Fractionation and leaching of heavy metals in soils amended with a new biochar nanocomposite

Novel chitosan-based barrier materials for environmental containment: Synthesis, characterization, and contaminant removal capacities and mechanisms

This study critically appraises employing chitosan as a composite with bentonite, biochar, or both materials as an alternative to conventional barrier materials. A comprehensive literature review was conducted to identify the studies reporting chitosan-bentonite composite (CBC), chitosan amended biochar (CAB), and chitosan-bentonite-biochar composite (CBBC) for effective removal of various contaminants. The study aims to review the synthesis of these composites, identify fundamental properties affecting their adsorption capacities, and examine how these properties affect or enhance the removal abilities of other materials within the composite. Notably, CBC composites have the advantage of adsorbing both cationic and anionic species, such as heavy metals and dyes, due to the cationic nature of chitosan and the anionic nature of montmorillonite, along with the increased accessible surface area due to the clay. CAB composites have the unique advantage of being low-cost sorbents with high specific surface area, affinity for a wide range of contaminants owing to the high surface area and microporosity of biochar, and abundant available functional groups from the chitosan. Limited studies have reported the utilization of CBBC composites to remove various contaminants. These composites can be prepared by combining the steps employed in preparing CBC and CAB composites. They can benefit from the favorable adsorption properties of all three materials while also satisfying the mechanical requirements of a barrier material. This study serves as a knowledge base for future research to develop novel composite barrier materials by incorporating chitosan and biochar as amendments to bentonite.

Study on the enhancement of citric acid chemical leaching of contaminated soil by modified nano zero-valent iron

This study aimed to evaluate the effect of modified nanoscale zero-valent iron (SAS-nZVI) on chemical leaching of lead and cadmium composite contaminated soil by citric acid (CA). The synthesized SAS-nZVI was used as a leaching aid to improve the removal rate of soil heavy metals (HMs) by CA chemical leaching. The effects of various factors such as SAS-nZVI dosage, elution temperature and elution time were studied. At the same time, the effect of chemical leaching on the basic physical and chemical properties of soil and the morphology of HMs was evaluated. The results show that when the SAS-nZVI dosage is 2.0 g/L, the leaching temperature is 25 °C, and the leaching time is 720 min, the maximum removal rates of Pb and Cd in the soil are 77.64% and 97.15% respectively. The experimental results were evaluated using elution and desorption kinetic models (Elovich model, double constant model, diffusion model). The elution and desorption process of Pb and Cd in soil by SAS-nZVI-CA fitted well with the double-constant model, indicating that the desorption kinetic process of Pb and Cd is a heterogeneous diffusion process, and the elution process is controlled by diffusion factors. After leaching with SAS-nZVI-CA, the physical and chemical properties of the soil changed little, the mobility and toxicity of HMs in the soil were reduced, and the HMs content in the leaching waste liquid was reduced. It can be concluded that SAS-nZVI enhances the efficiency of CA in extracting Pb and Cd from soil, minimizes soil damage resulting from chemical leaching technology, and alleviates the challenges associated with treating leaching waste liquid.

Nanomaterials in biochar: Review of their effectiveness in remediating heavy metal-contaminated soils

Biochar can be used for soil remediation in environmentally beneficial manner, especially when combined with nanomaterials. After a decade of research, still, no comprehensive review was conducted on the effectiveness of biochar-based nanocomposites in controlling heavy metal immobilization at soil interfaces. In this paper, the recent progress in immobilizing heavy metals using biochar-based nanocomposite materials were reviewed and compared their efficacy against that of biochar alone. In details, an overview of results on the immobilization of Pb, Cd, Cu, Zn, Cr, and As was presented by different nanocomposites made by various biochars derived from kenaf bar, green tea, residual bark, cornstalk, wheat straw, sawdust, palm fiber, and bagasse. Biochar nanocomposite was found to be most effective when combined with metallic nanoparticles (Fe₃O₄ and FeS) and carbonaceous nanomaterials (graphene oxide and chitosan). This study also devoted special consideration to different remediation mechanisms by which the nanomaterials affect the effectiveness of the immobilization process. The effects of nanocomposites on soil characteristics related to pollution migration, phytotoxicity, and soil microbial composition were assessed. A future perspective on nanocomposites' use in contaminated soils was presented.

