Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs’ immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

The Potential of Clover Green Amendment, Associated with Biochar, Activated Carbon or Ochre, for the Phytoremediation, Using Populus x. canescens, of a Former Mine Technosol

Metal(loid) soil pollution resulting from past and present mine activities is a serious environmental and health issues worldwide. Therefore, the remediation of those polluted areas has been a growing research interest over the last decades, especially the assisted phytoremediation. In this study, a pot experiment was set up, using a former mine technosol, highly polluted by As and Pb, to which biochar, activated carbon, or ochre was applied, alone or in combination to clover green amendment. Following amendment application, Populus x. canescens cuttings were planted. Results showed that all four amendments reduced soil acidity. However only the first three amendments immobilized As and Pb, while the green amendment drastically mobilized those two pollutants and none of the amendments improved plant growth. In conclusion, the association of clover green amendment to biochar, activated carbon, or ochre did not appear as an efficient remediation strategy in this case; although the aging of the amendments and degradation of the green amendment in the soil with time could have positive outcomes.

Immobilized Soybean Peroxidase Hybrid Biocatalysts for Efficient Degradation of Various Emerging Pollutants

In the present study, soybean peroxidase (SBP) was covalently immobilized onto two functionalized photocatalytic supports (TiO2 and ZnO) to create novel hybrid biocatalysts (TiO2-SBP and ZnO-SBP). Immobilization caused a slight shift in the pH optima of SBP activity (pH 5.0 to 4.0), whereas the free and TiO2-immobilized SBP showed similar thermal stability profiles. The newly developed hybrid biocatalysts were used for the degradation of 21 emerging pollutants in the presence and absence of 1-hydroxy benzotriazole (HOBT) as a redox mediator. Notably, all the tested pollutants were not equally degraded by the SBP treatment and some of the tested pollutants were either partially degraded or appeared to be recalcitrant to enzymatic degradation. The presence of HOBT enhanced the degradation of the pollutants, while it also inhibited the degradation of some contaminants. Interestingly, TiO2 and ZnO-immobilized SBP displayed better degradation efficiency of a few emerging pollutants than the free enzyme. Furthermore, a combined enzyme-chemical oxidation remediation strategy was employed to degrade two recalcitrant pollutants, which suggest a novel application of these novel hybrid peroxidase-photocatalysts. Lastly, the reusability profile indicated that the TiO2-SBP hybrid biocatalyst retained up to 95% degradation efficiency of a model pollutant (2-mercaptobenzothiazole) after four consecutive degradation cycles.