Application of Biochar Produced from Crop Residues on Trace Elements Contaminated Soils: Effects on Soil Properties, Enzymatic Activities and Brassica rapa Growth

Soil pollution by trace elements is a huge problem around the globe. In addition, heavy metal immobilization and primary productivity are two soil ecosystem services of contemporary importance to society. The goal of this study was to evaluate the effects of using olive pit and rice husk biochars as soil amendments for the immobilization of trace elements and on plant development growing in heavy metals-polluted soils under greenhouse conditions. The application of high doses (5% and 10%) of biochar significantly increased pH, water holding capacity and total C content of the soils. Dehydrogenase activity in the moderately acidic soil was greater than in the acidic soil due to the high concentration of metals and high acidity of the latter. The application of biochar reduced the β–glucosidase activity. Furthermore, the concentrations of CaCl2-extractable heavy metals significantly decreased in biochar amended pots, indicating metal immobilization, which was consistent with the increase in soil pH. Distribution of trace elements in the different fractions was modified after 65 days of incubation, independently of the treatment. The Cu and Zn contents in the oxidizable fraction were reduced with incubation, whereas Cd and Zn in the residual fraction increased. The reduction of bioavailable concentrations and increments in the residual or more stable fractions indicated less risk for the organisms in the environment. All biochars addition significantly increased the root-to-shoot ratio compared to the control soil. Particularly, 10% of amendment increased this ratio in the greatest extent. The application of 10% w/w of rice husk biochar produced at 500 °C was the most effective in restoring soil functionality and reducing the availability of heavy metals in the polluted soils.

Contribution of Nano-Zero-Valent Iron and Arbuscular Mycorrhizal Fungi to Phytoremediation of Heavy Metal-Contaminated Soil

Soil pollution with heavy metals has attracted increasing concern, which calls for the development of new remediation strategies. The combination of physical, chemical, and biological techniques can achieve more efficient remediation. However, few studies have focused on whether nanomaterials and beneficial microbes can be jointly used to facilitate phytoremediation. Therefore, we studied the role of nano-zero-valent iron (nZVI) and arbuscular mycorrhizal (AM) fungi in the phytoremediation of an acidic soil polluted with Cd, Pb and Zn, using sweet sorghum. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and mapping analyses were conducted to explore the mechanisms of metal immobilization by nZVI. The results showed that although both bare nZVI (B-nZVI) and starch-stabilized nZVI (S-nZVI) inhibited root mycorrhizal colonization, Acaulospora mellea ZZ successfully colonized the plant roots. AM inoculation significantly reduced the concentrations of DTPA-Cd, -Pb, and -Zn in soil, and the concentrations of Cd, Pb, and Zn in plants, indicating that AM fungi substantially facilitated heavy metal immobilization. Both B-nZVI and S-nZVI, ranging from 50 mg/kg to 1000 mg/kg, did not impede plant growth, and generally enhanced the phytoextraction of heavy metals. XRD, EDS and mapping analyses showed that S-nZVI was more susceptible to oxidation than B-nZVI, and thus had more effective immobilization effects on heavy metals. Low concentrations of nZVI (e.g., 100 mg/kg) and AM inoculation had synergistic effects on heavy metal immobilization, reducing the concentrations of Pb and Cd in roots and enhancing root Zn accumulation. In conclusion, our results showed that AM inoculation was effective in immobilizing heavy metals, whereas nZVI had a low phytotoxicity, and they could jointly contribute to the phytoremediation of heavy metal-contaminated soils with sweet sorghum.

Enhancing the Functional and Environmental Properties of Asphalt Binders and Asphalt Mixtures Using Tourmaline Anion Powder Modification

Due to its good piezoelectric and thermoelectrical properties, tourmaline anion powder (TAP) can be used as a potential modifier to improve the piezoelectric, thermoelectric, rheological, and mechanical properties of asphalt binders and asphalt mixtures, respectively. This study was conducted to investigate the functional, piezoelectric, and thermoelectric properties of a TAP-modified asphalt binder (TAPMA) and the corresponding asphalt mixtures. In the study, the TAPMA’s environmental friendliness, such as the volatile organic compound (VOC) adsorption and metal immobilization, were investigated. Compared to TAP at 3.95 pC/N, the piezoelectric constant of TAPMA was found to be 3.42 pC/N. In general, the results indicated that TAP could potentially improve the functional properties of asphalt binders and asphalt mixtures, including the piezoelectric and thermoelectrical properties. With respect to environmental enhancement, the asphalt binder VOC emission reduced to 50% after TAP addition. In terms of metal immobilization, the heavy metals Fe and Ti exhibited the best stability followed by the alkali metals Li, K and Na, and lastly, Ca and Mg, respectively. Nonetheless, the emission concentrations of all the metals were below the regulatory threshold. Furthermore, the study findings also indicated that TAPMA can potentially adsorb the tail gas emissions of vehicles and heavy metals.