Cd-related rectification. This methodological framework is expected to contribute to the precise management and differential sources control of Cd pollution and can be further extended to other pollutants for the precise targeting of key industries and hotspots during source pollution control in the future.

Impact of Soil Water on the Spectral Characteristics and Accuracy of Energy-Dispersive X-ray Fluorescence Measurement.

Soil water is a major interference in the on-site analysis of soil by energy-dispersive X-ray fluorescence. Apparent consequences of this interference include lowered readings for elemental concentrations and significant changes in spectral characteristics in wet soils compared with dry soils. A rigorous interpretation on this issue remains unresolved. Thus, this study evaluated the impact of soil water on the detection of Ca, Ti, Mn, Fe, Cu, Zn, As, Rb, Sr, and Pb. Specimens were prepared from 11 certified reference soils and 3 field soils with water contents from 0 to ∼40 wt %. Results from three commercial models revealed that the readings were subjected to respective internal quantification algorithms; therefore, they could not provide a fundamental perspective of this issue. We analyzed the spectra to examine the mechanism underlying this phenomenon. The spectra of wet soils feature elevated baseline, increased Compton and Rayleigh scatter peaks, and lowered characteristic peaks of elements. Previous studies attributed the lowered characteristic peaks to the absorption of fluorescent X-rays by water and considered soil water and dry soil as separate layers in the calculation. This work argues that wet soils should be treated as mixtures. Water becomes part of the soil matrix and leads to lower attenuation capability, which could be explained by the matrix effect. Meanwhile, the mass fraction of analytes is lowered because of dilution. Results confirmed that dilution lowers the characteristic peaks, whereas the matrix effect heightens them. When estimating the elemental concentrations on a wet weight basis, the matrix effect becomes the major interference. The Compton compensation method provided satisfying results on correcting the matrix effect caused by soil water on Zn, As, Rb, Sr, and Pb.

New isotopic evidence of lead contamination in wheat grain from atmospheric fallout.

Crops could accumulate trace metals by soil-root transfer and foliar uptake from atmospheric fallout, and an accurate assessment of pollution sources is a prerequisite for preventing heavy metal pollution in agricultural products. In this study, we examined Pb isotope rates to trace the sources of Pb in wheat grain grown in suburbs. Results showed that, even in zones with scarcely any air pollution spots, atmospheric fallout was still a considerable source of Pb accumulation in wheat. The concentration of Pb in wheat grain has poor correlation with that in farm soil. The Pb concentration in wheat grains with dust in bran coat was significantly higher than that in wheat grains, which indicates that Pb may accumulate by foliar uptake. The Pb isotope rate has obvious differences between the soil and atmospheric fallout, and scatter ratio is significantly closer between the wheat grain and atmospheric fallout. Atmospheric fallout is a more significant source of Pb concentration in wheat grains than in soil. As far as we know, this is the first study on the main sources of lead in grain crop (wheat) samples with isotope. This study aims to improve our understanding of the translocation of foliar-absorbed metals to nonexposed parts of plants.

Simulation of water removal process and optimization of aeration strategy in sewage sludge composting.

Reducing moisture in sewage sludge is one of the main goals of sewage sludge composting and biodrying. A mathematical model was used to simulate the performance of water removal under different aeration strategies. Additionally, the correlations between temperature, moisture content (MC), volatile solids (VS), oxygen content (OC), and ambient air temperature and aeration strategies were predicted. The mathematical model was verified based on coefficients of correlation between the measured and predicted results of over 0.80 for OC, MC, and VS, and 0.72 for temperature. The results of the simulation showed that water reduction was enhanced when the average aeration rate (AR) increased to 15.37 m(3) min(-1) (6/34 min/min, AR: 102.46 m(3) min(-1)), above which no further increase was observed. Furthermore, more water was removed under a higher on/off time of 7/33 (min/min, AR: 87.34 m(3) min(-1)), and when ambient air temperature was higher.