Performance of new biodegradable chelants in enhancing phytoextraction of heavy metals from a contaminated calcareous soil

Chelant-assisted phytoextraction has widely been exploited as a feasible option for removing heavy metals from the contaminated soils. Some synthetic chelants have shown promising performances for this option, but they have also revealed several negative environmental consequences. This study has sought to investigate the feasibility of two biodegradable eco-friendly chelants, namely methylglycinediacetic acid (MGDA) and N,N-Bis(carboxymethyl)-L-glutamic acid (GLDA), as compared to the resistant ethylenediaminetetraacetic acid (EDTA), in enhancing phytoextraction of Zn and Pb from a contaminated calcareous soil. For this purpose, a greenhouse experiment was carried out comparing the growth and metal absorption of maize (Zea mays L.) grown on soils treated with EDTA, MGDA, and GLDA chelants at 2, 4 and 8 mmol kg^- 1 levels. Results showed that the heavy metal uptakes by the plant shoots generally increased with increasing the chelant application level. Pb uptake by maize shoots increased from 10.6 mg plant^- 1 in control to 416, 398, and 416 mg plant^- 1 in the soils treated with 8 mmol kg^- 1 MGDA, GLDA, and EDTA, respectively. The corresponding increases in Zn uptake were from 100.9 mg plant^- 1 to 798.9, 718.9, and 530.4 mg plant^- 1 in the MGDA-, GLDA-, and EDTA-amended soils, respectively. Moreover, the amounts of water-extractable, and thereby potentially leachable, Pb and Zn in the post-harvest soil were considerably greater in the soil treated with EDTA than those treated with MGDA and GLDA. Therefore, MGDA and GLDA would be potential alternatives to environmentally-persistent EDTA for enhanced metal phytoextraction from contaminated soils.

Distinct Dispersion of As, Cd, Pb, and Zn in Farmland Soils near Abandoned Mine Tailings: Field Observation Results in South Korea

We investigated the characteristics of metal(loid) transport and dispersion in agricultural soils near an abandoned metal mine. Topsoil samples were collected from 162 sampling sites in the study area, including 1 in the mine tailing dumps, to analyze the total concentrations of As, Pb, Cd, and Zn. Subsequently, the metal(loid) transport and dispersion characteristics were investigated using geographic information system (GIS) technology. The results of this study clearly demonstrated the variation in the dispersal of As, Cd, Pb, and Zn from the mine tailing dumps to nearby agricultural soils and the element-specific spatial variability in their respective transport and dispersion characteristics. These findings suggested that compared with the migration behavior of Cd, Pb, and Zn, that of As has a farther-reaching impact on agricultural soils owing to its geochemical cycling in the soil and groundwater environment. This impact differed significantly in magnitude from that of the other investigated metals. Therefore, special consideration must be given to the migration behavior of As.

Synergistic immobilization of potentially toxic elements (PTEs) by biochar and nanoparticles in alkaline soil

Biochar and nanoparticle (NP) have the ability to sorb potentially toxic elements (PTEs) from soil and reduce toxicity and leaching into water bodies. However, there is need to tailor biochar formulations to soil types. In this study, we investigate the mobility and chemical forms of Cd, Cr, Cu, Ni, and Zn in a spiked, alkaline soil after amendment with combination of NPs (nano-Fe (NF), nano-clay (NC)) and biochars (almond shell 500 °C, walnut shell 400 °C) in different doses (0, 2.5, 5, and 10%). Many previous studies concluded biochar immobilized PTEs due to an increase in soil pH, which can be disregarded here (soil pH 7.9). In a twenty-week column leaching experiment biochar addition significantly decreased PTE leaching and NP addition further immobilized PTEs in most cases. On average almond biochar more effectively reduced Zn leaching and walnut biochar was more effective in decreasing the leaching of Cd, Cr, and Ni (e.g. 5% biochar reduced Cr leaching by 68%). Copper was immobilized effectively by both biochars. Nano-clay combined with walnut biochar performed best in all treatments, in particular for Cd, Ni, and Zn (e.g. 10% walnut biochar only and in combination with NC reduced Zn leaching by 14.2% and 58.5%, respectively). After amendment, PTEs were present in the Fe-Mn oxides, organic and residual fractions and less in the exchangeable fraction, reducing PTE availability and leachability. The results demonstrate that even for cationic PTEs that behave similarly in the environment optimal biochar-mineral formulations can differ.