Portable X-ray fluorescence (pXRF) spectrometers provide simple, rapid, nondestructive, and cost-effective analysis of the metal contents in soils. The current method for improving pXRF measurement accuracy is soil sample preparation, which inevitably consumes significant amounts of time. To eliminate the influence of sample preparation on PXRF measurements, this study evaluates the performance of pXRF measurements in the prediction of eight heavy metals’ contents through machine learning algorithm linear regression (LR) and multivariate adaptive regression spline (MARS) models. Soil samples were collected from five industrial sites and separated into high-value and low-value datasets with pXRF measurements above or below the background values. The results showed that for Cu and Cr, the MARS models were better than the LR models at prediction (the MARS-R² values were 0.88 and 0.78; the MARS-RPD values were 2.89 and 2.11). For the pXRF low-value dataset, the multivariate MARS models improved the pXRF measurement accuracy, with the R² values improved from 0.032 to 0.39 and the RPD values increased by 0.02 to 0.37. For the pXRF high-value dataset, the univariate MARS models predicted the content of Cu and Cr with less calculation. Our study reveals that machine learning methods can better predict the Cu and Cr of large samples from multiple contaminated sites.

Less attention was paid to the remediation of volatile organic compounds (VOCs) contaminated soil treated by thermal conduction heating (TCH) coupled with chemical oxidization. In this study, the lab-scale remediation experiments of trichloroethylene (TCE)-contaminated soil by TCH and TCH coupled with persulfate (TCH + PS) were performed to explore the influences of PS usage, temperature, reaction time, and the variation of soil properties. TCE was removed from contaminated soils using TCH with a temperature lower than boiling point, and the removal ratio of TCE reached 78.21% with a reaction time of 6h at 60 °C. In the TCH + PS treatments, the removal ratio increased to 87.60~99.50% when the PS dosage was increased from 7.0 mmol/kg to 17.5 mmol/kg at 60 °C. However, the usage efficiency of PS had no positive relationship with oxidant usage and temperature. The treatment with 14 mmol/kg PS after 3h at 50 °C had the highest PS usage ratio of 3.05. In addition, soil pH and soil organic matter (SOM) did not decrease significantly in the TCH-only treatment, while the content of SOM declined by almost 50% after the TCH + PS treatment. Overall, it was concluded that TCH + PS achieved higher removal efficiency, whereas TCH had less disturbance on soil pH and SOM. As such, the applicability of TCH-only or TCH + PS treatments is site-specific.

The coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants. While many studies have been done on the mutual toxicity of Cd2+ and Zn2+, few studies can be found in the literature focused on the interaction of Cd2+ and Zn2+ on soil clay fractions especially in terms of energy relationship. The binding energies of Cd2+ on boggy soil (Histosols) particles and Zn2+ on yellow brown soil (Haplic Luvisols) particles were the highest, while those of Cd2+ and Zn2+ on paddy soil (Inceptisols) particles were the lowest. These results indicated that Cd2+ and Zn2+ have a strong capacity to adsorb in the solid phase at the soil–water interface of boggy soil and yellow brown soil, respectively. However, both Cd2+ and Zn2+ adsorbed on paddy soil particles easily release into the solution of the soil suspension. Unlike the binding energy, the higher adsorption energies of ions in boggy and yellow brown soils showed a weak binding force of ions in boggy soil and yellow brown soil. A 1:1 ratio of Cd2+ to Zn2+ promotes the mutual inhibition of their retentions. Cd2+ and Zn2+ have high mobility and bioavailability in paddy soil and yellow drab soil (Ustalfs), whereas they have high potential mobility and bioavailability in boggy soil and yellow brown soil. In the combined system, Zn2+ had preferential adsorption than Cd2+ on soil clay fractions. Boggy soil and yellow brown soil have a low environmental risk with lower mobility and bioavailability of Cd2+ and Zn2+ while paddy soil and yellow drab soil present a substantial environmental risk. In the combined system, Cd2+ and Zn2+ restrain each other, resulting in the weaker binding force between ions and soil particles at a 1:1 ratio of Cd2+–Zn2+.

Less attention has been paid to soils with potentially toxic elements (PTEs) below agricultural land risk screening values, even though they continue to accumulate these elements. In this study, four typical areas in Ningxia were selected to determine the concentrations of cadmium (Cd) and arsenic (As) in 176 samples collected from 130 sampling sites across seven matrices. A refined mass balance model was developed by partitioning irrigation input into suspended-solid and supernatant phases and crop removal into grain and straw components. The model was used to analyze the contributions of various input and output factors and to predict future soil Cd/As concentrations and their related health risks via soil exposure. The input fluxes of Cd and As in the four regions ranged from 1.31 to 3.25 g·ha⁻¹·yr⁻¹ and 71.43 to 146.99 g·ha⁻¹·yr⁻¹, respectively, mainly contributed by irrigation water (40~64%), especially suspended solids in irrigation water, and atmospheric deposition (23~40%). The output fluxes of Cd and As were 0.89~1.43 g·ha⁻¹·yr⁻¹ and 13.81~33.86 g·ha⁻¹·yr⁻¹, respectively, dominated by crop harvesting (36~82%). The differences in input and output fluxes were mainly caused by the regional industrial structure and agricultural planting structure. The predicted results showed that soil Cd and As concentrations in all regions would not exceed regulatory limits after 100 years in the current scenario. A health risk assessment based on soil ingestion, dermal contact, and inhalation showed that the hazard indices for Cd and As were negligible, but their total carcinogenic risk reached notable levels. Over time, Cd-specific carcinogenic risk for children increased in several scenarios and transitioned from negligible to notable risk, with soil ingestion being the dominant exposure pathway. According to the results, targeted mitigation strategies, including the regulation of atmospheric deposition, optimization of irrigation water quality, and adoption of straw off-field practices, show potential to effectively limit the accumulation of potentially toxic elements in agricultural soils.