The effect of thermal treatment on the transformation and transportation of arsenic and cadmium in soil.

Thermal treatment can effectively decontaminate soils but alter their properties. Previous research mainly focused on volatile organic compounds and metals, i.e. Hg, neglecting non-volatile metal(loid)s. This study aimed to investigate Cd and As transformation during aerobic and anaerobic calcination. The results showed that both aerobic and anaerobic calcination increased soil pH by reducing soil organic matter (SOM) content, which also influenced the cation exchange capacity (CEC) and the leaching behavior of Cd and As in the soil. The total concentrations of Cd and As in the calcined soils varied depending on the calcination temperature and atmosphere. When the aerobic calcination temperature exceeded 700 °C, Cd volatilized as CdCl2, while anaerobic calcination at relatively low temperatures (600 °C) involved reductive reactions, resulting in the formation of metallic Cd with a lower boiling point. Similarly, As volatilized at 800 °C aerobically and 600 °C anaerobically. The formation of As-based minerals, particularly Ca3(AsO4)2, hindered its gasification, whereas anaerobic calcination promoted volatilization efficiency through the generation of C-As(III) based gaseous components with lower boiling points. Contrasting trends were observed in the TCLP-extractable Cd and As contents of the calcined soils. Over 70% of TCLP-extractable Cd contents were suppressed after thermal treatment, attributed to the elevated pH and reduced CEC of the soil, as well as volatilization. However, TCLP-extractable As contents increased with elevated temperatures, likely due to the desorption of AsO43- and re-adsorption of gaseous As2O3 during cooling. These findings have implications for assessing the environmental impact of thermal treatment and provide insights for remediation strategies concerning Cd and As-contaminated soils.

Effects of Zn in sludge-derived biochar on Cd immobilization and biological uptake by lettuce.

Considering the high Zn content of municipal sewage sludge and its competition with Cd during plant uptake due to their similar properties, the presence of Zn in sludge-derived biochar (SDBC) may affect Cd immobilization and uptake by plants. To confirm this, SDBC samples with different Zn contents were prepared and characterized. Their Cd immobilization behavior was studied by conducting batch sorption experiments, and their effects on Cd uptake by lettuce were explored by conducting hydroponic experiments. The results reveal that some Zn contained in the sewage sludge was transformed into ZnO during pyrolysis. The Brunauer-Emmett-Teller (BET) surface area of the SDBC samples containing 2324 mg kg-1 Zn (BC-2324) was 18.3 m2 g-1, which was 132% larger than that of the samples containing 1438 mg kg-1 Zn (BC-1438). The SDBC samples containing 1901 mg kg-1 (BC-1901) exhibited the highest Langmuir sorption capacity of 3476 mg kg-1, which is 115% higher than that of SB-1438. Furthermore, the lettuce remedied with SB-1901 exhibited 44% more biomass; lower peroxidase, catalase, and malondialdehyde activity; and 18.4% less Cd in the leaves of the lettuce than the lettuce remedied with BC-1438, suggesting the potential benefits of using Zn-rich SDBC for soil amendment.

Effects of light irradiation on the complexes of cadmium and humic acids: The role of thiol groups.

In natural environments, humic substances (HS) play a key role in the control of Cd stability by forming Cd-HS complexes. In this study, we investigated the influence of the photo-irradiation on the Cd levels remaining in Leonardite humic acid (HA) standard solution after chemical precipitation. The kinetic experiment showed that after the irradiation of ultraviolet light, especially UVC, Cd-HA complexes become more stable. The coexisting Cd in the HA solution under light irradiation seemed to enhance the stability of Cd-HA complexes. Both Fourier transform infrared and X-ray photoelectron spectrometric spectra indicated an increase in the thiol amounts in the HA samples treated by light irradiation. Therefore, we developed a fluorescent method to quantify thiol groups in HA with N-(1-pyrenyl) maleimide (NPM). The HA samples were initially spiked with known amounts of glutathione, the reference containing thiol groups, and then was titrated with various amounts of NPM. The measured thiol amounts in the HA treated by UV were found to significantly increase, and the increment was almost equivalent to those of Cd level in HA solution after chemical precipitation. Therefore, it was concluded that the increased thiol groups in HA induced by ultraviolet irradiation resulted in the formation of more stable Cd-HA complexes in solution.